TiO2(110) Interface under Water-Gas Shift Reaction

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Nature of Ptn/TiO2(110) Interface under Water-Gas Shift Reaction Conditions: A Constrained ab Initio Thermodynamics Study Salai Cheettu Ammal and Andreas Heyden* Department of Chemical Engineering, University of South Carolina, 301 South Main Street, Columbia, South Carolina 29208, United States

bS Supporting Information ABSTRACT: The electronic structure of small Ptn (n = 18) clusters supported on the stoichiometric and partially reduced rutile TiO2(110) surface have been investigated using density functional theory. Pt atoms prefer to form a close-packed structure with (111) facet near an oxygen vacancy of the TiO2 support and a less dense structure with (100) facet away from oxygen vacant sites. The main focus of this study is on identifying a realistic catalyst model for the Pt/TiO2 interface under watergas shift (WGS) reaction conditions. Constrained ab initio thermodynamic simulations on the stability of oxygen vacancies and formation of adsorbed gas phase molecules such as oxygen, CO, and hydrogen at the metal/oxide interface reveal that under WGS reaction conditions the formation of surface oxygen vacancies are thermodynamically favorable, platinum oxide species (PtOx) can easily be reduced and should not be present, CO adsorbs only weakly on interfacial Pt atoms, and CO poisoning of these sites should be less important. While hydrogen generally interacts weakly with interfacial Pt atoms, it forms very stable hydride species on Pt atoms neighboring an oxygen vacancy of the TiO2(110) support, possibly negatively affecting the WGS reaction rate.

1. INTRODUCTION For heterogeneously catalyzed reactions with more than one key surface intermediate, it is likely that multiphase catalysts have a significant advantage over conventional monophase catalysts since each phase can potentially be adjusted independently to activate a key reaction step.1,2 Rational design of multiphase catalysts and the advancement of heterogeneous catalysis relies ultimately on an atomic-scale understanding of the structure performance relationship of each active site in the multiphase system. Modern electronic structure theory methods such as density functional theory (DFT) have been used as a standard tool for this task. It is important to remember, though, that conventional DFT is a zero-temperature, zero-pressure technique that provides in most catalysis applications “only” the atomic structure and total energy of a critical point on the potential energy surface (PES) of a predefined system. Thus, DFT calculations are not able to directly quantify the importance/relevance of a model system (critical point on the PES) under realistic reaction conditions. In other words, conventional DFT calculations at 0 K and vacuum conditions are not able to identify the composition and geometry of an active site of a catalyst under experimental reaction conditions, which again is a prerequisite for modeling reactions of real catalysts and reaching a microscopic understanding of heterogeneous catalysis. Detailed experimental observations are usually required. In addition, constrained ab initio atomistic thermodynamics calculations, r 2011 American Chemical Society

which employ DFT information and compute Gibbs free energies, can be used to identify relevant structures and compositions under realistic temperatures and pressures that constitute starting points for a later detailed investigation of reaction pathways. Although this methodology assumes equilibrium of a specific set of reactions that might not necessarily be achieved experimentally, a number of researchers were able to understand the nature of active catalytic sites under specific reaction conditions with this technique.39 In the present study, we have employed this technique to investigate the nature of the active sites at the Pt/TiO2(110) interface that has been proposed to be an efficient catalyst for the water-gas shift reaction (WGS, CO + H2O h CO2 + H2).1014 The WGS reaction has historically been an important industrial process for the synthesis of ammonia,15 for hydroprocessing of petroleum,16 in methanol steam reforming,1719 and in fuel cell applications for providing clean hydrogen and maximizing the hydrogen yield.2026 The heterogeneously catalyzed WGS reaction is moderately exothermic (ΔH = 41.1 kJ/mol) and often equilibrium limited in industrial applications. Therefore, low CO levels can only be achieved at low temperatures where the kinetics is unfavorable.27 Recent studies have shown that Received: June 22, 2011 Revised: August 17, 2011 Published: August 23, 2011 19246

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noble metals (e.g., Pt, Au) supported on reducible oxides show excellent activity and selectivity for the low-temperature WGS reaction.10,1214,2832 It has been proposed that these WGS catalysts are bifunctional in that the noble metal adsorbs/ activates CO and the oxide support activates H2O.13,20,26,32,33 In other words, it is believed that the reaction occurs at the threephase boundary (TPB) of a gas phase, reducible oxide support, and metal cluster. This conclusion was reached by noticing that the reaction rate scales linearly with the total length of the TPB,3436 and that nearly metal-free ceria catalysts37,38 and inversely supported catalysts of TiOx and CeOx nanoparticles grown on a Au(111) substrate39 display significant catalytic activity for the WGS reaction. Lastly, it has been shown that the oxide support strongly influences the catalytic performance in terms of activity and stability. Various reaction pathways, such as the redox, carboxyl, formate, or carbonate mechanism, have been proposed for the WGS reaction catalyzed by noble metals supported on CeO2, TiO2, and ZrO2. Noble metals with different oxidation states and particle sizes have been proposed as active sites for the low-temperature WGS. In particular, oxidized Pt and Au species on reducible oxides attracted much attention. On the basis of experimental investigations of the WGS reaction catalyzed by Au and Pt supported on CeO2 and Fe3O4, Stephanopoulos et al.37,40 concluded that nanoparticles, clusters, and atoms (ions) of Pt or Au coexist on these surfaces, but that the chemistry takes place on atomically dispersed Ptδ+ or Auδ+ species and involves neighboring OH groups. On irreducible oxide supports, such as Al2O3 or SiO2, alkali-stabilized PtOHx species were proposed as active sites.41 Altogether, it seems likely that the most important mechanism changes with changes in oxide support, noble metal, temperature, and gas composition. Thus, understanding the nature of the interaction between the metal and oxide support at the interface under realistic WGS reaction conditions is crucial to identifying the active site and understanding the unique low-temperature activity and selectivity of these novel multiphase catalysts. In the present study, we use theoretical methods based on DFT and constrained ab initio thermodynamics simulations to investigate the nature of the Pt/TiO2(110) interface under WGS reaction conditions. In particular, we try to answer the following questions: (1) Is the TiO2 support reduced under reaction conditions? Does the presence of Pt clusters help create oxygen vacancies on the TiO2 surface? (2) What is the oxidation state of the metal particles under reaction conditions? (3) What is the effect of H2 on the Pt/TiO2 interface? (4) What is the importance of CO poisoning for interfacial Pt/TiO2 sites? Finally, this systematic study will provide helpful insights into the design of catalyst models for a future mechanistic study of the WGS reaction at the Pt/TiO2 interface. This paper is organized as follows: After the computational details are described in section 2, the results are discussed in section 3. First, we analyze the binding mechanism of Ptn (n = 18) clusters on the stoichiometric and partially reduced TiO2(110) surface. Then, we test the effect of Pt clusters on the reducibility of the TiO2(110) surface. Next, the nature of the Pt/TiO2 interface is studied under WGS reaction conditions by analyzing the thermodynamic stabilities of adsorbed oxygen, CO, and hydrogen. Finally, conclusions are summarized in section 4.

(PAW) method of Bl€ochl, as implemented in the VASP program.4245 We chose the PerdewBurkeErnzerhof (PBE)46 functional within the generalized gradient approximation (GGA) to describe exchange and correlation effects. The number of valence electrons considered for Ti, O, and Pt are 10, 6, and 10, respectively. We used a soft pseudopotential and a hard pseudopotential for describing the oxygen atoms. The cutoffs in the plane-wave expansion were 350 eV in calculations involving the soft oxygen pseudopotential and 400 eV in the case of a hard pseudopotential. The energy differences between the results obtained with the two pseudopotentials are less than 0.1 eV, which is smaller than the expected accuracy of DFT. Bulk rutile TiO2 has a tetragonal structure with two formula units (TiO2) per unit cell. The calculated PAW-PBE lattice constants, a = 4.603 Å and c = 2.945 Å with the soft oxygen potential and a = 4.649 Å and c = 2.971 Å with the hard oxygen potential, are in close agreement with the experimental values of 4.593 and 2.958 Å.47 These lattice parameters were used to construct stoichiometric (4 2) and (6 2) supercell (110) surface slabs using a three-dimensional slab model of finite thickness (12.4 Å; corresponding to four triple layers). The vacuum space above the slab was 17 Å thick. All atoms in the solid and the clusters were relaxed except for those in the bottom triple layer of the TiO2(110) slab which were fixed in bulk positions. Adsorption of Ptn clusters have been considered only on one side of the TiO2 slab. Many starting structures corresponding to the adsorption of Ptn on the stoichiometric surface and on the partially reduced surface have been fully optimized, without symmetry constraints, using a conjugated-gradient algorithm. Electronic energies were converged to 105 eV, and ionic relaxations were considered converged when the forces on the ions were less than 0.02 eV/Å. A (2 2 1) MonkhorstPack k-mesh was used for structure relaxations. The Brillouin zone was sampled only at the Γ point whenever the larger (6 2) surface slab was used. Dipole and quadrupole corrections to the energy were taken into account using a modified version of the Markov and Payne method.48 Dipole correction to the surface models was included solely in the surface normal direction. Harris Foulke-type corrections49,50 have been included for the forces. Fractional occupancies of bands were allowed using a window of 0.05 eV and the Gaussian smearing method. Density of state (DOS) calculations have been performed using the tetrahedron method with Bloch corrections and a k-point grid enlarged to 4 4 2. Finally, atomic charges were obtained using Bader’s analysis51 (including the core states) based on the numerical implementation developed by Henkelman et al.52 Every optimization took place in two steps. First, various starting structures were optimized using the soft oxygen pseudopotential and only the low energy structures within a window of 0.3 eV were reoptimized with the hard oxygen pseudopotential. It is noted that the supercell used in this study is large enough to keep the distance between a cluster and its periodic replica larger than 6 Å. The adsorption energies of Ptn clusters, Eads[Ptn], on the stoichiometric (S-TiO2) or reduced (R-TiO2) surface are calculated as

2. COMPUTATIONAL DETAILS We performed fully spin-polarized periodic DFT calculations using the frozen-core all-electron projector-augmented-wave

where E[Ptn/TiO2] and E[TiO2] are the total energies of S- or R-TiO2 with and without a Pt cluster on its surface, and E[Ptn] is the total energy of the Ptn cluster at its equilibrium geometry

Eads ½Ptn ¼ E½Ptn =TiO2 E½TiO2 E½Ptn

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ð1Þ

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in the gas phase. The energies of isolated Ptn clusters were calculated in a 20 Å cubic supercell including the Γ point only. Different possible structures and spin multiplicities were considered for the Ptn clusters in order to identify the most stable structure for each complex. Similarly, the clustering energy of adsorbed Ptn clusters is defined as Eclu ½Ptn ¼ ðE½Ptn =TiO2 E½TiO2 nE½PtÞ=n

ð2Þ

where E[Pt] is the total energy of an isolated Pt atom. To compute the vacancy formation energy (Evf) of the TiO2(110) surface, we used the following expression: Evf ¼ E½R-TiO2 þ

1 E½O2 E½S-TiO2 2

ð3Þ

where E[O2] is the energy of a gas phase oxygen molecule. In order to avoid difficulties associated with the GGA-DFT treatment of the triplet state of gas phase O2,53 the O2 energies are obtained from the H2O splitting reaction using the experimental reaction energy and calculated DFT energies of H2 and H2O in the gas phase.54 Next, electron density differences, ΔF, are calculated via frozen ΔF ¼ FPtn =TiO2 ðFfrozen TiO2 þ FPtn Þ

ð4Þ

where FPtn/TiO2 is the electron density for the adsorbatesubstrate system in its minimum energy configuration, Ffrozen TiO2 is the electron density of the surface kept frozen in the positions of the is the electron density of adsorbatesurface system, and Ffrozen Ptn the Ptn cluster at the same position of the adsorbatesurface system. For charge density analyses the dipole correction is included. To take environmental effects into account, the Gibbs free energy G(T,P) of the system is computed as a function of temperature, T, and pressure, P, as described elsewhere.5557 For a gassurface reaction (surface1 ( gas f surface2), the change in Gibbs free energy can be written as ΔG ¼ Gsurface2 Gsurface1 - Ggas

ð5Þ

where the Gibbs free energies (Gsurface1, Gsurface2) of solid phases are approximated by the computed DFT energies, and the Gibbs free energy of a gas phase molecule is calculated using P DFT 0 Ggas ðT, PÞ ¼ Egas þ Δμgas ðT, P Þ þ kB T ln 0 ð6Þ P where Δμgas(T,P0) can be calculated from the rotational, translational, and vibrational partition functions of the gas molecule. In order to avoid large errors associated with neglecting entropy contributions of adsorbed species, we have ignored the vibrational entropy of both the gas phase molecules and the surface species. We have also verified for a few characteristic systems relevant for this work that the inclusion of vibrational entropy contributions does not alter free energy differences by more than 0.1 eV under relevant conditions (see the Supporting Information).

3. RESULTS AND DISCUSSION The TiO2(110) surface is composed of atoms with different local environments such as five- and sixfold coordinated Ti atoms (Ti5c and Ti6c), threefold coordinated surface oxygen atoms (Os), and doubly coordinated bridging oxygen atoms (Ob) as shown in Figure 1a. Several recent papers reported shortcomings of DFT functionals based on the generalized gradient

approximation (GGA) in describing the correct electronic structure of the reduced TiO2(110) surface. Hybrid-DFT functionals5861 and DFT+U methods61,62 have been used to obtain a better electronic structure that yields a wider band gap for the reduced surface and a gap state due to localization of electrons on the Ti atoms neighboring the vacancy. We recently performed periodic electrostatic embedded cluster (PEEC) calculations with various functionals and showed that the shortcomings of GGA functionals affect the binding energies of Ptn clusters especially for the reduced surface.63 Hybrid-DFT functionals seem to have a major effect on the calculated properties whenever there are unpaired electrons present on the surface. Considering that the Ptn/TiO2 systems studied have no unpaired electrons at the interface of the Ptn cluster and TiO2 surface, we do not expect our conclusions or trends reported in this paper to be affected by the DFT functional. 3.1. Binding Mechanism of Ptn Clusters on the Stoichiometric Rutile TiO2(110) Surface. Figure 1 illustrates the lowest energy structures identified for the adsorption of small Pt clusters, Ptn (n = 18), on the stoichiometric TiO2(110) surface. More than 10 possible orientations have been considered for the adsorption of Ptn (n = 48) clusters in order to locate the most stable structure. It is noted that the binding site shown for the Pt monomer adsorption (on top of Os) is different from that reported by Iddir et al.6466 (fourfold hollow site over Ti5c, two Os and Ob). However, the adsorption energy difference for the hollow and on-top sites is less than 0.1 eV. Pt2 and Pt3 structures on the stoichiometric and reduced TiO2(110) surfaces have been analyzed by us in an earlier report,63 and these structures are included in the present analysis for comparison only. The structures of Ptn (n = 48) clusters on the stoichiometric TiO2(110) surface have not been reported previously in the literature; only structures of Aun clusters on the TiO2(110) surface have been studied with computational methods.6775 Aun clusters interact weakly with the stoichiometric TiO2 surface such that the (111) facet of Au is most stable (parallel to the (110) direction of the TiO2 surface). Considering that the most stable surface of Pt has also a (111) facet, one could expect that Ptn clusters form structures similar to Aun clusters on the TiO2(110) surface. However, our calculations suggest that Ptn clusters prefer a (100) facet parallel to the (110) TiO2 surface (Figure 1). Such a pattern arises from the strong interaction of Pt atoms with both the surface oxygen atoms and the Ti5c atoms (in contrast, Aun clusters interact only weakly with the surface through the bridging oxygen atoms). The strong interaction of Ptn clusters with the TiO2 (110) surface is further substantiated by adsorption energies that range from 1.8 to 4.3 eV (compared to Aun cluster adsorption energies in the range 0.4 to 1.5 eV68). A Bader charge analysis shown in Figure 1 indicates that there is a significant amount of charge transfer from the Ptn clusters to the stoichiometric surface of TiO2(110). In order to obtain further insights into the bonding mechanism of Ptn clusters to the TiO2(110) surface, we plotted in Figure 2 the total and atom-projected density of states (DOS) and electron density difference (EDD) for the Pt8/TiO2(110) surface. The DOS plots illustrate the strong interaction between the Pt8 cluster and the TiO2(110) surface. Metal-induced states in the band gap of the clean surface are predominantly Pt d states mixed with a small contribution from O 2p orbitals and Ti 3d orbitals, which are observed near the valence band of the surface (around 1.3 to 1.9 eV). 19248



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Figure 1. Adsorption energies in electronvolts of the lowest energy structures of Ptn (n = 18) adsorbed on a stoichiometric rutile TiO2(110) surface. The change in total electronic charge Q[Ptn] on the Pt cluster was obtained using Bader’s charge analysis.

EDD plots in different orientations are shown in Figure 2b,c for two isosurfaces at ΔF = 0.05 e/Å3 and ΔF = +0.05 e/Å3. Considering that the average (valence) electron density for TiO2 in the bulk is F = 44e/64.22 Å3 = 0.69 e/Å3, the isosurfaces represent an electron density change of ΔF/F = 7.2% which is significant. Bader’s charge analysis on this system reveals that the two central Pt atoms and one corner Pt atom in the first layer of the cluster that directly interacts with the surface have positive charges of +0.23, +0.15, and +0.13, respectively. The charges on the remaining Pt atoms are nearly zero. Such a charge distribution is also reflected in the EDD plots where charge depletion from the Pt atoms and charge accumulation near the Ti5c atoms (Figure 2b) and bridging oxygen atoms (Figure 2c) can be observed. Next, the EDD plots also show that there is no significant polarization beyond the first layer of the Pt cluster. Finally, the two central PtO bond lengths (2.09 Å) are only slightly larger than the sum of the covalent radii of Pt (1.30 Å) and O (0.73 Å), and two of the corner PtTi bond lengths (2.58 Å) are only slightly shorter than the sum of the covalent radii of Pt (1.30 Å) and Ti (1.32 Å), which again is in agreement with the charge polarization observed in the EDD plots and suggests a

covalent-type interaction between the Pt8 cluster and the TiO2(110) surface. 3.2. Binding Mechanism of Ptn Clusters to a Partially Reduced Rutile TiO2(110) Surface. The lowest energy structures of Ptn clusters (n = 18) adsorbed on a partially reduced (one oxygen vacancy per unit cell) rutile TiO2(110) surface are shown in Figure 3. Computed binding energies, binding sites, and geometries of the Pt monomer and dimer agree with those published previously.66 It is generally believed that the Pt clusters nucleate at the oxygen vacancies by placing one Pt atom at the vacancy. However, our calculations indicate that this is not the case for all small Pt clusters. Among the eight complexes studied, the Pt1, Pt2, Pt6, and Pt8 complexes have one Pt atom at the oxygen vacancy while the remaining complexes have two Pt atoms at the vacancy. There seems to be no general rule for the preference of such orientations. Similar “random” orientations for the nucleation of small Aun (n = 17) clusters on partially reduced TiO2(110) surfaces have been reported by Chretien and Metiu.67 They showed that the structural preferences of the Aun/ TiO2x complexes originate from the orientations of the frontier molecular orbitals of the free Au clusters. We believe that a similar 19249



Figure 2. (a) Total and atom-projected density of states (DOS) around the Fermi energy for Pt8 adsorbed on a stoichiometric TiO2(110) surface. (b, c) Charge density difference isosurfaces (0.05 e/Å3) for Pt8/ TiO2(110) in different orientations. Blue and red represent charge accumulation and depletion, respectively. The numbers shown in the two insets correspond to the Bader charges on the Pt atoms.

explanation could also hold for the Ptn/TiO2x complexes. Comparing the structures and binding energies of the reported Aun/ TiO2x system with those of the here reported Ptn/TiO2x system, the following observations can be made. (1) Ptn clusters bind more strongly with the reduced TiO2 surface than Aun clusters. (2) While the structures of the Ptn clusters are mostly planar and form two-layer structures for n = 68, Aun clusters prefer to form two- or three-layer structures with only very few Au atoms interacting with the surface. (3) A negative charge on the Aun clusters is observed only when the structure has one Au atom at the vacancy. In contrast, Ptn clusters adsorbed on the reduced TiO2 surface are always negatively charged (although there is a significant amount of charge transfer from the Pt atoms far from the oxygen vacancy to the Ti5c and bridging oxygen atoms). Next, we compare the structures of the Ptn clusters on the reduced TiO2(110) surface with those on the stoichiometric TiO2(110) surface. The binding energy of the Pt clusters with the reduced surface exceeds the binding energy of the clusters with the stoichiometric surface by about 0.80.9 eV. Exceptions to this rule are Pt1 and Pt2, whose binding energies are 1.75 and 1.51 eV higher than those on the stoichiometric surface, respectively. Also, while Pt clusters on the stoichiometric TiO2(110) surface display a (100) facet (Figure 1), Pt atoms near an oxygen vacancy follow a close-packed (111) arrangement (Figure 3). The coexistence of two different arrangements of Pt clusters is clearly seen in the Pt7 cluster (Figure 3). Interestingly, RuizMartinez et al.76 recently reported, using in situ infrared spectroscopy and adsorption microcalorimetry of adsorbed CO on a Pt/TiO2 catalyst, the presence of both (100) and (111) arrangements of Pt atoms on samples reduced at 473 and 773 K. To better understand the interaction of Pt clusters with the reduced TiO2(110) surface, we analyzed the total and atomprojected DOS of the Pt8/TiO2x surface illustrated in Figure 4a. Our calculations predicted,63 in agreement with previous periodic DFT calculations,62,67,77 that the creation of oxygen vacancies on the clean TiO2 surface results in the presence of electrons

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at the bottom of the conduction band. One Pt atom of the Pt8 cluster uses these electrons to form a covalent-type interaction with the Ti atoms at the vacant site. The energy of this bonding state is observed 0.4 eV below the conduction band, and the band gap is slightly larger than that of the Pt8/TiO2 surface (Figure 2a). The gap states originate predominantly from Pt d orbitals together with a small contribution from Ti 3d and O 2p orbitals. Bader’s charge analysis suggests that there is charge transfer from the surface Ti atoms surrounding the vacancy to the Pt atom that is negatively charged (0.51 e). Again, Pt atoms that interact predominantly with the Ti5c and Ob atoms are positively charged, indicating electron transfer from the Pt cluster to the surface. EDD plots shown in Figure 4b,c clearly demonstrate these charge transfer effects. In fact, the similarity observed between Figures 2c and 4c indicates the presence of covalent bond type interactions between the Pt atom and the two Ti atoms neighboring the oxygen vacancy. The PtTi bond length (2.5 Å) neighboring the oxygen vacancy is shorter than the sum of the covalent radii of Pt and Ti (2.62 Å). 3.3. Effect of Pt Clusters on the Reducibility of the TiO2(110) Surface. Most experimental studies1012,14,78 focused on understanding the mechanism of the Pt/TiO2 catalyzed WGS reaction suggest the involvement of surface oxygen atoms either through the direct formation of CO2 (redox mechanism) or through the formation of formate or carboxyl intermediates (associative mechanism). In all of these cases, oxygen is released from the surface leaving a vacant site at the Pt/TiO2 interface which constitutes an active site for water dissociation. Although TiO2 is known to be a reducible oxide, it is more resistant to reduction than CeO2. While various experimental studies7981 have shown that the partially reduced surface is thermodynamically stable under reducing conditions, computational studies58,62,63,67,82 suggest that the energy required to remove an oxygen atom from the TiO2(110) surface is between 3 and 5.3 eV. This vacancy formation energy is high compared to the small apparent activation barrier (∼0.30.5 eV) of the catalyzed WGS reaction. Thus, it has been suggested that the presence of transition metal atoms facilitates the creation of oxygen vacancies on the surface. In the following, we tested this hypothesis by calculating the oxygen vacancy formation energies (Evf) of the TiO2(110) surface in the presence and absence of Pt clusters. Evf in the presence of a Pt cluster (Ptn) has been calculated using eq 3 and using the energies of the most stable Ptn structures on the stoichiometric and reduced TiO2 surfaces. Table 1 displays the clustering energies of the adsorbed Ptn (n = 18) clusters on the stoichiometric and reduced TiO2(110) surfaces and the corresponding Evf values. We prefer to compare clustering energies instead of adsorption energies since the structures of Pt clusters on the stoichiometric and reduced TiO2 surfaces are quite different. The clustering energies for the Ptn (n = 28) clusters on both the stoichiometric and reduced surfaces decrease gradually and converge to within 0.1 eV for clusters with six to eight atoms. The clustering energy is lower on the reduced surface compared to the stoichiometric surface due to the strong interaction of the Pt atoms with the oxygen vacancy. Nevertheless, we find no preference for adding a Pt atom to an existing small Ptn cluster on the reduced versus stoichiometric surface. In other words, the effect of an oxygen vacancy on Ptn cluster adsorption is localized to the immediate Pt atom interacting with the vacancy. Next, the calculated vacancy formation energy for the clean surface (3.76 eV) is in good agreement with previously reported values62,67,82 for a similar 19250



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Figure 3. Adsorption energies in electronvolts of the lowest energy structures of Ptn (n = 18) adsorbed on a partially reduced rutile TiO2(110) surface. The change in total electronic charge Q[Ptn] on the Pt cluster was obtained using Bader’s charge analysis.

oxygen vacancy concentration and supercell dimension. Evf values shown in Table 1 for the Ptn/TiO2 interface illustrate clearly that adsorbed Pt clusters promote the reducibility of the TiO2(110) surface. The vacancy formation energy increases with increase in Pt cluster size and converges about 0.70.9 eV below the vacancy formation energy of the clean TiO2 surface. The enhanced reducibility of smaller metal clusters is due to their stronger adsorption on the reduced surface. Previously published hybrid DFT calculations show an even more pronounced effect, where the Evf of the TiO2(110) surface decreased by 2.27 and 1.26 eV in the presence of Pt2 and Pt3 clusters, respectively.63 To evaluate the relative thermodynamic stability of oxygen vacancies at the Pt/TiO2 interface under various environments, we performed constrained ab initio thermodynamic simulations and calculated the free energies of different systems. In an oxygen atmosphere, the free energy for the formation of one oxygen defect was calculated using ΔG ¼ ETiO2x ETiO2 þ

1 EO þ ΔμO ðT, PÞ 2 2

pressure of O2: ΔμO ðT, PÞ ¼

1 P ΔμO2 ðT, P0 Þ þ kB T ln 0 2 P

ð8Þ

Figure 5a illustrates the change in Gibbs free energy for the formation of oxygen vacancies on the clean TiO2(110), Pt3/ TiO2(110), and Pt8/TiO2(110) surfaces as a function of oxygen partial pressure and temperature [ΔμO(T,P)]. It is evident that the formation of oxygen vacancies on any of these surfaces is unlikely under oxidizing conditions except at very high temperatures. Under reducing conditions such as WGS conditions, the surface is possibly in equilibrium with CO or H2 TiO2 þ CO ðH2 Þ f TiO2x þ CO2 ðH2 OÞ

ð9Þ

and the Gibbs free energy can be expressed as ΔGðT, PÞ ¼ ETiO2x þ ECO2 ðH2 OÞ ETiO2 ECOðH2 Þ þ ΔμCO2 ðH2 OÞ ðT, P0 Þ ΔμCOðH2 Þ ðT, P0 Þ ! PCO2 ðH2 OÞ þ kB T ln ð10Þ PCOðH2 Þ

ð7Þ

and assuming that gas-phase oxygen is well described by the ideal gas law, ΔμO(T,P) can be related to the temperature and partial 19251



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Table 1. Clustering Energies (Eclu) of Pt Clusters on the Stoichiometric and Partially Reduced TiO2(110) Surfaces and Effect of Metal Clusters on the Oxygen Vacancy Formation Energy (Evf) of the TiO2 Surface Eclu (eV) adsorbed Pt cluster clean surface

Figure 4. (a) Total and atom-projected density of states (DOS) around the Fermi energy for Pt8 adsorbed on a partially reduced TiO2(110) surface. (b, c) Charge density difference isosurfaces (0.05 e/Å3) for Pt8/ TiO2x(110) in different orientations. Blue and red represent charge accumulation and depletion, respectively. The numbers shown in the two insets correspond to the Bader charges on Pt atoms.

(It is noted that eqs 10 and 7 are equivalent but considering that the O2 partial pressure is very low under reducing conditions and is difficult to estimate we found eq 10 more convenient to use.) Figure 5b,c illustrates the Gibbs free energy for an equimolar gas mixture (CO/CO2 or H2/H2O) as a function of temperature. In agreement with experimental observations, there are only a few point oxygen vacancies on the TiO2(110) surface even under reducing conditions and temperatures above 500 °C. However, the presence of Pt clusters seems to promote the reducibility of the TiO2 surface and the formation of oxygen vacancies at the Pt/ TiO2 interface is thermodynamically favorable under reducing conditions. This observation is in agreement with ESR and NMR measurements which show that indeed Ti3+ ions are formed when Pt/TiO2 is reduced at temperatures as low as 250 °C.8385 Similarly, Panagiotopoulou et al.11 performed temperature-programmed-reduction (TPR) experiments with CO on the Pt/ TiO2 system and noticed that the TiO2 surface is reduced at low temperatures. All of these results indicate that the Pt/TiO2(110) interface is likely partially reduced under WGS reaction conditions, which is important for the redox mechanism (direct CO2 formation). Finally, it is interesting to note that ΔG is only slightly negative (0.1 to 0.2 eV) for the reduction of the Pt/ TiO2 interface by hydrogen in the relevant WGS reaction temperature range (500600 K). The small negative ΔG indicates that it is possible for water to dissociate at an oxygen vacancy (reverse reaction) to form H2 and heal the vacant site created by CO which completes the catalytic redox cycle. 3.4. Stability of Platinum Oxide Species under WGS Reaction Conditions. Experimental studies on Pt crystallites dispersed on TiO2, Al2O3, CeO2, and their mixtures suggest that various platinum oxides, including PtsO (surface platinum oxide), PtO, and PtO2, are stable under various reaction conditions.8688 Stable surface metal oxides and interfacial metal oxide support species have been suggested to be important for the low-temperature (partial) oxidation and the WGS reaction. As a result, we investigated

TiO2

TiO2x

Evf (eV)

3.76

Pt

2.00

3.75

2.00

Pt2

2.83

3.58

2.25

Pt3

3.47

3.77

2.87

Pt4

3.60

3.84

2.83

Pt5

3.73

3.90

2.92

Pt6 Pt7

3.82 3.87

3.96 3.98

2.92 3.02

Pt8

3.95

4.05

2.96

the stability of such platinum oxides under WGS reaction conditions using the constrained ab initio thermodynamic approach. The adsorption energies of a single oxygen atom at various positions on Pt8/TiO2 and Pt8/TiO2x have been calculated to locate the most stable oxygen adsorption site. In order to avoid any spurious interaction between the adsorbents, we used a (6 2) surface supercell for these calculations. Our computations indicate that the oxygen atom binds strongly at the bridging position (br) between the top two Pt atoms in both systems. Figure 6 illustrates the optimized structures for the most stable position of the oxygen atom on the Pt clusters (br) and at the Pt/ TiO2 interface (int). The adsorption energies shown in Figure 6 have been calculated with reference to the energy of the oxygen molecule. 1 Eads ½O ¼ E½OPt8 =TiO2 E½Pt8 =TiO2 E½O2 2

ð11Þ

Oxygen adsorption at the PtPt bridge position is stronger (Eads = 2.38 eV) on the stoichiometric surface than on the partially reduced surface (Eads = 1.54 eV). This concurs with the difference in the charge distribution of the Pt clusters adsorbed on these two surfaces. Pt8 clusters adsorbed on the stoichiometric surface are positively charged (Figure 1), which attracts the electronegative oxygen atom more strongly. In contrast, the Pt8 clusters on the reduced surface is negatively charged (see Figure 3) due to transfer of electrons from the oxygen vacant site to the Pt cluster and thus binds the oxygen atom more weakly. Considering that the vacancy formation energy for the Pt8/TiO2 surface is about 3 eV (Table 1), we could expect that adsorbed oxygen atoms on the Pt8/TiO2x surface migrate and heal the oxygen vacant site. The adsorption energy of an oxygen atom at the metalsupport interface is less dependent on the charge distribution of the Pt cluster and is significantly smaller due to the fact that these Pt atoms are already partially oxidized by the oxide support. Gibbs free energies for the addition of an oxygen atom to the Pt8/TiO2 system in an oxidizing atmosphere have been calculated using 1 ΔG ¼ EOPt8 =TiO2 EPt8 =TiO2 EO2 ΔμO ðT, PÞ 2 19252

ð12Þ



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Figure 5. (a) Gibbs free energy (ΔG) for the formation of an oxygen vacancy on the TiO2 surface (TiO2 f TiO2x + 1/2O2) versus oxygen chemical potential (ΔμO). (b, c) Gibbs free energy (ΔG) for the formation of an oxygen vacancy on the TiO2 surface under reducing conditions (TiO2 + CO/H2 f TiO2x + CO2/H2O) versus the difference in chemical potentials ΔμCO2 ΔμCO and ΔμH2O ΔμH2, respectively.

thermodynamically favorable over a wide range of the O2 chemical potentials. This suggests that, in an oxidizing atmosphere, surface Pt atoms and the Pt/TiO2 interface might be enriched by atomic oxygen. A similar stability of adsorbed oxygen atoms at the Au/TiO2(110) interface has been reported by Laursen and Linic9 and Matthey et al.73 Next, we investigated the stabilities of these platinum oxides under WGS reaction conditions. Analogous to eqs 9 and 10, the stability of PtO species under reducing conditions has been evaluated using OPt8 =TiO2 þ CO ðH2 Þ f Pt8 =TiO2 þ CO2 ðH2 OÞ

ð13Þ

ΔGðT, PÞ ¼ EPt8 =TiO2 þ ECO2 ðH2 OÞ EOPt8 =TiO2 ECOðH2 Þ þ ΔμCO2 ðH2 OÞ ðT, P0 Þ ΔμCOðH2 Þ ðT, P0 Þ ! PCO2 ðH2 OÞ þ kB T ln ð14Þ PCOðH2 Þ Figure 6. Adsorption energies (Eads) and optimized structures for the adsorption of one oxygen atom on Pt8/TiO2(110). Structures (a) and (c) correspond to the adsorption of an O-atom at the top PtPt bridge (br) position on Pt8/TiO2 and Pt8/TiO2x respectively; (b) and (d) correspond to the adsorption of an O-atom at the metalsupport interface (int) on Pt8/TiO2 and Pt8/TiO2x, respectively.

Figure 7a shows calculated Gibbs free energies for the formation of the four structures illustrated in Figure 6 as a function of oxygen pressure and temperature [ΔμO(T,P)]. The free energy graphs demonstrate that the formation of PtO species is

Parts b and c of Figure 7 illustrate the stabilities of PtO species in the presence of equimolar CO/CO2 and H2/H2O gas phases, respectively. Clearly, platinum oxide species are not stable under these reducing conditions even at very low temperatures. In fact, one could estimate that the Eads[O] value should exceed 3 eV in order for the investigated PtO species to be stable under WGS reaction conditions. This suggests that platinum oxide species initially formed during the preparation of the Pt/TiO2 catalyst will react with CO (or H2) to form CO2 (or H2O) with time-on-stream and the catalyst will be free of any extra oxygen 19253



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Figure 7. (a) Gibbs free energy (ΔG) for the adsorption of an oxygen atom on the Pt8/TiO2 surface (Pt8/TiO2 + 1/2O2 f OPt8/TiO2) versus oxygen chemical potential (ΔμO). (b, c) Gibbs free energy (ΔG) for the reduction of the OPt8/TiO2 surface by CO and H2 (OPt8/TiO2 + CO/H2 f Pt8/TiO2 + CO2/H2O) versus the difference in chemical potentials ΔμCO2 ΔμCO and ΔμH2O ΔμH2, respectively. Refer to Figure 6 for the corresponding structures.

atoms under steady-state WGS reaction conditions. This observation is in agreement with results from Panagiotopoulou et al., who performed temperature-programmed-reduction experiments of Pt/TiO2 by H2 and CO and report that the PtOx particles are reduced at temperatures below 250 °C in a H2/He stream and between 90 and 120 °C in a CO stream.11 Considering that the WGS reaction occurs generally in the temperature range between 200 and 300 °C, it is thus reasonable to assume that there will be no adsorbed oxygen atoms on the Pt cluster under these conditions and adsorbed PtO species should not be important in the reaction mechanism of the WGS. 3.5. Extent of CO Coverage on Pt/TiO2 under WGS Reaction Conditions. It is well-known that CO interacts strongly with Pt metal atoms compared to other noble metals such as Au. The average heat of CO adsorption is strongly dependent on the coverage and varies from about 1.6 eV at low coverage to about 0.8 eV at high coverage on Pt surfaces.89 The interaction of CO was found to be even stronger on simple Pt clusters or electronically modified Pt atoms such as bimetallic or oxide supported Pt clusters.9092 CO was found to be the most abundant intermediate in the WGS reaction catalyzed by supported Pt nanoparticles, and on the basis of experimental and microkinetic studies of the WGS reaction mechanism on Pt(111), Grabow et al.93 identified the CO coverage on Pt(111) to be approximately 2/3 monolayer (ML). The microkinetic analysis further showed that CO inhibits the WGS reaction rate (reaction order 0.37) and reducing the CO adsorption strength has a significant effect on increasing the reaction rate. (It is noted, though, that slightly positive CO reaction orders for the WGS have also

been reported for various supported Pt catalysts.14,9496) Considering that reducible oxide supported metal particles have some advantages for the WGS compared to the Pt(111) surface, we speculated that the Pt atoms at the metaloxide interface would be affected differently by CO and we investigated the adsorption of CO at various coverages and sites on the Pt8/TiO2(110) surface under WGS reaction conditions. Table 2 displays calculated CO adsorption energies at different CO coverages (0.13 and 0.5 ML), and Figure 8 shows optimized structures with up to four CO molecules (0.5 ML). Our calculations predict that a single CO molecule prefers to adsorb strongly (Eads = 2.7 eV) on the top two Pt atoms forming linear bonds. The vibrational frequency calculated for this adsorbed CO molecule (2013 cm1) is reasonably close (though slightly too low) to the experimentally observed frequency for linearly bonded CO on the Pt/TiO2 catalyst (2060 cm1).11,14,76 The adsorption energy for the bridge-bonded CO on the top two Pt atoms is slightly lower than that for the linear one (2.67 eV), and the calculated frequency (1839 cm1) is in close agreement with the experimental value (1837 cm1) observed from bridgebonded CO on Pt0.11 Furthermore, three different interfacial sites described in Figure 8 are considered for CO adsorption, and the adsorption energy at these sites decreases in the order interface(1) > interface(2) > interface(3). The computed frequency for site interface(1) (1745 cm1), which is a threecenter-hollow site, is in close agreement with the experimental value reported by Kalamaras et al.14 for bridge-bonded CO (1760 cm1), and for site interface(2) the CO frequency (1858 cm1) is again similar to the reported value for the 19254



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Table 2. Calculated Adsorption Energies (Eads) per CO Molecule and CO Vibrational Frequency for Various CO Coverages on Pt8/TiO2(110) CO frequency species CO

site

(cm1)

Eadsa (eV)

top (linear)

2.74

2013

top (bridge)

2.67

1839

interface(1), 3c-hollow

2.07

1745

interface(2), PtPt bridge interface(3), PtOb bridge

1.51 0.76

1858 1704

4CO(1)

top and interface(1)

2.04 (1.30)

2042, 1758

4CO(2)


1.87 (0.97)

2027, 1887

4CO(3)


1.71 (0.65)

2027, 1690

a

The values given in parentheses are calculated using eq 15 and correspond to the adsorption of one CO molecule at the interfaces in the presence of two CO molecules at the Pt-top positions.

bridge-bonded CO on Pt0 (1837 cm1).11 Next, Green et al.97 performed temperature-programmed-desorption experiments after dosing CO on Pt/TiO2 and observed the formation of CO2 even when there was no extra oxygen added. This observation was explained by the spillover of CO from the metal particles to the support where they react with surface oxygen to form CO2. The adsorption of CO at the site interface(3) (PtOb bridge) corresponds to this CO spillover intermediate, and the optimized structure is somewhat close to a CO2 molecule with a COb bond distance of 1.37 Å. The calculated adsorption energy is lowest at this position (0.76 eV), and the CO frequency (1704 cm1) is again close to the experimental value 1690 cm1 that has been assigned to surface formate or carbonate species.11 Next, we investigated the adsorption of four CO molecules (0.5 ML) at different positions. All structures considered have two CO molecules linearly bonded on the top two Pt atoms as we expect that these sites will always be covered due to their strong interaction with CO. Adsorption of the remaining two CO molecules was considered at three different interfacial sites (see Figure 8). Table 2 illustrates that the average CO binding energy decreases with increase in CO coverage. The calculated vibrational frequencies are again in very good agreement with experimental values (especially also for the linearly bonded CO). Also, we report differential adsorption energies for adsorbed interfacial CO molecules when the top two surface Pt atoms are covered with CO. These energies are calculated using the following expression: 1 Eads ¼ ðE4COPt8 =TiO2 E2COPt8 =TiO2 2ECO Þ 2

ð15Þ

CO adsorption at interfacial Pt atoms decreases with increasing CO coverage (although the differential adsorption energy is approximately constant for interfacial CO molecules with the top two Pt atoms covered by CO). The CO binding energy at the interface(1) site (1.30 eV) is close to the binding energy on the Pt(111) surface (1.37 eV) for a coverage of 0.55 ML. Also, the binding energy is less than 1 eV for the other two interfacial sites. In other words, CO poisoning is reduced at interfacial Pt atoms and these sites are possibly efficient active sites for the WGS reaction. Finally, Figure 9 shows the Gibbs free energy for the formation of adsorbed CO species as a function of CO chemical potential. Adsorbed CO species on the top two surface Pt atoms are stable even at a very low CO chemical potential. However, adsorbed CO species at interfacial Pt sites become unstable

Figure 8. Optimized structures for the adsorption of CO on Pt8/ TiO2(110). The adsorption sites are specified in each structure.

except at very high CO partial pressures. This again suggests that surface Pt atoms will be covered by CO under WGS reaction conditions and only the interfacial Pt atoms will be available for catalysis. The Pt sites away from the metaloxide interface possibly act as a CO reservoir. 3.6. Effect of Hydrogen on Pt/TiO2 under WGS Reaction Conditions. As shown experimentally11 and in section 3.3, the Pt/TiO2 interface is reduced by hydrogen at temperatures below 250 °C. The reduction of the TiO2 support is generally explained by the dissociative adsorption of hydrogen on the metal followed by spillover of hydrogen from the metal to the support. This spillover process and its reverse are important steps in the WGS reaction mechanism for the formation of surface OH groups (or oxygen vacancies) and the H2 product. Analogous to CO, H2 is also known to have a negative effect on the WGS reaction rate with a reaction order of 0.39 on Pt catalysts93 and 0.5 to 0.7 on Pt/TiO2 catalysts.14,96 The negative reaction order on the supported Pt catalysts with respect to H2 has mainly been attributed to the competitive adsorption of H2 with CO. It has been proposed that atomic hydrogen is together with adsorbed CO the dominant adsorbed surface species on Pt sites under WGS reaction conditions. A somewhat different explanation has been given by Azzam et al.,96 who proposed that hydrogen inhibits the WGS reaction rate by suppressing the formation of OH groups on TiO2, either by shifting the equilibrium (Ti3+ + H2O T Ti3+OH + 1/2H2) or by interaction of hydrogen with vacancies which results in trapped ionic hydrogen (Ti4+H) species. To shed more light on this issue, we investigated the adsorption of hydrogen atoms at various sites on the Pt8/TiO2 interface and analyzed the thermodynamic stabilities of these structures under WGS reaction conditions. The adsorption of two hydrogen atoms at three different positions (Figure 10) has been considered for this investigation, and the adsorption energies per hydrogen atom have been calculated with reference to the energy of a H2 molecule. 1 Eads ¼ EHPt8 =TiO2 EPt8 =TiO2 EH2 2 19255

ð16Þ



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Figure 9. Gibbs free energy (ΔG) of CO adsorption on Pt8/TiO2(110) versus CO chemical potential (ΔμCO). Refer to Figure 8 for the corresponding structures.

Our calculations predict that adsorption of hydrogen atoms is stronger at the top two surface Pt atoms (Eads = 0.68 eV) than at the interfacial Pt atoms (Eads = 0.18 eV) or surface oxygen atoms (Eads = 0.13 eV). The very similar adsorption energies at the latter two sites indicate that both forward and reverse hydrogen spillovers are feasible. Figure 10 a illustrates the Gibbs free energies for hydrogen adsorption as a function of hydrogen chemical potential. Adsorbed hydrogen atoms at the top two Pt atoms are stable at room temperature even at a very low hydrogen partial pressure. At the same time adsorbed hydrogen atoms are not stable under most reaction conditions at interfacial Pt and oxygen atoms, suggesting that these sites are available for the WGS reaction. Next, Figure 10 b shows Langmuir adsorption isotherms for competitive hydrogen and CO adsorption on the top two noninterfacial Pt atoms. KCO PCO pffiffiffiffiffiffiffi; 1 þ KCO PCO þ KH PH2 pffiffiffiffiffiffiffi KH PH2 θH ¼ pffiffiffiffiffiffiffi 1 þ KCO PCO þ KH PH2

θCO ¼

ð17Þ

It is apparent that at all relevant WGS reaction conditions the noninterfacial Pt atoms are covered by CO and not hydrogen (for larger Pt clusters we expect a higher hydrogen coverage on noninterfacial Pt atoms). In order to obtain more insights into the negative reaction order of the WGS with respect to H2, we investigated the stabilities of interfacial OH groups and adsorbed hydrogen atoms on interfacial vacancies as a function of hydrogen partial pressure. Figure 11 a illustrates the phase diagram (ΔG = 0 curve) for the reaction Ti3+ + H2O T Ti3+OH + 1/2H2. Above 400 K interfacial OH groups are not stable under most WGS reaction conditions, and as proposed by Azzam et al.,96 it is thermodynamically possible that H2 suppresses the formation of interfacial OH groups which are important intermediates in the WGS reaction mechanism and which might explain the negative hydrogen reaction order. Next, we investigated the stability of ionic hydrogen (Ti4+H) species at the oxygen vacancy under WGS reaction conditions. In the absence of a Ptn cluster, hydrogen atoms form a weak bridging interaction with the Ti atoms at the vacancy site of the TiO2(110) surface (dTiH = 1.9 Å). The adsorption energy calculated with reference to the energy of

Figure 10. (a) Gibbs free energy (ΔG) for the adsorption of hydrogen on Pt8/TiO2(110), Pt8/TiO2 + 1/2H2 f HPt8/TiO2, versus the hydrogen chemical potential (ΔμH). (b) Equilibrium surface coverage of CO and hydrogen as a function of PCO and temperature at constant H2 partial pressure (PH2 = 1 atm).

a hydrogen atom is 2.1 eV and it is +0.21 eV calculated with reference to the hydrogen molecule (Ti3+ + 1/2H2 T Ti4+H). In agreement with an earlier report,98 the adsorption of a hydrogen atom on the bridging oxygen atom is about 0.28 eV more stable than the formation of a hydride species. However, in the presence of a Pt cluster, hydrogen interacts strongly with the Pt atom neighboring the oxygen vacancy (Figure 11b) which transfers a negative charge to the hydrogen atom (Q[H] = 0.3). The adsorption energy calculated with reference to the energy of a hydrogen molecule for the Pt8/TiO2x system is 0.51 eV and that for the CO-covered system (2COPt8/TiO2x) is 0.87 eV. The free energy curve shown in Figure 11 b illustrates that these hydride species are thermodynamically stable under WGS reaction conditions even at low hydrogen partial pressures. To shed more light on the possible importance of hydride species under WGS conditions, we calculated Langmuir adsorption isotherms for competitive hydrogen and H2O adsorption at interfacial oxygen vacancies. K2OH PH2 O pffiffiffiffiffiffiffi; 1 þ K2OH PH2 O þ KH PH2 pffiffiffiffiffiffiffi K H P H2 ¼ pffiffiffiffiffiffiffi 1 þ K2OH PH2 O þ KH PH2

θ2OH ¼ θH

ð18Þ

The dissociative adsorption of H2O at the vacant site leads to the formation of two surface OH species, which is likely an important step in the WGS reaction mechanism. Figure 11 c indicates that increasing the hydrogen partial pressure promotes the formation of hydride species rather than the formation of surface OH groups. Overall, the presence of hydrogen seems to affect the stability of interfacial OH groups that are important intermediates for both the redox and carboxyl pathways of the 19256



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favorable and Pt clusters are (noninterfacial) surface oxygen free. Furthermore, noninterfacial surface Pt atoms are likely covered/ poisoned by CO and only interfacial Pt atoms are available for reaction. Adsorption of hydrogen atoms, at both interfacial Pt and oxygen atoms, are weak suggesting that hydrogen spillover from the metal to the surface and also hydrogen spillover from the reverse process are thermodynamically possible under WGS reaction conditions. Finally, we test and confirm the hypothesis from Azzam et al.96 that the negative hydrogen reaction order of the WGS on Pt/TiO2 might originate from hydrogen strongly interacting with interfacial oxygen vacancies forming hydride species that displace interfacial OH groups.

’ ASSOCIATED CONTENT

bS

Supporting Information. Effect of vibrational entropies on Gibbs free energies for the adsorption of an oxygen atom and CO molecule on the Pt8/TiO2 surface versus oxygen and CO chemical potentials, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Figure 11. (a) Phase diagram for hydrogen induced oxygen vacancy formation from interfacial OH groups on Pt8/TiO2(110). (b) Gibbs free energy (ΔG) for the adsorption of hydrogen at the oxygen vacancy of Pt8/TiO2x(110) versus the hydrogen chemical potential (ΔμH). (c) Equilibrium surface coverage (Langmuir isotherms) of hydrogen adsorption at the oxygen vacancy versus water adsorption and formation of two surface OH groups as a function of PH2 and temperature at constant H2O partial pressure (PH2O = 1 atm).

WGS. Future mechanistic studies that include all relevant rate processes will hopefully explain the precise reason for the negative hydrogen reaction order.

4. CONCLUSIONS Constrained ab initio thermodynamic simulations have been performed to investigate the nature of the Pt/TiO2 interface and to determine a realistic catalyst model for a future mechanistic investigation of the WGS reaction. Pt clusters bind strongly on both the stoichiometric and partially reduced TiO2(110) surfaces through covalent bond type interactions. The clustering of Pt metal atoms near oxygen vacancies on the TiO2 surface follows a close-packed arrangement with (111) facet, while away from oxygen vacancy defects a less dense arrangement with (100) facet is preferred. The strong interaction of Pt clusters with the TiO2 surface promotes the reducibility of the surface. This effect seems to decrease with increase in Ptn cluster size and converges with the cluster size at n = 68. In an oxidizing atmosphere, oxygen vacancies on the TiO2(110) surface should not play an important role in most reactions; instead oxidized Pt species might be important. Under reducing WGS reaction conditions, the formation of oxygen vacancies at the Pt/TiO2 interface is thermodynamically

’ ACKNOWLEDGMENT This work was supported by the National Science Foundation under Grant CBET-0932991 and in part TeraGrid resources provided by the National Center for Supercomputing Applications (NCSA), Louisiana Optical Network Initiative (LONI), the National Institute for Computational Sciences (NICS), Texas Advanced Computing Center (TACC), and Purdue University under Grant TG-CTS090100. Furthermore, a portion of this research was performed at the U.S. Department of Energy facilities located at Oak Ridge National Laboratory (CNMS2009-248), at the National Energy Research Scientific Computing Center (NERSC), and at EMSL, located at Pacific Northwest National Laboratory (Grant Proposal 34900). Finally, computing resources from the USC NanoCenter, USC’s High Performance Computing Group, and the Minnesota Supercomputing Institute for Advanced Computational Research are gratefully acknowledged. ’ REFERENCES (1) Cheng, J.; Hu, P. J. Am. Chem. Soc. 2008, 130, 10868. (2) Norskov, J. K.; Bligaard, T.; Hvolbaek, B.; Abild-Pedersen, F.; Chorkendorff, I.; Christensen, C. H. Chem. Soc. Rev. 2008, 37, 2163. (3) Reuter, K.; Scheffler, M. Phys. Rev. B 2003, 68, No. 045407. (4) Rogal, J.; Reuter, K.; Scheffler, M. Phys. Rev. B 2004, 69, No. 075421. (5) Reuter, K.; Frenkel, D.; Scheffler, M. Phys. Rev. Lett. 2004, 93 No. 116105. (6) Inoglu, N.; Kitchin, J. R. J. Catal. 2009, 261, 188. (7) Greeley, J.; Norskov, J. K. J. Phys. Chem. C 2009, 113, 4932. (8) Norskov, J. K.; Scheffler, M.; Toulhoat, H. MRS Bull. 2006, 31, 669. (9) Laursen, S.; Linic, S. J. Phys. Chem. C 2009, 113, 6689. (10) Iida, H.; Igarashi, A. Appl. Catal., A 2006, 298, 152. (11) Panagiotopoulou, P.; Christodoulakis, A.; Kondarides, D. I.; Boghosian, S. J. Catal. 2006, 240, 114. (12) Azzam, K. G.; Babich, I. V.; Seshan, K.; Lefferts, L. J. Catal. 2007, 251, 153. 19257


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TiO2(110) Interface under Water-Gas Shift Reaction

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