Adsorption of Atomic and Molecular Oxygen on the Au (321) Surface

J. Phys. Chem. C 2007, 111, 17311-17321

17311

Adsorption of Atomic and Molecular Oxygen on the Au(321) Surface: DFT Study Jose´ L. C. Fajı´n,† M. Nata´ lia D. S. Cordeiro,‡ and Jose´ R. B. Gomes*,† CIQ-UP, Centro de InVestigac¸ a˜ o em Quı´mica, Faculdade de Cieˆ ncias, and REQUIMTE, Faculdade de Cieˆ ncias, UniVersidade do Porto, P-4169-007 Porto, Portugal ReceiVed: May 16, 2007; In Final Form: September 4, 2007

Spin-polarized density functional theory based calculations within the GGA/PW91 exchange-correlation functional have been carried out to investigate the interaction of atomic and molecular oxygen on the Au(321) surface modeled by a periodic supercell approach. It was found that the atoms prefer interaction with surface cavities, while in the case of molecules, adsorption is more favorable if the molecular axis is parallel to the surface terraces on bridge or nearby bridge sites. Further, the interaction of separated atoms with the Au(321) surface may well induce, at a relatively low energetic cost, strong surface relaxation. The dissociation of molecular oxygen on the Au(321) surface is exothermic, and the barrier for the dissociation reaction, determined using the climbing-image nudged elastic band method, is 1.00 eV and with an interatomic oxygen distance for the transition states of ∼1.91 Å.

1. Introduction Heterogeneous catalysis on gold surfaces with different Miller indices and on supported small gold nanoparticles has been the aim of many works in past years.1 The oxidation of carbon monoxide at low temperature was intensively studied due to the uncertainty in the reaction mechanism.1 It is not clear yet whether the oxidation is due to the intrinsic characteristics of the gold clusters or due to their changed properties by interaction with the support. To reach a deeper understanding of the role of gold catalysts on CO oxidation at low temperature, Mavrikakis et al.2 used three different modelssperiodic Au(111) and Au(211) slabs intended to mimic pure gold catalysts and a twolayer cluster used as a model for a gold nanoparticlestogether with a density functional theory (DFT) approach. They suggested that the high step densities and/or strain effects, due to the mismatch at the Au-supported interface, are in part the cause of the unusual catalytic activity of the highly dispersed gold particles. A key step in the CO oxidation on gold surfaces is the activation of reactive oxygen species; this activation can occur by the dissociative adsorption of molecular oxygen, by direct deposition of atomic oxygen on the surface, by hydroxyl or by hydroperoxyl surface intermediates if coadsorbed water exists, or by direct reaction with molecular oxygen coadsorbed on the surface. Liu et al.3 demonstrated by carrying out DFT calculations on several different Au surfaces that the oxidation of the carbon monoxide may occur following one of two different mechanisms, that is, by reaction with molecular oxygen forming a transition state involving four atoms (O2 + CO) or by reaction with atomic oxygen, in which case previous dissociation of molecular oxygen is necessary. The lowest energy barrier for the dissociation of molecular oxygen was found on the steps of the Au(211) surface, being 0.93 eV. On the other surfaces studied, the barrier increases up to 1.16 eV on Au(221) and 2.23 eV on Au(111), showing that on the latter surface the * Corresponding author. Phone: +351 220402514. Fax: +351 220402659. E-mail: [email protected]. † CIQ-UP, Centro de Investigac ¸ a˜o em Quı´mica. ‡ REQUIMTE.

dissociation of molecular oxygen is almost unfeasible. The most favorable paths for the reaction of carbon monoxide with atomic oxygen or with molecular oxygen have barriers of 0.25 and 0.46 eV, respectively, on the steps of the Au(221) surface. In 2003, Xu et al.4 reported the use of a periodic generalized-gradient approach (GGA), along with the PW91 density functional, to study the dissociation of molecular oxygen on strained and unstrained gold surfaces with and without steps. They found that molecular oxygen only adsorbs on the Au(111) surface when it is stretched by 10%; its adsorption energy is -0.08 eV and the energy required for its dissociation is computed as 1.37 eV. On the Au(211) surface the adsorption on the unstrained and strained surfaces is favorable, with binding energies of -0.15 and -0.26 eV, respectively. The energy barriers for oxygen dissociation are 1.12 eV on the unstrained surface and 0.63 eV on the strained one. Also in 2003, several Au(n)O2(0,-) (n ) 2-8) complexes were investigated with a DFT approach by Yoon et al.,5 who obtained higher binding energies for the anionic complexes than for the neutral ones. Dissociative adsorption is favored when n g 4 and molecular oxygen adsorption is favored when n e 3 for both charged and uncharged complexes. The lowest energy barrier for the dissociation of oxygen was calculated as 1.37 eV for the Au6O2complex. Experimentally, Visart de Bocarme´ et al.6 concluded from their mass spectroscopic measurements that there is neither adsorption of molecular oxygen nor dissociation (yielding surface atomic oxygen) on several gold facets (different Miller indices with few point defects) of the crystal under the experimental conditions (pressure lower than 10-4 mbar and temperature between 300 and 350 K). Further, the use of a static electrostatic field does not enhance molecular oxygen dissociation or adsorption. These authors have also carried out DFT calculations (B3LYP exchange-correlation functional) on a three-layer Au10(111) cluster, consisting of one, six, and three Au atoms per layer, reaching much smaller interaction energies, lower than 0.023 eV, for the interaction of molecular oxygen with the cluster model, supporting their experimental findings. The combination of the PW91 approach with a Au1/Au(100)

10.1021/jp073796y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/25/2007

17312 J. Phys. Chem. C, Vol. 111, No. 46, 2007 periodic slab model resulted also in the small interaction energy of 0.25 eV. They also noticed that the inclusion of electric fields in the DFT calculations increases the energy barrier computed for the dissociation of O2, which is in agreement with the field desorption mass spectra. Kim et al.7 concluded from their temperature programmed desorption (TPD) studies employing several spectroscopic techniques that the unstrained Au(211) surface does not have a special reactivity for oxygen dissociation even under highpressure conditions. An important finding from their experiments is the evidence for a slightly stronger binding of oxygen adatoms on surface steps than on terraces. On the most favorable step sites, the desorption activation energy for step-adsorbed oxygen is 1.47 eV. Finally, these authors suggest that the presence of Au(211) facets or other facets with higher densities of steps cannot explain the novel catalytic activity of supported gold nanoparticles. A different picture seems to arise from the experimental works of Xu et al.1l and Min et al.1j The former authors found that unsupported nanoporous gold (NPG) can catalyze the oxidation of carbon monoxide, suggesting that the catalytic activity of the gold nanoparticles arises from its structure. They also observed the formation of gold oxides. Min et al.1j observed that the rate of CO2 production depended on the form of the oxygen deposition on Au(111). They identified three different types of oxygen deposition on the Au(111) surface, i.e., chemisorbed oxygen, a surface oxide, and a bulk oxide. The formation of these types of deposited oxygen depends on the temperature and on the oxygen coverage. They also demonstrated that these three types of oxygen deposition have different reactivities, with the chemisorbed oxygen being the most active for CO oxidation and the bulk oxide the least reactive. Interestingly, the highest rate of CO oxidation is found at low temperature and for the maximum density of chemisorbed oxygen. The mechanism of CO oxidation catalyzed by gold surfaces may be of even higher complexity if water is present on the surface. In this situation, other oxygen types are possible, such as those studied recently by Vassilev et al.8 These authors have also employed the PW91 exchange-correlation functional in the study of the relative stability of several types of oxygencontaining species (O2, O, OH, OOH, and H2O2) and concluded that O-O bond dissociation is thermodynamically possible for adsorbed O2, OOH, and H2O2 species and that hydroperoxyl could be an important intermediate during the electrochemical reduction of oxygen on Au(100) or Au(111) electrodes. This suggests that other surface species rather than those studied by Min et al. may change drastically the reactivity of gold surfaces toward CO oxidation and that further computational and experimental investigations in this area are still needed. The main aim of the present work is to contribute to a better understanding of the catalytic properties of gold surfaces on the dissociation of molecular oxygen, which seems to be a key step in the reaction of CO oxidation. To accomplish that objective, a high Miller index gold surface, namely Au(321), is considered. This stepped surface has Au atoms forming a zigzag step line and presenting a rather high heterogeneity of adsorption sites, i.e., surface positions in the middle of terraces, nearby kinks, or steps, resembling more a real catalytic surface than other surfaces with lower Miller indices. This paper is organized as follows. The computational methods are described in detail in section 2, while the calculated results are reported and discussed in section 3. Finally, in the last section, the most important conclusions are summarized.

Fajı´n et al. 2. Computational Details 2.1. DFT Approach. The density functional theory calculations have been carried out by means of the VASP 4.6.3 computer code.9 These calculations resorted to the GGA-PW91 functional proposed by Perdew et al.10 The projected augmented wave (PAW) method due to Blo¨ch11 and further implemented by Kresse and Joubert12 was employed to describe the effect of core electrons on the valence shells together with a plane-wave basis set used to span the valence electronic states. The cutoff energy for the plane waves was 415 eV. During the calculations, quasi-Newton and conjugate-gradient algorithms were used to relax the positions of the ions. A spin-polarized approach has been used throughout all the calculations, and the relaxations were stopped when the change in the total energy between successive steps was less than 0.0001 eV. This approach is faster in terms of computing time and yields practically the same results, energy and geometric differences less than 0.01 eV and 0.01 Å, as those coming from calculations using a force convergence criterion of 2 > 3. 3.2. Adsorption of Molecular Oxygen. The adsorption of undissociated molecular oxygen with the O-O bond placed normal or parallel to the terraces (or steps) on the Au(321) surface was studied for several combinations of sites shown in Figure 2. The full set of energetic and geometric results are given in Table 2. The interaction is more favorable if it occurs with the oxygen molecule parallel to the surface plane. Nevertheless, the difference between the adsorption energies in the case of the most favorable configuration for molecular oxygen parallel or normal to the surface is only 0.16 eV. Even this is a small energetic difference, as can be seen in Figure 3, there are important electronic differences between the two configurations. In the case of molecular oxygen adsorbed horizontally, there is clear loss of electron density in the nearby metal atoms and also in the regions between the oxygen atoms with a gain of electron density above and below the oxygen atoms. When molecular oxygen is adsorbed with its axis vertical to the surface, and considering the same cutoff for the isodensity surfaces, only a local reorganization of the electron density

around the oxygen atoms is observed. According to Bader partitioning, the results above are in agreement with the 60% larger negative charge found for horizontal molecular oxygen than for vertical molecular oxygen adsorbed on the surface. The most favorable configuration is when the O2 molecule interacts with the bridge connecting atoms 1 and 2 (b1-2). The adsorption on this position is characterized by an energy, calculated using eq 2, of -0.17 eV and a distance between the two oxygen atoms of 1.31 Å. The distances to the nearest surface gold atoms are 2.23 and 2.38 Å for atoms 1 and 2, respectively. It must be pointed out here that, after optimization, the final position of the oxygen molecule is above a bridge site (short bridge or cross-bridge) and, therefore, the number of possible adsorption configurations is reduced. Again, the comparison of the present results for the relaxed Au(321) with those computed previously for the stretched or unstretched Au(111) and Au(211) surfaces shows that the interaction energies computed for O2 adsorbed on Au(321) are more favorable than for O2 adsorbed on the unstretched Au(111) or Au(211) surfaces.4 In the case of the 10% stretched surfaces considered by Xu et al.,4 the adsorption of molecular oxygen on the relaxed Au(321) surface is 0.09 eV more favorable than that on the 10% stretched Au(111) surface and 0.09 less favorable than that on the 10% stretched Au(211) surface. Most importantly, the fact that computed adsorption energies for the kink and steps sites and also for some terrace sites are negative, hence favorable, suggests that the movement of molecular oxygen on the Au(321) surface might be possible as long as barrier heights for transition between sites are expected to be small. In fact, the energy barrier for atomic oxygen diffusion on the Au(111) surface has been very recently calculated by Lilley and Meyer as 0.39 eV.25 This value is identical to 12% of the adsorption energy computed for the oxygen atom on the most stable hole site a, which is 0.40 eV. The rule that 12% of the binding energy of the adsorbed state is fairly the diffusion activation energy barrier has been recently introduced by Nilekar et al.26 after the computation of adsorption and diffusion barrier energies for 26 different adsorbate/surface pairs on seven different transition metals deliberately chosen to cover a wide range of adsorbate binding energies (-7 to 0 eV). Finally, using the present methodology, configurations for molecular oxygen that could be linked with physisorbed states, such as that reported previously by Gravil et al. for molecular oxygen adsorption on the Ag(110) surface,27 were not found. 3.3. Oxygen Dissociation. The possible final states for dissociated molecular oxygen on the Au(321) surface model

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TABLE 3: Adsorption Energies (eV) and Distances (Å) between Oxygen Atoms and between Oxygen and Nearest-Neighbor Gold Atoms for Dissociated Oxygen Adsorbed on the Au(321) Surfacea oxygen dissociation positionb

Eads

distancesc Oa-Ob; Oa-Au; Ob-Au

hole “a”-hole “f” hole “b”-b2-1 hole “a”-b1-4 hole “a”-hole “g” hole “b”-hole “g” surf. oxid. (kink) b1-2-hole “e” b1-2-hole “f” top “1”-hole “e” surf. oxid. (terrace) top “1”-hole “c” top “1”-hole “d”

0.61 0.41 -0.23 0.82 0.71 0.00 0.67 0.89 1.41 0.04 1.50 1.17

3.27, 2.10 (2); 2.12 (3) 3.10; 2.00 (1); 2.10 (3) 3.18; 2.01 (1); 1.96 (1) 3.14; 2.10 (3); 2.03 (1) 2.82; 2.05 (1); 2.07 (3) 2.93; 2.00 (1); 1.96 (1) 3.35; 2.01 (1); 2.12 (4) 3.17; 2.09 (1); 2.01 (2) 3.99; 1.90 (1); 2.12 (3) 2.74; 2.00 (1); 2.01 (1) 3.76; 1.88 (1); 2.12 (2) 3.86; 1.92 (1); 2.13 (3)

oxygen dissociation positionb

Eads

distancesc Oa-Ob; Oa-Au; Ob-Au

top “2”-hole “f” hole “a”-hole “e” hole “b”-hole “c” hole “b”-hole “e” hole “a”-hole “d” hole “c”-hole “g” hole “d”-hole “h” flat surf. hole “d”-hole “g” hole “d”-hole “f” top “2”-hole “e” hole “g”-hole “f”

1.61 0.53 1.09 0.71 0.64 0.10 -0.35 0.66 0.41 1.40 1.59 0.68

3.69; 2.10 (1); 1.94 (2) 3.34; 2.08 (1); 2.12 (4) 3.15; 2.06 (1); 2.13 (3) 3.92; 2.06 (1); 2.17 (4) 3.07; 2.07 (1); 2.09 (3) 2.68; 2.00 (2); 1.99 (2) 3.12; 2.01 (2); 2.02 (2) 2.76; 2.09 (1); 2.08 (3) 3.53; 2.07 (3); 2.01 (1) 2.05; 1.97 (1); 2.16 (3) 3.52; 1.93 (2); 2.12 (3) 2.86; 2.16 (3); 2.04 (2)

a Numbers in italics and inside parentheses are labels for surface gold atoms; cf. Figure 2. b First and second terms denote the adsorption sites for Oa and Ob atoms, respectively. Three special cases were found, two resembling surface oxidation (surf. oxid.) with Au-Au bond cleavage, and one other with strong deformation of the surface (flat surf.); they are depicted in Figure 4. c The use of a periodic approach means that in, some cases, the shortest O-O distances are for oxygen atoms belonging to different unit cells.

used have been studied by consideration of two oxygen atoms adsorbed on the same unit cell used in the studies presented in previous sections. In other words, the configurations that will be presented below refer to atomic oxygen surface coverage that is double that reported in subsection 3.1; hence, the positioning of two oxygen atoms on two identical sites is not possible. The energetic and geometric data for dissociated oxygen on the relaxed Au(321) surface appear in Table 3. The adsorption energies were calculated with respect to the energies of the relaxed clean surface plus the energy of the O2 molecule in the gas phase, eq 3. As can be seen in Table 3, 24 different combinations of sites have been considered and only in two situations the calculated interaction energy is favorable. One of these two cases is that where one of the oxygen atoms is adsorbed on one cavity from the terrace and is labeled in Figure 2 as “d”, while the other is a nearby hollow site from the step labeled “h” in the same figure. A representation of this most stable configuration is given in Figure 4, and the computed adsorption energy is -0.35 eV. The distances between the oxygen adsorbed in hole d with respect to the nearest-neighbor 2, 3, and 4 atoms are 2.01, 2.20, and 2.21 Å, respectively; the distances from the oxygen adsorbed in the step cavity h to the atoms 1, 2, and 5 are 2.06, 2.02, and 3.35 Å, respectively. The other situation where the interaction energy, calculated using eq 3, is negative is that where one of the oxygens is on an fcc hollow site a with another on the bridge connecting atoms 1 and 4. The O-O distances are 4.47 Å considering atoms on the same terrace and 3.18 Å considering atoms on different terraces connected by a single step. In the latter case, the adsorption energy has a value of -0.23 eV. Importantly, the calculations show two situations where the interaction energy for two coadsorbed oxygen atoms on the Au(321) model is more negative than the sum of the interaction energies for separated oxygen atoms deposited on the identical sites, that is, the sum of the interaction energies given in Table 1. One of those two cases is the most stable configuration given in Table 3, that is, for oxygen atoms deposited on holes d and h, which is represented in Figure 4. The second configuration is that with the oxygen atoms adsorbed on holes c and g. The energy gains with respect to the sum of the interaction energies compiled in Table 1 for oxygen atoms adsorbed on holes d and h and on cavities c and g are 0.60 and 0.52 eV, respectively. Interestingly, in the former situation, the interaction energy of -0.35 eV is -0.03 eV more negative than the twice the value

of the interaction energy for atomic oxygen on hole a, which is the most stable adsorption site for atomic oxygen as reported in Table 1. The main reasons beyond these enhanced interaction energies for coadsorbed oxygen atoms are the transfer of charge from the surface to the oxygen atoms and the strong induced relaxation of the surface. In the most stable configuration, atom 2 moves upward by ∼0.6 Å, suggesting that the O-Au[atom 2]-O interaction is rather strong; this reconstruction seems to be helped by the surface shape where some surface atoms have a much lower coordination than those belonging to the bulk. The analysis of the other combinations of adsorption sites considered shows that, in absolute values, the rest of adsorption energies are generally larger than the absolute values of the interaction energies stated above for the two favorable configurations. There are three exceptions where the interaction energy is positive but not higher than 0.1 eV; in two of these three cases, the structures obtained after optimization show elongations of Au-Au bonds larger than 20% of the optimum value found in the case of the clean Au(321) surface and, in one other case, one of the oxygens is adsorbed on a hcp c site and the other on cavity g of the (100) facets near the bridge connecting atoms 1 and 2. The former two situations where there are strong elongations of Au-Au bonds, suggesting surface oxidation, are shown in Figure 5a,b. The most important finding here and at the coverage considered, i.e., an unit cell with two oxygen atoms per four gold atoms from the outermost layer, is that the most favorable configurations with both oxygens on the terrace introduce significant deformation of the surface. Else, the most favorable configurations are those having one of the two oxygen atoms interacting with the steps. During the optimization procedure, a curious case appeared where the steps seemed to disappear by the adsorption of oxygen; the optimized flat surface is shown in Figure 5c. The energetic difference to the most favorable situation is about 1 eV, and the O-O shortest distance is 2.76 Å. In this case, two oxygen atoms are interacting with the atom 3 in the middle of the terrace and, as a consequence of those interactions, atom 3 moves upward with a significant increase of the Au-Au bond length connecting atom 3 and an internal atom denoted as X; in the clean surface the distance is 3.05 Å and in the case of the flat surface the distance is increased to 3.60 Å. Further, the distances between atoms 1 and 2 in the case of the clean surface (Figure 1) change from 2.83 and 2.86 Å to 2.69 and 2.83 Å (Figure 5c).

Oxygen Adsorption on Au(321) Surface

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Figure 5. Lateral and top views of differently reconstructed Au(321) surfaces by adsorption of oxygen atoms. The ratio between oxygen and surface gold atoms is 1/2. Distances are in angstroms and angles are in degrees. (a) The two oxygen atoms are interacting with the kink, bridging atom 1, and atoms 2 or 3. (b) The two oxygen atoms are interacting with the terrace, bridging atom 3, and atoms 1 or 2. (c) The two oxygen atoms interact with the surface in such a way that it resembles a flat surface (the dashed lines illustrate the terrace where atoms 1, 2, 3, and 4 were originally placed in the case of the clean Au(321) surface).

The formation of the flat configuration depicted in Figure 5c resembles a gold oxide adlayer. These kinds of metal oxide adlayers were found to be rather stable and suggested to have a critical role in the catalytic activity of metals, but a clear understanding about the initial steps that lead to the formation of these films is still lacking.28 Recent experimental and computational studies show that, under the oxygen-rich conditions of oxidation catalysis, these structures could be very stable with respect to the clean catalysts and gaseous molecular oxygen, and many possible structures for oxygen deposition on metal surfaces are being determined.28-30 The formation of oxides on gold surfaces was experimentally corroborated in regimes of high oxygen coverage.1j,l Here, the association of high oxygen coverage with oxidation and reconstruction of the gold surface was tested by using a 4 times larger unit cell with 60 gold atoms and keeping two oxygen

atoms per cell. In the case of the structure shown in Figure 5b, denoted as surface oxidation at the terrace in Table 3, i.e., with the atoms initially adsorbed on cavity b or f in the case of the unreconstructed surface, the increase of the distance between atoms 4 and 3 is not observed suggesting that low oxygen coverage prevents the reconstruction of the surface. Further, the interaction energy for the adsorption of oxygen atoms on holes b and f on this larger cell is 0.99 eV, supporting the experimental evidence in which the oxidation of the surface is linked with high oxygen coverage. However, the picture arising from the use of the larger unit cell to study the “flat surface” shown in Figure 5c is somewhat different, as shown in Figure 6. In fact, it is found that surface atom 3 interacting with the two oxygen atoms is moved upward from the surface as shown in Figure 6, which suggests that oxidation of the surface is possible when two oxygen atoms interact directly with the same surface atom.

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Figure 6. Optimized configuration for two oxygen atoms interacting with 1/4 of the surface atoms 3 on the larger 2 × 2 × 1 unit cell. Distances between atom 3 and neighboring 1, 2, and 4 surface atoms are given in angstroms.

TABLE 4: Summary of O2 Dissociation on the Au(321) Surfacea,b dissociation path

Ea (eV)

dO-O (Å)

ME (eV)

ν (cm-1)

along the step across b1-2 across b2-1

1.20 1.21 1.00

1.93 1.88 1.91

0.48 0.56 0.65

470 682 620

a

The energy of the transition state structures with respect to the clean Au(321) surface and gaseous molecular oxygen is Ea + ME 0.17 eV. b Ea is the energy barrier, ME the migration energy, and dO-O the distance between the oxygen atoms.

The Au3-Au1,2,4 distances increase by 0.15-0.35 Å while that to the AuX atom increases from 3.6 Å, Figure 5c, to 4.4 Å, Figure 6. The calculated charges on the oxygen atoms and also on the surface gold atom 3 are identical to those computed using the smaller cell. Bader’s charges are -0.8 au in the oxygen atoms and +0.5 au in the case of atom 3. 3.4. Reaction Profile for Oxygen Dissociation on Au(321). The strategy used in the present study for the determination of the profile for the reaction of oxygen dissociation catalyzed by the Au(321) surface considered all the possible starting geometries for molecular oxygen with its axis horizontal to the surface and compiled in Table 2. Exceptions were those structures for which the calculated interaction energies were above 0.2 eV. The consideration of a quite large number of initial geometries was due to the fact that energetic differences between the possible adsorption configurations were small and were expected to be much smaller than the energetic barriers for oxygen dissociation. In an initial phase of the work, we could easily detect that the reaction path for some initial configurations seemed to follow the same direction, which diminished the number of possible routes, and in fact, only three routes were found. The transition state (TS) configurations determined with the cNEB calculations were further optimized in order to reduce the forces acting on the atoms and were characterized as real TS structures by calculation of vibrational frequencies. The final results are compiled in Table 4, with the TS configurations shown in Figure 7. From the values already discussed in previous subsections and given in Tables 2 and 3, the reaction of oxygen dissociation on the Au(321) is thermodynamically favorable; the results in those two tables show an exothermic reaction (-0.18 eV) if we consider the most stable molecular oxygen adsorption on the surface as the initial state and the most stable dissociation as the final state. This energetic difference includes the energy required for the translational movement of the adsorbed species on the surface, i.e., the energy required to move molecular oxygen to the place where O-O bond breaking occurs and also

Figure 7. Structures of the three different transition states for the reaction taking place (a) along the step, (b) on the step across bridge 1-2, and (c) on the step across bridge 2-1. Distances are in angstroms and angles are in degrees.

for the migration of the dissociated oxygen atoms to the most stable final positions. Thus, in Table 4, the reaction barrier energies (RBEs; Ea’s) are obtained from the energy of the transition state structures minus the energy of the initial state that are not the same as the optimized positions reported in Table 2. The energy required to go from the most stable configuration reported in Table 2 for molecular oxygen adsorbed on the Au(321) surface to the initial states is the migration energy (ME); ME’s are also listed in Table 4. Three different dissociation paths were obtained; the first is located along the step and around atom 1 (shown in Figure 7a). This dissociation path is characterized by an energy barrier of

Oxygen Adsorption on Au(321) Surface

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Figure 8. Representation of the dissociation path across the bridge 2-1 (b2-1) of the Au(321) surface. Dashed marked circles on the left-hand side show the preferred geometry for O2 adsorbed on the surface (these two circles are separated from each other to permit a better understanding of the atomic positions), while dashed circles are used to illustrate the positions of the most stable adsorption configuration for dissociated oxygen. The reaction path appears as full-line circles, and white is used for the positions of the rotated initial state, light gray is used for the transition state positions, and black is used for the final state positions. The inset shows the energy variation along the reaction coordinate.

Figure 9. Side (top) and front (bottom) views of the calculated electron density differences of adsorbed molecular oxygen (a), of the transition state (b), and of coadsorbed oxygen atoms (c) for the reaction of molecular oxygen dissociation across the bridge 2-1 (b2-1) of the Au(321) surface. Dark and light gray represent electron density difference isosurfaces corresponding to electron energy gain and electron energy loss with respect to the separated systems.

1.20 eV, and the O-O interatomic distance of the transition state structure is 1.93 Å. The other two dissociation paths are located on the step across the bridges defined by atoms 1 and 2, 1-2 (b1-2) and 2-1 (b2-1), that is, with one oxygen atom interacting with the bridge defining the (110) and (100) facets, respectively. The transition state structures for these two reaction paths are shown in Figure 7b and Figure 7c, respectively. The former transition state has an energy barrier of 1.21 eV and the O-O distance is 1.88 Å. The latter is the one with the lowest energetic barrier, 1.00 eV; the O-O interatomic distance for the transition state is 1.91 Å. The combination of RBEs and

ME’s reported in Table 4 show that the dissociations along the step and across the bridge b2-1 need almost the same energy, 1.68 and 1.65 eV, respectively. These values are not far from the global energetic variation calculated for the reaction across the bridge b1-2, almost suggesting that any of the three reaction paths is possible for the dissociation of molecular oxygen catalyzed by the Au(321) surface. Detailed positions for the oxygen species interacting with the surface along the reaction path with the lowest energy barrier, i.e., dissociation across the bridge 2-1 (b2-1), are shown in Figure 8. Along the reaction, the charges on the oxygen atoms

17320 J. Phys. Chem. C, Vol. 111, No. 46, 2007 become more negative as expected on going from molecular oxygen to dissociated oxygen atoms adsorbed on the Au(321) surface. Interestingly, the charge variation on going from adsorbed molecular oxygen to adsorbed oxygen atoms (see Figure 8) follows an almost linear behavior. The charges associated with each of the O atoms obtained according to Bader partitioning along the reaction path are -0.179 and -0.193 (adsorbed O2), -0.344 and -0.322 (initial state), -0.516 and -0.533 (transition state), -0.696 and -0.719 (final state), and -0.779 and -0.804 (dissociated oxygen), showing that charge transfer from the surface assumes an important role during the reaction of dissociation of molecular oxygen. A better interpretation of what is happening during the dissociation of molecular oxygen is obtained from the views of the electron density differences shown in Figure 9. It is easily seen that in the initial state there is a depletion of charge in the region between the oxygen atoms and that the negative charge comes essentially from the nearest surface gold atoms which belong to the bridge b1-2. In the case of the transition state structure, besides depletion of charge in the gold atoms from the bridge, regions of electron density loss are also observed for the atoms in the lowest part of the step. Finally, these findings are even more evident in the case of the final state configuration. The calculated barriers for molecular oxygen dissociation on the Au(321) surface are very similar to those reported recently by Xu and Mavrikakis4 and also by Liu et al.3 for the Au(211) surface. These barriers are indeed much smaller than those computed for the planar Au(111) surface.3 However, a direct comparison with those works is not easy since in the former a slightly different strategy was used and in the latter a different exchange-correlation functional was considered. Nevertheless, as concluded from a very recent experimental work,1l the unique catalytic properties of gold seem to be due only to its particular structure. In that work due to Xu et al., the surface irregularity of the most active a-NPG catalyst is much more evident than that of the less active f-NPG catalyst. Therefore, from the intrinsic structures of these nanoporous gold catalysts it seems that stepped gold surfaces would be more active than flat ones due to the increased irregularity of the former surfaces, which is also corroborated by the theoretical work. However, much more work is needed in this area since another contemporary experimental study7 suggested that molecular oxygen does not dissociate on the Au(211) surface even under high pressures of oxygen and temperatures of 300-450 K. 4. Conclusions A slab periodic model has been used to study the interaction of both atomic and molecular oxygen with a stepped Au(321) surface. In the case of the adsorption of oxygen atoms, differing surface coverage has been considered. First, the study focusing on the adsorption of a single oxygen atom on a unit cell consisting of 15 gold atoms and four atomic layers parallel to (111) terraces showed that the interaction was stronger on the cavities. It was found that the fcc hollow site near the edge of the step connecting (111) terraces was the most stable site. In particular, the energy of the oxygen atom interacting with that site was more negative than the energy of the separated slab plus half the energy of the oxygen molecule; in other words, oxygen dissociation is thermodynamically favorable on this catalytic surface. Then, on the same unit cell, two oxygen atoms were deposited on the surface. Since there is only an fcc hollow site near the edge of the step, the preferred configurations still involve adsorption on cavities but not necessarily the same cavity found to be the most stable when only a single atom

Fajı´n et al. was deposited on the same slab. Interestingly, in one of the preferred configurations, one of the oxygens is positioned above a hollow site in the middle of the (111) terrace while the other one is interacting directly with the step, that is with the (110) facets. In the case of the second most stable configuration, the two oxygens are placed near the edge of the step: one adsorbed on a hollow site and the other one on a step bridge connecting two consecutive (111) terraces. Since several different starting configurations have been studied, some resulted in quite interesting structures with oxygen atoms interacting with a single surface gold atom, weakening Au-Au interaction, and resembling a sort of an initial stage of surface oxidation. Surface reconstruction is achievable at a low cost, 0.66 eV, when compared with the energy of the free slab and the gas-phase O2 molecule, and a practically planar surface was found. The adsorption of molecular oxygen on the Au(321) surface is more favorable if the O-O axis is planar to the surface terraces. However, since the interaction energy is small, the energetic difference between the most stable configuration with the axis planar to the surface (above a bridge site) and the most stable one with the axis normal to the surface (above a top site) is only 0.16 eV. The global profile for oxygen dissociation on the Au(321) surface was obtained from the determination of transition state structures for that reaction. The most favorable reaction path is that starting from the preferred adsorption configuration for the oxygen molecule, i.e., a bridge site at the edge of the terrace, and involves a previous rotation of the molecular axis. The energy of the transition state structure is 1.00 eV above that of the initial rotated oxygen molecule, the interatomic distance in the transition state is 1.91 Å, and one of the oxygens is almost falling down to the nearby terrace and the other is holding onto the top of the step. At the end of the reaction, the oxygen atoms are adsorbed near the step but on different terraces. These results are similar to those obtained previously for the same reaction on the Au(211) surface, showing the importance of the surface steps in the oxygen activation on gold surfaces. However, the use of the present results requires some caution since previous experimental work showed the absence of molecular oxygen dissociation on the Au(211) surface even under high-pressure (700 Torr) conditions with the sample at 300-450 K. Nevertheless, both experimental and theoretical works show unequivocally that step sites do bind oxygen adatoms more tightly than do terrace sites. Acknowledgment. Thanks are due to the Fundac¸ a˜o para a Cieˆncia e Tecnologia (FCT), Lisbon, Portugal, and to FEDER for financial support to CIQUP and to REQUIMTE. J.L.C.F. and J.R.B.G. acknowledge FCT for Grants SFRH/BPD/27167/ 2006 and SFRH/BPD/24676/2005, respectively. References and Notes (1) (a) Grzybowska-Swierkosz, B. Catal. Today 2006, 112, 3. (b) Chen, M.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (c) Arenz, M.; Landman, U.; Heiz, U. ChemPhysChem 2006, 7, 1871. (d) Manzoli, M.; Boccuzzi, F.; Chiorino, A.; Vindigni, F.; Deng, W.; Flytzani-Stephanopoulos, M. J. Catal. 2007, 245, 308. (e) Kim, J.; Samano, E.; Koel, B. E. J. Phys. Chem. B 2006, 110, 17512. (f) Qian, L. H.; Wang, K.; Li, Y.; Fang, H. T.; Lu, Q. H.; Ma, X. L. Mater. Chem. Phys. 2006, 100, 82. (g) Pala, R. G. S.; Liu, F. J. Chem. Phys. 2006, 125, 144714. (h) Wang, Y.; Gong, X. G. J. Chem. Phys. 2006, 125, 124703. (i) Stiehl, J. D.; Gong, J.; Ojifinni, R. A.; Kim, T. S.; McClure, S. M.; Mullins, C. B. J. Phys. Chem. B 2006, 110, 20337. (j) Min, B. K.; Alemozafar, A. R.; Pinnaduwage, D.; Deng, X.; Friend, C. M. J. Phys. Chem. B 2006, 110, 19833. (k) Sadek, M. M.; Wang, L. J. Phys. Chem. A 2006, 110, 14036. (l) Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42. (2) Mavrikakis, M.; Stoltze, P.; Nørskov, J. K. Catal. Lett. 2000, 64, 101.

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