Graphene-based Carbocatalysis: Synthesis, Properties and Applications: Volume 1
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Graphene-based Carbocatalysis - Pinki Bala Punjabi
Introduction to Carbocatalysis
Pinki Bala Punjabi¹, *, Sharoni Gupta¹
¹ Department of Chemistry, University College of Science, Mohanlal Sukhadia University, Udaipur-313001, Rajasthan, India
Abstract
Carbocatalysis has emerged as a promising field of catalysis. The exceptional surface morphology, pore distribution, thermal conductivity, chemical inertness, electrical property and renewability of carbon materials have rendered them suitable for various catalytic processes namely, photocatalysis, electrocatalysis, biocatalysis and chemical catalysis. Therefore, the introductory chapter on carbocatalysis describes the useful properties of carbonaceous materials which govern their catalytic behaviour. Moreover, synthetic approaches for the fabrication of diverse carbon polymorphs such as active carbon, graphite, fullerene, glassy carbon, carbon black, carbon nanotubes, carbon nanofibres, nanodiamonds, carbon nano-onions, and graphene have also been briefly discussed in this chapter. The scope of carbocatalysts over broad areas has also been elucidated by quoting instances.
Keywords: Carbocatalysis, Electrical Properties, Surface Properties, Sustainability, Synthesis, Thermal Conductivity.
* Corresponding author Pinki Bala Punjabi: Department of Chemistry, University College of Science, Mohanlal Sukhadia University, Udaipur-313001, Rajasthan, India; E-mail: pb_punjabi@yahoo.com
INTRODUCTION
Catalysis is the backbone of chemical transformations which facilitates a sustainable and efficacious means to convert starting raw materials to useful chemical compounds. One of the twelve principles of green chemistry states that the use of catalysts should be encouraged as these enhance selectivity, reduce the formation of by-products, reaction time and energy requirements
[1]. Therefore, catalytic transformations have become an integral part of environmentally benign chemical processes. Homogenous and heterogeneous catalysts are the two main classes of catalysts that over the past several decades have served as a driving force in achieving high performance conversions by abating the activation energy. Both homogeneous and heterogeneous catalysts have their high points, for instance, homogeneous catalysts have excellent catalytic efficiency owing to
readily accessible active sites [2] while heterogeneous catalysts are quite stable and easy to separate and recover [3]. However, with growing awareness of environmentally friendly protocols, the use of conventional catalysts has become a debatable topic. This is primarily because the traditional catalysis employs acids, bases, transition and non-transition metals which are toxic, expensive, get easily deactivated by impurities and non-reusable.
Sustainability being an essential element of modern chemistry has coerced researchers and chemists globally to look for renewable and cost-effective catalytic systems. The recent emergence of graphene based catalysts has paved way for a green alternative to conventional catalysts. Carbocatalysis viz. catalysis driven by carbonaceous materials is not a novel avenue of study. Several carbon based catalyst have been used in the past. Perhaps the beginning of carbocatalysis can be traced back to the year 1854 when Stenhouse observed that activated carbon was capable of oxidizing a mixture of organic gases coming from putrefied biomass [4]. Since then there have been several reports where carbon allotropes were used as catalytic materials. In 1925, Rideal and Wright employed charcoal as a catalyst for the oxidation of oxalic acid [5]. In the 1930s, Kutzelnigg and Kolthoff separately confirmed that activated charcoals were highly suitable for catalyzing the aerobic oxidation of ferrocyanide to ferricyanide [6, 7]. Thereafter in 1979, charcoal was used to catalyze the oxidative dehydrogenation (ODH) reaction of ethylbenzene to styrene [8]. Also, graphite catalyst was employed for the first time by Lucking and co-workers for the oxidative cleavage of 4-chlorophenol, which yielded CO2, H2O, and HCl [9]. In another instance, Fortuny et al. have also reported oxidation of phenol using active carbon as a catalyst [10]. Despite the good catalytic performance of carbonaceous materials attributable to their versatile porosity and surface properties, carbocatalysis did not achieve the desired level of attention until the discovery of fullerene, carbon nanotubes, and graphene. Further, mostly the carbon polymorphs such as charcoal [11], graphite [12], carbon black [13], and diamond [14] have been used as support in catalytic systems because of their advantages including ease of metal immobilization on carbon, resistance to attack by acids and bases, simple as well as cost-effective preparation, high thermal stability and straightforward recovery of the active phase. Regardless of the beneficial features, some shortcomings such as lack of active sites, microporous structure, hydrophobic character and poor resistance to oxidation stalled the growth of carbon based catalysts in earlier days [15]. However, the recent developments in the synthetic chemistry of carbon have allowed easy modification of the pore size distribution, surface chemistry, and hydrophilicity to afford carbocatalyst with desirable electronic and physicochemical properties, large surface to weight ratio, and better biocompatibility.
The discovery of fullerenes in 1985 was a landmark in the field of carbon science [16]. The unique three dimensional structure of fullerene comprises intricately arranged five and six membered rings of covalently bonded carbon atoms arranged to form spherical shape. The high conductivity (narrow band gap) of fullerene arises from the pi electron delocalization within carbon layers which favours its catalytic behaviour. Functionalization of fullerenes also aids in tuning their electrical properties and serves as a precursor for the preparation of fullerene based nanocatalyst. Ever since their discovery, fullerene based catalysts have been used to catalyze several reactions such as reduction [17], oxidation [18], photodegradation [19], dehydration [20], proton transfer [21], deallylation [22], etc.
The birth of nanotechnology proved to be a boon for carbocatalysis. The designing of nanodimensional carbon tubes with readily accessible sites on large surface area revolutionized the field of catalysis. The breakthrough invention of carbon nanotubes in 1991 by Iijima [23] followed by its use as catalyst support in 1994 [24] led to a new generation of catalysts dominated by carbon nanomaterials. Carbon nanotubes belong to the fullerene family and are basically graphite sheets rolled in the form of cylindrical shapes. The excellent electrical conductivity, striking mechanical and thermal stabilities, tolerance against poisoning effect, easily modifiable surface area, and ease of functionalization of carbon nanotubes make them promising catalytic systems [25]. Carbon nanotubes have been widely applied and employed in heterogeneous catalysis [26], photocatalysis [27], and electrocatalysis [28].
One of the lightest and strongest known materials, graphene is a single layered allotrope of carbon that has honeycomb structure and is the most famous carbocatalyst of recent times. The discovery of graphene dates back to 1859 when Brodie while examining the structure of graphite, observed graphene and named it graphone
[29]. In 2004, the isolation of graphene sheets by Novoselov et al. brought about a storm in the field of carbonaceous materials [30]. The extraordinary physicochemical and thermoelectrical properties of graphene such as large specific surface area (theoretically 2630 m²/g for single-layer graphene) [31], amazing electronic properties and electron transport potential [32, 33], unparalleled flexibility and impermeability [34, 35], sturdy mechanical property [36] and excellent thermal and electrical conductivities [37, 38] attracted enormous interest and resulted in an explosion of research studies in the domain of graphene based materials. In 2009, Lu et al. highlighted the use of gold embedded graphene based catalyst for oxidation of CO [39]. Jafri et al. also reported nitrogen doped graphene nanoplatelets as a potent catalyst for oxygen reduction reaction [40]. In 2010, Dreyer et al. demonstrated graphene oxide as a compatible carbocatalyst for oxidation and hydration reactions [41]. In the same year, Li and his research group reported electro-oxidation of methanol promoted by Pt nanoparticles on reduced graphene oxide [42]. In another instance, Gao and co-workers used reduced graphene oxide as carbocatalyst for the reduction of nitrobenzene [43]. Since then, a plethora of literature has been available which showcases the abilities of graphene and its derivatives as potent carbocatalysts.
Thus, a bountiful of reusable and economically viable carbonaceous materials are available today which serve as heterogeneous and semi-homogeneous catalysts as well catalytic supports for a range of reactions that find significance in different areas namely, chemical manufacturing, environmental remediation, biomass production, water splitting, energy conversion and storage devices. In the present chapter, an effort has been made to acquaint the readers with the properties, syntheses and scope of carbocatalysts (Fig. 1).
Fig. (1))
Properties, syntheses and scope of carbocatalysts.
PROPERTIES OF CARBOCATALYSTS
Now let us delve into the properties of carbon materials that render them suitable as catalytic systems.
Surface Properties
It is well known that for any catalyst, surface features are crucial in governing its catalytic activity. The surface properties of carbon materials depend on the spatial arrangement of carbon atoms which in turn are based on the method of preparation or treatment used to synthesize these materials. The presence of a high density of pores and large surface area on carbonaceous materials play a key role in the adsorption of reactant molecules [44]. The introduction of defects in carbon atom layers present in carbon based materials and eases of their functionalization make their surface apt for catalysis. These structural defects particularly on the edges serve as active sites for binding of reaction molecules being easily accessible.
The defects in carbon materials can be introduced by thermal treatment or doping. For instance, Qiu et al. have reported that the amount of surface defects is directly related to the catalytic efficiency of activated carbons [45]. They used a simple thermal treatment method aided by a nitrate solid-phase oxidative method to increase the amount of surface defects on the activated carbons and observed their catalytic effect on the hydrochlorination of acetylene. It was reported that this treatment removed oxygen moieties and introduced defects on the edges of active carbons. Armchair and zigzag edges were observed in active carbon due to the stacking arrangement of nanosized graphitic layers, thereby populating the edges with active sites. In another study, it was reported that selective doping of carbon materials with heteroatoms induces catalytically beneficial defects. Chen et al. doped carbon nanosheets with B-N pairs and employed them as a catalyst to promote ammonia synthesis [46]. It was highlighted that the B-N doped carbon nanosheets showed superior activity compared to metallic catalysts for the nitrogen reduction reaction. The better electrocatalytic behavior of B-N doped carbon nanosheets was attributed to the fact that doping resulted in generating sufficient active sites at the edge carbon atoms.
The functionalization of carbon surfaces causes changes in the surface energy as well as chemical reactivity of the materials. Surface functional groups may increase free valences on the edges of the external surface, thereby increasing the catalytic activity [47]. The functional groups also assist in tuning the hydrophilicity of carbon materials and are capable of altering the acidic and basic character of these materials [48, 49]. Oxygen, nitrogen, hydrogen, sulphur, phosphorous and halogens functionalities are commonly used for this purpose. Chen et al. functionalized carbon nanotubes with oxygen functional groups [50]. Several oxidants like H2O2, KMnO4, HNO3/H2SO4 were used to functionalize the nanotubes. After functionalization, the pHpzc (pH at which net surface charge on carbon is zero) was measured by the chemical titration method [51]. It was reported that after functionalization the carbon surface became acidic due to the presence of functional groups like carboxyls which decreased the pHpzc value making the surface more hydrophilic [52-54]. It was also reported that the presence of carboxylic functional groups also enhanced the dispersion of Pd and Pt metals on CNTs via the formation of chemical bonds, which hindered the agglomeration metal particles on CNTs and minimized the sintering propensity of the formed Pd–Pt particles across the CNT surface in the calcination and reduction process [55, 56]. This increased uniform dispersion of Pd-Pt on functionalized CNTs resulted in improved catalytic efficiency for hydrogenation of naphthalene to tetralin. In another study, it was reported that the functionalization of sulphonic acid group on graphene oxide improved its hydrophilicity as well as permeability [57].
It has been also reported that surface functionalization also improves the adsorption property of carbon materials. Sun et al. functionalized carbon beads via surface amination treatment and used it to study CO2 capture [58]. It was demonstrated that the surface modifications enhanced the adsorption of CO2 without compromising the desirable spherical morphology and mechanical strength of the activated carbon beads. This was attributed to surface modification of carbon, greater number of N-H functionalities resulted in stronger binding between CO2 and C-N-H groups than that of C=N-C.
A study conducted by Arrigo et al. showed that the functionalization of nanocarbons aids in the regulation of their acidic/basic character [49]. They treated oxidized carbon nanofibres with ammonia and evaluated the surface acid-base properties. It was reported that the samples contained both O and N species on the surface which served as acidic and basic sites. It was further illustrated that the population of O and N functionalities depended on temperature and could be controlled to modify the acid-base properties of the carbon surface. At 473 K, nitrogen moieties were incorporated on the surface as a consequence of the reaction of carboxylic acid sites with ammonia, thereby generating a bifunctional hydrophilic material with both acidic and basic sites on the carbon surface. Further raising the temperature to 873 K resulted in strong basic sites leading to a hydrophobic carbon surface. Thus, it was proved that altering the surface functionalization once can modify the acid-base nature of the carbon surface.
The specific surface area of materials is one the most imperative factors that directly affect catalysis. Greater surface area means better ease of access to active sites. The size reduction of particles causes the enhancement in the surface area of a substance. The surfacing of nanotechnology has resulted in the development of methods through which one can get nanodimensional materials with extremely large surface area. The nanocarbon materials with higher surface to volume ratio have therefore emerged as very effective carbocatalysts. For example, Pan et al. prepared porous N-doped carbon nanotubes (NCNTs) by pyrolysis of polypyrrole nanotubes and KOH activation at different temperatures 800 °C, 900 °C and 1000 °C [59]. The prepared nanotubes were used as electrocatalysts for the oxygen reduction reaction. A BET analysis was conducted to deduce the surface area of the nanotubes. It was reported that the specific surface area of polypyrrole nanotubes, NCNT-800, NCNT-900, and NCNT-1000 was 26, 1057, 1402, and 1137 m²/g, respectively. The assessment of ORR in a standard three electrodes cell showed that the NCNT-900 and NCNT-1000 were the most efficient catalysts in promoting the reactions. Furthermore, the nanotubes exhibited superior catalytic activities as compared to the commercially available Pt/C catalyst. It was thus highlighted that the high surface areas in porous nanotubes afforded active sites for diffusing O2 and KOH electrolyte thereby enhancing the rate of ORR.
Pore Size Distribution
The porosity of carbon materials is another important factor that influences their catalytic performance. Porous carbon materials can be classified as microporous (<2 nm), mesoporous (2-50 nm), and macroporous (>50 nm) on the basis of their pore diameters [60]. The pore volume and pore-size distribution play a crucial role in shape-selective catalysis [61]. Porosity in a material enhances catalytic activity not only by increasing accessibility to active sites albeit also by facilitating the mass transfer. Thus, by tuning the porosity one can induce desired catalytic properties in a material. Ferrero et al. investigated nitrogen-doped carbon microspheres with tunable porosity as electrocatalysts for the oxygen reduction reaction (ORR) [62]. It was reported that the catalyst was prepared by nanocasting using pyrrole as N-containing carbon precursor and porous silica microspheres as a template. Using two different silica samples as templates, the porosity of the microspheres was tuned from a micro- to a mesoporous network. The prepared catalytic system was applied to ORR and it was found that mesoporous electrocatalyst was more efficient than microporous one. This result was attributed to better accessibility of the active sites in the case of the mesoporous microspheres compared to the microporous ones. This is because in microporous materials the diffusion of reactant molecules becomes restricted due to small pore size and volume. It was further deciphered that the improved accessibility to the active sites on the surface of the mesoporous microspheres led to increase in mass-transfer reactions during the electrochemical process, thereby increasing the rate of catalytic reduction. Consequently, it can be said that pore size and volume are significant contributing factors in carbon based catalysis.
Thermal Conductivity
Thermal conductivity is another important criterion in carbocatalysis. Thermal conductivity is the ability of an object to conduct heat. Heat transfer is an important phenomenon in catalysis, particularly for strongly endo- and exothermic reactions in which fast heat management is advantageous for active sites [63]. Heat conduction in materials usually occurs due to the transmission of lattice vibrations known as phonons [64]. Carbonaceous materials like graphite, diamond, graphene, carbon fibres, and carbon nanotubes are well known for their thermal properties. The arrangement and coordination of carbon atoms along with defects in these materials are the main reasons for high thermal conductivities [65, 66]. A thermally conductive cobalt loaded graphite based catalyst for low temperature based highly exothermic Fischer-Tropsch synthesis (FTS) was reported [67]. It was reported that the use of this catalyst resulted in high productivity of hydrocarbons. This is because the incorporation of exfoliated graphite in the catalyst provides an amalgamation of extensive surface with high thermal conductivity and durability for the gasification process. Thermal analyses of graphite and aluminium based catalysts showed that the thermal conductivity of graphite is twice more than aluminum based catalyst and 30 times greater than a cobalt catalyst without heat conducting agents. It was further revealed that due to the high thermal stability of graphite, the catalyst survives overheating while the aluminum based counterpart is limited to 450 °С or even lower temperature because of irreversible effects such as metal pre-melting and oxidation. Hence, it is clear that thermally conductive as well as stable catalysts are perquisite for promoting reactions that require conduction of heat energy or heat removal.
Electrical Properties
To understand the electrical properties of carbonaceous materials, first, it is imperative to clarify the concepts of electrical resistivity and conductivity. Electric resistivity is a fundamental property of a material due to which it opposes the flow of electric current [68]. Mathematically, electric resistivity (ρ, Ωm) is defined as the ratio of electrical field (E, Vm-1) inside the material to the electric current density (J, Am-1), eq. (1). In other words, the resistivity is the resistance (R) offered by a material with unit cross sectional area (A) and unit length (l), eq. (2),
Electric conductivity (σ, Ω-1m-1 or Sm-1) is the property of a material by the virtue of which it conducts electricity. It is the reciprocal of electric resistivity. Consequently, both the terms resistivity and conductivity are interrelated and form the basis of the classification of materials [68]. On the basis of electric conductivity or resistivity, materials can be classified as conductors (materials that conduct electric current), insulators (materials that resist electric current) and semi-conductors (materials that intermediate between conductors and insulators).
The electrical properties of carbon materials are very much dependent on the structural attributes and type of treatment given to them. Different allotropes of carbon show different degrees of conductivities due to structural differences [69]. For instance, diamond exhibits low conductivity because each carbon atom is sp³ hybridized and bonded to four carbon atoms by strong covalent bonds. Therefore, no electrons are available for conduction. It has a wide band gap of 5.5 eV [70]. While in graphite the sp² hybridized carbon atoms are arranged as two-dimensional hexagonal lattice structure with carbon sheets stacked one above another and each carbon has one free electron. The sheets are held by weak van der Waals forces, electrons freely move throughout the layered structure. Graphite, thus, has a small band gap (40 meV) between valence and conduction bands and behaves as a conductor similar to metal [71].
Fullerene, a carbon polymorph, is a large caged polyhedral molecule with either fcc or bcc packing of atoms. In fullerene sp² carbons are directly bonded to three neighbors in an arrangement of five- and six-membered rings. Fullerenes are basically semiconductors, however, by doping heteroatoms, superconductivity can be induced [72].
Carbon nanotubes (CNTs) are rolled-up graphene sheets with cylindrical structure. These are of two kinds, single-walled carbon nanotubes (SWCNT) with a diameter in the nanometer range and multi-walled carbon nanotubes (MWCNT) are nested single-walled carbon nanotubes held together by van der Waals forces with a diameter of less than 100 nm. The interlayer distance in multi-walled carbon nanotubes is 0.34-0.39 nm [73]. SWCNTs can have an armchair, chiral, and zigzag geometries depending on the wrapping of cylinders while MWCNTs are of two types Russian Doll model and the Parchment model [74]. When the diameter of the outer nanotube is greater than the inner nanotube then, it is called the Russian Doll model. Whereas, when a single graphene sheet wraps around itself multiple times just like a scroll of paper, it is called the Parchment model. On the basis of structural symmetry, carbon nanotubes can exhibit conducting or semiconducting nature [75].
Graphene is a two dimensional single sheet of carbon atoms arranged like a honeycomb lattice and displays unusual electrical properties. In graphene, each C atom is linked to three other carbon atoms, leaving an electron freely available for electronic conduction. These mobile electrons are called pi (π) electrons and are located above and below the graphene sheet. Graphene is a zero-gap semiconductor because its conduction and valence bands meet at the Dirac points, where the effective mass of electrons (or holes) is zero. The free electrons in graphene interact with the periodic potential of the honeycomb lattice, generating new quasiparticles that have lost their mass or rest mass and hence, called massless Dirac fermions. Thus, graphene conducts electrons with high velocity and the lattice symmetry inhibits direct backscattering of electrons, enhancing the electrical mobility and conductivity [76].
The remarkable electric conductivities of graphite, fullerenes, nanotubes, and graphene make them highly suitable as a catalyst for various redox reactions. The electrical properties of carbon materials arising from their unique structures can further be tuned by functionalization, heteroatom doping and controlling the temperature of graphitization or carbonization [77].
Functionalization of carbon surface alters the electrical behaviour of carbon material by modifying their structure. In one such instance, Suslova et al. have reported that the electrical conductivity of nitrogen doped carbon nanotubes increased considerably with an increase in oxygen functionalities on their surfaces owing to elevation in the density of defects in their structural arrangement [78]. In another example, it was shown that graphitized carbon nanofibre bearing oxygen functional groups demonstrated high electrocatalytic activity as bifunctional electrode for vanadium redox flow battery [79]. The high catalytic performance of carbon nanofibres was attributed to an increase in ion diffusion and electron transfer processes owing to improved electrical conductivity and surface area due to presence of surface functionalities.
Doping is a method by which introduction of heteroatom results in generation of holes or addition of an extra electron in a substance. This process leads to decrease in band gap of valence and conduction bands, thereby, improving the conductivity of semiconductors. Heteroatom doping is therefore a strong tool for tuning the electrical conductivities of carbon materials. Han et al. studied the effect of N-doping in carbon nano-onions [80]. The N-doped carbon nano-onions (N-CNOs) were prepared by a one-step, in situ flame synthesis technique using clarified butter and acetonitrile as precursors. The prepared N-CNO was than employed as catalyst for ORR process in a microbial fuel cell. It was reported that the catalytic activity of N-CNO was 5.4 times higher than CNO. This high performance was ascribed to the presence of nitrogen atoms in carbon framework. Through electrochemical impedance spectroscopy, it was elucidated that the conductivity of N-CNO is higher than CNO due to N-doping. It has been explained that the sp²–C atoms adjacent to N atoms and N atoms stimulate the electrostatic adsorption of oxygen molecules leading to sharp reduction in the kinetic barrier for the ORR process [81]. Simultaneously, N-doping causes electron excess in the delocalized π-system which results in enhancement of electrical conductivity. Finally, the high electrical conductivity induces electron transfer during catalysis, assisting the contribution of C-N active sites and improving the ORR activity on the whole [82]. As a result, the high ORR catalytic performance of N-CNO can be attributed to the high electrical conductivity arising from N-doping.
It has also been reported that thermal [83] and graphitization treatments [84] generally result in metallic conductivity in carbon materials while oxidation induces semi-conductivity due to π-electron delocalization [85]. Therefore, by applying different treatments one can tune the electrical properties of carbocatalysts.
SYNTHETIC STRATEGIES FOR CARBOCATALYSTS
As is well known that synthetic methods greatly influence the properties of a material, therefore, in this section some strategies employed for synthesis of carbocatalyst have been discussed. So many methods for synthesis of carbon materials are reported that a whole chapter can be devoted to the synthetic aspects of carbon allotropes. However, since the book deals with graphene here we have highlighted some important methods only.
Active Carbon
Activated carbon is highly porous form of carbon. Coal, wood, agricultural wastes or biomass, ionic liquids, salt solutions, etc. are used as precursors to prepared activated carbon or charcoal [86]. Physical and chemical activation methods are commonly used to prepare active carbon materials. Before activation, carbonization and de-ashing is performed as pre-treatment. In carbonization the raw materials are subjected to heat treatment or pyrolysis at high temperatures around 573.15 K to 1173.15 K in order to eliminate non-carbon materials and volatile impurities. This result in formation of char after rearrangement of carbon atoms into graphitic sheets stacked above one another to form rigid microcrystals with voids which constitute the pore structure [44]. Ahead of activation, ash and mineral content of the carbon materials is reduced by de-ashing or demineralization technique via chemical leaching. In this process, acid and alkali solutions are used to demineralize the carbon raw materials [87]. For instance, Ahmad et al. performed acid treatment of cocoa shell pellets with 1M HCl which reduced the ash content to less than 10% [88]. In another study, it was reported that high ash content reduced the adsorption property of activated charcoal prepared from stone coal ore hence, to improve the adsorption. The stone coal ore was pre-treated with HF and H2SO4. The acid treatment reduced the contents of mineral atoms like Fe, Mg, Ca, Si, etc. resulting in stone coal ore with 74.76% carbon content [89]. Similarly, KOH promoted de-ashing of rice husk was reported by Shen and Fu which resulted in highly porous biochar [90].
Next step involved in formation of activated carbon is activation. Activation can be carried out in two methods: physical and chemical. Physical activation includes use of carbon dioxide, air, steam or other gaseous agents at 1073.15 to 1273.15 K to remove oxides of carbon from char and improving the porosity of carbonaceous materials. Zhou and co-workers have reported preparation of highly porous activated carbon from waste tea via physical activation with steam [91]. It was further highlighted that typically micropores were developed when the activation temperature was below 800 °C while activation above 800 °C enriched the carbon with both micropores and mesopores.
Chemical activation involves single-step carbonization and activation of carbon materials by impregnating them with chemical agents such as KOH, ZnCl2, H3PO4, FeCl3, CaCl2, H2SO4, NaOH, MgCl2, K2CO3, HNO3, etc., followed by thermal treatment [86]. The chemical agents due to dehydrating property result in generation of a rigid framework which is less prone to reduction in volume during carbonization, thereby generating excellent yields of activated carbon [92, 93]. Moreover, since chemical activation is performed at lower temperature than physical activation, it offers better control of porous structure [94]. However, from environment point of view physical activation is better as it avoids use of chemical agents while chemically activated materials contain some amount of inorganic matter [95]. Recently, Chen et al. have demonstrated synthesis of activated carbon with large surface area from tobacco stem [96]. Chemical activation was performed using KOH, K2CO3 and ZnCl2 as activating agents. It was reported that ZnCl2 caused higher activation of tobacco stem compared to KOH and K2CO3. Further, increasing the concentration of ZnCl2 enhanced the development of new pores and the widening of existing pores. Furthermore, activation of carbon with K2CO3 resulted in high oxygen functionalities on the surface. Thus, it is evident that nature of chemical agent and concentration affects the activation process as well as surface properties.
Graphite
Synthetically, graphite is prepared via thermal treatment of petroleum coke or coal-tar. Hydrothermal methods involving treatment of polymers and metal carbides can also be used to produce graphitic materials. Of late, graphite encapsulated nanoparticles have also emerged as potent carbocatalysts. These can be fabricated by several methods such as pyrolysis, template synthesis, chemical vapor deposition, arc discharge, laser ablation technique, etc. Li et al. have demonstrated hydrothermal synthesis of graphite encapsulated molybdenum carbides [97]. The graphite encased molybdenum carbide nanoparticles were prepared by the hydrothermal carbonization of a mixture of glucose solution and ammonium molybdate followed by temperature programmed reduction under mild conditions. The prepared graphite encapsulated nanoparticles showed high catalytic efficiency for conversion of guaiacol to phenolic compounds in methanolic medium. In another study, graphite encapsulated gold nanoparticles displaying excellent electrocatalytic activities towards oxidation reduction reaction were prepared by laser ablation method. The process of laser ablation involved installation of Au target in a vessel containing graphite precursor- acetone (16 mL). Thereafter, the revolving Au target was ablated by a focused beam of laser with wavelength of 1064 nm, 10 Hz pulse repetition rate and 100 mJ pulse energy for 30 min. Finally, the graphite encased Au nanoparticles were obtained by centrifugation followed by freeze drying. Thus, laser ablation technique afforded a facile and efficient route to synthesis of graphite encased nanoparticles.
Recently, expanded graphite has been employed as catalyst for oxidation reaction [98]. Expanded graphite is a type of graphite with interlayer spaces. The expanded graphite is prepared by oxidation of natural graphite flakes using acids like nitric acid and sulfuric acid and oxidants like potassium permanganate, chromic acid, hydrogen peroxide, etc. Then, the mixture is subjected to high temperature which causes expansion and swelling of graphite resulting in separation of layers leading to formation of its expanded form.
In recent past, hydroxylated high surface area graphite has also been used as carbocatalyst for ozonation. Usually, oxygen functionalized graphite are prepared via oxidation using strong acids and mechanical or thermal exfoliation methods. In a method reported by Bernat-Quesada et al., the water soluble graphite was synthesized by oxidation of graphite by nitric acid [99]. Dispersion of graphite in nitric acid followed by heating at 83 °C for 20 h followed by filtration and drying resulted in formation of oxygen functionalized graphite on edges as well as basal planes [48]. In another method, the workers used ball milling technique to fabricate hydroxylated graphite. In this method, graphite, KOH and water were loaded in a grinding jar of planetary ball mill. The jar was agitated at 300 rpm for 10 hours at ambient temperature. Later, the mixture was filtered and dried to obtain graphite functionalized with oxygen moieties on the edges.
Xu et al. have also reported the synthesis of functionalized graphite felts as catalyst for oxidation of hydrogen sulphide [100]. The commercial graphite felts bearing oxygen functionalities were prepared by treatment with nitric acid. According to the method, graphite felts were first heated at 500 °C for 1 hour in air. Then, the felts were loaded in a tubular reactor and heated at 250 °C using an external electric furnace [101]. The reactor loaded with the felts was connected to a round bottom flask (RBF) containing 65% HNO3 solution (150 mL). The temperature of the RBF was maintained at 125 °C and the solution was continuously stirred magnetically. The gaseous acid passed through the felts for several hours causing their oxidation. The sample was then washed using deionized water and dried at 130 °C overnight. The acid treated functionalized graphite felts exhibited large surface area and it was concluded that the structural defects in graphite felts as well as oxygen groups on them served as active sites for oxidation of hydrogen sulphide.
Fullerenes
Fullerenes and their composites are widely used as catalysts. Fullerenes are giant caged carbon allotropes that are generally prepared by arc discharge and laser ablation of graphite, and combustion of hydrocarbons. In a typical arc discharge method, electric current is struck between graphite or carbon anode and cathode electrode in an inert atmosphere. The electric discharge produces soot which is collected and fullerene is then extracted from the soot by solvent extraction method, sublimation or supercritical fluid extraction technique. Generally for fullerene synthesis, an alternating current between the graphite electrodes in He atmosphere of about 200 Torr is used to produce soot containing C60 and C70 molecules [102].
The laser ablation technique makes use of a pulsed laser beam which is focused on the surface of carbon sample at high temperature (usually 1000 °C). Thus, the ablation of carbon results in fragments which form fullerenes [103]. Also, hydrocarbon combustion in presence of electric field acting on the flame is widely used to achieve soot-containing fullerene [104].
Additionally, several fullerene based complexes are also well known as catalysts and can be prepared by different methods. For instance, Sabounchei and co-workers have prepared mono- and bidentate palladium(0)- [60]fullerene based complexes as catalyst for Mizoroki-Heck coupling reaction [105]. The synthetic strategy involved a Schlenk tube containing C60 (0.05 mmol, 0.036 g) and Pd(dba)2 (0.05 mmol, 0.029 g) in 15 mL of toluene. The reaction mixture was stirred for 15 min at room temperature during which the color changed from purple to black. Thereafter, a phosphorus ylide solution (0.05 mmol) in toluene (5 mL) was injected into the reaction solution via a syringe pump. Further, a 2 hour stirring was continued at ambient temperature which resulted in deep green solution. Dark green crystals were obtained by treating the solution with n-hexane overnight and later washing with diethyl ether.
Further, several fullerene based composites fabricated by sol-gel method, chemical reduction, ultrasonication, hydro- or solvothermal technique, etc. serve as useful catalysts [106]. For example, Liu et al. have reported synthesis of silver (I) doped fullerene by liquid- liquid interfacial precipitation method [107]. The method involved mixing of saturated solution of fullerene (approx. 2.8 mg/mL) in toluene with isopropyl alcohol saturated with silver nitrate at ambient temperature. The bilayer so formed was kept still under room conditions for 3 hours. Thereafter, centrifugation of the mixture at 7000 rpm for 5 minutes resulted in precipitation of crystalline solid which was dried under nitrogen stream. Then, chemical reduction the AgNO3 complex encapsulated fullerene crystals with hydrazine hydrate solution (1.3% wt.) resulted in formation of Ag(I)-fullerene composite. Finally, the composite was washed with deionized water and ethanol to remove excess silver ions. The prepared Ag(I)-fullerene composite was used as a catalytic system for reduction of 4-nitrophenol and photo-degradation of orange G dye.
A self assembled method based on hydrothermal mode for synthesis of amine functionalized fullerene decorated with palladium nanoflowers was reported by Li and Han [108]. Initially, amine functionalized fullerene was prepared by dissolving ethylenediamine in ethanol-water system (5:1). Then this solution was added drop by drop to toluene saturated with C60 fullerene and sodium hydroxide with continuous stirring. The stirring was continued for 5 days in argon atmosphere at room temperature, later the solution was filtered, centrifuged, and washed with ethanol and distilled water for a number of times. Eventually, dark brown solid products were obtained after drying in vacuum for 24 hours at 60 °C. For preparation of composite of fullerene, palladium chloride was dissolved in HCl and DMF and mixed with functionalized fullerene. The reaction mixture was then placed in autoclave for 6 hours at 140 °C in nitrogen atmosphere. The obtained product was washed with ethanol and vacuum dried overnight to yield amine functionalized fullerene decorated with palladium nanoflowers which displayed excellent electrocatalytic effect for reduction of p-nitrophenol.
Glassy Carbon
It is a type of carbon material which combines characteristics of both glass and ceramic materials with that of graphite. It is a commonly used electrocatalyst or electrode material. The strategy for fabrication of glassy carbon is based on pyrolysis of polymeric materials at elevated temperatures of about 3000 °C [44, 109]. The mechanism basically involves breakdown of carbon-heteroatom bonds in polymers and generation of C-C bonds [110-112]. Initially, hydrocarbon radicals are generated with their highest concentration at around 600 °C [113] and after 800 °C or higher temperature, a network of graphene fragments develop [114]. Further, annealing process results in formation of cross-linked carbon network and removal of volatile by-products like oxides of carbon, water, methane and small hydrocarbons [115]. For instance, Jurkiewicz et al. prepared glassy carbon by carrying out pyrolysis of polymeric furfuryl alcohol under Ar gas flow at varied temperatures ranging from 600 to 2500 °C at different heating rates [116]. The carbonized samples were then cooled in argon atmosphere resulting in glassy carbon with fullerene-like or nanotube-like elements in its structure. It was also reported that pyrolysis up to 1000 °C resulted in increase in fullerene-like non-planar sp ² carbon bonds. But on further increase in pyrolysis temperature the amount of non-planar sp ² bond content decreased.
Recently, a nitrogen enriched glassy carbon was prepared as an electrocatalyst for hydrogen evolution reaction by Thirukumaran and his research group [117]. Initially polybenzoxazine was synthesized by step-wise polymerization of benzoxazine. The carbonization of polybenzoxazine at 600 °C for 5 hours under nitrogen atmosphere was performed. The resultant carbonaceous material was then activated by adding KOH solution followed by water elimination through evaporation at 120 °C. Subsequently, the carbon material was heated in a tubular furnace for an hour at 600 °C to obtain nitrogen enriched glassy carbon. This method thus offered an efficient method for synthesis of glassy carbon electrocatalyst.
Carbon Black
Carbon black is composed of colloidal carbon particles and is usually synthesized via partial combustion or thermal decomposition of petroleum products. Carbon blacks are extensively used as electrocatalysts for redox processes. Diverse thermal methods for carbon black synthesis are summarized below [118].
Channel Black Process
In this process, combustion of natural gases using a continuous small wide luminous fan-shaped flame occurs. This process yields carbon black with a particle size of approximately 10-30 nm.
Gas Black Process
It involves thermal oxidative decomposition, where the formation of carbon black takes place in diffusion flame.
Thermal Black Process
In the thermal black process, thermal combustion of natural gas into elemental carbon and hydrogen takes place at high temperatures to produce carbon black and tail gas. The sizes of obtained carbon black particles are about 180 nm.
Acetylene Black Process
The method involves constant thermal decomposition of acetylene gas in the absence of oxygen gas to generate carbon black as aerosols.
Lamp Black Process
It includes partial combustion of carbon materials such as resins, fatty oils and acids, coal tar, etc., resulting in the deposition of carbon in the form of soot. Carbon black particles so obtained range in the size from 110 to 120 nm.
Furnace Black Process
In this process, aromatic oils are pyrolized by exposing them to a hot gas stream to produce carbon blacks with particle sizes of about 20-100 nm.
Besides these methods, carbon black is also obtained by plasma synthesis. An arc discharge plasma method coupled with thermal pyrolysis for the synthesis of carbon black was demonstrated by Sun et al. [119]. An apparatus composed of a plasma generator, thermal pyrolysis device, gas injector, power supply and collecting device was used for this purpose. Thermal plasma was generated by electrodes supplied with 10 kV power. Argon was injected into the device for 5 min. to firstly drain out oxygen and then mixed with propane at definite rate of volume. As the propane gas flowed into the plasma region, carbon black was produced instantly. Then the thermal pyrolysis furnace pyrolyzed the un-reacted propane. Finally, carbon black was collected from the bottom of sand core funnel connected with a vacuum pump.
Functionalized carbon black has also gained attention as catalyst. Recently, oxidized carbon black has been employed as a catalyst for the synthesis of benzodiazepines [119]. The oxygen functionalized carbon black was prepared by Hummer’s method. In this method, sulphuric acid (120 mL) and sodium nitrate (2.5 g) were mixed with 5g of carbon black in a round bottom flask kept on an ice bath. The suspension was magnetically stirred and then 15 g potassium permanganate was added gradually. The mixture was subjected to heating at 35 °C and stirred for about 24 hours. Then, 700 mL deionized water was added in small portions resulting in black slurry for carbon black under stirring. Finally, 5 mL of hydrogen peroxide solution (30 wt%) was added to the slurry. The obtained sample was poured into deionized water and then centrifuged for 15 min at 10000 rpm. The sample was then washed with deionized water and HCl (5 wt %) and later dried to obtain oxidized carbon black. The optimal conditions for highly graphitized carbon black production were a discharge current of 0.8 A, 120 L/h of argon flow rate, 80 ml/min of propane flow rate and pyrolysis temperature at 700 °C.
Also, carbon black based composites to have found applications as electrocatalysts. In a work conducted by Lim et al., TiO2/carbon black composite was prepared and used as a catalytic electrode in dye-sensitized solar cells [120]. The composite formation involved pulverization of carbon black followed by its calcination in a muffle furnace at 500°C for 2 hours by placing the carbon black in 80-unit mesh. The calcinated carbon black was again ground and thermally treated at 300 °C for 2 hours to obtain carbon black powder with a particle size of about 80 nm. Then, nano-TiO2 was prepared by the modified Burnside method. Finally, TiO2/carbon black composite was fabricated by mixing carbon black powder with TiO2 NPs followed by ultrasonication for 10 min. at 750 W. Then, the addition of 100 μl of Triton X-100 to the mixture followed ultrasonic treatment for 10 min. yielded the TiO2/carbon black composite. The prepared composite was subsequently used as the counter electrode material in solar cells.
Carbon Nanomaterials
Carbon Nanofibres
Carbon fibres are carbon containing fibres of diameter of the order of few micrometers that find applications in industries due to their enormous tensile strength and thermo-chemical conductivity and stability. In last few years nanodimensional carbon fibres have attracted huge attention in the field of catalysis due to their advanced chemical and electrical activities along with large surface area. Carbon nanofibres are usually prepared by chemical vapour deposition method (CVD), catalytically promoted CVD (CCVD), electrospinning and template synthesis techniques. CVD is a well acknowledged method for synthesis of 2-dimensional nanomaterials. The method is based on decomposition of vapour or gaseous precursors on a substrate for preparation of materials at elevated temperature in presence of vacuum. The CVD method was explored by Hulicova-Jurcakova et al. for synthesis of carbon nanofibre [121]. Carbon precursors like methane and acetylene were used for this process and nickel supported on γ-alumina was used as substrate. The reaction was carried out in tubular furnace with a heating rate of 10 °C/min at 500°C and 700°C, respectively. The obtained carbon samples were purified in reflux system using 5 M nitric acid solution for 6 h, the obtained solution was filtered and dried to obtain carbon nanofibres. Highly graphitic carbon nanofibres were obtained by this CVD method.
Another important method that has emerged for nanofibre synthesis is CCVD, in which catalyst based chemical vapour decomposition, is carried out. Miniach and co-workers have reported a CCVD method for synthesis of carbon nanofibres [122]. They used hydroxyapatite-supported nickel catalyst (Ni/HAp) on which methane decomposition occurred resulting in the formation of herringbone