Tungsten-Based Catalysts for Environmental Applications

catalysts Review Tungsten-Based Catalysts for Environmental Applications Fabien Can *, Xavier Courtois * and Daniel Duprez CNRS, UMR7285, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Université de Poitiers, 4 rue Michel Brunet, CEDEX 09, 86073 Poitiers, France; daniel.duprez@univ-poitiers.fr * Correspondence: fabien.can@univ-poitiers.fr (F.C.); xavier.courtois@univ-poitiers.fr (X.C.) Abstract: This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2 ) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3 -based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).



 Keywords: tungsten; WO3 ; deNOx ; VOC; photocatalysis; sensor Citation: Can, F.; Courtois, X.; Duprez, D. Tungsten-Based Catalysts for Environmental Applications. Catalysts 2021, 11, 703. https:// 1. Introduction doi.org/10.3390/catal11060703 Tungsten is the metal having the highest melting point (3410 ◦ C) and a density (19.3 g cm−3 ) close to that of gold (19.32 g cm−3 ). α-W (lattice type: body-centered cube) is the crystallographic stable form of tungsten (lattice parameter: 0.3165 nm) [1]. β-W is a metastable cubic (type A15) form of tungsten, first found in products of WO3 reduction by hydrogen [2]. γ-W is an fcc form of tungsten only detected in thin film. Owing to its unique physical properties, tungsten is widely used in refractory alloys. While group 8–11 elements are currently used as nanoparticles of metals in catalysis, W-based heterogeneous catalysts contain tungsten in the form of oxides, sulfides, carbides or heteropolytungstates [3]. Although less employed than molybdenum sulfides, tungsten sulfides are essential components of hydrotreating catalysts [4,5]. More recently, catalysts and photocatalysts based on tungsten sulfides were developed for application in hydrogen production by water splitting [6–8]. Utilization of tungsten carbides for electrochemical applications was also developed in the domain of hydrogen evolution reaction [9,10]. Due to their high solubility in water, heteropolytungstates serve as precursors of tungsten in the preparation of W-based catalysts [11,12]. They are also utilized in many organic syntheses, especially in oxidation reactions [13]. Associated with metals such cobalt, heteropolytungstates are also excellent water-splitting catalysts [14]. Tungsten-based catalysts are currently utilized for environmental applications (e.g., DeNOx , oxidation in gas and in liquid phases, sensors, photocatalysis). In these catalytic systems, tungsten is nearly always in an oxidized form, either as tungsten oxide particles or in strong interaction with specific supports or dopants. The structures of tungsten oxides, supported or not on other oxides, will be reviewed in Section 2. One of the specificities of tungsten oxides is their acid properties that are examined in Section 3. Catalytic Academic Editors: Annemie Bogaerts and Jean-François Lamonier Received: 11 May 2021 Accepted: 28 May 2021 Published: 2 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Catalysts 2021, 11, 703. https://doi.org/10.3390/catal11060703 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 703 2 of 78 applications are then reviewed in the following sections: selective catalytic reduction of NOx by ammonia over WO3 -V2 O5 and other WO3 -supported catalysts (Section 4), other DeNOx applications (Section 5), total oxidation of pollutants in gas phase and gas sensors (Section 6) and total oxidation of pollutants in liquid phase with special insight into photocatalytic processes (Section 7). Selective oxidation in gas or liquid phase (for instance, in ε the cyclohexanol-to-ε-caprolactone process), as well as the area of heteropolytungstates, will not be reviewed in these sections. Readers interested in these topics are invited to read the recent review by Dai et al. [15]. Due to the high number of works published on the catalytic properties of tungsten-based catalysts, Sections 3–7 will be restricted to the 2014–2020 period, occasionally including some earlier reference papers. 2. Structure of Tungsten-Based Catalysts Although it is important to stress that the bulk crystal structure may have a limited impact on catalysis since reactions occur at the surface of the crystalline solids, which may not exhibit the same chemical composition as the bulk, the structures of tungsten-based catalysts are summarized in this section to offer an in-depth introduction to the materials presented in this review. 2.1. Structure of Unsupported Tungsten Oxides No less than seven tungsten oxides were identified in the tungsten–oxygen system [1,16]. However, three oxides are currently more often encountered in catalysis and in β most applications [3]: WO3 (yellow oxide), WO2.9 or β-tungsten oxide (blue oxide) and WO2.72 or γ-tungsten oxide (violet oxide). γ 2.1.1. Tungsten Trioxide Tungsten trioxide has several allotropic forms, mainly two monoclinic structures α (α-WO3 and γ γ-WO3 ), a triclinic structure β (β-WO3 ) and a hexagonal form (h-WO3 ). All these forms of WO3 consist of corner-sharing octahedra of WO6 units. Monoclinic and triclinic structures form a quasicubic arrangement of the octahedra. They differ by the angles of distortion between the adjacent octahedra. Triclinic WO3 is stable only at low temperature (T < 17 ◦ C) [17]. Ab initio calculations showed that the ideal cubic arrangement of WO6 blocks is not stable and would tend to tetragonal structure [18]. Sun et al. recently compared the potentialities of monoclinic WO3 (m-WO3 ) and hexagonal WO3 (h-WO3 ) in electrochemical and gas adsorption applications [19]. The structures of the two forms of WO3 are represented in Figure 1. Figure 1. Structure of hexagonal WO3 (a) and monoclinic WO3 (b) projected along the (001) direction. Reprinted from Sun et al. [19] with permission from the American Chemical Society. While monoclinic WO3 possesses arrangements of small rhomboid tunnels whose great diagonals are alternatively along x and y axes, hexagonal WO3 has a network of large Catalysts 2021, 11, 703 3 of 78 hexagonal tunnels alternating with small triangular ones. Accessibility of internal surface is then better for h-WO3 than for m-WO3 . This was evidenced in the work of Sun et al. by a higher specific capacitance of h-WO3 with facilitated proton transportation along the tunnels. − Szilágyi et al. investigated the behavior of m- and h-WO3 for the photocatalytic reaction of methyl orange decomposition [20]. The two materials were prepared by annealing hexagonal ammonium tungsten bronze in air or N2 . Hexagonal WO3 is formed by annealing the tungsten bronze (NH4 )x WO3−y at moderate temperature (470–550 ◦ C), while − monoclinic WO3 is formed at higher temperatures (600–650 ◦ C). Decomposition processes of the hexagonal ammonium tungsten bronze were earlier investigated by the same group, who showed that residual ammonia and/or ammonium ions are vital for stabilizing the hexagonal structure [21]. Blue oxides are formed when (NH4 )x WO3−y is decomposed in N2 , which shows the presence of reduced tungsten oxides. On the contrary, yellow oxides are obtained by decomposing the tungsten bronze in air. However, XPS of the oxidized materials shows that small amounts of W5+ and W4+ are still present in the h-WO3 ox, while tungsten in m-WO3 ox is entirely in the form of W6+ . Monoclinic WO3 ox is more active than hexagonal WO3 ox in the photocatalysis reaction because the electron–hole recombination is less probable in WO3 having 100% W6+ (no defect sites). In addition to XRD, Raman spectroscopy is a good technique to distinguish hexagonal and monoclinic WO3 phases (Figure 2). XRD spectra are consistent with m-WO3 (ICDD file 43-1035) and h-WO3 − (ICDD file 85-2460). Peak width broadening observed on reduced samples is indicative of − distortion and creations of defects in the structure. O-W-O stretching bands allow to clearly distinguish m-WO3 (two bands at 714 and 806 cm−1 ) and h-WO3 (three bands at 651, 693 and 786 cm−1 ). By contrast, O-W-O deformation vibrations, in the 260–330 cm−1 range, are virtually recorded at the same wavenumber for the two forms of WO3 . For that reason, deformation vibration bands cannot be used for the characterization of m- and h-WO3 . Figure 2. XRD and Raman spectra of monoclinic WO3 (m-WO3 ) and hexagonal WO3 (h-WO3 ) oxidized (air treatment) and reduced (N2 treatment). Reprinted from Szilágyi et al. [20] with permission from Elsevier. If the presence of ammonium ions allows h-WO3 to be stabilized, it is also a key factor in controlling the shape of nanorod crystals [22]. NH4 + ions are preferably adsorbed on the (001) facets and block the crystal growing along this axis (Figure 3). Hexagonal nanowires − Catalysts 2021, 11, 703 4 of 78 of WO3 were also prepared as anodic materials of Li-ion batteries [23]. Hydrothermal treatment of lithium tungstate–lithium sulfate mixed solutions in HCl medium leads to typical nanowires, as shown in Figure 4. Figure 3. Effect of ammonium ions on the growth orientation of hexagonal single crystal of WO3 nanorods (hydrothermal synthesis from ammonium tungstate). Reprinted from Zhu et al. [22] with permission from the Royal Society of Chemistry. Figure 4. TEM image of h-WO3 nanowires (left) and HRTEM image of an individual wire growing in the (001) direction. Reprinted from Gu et al. [23] with permission from Elsevier. Crystal structures, phase transition and physical properties of nanostructured WO3 oxides have been reviewed by Zheng et al. [24]. Numerous synthesis methods were reported for the preparation of films, ribbons, nanowires, nanotubes, nanorods, etc. In many cases, hexagonal WO3 is observed in these nanostructures. For instance, leaflike nanoplatelets were prepared by dehydration of orthorhombic tungstite H2 WO4 to monoclinic WO3 [25]. 2.1.2. Tungsten Trioxide WO2.9 Blue Oxide (β-Tungsten Oxide W20 O58 ) and Other Suboxides WO3−δ (δ < 0.13) Tungsten suboxides phases with formulae Wn O3n−1 and Wn O3n−2 were first described by Magnéli in 1950 [26,27]. They were fully described by Tilley in 1995 [28]. The lattice (symmetry P2/m) consists of blocks of WO6 octahedra joined by sharing corners (as in Catalysts 2021, 11, 703 −δ δ β 5 of 78 − − m-WO3 ) with certain octahedra sharing edges along a zig-zag dislocation line, as illustrated in Figure 5. β Figure 5. Structure of β-tungsten oxide WO2.9 , consisting of WO6 blocks joined by sharing corners (ideal WO3 oxide) or by sharing edges. WO6 octahedra sharing edges create zig-zag distortion inside the structure. Tungsten atoms located along the zig-zag stripes are labeled. Reprinted from [29]. WO3−−x suboxides are formed when WO3 is treated in H2 or alcohol at moderate temperature (>400 ◦ C). First, oxygen vacancies are created without a definite compound being formedβ [30]. β-Tungsten oxide is usually prepared by heating a mixture of WO3 and tungsten powder at high temperature (~1000 ◦ C), by reduction of WO3 in hydrogen around 650 ◦ C [1,27] or by reduction of ammonium paratungstate ((NH4 )10 H2 W12 O42 , 4H2 O) in hydrogen from 430 to 650 ◦ C [31]. It should be noted that the color of ultrafine “WO3 ” powders may change from yellow for the greatest particle sizes (>50 nm) to blue for the finest particles (<10 nm) [32]. In the meanwhile, the structure of ultrafine powders changes from triclinic (>50 nm) to cubic (<10 nm). A cubic phase of blue oxide nanosheets was synthetized by direct decomposition of H2 WO4 ·H2 O in H2 /N2 [33]. The cubic phase is stable up to 280 ◦ C and tends to form a monoclinic phase, complete at 410 ◦ C. Structures similar to those shown in Figure 5 are described as crystallographic shear (CS) by Tilley [28]. By varying the number and orientation of WO3 blocks sharing edges, it seems possible to reach WO2.889 (W18 O52 ) composition. 2.1.3. WO2.72 Violet Oxide γ(γ-Tungsten Oxide W18 O49 ) and Other Suboxides WO −δ −δ 3δ (δ > 0.2) More complex structures are obtained when the O/W ratio decreases to less than 2.88. Solonin et al. described a suboxide WO2.8 directly derived from hexagonal WO3 [34]. They named it h-WO2.8 ; it is formed at the initial stage of h-WO3 reduction. Mesostructured WO2.83 (W24 O68 ) was recently prepared by Cheng et al. by using KIT6 silica as a hard template [35]. XRD confirmed the formation of monoclinic WO2.83 (PDF # 36-0103). This result contrasts with the formation of WO2.9 and WO2.72 by reduction of bulk WO3 (vide infra). Cheng et al. showed that the reduction of meso-WO3 to meso-WO2.83 occurred at low temperature due to a fast hydrogen diffusion through the mesoporosity (Figure 6). Catalysts 2021, 11, 703 6 of 78 Figure 6. Compared reduction of bulk WO3 and meso-WO3 (a) and corresponding H2 -TPR profiles (b). From Cheng et al. [35] with permission from the Royal Society of Chemistry. Violet oxide is generally formed by reduction of WO3 in hydrogen at 900 ◦ C in the presence of water vapor [1]. Sarin showed that WO2.72 is the most stable oxide formed by reduction of WO3 [36]. Intermediary blue oxide WO2.90 is difficult to observe. As shown in Figure 7, violet oxide tends to form whiskers [37] or nanoneedles [38]. This 1D morphology seems to be characteristic of WO2.72 oxide. While ultrasmall nanoparticles of WO3 are not easily prepared, Soultanidis et al. succeeded in preparing 1.6 nm particles by thermal decomposition of ammonium metatungstate in oleyamine [39]. XRD revealed that these particles are mainly composed of the W18 O49 phase (JCPDS 05-0392). Interestingly, XPS analysis can be performed to assess the chemical composition of tungsten oxide nanopowders. The x value in WO3−x can be determined by means of the ratio between W5+ 4f and W6+ 4f-states. For instance, three series of WO3−x were synthesized by Shpak et al. at atmospheric pressure by electric explosion of wires (EEW) with different proportions between argon and oxygen [40]. In Figure 8, WO3−x , WO2.9 and WO2.72 nanopowder compositions were determined using the W6+ response (comps. d-d, W4f7/2 at 35.7 eV; comps. e-e, W4f7/2 at 36.1 eV for hydroxide) and the W5+ -response (comps. c-c, W4f7/2 at 34.8 eV). Catalysts 2021, 11, 703 7 of 78 − − − Figure 7. Morphology of violet oxide WO2.72 : random-plate-like whiskers prepared by Pfeifer et al. [37] (left, reprinted with permission from Elsevier); nanoneedles used by Wu [38] from commercial samples (right, reprinted with permission from Elsevier). − − − Figure 8. Peak synthesis for W4f-level XPS spectrum of tungsten atoms. The maxima of peak couples correspond to W4f7/2 and W4f5/2 levels of tungsten atoms for W5+ states of oxide (comps. c-c, W4f7/2 at 34.8 eV) and W6+ states of oxide (comps. d-d W4f7/2 at 35.7 eV) and hydroxide (comps. e-e, W4f7/2 at 36.1 eV). From Shpak et al. [40], with permission from Elsevier. 2.1.4. Tungsten Dioxide (WO2 ) Tungsten dioxide is a rutile-like oxide, but contrary to TiO2 , WO2 has a monoclinic structure with distorted octahedron of oxygen ions (C5 2h , P21 /c). This was confirmed by Ben-Dor and Shimony, who succeeded in preparing WO2 monocrystals [41]. The monoclinic structure was later refined by powder neutron diffraction (PDF# 32–1393) [42]. An orthorhombic form of WO2 was also reported (PDF# 82–728); it was evidenced in sonochemical preparation [43]. Most preparations of WO2 were carried out by annealing of WO3 at high temperatures (up to 1000 ◦ C). A facile preparation at low temperature (500 ◦ C) was reported by Coşkun and Koziol by hydrolysis of WCl6 in the presence of NaBH4 [44]. Catalysts 2021, 11, 703 8 of 78 Compared to other tungsten oxides, WO2 presents relatively high conductivity properties. The energy bands and density of states from theoretical calculations confirm a metal-like behavior of WO2 [45]. 2.2. Structure of Supported Tungsten Oxides The structure of tungsten species dispersed on oxide supports depends on both the nature of support and the concentration of tungsten. In 2007, Knowles et al. reported a detailed overview of the surface chemistry of supported oxide, especially tungsten oxide [46]. WO3 is more easily dispersed on alumina than on silica, on which WO3 tends to form agglomerated crystallites [47]. The surface structure of tungsten oxide on various oxide supports (Al2 O3 , TiO2 , Nb2 O5 , ZrO2 , SiO2 and MgO) was investigated by Raman spectroscopy by Kim et al. [48]. WO3 on Al2 O3 , TiO2 , Nb2 O5 and ZrO2 exhibits strong bands at 1005–1020 cm−1 characteristic of terminal W=O mono-oxo species. Bands in the 800–960 cm−1 spectral region are also observed. They are attributed to W-O-W species whose intensity increases with the tungsten surface coverage. Bands at 1005–1020 cm−1 are not observed in Raman spectra of WO3 /SiO2 and WO3 /MgO. Two intense bands at 802 and 715 cm−1 are visible on the spectra of 5%WO3 /SiO2 . These bands are characteristic of the formation of crystalline WO3 , due to the poor dispersion of WO3 on this support. Raman spectra of WO3 /MgO are more complex and suggest the formation of crystalline MgWO4 (as well as CaWO4 due to Ca impurity in MgO). In situ Raman spectroscopy coupled to 18 O/16 O exchange allowed Lee and Wachs to precisely identify the surface structure of different oxides (including WO3 ) dispersed on silica [49]. In this study, WO3 /SiO2 showed Raman bands at 968-985-1014 cm−1 shifted to 920-935-963 cm−1 in totally 18 O-exchanged solids. Monoxo and dioxo tungstate species were formed, while crystallized WO3 was virtually absent (contrary to the previous study). Decisive information on the structure of supported tungsten catalysts can be obtained by UV–Vis. Ross-Meedgarden and Wachs showed that the electronic edge energy (Eg) can be correlated to the nature of W surface species, especially the number of nearest cations surrounding the central M cation [50]. The number of covalent W-O-W bonds around the central W(VI) cation would be given by Equation (1): NW-O-W = 11.89 − 2.37Eg (1) with Eg varying from 5.5 eV for isolated WO4 species (as bi-grafted, di-oxo WO4 surface sites [51]) to 2.7 eV for crystallized WO3 3D structures. Though WO3 tends to form nanocrystals on silica, there remains an amorphous fraction more or less dispersed. Chauvin et al. developed a methodology coupling XRD and Raman studies for the quantification of W species: surface dispersed WOx (monomeric and oligomeric), amorphous WO3 and crystallized WO3 [52]. On silica, only dispersed W species (monomeric and polymeric) would be formed below 1 W nm−2 . For higher concentrations, WO3 nanoparticles (amorphous and crystallized) are evidenced. This contrasts with other supports (Al2 O3 [53,54], TiO2 [55,56], ZrO2 [57–59]) for which tungsten impregnation up to 4–5 W nm−2 leads to well-dispersed catalysts. On these supports, WO3 nanocrystals are generally observed in the 4.5–9 W nm−2 loading range, while large bulklike WO3 crystals can be observed beyond 9 W nm−2 [60]. In the last decades, the WO3 -ZrO2 catalytic system has been the subject of a huge number of investigations owing to its excellent performances in reactions requiring strong acid sites [61,62]. Recent investigations by HAADF [63] and HRTEM [64] allowed obtaining detailed pictures of the various tungsten species populating the zirconia surface (Figure 9a,b). Catalysts 2021, 11, 703 9 of 78 Figure 9. Models of tungsten surface species evidenced (a) by STEM and HAADF (Zhou et al. [63], reprinted with permission from Elsevier) and (b) by HRTEM (De Angel et al. [64], reprinted with permission from Elsevier). It should be noted that WO3 clusters (crystallized or not) may contain Zr atoms from the support (Figure 9a). Tungstate species depicted in Figure 9 are in agreement with the model of growth mode of W species when the surface density exceeds 5 W nm−−2 [65]. Preparation of tunsgtated zirconia catalysts from Lindqvist-type complexes containing (W5 O18 Zr)2−− moieties confirmed the structure of such surface species containing both W and Zr atoms, as well as their role in acid catalysis [66]. Structures of oxo-W(VI) species deposited on TiO2 solids were extensively studied by Tribalis et al. by Raman spectroscopy [56]. Molecular configuration of W surface species depends on the temperature of 1 (Ti OH···OWO , main species at calcination in O2 . Raman features shifted from 930 cm− 2 3 − −1 (Ti-O-WO at 100 ◦ C), then 970 cm−1 ((Ti-O) -W(=O) at 120–250 ◦ C) 25 ◦ C) to 950 cm 3 2 2 − − and finally 1030−cm−1 for the trisubstituted mono-oxo (Ti-O)3 -W=O at 430 ◦ C. Though W dispersion is not easily obtained on amorphous silica, recent developments of mesostructured silicas (MCM 41 [67,68], KIT-5 and KIT-6 [69,70], SBA-15 and SBA-16 [71–73]) has allowed inserting significant amounts of tungsten while limiting the formation of WO3 nanocrystals. This technique has been extended to spongelike silicate TUD-1 discovered at Delft University [74]. Isolated WO4 2- species could be anchored on Ti-modified TUD-1 up to 30% WO3 [75]. Other techniques including aerosol-assisted sol–gel processes [76] or grafting of tungsten-containing molecules on silica [77,78] were also developed to obtain highly dispersed “single-site” W species. Catalysts 2021, 11, 703 10 of 78 As a final statement, critical insights of tungsten-based materials reveal no less than seven tungsten–oxygen systems, with three arrangements mainly encountered in the catalysis field: WO3 , WO2.9 (β-tungsten oxide) and WO2.72 (γ-tungsten oxide). WO3 presents several allotropic forms: monoclinic, triclinic and hexagonal structure. The structure impacts the photocatalysis activity since the electron–hole recombination is favored by the absence of defects. Tungsten suboxide phase (WO2.9 ) is formed under H2 treatment and exhibits oxygen vacancies. More complex structures are obtained when the O/W ratio decreases to less than 2.88, and WO2.72 is the most stable oxide formed by reduction of WO3 . 3. Acidity of Supported Tungsten Oxide Catalysts Tungsten oxide WO3 is among the most acidic transition metal oxides [79]. Very few studies have been devoted to the acid properties of unsupported tungsten oxides. Kanan et al. studied the change of acid site concentration upon dehydroxylation/dehydration of the surface of monoclinic WO3 [80]. They concluded that the changes in Brønsted/Lewis (B/L) acid site concentration were not related to the degree of dehydroxylation but rather to a mild reduction of the tungsten oxide surface. Choi et al. investigated surface acidity of mesoporous WO3 synthesized using KIT-6 as a hard template [81]. Acid site characterization (NH3 -TPD, FTIR of adsorbed pyridine) showed that mesoporous WO3 exposes predominantly Lewis acid sites associated with W6+ and probably cus-W6+ (cus = coordinatively unsaturated sites). A similar study performed by Kasian et al. showed that acid site concentration of mesoporous tungsten oxide was increased by a mild reductive treatment in hydrogen at 250 ◦ C [82]. Yue et al. studied the reactivity of m-WO3 , h-WO3 , hexagonal tungsten bronze (HATB) and (NH4 )0.33−x WO3−z for the hydroconversion of n-heptane reaction (Pd was added to the W oxides as hydrogenation function) [83]. The tungsten oxides were reduced in H2 at 440 ◦ C, 35 bar. The n-C7 reactivity revealed the formation of strong Brønsted acid sites, certainly linked to the reduction of W6+ to lower valence states, the protons allowing the neutralization of negative charges localized on W-O-W formed with W(6-x)+ ions. Li et al. investigated the behavior of MoO3 or WO3 clusters in the conversion of labeled ethanol (CH2 CH3 OD) [84]. Dehydration and dehydrogenation can occur. The overall alcohol conversion would be correlated to Lewis acidity, while the selectivity (dehydration vs. dehydrogenation) would reflect the redox properties of the oxide (i.e., the propensity of Mo or W to reduce from VI to V state in the reaction medium). The reaction was carried out with (MO3 )3 trimers supported on graphene. Other supports and other reactions were tested with contrasted differences between Mo and W oxides depending on the substrate (see the review [85]). 3.1. General Overview Barton et al. reported that supported tungsten oxide catalysts are able to generate Brønsted acid sites when W6+ species are replaced by tungsten cations with lower valency [86]. A general overview of the acidity of tungstated oxides was performed by Zaki et al., who compared the relative strength of acid sites of 10% WO3 samples deposited on Al2 O3 (69 m2 g−1 ), TiO2 (34 m2 g−1 ) and SiO2 (136 m2 g−1 ) [87]. All samples were calcined at 500 ◦ C. Well-dispersed monotungstates and polytungstates were the most abundant surface species on WO3 /Al2 O3 , while 3D polytungstates were observed on WO3 /TiO2 . On WO3 /SiO2 , tungstosilicates and crystallized WO3 would be the dominant surface structures. Acid sites were monitored by pyridine adsorption, which can lead to four species: LPy (Lewis acid bound Py), BPy (Brønsted acid bound Py), HPy (hydrogen-bonded Py) and PPy (physically adsorbed Py). FTIR spectra of adsorbed pyridine were recorded at room temperature (Figure 10) and after desorption at 100, 200 and 300 ◦ C. Catalysts 2021, 11, 703 11 of 78 Figure 10. FTIR spectra of adsorbed pyridine at room temperature on Al2 O3 (Al), TiO2 (Ti), SiO2 (Si) and 10%WO3 on these supports (code: WAl, WTi and WSi, respectively). From Zaki et al. [87] with permission from Elsevier. Hydrogen-bonded pyridine is the main species at RT on WO3 /SiO2 , but it is unstable and tends to disappear upon heating. Lewis acid sites (LPy) and hydrogen-bonded pyridine (HPy) are clearly observed on WO3 /Al2 O3 , while both BPy and LPy sites are identified on WO3 /TiO2 . Brønsted sites would be very strong on WO3 /TiO2 (still intense upon heating at 300 ◦ C). The work of Zaki et al. [87] clearly underlines the critical role of the support of tungsten oxides, both in the nature of W species and in their respective acidity. 3.2. Silica-Supported Tungsten Oxides Due to the quasiabsence of acid sites on silica [88], WO3 /SiO2 materials were often used to investigate the acidity of tungsten oxides. In their study on WO3 /SiO2 catalysts (0 to 11.7 wt.% W on a silica of 200 m− 2 g−1 , i.e., 0 to 2.4 W− nm−2 ), Chauvin et al. monitored the acidity of the samples by FTIR spectroscopy of adsorbed 2,6-dimethylpyridine (lutidine) [52]. The spectra reported in Figure 11a show the presence of Brønsted sites (bands at 1643 and 1630 cm−1 ). Lewis sites that should give bands at 1620–1600 cm−1 are clearly Catalysts 2021, 11, 703 12 of 78 − − absent from the samples. Amounts of Brønsted sites linearly increase with the WO3 surface density up to 1.5 W −nm−2 (Figure 11b). Figure 11. (a) FTIR spectra following lutidine adsorption and evacuation at 150 ◦ C over WO3 /SiO2 of various W surface densities; (b) amounts of B sites measured after lutidine evacuation at 150 ◦ C (black circles), 200 ◦ C (open circles) and 250 ◦ C (black squares). From Chauvin et al. [52] with permission from the American Chemical Society. The acidity of WO3 /SBA-15 oxides was characterized by Hu et al. using pyridine adsorption, solid-state NMR and quantum chemistry calculation [89]. The study was completed by measurements and calculations of 15 N NMR chemical shift tensors of pyridine interacting with tungsten oxides and silica. It was shown that W-OH-W dimers are the Catalysts 2021, 11, 703 13 of 78 principal Brønsted acid sites. W-OH monomers and silanols are very stable with minimal Brønsted acidity. Contrasting with the work of Hue, Gonzalez et al. found quasiexclusively Lewis acid sites in a series of WO3 /SBA-15 materials [90]. Acidity was characterized by pyridine adsorption monitored by FTIR. Bands at 1595 and 1445 cm−1 attributed to pyridine coordinated to Lewis acid sites are much more intense than the 1545 cm−1 band characteristic of protonated pyridine bound to Brønsted acid sites. However, even though it is relatively weak, the Brønsted site density seems very stable when the temperature of pyridine adsorption is increased from 50 to 100 ◦ C, while the number of Lewis sites is strongly affected (Table 1). Table 1. Acid site density on SBA-15 and WO3 /SBA-15 measured by pyridine adsorption at 50 and 100 ◦ C. The number of sites is calculated from the integrated area of the band at 1545 cm−1 for Brønsted sites and at 1445 cm−1 for Lewis sites. From González et al. [90]. Brønsted Acid Sites Lewis Acid Sites Sample T (◦ C) SBA-15 50 ◦ C 100 ◦ C 0 0 152 67 5%WO3 /SBA-15 50 ◦ C 100 ◦ C 24 24 920 92 15%WO3 /SBA-15 50 ◦ C 100 ◦ C 34 31 1102 350 25%WO3 /SBA-15 50 ◦ C 100 ◦ C 15 12 1246 458 (µmol g−1 ) (µmol g−1 ) The reasons for the discrepancy between the work of Hu et al. [89] and that of González et al. [90] are not clear. They may originate from the nature of the tungsten precursor (WCl6 in toluene for Hu et al. and ammonium metatungstate for González et al.), the final temperature of calcination (400 ◦ C for Hu et al. and of 600 ◦ C for González et al.) or the method of acid site detection (NMR for Hu et al. and FTIR for González et al.). This question should certainly be reconsidered in the future. WO3 /SiO2 catalysts were prepared by Kulal et al. for liquid phase nitration of aromatics [91]. A sol–gel technique using ammonium metatungstate and ethyl silicate 40 was employed for the preparation. FTIR of adsorbed pyridine revealed the presence of both Lewis and Brønsted sites. Kulal et al. attributed the formation of Brønsted sites to the presence of polytungstate species, while Lewis sites would require rather isolated W ion centers. The B/L ratio increased with the W loading in the catalysts. At low loading (<5% W), there were virtually no Brønsted sites. Bhaumik et al. developed silica-supported WO3 and Ga2 O3 catalysts for lignocellulosic biomass to furfural processes [92]. Although the acidity was not characterized in detail, the authors of this work stated that Lewis sites are the most abundant acid sites on the catalysts. Tungsten species are generally added by impregnation on the silica support. They may also be incorporated during the preparation of the mesoporous silica. Zhu et al. prepared WZr-KIT6 with various Zr/W ratios by addition of pluronic triblock copolymer to a mixture of tetraethoxysilane (TEOS), ammonium tungstate and ZrOCl2 [93]. Acidity of the materials was measured by adsorption of pyridine monitored by NMR. Both Brønsted and Lewis acid sites coexisted on the catalysts. Lewis acid sites would be mainly linked to the presence of zirconium while tungsten is necessary to generate strong Brønsted acid sites. The materials were tested in ethanol dehydration. Interestingly, it was shown that calcination of the coke catalysts regenerated Brønsted sites, which became even stronger than those on the fresh materials. Attempts to modify the activity of WO3 /SiO2 catalysts by means of gold nanoclusters revealed that the acidity of tungsten oxide is not modified by Au (methanol transformation tests) [94]. WO3 /SiO2 is highly selective for dimethyl ether, while the presence of gold Catalysts 2021, 11, 703 14 of 78 is necessary for the formation of methyl formate and dimethoxymethane. This result suggests that pure WO3 possesses only acid sites and virtually no redox sites. The addition of WO3 was recently used to give acidity to Pt/SiO2 for glycerol hydrogenolysis to 1,3-propanediol [95]. Tungsten oxide would be present as W25 O73 with a high proportion of W5+ . It also maintains a high dispersion of platinum. 3.3. Alumina-Supported Tungsten Oxides Acidity of WO3 /Al2 O3 was studied in the 1980s and 1990s by Soled et al. [96] and Zhang et al. [97]. While alumina possesses virtually no (or very weak) Brønsted sites, the addition of tungsten oxide generates both Brønsted and Lewis sites. Most probably, WO3 titrates the strongest Lewis sites on alumina to form Brønsted sites. The number and strength of these Brønsted sites increase with the temperature of calcination [96]. The proportion of Brønsted sites also increases with the tungsten loading (Table 2). Table 2. Effect of W loading on the number of Lewis and Brønsted sites of WO3 /Al2 O3 (152 m2 g−1 ). Acid site distribution was determined by a modified Benesi colorimetric technique. From Zhang et al. [97]. WO3 Loading Lewis Sites Brønsted Sites Lewis Sites Brønsted Sites (wt.%) (µmol g−1 ) (µmol g−1 ) (%) (%) 0 190 14 93 7 2.5 207 69 75 25 10 220 125 64 36 30 242 182 57 43 Combining lutidine adsorption and other spectroscopic investigation, Chen et al. obtained precise information about the change in acid site concentration with respect to the W surface density [54]. Their results are summarized in Figure 12, which shows that monomeric tungsten species contain quasiexclusively Lewis acid sites, while Brønsted sites are relatively abundant on polymeric species appearing for W density above 1.4 W nm−2 . The acid properties of WO3 -Al2 O3 catalysts were recently revisited by Kitano et al., who showed that the formation of Brønsted sites depended on the temperature of calcination [98]. They proposed that Brønsted sites are predominantly located at the boundaries between WO3 monolayer domains (Figure 13). Potassium-doped WO3 /Al2 O3 catalysts are able to transform gas mixtures such as CO2 /H2 S/H2 , methanol/H2 S or CH4 /H2 S/CO2 into valuable products such as methyl mercaptan (CH3 SH) [99–101]. All these reactions were reviewed by Taifan and Baltrusaitis [102]. Structural and acid–base properties of K2 O-WO3 -Al2 O3 catalysts were recently investigated by Wang et al. [103], Zhu et al. [104] and Kiani et al. [105]. Raman spectroscopy was used to differentiate and detect crystalline WO3 (bands at 273, 719 and 809 cm−1 ), crystalline K2 WO4 (bands at 324 and 927 cm−1 ) and dispersed surface oligomeric WOx species on alumina (strong band at 1021 cm−1 ). Conformation and position of most of these Raman bands were confirmed by DFT calculation [105]. Surface acidity was probed by NH3 -TPD [103] or NH3 adsorption monitored by FTIR [104], while basicity was probed by CO2 -TPD. K2 O suppressed both Brønsted and Lewis acidity from the surface WOx species or exposed Al2 O3 sites [104]. Adsorption of CO2 , nil on unpromoted WO3 /Al2 O3 (no basic sites), significantly increases on K2 O-promoted catalysts. Residual acidity could play a major role in catalytic activity, while basicity would be essential for a good selectivity for methyl mercaptan [103]. Catalysts 2021, 11, 703 15 of 78 − − − − Figure 12. Correlation between amounts of (a) polymeric species, (b) Brønsted acid sites and (c) Lewis acid sites and the tungsten density on WOx /Al2 O3 catalysts. Amounts of polymeric species were − deduced from the integration of the Raman band at 1022 cm−1 . B and L concentrations were deduced from lutidine adsorption (FTIR). From Chen et al. [54] with permission from Elsevier. Figure 13. Effect of the temperature of calcination on the formation of Brønsted acid sites at the junction between WOx monolayer and alumina. From Kitano et al. [98] with permission from John Wiley and Sons. 3.4. Zirconia-Supported Tungsten Oxides The system WO3 -ZrO2 was earlier reported to possess strong acid sites with H0 < −14.7 [106]. This “super” acidity makes the catalyst able to replace concentrated sulfuric acid for alkylation and isomerization reactions of alkanes [62,106] or aromatics [61,107]. Acidic sites certainly play a great role in these reactions, even though redox properties − of WO3 -ZrO2 can be put forward for explaining the enhanced activity of reduced materi− − Catalysts 2021, 11, 703 16 of 78 als [108]. The effect of the preparation method on the acidity of WO3 -ZrO2 was studied by Santiesteban et al. [109]. Acid sites were titrated by adsorption of 2,6-dimethyl pyridine. − − Brønsted sites (0.054 meq g−1 ) were more abundant than Lewis sites (0.013 meq g−1 ), but −1 − only 20% of Brønsted sites were strong acid sites (0.011 meq g ) in proportion close to Lewis sites. The most efficient preparation consists of reflux/impregnation of ZrO(OH)2 and H2 O with ammonium metatungstate. The acid site structure (Scheme 1) is similar to the model proposed by Afanasiev et al. in 1994 [110]. O OH O W O Zr O W Scheme 1. Acid site model of WO3 -ZrO2 with Lewis site born by tungsten oxo species and Brønsted site on zirconium in the vicinity of tungsten atoms. Adapted from Santiesteban et al. [109] and Afanasiev et al. [110]. Brønsted/Lewis (B/L) site ratio in WO3 -ZrO2 catalysts was measured by Baertsch et al. for various tungsten loadings and in various conditions of pretreatment [111]. Acid sites were evaluated by adsorption/desorption of NH3 or pyridine. Complementary information about the redox sites was obtained by adsorption of O2 and H2 , while the number of hydroxyl groups was measured by D2 /OH isotopic exchange. Brønsted sites were virtually absent on low-loaded catalysts where tungsten was in the form of monomeric species. The B/L ratio gradually increased with the tungsten loading. The presence of H2 during NH3 adsorption led to the generation of strong Brønsted sites. Platinum nanoparticles used in bifunctional Pt/WO3 -ZrO2 catalysts would also be a source of Brønsted site by H spillover between Pt and WO3 [112]. The surface chemistry and acidity of WO3 -ZrO2 catalysts for skeletal isomerization of alkanes were discussed by Di Gregorio and Keller, who suggested that a condensation phenomenon between Lewis and Brønsted sites can occur during the calcination treatment [113] (Figure 14). A combined theoretical–experimental study on the acidity of tungstated zirconia was performed by Galano et al. [114]. They confirmed that the Lewis acidity (band at 1444 cm−−1 for pyridine coordinated to L sites) and Brønsted acidity (band at 1539 cm−−1 for protonated pyridine) is a function of the degree of WO3 polymerization. The number of Brønsted sites increased while that of Lewis sites decreased as the W centers shifted from tetrahedral to octahedral coordination, i.e., when the degree of WOx condensation increased. In parallel, the strength of the Brønsted acid sites tended to decrease. A maximum of Brønsted acidity was observed for a W loading of 7 W nm−−2 . Recent investigations of tungstated zirconia designed for acid-driven catalytic reactions showed that active sites are three-dimensional distorted Zr-WOx clusters of 0.8−1 nm size [59]. Optimum structure for a better activity − tt is obtained when W is in a distorted octahedral environment in close contact with Zr cations. Calcination temperature (up to 800 ◦ C) and nature of zirconium oxyhydroxide used as support play a major role in obtaining higher Brønsted acidity [115]. Though acid properties are generally characterized by adsorption of basic molecules (e.g., NH3 , pyridine) monitored by FTIR, other techniques are available. Li et al. showed that 31 PNMR using TMPO (trimethyl phosphine oxide) as a probe molecule reveals the presence of Brønsted sites on WO3 -ZrO2 prepared by a sol–gel method with Zr(OBu)4 and WCl6 as reagents [107]. TMPO gives a broad band from 40 to 90 ppm, while chemical shift above 57 ppm is characteristic of Brønsted sites. Catalysts 2021, 11, 703 17 of 78 Figure 14. Condensation of Lewis and Brønsted sites to form new surface tungstate species on WO3 /ZrO2 catalysts. Adapted from [113] with permission from Elsevier. Yttrium doping of zirconia is often used to stabilize the tetragonal structure of ZrO2 . The effect of yttrium on the generation of acid sites on WO3 -ZrO2 was investigated by Yamamoto et al. [116]. Acidity of the catalysts (15% WO3 ) was characterized by means of model reactions: n-butane skeletal isomerization, alkylation of anisole with benzyl alcohol, and 2-butanol decomposition. Acidity increased with yttrium content up to 4% Y. In the meanwhile, the tetragonal-to-monoclinic ratio of zirconia increased to reach 85% of tetragonal phase for 4% Y. This proves that t-ZrO2 is more suitable than m-ZrO2 for stabilizing WOx species with the highest acidity. Above 4% Y, the catalyst acidity decreased due to the excessive formation of inactive Y-W-O species. The beneficial role of tetragonal zirconia was demonstrated for different applications of WO3 -ZrO2 catalysts, such as hydrolysis of cellobiose [117] or viscoreduction of heavy crude oil [118]. The mode of preparation of the zirconia support can greatly affect the acidity of the materials. Sol–gel synthesis and extraction of the solvent in supercritical conditions give the most active catalysts for organic reactions of industrial interest: acylation of veratrole with acetic anhydride and acylation of anisole with benzoic anhydride [119]. Doping zirconia Catalysts 2021, 11, 703 18 of 78 with phosphorus was extensively studied by Miao et al. [120,121]. WO3 was deposited on mesoporous zirconium oxophosphate [120] or directly incorporated in the one-pot preparation of meso-ZrPOx to synthesize mesoporous WZrP materials [121]. The catalysts were tested in benzylation of anisole. Unfortunately, the effect of phosphorus could not be clearly established due to the absence of reference WO3 /ZrO3 catalyst in these studies. However, the mode of preparation and the mesoporous texture give the WZrP materials a high activity and a high stability. The nature of acid sites is similar to the acidity spectrum characterized in WO3 /ZrO2 : W-free materials possess Lewis sites (83 µmol g−1 ) and very few Brønsted sites (19 µmol g−1 ). Brønsted site density increases up to 55 µmol g−1 for W/Zr ratio of 0.2. 3.5. Titania-Supported Tungsten Oxides WO3 -TiO2 is also an acid catalyst with activity for n-alkane isomerization [55]. These mixed oxides possess both Lewis sites ascribed to surface Ti4+ species and strong Brønsted sites generated by WO3 nanoclusters. WO3 -TiO2 was proven to have excellent performances in the reaction of sorbitol transformation into biofuels [122]. The reaction proceeds via successive steps of dehydration and hydrogenation, which requires metal–acid bifunctional catalysts. WO3 -TiO2 (acid function) associated with Pt/ZrO2 (metallic function) gave the best performance for the production of C5-C6 hydrocarbons. The main application of WO3 -TiO2 (associated with V2 O5 ) is for the NOx abatement by the NH3 -SCR process [123–125]. While the chemical state of vanadium and tungsten seems to be crucial for a good DeNOx activity [126], acid properties could also play a primary role in the reaction [123,127]. The characterization of V2 O5 -WO3 /TiO2 is examined in Section 4. 3.6. Conclusions Evaluation of the acidity of supported WOx obviously depends on the used probe molecule because of its own basic strength. For instance, NH3 , lutidine and pyridine, the most popular probe molecules for acidity evaluation, display pKa values of 9.23, 6.65 and 5.23, respectively. Consequently, they do not all exhibit the same sensibility toward the Brønsted (or acidic hydroxyl group) and Lewis acid sites (coordinatively unsaturated sites). In addition, the acidity of WOx species clearly depends on the W surface coverage. Indeed, dispersed phase surface sites (WO4 , WO5 ), poorly crystalline WO3 nanoparticles and bulklike crystalline WO3 all show unique acidity. For instance, crystalline WO3 exhibits Brønsted acidity, not encountered on dispersed WO4 sites. Consequently, the acidity of catalysts strongly depends on the obtained WOx surface species, which depend on the preparation protocol and the tungsten loading. Brønsted acid sites are reported to be mostly dependent on the presence of polytungstate polymeric species. Lewis acid sites are associated with coordinatively unsaturated W6+ sites or isolated W ions centers. The B/L ratio depends on the support. Lewis sites are the most abundant acid sites on WO3 /SiO2 catalysts. Addition of tungsten oxide on alumina generates both Brønsted and Lewis sites, and the proportion of Brønsted sites increases with the tungsten loading. The stronger acid sites are encountered on the WO3 -ZrO2 system, where a condensation phenomenon between Lewis and Brønsted sites can occur during the calcination treatment. New surface tungstate species appear with various degrees of WO3 polymerization, leading to octahedral coordination that promotes an increase in the amount of Brønsted acid sites. 4. WO3 Catalysts for DeNOx Applications: WO3 -V2 O5 /TiO2 and Other Tungsten-Based Materials Nitric oxide abatement in nitric acid plants was initially carried out by NH3 -SCR over V2 O5 -TiO2 catalysts. Very soon, it was proven that adding WO3 (or MoO3 ) to the catalyst formulation led to a dramatic improvement of the performance. These V-W/TiO2 catalysts were also implemented in DeNOx autocatalyst processes, especially for heavyduty engines, working with NO2 /NOx inlet ratio of 0 or 0.5, related to the standard and fast SCR stoichiometry, respectively. The role of WO3 was summarized by Chen and Yang in 1992 [123]: (i) it increases the activity and widens the temperature window; (ii) it increases Catalysts 2021, 11, 703 19 of 78 the resistance to various poisons (alkalis, arsenic); (iii) it reduces ammonia oxidation by O2, which is a nondesired reaction (especially possible at elevated temperature, T > 400 ◦ C). Kompio et al. also showed by different techniques (e.g., Raman spectroscopy, EPR, H2 TPR) that tungsten allowed an increase in V-O-V species by confining vanadia in small clusters [128]. Most of these effects were confirmed and detailed in a recent review by Lai and Wachs [129], who stressed three important features of the reaction: (i) the role of V5+ surface species as active sites: WO3 is not active per se but promotes the reaction by vanadia by formation of oligomeric vanadia (V2 O5 ) sites or crowding; (ii) the specific role of Brønsted acid sites in NH3 activation: most of the Lewis sites are converted to Brønsted sites in the presence of moisture at 250 ◦ C; (iii) the detailed mechanisms reported by Topsoe and Dumesic et al. in the 1990s for V2 O5 -TiO2 [130,131] (see Figure 15) seemed to be still valid in the 2010s for V2 O5 -WO3 /TiO2, as also recently summarized by Han et al. [132]. Figure 15. Mechanism of NH3 -SCR over V2 O5 -TiO2 . It combines NH3 activation over Brønsted acid sites and a reoxidation of V4+ to V5+ (Brønsted V4+ -OH to vanadyl V5+ = O species). Insertion of NO into ammonium ion would be a key step of the mechanism. Reprinted from [130] with permission from Elsevier. This mechanism, however, contrasts with the amide–nitrosamide mechanism proposed by Lietti et al. [133] in which NH3 is predominantly activated on Lewis sites as amide species (Equation (2)). The main steps of this mechanism lead to the formation of NH2 NO (nitrosamide) (Equation (3)), which further decomposes into N2 + H2 O. 4+ H3 N:→V5+ − O2− ⇒ V − NH2 + OH− − −  − 4+ V4+ − NH −2 + NO ⇒ V − NH − 2 NO − (2) (3) Another important feature to consider is the formation of N2 O. Nitrous oxide is a potent greenhouse gas with a global warming potential of 295–300 times that of CO2 [134,135]. Its concentration in exhaust gases should be controlled. Djerad et al. reported that N2 O is formed at high temperature (T > 300 ◦ C) in the NH3 -SCR process over V2 O5 WO3 /TiO2 [136]. Nitrous oxide concentration increases with the vanadium loading in the catalyst and is formed via NH3 oxidation (Equation (4)). 2NH3 + 2O2 → N2 O + 3H2 O (4) Liu et al. showed that the use of titania supports with high surface area would be a key to reduce N2 O formation [137]. Vanadium being more dispersed on these supports, ammonia oxidation to N2 O could be favored on polymerized vanadyl species. Catalysts 2021, 11, 703 20 of 78 4.1. Mechanisms and Surface Intermediates The nature of active sites and the surface intermediates were investigated by Zhu et al. by in situ IR spectroscopy [138]. The first step of the mechanism would be the adsorption of ammonia on acid sites either as NH3ads (on Lewis (L) sites) or NH4 + (on Brønsted (B) sites). The surface VOx sites (V5+ L and V5+ B ) are the active sites for the reaction, tungsta sites (W6+ L and W6+ B ) being significantly less active. Both B and L sites participate in the reaction. However, if Brønsted sites are more abundant, Lewis sites possess the highest turnover frequency. Overall, ammonia adsorption is part of a complex NOx removal mechanism and results in hydrogen abstraction to provide –NH2 amide-type species. Hydrogen migration is involved in both acidity conversion for Lewis to Brønsted acid site transformation at T > 300 ◦ C and reoxidation for the redox loop of the DeNOx reaction [139]. In addition, coordination of ammonia on Lewis acid sites does not prevent the dissociative H2 O chemisorption and the formation of Brønsted acid sites, leading to adsorbed NH4 + species [140]. Surface acidity and redox properties, as well as their effect on SCR activity, were investigated by Zhao et al. on a series of Vn W-TiO2 with various vanadium loadings (1–13 wt.%), while keeping the tungsten loading constant (8 wt.% WO3 ) [141]. The main characteristics of the series are reported in Table 3. The highest activity observed for the V9W-TiO2 sample is correlated with both higher V4+ /V5+ ratio and higher Brønsted concentration sites. Redox properties do not seem to play a major role. Additional conclusions resulting from experimental analyses and DFT calculations show that the structure of vanadyl species plays a crucial role in DeNOx performances; polymeric species demonstrate higher NH3 -SCR activity than monomeric vanadyl compounds [142]. Besides, based on XPS results, low-valence vanadium species (V4+ and V3+ ) were recently claimed to enhance SCR activity compared to the V5+ form [143]. Both low-valence vanadium species and polymeric surface VOx species drive the NH3 -SCR activity at low temperature (T < 350 ◦ C), as previously mentioned in [142]. Table 3. Vanadium state and acid and redox properties of a series of VnW-TiO2 catalysts (n: vanadium loading (%), 1 ≤ n ≤ 13; [WO3 ] = 8%). TiO2 (120 m2 g−1 ) mainly as anatase. Acid site concentrations were measured by pyridine adsorption monitored by FTIR (bands at 1580 and 1633 cm−1 for Brønsted sites; bands at 1597 and 1438 cm−1 for Lewis sites). H2 -TPR peaks are between 535 and 569 ◦ C, while O2 -TPO gives peaks around 450–460 ◦ C. The mean rate of reduction (νTPR) and oxidation (νTPO) are given in the table. SCR activity was measured between 150 and 420 ◦ C in a flow of 1000 ppm NO + 1000 ppm NH3 in 5% O2 (WHSV: 20,000 h−1 ). NO conversion at 200 ◦ C is representative of the activity order. From Zhao et al. [141]. Catalyst V1W/TiO2 V5W/TiO2 V7W/TiO2 V9W/TiO2 V11W/TiO2 V13W/TiO2 V4+ /V5+ B Sites L Sites νTPR νTPO SCR Activity: NO Conv. at 200 ◦ C (XPS) (µmol m−2 ) (µmol m−2 ) (nmol m−2 s−1 ) (nmol m−2 s−1 ) (%) 0.45 0.99 1.47 1.73 0.88 0.50 28 57 149 164 140 98 131 153 292 318 325 485 2.4 4.6 7.5 8.6 8.8 1.1 0.95 1.4 2.1 2.9 7.6 1.0 19.5 38.0 41.5 60.5 51.5 44.0 The crystallinity of WO3 has a significant effect on vanadia dispersion [127]. Twodimension VOx moieties have a tendency to anchor onto the titania surface in the vicinity of WOx . Small WO3 crystallites are preferable to WOx amorphous layer occupying a large surface of titania. Nevertheless, a compromise should be found because bulk tungsten oxide is less acidic than the dispersed oxide. Interactions between tungsten oxide and vanadia were reinvestigated by Kompio et al. to better understand the promoter effect of tungsten on SCR activity during heat treatments and thermal stress [144]. The study was carried out on a V1.5-W10/TiO2 catalyst (anatase, 140 m2 g−1 after low-temperature calcination at 350 ◦ C). Heat treatments generally provoke catalyst deactivation. However, several maxima of activity were observed during the Catalysts 2021, 11, 703 21 of 78 global decrease in activity when the support BET area reached 42 m2 g−1 and 20 m2 g−1 . New surface sites were then created because isolated vanadia species, less active, were replaced by more active bridged V-O-V sites. Similar active species were proposed by Kwon et al. for non-tungstated V/TiO2 catalysts calcined at 600 ◦ C [145]. Marberger et al. underlined the fact that the presence of water is required to judge the thermal stability of the catalysts [146]. Water accelerates sulfate group removal (generally present in commercial titania), which changes the acidity of the catalysts. Water also accelerates the loss of BET surface area and V volatility. However, curiously, it tends to increase the vanadium dispersion (Table 4). Authors recently confirmed the mobility of the VOx and WOx species of the catalyst exposed to hydrothermal aging together with various structural changes of surface acidity [146]. Additionally, Liang et al. [147] observed that gas flow containing SO2 and H2 O decreased the number of NH3 adsorption sites, active component content, specific surface area and pore volume over F-containing V2 O5 -WO3 /TiO2 catalysts. Table 4. Comparison of dry (10% O2 in N2 ) and wet (10% O2 + 10% H2 O in N2 ) aging of a commercial V2 O5 (2%)-WO3 (10%)/TiO2 catalyst. Vanadium emissions and VOx surface coverage (based on 7.9 VOx nm−2 for one monolayer). From Marberger et al. [146]. Gas Feed 550 ◦ C 600 ◦ C 650 ◦ C V release (µg m−3 ) dry wet 1.0 2.1 1.4 49.3 34.2 201 VOx coverage (%) dry wet 24 27 32 37 42 44 Another possible cause of deactivation is the transformation of anatase to rutile, a poorer support of vanadia and tungsta for the SCR reaction. Promotion of titania by silica can prevent rutile formation above 600 ◦ C and give more stable catalysts [148,149]. A commercial catalyst named VSCR1 has been compared to a catalyst doped with 4–5% SiO2 , named VSCR2. Figure 16 shows the global behavior of the two catalysts when they are calcined at 600, 700 and 800 ◦ C. A dramatic improvement of the catalyst stability is obtained by Si doping. Maintaining the anatase phase is correlated with a decrease in the WO3 and V2 O5 phase segregation into monoclinic tungsten oxide and vanadia crystallites. It seems that SiO2 tends to segregate as bulky silica crystallite at high temperature (>600 ◦ C) [149]. Only small polymeric entities formed by a minute amount of silica can prevent catalyst deactivation. Deactivation is often observed after hydrothermal treatments at high temperature. However, this is not the case in all circumstances. Chen et al. prepared V-W/TiO2 catalysts in two steps: (i) WO3 /TiO2 was first prepared by mixing metatitanic acid and ammonium paratungstate; (ii) the solid, dried and calcined at 550 ◦ C, was then impregnated with 1% V2 O5 using metavanadate dissolved in monoethanolamine [143]. The resulting catalyst was finally hydrothermally treated at 750 ◦ C. This treatment induced a decrease in surface area but resulted in considerably higher SCR activity. In fact, more V4+ and V3+ species were found on the catalyst treated at 750 ◦ C, which explains its better SCR performances. WO3 -TiO2 catalysts, without vanadia addition, were investigated by Shin et al. [150]. Tungsten oxide was well dispersed in the interlayer between the grains of titania, avoiding TiO2 sintering. It created surface acid sites (mainly Brønsted), giving the material a good SCR activity. Catalysts 2021, 11, 703 22 of 78 Figure 16. Changes of the catalyst characteristics upon temperature aging: reference V-W/TiO2 catalyst (a) and Si-doped catalyst (b). From Beale et al. [148]. 4.2. Influence of the Preparation Method: Powder and Monolithic Catalysts Powder V-W/TiO2 catalysts are commonly prepared by impregnation of anatase with aqueous solutions of ammonium metavanadate (NH4 VO3 ) and ammonium metatungstate hydrate (NH4 )6 (H2 W12 O40 )·xH2 O or paratungstate hydrate (NH4 )10 (H2 W12 O42 )·4H2 O precursors. He et al. evaluated the role of the synthesis method and showed that coprecipitation of the three elements—V, W and Ti—gave the best results in terms of SCR activity [151]. Coprecipitated catalysts possess new O-VO3 and O-WO4 sites that enhance the ammonia adsorption capacity. The origin of the best results obtained with coprecipitated catalysts is not clear. Titania is present as poorly ordered anatase with many defects, while acidity is not strengthened with respect to impregnated catalysts. Tungsten– titanium pillared clays were shown to be excellent supports of vanadia for the NH3 -SCR reaction [152]. A great advantage of this preparation method is the better control of the intimacy of contact between W and Ti, which are both inserted in clay pillars. The addition of S or N compounds during the preparation of TiO2 clearly enhanced the performances. SCR catalysts should be deposited on monoliths before practical use in DeNOx processes. Differences in color, structure and local activity of the V-W/TiO2 deposit depending on its location (center, monolith periphery) were studied by Wang et al. on commercial honeycomb catalysts [153]. As these catalysts were designed for denitrification of flue gases, NH3 -SCR was tested in presence of SO2 . In the monolith, initially yellow, some grey areas may appear in used catalysts. This due to changes of the vanadium valence state Catalysts 2021, 11, 703 23 of 78 from +5 (V2 O5 is bright yellow) to +4 (VO2 is yellow darker, almost brown) and +3 (V2 O3 is brown). This study confirms that the V-W/TiO2 is tolerant to a certain amount of SOx . Monoliths pretreated in an acidic medium seem to give good impregnation characteristics [154]. The fabrication process is illustrated in Figure 17. Best results in NH3 -SCR were obtained with the monolith impregnated three times with 3% V + 10% W. Figure 17. Preparation of V-W/TiO2 monolithic catalysts. The ceramic is pretreated in nitric acid and calcined at 500 ◦ C. It is then immersed in a mixture of solutions A and B consisting of butyl titanate in ethanol (sol. A) and metavanadate + ammonium tungstate in ethanol/nitric acid (sol. B). It is dried at 80 ◦ C and calcined at 500 ◦ C. From Zhao et al. [154] with permission from Elsevier. In the NH3 -SCR technology dedicated to automotive applications, ammonia is generally provided by urea decomposition/hydrolysis, even though some applications using liquid ammonia have been envisaged [155]. Combining WO3 -V2 O5 /TiO2 (upstream) and Cu-zeolite (downstream) catalysts was proven to give excellent performances in urea-SCR technology [156]. Deterioration of activity of the W-V/TiO2 catalyst above 270 ◦ C was compensated by the high performance of the zeolite catalyst. Lower N2 O selectivity was observed in the dual catalyst. Similar results were obtained by combining WO3 -V2 O5 /TiO2 and Fe-zeolite catalysts [157]. 4.3. Poisoning of V2 O5 -WO3 /TiO2 Catalysts Apart from the physical deactivation of WO3 -based catalysts due to its working time (e.g., sintering, volatilization of active elements), the system can suffer from chemical poisoning [158]. In 2008, Kröcher et al. reported a detailed study on the deactivation of V2 O5 -WO3 /TiO2 catalysts by inorganic impurities of lubricants, biodiesel or urea solutions [159,160]. Among all the poisons tested, potassium had the strongest effect on both catalytic activity and N2 O selectivity (at 500 ◦ C). The poisons can be ranked as follows: K >> Ca >> Mg > Zn > P. These results prompted researchers to focus their investigations on K and Ca poisoning (Section 4.3.1). Another poison present in exhaust gases or flue gases is sulfur dioxide. The effect of SO2 was also investigated by several authors, as presented in Section 4.3.2; poisoning by arsenic is presented in Section 4.3.3. 4.3.1. Potassium and Calcium Poisoning Potassium is a poison of acid sites and inhibits NH3 adsorption. Xie et al. investigated the poisoning of a W-V/TiO2 catalyst exposed to the flue gas of a coal-fired power plant [161]. They showed that alkali contaminants (mainly K) contained in the flue gas preferentially poison the vanadium sites (V5+ –OH and/or V5+ =O) rather than the sites Catalysts 2021, 11, 703 24 of 78 associated with tungsten oxide or TiO2 . A detailed kinetic analysis reveals that NH3 adsorption/desorption, NH3 oxidation and DeNOx activity are all affected by K poisoning. Similar tendencies were demonstrated by Siaka et al. [162], who claim that potassium preferentially neutralizes strong acid sites and alters V=O redox sites, leading to stabilization of well-dispersed VOx species. Contrasting with the results of Xie et al., Chen et al. concluded that tungsten oxide can serve as a sacrificial agent protecting vanadia from severe K poisoning [163]. The combined effect of potassium and chloride ions showed that KCl is a more severe poison than KOH [164]. A higher amount of potassium was fixed by the catalyst when KCl was used as a potassium precursor. Better performance and especially higher potassium tolerance were obtained by doping the V2 O5 /TiO2 catalyst with 15% HPA (H3 PW12 O40 , H4 SiW12 O40 or H3 PMo12 O40 ) instead of 10% WO3 [165]. HPA does alter vanadium dispersion and does not increase N2 O formation. A higher concentration of acid sites could explain the higher K tolerance. By studying multielement poisoning systems, Mia et al. [166] found that phosphorous–potassium combination results in lower deactivation than single potassium poisoning. The advanced explanation is that new active sites generated by phosphorous react with potassium to liberate V-OH acidic sites. Concomitantly, alkali species may provide additional basic sites for NO2 adsorption, enabling gaseous ammonia to react with adsorbed NOx compounds for fast SCR stoichiometry [167]. Finally, potassium resistance may be strengthened by Ce and Cu addition over V2 O5 -WO3 /TiO2 SCR materials due to an enhancement of V5+ amount and active oxygen species [168]. Calcium poisoning was investigated in detail by Li et al. [169,170]. The degree of poisoning follows the order CaCO3 > CaO > CaSO4 . It is linked to the ability of the precursor to form calcium tungstate [170]. CaCO3 decreases oxygen availability, vanadium reducibility and acid site concentration. If calcium sulfate is a less severe poison than CaO and CaCO3 , it tends to increase the N2 O selectivity. The catalysts can be regenerated by using specific treatment based on 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) liquid under weak acid environment [169]. Odenbrand investigated the effect of CaSO4 on kinetic parameters of NH3 -SCR over W-V-TiO2 catalysts [171]. He concluded that Ca introduced into the pore system by impregnation strongly poisons the catalyst, but in a different way from that when it is coming from the engine during operation. CaSO4 affects both activation energy and NH3 heat of adsorption. A moderate effect on N2 O selectivity was observed. 4.3.2. Sulfur Poisoning Tungsten-promoted V2 O5 /TiO2 catalysts are not resistant to SO2 for the NH3 -SCR reaction. Antimony oxide [172], molybdenum oxide [173] or iron oxide [174] are better promoters of these catalysts for sulfur resistance. Xu et al. confirmed the superiority of V2 O5 -Sb2 O3 /TiO2 compared with the commercial V2 O5 -WO3 /TiO2 catalyst for SO2 resistance [175]. Sulfur deactivation is due to the formation of a surface layer of NH4 HSO4 [176]. Compared to tungsten, Sb-doped catalysts have a lower activity for SO2 oxidation to SO3 , which leads to weaker deposit of ammonium sulfate. Moreover, it seems that NH4 HSO4 is more reactive with NO when it is deposited on V2 O5 -Sb2 O3 /TiO2 [175]. The sulfur resistance of iron-promoted WO3 /TiO2 is also reported in [177], where non-vanadiumbased catalysts present thorough regeneration. Nobia-doped materials also exhibit significant sulfur resistance [178]. Nb2 O3 -CeO2 /WO3 -TiO2 (NbCeWTi) catalyst presents mainly ammonium hydrogen sulfate species after H2 O and SO2 exposition that protect Cex+ active sites. SO2 resistance of V2 O5 -WO3 /TiO2 catalysts can be improved by the addition of WO3 graphene nanocomposite [179]. WO3 remains well dispersed on graphene, which tends to decrease the SO2 oxidation activity. Moreover, WO3 acidity is preserved on graphene, essential for a good SCR activity. In another attempt, sulfur resistance was successfully increased by the addition of barium sulfate [180]. BaSO4 contributed to reinforcing the acidity of the catalysts while blocking the SO2 oxidation activity. Catalysts 2021, 11, 703 25 of 78 4.3.3. Arsenic Poisoning Arsenic is present in the gas phase of power plants and may reach concentrations of up to thousands of micrograms of As2 O3 per cubic meter [181]. Power plant gases can be treated with limestone to reduce arsenic concentration. The compound thus formed is (Ca3 (AsO4 )2 ), and it may be condensed or adsorbed on ashes, decreasing the final As concentration to less than 100 µg m−3 . However, even at this concentration, As remains a severe poison for NH3 -SCR catalysts. Deactivation of V2 O5 -WO3 /TiO2 catalysts was investigated by Peng et al. [182,183] and Kong et al. [184]. Arsenic oxides decrease Lewis acidity and tend to form very unstable As-OH Brønsted sites. At a certain concentration, As2 O3 is oxidized to As2 O5 , much less active for NH3 -SCR but more active for NH3 oxidation, leading to N2 O in significant amounts. As2 O5 forms a dense layer at the catalyst surface, which reinforces the deactivation effect of arsenic. It was proven that catalysts promoted by Mo instead of W are more resistant to As poisoning. Synergetic poisoning effect of arsenic with potassium was evaluated in [185,186]. Results indicated that the coeffect of As + K was more dramatic than the additive effect of a single poison. As previously discussed, arsenic loading leads to the formation of As-OH acid sites, which can be neutralized by potassium deposits. In addition, the effect of charge-compensating anions was also investigated, with the following enhancement ranking sequence: SO4 2− < Cl− << NO3 − . Finally, catalysts poisoned by arsenic could be almost regenerated by sulfuric acid treatment, leading to the recovery of Lewis and Brønsted acid sites. However, the main drawback remains the formation of chelating bidentate sulfates and the loss of vanadium species [187]. Alkali solution treatment by single sodium carbonate [188] or with sodium hydroxide combined with sulfuric acid [189] also demonstrated interesting regeneration results. 4.3.4. Metal Release and Reuse of V-W-TiO2 Catalysts A cause of slow but irreversible deactivation of V2 O5 -WO3 /TiO2 catalysts is the metal release at high temperature (T > 600 ◦ C) [190,191]. Vanadium VO(OH)3 and tungsten WO2 (OH)2 oxo-hydroxides are responsible for the greatest part of the metal release. This effect is more important on agglomerated vanadium and tungsten particles, especially when the support undergoes severe sintering. Therefore, metal release is significantly reduced with catalysts supported on stabilized titania. Tungsten can be recovered from spent catalysts by a chemical treatment consisting of an alkaline leaching–ion exchange method [192]. Spent catalysts were also used to prepare new W-V catalysts. Huo et al. reported a feasible preparation method for a visible-light-sensitive BiVO4 /Bi2 WO6 heterojunction photocatalyst from waste SCR catalysts [193]. 4.4. Cerium-Promoted WO3 Catalysts for NH3 -SCR Promotion by ceria was studied in detail in the last decade. Redox properties [194,195] and oxygen mobility on ceria [196,197] could have a major impact on the NH3 -SCR reaction [198]. Ceria could be added to V2 O5 -WO3 /TiO2 catalysts, WO3 /TiO2 catalysts (without vanadia) or WO3 catalysts (without vanadia and titania). 4.4.1. Ceria Added to V2 O5 -WO3 /TiO2 The addition of ceria to V2 O5 -WO3 /TiO2 for the NH3 -SCR reaction was investigated by Chen et al. [199]. The great merit of ceria is the enhancement of the DeNOx activity at low temperature (below 300 ◦ C). For instance, with a catalyst having typical composition close to that of commercial materials (1% V2 O5– 9% WO3 ), a NOx conversion of 52% is obtained at 200 ◦ C in the following conditions: 500 ppm NO, 500 ppm NH3 , 3% O2 and 28,000 h−1 . Addition of 5% CeO2 increases NO conversion up to 88%, while manganese oxide has virtually no effect (NO conversion = 50%), and iron oxide decreases NO conversion down to 15%. Vanadium oxide being responsible for the formation of N2 O at high temperature, attempts were made by Chen et al. to significantly reduce the vanadium loading down to 0.1% while maintaining a good activity by doping the V0.1 W6 Ti with ceria [199]. Above Catalysts 2021, 11, 703 26 of 78 5% CeO2 , activity of V0.1 W6 CexTi becomes equal or superior to that of V1 W9 Ti at 200 ◦ C. In the meanwhile, N2 O formation is virtually suppressed on the low-loaded vanadium catalysts. This effect is still more marked on Ce-doped catalysts. Addition of ceria leads to complex features since the basicity of titania seems to be reinforced while there are stronger and more active Brønsted acid sites on VWTi. Series of CeO2 -V2 O5 -ZrO2 /WO3 -TiO2 catalysts with different loadings of ceria were prepared by Wang et al. [200]. They confirmed the specific role of ceria in improving both the low-temperature activity and the N2 selectivity at high temperature (less N2 O). However, the zirconia-free catalyst (CexV1/W8Ti) has a poor hydrothermal stability (750 ◦ C, 10% H2 O, 12 h). Addition of zirconia (10%) significantly reinforces the stability of the ceriapromoted catalyst. Ceria also has a great influence on the reaction mechanism. Ce-doped catalysts (CeVZr/WTi) are extremely sensitive to the presence of NO2 in the reaction gases and are much more active in fast SCR conditions (Equation (5)) than in standard conditions (Equation (6)). The influence of NO2 is limited on the cerium-free catalysts (VZr/WTi), suggesting that NO2 does not readily react via the fast SCR reaction in absence of ceria. NO + NO2 + 2NH3 → 2N2 + 3H2 O (5) 1 O2 + 2NH3 → 2N2 + 3H2 O (6) 2 The group of Tianjin University investigated the method of introduction of cerium and vanadium on the commercial 10% WO3 /TiO2 support [201]. Ceria was introduced either by impregnation of cerium nitrate (IMP) or by deposition–precipitation of cerium nitrate in the presence of ammonia (DP). Characteristics and performances of the catalysts are reported in Table 5. NO + Table 5. Comparison of VCe/WTi DP, VCe/WTi IMP and cerium-free catalyst in NH3 -SCR. Vanadium was introduced by impregnation on the support (VWTi) or on the Ce-doped support using the VO(CO2 )2 complex (prepared by reaction of V2 O5 powder with oxalic acid). SCR conditions: 500 ppm NO, 500 ppm NH3 , 5% O2 and 4% H2 O. From [201]. BET Area Ce3+ (%) NO Conv. N2 O (m2 g−1 ) (from XPS Data) at 250 ◦ C (%) at 550 ◦ C (ppm) VWTi 80 - 35 14 VCeWTi IMP 80 44 42 8 VCeWTi DP 83 53 67 5 Catalyst The catalyst prepared by deposition–precipitation shows higher performances for the NH3 -SCR reaction. This is not due to changes in the textural properties but to specific properties of ceria, more reducible in VCeWTi DP. This catalyst also possesses more acidic sites (mainly Lewis sites) associated with surface wolframyl and ceria species. Youn et al. studied the role of the order of impregnation of vanadium and cerium–tungsten [202]. They showed that vanadium impregnated first (CeW/V/Ti) gave better performances. Similar topics were recently explored by Liu et al. [203], who demonstrated that the impregnation sequence of W and Ce influence the NH3 adsorption capacity and the Ce3+ /(Ce4+ + Ce3+ ) ratio in correlation with catalytic results. Addition of copper to ceria (Cu-Ce-W-V/Ti) reinforces the beneficial effect of ceria at low temperature [204]. The specific role of ceria on the sulfur resistance of V2 O5 -WO3 /TiO2 catalysts was investigated by Liang et al. [205]. Ceria helps to improve DeNOx activity and sulfur resistance. A 3% loading of ceria is optimal for better performances. Higher loadings of ceria are detrimental: they provoke a decrease in surface area and partial covering of vanadia by ceria. Complex formulae combining Ce and Mn doping were described by Zhao et al. Ce and Mn coaddition confers excellent H2 O and SO2 resistance on V2 O5 WO3 /TiO2 catalysts [206]. SO2 is mainly adsorbed on Mn4+ and Ce4+ cations, making the Catalysts 2021, 11, 703 27 of 78 active surface of V2 O5 -WO3 free. A multiplicity of valence states of the elements would be beneficial to a high NO conversion. Additionally, codoping of Ce4+ and Zr4+ enhances the tolerance to alkali metals of V2 O5 -WO3 /TiO2 catalysts, as K-poisoning resistance occurs by blocking potassium in the form of Ce-O-K structure [207]. 4.4.2. Ceria Added to WO3 /TiO2 (Without Vanadium) A number of works were carried out on vanadium-free catalysts to better study the interaction between ceria and tungsten and its role in the SCR reaction. Chen et al. prepared CeO2 /TiO2 and CeO2 -WO3 /TiO2 catalysts by an ultrasonic method and investigated the reactivity of adsorbed ammonia with NO + O2 by DRIFT [208]. Comparison of CeO2 /TiO2 and CeO2 /WO3 /TiO2 catalysts shows that tungsten has a dramatic effect on the SCR reaction: (i) it allows conversion of NO at lower temperatures (50% conversion is reached at 165 ◦ C on CeWTi instead of 225 ◦ C on CeTi); (ii) it increases the selectivity for N2 at high temperature (less N2 O). Tungsten increases the number of Brønsted sites (virtually absent on CeTi) and tends to accelerate the cerium reduction (more Ce3+ in CeWTi). Another report by Chen et al. confirmed the important role of tungsten by increasing the oxidation activity of NO to NO2 , allowing the catalyst to work in fast SCR conditions [209]. Interaction between ceria and tungsta seems to play a decisive role in SCR activity and selectivity. Geng et al. compared WTi and CeWTi catalysts and showed that ceria is effective for the inhibition of N2 O formation when it is added to WO3 -TiO2 [210]. Great efforts were made in the last decade to improve the preparation of cerium-based tungsten catalysts. Michalow-Mauke et al. developed a preparation method based on flamespray (FS) pyrolysis of Ce, W and Ti precursors in tetrahydrofuran [211,212]. Compared to catalysts prepared by impregnation, catalysts prepared by FS synthesis show superior performances. This is due to a better dispersion of ceria and tungsta and to a greater interaction of these elements with titania leading to highly active Ce–O–W (especially Ce3+ – O–W6+ ) and Ce–O–Ti sites [212]. The fact that FS synthesis mainly produces rutile instead of anatase does not seem to hamper the performance of the catalysts prepared by this technique. Sol–gel techniques were also used by Jiang et al. to prepare CeO2 -WO3 /TiO2 catalysts [213]. Butyl titanate was dissolved in a solution of ethanol/water/nitric acid and then mixed with cerium nitrate and ammonium metatungstate. Different catalysts were prepared with 10% WO3 and various loadings of ceria (from 0 to 40%). DeNOx activity of these materials is visualized in Figure 18. Figure 18. SCR activity of CexW10Ti catalysts prepared by a sol–gel method using butyl titanate as Ti precursor. Effect of cerium loading on NO conversion. Reaction conditions: 1000 ppm NO, 1000 ppm NH3 , 3% O2 . From Jiang et al. [213] with permission from Elsevier. The catalyst with 20% CeO2 shows the best performance in NH3 -SCR. It has also a good resistance to SO2 /H2 O when 500 ppm SO2 and 10% H2 O are added to the reactant mixture. The same optimal composition (20% CeO2 ) had already been observed on a Catalysts 2021, 11, 703 28 of 78 series of Cex W20 TiO2 catalysts prepared by homogeneous precipitation with urea aqueous solution [214]. In every case, these CeO2 -WO3 -TiO2 catalysts show exceptional resistance to SO2 . Improved methods of preparation of CeO2 -WO3 /TiO2 catalysts were recently re−δ ported, either using H2 O2 as a promoter of active sites [215] or leading to 2D materials (Ce0.184 W0.07 Ti0.748 O2−δ nanofibers prepared by electrospinning) [216]. H2 O2 modification improves both the BET surface area and the number of Brønsted sites, while nanofibers seem ideal for a good performance at low temperature owing to the formation of a great number of oxygen vacancies. Salazar et al. investigated the method of impregnation of Ce and W on TiO2 [217]. They showed that successive impregnation (with intermediary calcinations) of Ce on W/Ti led to higher performances than when catalysts were prepared by coimpregnation. Successive impregnation favors the formation of Ce-O-W bridges, which play an essential role in the SCR reaction. No clear correlation was observed between the SCR activity and the degree of Ce reduction or the number of Brønsted sites, even though these parameters are probably important for a good DeNOx activity. Monolithic catalysts were prepared and tested by Cao et al. [218]. A TiO2 -SiO2 powder was mixed with aqueous solutions of ammonium paratungstate and cerium nitrate. Ammonia was added up to pH = 10 to form a slurry which was extruded, dried and calcined at 550 ◦ C. Silica was added to titania to increase its thermal stability. Excellent performances were obtained, superior to those of powder catalysts especially at high temperatures. 4.4.3. Ceria Added to WO3 (Without Vanadia or Titania) In order to avoid complex interactions between Ce, W, V and Ti, CeO2 -WO3 catalysts were prepared and tested in NH3 -SCR. Zhang et al. compared the behavior of ceria doped with various acid promoters: phosphotungstic, silicotungstic and phosphomolybdic acids or ammonium sulfate [219]. Ceria doped with phosphotungstic (CeO2 -P-W) or silicotungstic acid (CeO2 -Si-W) showed the highest NOx conversion (Figure 19). However, CeO2 -P-W exhibited the best N2 selectivity over the whole temperature range, especially at T > 450 ◦ C where N2 O could be formed. Figure 19. NOx conversion over acid-promoted ceria catalyst. Ceria promoted by tungstic acids is much more active (800 ppm NH3 + 800 ppm NO + 5% O2 , GHSV = 60,000 h−1 ). From Zhang et al. [219] with permission from Elsevier. The mode of preparation of ceria for the synthesis of P-W/CeO2 catalysts was further investigated by Song et al. [220]. They compared hydrothermal (cerium nitrate + glucose + acrylic acid aged at 160 ◦ C in autoclave), sol–gel (cerium nitrate in citric acid) and precipitation (cerium nitrate solution + ammonium carbonate) techniques. The P-W/CeO2 Catalysts 2021, 11, 703 29 of 78 catalyst with ceria prepared by hydrothermal technique gave the highest performances. It combined the highest BET area, the highest Ce3+ concentration and an adequate balance between Brønsted and Lewis acid sites. Wang et al. showed that the ceria morphology (cubes, particles, rods) would have a great impact on the performance of W-CeO2 catalysts for the NH3 -SCR reaction [221]. Ceria supports were prepared by hydrothermal methods according to the synthesis procedures developed by Peng et al. [222]. Ceria nanorods were prepared from cerium acetate, while nanoparticles and nanocubes were synthesized using cerium nitrate. Nanocubes expose preferentially (100) planes while nanorods expose both (110) and (100) planes. Contrasting with these surface structures, (111) planes are essentially found with nanoparticles. Nanoparticles are more active than nanocubes and nanorods for the NH3 -SCR reaction, which tends to prove that WO3 attached to (111) faces of ceria gives the best performances for NOx abatement. Modification of ceria by manganese shows that N2 O can be avoided by the incorporation of WO3 into the catalyst. Better performances, with a good SO2 and CO2 resistance, were obtained with W0.1 Mn0.4 Ce0.5 composition [223]. One of the roles of ceria is to promote redox properties by oxygen mobility improvement. As cerium–zirconium oxides have superior properties of reduction [194,224,225], it was logical to replace pure ceria with cerium–zirconium oxides in the preparation of supported WO3 catalysts. These formulations were explored by Ning et al. [226]. Methods of preparation of CeO2 -ZrO2 -WO3 (CZW) catalysts were similar to those developed by Song et al. for tungsten catalysts supported on ceria [220]. Catalysts prepared by hydrothermal methods show the best performance for the NH3 -SCR reaction. This is due to a higher tungsten dispersion and to the coexistence of Brønsted and Lewis acid sites, while CZW catalysts prepared by other techniques possess only Lewis sites. Acid properties of CeO2 -ZrO2 -WO3 catalysts are greatly influenced by the state of tungsten oxide [227]. Moreover, the formation of amorphous W species resulted in the abundance of Ce3+ and oxygen vacancies with a correlative increase in the NO oxidation activity. Recently, great efforts were devoted to reinforcing the stability of SCR catalysts. Liu et al. showed that doping WO3 /Ce0.68 Zr0.32 O2 with silica significantly improved the thermal stability of the catalyst (10% H2 O, 800 ◦ C) [228]. Silica allows maintaining the acidity of the catalyst after thermal treatment. Moreover, it inhibits the formation of cerium tungstate Ce2 (WO4 )3 , which is detrimental to the catalyst performances. Similar effects were obtained by doping the CeZrOx support with alumina in monolithic catalysts [229]. The Cex Zr1−x O2 composition also impacts the WO3 /Cex Zr1−x O2 behaviors. The characterization of 9% WO3 /Cex Zr1−x O2 catalysts with various ZrO2 weight ratios in Cex Zr1−x O2 (30%, 42%, 60% and 80%) indicated that the increase in the zirconium content enhanced the acidity (number and strength of acidic sites). Accordingly, the NH3 -SCR activity also increased [230]. Note that after WO3 addition, the basic NOx storage sites of Cex Zr1−x O2 were fully altered while the oxygen storage capacities (OSC) were dramatically decreased for all samples. 4.5. Iron-Promoted WO3 Catalysts for NH3 -SCR Iron catalysts being promising SCR catalysts, especially when Fe is inserted in zeolites [231], it was logical that researchers attempt to associate Fe and W for the DeNOx reaction. Fe-W mixed oxides were studied for this application by Li et al. [232] and Wang et al. [233]. Catalysts were prepared by ammonia precipitation of ammonium paratungstate (APT) and iron nitrate [232], or by urea precipitation of ammonium metatungstate (AMT) and iron nitrate [233], which may explain some differences in performances for the two series of catalysts. Activity maximum was observed by Li et al. with the FeW5 sample (i.e., having a Fe/W molar ratio of 5), while the optimal performances were found by Wang et al. for a Fe/W ratio of 3 (Fe0.75 W0.25 Ox sample). Location of acid sites would also be different: B sites on tungsten (W-OH) and L sites on iron (Li et al.), or B sites on FeWO4 and L sites on Fe2 O3 , i.e., Fe oxide not associated with tungsten (Wang et al.). In fact, Wang et al. proposed that the good performances of Fe0.75 W0.25 Ox would be due to a fine interaction between Fe2 O3 and FeWO4 with an easier electron transfer from W6+ sites to Fe3+ sites, which favors Catalysts 2021, 11, 703 30 of 78 the formation of NO2 . Wang et al. also reported that their mixed oxide catalysts were extremely resistant to SO2 poisoning, especially at high temperature (T > 300 ◦ C) [234]. Similar conclusions were reported for tungsten-free catalysts, where Fe0.1 V0.1 TiOx catalyst showed the optimal NH3 -SCR performance and excellent SO2 resistance [174]. Direct impregnation of tungsten on hematite Fe2 O3 was reported by Liu et al. [235]. Hematite was first prepared by urea precipitation of Fe nitrate. The solid (dried and calcined at 500 ◦ C) was then impregnated with ammonium metatungstate solution in oxalic acid. Optimal performances were observed with the 5% WO3 /Fe2 O3 sample. The reverse impregnation (iron on WO3 nanorods) was studied by Li et al. [236]. It is difficult to compare the two methods of preparation. Nevertheless, both led to comparable DeNOx activity with a 50% NO conversion around 275 ◦ C, even though the catalyst evaluation was carried out in somewhat different conditions. Iron–tungsten was also supported on zirconia [237] or cerium–zirconium oxide for monolith preparation [238]. In absence of iron, supported tungsten shows a good ammonia oxidation activity but virtually no SCR activity. The presence of iron is required to create SCR catalysts whose activity is linked to the formation of Fe3+ Lewis sites. The main role of these sites would be to promote NO oxidation to NO2 , the first step in the SCR mechanism. These studies on ZrO2 and CeZrOx were carried out with relatively high W loading (Fe/W molar ratio close to 1). Magnetic iron oxides doped with tungsten and cerium were also prepared as SCR catalysts [239]. In these materials, iron is mainly in the form of γ-Fe2 O3 and α-Fe2 O3 , but other forms can be present since W and Ce tend to create highly dispersed iron species. Compared to previous catalysts synthesized over zirconia or CeZrOx , higher iron loadings were used to prepare FeCeWOx catalyst (most active material: Fe0.90 Ce0.05 W0.05 and Fe0.85 Ce0.10 W0.05 ). Copper being an active component of SCR catalysts, it was tempting to promote iron catalysts with copper. Ma et al. reported a one-pot preparation of Cu0.02 Fe0.2 Wx TiO2 materials (x varying from 0.01 to 0.03), which were tested in the SCR reaction [240]. A moderate amount of tungsten significantly improves stability and acid and redox properties, leading to superior performances for the SCR reaction, even in the presence of water and SO2 (best formula: Cu0.02 Fe0.2 W0.02 TiO2 ). Complex catalysts including several promoters (Fe, V, Mn, W, Ce) were also developed [241]. It appears that multiple redox pairs (Fe2+ /Fe3+ , V4+ /V5+ , Mn2+ -Mn3+ /Mn4+ ) would play an important role in the reaction. 4.6. Manganese-Promoted WO3 Catalysts for NH3 -SCR Titania-supported manganese tungstate catalysts were prepared by Kong et al. in order to replace the standard V-W/TiO2 catalyst for the NH3 -SCR reaction [242]. The solution combustion method with Mn, W and Ti precursors mixed with glycine is suitable for preparing high-surface-area materials (up to 280 m2 g−1 ). Mn/TiO2 without W showed a high activity at low temperature, which declined rapidly above 300 ◦ C. Tungsten allowed enlarging the activity window up to 400 ◦ C. The highest performance was obtained with the Mn0.1 W0.05 Ti0.85 O2−δ catalyst. Similar materials were reported by Shin et al. [243] and Wang et al. [244]. In these studies, high tungsten contents were used (15% in [243] and 25% in [244]), which seems beneficial to improve the performance at low temperature. For instance, Wang et al. obtained a 50% NO conversion at 40–50 ◦ C and 100% NO conversion from 80 to 280 ◦ C over the W0.25 Mn0.25 Ti0.5 O2 catalyst. The role of tungsten is to facilitate Mn redox capacity, leading to high concentration of Mn4+ and reactive oxygen species (Figure 20). Catalysts 2021, 11, 703 31 of 78 Figure 20. Characterization of Mn/TiO2 and W-Mn/TiO2 catalysts by XPS: Mn (a) and O (b) photopeaks. Presence of tungsten favors Mn oxidation to Mn 4+ and formation of O reactive species α (Oα ). NO oxidation to NO2 is improved on W-Mn/TiO2 favoring the fast SCR reaction. From Wang et al. [244] with permission from Elsevier. Additionally, a study investigating the vanadium loss from V-W/TiO2 catalysts was conducted in [245] and reports that manganese is the best candidate to make up for the loss of SCR activity caused by the decrease in V2 O5 loading (50%), in opposition to other transient metals as Nb, Co, Cr, Cu or Ce. Note that similar behaviors are denoted for tungsten-free catalysts: Mn-loaded catalyst (Mn5 V1 Mo3 Ce7 /Ti) exhibits the optimal SCR performance associated with a large number of acid sites and high redox properties [246]. Many studies were devoted to the promotion by cerium of manganese-based SCR catalysts. Nie et al. investigated the effect of several acidic oxides (Nb2 O5 , WO3 and MoO3 ) on the performance of MnOx -CeO2 catalysts [247]. NbCeTi and WCeTi are very active at 200 ◦ C, while MoCeTi is active only at high temperature. WO3 -promoted catalyst has the broadest operation window with significant NO conversion above 300 ◦ C. Ma et al. investigated the effect of WO3 doping on the performances of MnOx -CeO2 catalysts for the SCR reaction [223]. The concentration of manganese in ceria was kept constant (Mn/Ce = 0.4) while the concentration of tungsten was varied from 0.03 to 0.2. Undoped Mn0.4 Ce was active only at low temperature: its activity decreased sharply above 200 ◦ C. WO3 -doped catalysts showed far better activity above 200 ◦ C, with an optimum for the W0.1 Mn0.4 Ce sample. Mn0.4 Ce has only Lewis acid sites. Doping with tungsten generates Catalysts 2021, 11, 703 32 of 78 Brønsted sites allowing activity at high temperature. Moreover, High SO2 resistance is also achieved on W0.1 Mn0.4 Ce by suppression of SO2 oxidation activity. Promotion by tin of MnOx -CeO2 support seems to favor low-temperature SCR activity [248]. High NO conversion is obtained on the SnMnCeOx support (50% at 60 ◦ C but only 50% at 300 ◦ C). Addition of tungsten shifts the conversion profile to higher temperatures with good activity and excellent N2 selectivity up to 300 ◦ C. In several studies by the group of Hong (South Korea), WMnCe-based catalysts were supported on titania [249–251]. In the first two studies, WMnCeTi samples were prepared by wet impregnation without any control of the pH, while in the third study, a strict control of the pH between 2.8 and 1.7 was applied by addition of oxalic acid. Controlling the pH of the slurry around 1.7 leads to catalysts more active for the NH3 -SCR. This result is linked to the fact that an acidic pH increases the formation of surface Mn4+ and Ce3+ , while NO would be less strongly adsorbed. A good synergetic effect was observed with zirconia as support for manganese. Increasing the concentration of tungsten on MnZrOx allows obtaining catalysts more active and more selective for N2 up to 400 ◦ C [252]. XPS results and DRIFT studies on the most active catalyst (15%W/MnZrOx ) showed that the redox cycle (Equation (7)) would promote the electron transfer between W and Mn, contributing to NH3 activation. Mn4+ − O2− −− W5+ ⇔ Mn3+ − O−2− − W6+ − −  − − (7) Developing active catalysts over a wide range of temperatures is a challenge for the NH3 -SCR reaction. Combining two catalysts (MnOx -CeO2 /TiO2 (MnCeTi) and V2 O5 WO3 /TiO2 (VWTi)), Zhang et al. were able to obtain high NO conversion from 150 to 400 ◦ C [253]. The best configuration was obtained when VWTi was set at the fore part and MnCeTi at the rear part of the catalyst bed (Figure 21, CC-B curve). MnCeTi is much more active than VWTi for the NO oxidation reaction, allowing work in fast SCR conditions over a wide range of temperatures. MnCeTi is also more active for the NH3 oxidation reaction, which has a less detrimental effect in CC-B configuration (VWTi + MnCeTi). Figure 21. Combination of MnOx -CeO2 /TiO2 (MnCeTi) and V2 O5 -WO3 /TiO2 (VWTi) catalysts for the NH3 -SCR reaction. Configuration CC-A: MnCeTi first and then VWTi; configuration CC-B: reverse position; configuration CC-C: physical mixture of the two catalysts. From Zhang et al. [253] with permission from John Wiley and Sons. Catalysts 2021, 11, 703 33 of 78 5. Other DeNOx Applications of WO3 -Doped Catalysts 5.1. Tungsten Catalysts for the NOx Trap–SCR Coupled System One of the technologies for NOx abatement in diesel engine exhausts is the NOx trap system, also called NOx storage reduction (NSR) or lean NOx trap (LNT) [254]. The catalyst (typically Pt/BaO-Al2 O3 ) works according to a sequential operation: (i) during one or two minutes, NOx from the exhaust gases is stored on the catalyst as nitrate and nitrite species; (ii) during a few seconds, hydrocarbons are added to the exhaust gases for the reduction step, and adsorbed nitrates and nitrites are reduced to N2 . However, some ammonia may also be produced (ammonia slip), which should be eliminated on an ammonia oxidation catalyst. An alternative solution would be to use this in situ produced ammonia on an SCR catalyst to transform the residual NOx not yet converted. This is the NOx trap–SCR coupled system [255]. Several studies considered the coupling of Pt-BaO-Al2 O3 (NSR catalyst) with Cu-zeolites (SCR catalyst) [256–259] or with Fe-zeolites [260,261]. Can et al. investigated the use of WO3 /CeZrOx (as SCR material) coupled to a usual Pt/BaO/Al2 O3 catalyst [230]. The effect of adding an SCR catalyst (WO3 /Ce0.58 Zr0.42 O2 ) downstream of the NSR catalyst is depicted in Figure 22. SiC being inert for all the reactions, NSR + SiC represents the performance of the NSR catalyst alone. The effect of adding an SCR catalyst to the NSR material is clearly visible in the figure: more NOx is converted and more NH3 produced on the first bed is consumed. N2 O is never produced on the NSR catalyst alone or on the coupled system NSR+SCR. In fact, a part of the ammonia produced on the NOx -trap catalyst reacts with oxygen (NH3 oxidation reaction or SCO). Fortunately, both SCR and SCO are selective for N2 on these materials. Figure 22. Analyses of nitrogen-containing gases during lean/rich oscillations. The catalysts were exposed to 500 ppm NO, 10% O2 , 10% H2 O and 10% CO2 during the lean period (60 s) and to 3% H2 , 10% H2 O and 10% CO2 during the rich period (3 s). From Can et al. [230] with permission from the American Chemical Society. Catalysts 2021, 11, 703 34 of 78 Other tungsten catalysts (WO3 /Al0.2 Ce0.4 Ti0.4 , WO3 /Al0.2 Ce0.16 Zr0.32 Ti0.32 and WO3 / Al0.1 Si0.1 Ce0.16 Zr0.32 Ti0.32 , selected among 30 formulations) were also tested in the NOx trap–SCR coupled process [262]. Ammonia release and its use in oxidation (SCO) or reduction (SCR) is shown in Figure 23 for four configurations of NOx trap–SCR coupled systems. Configuration 4 with silica-containing SCR catalyst offers the best performances, especially at 300 ◦ C. Presence of silica increases Lewis acidity and more strongly increases Brønsted acidity, which can explain the good behavior of the Si-doped catalyst.   Figure 23. Ammonia release and use in SCO or SCR on different NOx trap–SCR systems. In on  corresponds  Configuration every case, 1%Pt/10%BaO-Al2 O3 (Pt-BaAl) is used as NOx trap catalyst. corre  to P      alone),  to   0.4 ,  trap alone), to Pt-BaAl + WO /Al Ce Ti , to Pt-BaAl + sponds to Pt-BaAl + SiC (NO  x 3 0.2 0.4 0.4 WO3 /Al0.2 Ce0.16 Zr0.32 Ti0.32 and  to Pt-BaAl + WO3 /Al0.1 Si0.1 Ce0.16 Zr0.32 Ti0.32 . The catalysts were exposed to the same lean/rich oscillations as in Figure 22. From Can et al. [262] with permission from Elsevier. 5.2. Tungsten Catalysts for NOx Reduction by Other Reductants 5.2.1. Reduction by Hydrogen (H2 -SCR) NOx reduction by hydrogen is well adapted to depollution of sites where hydrogen is available (e.g., refineries, H2 plants) [263]. The authors of [263] investigated the H2 SCR over WOx -ZrCe (zirconium rich) and WOx -CeZr (cerium rich) catalysts. Reaction conditions were 520 ppm NO + NO2 (NO/NO2 = 9), 5% O2 and 10% CO2 . The reaction can be written as follows (Equation (8)): NO + H2 → 1 N2 + H2 O 2 (8) However, formation of nitrous oxide (Equation (9)) and undesired hydrogen oxidation (Equation (10)) may be observed: 2NO + H2 → N2 O + H2 O (9) 1 O2 → H2 O (10) 2 Maximum activity is obtained at 250 ◦ C over WOx -CeZr and at 300 ◦ C over WOx -ZrCe. The Zr-rich catalyst WOx -ZrCe shows better performances if the whole 150–600 ◦ C temperature range is considered. This could be due to a higher concentration of acid sites on WOx -ZrCe than on WOx -CeZr (about twice). Selectivity for N2 is close to 80–90%, with maximum formation of N2 O around 20 ppm over WOx -CeZr and 15 ppm over WOx -ZrCe. Addition of 7% water slightly decreases the NOx conversion but improves the N2 selectivity. H2 + Catalysts 2021, 11, 703 35 of 78 Platinum is one of the most active metals for the NO reduction by H2 at low temperature [264–266]. Platinum activity and selectivity are extremely sensitive to the nature of support. For instance, Pt/ZrO2 is superior to Pt-Al2 O3 (more active and more selective for N2 ), and promotion by WO3 improves the performances of Pt/ZrO2 [267]. NO2 and nitrate species are thought to be essential intermediates in the reaction [265]. However, other authors found that NO adsorption and decomposition on Pt would be the most important step of the reaction [268,269]. Maximum NO conversion is observed between 90 and 130 ◦ C. Zhang et al. reported a NO conversion of 91% at 110 ◦ C over 0.1%Pt-1%W-HZSM-5 in the following reaction conditions: 910 ppm NO + 90 ppm NO2 + 5000 ppm H2 + 10% O2 . Tungsten oxide is thought to maintain Pt in the metallic state even in highly oxidizing conditions. It suppresses NO2 adsorption on Pt and inhibits the formation of nitrate species. It accelerates the dissociation of NO and H2 on Pt. The main drawback of H2 -SCR is the lack of N2 selectivity due to ammonia formation. Platinum being a poor NH3 -SCR catalyst, this ammonia cannot be used for increasing the selective conversion of NO to N2 . By contrast, Pt-WO3 is an excellent catalyst for selective oxidation of NH3 (SCO), but only at higher temperatures (250–300 ◦ C) [270]. Replacing Pt by Ru does not significantly improve the SCO reaction at lower temperatures [271], which does not allow selective conversion of ammonia formed in H2 -SCR. 5.2.2. Reduction by Ethanol (C2 H5 OH-SCR) Alumina-supported silver catalysts are very active for the NOx reduction by oxygenated compounds, especially ethanol or acetone [272]. It seems that the formation of isocyanate species (-NCO) is a key step of the ethanol-SCR reaction over Ag/Al2 O3 [273,274] even though many other species may be detected [275,276]. This remarkable activity of silver catalysts for ethanol-SCR prompted Barreau et al. to imagine a combination between Ag/Al2 O3 and a NH3 -SCR catalyst (WO3 /CeZrO2 ) using C2 H5 OH-NH3 mixture for NOx reduction [277,278]. The effect of silver alone for the C2 H5 OH-SCR and for the C2 H5 OHNH3 -SCR and finally the effect of adding WO3 /CeZrO2 are illustrated in Figure 24. Figure 24. NOx conversion and NH3 outlet concentration in C2 H5 OH-SCR (blue) and C2 H5 OH + NH3 -SCR (red) on 2%Ag/Al2 O3 and effect of adding 6%WO3 /Cex Zry O2 to Ag/Al2 O3 in the C2 H5 OH + NH3 -SCR reaction (green). Mixture: 400 ppm NO + 1200 ppm C2 H5 OH + 400 ppm NH3 (when present) + 10% O2 + 10% CO2 + 8% H2 O. From [277] with permission from Elsevier. Silver alumina alone possesses a significant activity for the ethanol-SCR reaction (blue curve). The activity is enhanced by the presence of ammonia (red curve). In the dual bed (Ag + W) system (green curve), an increased conversion of NOx is observed, and simultaneously the outlet concentration of ammonia is decreased. Catalysts 2021, 11, 703 36 of 78 The respective roles of Ag/Al2 O3 and WO3 /CeZrO2 in this complex reaction were summarized in the review by Barreau et al. [279]. Figure 25 illustrates the reaction pathway identified in the ethanol-NH3 SCR reaction over the Ag/W dual bed. Note that WO3 /CeZrO2 is then more suitable than a copper-exchanged zeolite (2.5% Cu–FER) as an SCR catalyst. Despite a significantly higher ammonia conversion rate using the zeolite, NOx abatement is lower because ammonia and ethanol strongly interact together on Cu2.5–FER [278]. Figure 25. Reaction pathway identified in the ethanol-NH3 SCR reaction over Ag/Al2 O3 + WO3 /CeZrOx dual bed. Hydrogen species are key intermediates in the reduction of NO on Ag. The fast SCR is the main reaction over WO3 /CeZrOx , but acetaldehyde formed over Ag tends to produce undesired reactions. Oxidation of NH3 is observed at high temperature. Fortunately, this reaction is totally selective for N2 over the tungsten catalyst. From Barreau et al. [279] with permission from Elsevier. 5.3. Conclusions Tungsten-based catalysts are highly referenced as active samples in NOx reduction abatement for a wide range of reducers or processes. Commonly used in V-W/TiO2 catalysts for NH3 -SCR application, WO3 is known to increase the activity, widen the temperature window, improve the resistance to various poisons and lower ammonia oxidation activity by O2 . Tungsten is also involved in V-O-V species formation by confining vanadia in small clusters leading to oligomeric vanadia (V2 O5 ) sites, demonstrating higher NH3 -SCR activity than monomeric vanadyl compounds. WO3 is also associated with ceria as redox support, with or without vanadia or titania, for low-temperature activity and N2 O limitation emission. Tungsten increases the number of Brønsted sites, favors the formation of Ce-O-W bridges and tends to accelerate the cerium reduction. 6. Total Oxidation of Volatile Organic Compounds in Gas Phase and Gas Sensors Owing to the multiple oxidation states of tungsten and rapid diffusion of surface oxygen, WO3 is also a candidate for oxidation reaction in gas phase [280], VOC oxidation (Section 6.1) and gas sensor application (Section 6.2). 6.1. VOC Oxidation in Gas Phase on Tungsten Catalysts The first part of this section is focused on the non-photocatalyzed reactions. However, WO3 is response-sensitive to light up to 480 nm, which also makes it a good candidate Catalysts 2021, 11, 703 37 of 78 for photoassisted VOC oxidation (Section 6.1.2). Note that WO3 -based photacatalysts for liquid phase applications are depicted in Section 7. 6.1.1. VOC Oxidation on Tungsten Catalysts (Non-Photocatalyzed Reactions) Balzer et al. showed that WO3 alone (7.5 m2 g−1 ) can efficiently catalyze BTX oxidation [281]. The reactivity of the different hydrocarbons is as follows: benzene (T50 = 250 ◦ C) > toluene (T50 = 340 ◦ C) > m-xylene (T50 = 420 ◦ C) ≈ p-xylene (T50 = 430 ◦ C). Oxidation activity of WO3 is linked to the presence of W6+ , W5+ and W4+ surface species generating reactive oxygen species. However, WO3 is generally used in supported catalysts. Pansare et al. investigated NH3 and toluene decomposition on tungsten carbide (WC) and tungstated zirconia (WZ) [282]. Both WC and WZ catalysts were active for the simultaneous decomposition of NH3 and toluene at 700 ◦ C in the presence of H2 , CO, CO2 and H2 O. Benzene is formed by steam dealkylation of toluene. The WO3 -V2 O5 -TiO2 catalyst usually employed for NH3 -SCR reaction (see Section 4) was proven to possess good oxidation activity for VOC abatement. Debecker et al. studied the total oxidation of benzene and chlorobenzene on a catalyst containing 3% WO3 (or MoO3 ) supported on titania variously loaded with vanadia (3 to 10% V2 O5 ) [283]. The main results of this study are summarized in Table 6. Table 6. Total oxidation of benzene and chlorobenzene (CB) at 300 ◦ C on V2 O5 -TiO2 , 3%WO3 -V2 O5 TiO2 or 3%MoO3 -V2 O5 -TiO2 catalysts. Reaction conditions: 100 ppm benzene or chlorobenzene, 20% O2 in He. VVH = 37,000 h−1 . Adapted from [283]. Benzene Conversion @ 300 ◦ C (%) CB Conversion @ 300 ◦ C (%) Vanadium Loading 3% V2 O5 5% V2 O5 10% V2 O5 10% V2 O5 No promoter 23 71 86 75 WO3 -promoted 34 77 98 93 MoO3 -promoted 43 83 95 93 Vanadium oxide was then the active phase for VOC oxidation. However, tungsten and molybdenum oxides significantly increased V2 O5 activity. It is worth noting that promotion by WO3 allowed reaching almost 100% conversion of benzene with the 10% V2 O5 catalyst. WO3 and MoO3 promotion were also beneficial for chlorobenzene conversion, even though total oxidation cannot be reached at 300 ◦ C. Benzene oxidation over WO3 -V2 O5 -TiO2 catalysts was also investigated by Lu et al. [284]. Traces of benzene (1–10 ppm) were treated in a gas containing CO2 , CO, O2 and H2 O, simulating a flue gas issued from a methane burner. HCl (50 ppm), SO2 (400 ppm), NO (300 ppm) and NH3 (360 ppm) were added to simulate a waste incineration flue gas atmosphere. Due to the very low concentration of benzene, its conversion was measured by resonance-enhanced multiphoton ionization time-of-flight MS (REMPI-TOFMS), a technique well adapted to analyze minute traces of hazardous air pollutants [285]. The most active catalyst was supported on a highsurface-area TiO2 (166 m2 g−1 , quasipure anatase) with a low loading of vanadia (0.8 wt.%) promoted by 6 wt.% WO3 . About 80% of benzene could be eliminated with little variation of efficiency when other pollutants (NO, NH3 , HCl, SO2 ) were present. Other tungsten-based catalysts were employed for VOC abatement. For instance, magnesium tungstate (MgWO4 )-based catalysts were evaluated by Gancheva et al. in CO and hydrocarbon oxidation [286]. Four catalysts were tested: pure MgWO4 (5.2 m2 g−1 ), MgWO4– 3%WO3 (4.0 m2 g−1 ) and these materials promoted by 0.5% Pd. Without palladium, pure MgWO4 was more active than the same support enriched in WO3 . With Pd-promoted catalysts, two opposite behaviors were observed: MgWO4 /3%WO3 /0.5%Pd was the most active catalyst in hydrocarbon combustion (toluene, n-hexane) while MgWO4 /0.5%Pd (with no tungstate) was much more active for CO oxidation. Gancheva et al. proposed that CO and HC would be activated on MgWO4 and WO3 by two different mechanisms depending on Catalysts 2021, 11, 703 38 of 78 the nature of the reactant CO or HC. The role of palladium would be to favor O2 adsorption and activation. Catalytic abatement of trichloroethane (TCE) was performed on complex materials consisting of W-Mo bronzes (W-Mo/Nb/V/P) [287]. The best performances were obtained on the mixed W-Mo bronze with a W/Mo ratio close to 1. Complete composition (based on W + Mo = 1) was Mo(0.54)/W(0.46)/Nb(0.41)/V(0.20)/P(0.08). A 50% decomposition of TCE was achieved at 300 ◦ C on this material, while a reference zeolite (HMOR) gave 50% conversion at 470 ◦ C. The performance of this bronze catalyst was linked to its acidity (B sites only present on W-containing materials) and to its high oxygen mobility in the bulk, as revealed by 16 O/18 O exchange. Tungsten oxide is also a good promoter of diesel oxidation catalyst (DOC) allowing increasing activity of noble meta—alumina catalyst: a gain of 20 ◦ C on the light-off conversion of HCs was attained with 1% WO3 on the reference PtPd/Al2 O3 catalyst [288]. For ambient air purification, the very mild operating conditions require the development of fine structures/morphologies to improve the number of surface oxygen vacancies. With this aim, formaldehyde oxidation can be performed at room temperature by 1% Pt-doped WO3 nanoflakes assembled into hollow microspheres (23 m2 g−1 ), in which the porous architecture promotes diffusion and adsorption of HCHO [289]. However, photoassisted catalysis appears more suitable for the very mild operating conditions. 6.1.2. WO3 -Based Photocatalysts for VOC Oxidation WO3 -based materials as photocatalysts are mostly dedicated to reactions occurring in liquid phase (Section 7, in which general information about WO3 -based photocatalysts is provided). However, few recent studies reported that such catalysts also demonstrated intersecting behaviors in gas phase, especially for ambient air purification. To overcome drawbacks of bare WO3 , such as photocorrosion and unsuitable bandgap structure for the reduction of molecular oxygen, improvement can be obtained by structure/morphology control and/or doping. For instance, WO3 nanoparticles obtained by gas phase method and annealing at 600 ◦ C were reported to be much more active than commercial WO3 in acetaldehyde oxidation. Oxidation rate can be further significantly improved by addition of ZrO2 , which acts as a sorbent for the acetic acid intermediate species to release the WO3 surface. Moreover, Pt or Ru addition allowed the total mineralization into CO2 and H2 O [290,291]. A physical mixture of WO3 and CeO2 also exhibited excellent photocatalytic activity in acetaldehyde oxidation, which was attributed to the electron scavenging property of CeO2 aiding charge carrier separation [292]. WO3 can be also associated with the other usual photocatalyst, namely TiO2 . To improve the visible light sensitivity of WO3 nanoparticles impregnated into a commercial TiO2 powder, Balayeva et al. successfully added Fe(III) nanoclusters. The observed enhancement in acetaldehyde photooxidation was then attributed to the promotion of multielectron reduction processes [293]. However, Caudillo-Flores et al. pointed out that a cautious approach must be adopted when interpreting the results. They prepared TiO2 /WO3 samples with various W/Ti atomic ratios from 0 to 0.5, leading to various structures ranging from truly doped samples in which tungsten was exclusively located in lattice positions of the anatase structure to composite catalysts where nanosized tungsten species were supported over TiO2 . The authors highlighted that both the reaction rate (in toluene and styrene photo-oxidation) and the apparent quantum efficiency can lead to misleading results in terms of the most active TiO2 /WO3 sample(s) as well as the (positive/negative) magnitude in comparison with bare titania reference [294]. Implementation of photocatalysts in building windows is of major interest in air purification. Li et al. proposed a g-C3 N4 @Csx WO3 heterostructure as a coating for a multifunctional smart window for UV-isolating, Vis-penetrating, NIR-shielding and photocatalytic activity. These composites display excellent formaldehyde and toluene decomposition properties. The shielded NIR light is used instead of wasted as heat, while the C3 N4 @Csx WO3 structure promotes the separation of charge carriers and then enhances photocatalytic oxidation. Moreover, the small polaron can jump from localized states to Catalysts 2021, 11, 703 39 of 78 the conduction band of Csx WO3 under NIR irradiation (730–1100 nm), resulting in an NIR-catalytic reduction [295]. Note that this kind of structure was also proposed as a photocatalyst for water purification (see Sections 7.6.3 and 7.6.4). 6.2. Gas Sensors Using Tungsten-Based Catalytic Materials Since the mid-1950s, gas sensors have experienced great development for environmental and safety applications. Gas sensors are based on the measurement of electrical behaviors caused by chemical changes. Expected properties are sensibility, selectivity, stability, repeatability and response time. To build low-cost sensors, the measurement of the resistivity of heated semiconductor oxides is suitable thanks to the cost of raw materials and the convenience of microelectronic integration. Details of the internal structure of the sensors, including the heater, electrodes and external circuit, are out of the scope of this review. The sensing is related to changes in the electrical conductivity, which are mainly attributable to changes in the oxygen concentration at the oxide surface. Many oxides exhibit conductivity changes in presence of gases, but most of the commercialized sensors are based on SnO2 , WO3 or ZnO. As reported in the recent review by Dong et al., a huge number of works have been devoted to tungsten oxide based sensors [296]. Tungsten oxide is sensitive to many gases such as O2 , O3 , CH4 , CO, H2 , NH3 , C3 H8 , NO, NO2 and H2 S, with operating temperature in the 250–450 ◦ C range [297,298]. WO3 is an n-type oxide; i.e., the adsorption of surface oxygen atoms form an electron depletion region (Equations (11)–(13)), depending on the temperature of the material, leading to a potential barrier. Consequently, the adsorption of an oxidizing gas increases the resistivity of WO3 [299,300]. O2 + e− → O2 − (low temperature) (11) O2 − + e− → 2O− (medium temperature) (12) O− + e− → O2− (high temperature) (13) The formation of oxygen anions (Equation (13)) should be avoided (via the temperature control of the sensor) because of their tendency to be incorporated in the bulk of the material. Recent studies about WO3 -based sensors are mainly dedicated to NO2 detection (Section 6.2.1), but other gases are also considered (Section 6.2.2). 6.2.1. WO3 -Based Sensor for NO2 Detection Since WO3 is an n-type semiconductor, NO2 adsorption leads to anionic adsorbates on the surface of WO3 according to reaction (14): NO2 (gas) + e− (from surface) ⇋ NO2− (14) Han and Yin performed density functional theory (DFT) calculations to study the adsorption characteristics and electron transfer of nitrogen dioxide on O- and WO-terminated WO3 (001) surfaces with oxygen vacancies [301]. It was found that NO2 is (i) oxidized into nitrate on the bridging oxygen atom from an oxygen defect of the O-terminated WO3 (001) surface and (ii) dissociated on a WO-terminated (001) surface: one oxygen atom from NO2 fills the oxygen vacancy, and the resulting NO fragment is adsorbed onto a W atom. Both of these adsorption models are responsible for an increase in the electrical resistance of WO3 . In a very recent study, Yang et al. showed by in situ DRIFT spectroscopy that NO2 and NO exhibit similar interaction with the surface of tungsten oxide; both nitrogen oxides were detected as oxidizing gases [302]. The structure/morphology of WO3 is a key factor in the sensor response, and this topic is particularly developed in the recent literature. For instance, comparison of Ni-doped WO3 nanowires and nanosheets shows that nanowires exhibited a rapid response time (66 s) but a slow recovery time (204 s) due to a low NO2 desorption rate from the internal Catalysts 2021, 11, 703 40 of 78 porous structure of nanowires. On the contrary, the recovery time over nanosheets was shorter (126 s), thanks to a lower surface area and a less porous structure [303]. The effect of the structure on the sensor response for undoped WO3 was also evidenced by Li et al. [304]. An enhancement of the gas sensing properties was obtained by the formation of self-assembled hierarchical hollow spheres. Such structure enhances the sensitivity to NO2 . At 140 ◦ C, a response was recorded for NO2 concentration of 18 ppb. This was attributed to a high specific surface area (7 m2 g−1 ) and to a wide range of pore size distribution (from 3 to 60 nm) that promotes the gas diffusion. Table 7 summarizes the latest developments in the synthesis of controlled undoped WO3 structures for high-sensitivity NO2 gas sensors (papers published in 2019–2020). Table 7. Latest developments in the synthesis of controlled undoped WO3 structures for highsensitivity NO2 gas sensors (papers published in 2019–2020). WO3 Structure/ Morphology Detection Limit @ Optimal T Remarks Ref. Nanoparticles (~40 nm) 100 ppb @ 25–50 ◦ C Ionic liquid-assisted synthesis, samples calcined at 500 ◦ C for 2 h. [305] Microflowers assembled from nanoplates 125 ppb @ 105 ◦ C Outer diameters of ~2 µm, composed of nanoplates with the average pore size of 10.9 nm [306] Nanowire-assembled WO3 nanomesh 50 ppb @ 160 ◦ C WO3 nanomesh, assembled from single-crystalline WO3 nanowires [307] Nanowires (2 nm) 930 ppb @ 100 ◦ C Nanosheets 300 ppb @ 100 ◦ C Comparison of monoclinic, triclinic and hexagonal WO3 nanosheets; best performances obtained with triclinic WO3 [309] Mesoporous WO3 60 ppb @ 500 ◦ C Diameter: 7 nm; specific surface area: 209 m2 g−1 , prepared from SBA-15 as the hard template [310] [308] Improvement in NO2 detection can also be obtained by both tuning the tungsten morphology and creating interactions with a support. For instance, Ma et al. showed that tungsten oxide nanorods (diameters of 50–150 nm and lengths of 5–20 mm) supported on porous silicon (PS) had a good response to NO2 and good recovery characteristics at room temperature. The lowest detected NO2 concentration was 250 ppb [311]. The same authors also synthesized nanowires with diameters of 20–30 nm and lengths of 1–2 µm which were grown directly on the porous silicon through thermal annealing of tungsten film. The sensor responded well to NO2 compared to other gases since the NO2 response was three times higher for 2 ppm NO2 than for 50 ppm NH3 , 100 ppm ethanol or 100 ppm acetone at 150 ◦ C [312]. Modulations of the potential barriers at both homo- and heterojunctions between porous silicon and tungsten oxide were proposed to be responsible for the good sensibility at a low operating temperature (100 ◦ C) [313]. Excellent gas-sensing behaviors were obtained with n-WO3−x /n-PS nanocomposite prepared by sputtering tungsten oxide films on a high specific surface area porous silicon substrate [314]. This sensor exhibited an anomalistic p-type semiconducting behavior, which was supposed to improve the amplification effect of the heterojunction between WO3 nanowires and the PS composites (PS forms p–n heterointerface with n-type WO3 nanowires). The XPS analysis indicated the presence of large amounts of surface oxygen vacancies that directly contributed to the sensor response. NO2 concentration as low as 30 ppb was then detected, and Figure 26 illustrates that the NO2 selectivity vs. other gases was especially interesting. To improve the control of the WO3 nanowire synthesis on silicon microelectromechanical systems (MEMS), Lee et al. recently developed a fabrication process by stress-induced method, in Catalysts 2021, 11, 703 41 of 78 which the growth position of the WO3 nanowires can be controlled by patterning of the WO3 seed film [315]. Finally, note that competition between different gases may influence the recovery time. For instance, WO3 film demonstrated a longer recovery time towards NO2 compared to NH3 [316]. Figure 26. n-WO−3−x /n-porous silicon sensor responses to various gases at RT. From Li et al. [314] with permission from Elsevier. Doped WO3 sensors: Sensor improvement can also be obtained by WO3 doping. The addition of precious metals to WO3 is performed to improve the gas–sensor interaction, as well as the response and recovery times, and decrease the operating temperature. Gold is the most represented metal in the recently reported works, probably because Au-doped WO3 -based sensors exhibit overall enhancement in NO2 sensing performances (response, detection limit and response/recovery times) [317,318] but also because they are only slightly affected by humidity. In fact, measurement in a humid atmosphere is one drawback of the semiconductor oxide-based sensors, and WO3 is no exception [319]. Water adsorption leads to the oxidation of the WO3 lattice because water fills the oxygen vacancies (operando DRIFT experiments), and consequently the resistance of the WO3 -based sensor increases [320]. The selectivity of the sensor is then affected, with lower signals toward NO2 and higher sensor signals to CO. Sevastyanov et al. showed that water dependency can be virtually avoided by gold addition in the bulk and on the surface of WO3 . The conductivity in pure air of such Au/WO3 :Au material increased only 1.1–1.2 times when the absolute humidity raised from 2 to 16 g m3 (it increases 6–7 times for Pt/SnO2 :Sb films in the same humidity range). Measurements with 0.45–10 ppm NO2 showed that the Au/WO3 :Au sensor response did not depend on humidity [321]. This behavior toward humidity was confirmed on Au-WO3 core–shell-structured nanospheres, associated with excellent NO2 selectivity and long-term stability [322]. The significant performance improvement in NO2 detection of Au/WO3 compared to bare WO3 was attributed by Hang et al. to a combined sensing mechanism: the surface Au nanoparticles dominated chemical sensitization while interbedded Au nanoparticles induced electronic sensitization [323]. An activation of Au/WO3 :Au thin films by laser diode radiation instead of constant heating was proposed by Almaev et al. [324] to reduce the response time to NO2 by photodesorption. Moreover, holes generated in the near-surface region of WO3 film by optical transitions were supposed to favor the photodesorption of chemisorbed O2 - species, leading to the absence of response to reducing gases and change in oxygen concentration. Results obtained with platinum-doped WO3 appear possibly contradictory. Chmela et al. reported that platinum addition to WO3 nanowires (<100 nm) caused detrimental effects, with lower sensitivity and selectivity toward NO2 compared with the unfunction- Catalysts 2021, 11, 703 42 of 78 alized systems, associated with better sensing properties toward C2 H5 OH [325]. On the opposite, with nitrided WO3 (WOx Ny nanofibers), platinum addition enhanced the gas sensing characteristics by (i) lowering the operating temperature; (ii) enhancing the sensor reversibility at 50 ◦ C; and (iii) exhibiting an exceptional selectivity toward NO2 against interfering molecules such as C2 H5 OH, C7 H8 , CH4 , CO, NH3 and NO [326]. Silver was also recently reported as a promising dopant. The optimal calcination temperature was found at 500 ◦ C to obtain the larger response, better selectivity, faster response/recovery time and better long-term stability for NO2 [327]. With 0.5%Ag-WO3 [328], the sensor shows higher NO2 sensing response at 200 ◦ C and higher selectivity for NO2 in the presence of different interfering gases (NH3 , acetone, SO2 , methanol, CO2 , NO). Significant decreases in gas responses were observed with high silver loading (5 and 10 mol%) because the silver crystallite size became too large and hindered their catalytic effects [329]. Comparing Ag-, Pd- and Pt-doped WO3 nanoplates, Li et al. showed that Pd-WO3 exhibited the highest response to NO2 while Ag-WO3 exhibited the fastest response speed [330]. The promotional effect of palladium was attributed by Liu et al. to the Schottky barrier between Pd and WO3 [331]. WO3 -based sensors doped with other oxides have attracted little attention in the recent literature. Only a few recent studies report improvements of WO3 -based sensors by doping with oxides such as Sn, Sb and Fe. The main characteristics of these studies are summarized in Table 8. The observed improvements were generally attributed to an increase in the oxygen vacancies on the sensing surface. Table 8. Oxide-doped WO3 -based sensors for NO2 detection. Sensor Material Detection Limit @ Optimal T RuO2 /WO3 nanowires Remarks Ref. Improved selectivity by enhancement of the electron depletion layer due to the formation of RuO2 /WO3 Schottky junctions [332] (2014) Fe-doped WO3 hollow nanospheres. 10 ppb @ 120 ◦ C The light distortion in the WO3 crystal lattice by Fe doping produced interesting defects for gas sensing, with more oxygen vacancies [333] (2018) WO3 -In2 O3 nanocomposites @ 140 ◦ C Sol–gel preparation method; also active for CO detection at 240 ◦ C [334] (2019) @ 30 ◦ C Improvement in NO2 detection attributed to abundant structural defects derived from Sb doping, reduced bandgap and the 3D hierarchical microstructure [335] (2018) 5 ppb NO2 @ 100 ◦ C Optimal loading: 2 wt.% Sn; introduction of Sn ions resulted in shorter response and recovery times, attributed to the increased number of oxygen vacancies on the sensing surface [336] (2018) 3D hierarchical structured Sb-doped WO3 Sn-doped WO3 nanoplates 6.2.2. WO3 -Based Sensors for Detection of Gasses Other Than NO2 WO3 -based sensors were also investigated for the detection of gases other than NO2 . With this aim, only a few studies have dealt with undoped WO3 . Nanostructured WO3 was evaluated for H2 S [337] or Cl2 [338] detection, while thin film was found to be sensitive to acetone [339]. However, for this latest gas, a comparative study showed that SnO2 is the best thin film compared to tungsten oxide or tin-doped tungsten oxide [340]. As described in Section 6.2.1, water adsorption on semiconductor oxides usually affects the measurements. However, this behavior can be used to develop sensors for relative humidity measurement. With this aim, WO3 is usually associated with TiO2 in the recently published studies. Zanettia et al. examined the influence of the WO3 loading (0–10 mol%) on nanopowdered samples prepared by the polymeric precursor Catalysts 2021, 11, 703 43 of 78 method [341]. Best results in humidity measurements (in the 15–85% relative humidity range) were obtained with 2 mol% WO3 . This was attributed to the best compromise between the increase in acid sites and the mean pore size and pore size distribution. Faia et al. studied the influence of V2 O5 doping for TiO2 -WO3 sensors [342]. A p- to n-type transition still occurred for the doped sensors with the lower V2 O5 content, while the sensor with the higher V2 O5 content exhibited a typical n-type behavior with humidity increase. In addition, the electrical response to the relative humidity (10–100%) depended on the changes in the fabrication route (sintering temperature), which influences the final structure. Section 6.2.1 also shows that NO2 detection can be improved by the addition of precious metals to WO3 -based sensors. Nevertheless, these modifications are rather dedicated to the detection of reductant gases such as NH3 , H2 , H2 S and CO. Palladium–WO3 samples were prepared by spray pyrolysis by Gobole et al. The optimal palladium loading was found to be 3 wt.% Pd in WO3 for the detection of NO2 , SO2 and NH3 at 100, 200 and 225 ◦ C, respectively. For all of the studied gases, the response and recovery times for concentrations up to 750 ppm were fast, in the ranges of 0.5–1.25 s and 1–6.7 s respectively [343]. Tungsten trioxide nanowires decorated with iridium oxide nanoparticles resulted in remarkable changes in the morphology and defects of tungsten oxide nanowires. Such a sensor was found to be sensitive towards ethanol, NH3 , H2 , H2 S and NO2 [344]. Tungsten oxide functionalized with gold or platinum nanoparticles was synthesized by Vallejos et al. using a single-step method via aerosol-assisted chemical vapor deposition. The metal additions allow the discrimination of C2 H5 OH, H2 and CO gases, which are present in proton-exchange fuel cells. Particularly, Pt-functionalized tungsten oxide films allow H2 detection at 250 ◦ C, whereas nonfunctionalized tungsten oxide films detected low CO concentration (100 ppm) at a lower temperature (150 ◦ C) [345]. Platinum– tungsten oxide can also be used to improve the CO electro-oxidation activity employed in the electrochemical sensor [346]. The interaction between platinum and tungsten oxide was enhanced by a reductive heat treatment, leading to a significant negative shift in CO oxidation potential. A portable CO sensor device built with Pt/WOx /C exhibited a higher sensitivity, faster response time and good linearity within 50 ppm CO, compared with a usual Pt/C-based sensor. Xu et al. showed that triethylamine vapor is efficiently detected by Ag/Pt/W18 O49 hybrid nanowire sensor, with a detection limit of 71 ppb. The ternary hybrid showed better behavior in detecting triethylamine than the binary Ag/W18 O49 and Pt/W18 O49 nanowires [347]. This excellent performance was attributed to the dual sensitization mechanism, i.e., a synergy of both electronic and chemical interactions. Pt-catalyst-loaded tungsten oxide is also a good candidate for optical hydrogen gas sensor applications due to its gasochromic behavior (Pt/WO3 turns blue in H2 atmosphere while its electrical conductivity also changes). Yamaguchi et al. investigated the influence of partial pressures of hydrogen and oxygen gases on a Pt/WO3 -based sensor prepared by a sol–gel method [348]. Unfortunately, the optical absorbance of the film exhibited a nonlinear relationship with the H2 concentration in absence of oxygen. However, the absorbance and electrical conductivity increased proportionally with the H2 concentration in presence of oxygen, but both parameters strongly depended on the oxygen partial pressure. According to the relationship between the gasochromism and the oxygen concentration, the authors demonstrated that Pt/WO3 is able to detect hydrogen gas concentrations in a low oxygen gas concentration atmosphere. 6.3. Conclusions In summary, WO3 is active in catalytic or photocatalytic oxidation of volatile organic compounds. Catalytic oxidation activity of WO3 is linked to the presence of W6+ , W5+ and W4+ surface species generating reactive oxygen compounds. In supported catalysts, tungsten oxides promote the activity of the V2 O5 active phase. Many pollutants are reported in studies considering the catalytic abatement of VOCs, such as formaldehyde, hexane, Catalysts 2021, 11, 703 44 of 78 trichloroethane, chlorobenzene, benzene, toluene and xylene. W-based samples are also largely studied for gas sensors, mainly in NO2 detection. WO3 is an n-type semiconductor, and its electrical conductivity varies with the oxygen concentration at the oxide surface. In addition, great recent efforts have been made in the development of the synthesis of controlled WO3 structure/morphology for high-sensitivity gas sensors. 7. Pollutant Remediation in Liquid Phase (Photocatalysis) The use of WO3 -based catalyst for pollutant remediation in liquid phase mainly concerns the oxidation of organic compounds via photocatalytic processes. Only a few recent studies have dealt with WO3 -based catalysts for non-photocatalytic application, such as H2 O2 electrogeneration over WO2.72 /Vulcan XC72 gas diffusion electrode [349], hydrolysis of waste bottle PET in supercritical CO2 assisted by acidic catalysis over WO3 -TiO2 [350] or heavy metal ion adsorption on inorganic–organic hybrid WOx –ethylenediamine nanowires [351]. However, this section is focused on the main use of tungsten-based catalysts in liquid phase, namely photocatalysis. Compared to TiO2 , which responds only to UV light, WO3 is visible-light-responsive. The bandgap between the valence band (VB) and the conduction band (CB) is 2.6–2.8 eV. However, bare WO3 is not very active due to the fast recombination of photogenerated electrons and holes. Fortunately, the photocatalytic activity of WO3 can be improved in five main ways: (i) the control of the WO3 structure/morphology to obtain nanoporous structures with large surface area and fast pollutant diffusion; (ii) surface hybridization with graphene to obtain large specific surface areas and improvement in the charge transfer; (iii) coupling with other semiconductors (TiO2 ) to enhance the photoinduced charge separation efficiency; (iv) doping by noble metal, which works as an electron pool and catalyzes O2 activation. WO3 as a photocatalyst has been the subject of three recent reviews (“WO3 -based photocatalysts: morphology control, activity enhancement and multifunctional applications” by Dong et al. (2017) [352]; “Adsorptive removal and photocatalytic degradation of organic pollutants using metal oxides and their composites: A comprehensive review” by Gusain et al. (2019) [353]; “Tungsten oxide-based visible light-driven photocatalysts: crystal and electronic structures and strategies for photocatalytic efficiency enhancement” by Quan et al. (2020) [354]). Consequently, the section is mainly focused on the more recent studies. Evaluated molecules are mainly dyes, but other organics that are of major interest for water treatment are also considered, such as active molecules (medicinal and plant protection products) or pathogens. 7.1. Undoped WO3 Photocatalysts As previously introduced, bare WO3 is poorly active as a photocatalyst, but structured/hierarchized WO3 exhibits interesting behaviors. Monoclinic WO3 phase was often reported as a suitable structure for photocatalysis. Noticeably, Xie et al. showed in 2012 that monoclinic WO3 exhibits preferentially the suitable high-surface-energy {002} facet [355]. Indeed, the simple thermodecomposition of H2 WO4 leads first to the cubic phase, but the monoclinic structure is formed with increasing time and/or temperature. After 30 min at 450 ◦ C, the resulting WO3 oxide shows 90% monoclinic phase [356]. The phase transformation is associated with a decrease in the specific surface area and an increase in oxygen vacancies. The electron transport is then improved, favoring the photocatalytic activity (ibuprofen removal by ozonation under visible-light radiation). Various preparation methods have been developed to improve the WO3 photocatalyst. Chen et al. evaluated different fuels (glycine, urea, urea and citric acid) to synthesize nanoscale tungsten oxides (nanoparticles, nanorods and nanoneedles) by the solution combustion synthesis method. Monoclinic WO3 was obtained with urea, while a mixture of monoclinic W18 O49 and WO3 structures was obtained with glycine when the molar ratio glycine/ammonium paratungstate was 10 or higher. Such W18 O49 + WO3 mixture was also obtained with improved porosity using a mixture of urea and citric acid because the Catalysts 2021, 11, 703 45 of 78 reactants generated more gases and the combustion reaction was more vigorous (production of larger holes). Accordingly, this sample exhibited the best activity in methylene blue degradation [357]. Nanoparticles of monoclinic WO3 can be also synthesized from Na2 WO4 and PVP 70,000 (polyvinylpyrrolidone) as surfactant and template [358]. The synthesis was carried out at 180 ◦ C in a Teflon autoclave. The best morphology for the photocatalytic efficiency in degradation of rhodamine B (RhB) under visible light was the block-shaped morphology. The authors proposed that the high photocatalytic efficiency could be attributable to the reducing activity of PVP which led to the formation of oxygen vacancies, beneficial for the capture of photoelectrons and the generation of superoxide radicals. Monoclinic WO3 nanoplates (100–170 nm in side length and 30–50 nm in thickness) can be also obtained by a one-step template-free hydrothermal route, from a mixture of aqueous solutions of Na2 WO4 · and HCl maintained at 180 ◦ C for 12 h in an autoclave [359]. The optimal calcination set-up was found to be 600 ◦ C for 2 h. The resulting catalyst allowed the degradation of rhodamine B with an activity 5 times higher than that of a commercial WO3 powder. It was proposed that the high concentration of -OH species was responsible for the enhancement of photocatalysis. However, the superiority of the monoclinic phase has been questioned very recently by Zhang et al. [360]. They studied the photocatalyzed degradation of rhodamine B (RhB) over monoclinic (m-WO3 ) and hexagonal (h-WO3 ) tungsten oxide. Samples were synthesized via acid precipitation process and decomposition of H2 WO4 at different temperatures in air or N2 to obtain oxidized or partially reduced oxidation states. After irradiation, the RhB removal efficiency classification was as follows: m-WO3 (22%) < m-WO3−x (48%) < h-WO3−x (76%) ≤ h-WO3 (80%). It is important to note that the specific surface areas of the hexagonal samples (52–55 m2 g−1 ) were approximately twice those of m-WO3 . It also appeared that the impact of the oxidation state of tungsten was more significant for the monoclinic phase than for the hexagonal one. In fact, the impact of the hexagonal/monoclinic-WO3 ratio (h/m) in the photocatalytic activity was clearly pointed out by Lu et al. [361]. To tune the h/m ratio, they used a sol–gel preparation method with K2 SO4 as stabilizing agent and controlled calcination time and temperature. The observed optimal h/m ratio was close to 70/30, for which the activity in rhodamine B degradation was multiplied by a factor of 7.4 compared to pure m-WO3 . This improvement was attributed to the formation of a phase junction between h-WO3 and m-WO3 , which exhibits high efficiency for the separation and transfer of photoexcited electron–hole pairs (electrochemical impedance measurements). New innovative designs are also a way to produce efficient WO3 photocatalysts. For instance, monoclinic nanocuboids, synthesized by hydrothermal treatment of commercial WO3 with H2 O2 , showed high amounts of coordinative unsaturated W sites suitable for the photocatalytic degradation of methylene blue [362]. Other morphologies such as diskshaped WO3 (D-WO3 , monoclinic phase) were recently reported as attractive for acetic acid mineralization (after 0.1% Pt addition). To obtain such samples, aqueous solution of (NH4 )10 W12 O41 · was mixed with HNO3 at 75 ◦ C. After maturation, the resulting powder was calcined 3 h at various temperatures to vary the physical properties. As expected, the crystallinity of D-WO3 increased with the calcination temperature while the specific surface area decreased. The highest photocatalytic activity was obtained with the sample calcined at 600 ◦ C [363]. Another recent approach was to design a single tungsten atom oxide photocatalyst, supported on poly(ethylene oxide), as shown in Figure 27a [364]. In this case, the photogenerated electron transfer process is enabled by an electron in the spin-up channel excited from the highest occupied molecular orbital to the lowest unoccupied molecular orbital +1 state, which can only occur with single tungsten atom oxide with W5+ (Figure 27b). The resulting catalyst exhibited a degradation rate of 0.24 s−1 for dyes (methyl orange, methyl red, dimethyl yellow; mechanism illustrated in Figure 27c for methyl orange), which is two orders of magnitude higher than those of available photocatalysts. Additionally, the reactions between oxygen with photogenerated electrons (e− + O2ads → O2 − •) and between adsorbed water and photogenerated holes (h+ + H2 Oads → H+ + OHads •) produce reactive oxidizing species such as superoxide anions (O2 − •), hydrogen − Catalysts 2021, 11, 703 − → − 46 of 78 → − peroxide (H2 O2 ) and hydroxyl radicals (OH•). Reactive oxidizing species are responsible for the degradation of aqueous organic compounds, besides the direct decomposition of organic compounds with photogenerated holes. Figure 27. (a) A Cs-corrected STEM-HAADF image showing monodispersed single tungsten atoms. (b) TD-DFT calculated the occupations and transitions between states in W6+ (left part) and separately ↑ spins. ↓ Dashed lines connect spin-up and -down channels in W5+ (middle part). Arrows ↑ and ↓ denote in the same band. The HOMO, LUMO and LUMO+1 bands for W5+ have been decomposed (right part) according to the tungsten atomic orbitals. (c) Photocatalytic degradation reaction (methyl orange): upon photoexcitation, the hole dissociates an H2 O into H+ and ·OH. The electron⋅ cleaves a C-N bond in the azo-benzenesulfonic group. The resulting H+ then reacts with 4-(2λ2-diazenyl)-N,Nλ ⋅ dimethylaniline (green) to form N,N-dimethylaniline, while the ·OH reacts with benzenesulfonic acid radical (blue) to form p-hydroxy benzene sulfonic acid. From [364] with permission from John Wiley and Sons. It is important to note here that dye decolorization and mineralization have to be distinguished. This point is highlighted for instance in the paper of Cheng et al. about the treatment of effluent from oil palm agroindustry with tungsten oxide photocatalysts [365]. With the optimum catalyst loading (at 0.5 g/L), the decolorization reached 96.2% but the mineralization reached only 51.1%. To improve the efficiency of treatment by photocatalysis, tungsten oxide can be also used as a photoelectrocatalyst, i.e., implemented at the surface of a photoanode. The idea is to promote the photoelectron transfer to dissolved oxygen, which is a drawback of WO3 photocatalysis. To enhance charge separation, electrochemical and photocatalytic techniques can be used together. Zheng et al. synthetized nanoporous, monoclinic WO3 photoanodes from a tungsten foil (0.1 mm thickness) that was anodized at 50 V in 0.25 wt.% HF electrolyte and subsequently annealed at 500 ◦ C. It was showed that methyl orange mineralization rate was higher in photoelectrocatalysis (at E = 1 V vs. SCE in NaH2 PO4 electrolyte) than in photocatalysis or electrocatalysis [366], as illustrated in Figure 28. The high efficiency of the WO3 photoelectrocatalyst was also demonstrated for the abatement of pathogens such as Escherichia coli [367]. Catalysts 2021, 11, 703 47 of 78 Figure 28. Methyl orange degradation rate by different processes. Experimental set-up: nanoporous − lamp), oxidation potential of 1 V monoclinic WO3 anode, irradiating intensity of 0.1 W cm−2 (xenon vs. SCE (NaH2 PO4 electrolyte). From Zheng et al. [366] with permission from Elsevier. 7.2. WO3 –Carbon Photocatalysts As previously mentioned, the fast electron–hole recombination is a major drawback of WO3 for photocatalysis application. In addition to the synthesis of structured WO3 with controlled morphology, another way to improve the photocatalysis efficiency is to associate WO3 with carbon-based materials that exhibit interesting high specific surface areas. Among the various possible structures of carbon-based materials, graphene appears the most popular. Preparation of WO3 –graphene composite can be performed by different methods. Recent studies reported techniques such as the electrospinning technique to obtain a porous tungsten oxide nanoframework with graphene film [368]; a one-pot synthesis, first mixing sodium tungstate and nitric acid and then adding tetraphenylporphyrinn, graphene and citric acid (final calcination in the 350–550 ◦ C temperature range) [369]; or a method based on pulsed laser ablation in liquid phase: WO3 nanoparticles suspended in water were mixed with a suspension of monolayer graphene and irradiated for 30 min by the pulsed laser beam of 355 nm. During the laser irradiation, the photoinduced electrons in the WO3 reduced graphene, and WO3 nanoparticles were anchored on the graphene sheets [370]. The synthesized film reported in [368] showed the ability to adsorb aromatic molecules, extensive light absorption range, significant light trapping and efficient charge carrier separation properties. Consequently, this sample allowed a high activity in the photodegradation of rhodamine B compared to bare WO3 and TiO2 nanomaterials. The in situ prepared nanocomposites made of tetraphenylporphyrin/WO3 /reduced graphene [369] showed specific surface areas around 450 m2 g−1 and spherically shaped nanoparticles of monoclinic WO3 when the appropriate tetraphenylporphyrin loading and calcination temperature of 350 ◦ C were used. The visible radiation absorption was confirmed with a bandgap energy of 2.14 eV. The photocatalytic degradation rate of 20 ppm Acid Blue 25 was 85% in 3 h at pH = 4 (UV–Vis and TOC analyses). This behavior was attributed to the visible-light absorption properties and to the high separation rate of photogenerated charge carriers. Similar conclusions were obtained with the samples prepared by the pulsed laser ablation evaluated in methyl blue degradation [370]: WO3 -reduced graphene samples showed much better visible-light absorption and less photogenerated charge recombination than pure WO3 . Catalysts 2021, 11, 703 48 of 78 Other carbon-based materials were also recently developed. Amorphous carboncoated tungsten oxide was obtained via pyrolysis of hybrid polyoxometalates (hybrid POMs) in nitrogen atmosphere [371]. Compositions, microstructures and concentrations of oxygen vacancies were closely related to the species of organic amines in the hybrid POMs. The presence of defects (oxygen vacancy) was responsible for the improvement in the degradation of dyes (methylene blue, methyl orange and rhodamine B) compared with WO3 or TiO2 photocatalysts. WO3 /carbon nanotube (CNT) nanocomposites were proposed by Isari et al. for the abatement of tetracycline (antibiotic) and other pharmaceutical wastes in water [372]. Samples were synthesized via a sol–gel method (aqueous sodium tungstate + lactic acid + CNT+ HCl; the obtained gel was then transferred in an autoclave for 30 h at 190 ◦ C). The characterization techniques demonstrated the incorporation of CNTs into the WO3 framework and an efficient reduction in charge carrier recombination rate compared to the corresponding WO3 catalyst without CNT. The pollutant remediation was performed coupling visible light and ultrasound (US) irradiations (sono-photocatalysis). Ultrasonic irradiation favors HO• formation, mainly via water decomposition. It was found that 60 mg/L tetracycline could be perfectly degraded with the following set-up: WO3 /CNT: 0.7 g L−1 ; pH = 9; US power: 250 W m−2 ; light intensity: 120 W m−2 ; duration: 60 min. Trapping experiment results verified that HO• radicals and h(+) were the main oxidative species. 7.3. WO3 Photocatalysts Doped by Precious Metals or Silver As previously mentioned, the restricted application of tungsten oxide (WO3 ) is due to its low conduction band. Tungsten trioxide (WO3 ) is an n-type semiconductor with a small bandgap (Eg 2.6–2.8 eV). Consequently, WO3 has a limited ability to react with electron acceptors such as oxygen and has a high recombination rate of the photogenerated electron–hole pairs. Therefore, great attention has been paid to develop novel visible-lightdriven WO3 -based photocatalysts, and special attention has been paid to the crystal facet engineering of WO3 nanocrystals (Section 7.1). Additionally, doping with noble metal (Section 7.3.1) or silver (Section 7.3.2) is an effective way to enhance the absorption of sunlight and improve the photocatalytic efficiency together with material phase/morphology design to generate oxygen vacancies. 7.3.1. WO3 Photocatalysts Doped by Precious Metals Abe et al. [373] proposed the enhancement of photocatalytic properties of WO3 by loading Pt, which can trap electrons photogenerated from WO3 . The use of Pt as an electron scavenger on WO3 nanorods was also investigated in [374]. Platinum nanoparticles were loaded on WO3 nanorods with various mass ratios (0.1, 0.2 and 0.3) via a photoreduction process (PRP). The photocatalytic activity in aerobic oxidation of alcohols reached up to 98% for Pt/WO3 and 69% for WO3 , while no oxidation was observed in the absence of light. The highest photocatalytic performance was obtained for the mass ratio of 0.2. This enhancement in the photocatalytic activity after platinum loading was attributed to an extended lifetime of the generated electron–hole pairs compared to WO3 support. Additionally, Gunji et al. [375] reported the importance of controlling the metal deposition site on the photocatalyst surfaces to optimize the use of the photoexcited electrons and holes without the recombination of photogenerated carriers during the photochemical decomposition of organic pollutants. The authors proposed a cocatalyst based on the site-selective deposition of PtPb nanoparticles on oxidation sites and deposition of Pt nanoparticles on reduction sites. The synthesized PtPb/Pt/WO3 exhibited higher photocatalytic performance for the decomposition of acetic acid under visible-light irradiation (λ > 420 nm) than that observed using the conventional photodeposited Pt/WO3 or chemically deposited PtPb/WO3 . Platinum doping was also evaluated on tungsten trioxide for catalyzed oxidation reaction [289] or as an electrode for direct methanol fuel cells (DMFCs) [376]. Pt/WO3 catalyst layers were prepared by electrosynthesis of WO3 on a graphite (Gr) support. It was reported that the corresponding Pt/WO3 /Gr electrode has higher catalytic activity for λ Catalysts 2021, 11, 703 49 of 78 methanol oxidation than a commercial Pt/C catalyst. Interestingly, methanol oxidation is enhanced under visible-light illumination, which is attributed to both a synergy between Pt and WO3 active sites and the simultaneous occurrence of methanol photooxidation at WO3 sites. 7.3.2. WO3 Photocatalysts Doped by Silver The surface plasmon resonance (SPR) effect greatly contributes to enhancing the visible-light absorbance of catalysts by allowing resonant photons at the metal–dielectric interface to resist the restoring force of positive nuclei [377]. The SPR effect of silver was investigated by Ding et al. [378]. By studying the relationships between the locations of silver nanoparticles on different facets of hexagonal WO3 nanorods and the photocatalytic performance of the photocatalyst, Ding et al. reported that both the intrinsic nature of charge separation on the {001} facets of WO3 -110 nanorods and the SPR effect contribute to the enhancement of visible-light absorption and the decrease in the recombination of the photogenerated electron−hole − pairs. The highest efficiency in the degradation of methyl orange (MO) was obtained with 4.5 wt.% Ag/WO3 photocatalyst presenting dominant exposed {001} facets compared to {100} or {010} ones. A mechanism was proposed where photons are absorbed under visible-light irradiation and photogenerated electrons and holes are subsequently produced by a facet surface transfer, as summarized in Figure 29. Figure 29. Possible degradation mechanism of MO over 4.5 wt.% Ag/WO3 photocatalyst under visible-light irradiation. From [378] with permission from the American Chemical Society. The crucial role of oxygen defects was investigated by Wei et al. [379]. Tungsten oxide (WO3−X ) is a transition metal oxide that has rich substoichiometric compositions and possesses oxygen defects involved in photon–electron interactions. In combination with silver nanowires (Ag NWs) presenting surface plasmon resonance properties, the incident photon-to-electron conversion efficiency is enhanced together with the methylene blue (MB) photodegradation performance. In fact, the electron concentration in WO3−X depends mainly on the stoichiometric defect concentration. This means that oxygen deficiencies play a critical role in the decay of localized SPR phenomena. Morphology and properties of Ag-WO3 hierarchical materials were also investigated by Capeli et al. [380] for the degradation of rhodamine B (RhB) dye under 467 nm LED light irradiation. Three-dimensional WO3 catalysts decorated with silver nanoparticles (Ag NPs) − Catalysts 2021, 11, 703 50 of 78 were prepared by a one-step hydrothermal method in the absence of surfactant (WO3 •Ag).  The amount of Ag NPs is an important factor in the formation of various novel and complex WO3 3D hierarchical architectures, from 3D irregular-platelet-like building blocks, which evolve into 3D hexagonal building blocks, to three-dimensional hexagonal-footballlike and finally to 3D multibranched spiky ball-like microcrystals. The WO3 •0.20Ag 3D  hierarchical structure presented higher photodegradation of RhB dye solution compared to the individual WO3 3D material. This result was assigned to an engineering heterojunction between Ag NPs and WO3 semiconductor, which could enhance the light absorption (more photoexcited electrons) and suppress the photogenerated electron–hole pair recombination. Ag NPs act as an electron reservoir. Under light, photogenerated electrons (e– ) in the valence band (VB) are excited to the conduction band (CB), and electrons from the reservoir on the Ag NPs could be trapped by O2 molecules to generate superoxide radical (•O2 – ) species. Concurrently, a similar number of holes (h+ ) are generated in the valence band (VB), which are able to react with H2 O or OH– molecules to generate OH• radicals. Similar results were achieved by Gao et al. [381], who used silver to improve the properties of the WO3 . Ag nanoparticles (Ag NPs) were reported to enhance the production of negative oxygen ions (•O2 – radicals) under visible light by the prepared Ag/WO3 catalysts supported on wood. More precisely, Ag NPs redistribute the charge carriers, which could trap the photogenerated electrons and inhibit the recombination of excited electrons and holes (Figure 30). Figure 30. Schematic illustration of the mechanism of negative oxygen ion production upon the Ag/WO3 –wood. From [381] with permission from Elsevier. Promising plasmonic Ag/AgCl photocatalysts have been recently investigated. The SPR effect of silver is responsible for the broadened absorption in the visible-light region, and these properties were expanded to WO3 . For instance, Ma et al. [382] prepared Ag–AgCl/WO3 hollow spheres with a flowerlike structure that had superior visible photocatalytic activity because of their unique morphology. Adhikari et al. [383] also observed a photocatalytic activity enhancement of Ag/AgCl/WO3 powder prepared by a microwaveassisted hydrothermal method. However, most WO3 /Ag/AgCl photocatalysts reported were in powder form, thus having limited use in practical applications because tedious regeneration is required. Consequently, Fang et al. [384] prepared WO3 /Ag/AgCl films on a conventional glass substrate to reduce the regeneration cost. The WO3 film was prepared on a glass substrate by calcination of spin-coated W precursor. Ag/AgCl particles were then deposited on WO3 film by an impregnation–precipitation–photoreduction method. Excellent photocatalytic performances were obtained in the degradation of methyl orange Catalysts 2021, 11, 703 51 of 78 (MO) and rhodamine B (RhB) under visible light. It is proposed that Ag and AgCl greatly promoted the separation of photogenerated electron–hole pairs and improved the charge transfer efficiency of WO3 . Based on the photoelectrochemical test and radical trapping measurement, a Z-scheme mechanism for WO3 /Ag/AgCl is proposed where •O2 – and h+ play the major roles in photodegradation, while the effect of OH• could be neglected (Figure 31a). Consequently, Z-scheme composite photocatalysts have to be considered as an effective way to enhance the photocatalytic performance of catalysts. Li et al. [385] proposed plasmonic Ag/Ag2 WO4 /WO3 Z-scheme visible-light composite photocatalyst for the degradation of rhodamine B, methylene blue and methyl orange. A possible Z-scheme mechanism of the ternary composite was proposed under visible light where Ag particles produce SPR effect but also work as the charge transmission bridge (Figure 31b). Figure 31. (a) The possible photocatalytic mechanism of organic pollutant degradation on WO3 /Ag/AgCl film under visible-light illumination. From [383] with permission from Elsevier. (b) Schematic illustrating the proposed photocatalytic mechanism over the Ag/β-Ag2 WO4 /WO3 photocatalyst. From [385] with permission from Elsevier. Sahoo et al. [386] designed a Z-scheme WO3−X -Ag-ZnCr layered double hydroxide (LDH) photocatalyst for tetracycline degradation, based on the SPR effect of metallic Ag as redox electron mediator. The defects created by surface oxygen vacancy in WO3−X and the existence of Ag as electron transfer conductor facilitate the charge pair separation efficiency and enhance the photocatalytic activity. Catalysts 2021, 11, 703 52 of 78 7.4. WO3 –TiO2 -Based Photocatalysts Because of the poor activity of WO3 due to its rapid electron–hole recombination, coupling with other semiconductors to use the sunlight as a free light source is an attractive research area. Concomitantly, titanium dioxide (TiO2 ) is an interesting material for applications in the purification of air and water by removal of recalcitrant organic and inorganic pollutants such as synthetic dyes, phenols and chlorophenols, volatile organic compounds, detergents, solvents, heavy metals or pharmaceutical antibiotics. Unfortunately, due to the size of TiO2 bandgap (Eg 3.0–3.2 eV), bare TiO2 only works in conjunction with irradiation of limited wavelength (λirr < 385 nm) corresponding to ultraviolet (UV) light that accounts for only 6.8% of the solar spectrum, thus limiting its photocatalytic activity. The visible (Vis) range (400–760 nm) accounts for about 38.9%, and infrared radiation (IR, 760–3000 nm) makes up most of the remaining 54.3% [387]. In addition to the ability of materials to absorb solar spectrum, the effectiveness of photocatalytic systems also depends on the sample capacity to separately collect photogenerated electrons and holes. The high electron–hole recombination rate limits the photocatalytic performance of bare TiO2 . Consequently, several approaches were devoted to improving the photocatalytic performance of titania, especially by coupling with other semiconductors. The objective is to increase the separation of charge carriers by doping titania with metal oxide presenting both conducting band (CB) and valance band (VB) of higher or equal energy compared to TiO2 . Among the many possibilities, the addition of crystalline tungsten oxide (WO3 ) has interesting advantages. Its shorter bandgap (Eg 2.6–2.8 eV) compared to TiO2 (Eg 3.0–3.2 eV) requires longer wavelengths for the excitation and extends the photoresponse of TiO2 to the visible light region. The coupling with WO3 also induces energy levels enabling the electrons photogenerated in the conducting band of TiO2 to transfer into the CB of WO3 . Consequently, the photopromoted holes can diffuse from the CB of WO3 into the valence band of TiO2 . The surface acidity of the TiO2 catalyst can also be increased by adding WO3 , which facilitates the adsorption of OH− or H2 O molecules and targeted molecules on TiO2 and thereby improves the photocatalytic activity of TiO2 . WO3 –TiO2 combination also induces dark activity due to the energy storage ability of tungsten oxide. WO3 –TiO2 -based systems exist as (i) tungsten oxide doping of supported or hybrid materials (Section 7.4.1) and (ii) metal doping of WO3 –TiO2 materials (Section 7.4.2). 7.4.1. WO3 –TiO2 -Based Systems as Supported and Composite Photocatalysts The photocatalytic activity of a WO3 –TiO2 system is largely determined by its structure, which is significantly influenced by the preparation method of the catalyst. Consequently, both WO3 supported (WO3 /TiO2 ) and composite (TiO2 /WO3 ) materials are described. Supported photocatalysts. Dyes such as methylene blue, rhodamine B or methyl orange are often chosen as model pollutants, but the degradation of several other contaminants has also been evaluated. For instance, Gao et al. [388] obtained WO3 /TiO2 material with high surface area and unique morphology by synthesizing heterostructured photocatalysts from wood fibers through a two-step hydrothermal method and a calcination process. The wood fibers acted as carbon substrates, allowing the recombination probability of photoexcited charge carriers to be reduced, and also increased the transport of charges. It results in high performance of the WO3 /TiO2 –wood fibers as a UV-light or a visiblelight photocatalyst for degradation of rhodamine B, methylene blue, methyl orange or phenol. Ding et al. [389] also observed a photoinduced electron–hole separation effect over tungsten oxide (WO3 )/TiO2 core–shell nanowires for the degradation of rhodamine B. The degradation of RhB was also undertaken over WO3 /TiO2 and MoO3 /TiO2 composites [390]. The reduction of the electron–hole recombination rate by coupling TiO2 with tungsten oxide or molybdenum oxide led to high photocatalytic activity. The 5 wt.% WO3 /TiO2 composite was the more efficient sample due to the more efficient separation of charge carriers. The electron migration from the conduction band (CB) of TiO2 to the WO3 CB can Catalysts 2021, 11, 703 53 of 78 be offset by O2 and produces superoxide radical (O2 •) and hydrogen peroxide (H2 O2 ) and finally hydroxyl radical (OH•) to decompose RhB (Figure 32). Figure 32. Proposed mechanism of dye degradation in visible light by WO3 /TiO2 composite. From [390] with permission from Elsevier. Yang et al. [391] synthesized porous WO3 /TiO2 hollow microspheres by a spray drying method. The authors observed that the tungsten oxides mainly existed in a highly dispersed amorphous form on anatase when the loading amount of tungsten oxide was below 3 mol%. The improved photocatalytic activity in methylene blue and phenol photodegradation under UV-light irradiation over the WO3 /TiO2 catalyst mainly arises from the enhanced charge separation efficiency provided by the acidity induced by WO3 addition, rather than the improved light absorbance by highly dispersed amorphous tungsten oxides. WO3 /TiO2 materials, designed as nanocomposites, mixed oxides or supported photocatalysts, were also used to degrade malathion pesticide [392], imazapyr [393], sulfamethoxazole [394] or formic acid [395]. For instance, 2% WO3 /TiO2 prepared by sol–gel method showed excellent photocatalytic performance, achieving complete malathion degradation after 2 h [392]. Synthesis of mesoporous WO3 –TiO2 nanocomposites presented interesting behavior for imazapyr herbicide degradation under visible light and UV illumination. The overall photocatalytic efficiency of the 3% WO3 –TiO2 nanocomposite was 3.5 higher than for mesoporous TiO2 [393]. The WO3 –TiO2 nanocomposite showed both monoclinic and triclinic WO3 structures. Tungsten-promoted composite catalysts were also envisaged by Ioannidou et al. [394]. Experiments showed that the 4% W-TiO2 catalyst calcined at 700 ◦ C was the most active for sulfamethoxazole degradation (350 µg/L) under simulated solar irradiation, enhancing the rate of pristine TiO2 by 50%. Tungsten is believed to act as a trap of electrons, thus reducing the rate of electron–hole recombination and, consequently, increasing degradation rates. The addition of electron acceptors, such as hydrogen peroxide and sodium persulfate, in the reaction mixture also improved the catalyst activity. Photoactivity of WO3 –TiO2 mixed oxides prepared by a sol–gel method was tested under UV–visible irradiation in both the mineralization of formic acid in aqueous suspension and the gas phase oxidation of acetaldehyde [395]. The catalyst containing a 3% W/Ti molar ratio gave the best results, due to the formation of an intimately mixed oxide resulting in a better charge separation due to the migration of photoproduced holes from WO3 to TiO2 . WO3 supported samples were also synthesized using liquid phase plasma process for degradation of diethyl phthalate [396]. The best degradation performance was observed for bare TiO2 photocatalyst under UV light source, but modified TiO2 photocatalysts showed a 1.7–6.2 times higher degradation rate under blue light. Catalysts 2021, 11, 703 54 of 78 Composite photocatalysts. Composite (or hybrid) TiO2 /WO3 materials are another strategy to obtain an enhancement in the photon absorption and a dark activity due to both the storage of photogenerated electrons and a redox process. Khan et al. [397] prepared microsized hybrid TiO2 /WO3 samples (TWx; x is 0.025, 0.5, 0.075, 0.1 molar ratio of W precursor) by sol–gel and crash precipitation methods followed by spray drying. Evaluated in the photocatalytic degradation of methylene blue (MB), the hybrid TW0.075 was reported as the most active material, with 90% of dye degradation reached in 100 min with dark runs. It was reported that TiO2 and WO3 acted in synergy to increase the lifetime of electron–hole pairs and to decrease the recombination rate by electron diffusion from CB of TiO2 to WO3 . The authors proposed a mechanism for the transfer pathways of the photogenerated charge carriers in the hybrid powders (TWx) (Figure 33) where both hydroxyl radicals (OH•) and superoxide anions (O2 – ) play a major role. Figure 33. Proposed mechanism illustrating the transfer pathways of the produced charge carriers and various oxidizing species contributing to the degradation of MB. From [397] with permission from Elsevier. TiO2 /WO3 systems (TW) were also studied by Rimaldi et al. for ethanol and tetracycline (TC) degradation [398]. Analyses supported the parallel occurrence of several TC degradation pathways in the case of TW samples in which OH• radicals are involved, as previously mentioned in [397] for MB conversion. TiO2 -WO3 composites were also evaluated for bisphenol A (BPA) oxidation under simulated solar light [399]. A core–shell system composed of a layer of TiO2 nanoparticles on the surface of WO3 nanowires was also evaluated as an effective photocatalyst in the degradation of RhB [389]. Again, the photoinduced electron–hole separation effect between WO3 and TiO2 contributed to the improvement of the photocatalytic activity. WO3 -TiO2 photocatalytic systems can be also associated with other processes to obtain overall improvements. With the aim to associate photocatalysis with electrochemistry, WO3 Catalysts 2021, 11, 703 55 of 78 was associated with a TiO2 nanotube array to obtain a photoanode designed to improve photoelectrocatalytic (PEC) performance for the degradation of methylene blue [400]. A synergistic vacancy-induced self-doping effect and localized surface plasmon resonance (LSPR) effect were observed, confirming the importance of oxygen vacancies in improving PEC performance. WO3 –TiO2 -based catalysts were also evaluated for photocatalytic-assisted ozonation of pollutants. A 4% WO3 /titanate nanotube composite catalyst allowed the complete removal of emerging contaminants such as caffeine, metoprolol and ibuprofen in municipal wastewater in less than 40 min with TOC removal up to 64% after 2 h. Compared to bare TiO2 , WO3 addition induced a higher activity under visible-light radiation and an increase in the adsorption capacity of organic compounds [401]. 7.4.2. Metal-Doped WO3 –TiO2 -Doped Systems To decrease the fast electron–hole recombination and improve the interfacial charge transfer process of WO3 –TiO2 materials, the addition of noble metals or heteropolyoxometallates (POMs) has been successfully explored. Noble metals act as a sink of conduction band photoinduced electrons, avoiding their recombination and enhancing the photocatalytic process. Thus, TiO2 –WO3 –Au systems were evaluated by several authors. X-ray diffraction (XRD) data and Raman spectroscopy measurements usually indicated that TiO2 is mainly in anatase structure, while the WO3 can be amorphous or crystallized. Gold nanoparticles improve the material’s response in the visible domain [402]. Kovács et al. [403,404] studied the structural peculiarities of TiO2 /WO3 /Au materials. It was shown that each minor structural change in bulk or surface has a significant impact on the photocatalytic activity and intermediate formation dynamics, using phenol as a model pollutant. However, the gold deposition mode was found crucial over structural peculiarities, especially for W species. The resulting sample presented three WO3 forms in different proportions: amorphous, crystalline and doped. These forms manifested in different species (W6+ , W4+ , Wdefects ), which critically influenced the activity and the intermediate formation during the photodegradation. Consequently, gold-containing samples resulted in a modification of structural particularities, but the correlation between structural features and PCA or intermediate formation was strongly dependant on the synthesis pathway [404]. Platinum addition is reported to enhance the electron storage ability, the visible activity and the electron–hole separation of photocatalysts by the creation of Schottky-type junctions, thus facilitating the charge transfer at the catalyst–environment interface [405]. By varying the platinum amount from 0.2 to 1 wt.% in Pt/TiO2 -WO3 materials prepared through the spray drying method, the optimum amount for the degradation of methylene blue (MB) was found at 0.8 wt.% Pt. Hydrogen peroxide (H2 O2 ) and hydroxyl radicals (OH•) were found as the main species responsible for the aqueous degradation of the model pollutant. TiO2 maintained its tetragonal anatase phase and WO3 maintained its monoclinic structure. The corresponding Pt0.8 TW sample presented the lowest electron– hole recombination rate (photoluminescence measurements) and the highest amount of surface hydroxyl groups (FTIR experiments). The use of heteropolyoxometallates (POMs) is another interesting way to decrease the fast electron–hole recombination on TiO2 . POMs act as effective traps of photoinduced electrons, leading to visible-light-absorbing materials with high photocatalytic activity. Tungstosilicic acid (TSA; H4 SiW12 O40 ) was successfully used by Rengifo-Herrera et al. [406] to obtain the formation of a surface complex between the Keggin structure of TSA and the TiO2 surface, or the formation of Keggin–TSA/TiO2 composites. It resulted in visiblelight absorption of modified catalysts. The photocatalytic activity in the degradation of 4-chlorophenol depends on the TSA amount. FT-IR and FT-Raman characterizations indicated that the main heteropolyoxometallate species presented in the composites was the [SiW12 O40 ]4− anion, which exhibited a strong interaction with the TiO2 surface. Catalysts 2021, 11, 703 56 of 78 7.5. WO3 Associated with Transition Metal Oxides Other Than TiO2 (Zn, Fe, Sn, Mn, Ni, Mo, Co, Nb) In view of photocatalytic activity enhancement of WO3 catalysts, doped materials have attracted much attention, and the present section focuses on the modification of tungsten(VI) oxide by transition metal oxides (MOs). In fact, by coupling WO3 with another semiconductor, an improvement of charge separation in photoinduced charge transfer processes of n-type semiconductor is expected, leading to a drop in electron–hole (e− /h+ ) pair recombination and an increase in photocatalytic activities. 7.5.1. ZnO–WO3 Among the multitude of oxides considered, zinc receives great attention owing to its high light sensitivity and wide bandgap energy (3.37 eV). However, the photocatalytic activity of zinc oxide is limited to the ultraviolet light range of the solar energy spectrum, representing only 5–7% of the total sunlight energy. Another limitation is the rapid recombination of the electron–hole (e− /h+ ) cavity. To overcome these issues, surface modification of the structures of the ZnO nanoparticles has been proposed. The aim is to alter the bandgap energy, thus shifting the absorption band to the visible region in semiconductor systems. Doping ZnO by WO3 enables the electrons produced from ZnO to be trapped, thus preventing the fast recombination of electron–hole pairs and increasing the photocatalytic activity. WO3 has the advantage of narrow bandgap energy (2.2–2.8 eV), which could activate the zinc oxide. Such ZnO-WO3 materials can act as good candidates for the degradation of Direct Blue 15 (DB15) or methylene blue (MB) dye, organophosphorus pesticides such as diazinon, diclofenac and bacterial or fungal strains. Zinc oxide nanoparticles doped with 0.5% to 2% WO3 were synthesized by hydrothermal method to obtain samples where W was added into ZnO structure [407]. Photocatalytic activity for diazinon removal was reported to reach 99% for 2% WO3 -doped ZnO catalyst after 180 min of contact time. This result was attributed to a reduction in the network constant. Similar tungsten oxide doped ZnO nanoparticles were evaluated in the photocatalytic degradation of Direct Blue 15 [408]. Numerous parameters, namely pH, light intensity, dopant percentage, dye concentration and contact time, affect the process. It appeared that acidic pH resulted in higher efficiencies of the photocatalytic process. In addition, increasing the concentration of the WO3 dopant percentage from 1 to 5% w/v increased the degradation rate from 30.7 to 73.1%. Tungsten acted as a recombinant mediator of interfacial charge transfer, which resulted in a change in the photocatalytic efficiency of the doped ZnO. Sajjad et al. [409] revealed that the formation of Zn-O-W linkage was responsible for a redshift of the absorption peak to the visible region, resulting in a lower bandgap and a promotion of the separation of photogenerated carriers. In fact, the proposed energy state configuration showed dissimilar locations of conduction and valence bands of both semiconductor oxides in ZnO/WO3 composites. Due to a narrowing of the bandgap, the charge separation is facilitated and the rate of recombination is significantly reduced, resulting in improved photodegradation efficiency. In addition, the transfer of charge carriers due to the narrow bandgap can be confirmed by OH• radical measurement as proposed by Mugunthan et al. [410] for the degradation of diclofenac with ZnO-WO3 mixed oxide catalysts. Authors observed higher hydroxyl radical (OH•) generation when coupling ZnO and WO3 compared to bare ZnO. The benefit of the coupled system is mainly due to optimal loading of WO3 since good dispersion of WO3 in coupled oxides promotes the formation of heterojunction structures between the two coupled oxides. The optimal molar ratio of Zn:W = 10:1 was found for the degradation of diclofenac. Photocatalytic activity of Zn-doped WO3 (Zn-d-WO3 ) nanoparticles for antifungal and antibacterial degradation was studied by Arshad et al. [411]. Zn loading from 0.1 to 0.3% had a significant impact on both antibacterial/antifungal photocatalytic degradation and structural/morphological properties. Characterizations showed that the Zn-d-WO3 structure was triclinic, the particle shape was a mix of spheres and rods and the particle size was decreased as the Zn concentration increased from 0.1 to 0.3%. Photocatalytic activity of Zn-WO3 was evaluated in methylene blue (MB) dye degradation. Again, Zn Catalysts 2021, 11, 703 57 of 78 loading impacted the activity. The maximum dye degradation rate was achieved with Zn(0.3%)-WO3 sample, at 78% and 92% with 120 min of contact time for visible and UV irradiation, respectively. Again, it was proposed that after the generation of electron–hole pairs (e− /h+ ), the electronic vacancies or holes in the valence band are promoted due to the migration of electrons from the valence band to the conduction band. As a consequence, electron–hole pairs generate hydroxyl radicals (OH•) and superoxide ions (•O2 – ) that act as powerful oxidizing agents to mineralize the dye into CO2 and H2 O. Novel Pd/ZnWO4 nanocomposite materials for photocatalytic degradation of atrazine were evaluated by Al-Amshany et al. [412]. Photocatalytic activity of ZnWO4 was highly affected by the insertion of Pd into the composition. Palladium was reported to replace W in the ZnWO4 lattice leading to a sample presenting low bandgap, low electron–hole pair (e− /h+ ) recombination rate and high BET surface area. Consequently, high photocatalytic activity for atrazine degradation was obtained for 1.65 wt.% Pd/ZnWO4 nanocomposite, with 100% of degradation after 60 min of reaction time. 7.5.2. MO–WO3 (MO: Metal Oxides = Fe, Sn, Mn, Ni, Mo, Co) Other transition metal oxides such as Fe, Sn, Mn, Ni, Mo and Co were evaluated in the past few years to improve the photocatalytic activity of WO3 . Metal oxides act as electron sinks to capture the photoinduced electrons through their low energy trapping sites; they then reduce the electron–hole recombination and enhance charge separation. Fe2 O3 is a visible-light-driven material with a bandgap energy of 2.0–2.3 eV and a CB of 0.46–0.77 eV. Wang et al. [413] proposed a new active FeWO4 /Fe2 O3 di-modified WO3 for the degradation of methylene blue (MB), toluidine blue (TB), azure I (AI) and acridine orange (AO) under visible-light irradiation. Beneficially, Fe2+ , which is classically involved in the Fenton reaction favors the generation of OH• radicals and subsequent organic dye degradation. Over FeWO4 /Fe2 O3 -modified WO3 sample, FeWO4 and Fe2 O3 acted as electron traps. Consequently, a valence decrease of Fe3+ to Fe2+ led to the capture of photoexcited electrons of WO3 and enabled the Fenton reaction in the presence of trace H2 O2 . Consequently, di-modified WO3 samples showed high photocatalytic activity compared to bare WO3 . This assisted Fenton process was also described by Mwangl Ngigi et al. [414] for photocatalytic degradation of Methylparaben (MeP) over Fe-doped WO3 nanoparticles in presence of H2 O2 . The best formulation was 5 wt.% Fe-WO3, with 50.8% MeP degradation in the presence of H2 O2 . A hydroxyl radical mechanism was advanced to explain the efficiency and enhancement of iron-doped WO3 on visible-light activity of the organic pollutant removal. SnO2 is also used for bandgap tuning of nanomaterials to transform UV-light-active catalysts into visible-light-driven catalysts. Nevertheless, tin dioxide has a wider bandgap of 3.6 eV (UV-light-driven catalyst) with a high recombination rate of its photogenerated electron–hole pairs, which limits its use. These drawbacks can be overcome by tungsten doping to serve as a photogenerated charge trap, thus decreasing the recombination rate of e− /h+ pairs by the reduction of the bandgap. In this way, Ullah et al. [415] reported that tungsten-doped SnO2 (W@SnO2 ) nanospheres can be a preferred choice for visible light photocatalytic degradation compared to standard TiO2 (Degussa-P25). With the increase in the W content from 0 to 6 mol %, the bandgap of SnO2 was tuned from 3.6 to 2.8 eV, and the degradation of crystal violet dye reached 100% after 40 min of contact time. The formation of a nanocomposite of WO3 with manganese oxides is also a way to reduce the bandgap of tungsten oxide. The constitution of composite with Mn, indigo dye and reduced graphene oxide (RGO) was investigated by Ahmad et al. [416] as visible light photocatalyst for methylene blue (MB) dye removal. A synergetic association between WO3 and RGO was observed which increased the rate of charge transfer and limited the recombination of electron–hole pairs in the nanosized composite. As a consequence, the socalled Mn/indigo/RGO/WO3 presented improved photocatalytic performance compared to Mn/indigo/WO3 . Catalysts 2021, 11, 703 58 of 78 Well-crystalline NiO-WO3 nanoparticles were synthesized by Rosaline et al. [417] for the photocatalytic degradation of eosin yellow (EY) dye. A photocatalytic degradation rate of 95% was obtained after 180 min under visible-light irradiation. The authors proposed that photogenerated electrons of NiO possibly migrate into the conduction band of WO3 . Simultaneously, the holes move in the opposite direction, which greatly decreases the recombination of electron–hole pairs. Mo-doped WO3 nanowires for adsorbing methylene blue (MB) dye from wastewater were evaluated by Silveira et al. [418]. Samples presented hexagonal structure (h-WO3 ), and the Mo doping led to a change in morphology of h-WO3 nanowires from large bundles to narrow bundles when Mo loading increased from 4 to 15 at %. In fact, Mo6+ and W6+ have similar ionic radii and electron structures, which enables Mo to directly incorporate the crystalline lattice of WO3 to form a Mox W1−x O3 structure. As a consequence, the bandgap of WO3 can be reduced by substituting W by Mo in order to improve the photocatalytic activity with visible light, as demonstrated in [419]. In addition, Mo-doped WO3 samples also exhibited a considerable improvement in MB adsorption capacity [418]. The reduction in electron and hole recombination of WO3 can also occur with nanocomposites of cobalt(II, III) oxide and tungsten(VI) oxide. The synthesis of p–n heterojunction catalysts, such as Co3 O4 /WO3 , was claimed to enhance the photoactivity under UV- or visible-light radiation [420]. The strong reduction in photoexcited electrons and holes resulted in the enhancement in diclofenac sodium degradation. The highest degradation efficiency was obtained for 0.02 M of cobalt acetate (CW2), with 98.7% achieved for the degradation of 15 ppm diclofenac sodium at pH 10.7. Higher generation of hydroxyl radicals (OH•) associated with the enhancement of effective charge separation was still advanced to explain the high photocatalytic activity of the CW2 sample. Figure 34 shows the charge transfer proposed by Malefane et al. [420] and the p–n heterojunction formed to minimize photoexcited electron and hole recombination in CW2 nanocomposite. Figure 34. Proposed degradation mechanism in Co3 O4 /WO3 p–n heterojunction. From [420] with permission from Elsevier. 7.6. WO3 Associated with Other Systems (Post-Transition Metal Oxides and Metalloids, Rare Earths, Nitrides) 7.6.1. Post-Transition Metal or Metalloid–WO3 Systems Among the post-transition metals, bismuth and gallium were evaluated in recently published works. Bi2 WO6 /perylene diimide (PDI) composite photocatalyst was prepared and evaluated in phenol oxidation (and water decomposition to produce oxygen) under visible light [421]. Compared to the self-assembled perylene diimide and Bi2 WO6 , the composite showed an enhancement in the phenol degradation because the positions of conduction and valence Catalysts 2021, 11, 703 59 of 78 bands between Bi2 WO6 and perylene diimide favored the separation of the photogenerated carriers. The Bi2 WO6 /PDI composite exhibited an n–n-type heterojunction. BiFeWO6 /WO3 nanocomposites were studied by Priya et al. [422]. Various BiFeWO6 loadings from 1 to 3 wt.% deposited on WO3 nanorods were prepared by simple coprecipitation and hydrothermal treatment. Pure BiFeWO6 bandgap was measured at 2.0 eV, and the best photodegradation of rhodamine B under visible-light illumination was obtained with 1% BiFeWO6 /WO3 This sample exhibited a bandgap of 2.3 eV, while the bandgap increased to 2.9 eV for 3% BiFeWO6 /WO3 . The influence of the partial substitution of gallium by tungsten in a Ga2 Zr2 O7 fluorite-type nanosized material was recently examined in crystal violet degradation [423]. Ga2 Zr2−x Wx O7 samples with x = 0, 0.05, 0.1, 0.15 and 0.2 were prepared using the citrate technique. W0.15 substitution (optimal loading) allowed the shift of the absorption range to the visible-light range by decreasing the bandgap from 4.95 eV for Ga2 Zr2 O7 to 1.7 eV for Ga2 Zr0.85 W0.15 O7 . The crystal violet photocatalytic degradation rate was then multiplied by 20 (optimum operating conditions: pH = 9, 1 g L−1 catalyst, reaction duration of 300 min). Complete degradation was obtained with the addition of 25 mmol L−1 hydrogen peroxide. O2 center dot and holes were found to have a more important role in Ga2 Zr2−x Wx O7 systems compared with HO center dot. The excellent semiconductor tungsten selenide WSe2 was recently used as quantum dots (particle size: 7–8 nm) and associated with nitrogen-doped graphene oxide to prepare a composite photocatalyst by the mechanical stripping method [424]. The degradation of methylene blue under visible light with the composite (optimal WSe2 loading: 4.7 wt.%) was better than WSe2 and nitrogen-doped graphene oxide (N-GO) alone. The photocatalytic degradation rate of 50 mL at 1 × 10–4 mol·L−1 dye with only 0.01 g catalyst reached 93% after 1 h light exposure (300 W Xe lamp with a cutoff wavelength of 420 nm; reaction was monitored by a total organic carbon analysis). 7.6.2. Rare Earth–WO3 Systems Cerium is a very popular component in redox catalysis thanks to its ability to vary between CeIII and CeIV . Impregnation of 1 to 25 wt.% of Ce3+ ions onto WO3 led to a better absorption cross-section and a redshift in the band edges [425]. The decrease in the photoluminescence emission intensity and the suppression of the Raman active bands of WO3 indicated the recombination quenching ability of Ce surface states. For low cerium loading (≤5%), cerium was mainly found as CeIII , while CeIV was predominant for higher loadings. An enhancement in the charge retention ability was observed with the Ce loading increase. In comparison to pure WO3 , Ce-doped catalysts exhibited superior activity for 2-nitrophenol and 2-chlorophenol removal in natural sunlight exposure. For low cerium loadings, the surface oxygen bonded with CeIII serves as electron trapping and transfer centers, while the synergic composite mechanism is the dominating mode for high Ce loading [425]. Tahir et al. evaluated various other rare earths (REs) by doping WO3 with La, Gd and Er (RE molar contents of 2, 4 and 6%). Samples were obtained by coprecipitation at 80 ◦ C in acidified aqueous solution. Photocatalytic activity was measured in degradation of dyes (methyl orange, methyl blue, crystal violet), antibiotic (tetracycline) and antimetabolite (methotrexate) and compared to a 10% carbon nanodots–WO3 reference sample [426]. La, Gd and Er did not enter into the crystal lattice of WO3 (monoclinic and hexagonal phases) but were positioned in the interstitial sites, with La-O-W, Gd-O-W and Er-O-W bonds. The photocatalytic activity order was as follows: 2% Gd-WO3 > carbon nanodots–WO3 > 4% Er-W > 4% La-W > undoped WO3 . The best photocatalytic performance achieved with 2% Gd-WO3 could be attributed to its higher surface area, the inhibition of the chargecarrier recombination, the high surface hydroxyl content and the extended visible-light absorption region. Catalysts 2021, 11, 703 60 of 78 7.6.3. Nitride–WO3 Systems The graphite-like carbon nitride (g-C3 N4 ) is a metal-free polymer that is an n-type semiconductor. Its structural and physiochemical behaviors give it interesting properties in electrical and optical fields. The g-C3 N4 -based nanostructures are emerging materials for energy and environmental photocatalytic applications (e.g., photocatalytic water reduction and oxidation, degradation of pollutants, carbon dioxide reduction) [427]. Xiao et al. prepared Z-scheme WO3 /g-C3 N4 composite hollow microspheres by controlled in situ hydrolysis and a polymerization process (Na2 WO4 in deionized water with the addition of dicyandiamide C2 H4 N4 and glucose; the homogenized solution was then transferred into an autoclave and heated 200 ◦ C; for 20 h; after centrifugation and washing, the product was finally annealed at 550 ◦ C; for 3 h in air (Figure 35)) [428]. The shell of the hollow microspheres consisted of well-distributed WO3 and g-C3 N4 nanoparticles, favoring heterojunctions with numerous interfaces and highly exposed oxidation–reduction active sites. Various contents of g-C3 N4 were evaluated, and the best results were obtained with a molar loading of 4% dicyandiamide with respect to W. The lifetime of the charge carriers reached 2.23 ns, which is obviously prolonged compared to WO3 . This optimized Z-scheme system maintained the redox potential of the components. Combined with the long lifetime of holes and electrons, this photocatalyst showed enhanced degradation rates towards tetracycline hydrochloride (antibiotic agent) and ceftiofur sodium (antibacterial agent). Figure 35. Scheme of the in situ construction of WO3 /g-C3 N4 composite hollow microspheres (CHMs). From [428] with permission from Elsevier. g-C3 N4 /WO3 materials can be also shaped as thin films for water purification [429]. A bilayered structure has to be formed to ensure an enhanced charge carriers’ separation. Trapping experiments performed under UV illumination revealed that the activation of the photocatalytic oxidation of methylene blue (MB) came from both holes and superoxide radicals. Other nitrides such as tantalum nitride (Ta3 N5 ) were also recently evaluated in dye degradation. Ta3 N5 is one of the few visible-light-absorbing photocatalysts capable of overall water splitting (OWS). Ta3 N5 /W18 O49 nanocomposite fibers were prepared by the solvothermal technique to obtain a catalyst made of orthorhombic Ta3 N5 nanoparticles and monoclinic W18 O49 nanowires. This sample showed a doubled degradation rate of rhodamine B under white light compared to Ta3 N5 nanoparticles alone [430]. In fact, W18 O49 allowed dye adsorption before illumination. Using tert-butanol and p-benzoquinone as scavenger agents, it was demonstrated both hydroxyl and superoxide radicals contributed to the cleavage of rhodamine B. The superoxide species were involved in N-deethylation of the dye. The generation of both radical species on W18 O49 was due to the electronic transfer between W18 O49 and Ta3 N5 . Taking into account the recycling ability of the catalyst and Catalysts 2021, 11, 703 61 of 78 the simplicity of the synthesis process, the authors propose that this catalyst is a good candidate for the scale-up. 7.6.4. Photoreduction over WO3 Based Systems Photocatalytic water treatment is mainly focused on the oxidation of organic pollutants. However, due to its high toxicity, CrVI species reduction into CrIII is also a major concern. g-C3 N4 /WO3 thin-film materials presented in the previous section also exhibited Cr6+ elimination behavior under UV illumination [429]. The Cr6+ reduction rate was 6% and 93% after 120 min illumination for WO3 and g-C3 N4 /WO3 , respectively. In opposition with photocatalytic oxidation of methylene blue for which active species came from both holes and superoxide radicals, the main active species for chromium reduction are electrons. For the same chromium reduction, Thwala et al. evaluated the doping of WO3 by 1, 3 and 5 mol % of magnesium (one-pot preparation method) [431]. A slight increase (0.0069 nm) of d spacing was observed by HRTEM imagery after Mg addition, according to a decrease in the bandgap energy and a shift in the edge positions, also confirmed by XPS analysis (0.08 eV). The rate of recombination was greatly reduced upon doping (photoluminescence analysis). DFT calculations supported the recombination reduction rate due to the introduction of the Mg orbital. With 500 mL at 9.5 ppm CrVI maintained at pH = 1 and a catalyst weight of 0.1 g, 97% CrVI reduction rate was reached with 3 mol % Mg-WO3 after one hour under visible-light irradiation (990 W Xe lamp). 7.7. Conclusions WO3 -based photocatalysts benefit from a visible-light response due to a 2.6–2.8 eV bandgap between the valence band (VB) and the conduction band (CB). The challenge is to avoid the fast recombination of the photogenerated electrons and holes of bare WO3 by doping or promotion (with carbon, metal(s), TiO2 , etc.). Depending on the reaction conditions, the formation of reactive oxidizing species such as superoxide anions (O2 − •), hydrogen peroxide (H2 O2 ) and hydroxyl radicals (OH•) is reported. The WO3 -TiO2 system is of great interest for extending the photoresponse of TiO2 (Eg 3.0–3.2 eV) to the visible light region since the electrons photogenerated in the conducting band of TiO2 can transfer into the CB of WO3 . The photopromoted holes can thereafter diffuse from the CB of WO3 into the valence band of TiO2 . 8. Conclusions Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light. The bandgap of WOx is commonly stressed to 2.6–2.8 eV for crystallized WO3 3D structures but grows up to 5.5 eV for isolated WO4 species (such as bi-grafted, di-oxo WO4 surface sites) or supported WOx catalysts depending on the loading. From the operating conditions of the catalytic process, each of these behaviors is tunable by (i) controlling structure and morphology (nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres, etc.), (ii) thermal treatment under controlled atmosphere and (iii) interactions with other compounds (such as conductors (carbon), semiconductors (e.g., TiO2 ) and precious metals); WOx particles can be also dispersed on high specific surface area support. Based on these behaviors, WO3 -based catalysts were developed for environmental applications. One of the major advantages of the acidic and redox behaviors of WO3−x is their suitability for the abatement of NOx from stationary sources, associated mainly with TiO2 and V2 O5 (nitric acid plants first; treatment of incinerator and other combustion gases nowadays). WO3 -based catalysts also promote the abatement of some VOCs. Moreover, surface oxygen vacancies are also useful in the development of gas sensors, especially for NO2 detection. Modifications are suitable to improve sensibility, selectivity, stability, repeatability and response time. Besides, the recent literature shows a high interest in WO3 -based materials for photocatalysis applications. Bare WO3 is not very active due Catalysts 2021, 11, 703 62 of 78 to the fast recombination of photogenerated electrons and holes, but the photocatalytic activity of WO3 can be significantly improved by numerous associations to obtain suitable reactivity. 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