Surface Science 604 (2010) 1029–1033 Contents lists available at ScienceDirect Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c A periodic DFT study of water and ammonia adsorption on anatase TiO2 (001) slab Rezan Erdogan a, Olus Ozbek a,b, Isik Onal a,⁎ a b Department of Chemical Engineering, Middle East Technical University, Ankara, 06531, Turkey Chemical Engineering and Chemistry Department, Eindhoven Technical University, Netherlands a r t i c l e i n f o Article history: Received 8 December 2009 Accepted 10 March 2010 Available online 17 March 2010 Keywords: DFT Anatase Water Ammonia adsorption a b s t r a c t Water and ammonia adsorption mechanisms on anatase TiO2 (001) slab surface are investigated by means of periodic DFT approach. Molecular and dissociative adsorption energies for water are calculated to be − 15 kcal/mol and − 32 kcal/mol, respectively. Similarly, molecular and dissociative adsorption energies of ammonia on the same surface are found as − 25 kcal/mol and − 20 kcal/mol. A reverse result in this order is reached for the previous case of ONIOM cluster study (− 23 kcal/mol and − 37 kcal/mol, respectively). The vibration frequency values are computed for the optimized geometries of adsorbed water and ammonia molecules on anatase TiO2 (001) slab surface and compared with the values reported in the literature. © 2010 Elsevier B.V. All rights reserved. 1. Introduction TiO2 has been extensively studied for many years as a model metal oxide with a wide range of applications in catalysis, photochemistry, and electrochemistry [1]. Especially in catalysis, anatase phase of titanium dioxide is used much more often as a support or as a catalyst by itself [1–3]. It has been proposed by a number of researchers that the minority (001) surface is more reactive and plays a key role in the reactivity of anatase nanoparticles [4–7]. Since TiO2 exposed to air will always be covered by a water film, the presence of hydroxyl group can affect adsorption and reaction processes and also might enhance or diminish adsorption of other molecules such as CO, NH3, and O2 on the surface. NH3 adsorption on anatase TiO2 can be important for industrial catalytic reactions such as selective catalytic reduction (SCR) of NO [8] and photo-oxidation of NH3 over TiO2 [9,10]. Since periodic DFT involves heavy computations but it is also a more accurate method, it would be interesting and valuable to compare its results with those of ONIOM DFT method performed in the previous study [11]. There are some experimental studies with regard to the surface properties and adsorption reactions of water and ammonia on TiO2anatase surfaces. In the experimental studies by Ramis et al. [12] and Topsøe et al. [13] it was reported that fivefold coordinated titanium atoms (acting as Lewis acid sites) and surface oxygen atoms which have Bronsted base properties as the catalytic active sites of TiO2. With regard to the adsorption mechanism, the experimental studies [14–16] reported that water and ammonia are molecularly adsorbed on TiO2anatase surface. In a study by Srnak et al. [14], two states of water and ⁎ Corresponding author. Tel.: +90 312 210 2639; fax: +90 312 210 2600. E-mail address: ional@metu.edu.tr (I. Onal). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.03.016 two states of ammonia desorption values were observed and desorption activation energies of water and ammonia adsorbed states are estimated from vacuum TPD (temperature programmed desorption) studies as −11 and −18 kcal/mol and −14 and −27 kcal/mol, respectively. Munuera et al. [15] reported a heat of desorption value for water adsorption on TiO2-anatase as −12 kcal/mol. Sprinceana et al. [16] carried out a calorimetric study and reported a differential heat of −31 to −36 kcal/mol for ammonia adsorption on anatase titania. There are many theoretical studies about water adsorption reactions and surface properties on anatase TiO2 (001) and almost all of them [4,11,17–23] reported that dissociative adsorption mechanism is the favorable path for water adsorption on anatase TiO2 (001) surface. Vittadini et al. [4] reported that for 0.25 monolayer, water is adsorbed dissociatively with an adsorption energy − 37 kcal/mol by use of PW:DFT-GGA and Car–Parrinello calculations. By using periodic Hartree–Fock method, Fahmi and Minot [17] reported that water adsorbs on the titanium atom and then dissociates to give hydroxyl groups. Nair [18] determined molecular and dissociative water adsorption energy values on anatase TiO2 (001) by means of MSINDO-CCM (semiempirical molecular orbital method–cyclic cluster model) calculations as −19 and −33 kcal/mol, respectively. In the study of Jug et al. [19], molecular and dissociative water adsorption energies on anatase TiO2 (001) are calculated by means of MSINDO (semiempirical molecular orbital) method as −24 and − 50 kcal/mol, respectively. Arrouvel et al. [20] reported that water is mainly dissociated and adsorption energy varies strongly with increasing coverage from −39 to − 24 kcal/mol. In agreement with previous theoretical studies, Gong et al. [21] found that dissociative adsorption is favored, with average adsorption energies of − 29, −26, and −27 kcal/mol per H2O at 1/6, 1/3, and 1/2 ML coverages, respectively, by use of PW:DFT-GGA and Car–Parrinello methods. In our research group, Onal et al. [22] and Erdogan and Onal 1030 R. Erdogan et al. / Surface Science 604 (2010) 1029–1033 [11] performed two cluster studies with regard to the water and ammonia adsorption on anatase TiO2 (001) surface. The first study by Onal et al. [22] was carried out on a relaxed (001) cluster, and it was reported that non-activated dissociation of water takes place with an exothermic relative energy difference of 54 kcal/mol calculated via DFT-B3LYP/6-31G** method. In another similar study by Erdogan and Onal [11] an ONIOM cluster was used, and it was reported that water molecule dissociates on anatase TiO2 (001) surface by a non-activated process with an exothermic relative energy difference of 58 kcal/mol obtained by means of DFT/B3LYP/6-31G**-MM/UFF level calculations. Wahab et al. [23] reported molecular and dissociative water adsorption energy values on anatase TiO2 (001) as − 18 and − 25 kcal/mol, respectively by means of semiempirical MSINDO method. It was stated that the dissociated form of water molecule adsorption on anatase TiO2 surfaces is always more stabilized than the molecular form. Although the adsorption mechanisms of water on (001) anatase TiO2 have been extensively reported both in experimental and theoretical literature, there are only three theoretical studies accessible in the literature [11,22,24] concerning NH3 adsorption on the same surface. Two of these three works are our own research [11,22] and were carried out using non-periodic clusters. In the cluster study, Onal et al. [22] reported that on relaxed cluster surface dissociation occurs with a slight activation barrier of 3.6 kcal/mol and an adsorption energy value of − 36 kcal/mol. In the ONIOM cluster study by Erdogan and Onal [11], molecular and dissociative NH3 adsorption energy values were calculated as − 23 kcal/mol and − 37 kcal/mol, respectively. Calatayud et al. [24] determined a molecular adsorption energy value of − 19 kcal/mol by using periodic calculations, however; they did not indicate a dissociation case. The objective of the present study is to theoretically investigate adsorption of H2O and NH3 on TiO2-anatase (001) surface by means of a periodic slab model and density functional theory (DFT) approach. 2. Computational method and surface model The calculations are carried out by means of VASP [25] code, which uses periodic plane wave basis sets. The electronic interactions are described with PAW [26,27], and GGA (PW91) [28,29]. Except for the molecules and atoms in the gas phase, dipole corrections are included for the asymmetric slab calculations, where the top sides of the slabs are used. The cut-off energies and k-points (Monkhorst Pack) used are; 500 eV and (3 × 3 × 1) for 4 layer p(2 × 2) anatase TiO2 (001) slab (Fig. 1). All the results reported in this work are carried out by optimizing the respective structures until the net force acting on the Fig. 1. 4 layer p(2 × 2) TiO2 anatase (001) slab. atoms is smaller than 0.01 eV/Å. The atoms and the molecules in the gas phase are represented with the same level of precision by surrounding the species with a vacuum layer of minimum 10 Å in all directions. Anatase TiO2 (001) surface slab (Fig. 1) is prepared by initially optimizing the respective crystal and then cutting the crystal along (001) plane and placing a 15 Å vacuum layer above. The optimization of the bulk crystal is carried out to determine the optimal lattice parameters, which are calculated as a = b = 3.822 Å, and c = 9.670 Å. These values are in a good agreement with the experimentally reported values, 3.785 Å and 9.514 Å [30]. The adsorption energy is calculated for adsorbate on the clean surface as follows: Eads = Eadsorbate=surface –ðEadsorbate + Esurface Þ where Eadsorbate is the energy of the isolated molecule (H2O or NH3) in the vacuum. Esurface is the energy of a clean anatase (001) surface, and Eadsorbate/surface is the total energy of the molecule adsorbed together with the (001) anatase TiO2 surface. One side of the slab is occupied by adsorbed molecules. The coverage of adsorbed molecules is taken as θ = 0.25 with respect to surface Ti atoms. Following the optimizations to equilibrium geometries as described above, the vibrational frequencies of adsorbed surface species are also calculated. This is done by calculating the Hessian matrix based on a finite difference approach with a step size of 0.02 for the displacements of the individual atoms along each Cartesian coordinate. During the frequency calculations symmetry is excluded explicitly. The frequencies of the unfrozen surface atoms (phonons) are also calculated, however they are not reported herein. 3. Results and discussion 3.1. H2O adsorption on anatase TiO2 (001) For the adsorption reaction studies of small molecules (H2O, NH3), fivefold coordinated titanium atoms and the surface oxygen atoms are used as the catalytic active sites of TiO2 and illustrated in Fig. 2. By using a slab model and a periodic DFT approach, water adsorption on anatase TiO2 (001) surface is investigated. Molecular adsorption energy is evaluated from total energy calculations as −15 kcal/mol. Fig. 2 depicts the optimized geometry of molecular H2O adsorption on anatase TiO2 (001) slab model. For the molecular adsorption geometry, the distances between hydrogen and oxygen of water molecule (Hw–Ow) and oxygen of water molecule and fivefold coordinated titanium (Ow–Ti) are computed as 0.972 Å, and 2.171 Å, respectively. For the case of dissociative adsorption mechanism, it is observed that water molecule is dissociated on (001) anatase TiO2 surface with an exothermic relative energy difference of 32 kcal/mol. For the optimized geometry of this reaction (see Fig. 3), the distances between hydrogen and oxygen of water molecule (Hleft–Ow), hydrogen of water molecule and surface oxygen (Hright–Os), and oxygen of water molecule and fivefold coordinated titanium (Ow–Ti) are computed as 0.971 Å, 0.982 Å, and 1.892 Å respectively. Very similar results to those found by Vittadini et al. [4] are reached concerning the coverage and the structure of the bonds for dissociative adsorption. It is found that dissociative water adsorption occurs at 0.25 coverage with bond length increasing between the bridging oxygen and the Ti atom. This structure can be seen in Fig. 3. A comparison of calculated water adsorption energy values on (001) anatase TiO2 surface with the available theoretical and experimental literature is given in Table 1. A comparison of the computed vibration frequency values of dissociatively adsorbed H2O on (001) anatase TiO2 surface with the available experimental literature is given in Table 2. In Table 2, the change of frequencies upon adsorption of water molecule is also 1031 R. Erdogan et al. / Surface Science 604 (2010) 1029–1033 Fig. 2. Optimized geometry of molecular H2O adsorption on anatase TiO2 (001) slab model. a) Perspective view and b) top view. reported as compared with water molecule in the gas phase. As can be seen from this table, stretching frequency value of 3767 cm− 1 for dissociative water adsorption have a certain shift when compared with the water molecule in the gas phase (3847 cm− 1). The calculated stretching vibration value of 3767 cm− 1 is also in the experimentally predicted range (3600–3800 cm− 1) reported by [31] and agrees well with the experimental value of (3715 cm− 1) where (001) anatase surface was assumed [32]. 3.2. NH3 adsorption on anatase TiO2 (001) The most interesting issue in this study are the results obtained for ammonia adsorption on anatase TiO2 (001) surface. Adsorption energy for molecular ammonia on anatase TiO2 (001) slab model is computed from the total energy calculations as − 25 kcal/mol. Optimized geometry of this reaction is illustrated in Fig. 4 and it is found that ammonia is molecularly adsorbed on the surface having an N–Ti distance of 2.26 Å. For the dissociative mechanism of ammonia adsorption, the resultant energy calculation shows that ammonia molecule is dissociatively adsorbed on the surface with an exothermic relative energy difference of 20 kcal/mol. The optimized geometry of this interaction including the bond lengths is given in Fig. 5. The distances of N–Ti, N–H, and O–H are calculated as 1.918 Å, 1.017 Å, and 0.982 Å, respectively. As in the case of water, it is found that dissociative ammonia adsorption occurs at 0.25 coverage with bond length increasing between the bridging oxygen and the Ti atom (see Fig. 5). This is the analogous structure for the ammonia dissociative adsorption to what was found by Vittadini et al. [4] for water dissociative adsorption. As a result of the periodic DFT calculations in this study, it is found that molecular ammonia adsorption on anatase TiO2 (001) surface is Fig. 3. Optimized geometry of dissociative H2O adsorption on anatase TiO2 (001) slab model. a) Perspective view and b) top view. Table 1 A comparison of the calculated H2O adsorption energies on perfect anatase TiO2 (001) surface with the theoretical and experimental values in the literature. Authors and references Theoretical Fahmi and Minot 1994 [17] Vittadini et al. 1998 [4] Nair 2004 [18] Jug et al. 2005 [19] Arrouvel et al. 2004 [20] Gong et al. 2005 [21] Onal et al. 2006 [22] Erdogan and Onal 2009 [11] Wahab et al. 2008 [23] This study Experimental Srnak et al. 1992 [14] Munuera et al. 1999 [15] Method Adsorption energy, kcal/ mol Molecular Dissociative −14 −29 −19 −33 1/4 ML – −19 −24 – −37 −33 −50 −(24–39) 1/6 ML – −29 – – − 25 −26 −27 −54 Periodic PW:DFT-GGA Car-Parrinello 1/2 ML DFT MSINDO PW:DFT-GGA KS PW:DFT-GGA Car-Parrinello 1/3 ML 1/2 ML DFT-B3LYP/6-31G** Relaxed cluster DFT/B3LYP/6-31G**- ONIOM MD/UFF cluster MSINDO − 24 −58 − 18 −25 PW:DFT-GGA-PW91 1/4 ML − 15 −32 TPD Anatase TiO2 Anatase TiO2 − 11, − 18 − 12 TPD 1032 R. Erdogan et al. / Surface Science 604 (2010) 1029–1033 Table 2 Vibrational frequencies (cm− 1) of H2O molecule in gas phase, dissociatively adsorbed on anatase TiO2 (001) surface and in experimental literature. υOH a b stret. H2O in gas phase Dissociatively ads. H2O Experimental anatase TiO2 3847 3767 3600–3800a, 3715b Morterra et al. [31]. Primet et al. [32]. an energetically slightly more favorable process than dissociative adsorption (−25 kcal/mol vs. − 20 kcal/mol). A comparison of the computed adsorption energy values for the ammonia adsorption on the defect-free anatase TiO2 (001) surface with the available literature is given in Table 3. Experimental TPD [14] and XRD, BET [16] studies investigated NH3 adsorption on real TiO2 (anatase) catalyst surface and therefore the slight deviation is reasonable in view of the fact that (001) surface is minority in anatase as previously mentioned. However, molecular ammonia adsorption energy value (−25 kcal/mol) obtained from periodic DFT calculations in this study is quite comparable to the results of our preceding relaxed [22] and ONIOM [11] cluster studies (−27 kcal/mol and −23 kcal/mol, respectively). A significant difference is found in the relative stability of dissociative NH3 adsorption between this study (periodic DFT) and previous cluster studies [11,22]. Dissociative ammonia adsorption energy value (−20 kcal/mol) obtained in this study deviates significantly from the results of the previous relaxed [22] and ONIOM [11] cluster studies (−36 kcal/mol and −37 kcal/mol, respectively). As part of an analysis study, ONIOM cluster calculations are redone by using PW91 functional in Gaussian 03 instead of B3LYP functional; and the Fig. 5. Optimized geometry of dissociative NH3 adsorption on anatase TiO2 (001) slab model. a) Perspective view and b) top view. results give a very similar trend albeit with 3 kcal/mol higher adsorption energy for calculations involving the same PW91 functional used in periodic DFT calculations. This eliminates the possible effect of using a different functional. The most probable reason for these deviations may be related to differences between periodic DFT calculations and calculations involving small clusters with edge effects. A vibration frequency study for ammonia adsorption is also performed and a comparison of the vibrational properties of molecularly and dissociatively adsorbed NH3 molecule on the (001) surface with available experimental data reported for anatase surfaces Table 3 A comparison of the calculated NH3 adsorption energies on perfect anatase TiO2 (001) surface with the theoretical and experimental values in the literature. Authors and references Theoretical Calatayud et al. 2004 [24] Onal et al. 2006 [22] Erdogan and Onal 2009 [11] This study Fig. 4. Optimized geometry of molecular NH3 adsorption on anatase TiO2 (001) slab model. a) Perspective view and b) top view. Experimental Srnak et al. 1992 [14] Sprinceana et al. 1999 [16] Method PW:DFT-GGA KS Adsorption energy, kcal/mol Molecular Dissociative Act. barrier −19 – Relaxed cluster −27 DFT/B3LYP/6-31G** − 23 ONIOM cluster DFT/B3LYP/6-31G**MD/UFF PW:DFT-GGA-PW91 − 25 1/4 ML TPD XRD BET Anatase TiO2 Anatase TiO2 − 36 3.6 −37 2.7 −20 – − 14, −27 −(31–36) R. Erdogan et al. / Surface Science 604 (2010) 1029–1033 Table 4 Vibrational frequencies (cm− 1) of NH3 molecule in gas phase, molecularly and dissociatively adsorbed on anatase TiO2 (001) surface and in experimental literature. NH3 in gas phase Molecularly ads. NH3 υasym HNH stret. 3527, 3528 υsym HNH stret. 3405 δasym HNH bend. 1622, 1623 δsym HNH υHNH scis. bend. 1007 – 3510, 3535 (3400c) 3385 (3350c) 1573, 1609 (1599a, 1600c) 1094 (1190a, 1225a, 1215b) – Dissociatively ads. NH3 3581, 3613 3463 – – 1448 (1480c, 1540d) Experimental values are in parentheses and particular surface of the TiO2 (anatase) has not been reported. a Amores et al. [33]. b Teramura et al. [34]. c Schneider et al. [35]. d Lietti et al. [36]. in general is given in Table 4. In this table, the change of frequencies upon adsorption of ammonia molecule is also reported as compared with ammonia molecule in the gas phase. As this table shows, asymmetric stretching frequency data of 3510 cm− 1, 3535 cm− 1 and a symmetric bending frequency value of 1094 cm− 1 for molecular ammonia adsorption have a certain reasonable shift when compared with the ammonia molecule in the gas phase (3527 cm− 1, 3528 cm− 1 and 1007 cm− 1). Similarly, for molecular ammonia adsorption asymmetric bending (1573 cm− 1 and 1609 cm− 1) and symmetric stretching vibration (3385 cm− 1) data also exhibit a shift from the single ammonia molecule values (1622 cm− 1, 1623 cm− 1 and 3405 cm− 1). Although the experimental values of 1599 cm− 1 [33], 1600 cm− 1 [35], and 3350 cm− 1 [35] are in agreement with the above calculated values, other experimental values such as asymmetric stretching frequency (3400 cm− 1), symmetric bending frequency (1190 cm− 1) and asymmetric bending frequency (1599 cm− 1, and 1600 cm− 1) data deviate significantly from the corresponding calculated values. The deviations from the experimental studies could be expected since the particular surface of the TiO2 (anatase) on which NH3 adsorbed was not determined in this real catalyst surface, and it is known that (001) surface is minority in anatase. For the dissociative ammonia adsorption, a vibration frequency value (1448 cm− 1) of scissoring mode of NH2 species is approximately comparable with the experimental value of 1480 cm− 1 [35]. The frequency calculations also validate another important point that equilibrium geometries have only positive frequencies. 4. Conclusions The molecular and dissociative adsorption of water and ammonia on anatase TiO2 (001) surface represented by a slab model are investigated by use of periodic DFT calculations. DFT calculations indicate that H2O molecule is dissociated on anatase TiO2 (001) slab surface by a non-activated process with an exothermic relative energy difference of 32 kcal/mol. The dissociated form of water on anatase TiO2 (001) surface is energetically more favored than the molecular form in accordance with other theoretical and experimental studies. However, on the same surface it is found that molecular NH3 adsorption energetically may not be considered too different from dissociative adsorption (−25 kcal/mol vs. − 20 kcal/mol). Acknowledgments This research was supported in part by TÜBİTAK through TR-Grid e-Infrastructure Project. TR-Grid systems are hosted by TÜBİTAK ULAKBİM and Middle East Technical University. Visit http://www. grid.org.tr for more information. 1033 References [1] U. 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