DOI: 10.1002/cctc.201300105 Explicit Roles of Au and TiO 2 in a Bifunctional Au/TiO 2 Catalyst for the Water-Gas Shift Reaction: A DFT Study A. Hussain,* [a, b] J. Gracia, [c] B. E. Nieuwenhuys, [c] and J. W. Niemantsverdriet [c] Introduction &&Please provide authors’ first names and academic titles, e.g., Prof./Dr.&&Abbreviations in addresses defined, please check.&& The water–gas shift reaction, CO + H 2 O$CO 2 + H 2 , is the industrially applied route to produce H 2 from synthesis gas. In large-scale applications, the reaction is performed with Fe 2 O 3 -based catalysts at high temperatures (580–725 K) and with Cu–ZnO and Cu–Al 2 O 3 catalysts at low temperatures (470–520 K). [1, 2] For smaller-scale applications, Au-based cata- lysts are of interest because they demonstrate high activity at relatively low temperatures. [3, 4] On Cu–metal oxide (MO x ) catalysts, the mechanism proceeds through the adsorption of CO on Cu sites, followed by CO 2 for- mation with partial reduction of MO x , which helps in the disso- ciation of H 2 O on the created O vacancies. [5–7] It is difficult to imagine how Au alone can catalyze the water–gas shift reaction (WGSR). Although CO adsorbs readily on the more open and thus the reactive surfaces of Au, [8, 9] H 2 O activation is an energetically difficult step on these surfa- ces, [10, 11] which need the presence of MO x with sufficient reac- tivity, such as TiO 2 , CeO 2 , or Fe 2 O 3 . [12–14] Results that signify the activity of Au/MO x in the WGSR consistently indicate a bifunc- tional operation mode. The deposition of Au nanoparticles on MO x produces active catalysts for the WGSR. [14] Inverse cata- lysts, in which Au serves as a support for MO x nanoparticles, are also highly active. [15] This suggests that in addition to the particle size, [16] oxidation state, [17] and morphology of the Au particles, a cooperative action of Au and active oxides controls the activity. [18–20] A consensus on a mechanism seems to exist, in which the oxide helps in the dissociation of H 2 O [21, 22] and CO adsorption on the Au surface [23] whereas subsequent steps are believed to occur at the metal/oxide interface. [6, 15, 24] Au is directly active for CO oxidation in the presence of OH groups. [25–28] Under re- action conditions, a spillover of OH groups from the oxide phase to Au arises as a natural dynamic transfer for the specific synergy of reducible oxides with noble metals. [29] We have used DFT to explore the feasibility of a mechanism for the WGSR on Au/TiO 2 systems based on their mutual coop- eration. We model unconnected Au and TiO 2 surfaces, and we check separately their reactivity in the following approach: If the adsorption and activation of the reactant molecules is fea- sible on both noble metal and metal oxide systems and if the diffusion of intermediates is feasible on specific surfaces, then the OH species react with CO adsorbed on the low-coordinat- ed Au atoms or with OH at the interface to produce active O. [30] Similarly, H atoms diffuse on the anatase surface and react with Au particles to combine with H 2 . Our approach is computationally advantageous: We can apply periodic condi- tions to both TiO 2 and Au surfaces without the need of intro- ducing small, probably artificial, and computationally demand- ing nanoclusters into our models. Special caution must be ex- ercised in applying this scheme while checking the diffusion of the species on the surface models. [a] A. Hussain Theoretical Plasma Physics Division Pakistan Institute of Nuclear Science and Technology (PINSTECH) P.O. Nilore, Islamabad&&postal code?&&(Pakistan) E-mail : [email protected][b] A. Hussain Nano Science & Catalysis Division National Centre for Physics (NCP) Quaid-i-Azam University Islamabad&&postal code?&&(Pakistan) [c] J. Gracia, B. E. Nieuwenhuys, J. W. Niemantsverdriet Schuit Institute of Catalysis Eindhoven University of Technology P.O. Box 513, 5600 MB Eindhoven (The Netherlands) The water–gas shift reaction has been investigated by using DFT applied to Au(1 0 0), stepped Au(3 1 0), and TiO 2 anatase (0 0 1) surfaces. The results show that neither Au nor TiO 2 can catalyze the reaction by themselves. Of CO, CO 2 ,H 2 O, and H 2 , only CO adsorbs with moderate adsorption energy at low-co- ordinated sites, whereas other molecules interact only weakly with Au. The activation of H 2 O is impossible on Au surfaces. However, H 2 O adsorbs dissociatively on the anatase (0 0 1) sur- face and the diffusion of OH and H is feasible. The energetic data indicate that the rest of the process is possible on the Au surface. Two mechanisms were investigated and compared for the water–gas shift reaction, with H 2 O dissociation on the TiO 2 surface and diffusion of OH and H on Au surfaces in common. The &&latter&& is, in principle, the rate-limiting step. The first mechanism occurs through the disproportionation of two OH groups on Au into H 2 O and an O atom. The latter reacts with CO. In the alternative mechanism, CO combines with OH to give a COOH intermediate, which subsequently reacts with another OH group to form CO 2 and H 2 O. Finally, H atoms re- combine on the Au surface to complete the catalytic cycle. # 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2013, 5, 1 – 11 1 These are not the final page numbers! ÞÞ CHEMCATCHEM FULL PAPERS
Transcript
DOI: 10.1002/cctc.201300105
Explicit Roles of Au and TiO2 in a Bifunctional Au/TiO2Catalyst for the Water-Gas Shift Reaction: A DFT StudyA. Hussain,*[a, b] J. Gracia,[c] B. E. Nieuwenhuys,[c] and J. W. Niemantsverdriet[c]
Introduction
&&Please provide authors’ first names and academic titles,e.g. , Prof./Dr.&&Abbreviations in addresses defined, pleasecheck.&& The water–gas shift reaction, CO + H2O$CO2 + H2,is the industrially applied route to produce H2 from synthesisgas. In large-scale applications, the reaction is performed withFe2O3-based catalysts at high temperatures (580–725 K) andwith Cu–ZnO and Cu–Al2O3 catalysts at low temperatures(470–520 K).[1, 2] For smaller-scale applications, Au-based cata-lysts are of interest because they demonstrate high activity atrelatively low temperatures.[3, 4]
On Cu–metal oxide (MOx) catalysts, the mechanism proceedsthrough the adsorption of CO on Cu sites, followed by CO2 for-mation with partial reduction of MOx, which helps in the disso-ciation of H2O on the created O vacancies.[5–7]
It is difficult to imagine how Au alone can catalyze thewater–gas shift reaction (WGSR). Although CO adsorbs readilyon the more open and thus the reactive surfaces of Au,[8, 9] H2Oactivation is an energetically difficult step on these surfa-ces,[10, 11] which need the presence of MOx with sufficient reac-tivity, such as TiO2, CeO2, or Fe2O3.[12–14] Results that signify the
activity of Au/MOx in the WGSR consistently indicate a bifunc-tional operation mode. The deposition of Au nanoparticles onMOx produces active catalysts for the WGSR.[14] Inverse cata-lysts, in which Au serves as a support for MOx nanoparticles,are also highly active.[15] This suggests that in addition to theparticle size,[16] oxidation state,[17] and morphology of the Auparticles, a cooperative action of Au and active oxides controlsthe activity.[18–20]
A consensus on a mechanism seems to exist, in which theoxide helps in the dissociation of H2O[21, 22] and CO adsorptionon the Au surface[23] whereas subsequent steps are believed tooccur at the metal/oxide interface.[6, 15, 24] Au is directly activefor CO oxidation in the presence of OH groups.[25–28] Under re-action conditions, a spillover of OH groups from the oxidephase to Au arises as a natural dynamic transfer for the specificsynergy of reducible oxides with noble metals.[29]
We have used DFT to explore the feasibility of a mechanismfor the WGSR on Au/TiO2 systems based on their mutual coop-eration. We model unconnected Au and TiO2 surfaces, and wecheck separately their reactivity in the following approach: Ifthe adsorption and activation of the reactant molecules is fea-sible on both noble metal and metal oxide systems and if thediffusion of intermediates is feasible on specific surfaces, thenthe OH species react with CO adsorbed on the low-coordinat-ed Au atoms or with OH at the interface to produce activeO.[30] Similarly, H atoms diffuse on the anatase surface andreact with Au particles to combine with H2. Our approach iscomputationally advantageous: We can apply periodic condi-tions to both TiO2 and Au surfaces without the need of intro-ducing small, probably artificial, and computationally demand-ing nanoclusters into our models. Special caution must be ex-ercised in applying this scheme while checking the diffusion ofthe species on the surface models.
[a] A. HussainTheoretical Plasma Physics DivisionPakistan Institute of Nuclear Science and Technology (PINSTECH)P.O. Nilore, Islamabad&&postal code?&&(Pakistan)E-mail : [email protected]
[b] A. HussainNano Science & Catalysis DivisionNational Centre for Physics (NCP)Quaid-i-Azam UniversityIslamabad&&postal code?&&(Pakistan)
[c] J. Gracia, B. E. Nieuwenhuys, J. W. NiemantsverdrietSchuit Institute of CatalysisEindhoven University of TechnologyP.O. Box 513, 5600 MB Eindhoven (The Netherlands)
The water–gas shift reaction has been investigated by usingDFT applied to Au(1 0 0), stepped Au(3 1 0), and TiO2 anatase(0 0 1) surfaces. The results show that neither Au nor TiO2 cancatalyze the reaction by themselves. Of CO, CO2, H2O, and H2,only CO adsorbs with moderate adsorption energy at low-co-ordinated sites, whereas other molecules interact only weaklywith Au. The activation of H2O is impossible on Au surfaces.However, H2O adsorbs dissociatively on the anatase (0 0 1) sur-face and the diffusion of OH and H is feasible. The energeticdata indicate that the rest of the process is possible on the Au
surface. Two mechanisms were investigated and compared forthe water–gas shift reaction, with H2O dissociation on the TiO2
surface and diffusion of OH and H on Au surfaces in common.The &&latter&& is, in principle, the rate-limiting step. Thefirst mechanism occurs through the disproportionation of twoOH groups on Au into H2O and an O atom. The latter reactswith CO. In the alternative mechanism, CO combines with OHto give a COOH intermediate, which subsequently reacts withanother OH group to form CO2 and H2O. Finally, H atoms re-combine on the Au surface to complete the catalytic cycle.
For Au, we have used the stepped (3 1 0) and (1 0 0) surfaces,which were used in our previous study on the adsorption ofCO and NO.[8] Our choice includes flat and stepped surfaces be-cause they are expected to be present on Au particles or be-cause of their significant activity.[31–33] MOx was modeled withthe anatase (0 0 1) surface, which is known for its surface reac-tivity.[34, 35] We report a catalytic cycle based on H2O dissociationthat yields OH and H groups on MOx, which then diffuse onthe Au surface, provided the temperature is high enough. Herethey react further; the overall reaction on Au is CO + 2 OH!CO2 + H2O, and H atoms on Au recombine to form H2.
Computational Details
We used the Vienna ab initio simulation package,[36] whichgives an iterative solution to the Kohn–Sham equations ina plane-wave basis set. Plane waves with a kinetic energy lessthan or equal to 400 eV were used in the calculations. The ex-change-correlation energy was calculated within the general-ized gradient approximation proposed by Perdew and co-workers.[37, 38] The electron–ion interactions for C, O, H, Ti, andAu atoms were described by using the projector augmentedwave method developed by Blochl.[39] This method is essential-ly a scheme that combines the accuracy of all-electron meth-ods and the computational simplicity of the pseudopotentialapproach.[40]
The relative positions of the Au atoms were fixed initially asthose in the bulk, with an optimized lattice parameter of4.18 � (the experimental value is 4.08 �).[41] The optimized lat-tice parameter was calculated by using the face-centered cubicunit cell, and its reciprocal space was sampled with a (15 � 15 �15) k-point grid generated automatically by using the Mon-khorst–Pack method.[42] Similarly, the relative positions of theMOx ions were fixed initially as those in the bulk, with opti-mized lattice parameters of 3.80 and 9.49 � (the experimentalvalues are 3.78 and 9.51 �).[43] The optimized lattice parameterwas calculated by using the tetragonal anatase unit cell, andits reciprocal space was sampled with a (8 � 8 � 3) k-point grid.A first-order Methfessel–Paxton smearing function witha width less than or equal to 0.1 eV was used to account forfractional occupancies.[44] Partial geometry optimizations wereperformed, such as the RMM-DIIS algorithm.[45] Geometry opti-mizations were stopped when all the forces were smaller than0.05 eV ��1. Vibrational frequencies for transition states werecalculated within the harmonic approximation. The adsorbate–surface coupling was neglected, and only the Hessian matrixof the adsorbate was calculated.[46] The climbing imagenudged elastic band method was used here to determine min-imum energy paths.[47]
Closed-shell CO, CO2, H2, and H2O molecules were optimizedat the G point by performing non-spin-polarized calculations.Spin-polarized calculations were performed for open-shell spe-cies, H, O, OH, OOH, and O2. In any case, we used a 10 � 12 �14 �3 orthorhombic unit cell. Non-spin-polarized calculationswere performed for adsorbed species on Au or anatase.
Surfaces were represented within the slab model approxima-tion, with a vacuum gap greater than 10 �. We used a five-
metal layer model with top two relaxed for Au(1 0 0) and a 11-layer model (approximately equal to four layers of low-indexsurfaces)[8] with top four relaxed for Au(3 1 0). For the Au(1 0 0)slab, we used a p(2�2) unit cell, with the reciprocal space sam-pled with (5 � 5 � 1) k-point meshes. For the Au(3 1 0) slab, (3 �9 � 1) k-point meshes were used for sampling the reciprocalspace for a p(2�1) unit cell.
On anatase (0 0 1), the p(2�2) unit cell contains four each ofTi-5c, O-2c, and O-3c atoms on the surface. Each of these Ti-5catoms is bonded to two raised 2c and two 3c lowered Oatoms in the [1 0 0] and [0 1 0] directions, respectively. Ourp(2�2) unit cell consists of 16 � TiO2 units, and the reciprocalspace was sampled with (4 � 4 � 1) k-point meshes. Upper halfTi and O atoms in the unit cell were relaxed. Detailed figuresof the TiO2 surface are mentioned in a previous paper.[48]
Results
Adsorption on Au(3 1 0) and Au(1 0 0)
CO
CO adsorbs weakly on close-packed surfaces of Au; however, ifthe surface contains steps, adsorption energies become appre-ciable. On Au(3 1 0), CO bound linearly through the C atom tothe low-coordinated Au atoms at the step yields the highestadsorption energy (�0.73 eV). The molecule tilts to a directionbetween the normals of the facets forming the step, as shownin Figure 1 b. In addition, the two bridge positions on theAu(1 0 0) terrace yield stable adsorption geometries, with ad-sorption energies a few tenths of an electronvolt lower thanon the step (�0.55 eV). For details, we refer to previous work.[8]
Experimental investigations also stress on the low coordinationof Au for optimum activity.[30, 49]
Figure 1. a–c) Top and side views of the most stable adsorption geometriesof H2O, CO, and OH on Au(3 1 0). Top view of d) H on Au(3 1 0), e) H onAu(1 0 0), and f) OH on Au(1 0 0), along with adsorption energies and themost important geometrical parameters.
The adsorption of CO2 was studied for the molecule with itsaxis parallel with respect to the Au(3 1 0) surface on various lo-cations. As expected, and also on the basis of a DFT study ofCO2 on the Au(3 2 1) surface,[50] the molecule interacts onlyweakly with the Au(3 1 0) surface and the adsorption energieswith respect to CO2 in the gas phase are approximately zero.Consequently, the adsorbate-to-surface distance is large (>3 �)and the C�O bond lengths are equal to those calculated forgaseous CO2 (1.18 �). Thus, if CO2 forms on the surface, then itdesorbs instantaneously (as discussed below).
H2
Several sites on Au(1 0 0) and stepped Au(3 1 0) surfaces wereconsidered for the adsorption of H2, with its molecular axis par-allel and perpendicular to the surface. In all geometries, theadsorption energy was zero (�0.02 eV), which indicates no in-teraction. If the molecule forms on Au, for example, throughrecombination of H atoms, then it desorbs instantaneously(see below).
H2O
H2O also interacts weakly with Au surfaces. The most favorableposition is above the step of the Au(3 1 0) surface (see Fig-ure 1 a), with an adsorption energy of �0.23 eV, which corre-sponds to physisorption rather than chemisorption. The O�Hbond lengths in the adsorbed molecule (0.97 �) are equal tothose in the free H2O molecule, which indicates that the activa-tion of the molecule does not occur. Similarly, low interactionenergies resulted from the calculations of H2O &&dissocia-tion?&& on the Au(1 0 0) surface (�0.16 eV) and on the Au/Au(1 0 0) system (�0.22 eV), in which an additional Au atom isplaced in every fourth hollow site of a (2 � 2) unit cell on theAu(1 0 0) surface. These results are in good agreement withprevious studies of H2O adsorption on Au surfaces.[51, 52]
H2O dissociation is impossible, as any activation of an OHbond in the molecule costs more energy than the small ad-sorption energy of �0.23 eV, which causes H2O to desorb fromthe surface. The lowest activation energy found for the dissoci-ation of H2O is 1.34 eV, while the reaction is endothermic by atleast 1.0 eV, which depends on the configuration of OH and Hafter the dissociation. Our conclusion that H2O dissociation isnot feasible on Au surfaces is in agreement with that reportedin the literature: Wang et al.[51] reported an even higher barrierof 2.24 eV and an endothermic reaction energy of 1.77 eV forH2O dissociation on Au(111), whereas Liu and Rodriguez[52]
found an activation energy of 1.53 eV for H2O dissociation onAu(1 0 0). A smaller barrier of 0.59 eV has been computed (byusing DFT) on a Au4 cluster on CeO2(111) for H2O dissociation;nonetheless, activation energy is at least 0.1–0.2 eV higherthan the adsorption energy.[53] Experimental studies also indi-cate that H2O is not activated on Au unless predissociatedatomic O is present on the surface.[10, 54]
OH groups
OH adsorbs with binding energies of �2.31 and �2.11 eV onAu(3 1 0) and Au(1 0 0) surfaces, respectively (see Figure 1 c andf). These bridging OH groups correspond to the most stableconfigurations; however, adsorption on hollow sites is onlymarginally weaker. The diffusion of OH groups on the surfaceis possible without substantial barriers of less than 0.2 eV. Ad-sorption energies of �1.81 and �1.74 eV have been reportedfor OH on the face-centered cubic and bridge positions ofAu(2 11) and Au(111) surfaces, respectively.[55, 56]
H atoms
H atoms can occur on several sites of Au(3 1 0) and Au(1 0 0)surfaces, with a preference for bridging H (see Figure 1 d ande). Typical values for the adsorption energy are �2.0 to�2.3 eV. Notably, H atoms can diffuse freely on Au surfaces be-cause the differences between the various adsorption geome-tries are minimal. Our highest Eads of �2.24 eV on Au(1 0 0) is inagreement with the value of �2.25 eV reported in the litera-ture.[56]
O atoms
By ignoring for the moment the origin of O atoms (discussedbelow), we observe that O atoms bind to Au(3 1 0) andAu(1 0 0) surfaces with adsorption energies of �3.32 and�3.14 eV, respectively. On the Au(3 1 0) surface, the bridge onthe top of the step is the favored position; on the Au(1 0 0) sur-face, the hollow site is favored. The bridge positions yieldalmost equal adsorption energies (�3.14 and �3.10 eV).
Adsorption on anatase (0 0 1)
In an earlier paper, we described adsorption and surface reac-tions of H2O and H2O-derived species on anatase (0 0 1) indetail.[48] Here, we give a brief overview of the adsorbates in-volved in the WGSR. The most important species are shown inFigure 2.
Adsorption and diffusion of OH
The highest adsorption energy for OH groups (�1.85 eV) isfound for the tilted coordination of isolated OH on the top ofthe Ti-5c site. Adsorption on the O-3c position yields anenergy of �1.01 eV for the tilted geometry. However, thismode possesses one imaginary frequency and is thus a transi-tion state. Normal mode analysis indicates that the OH movesin the [0 1 0] direction. Placing OH on the hollow (+ 0.07 eV) orabove the O-2c (+ 0.39 eV) site is endothermic. A higher cover-age (two tilted OH groups per unit cell) on TiO2(0 0 1) decreasesthe adsorption energy per OH group from �1.85 to �1.56 eV.
With these adsorption energies, the diffusion of OH on theTiO2 anatase (0 0 1) surface is expected to be highly anisotropic(see Figure 3). Moving OH from the Ti-5c site in the [0 1 0] di-rection to the O-3c site, which is the transition state for diffu-
sion, yields a substantial barrier of 0.84 eV; however, the barrierin the [1 0 0] direction toward the O-2c site is much larger andof the order of 2.2 eV. Hence, the diffusion of OH occurs alongthe Ti–O-3c–Ti rows in the [0 1 0] direction.
Adsorption and dissociation of H2O
Anatase (0 0 1) readily dissociates H2O without any barrier intotwo nonequivalent tilted OH groups terminally bound to adja-cent Ti sites, whereby the latter O atom originates from a latticeO-2c atom (see Figure 2). The reaction is exothermic, and theproducts are stabilized by �1.46 eV with respect to H2O in thegas phase, which is in good agreement with the literature.[57, 58]
We also investigate the dissociation in the case of two H2Omolecules per (2 � 2) unit cell by positioning the H2O moleculeson adjacent Ti-5c sites in the [0 1 0] direction and find sponta-neous dissociation with an average adsorption energy of�1.28 eV per molecule. Notably, the increase in the adsorptionenergy of the terminal OH groups in the presence of atomic Hjumps to �3.79 eV for the resulting geometry after the dissoci-ation of one H2O molecule.
Considering the diffusion of OH groups in the [0 1 0] direc-tion from the dissociated H2O molecule, which is similar to thesituation of OH and H adsorbed on O-3c and O-2c sites, re-spectively, the mobility of OH groups is slowed down becauseof surface reduction and the H bond (see Figure 2 b and c). Inthis case, the diffusion barrier is 0.95 eV, which is 0.11 eVhigher than that for the isolated OH. By using the rule ofthumb that a barrier of 1.0 eV corresponds to a reaction tem-perature of approximately 400 K, the diffusion barriers caneasily be overcome at typical operating temperatures of ap-proximately 475 K for the WGSR on Au.
Adsorption and diffusion of H
We observe that the H atom bound on the bridging O (O-2c)atom in the tilted mode at 828 with respect to the surfacenormal yields the highest adsorption energy (�3.15 eV). Thisvalue agrees well with that of the literature.[59] Whether werefer to the situations shown in Figure 2 d and e as an ad-sorbed H atom or as a bridging OH group is a matter of se-mantics ; however, we prefer the former terminology. The Hatom can also adsorb on the O-3c (Eads =�2.48 eV) or thehollow (Eads =�2.3 eV) site. Coadsorption of two or four Hatoms in the unit cell decreases the adsorption energies per Hatom to �2.86 and �2.24 eV, respectively.
On the basis of the adsorption energies on different posi-tions, the energetically favorable route for H hopping is fromthe O-2c atom via the nearby O-3c atom to the next O-2catom, which would correspond to an endothermic step of ap-proximately 0.7 eV and an overall diffusion barrier of approxi-mately 0.9 eV. In addition, diffusion from the O-3c site via thehollow site seems possible as the adsorption energies on thesesites differ by less than 0.2 eV. This pathway would enable theH atom to diffuse over long distances in the [1 0 0] direction.Diffusion in the [0 1 0] direction toward the O-2c site is ener-getically less favorable.
Adsorption of CO
Adsorption of CO has been investigated on the top Ti-5c, O-3c,O-2c, and hollow sites of TiO2. In the most favorable configura-
Figure 2. Top: Dissociative adsorption of H2O on anatase (0 0 1): a) H2O disso-ciates without activation energy into a OH group (labeled 1) and a H atomand breaks a Ti�O bond to create a second OH group (labeled 2); b) if thefirst OH group diffuses away, the Ti�O bond is restored, resulting in c) alinear OH on a Ti site and a bridging OH between two Ti atoms (which canalso be seen as an adsorbed H atom on a bridging O atom). Bottom: d–f) Three geometries of adsorbed H atoms.
Figure 3. Anisotropy of OH diffusion on the anatase (0 0 1) surface. The acti-vation energy for OH diffusion in the [0 1 0] direction along the Ti–O-3c–Tirows is approximately 0.84 eV. Diffusion in the [1 0 0] direction, however, isenergetically unfavorable.
tion, the C atom is on the top of the Ti atom with an adsorp-tion energy of �0.55 eV at a coverage of 1=4 ML (1 ML = 4 COmolecules in the unit cell). Adsorption on other sites is sub-stantially weaker. In a DFT study of CO oxidation on TiO2(0 0 1),a lower adsorption energy of �0.23 eV has been reported ata coverage of 1 ML.[60] The presence of a coadsorbed H atom(bridging OH) or a terminal OH decreases the CO adsorptionenergy to �0.2 and �0.36 eV, respectively. The restricted ca-pacity of anatase (0 0 1) to adsorb CO limits CO oxidation onthe TiO2 surface because CO desorbs preferentially. We esti-mate CO2 formation and anatase (0 0 1) reduction through theformation of an O-2c vacancy. The process is endothermic by1.0 eV, a value that clearly exceeds the CO adsorption energyon TiO2 and Au surfaces we studied.
Comparison of adsorption on TiO2 anatase and on Au
A comparison of adsorption energies and diffusion barriers ofsome key species on Au and on TiO2 is presented in Table 1.
Isolated OH groups and O atoms adsorb more strongly onAu(3 1 0) and Au(1 0 0) surfaces than on the TiO2 surface. Rea-sonable differences appear from the dissociative adsorption ofH2O on TiO2, which is equivalent to OH + H coadsorptionstates. Cooperative effects increase the adsorption energies ofindividual OH and H atoms on anatase (0 0 1) to �3.79 and�5.09 eV, respectively. Notably, H2O dissociation on Au(3 1 0) isendothermic by 1.0 eV and exothermic by �1.46 eV on ana-tase. For CO the difference is smaller, though it adsorbs strong-ly on the stepped Au (3 1 0) surface. For isolated H atoms,bonding is stronger on the anatase (0 0 1) surface; however, ifthe coverage increases, the difference becomes smaller.
Because of the existence of moderate activation barriers forthe diffusion of OH and H atoms on anatase surfaces, whichenables sufficient mobility under reaction conditions, it is likelythat both OH and H atoms can diffuse on the entire MOx sur-face. In addition, the coadsorption energies for OH and H
atoms indicate that the movement of OH groups from MOx tothe Au particles is disfavored thermodynamically by approxi-mately 1.5 eV for the activation of a single H2O molecule or by1.3 eV for two. In a recent experimental study,[61] an active roleof the support (TiO2) is explained, in which it has been con-cluded that the support participates directly in activating H2Omolecules. The supports with higher catalytic rates bind H2O/OH species more strongly and have a higher coverage of thosespecies, which is in agreement with our calculations. In addi-tion, from operando FTIR experiments, it has been determinedthat the active Au sites are metallic in nature.[61] In the nextsections, we consider the reactions available on Au.
Elementary reactions
Formation of H2
Energy diagrams for the recombination of two H atoms onAu(3 1 0) and Au(1 0 0) are shown in Figure 4. From these dia-
grams, we first calculate the adsorption energy of two Hatoms in several coadsorbed configurations. Values are in therange of �3.87 to �4.11 eV (compared with �4.0 to �4.6 eVfor two isolated H atoms), which indicate that interactions be-tween the two H atoms are weakly repulsive. These coadsorp-tion energies agree well with the results reported by Nørskovand co-workers.[55] For the reaction on the Au(3 1 0) surface theactivation barrier was lowest (0.48 eV), whereas the reaction onthe Au(1 0 0) surface has an activation energy of 0.75 eV (seeTable 2). The reaction is mildly exothermic, and, as mentionedearlier, the H2 molecule does not interact with the surface andis expected to desorb instantaneously. We conclude that onceH atoms are available on Au surfaces (e.g. , as a result of H2Odissociation on MOx and subsequent diffusion at the periphery
Table 1. Comparison of adsorption energies and diffusion barriers on goldand anatase.&&column headings ok?&&
[a] Molecules of adsorbate per (2 � 2) unit cell.&&ok?&&Figure 4. Initial (IS), transition (TS), and final (FS) states of H2 formation fromcoadsorbed H atoms on Au(3 1 0) and Au(1 0 0) surfaces. The energy axis onthe left shows Eads values with respect to two gas-phase H atoms, whereasthe zero of the right axis corresponds to H2 in the gas phase.
or on the Au particles), H2 formation is relatively facile on theAu(1 0 0) surface and especially on the stepped Au(3 1 0) sur-face.
However, H2 formation on the anatase (0 0 1) surface leadsto endothermic reactions with high activation barriers of theorder of 2 eV. Only at high coverages and with H atoms on en-ergetically less favored adsorption sites, such as one H atomon the Ti-5c site and the other on the O-2c site, activationenergy decreases to the values of the order of 1.2 eV. However,it is difficult to understand how such a high coverage couldresult from H2O dissociation, as also noted in other theoreticaland experimental studies.[57, 58] We therefore conclude that H2
formation from atomic H is energetically feasible on the Ausurface but difficult—though not impossible—on TiO2.
Reaction of OH groups on Au
Au has a high activity for CO oxidation in alkaline media, andOH adsorption leads to CO oxidation in solution.[62] The factthat Au/TiO2 catalyzes CO oxidation in the vapor phase maywell come from a similar promotional effect of OH ions. In ad-dition, the effect that calcination has on the catalytic activity ofAu may be related to the OH density on MOx.
[63] Direct O�Hbond breaking on Au surfaces is difficult and thermodynami-cally unfavorable. As shown in Figure 5, OH adsorbed at thestep of the Au(3 1 0) surface faces a barrier of 1.8 eV for O�Hbond breaking and the overall reaction is endothermic. Similar-ly, high activation barriers and endothermicities have been re-ported for OH on Cu.[1] Hence,the direct disproportionation ofadsorbed OH groups is energeti-cally unfavorable.
Next, we consider dispropor-tionation of two OH groups,which is inherently more favora-ble as it creates H2O. The lowerpart of Figure 5 illustrates thefeasibility of this route. Notably,the formation of two O atomstogether with H2 is not favora-ble.[22]
In the most favorable OH +
OH coadsorption configuration,which is shown in the initialstate of Figure 6, both OHgroups are tilted. One OH isbound at the step and the other
near the hollow site of the Au(1 0 0) terrace. This coadsorbedstate is at �4.0 eV, which is slightly repulsive compared to twoOH groups at an infinite separation (�4.5 eV). However, thiscoadsorption energy is �0.35 eV more favorable than the onereported for Au(111).[56] The activation barrier for the reactionto H2O + O is 0.1 eV and should be added to the energyneeded to bring the two OH groups together. The resultingbarrier amounts to 0.6 eV. In the transition state shown inFigure 6, the OH group at the step has moved toward the OHgroup on the terrace. This configuration reduces the distancebetween the H atom of the former and the O atom of the OHgroup on the terrace from 2.9 to 1.85 � in the transition state.H2O forms and leaves an O atom near the step. The reaction isslightly exothermic (�0.06 eV), which implies that the reversereaction, that is, H2O dissociation on an O-covered surface,should not be difficult. An experimental study of H2O adsorp-
Table 2. &&Activation barrier (Ea), reaction energy (DE), H�H bondlength (dH�H), and imaginary frequency in transition states (n)&& onAu(1 0 0) and Au(3 1 0) surfaces for H2 formation.
Figure 5. Reaction pathways for atomic O formation on Au(3 1 0). Top: Directdisproportionation of OH is energetically uphill. Bottom: Disproportionationof two OH groups into H2O and an adsorbed O atom is energetically favora-ble. Adsorption energies are given with respect to the gas-phase OH andthe empty Au(3 1 0) slab.
Figure 6. Potential energy diagram for the reaction between OH and CO on Au(3 1 0) on the basis of the forma-tion of atomic O as an intermediate. Zero energy corresponds to Au(3 1 0) + coadsorbed OH + gas-phase CO. Spe-cies within parentheses are in proximity.
tion on O/Au(111) indicates the formation of OH groups fol-lowed by the subsequent formation and desorption of H2O athigher temperatures.[54] Our result is also in good qualitativeagreement with that of a previous study on Au(111), in whichthe reaction proceeded even without a barrier.[56]
We conclude that the disproportionation of OH groups pro-vides a straightforward and energetically feasible pathway togenerate O atoms on the Au(3 1 0) surface. Date et al. proposeda reaction of OH groups at the periphery of Au nanoparticleson MOx.
[26]
Reaction of CO and O on Au
The reaction path between ad-sorbed CO and atomic O on theAu(3 1 0) surface is illustrated inFigure 6. Because a significantvariation in the energies of dif-ferent CO + O coadsorbate sys-tems is not found, we calculatethe activation barrier for CO oxi-dation for the arrangement withCO on its most preferred posi-tion at the step and O bound tothe (1 0 0) hollow site on the ter-race. In the transition state, theO atom remains in the hollowsite while CO moves toward it toform CO2. The activation barrierfound is quite small (0.04 eV). Liuet al.[64] reported a barrier of0.25 eV on Au(2 2 1) and 0.68 eVon Au(2 11). The lowest barrierreported in Ref. [50] on Au(3 2 1)is 0.01 eV, which is in agreementwith our calculations. The reac-tion is highly exothermic by�2.85 eV. This is 0.25 eV morenegative than that reported inRef. [50] . Because CO2 interaction with this surface is weak, itdesorbs immediately after its formation, as discussed above.
Reactions of CO and OH
The migration of OH groups from TiO2 to Au seems difficultand may well be the rate-limiting step for the entire reaction.We therefore expect that the coverage of OH on Au is low andthe reaction of two OH groups is less likely. Alternative ways toform CO2 are conceivable as well. First, the potential reactionof CO with OH groups on the TiO2 anatase (0 0 1) surface wasconsidered. Notably, COOH is not stable on this surface. Themost stable coadsorption configuration of CO and OH de-creased the adsorption energy of CO from �0.55 to �0.36 eV.For the reaction of CO with OH on anatase (0 0 1), with COmoving toward OH, CO was destabilized further and wasfound to desorb without the reaction. Second, the reaction ofOH with CO on the Au surface is feasible. In principle, it can
produce HCOO, COOH, and CO3 species as key intermedi-ates.[54] Although HCOO and CO3 species have been detectedin many experiments studying the WGSR on metal/oxide cata-lysts,[65] the COOH intermediate is favored in DFT and experi-mental studies for the WGSR on Au surfaces.[10, 52, 54] In a photo-emission and IR spectroscopy study,[54] because of CO and OHinteractions on Au(111), a COOH intermediate with short life-time was formed but HCOO and CO3 intermediates were notdetected.
In our calculations of the CO and OH interaction onAu(3 1 0), COOH appeared as a stable species, with a barrier of0.18 eV and a reaction enthalpy of �0.91 eV (see Figure 7). Inthe transition state, the C atom of CO and the O atom of OH
are 2.31 � apart and the bond has an imaginary frequency of161i cm�1. An activation barrier of 0.32 eV has been reportedfor this reaction on Au(111) by Ojifinni et al.[10] Hence, COOHformation appears feasible on Au. For the decomposition ofCOOH into CO2 and H, the most stable coadsorbed configura-tion of CO2 and H is stabilized by 0.19 eV. Hence, COOH forma-tion and decomposition are exothermic and thermodynamical-ly favorable. However, the decomposition of COOH is not facileand requires a barrier of 1.20 eV. A lower activation barrier of0.93 eV for COOH decomposition has been reported onAu(111).[10] In the transition state, the O atom of the COOHcomplex and the H atom are at a distance of 2.65 � and thebond has an imaginary frequency of 89i cm�1. CO2 desorbsupon formation and H2 forms as discussed above.
Alternatively, the COOH fragment may also react witha second OH group to form CO2 and H2O in a facile reaction(see Figure 7). For this highly exothermic reaction, a transition
Figure 7. Reaction profile for the carboxyl mechanism on Au(3 1 0). COOH formation and decomposition into CO2,H, and reaction to CO2 and H2O are presented. Zero energy is as in Figure 6.
state was not found. For Au(111), a barrier of 0.28 eV was re-ported for this step.[10]
Discussion
Mechanisms for the WGSR on precious metals and metaloxides can be classified into three types based on the reactionintermediates; these are summarized in Table 3.[1, 10, 66, 67] In theredox mechanism, active O atoms are produced through thecomplete dissociation of H2O into atomic O and H or throughthe disproportionation of OH groups. In the formate mecha-nism, the process involves OH groups from H2O dissociationcombining with CO to form a HCOO intermediate, which thendecomposes into CO2 and H.[68] Similarly, in the carboxyl mech-anism, CO reacts directly with OH to form the COOH inter-mediate, which decomposes into CO2 and H or reacts furtherwith another OH group to produce H2O and CO2.[69] Whetherthese reaction steps occur on the noble metal, on the activeMOx, or at their interface is a matter of debate.
Adsorption of CO and H2O, dissociation of H2O, and forma-tion of H2 are important steps in the WGSR. As it is unlikelythat H2O dissociation occurs on Au,[10, 11] MOx activates H2O.[61] Akey question is whether MOx is reduced directly by CO to formCO2 or whether an O-containing species diffuses from MOx toAu through the Au/TiO2 interface, at which the reaction withCO follows. The advantage of the two-step mechanism is thatonce active O is present on the noble metal, subsequent reac-tions have low activation barriers.
Bunluesin et al. studied the WGSR on CeO2-supported Pt, Pd,and Rh and suggested that CO adsorbed on the preciousmetal is oxidized with CeO2, which in turn is oxidized byH2O.[70]
Inverse model catalysts consisting of TiO2 or CeO2 nanoparti-cles covering 20–30 % of the surface of Au(111) demonstratedhigh activity for the WGSR.[15] The reaction occurs via H2O dis-sociation on the O vacancies of MOx and CO adsorption on Ausites located nearby. Subsequent reaction steps occur at themetal/oxide interface. Our major concern with these mecha-nisms is the lower adsorption energy of CO compared to thehigher activation barrier for the direct reduction of MOx. Weshould always consider that CO oxidation and desorption arein competition. We determine that the formation of O vacan-cies and CO2 creation on anatase (0 0 1) is endothermic by1.0 eV and considerably higher than the CO adsorption ener-gies on different Au surfaces, which are �0.75 eV at most.Hence, CO is expected to desorb before it reacts. In an experi-mental study, Weststrate et al. demonstrated that CO desorp-
tion on Au/CeO2 catalysts occurs between 100 and250 K,[9] which supports the view that CO adsorptionenergies hardly exceed 1.0 eV.[71] Rodriguez et al. re-ported a theoretical barrier for CO oxidation, and alsorestoration of O vacancies, that exceeds the CO ad-sorption values on Au supported on TiO2.[14, 15] In con-trast, CO adsorbs sufficiently strongly for subsequentoxidation if active O is present on Au.[53]
Hence, we propose that the high catalytic activityof Au/MOx in various oxidation reactions is caused by
the presence of active O species, most probably OH groups,on the Au surface while the role of MOx is to provide suchactive O species. These OH groups either disproportionate onAu to yield O atoms or react directly with CO to form CO2 andH2O.
H2 is formed from adsorbed H atoms on TiO2, which diffuseat the interface and onto Au; there they recombine to H2 witha modest activation energy of 0.48 eV. The formation of H2 onTiO2 is not feasible, because it would incur an activation barrierof the order of 2.0 eV.[48]
Conclusions
The possible roles of Au and TiO2 in the water–gas shift reac-tion were examined by using DFT. Although CO adsorbs witha substantial strength on the stepped Au surface, H2O interactsonly weakly and cannot dissociate on Au. In contrast, anatase(0 0 1) dissociates H2O spontaneously with a dissociative heatof adsorption of �1.46 eV. High activation barriers for H2 for-mation on the MOx surface (2.0 eV) suggest that H atoms mustfirst diffuse &&from&& the Au/TiO2 interface. On Au surfa-ces, recombination is thermodynamically favorable witha small barrier of 0.48 eV. The diffusion of the dissociationproducts of H2O—coadsorbed OH + H—on the TiO2 surface tothe metal has a substantial barrier of the order of 1.0 eV.Active MOx provides a route with a reduced barrier for H2O dis-sociation, which is not available on Au. Complementarily, Auprovides sufficiently strong CO binding and moderate CO oxi-dation barriers. A spillover of H and OH groups appears as ratelimiting, which also indicates that the reaction occurs preferen-tially at the periphery of the Au/TiO2 interface.
Two mechanisms were investigated and compared for COoxidation on Au in the water–gas shift reaction. In the firstmechanism, two OH groups form H2O and an adsorbed Oatom; the latter reacts with CO without a barrier to form CO2,which desorbs instantaneously. The second mechanism in-volves the reaction of CO and OH on the Au surface to formCOOH. In this carboxyl mechanism, an additional OH groupreacts with COOH to form CO2 and H2O. In both cases, the cat-alytic cycle corresponds to the overall reaction 2 H2O + CO!H2O + CO2 + H2, in which both aforementioned mechanismsoccur interchangeably. The whole process is illustrated inFigure 8.
Table 3. Proposed mechanisms on precious metals and metal oxides for the water–gas shift reaction.
We thank NCF&&please write NCF in full&& (grant no. SG-06-2-202) for computer time at the Huygens Super Computer. A.H.acknowledges financial support from the Pakistan Higher Educa-tion Commission to enable his stay at the Eindhoven Universityof Technology.
[1] K.-O. Hinrichsen, K. Kochloefl, M. Muhler, in, Handbook of Heterogene-ous Catalysis, Vol. 6, 2nd ed. (Eds. : G. Ertl, H. Knçzinger, F. Sch�th, J. Weit-kamp), Wiley-VCH, Weinheim, 2008, p. 2905.
[2] J. H. Lin, V. V. Guliants, ChemCatChem 2011, 3, 1426 – 1430.[3] J. A. Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Perez, Angew. Chem. 2007,
119, 1351 – 1354; Angew. Chem. Int. Ed. 2007, 46, 1329 – 1332.[4] M. Haruta, CATTECH 2002, 6, 102 – 115.[5] J. A. Rodriguez, J. Graciani, J. Evans, J. B. Park, F. Yang, D. Stacchiola,
S. D. Senanayake, S. G. Ma, M. Perez, P. Liu, J. F. Sanz, J. Hrbek, Angew.Chem. 2009, 121, 8191 – 8194; Angew. Chem. Int. Ed. 2009, 48, 8047 –8050.
[6] J. B. Park, J. Graciani, J. Evans, D. Stacchiola, S. D. Senanayake, L. Barrio,P. Liu, J. F. Sanz, J. Hrbek, J. A. Rodriguez, J. Am. Chem. Soc. 2010, 132,356 – 363.
[7] I. Atake, K. Nishida, D. Li, T. Shishido, Y. Oumi, T. Sano, K. Takehira, J. Mol.Catal. A-Chem. 2007, 275, 130 – 138.
[8] A. Hussain, D. Curulla Ferre, J. Gracia, B. E. Nieuwenhuys, J. W. Niemants-verdriet, Surf. Sci. 2009, 603, 2734 – 2741.
[9] C. J. Weststrate, A. Resta, R. Westerstrom, E. Lundgren, A. Mikkelsen,J. N. Andersen, J. Phys. Chem. C 2008, 112, 6900 – 6906.
[10] R. A. Ojifinni, N. S. Froemming, J. Gong, M. Pan, T. S. Kim, J. M. White, G.Henkelman, C. B. Mullins, J. Am. Chem. Soc. 2008, 130, 6801 – 6812.
[11] M. A. Henderson, Surf. Sci. Rep. 2002, 46, 1 – 308.[12] D. Cameron, R. Holliday, D. Thompson, J. Power Sources 2003, 118, 298 –
303.[13] H. Sakurai, A. Ueda, T. Kobayashi, M. Haruta, Chem. Commun. 1997,
271 – 272.[14] J. A. Rodriguez, J. Evans, J. Graciani, J. B. Park, P. Liu, J. Hrbek, J. F. Sanz,
J. Phys. Chem. C 2009, 113, 7364 – 7370.[15] J. A. Rodriguez, S. Ma, P. Liu, J. Hrbek, J. Evans, M. Perez, Science 2007,
318, 1757 – 1760.[16] S. H. Overbury, V. Schwartz, D. R. Mullim, W. F. Yan, S. Dai, J. Catal. 2006,
241, 56 – 65.[17] J. C. Fierro-Gonzalez, B. C. Gates, Catal. Today 2007, 122, 201 – 210.[18] X. Lin, N. Nilius, M. Sterrer, P. Koskinen, H. Hakkinen, H. J. Freund, Phys.
Rev. B 2010, 81, &&page numbers?&&.[19] G. J. Hutchings, Gold Bull. 2009, 42, 260 – 266.[20] D. Widmann, Y. Liu, F. Schuth, R. J. Behm, J. Catal. 2010, 276, 292 – 305.[21] F. Romero-Sarria, A. Penkova, L. M. Martinez, M. A. Centeno, K. Hadjiiva-
nov, J. A. Odriozola, Appl. Catal. B 2008, 84, 119 – 124.[22] Y. Chen, J. Cheng, P. Hu, H. F. Wang, Surf. Sci. 2008, 602, 2828 – 2834.[23] F. Vindigni, M. Manzoli, A. Chiorino, F. Boccuzzi, Gold Bull. 2009, 42,
106 – 112.[24] W. L. Deng, C. Carpenter, N. Yi, M. Flytzani-Stephanopoulos, Top. Catal.
2007, 44, 199 – 208.[25] G. C. Bond, D. T. Thompson, Gold Bull. 2000, 33, 41 – 51.[26] M. Dat�, M. Okumura, S. Tsubota, M. Haruta, Angew. Chem. 2004, 116,
2181 – 2184; Angew. Chem. Int. Ed. 2004, 43, 2129 – 2132.[27] S. T. Daniells, A. R. Overweg, M. Makkee, J. A. Moulijn, J. Catal. 2005,
230, 52 – 65.[28] T. Tabakova, V. Idakiev, K. Tenchev, F. Boccuzzi, M. Manzoli, A. Chiorino,
Appl. Catal. B 2006, 63, 94 – 103.[29] J. A. Jaksic, N. V. Krstajic, L. M. Vracar, S. G. Neophytides, D. Labou, P. Fa-
laras, M. M. Jaksic, Electrochim. Acta 2007, 53, 349 – 361.[30] M. Shekhar, J. Wang, W. S. Lee, M. C. Akatay, E. A. Stach, W. N. Delgass,
F. H. Ribeiro, J. Catal. 2012, 293, 94 – 102.[31] L. M. Molina, B. Hammer, Phys. Rev. B 2004, 69, 155424.[32] C. J. Weststrate, E. Lundgren, J. N. Andersen, E. D. L. Rienks, A. C. Gluhoi,
J. W. Bakker, I. M. N. Groot, B. E. Nieuwenhuys, Surf. Sci. 2009, 603,2152 – 2157.
[33] C. P. Vinod, J. W. Niemantsverdriet, B. E. Nieuwenhuys, Phys. Chem.Chem. Phys. 2005, 7, 1824 – 1829.
[34] U. Diebold, Surf. Sci. Rep. 2003, 48, 53 – 229.[35] X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie, L. S. Zheng, J. Am. Chem. Soc.
2009, 131, 3152 – 3153.[36] G. Kresse, J. Hafner, Phys. Rev. B 1993, 47, 558 – 561.[37] J. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244 – 13249.[38] J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson,
D. J. Singh, C. Fiolhais, Phys. Rev. B 1992, 46, 6671 – 6687.[39] P. E. Blçchl, Phys. Rev. B 1994, 50, 17953 – 17979.[40] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758 – 1775.[41] Structure Data of Elements and Intermetallic Phases, Landolt-Bornstein,
New Series, Vol. IIIb, Springer, Berlin, 1971.[42] H. J. Monkhorst, J. D. Pack, Phys. Rev. B 1976, 13, 5188 – 5192.[43] M. Horn, C. F. Schwerdt, E. P. Meagher, Z. Kristallogr. 1972, 136, 273 –
281.[44] M. Methfessel, A. T. Paxton, Phys. Rev. B 1989, 40, 3616 – 3621.[45] P. Pulay, Chem. Phys. Lett. 1980, 73, 393 – 398.[46] J. D. Head, Int. J. Quantum Chem. 1997, 65, 827 – 838.[47] G. Henkelman, B. P. Uberuaga, H. Jonsson, J. Chem. Phys. 2000, 113,
9901 – 9904.[48] A. Hussain, J. Gracia, B. E. Nieuwenhuys, J. W. Niemantsverdriet, Chem-
PhysChem 2010, 11, 2375 – 2382.[49] J. Huang, L. C. Wang, Y. M. Liu, Y. Cao, H. Y. He, K. N. Fan, Appl. Catal. B
2011, 101, 560 – 569.
Figure 8. The potential energy diagram summarizing the whole process ofthe water–gas shift reaction for unconnected Au(3 1 0) and TiO2(0 0 1) surfa-ces. Both the redox and carboxyl mechanisms and the evolution of H2 onAu(3 1 0) are incorporated. Notably, steps A and B are energetically based&&favorable?&& routes. In practice, periphery would also play a role andthese barriers could be different. Species shown in parentheses representthe coadsorption of these species, which demonstrate a repulsive effect oncoming closer. Zero level corresponds to gas-phase H2O molecules.
[50] J. L. C. Fajin, M. N. D. S. Cordeiro, J. R. B. Gomes, J. Phys. Chem. C 2008,112, 17291 – 17302.
[51] G. C. Wang, S. X. Tao, X. H. Bu, J. Catal. 2006, 244, 10 – 16.[52] P. Liu, J. A. Rodriguez, J. Chem. Phys. 2007, 126, 164705.[53] Z. P. Liu, S. J. Jenkins, D. A. King, Phys. Rev. Lett. 2005, 94, 196102.[54] S. D. Senanayake, D. Stacchiola, P. Liu, C. B. Mullins, J. Hrbek, J. �. A. Ro-
driguez, J. Phys. Chem. C 2009, 113, 19536 – 19544.[55] T. Bligaard, J. K. Nørskov, S. Dahl, J. Matthiesen, C. H. Christensen, J. Se-
hested, J. Catal. 2004, 224, 206 – 217.[56] S. Kandoi, A. A. Gokhale, L. C. Grabow, J. A. Dumesic, M. Mavrikakis,
Catal. Lett. 2004, 93, 93 – 100.[57] A. Vittadini, A. Selloni, F. P. Rotzinger, M. Gratzel, Phys. Rev. Lett. 1998,
81, 2954 – 2957.[58] J. Blomquist, L. E. Walle, P. Uvdal, A. Borg, A. Sandell, J. Phys. Chem. C
2008, 112, 16616 – 16621.[59] M. Menetrey, A. Markovits, C. Minot, Surf. Sci. 2003, 524, 49 – 62.[60] B. Mguig, M. Calatayud, C. Minot, THEOCHEM 2004, 709, 73 – 78.[61] M. Shekhar, J. Wang, W. S. Lee, W. D. Williams, S. M. Kim, E. A. Stach, J. T.
Miller, W. N. Delgass, F. H. Ribeiro, J. Am. Chem. Soc. 2012, 134, 4700 –4708.
[62] P. Rodr�guez, A. A. Koverga, M. T. M. Koper, Angew. Chem. 2010, 122,1263 – 1265; Angew. Chem. Int. Ed. 2010, 49, 1241 – 1243.
[63] M. Maciejewski, P. Fabrizioli, J. D. Grunwaldt, O. S. Beckert, A. Baiker,Phys. Chem. Chem. Phys. 2001, 3, 3846 – 3855.
[64] Z. P. Liu, P. Hu, A. Alavi, J. Am. Chem. Soc. 2002, 124, 14770 – 14779.[65] R. Burch, Phys. Chem. Chem. Phys. 2006, 8, 5483 – 5500.[66] R. J. Gorte, S. Zhao, Catal. Today 2005, 104, 18 – 24.[67] G. Bond, Gold Bull. 2009, 42, 337 – 342.[68] T. Shido, Y. Iwasawa, J. Catal. 1993, 141, 71 – 81.[69] J. L. Gong, C. B. Mullins, Acc. Chem. Res. 2009, 42, 1063 – 1073.[70] T. Bunluesin, R. J. Gorte, G. W. Graham, Appl. Catal. B 1998, 15, 107 – 114.[71] H. F. Wang, X. Q. Gong, Y. L. Guo, Y. Guo, G. Z. Lu, P. Hu, J. Phys. Chem. C
A. Hussain,* J. Gracia, B. E. Nieuwenhuys,J. W. Niemantsverdriet
&& –&&
Explicit Roles of Au and TiO2 ina Bifunctional Au/TiO2 Catalyst for theWater-Gas Shift Reaction: A DFT Study
Acting shifty with DFT: The water–gasshift reaction is studied by using DFTapplied to Au(1 0 0), stepped Au(3 1 0),and TiO2 anatase (0 0 1) surfaces. NeitherAu nor TiO2 catalyze the reaction bythemselves. Two mechanisms are inves-tigated and compared for CO oxidationon Au in the water–gas shift reaction,both featuring H2O dissociation on theTiO2 surface and diffusion of OH and Hon Au surfaces, which is the rate-limit-ing step.&&Table of Contents entryok?&&
