A DFT Study - American Chemical Society

Feb 11, 2013 - TPPD, Pakistan Institute of Nuclear Science & Technology ... University, Islamabad and Pakistan Institute of Engineering and Applied Sc...
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Beneficial Effect of Cu on a Cu-Modified Au Catalytic Surface for CO Oxidation Reaction: A DFT Study Akhtar Hussain* TPPD, Pakistan Institute of Nuclear Science & Technology (PINSTECH), P. O. Nilore, Islamabad, Pakistan National Centre for Physics (NCP), Quaid-e-Azam University, Islamabad and Pakistan Institute of Engineering and Applied Sciences (PIEAS), Islamabad, Pakistan ABSTRACT: Density functional theory calculations within the generalized gradient approximations were employed to examine the molecular oxygen adsorption, decomposition, and CO oxidation upon a number of Cu modified Au surfaces cleavaged in (100) orientation. Amount of Cu was varied on the Au slab to optimize the model, which serves as the best one to investigate the synergic effect between Au and Cu for CO oxidation process. The adsorption energy of O2 varies between −0.61 and −2.20 eV depending on the surface composition. O−O bond activation up to 1.86 Å has been observed for the most favorable configuration. CO adsorbs preferably on top configuration of the Cu−Au catalytic surface with an Eads of −1.12 eV. The atomic O adsorption is strongly site specific, 4-fold hollow being the most favorable. A coincident activation barrier of 0.60 eV has been found employing constraint minimization and cNEB method for CO2 formation. Initially, a stable O−CO complex is formed, which leads to CO2 formation without the expenditure of any significant energy. Both the O−CO complex formation and its conversion to CO2 are thermodynamically favorable processes. The process has been repeated on a pure Cu(100) surface to distinguish the role of Au. CO on a bridge yields the highest adsorption energy −0.97 eV, being slightly less stable on other locations. O2 has strong adsorption on this surface as well, but bond activation and Eads are comparatively less. The minimum energy path follows a similar route to CO2 as on a Au−Cu surface but with higher activation energy and no exothermicity. CO2 created desorbs by surmounting a small barrier through an exothermic process. Comparison suggests that the Cu-modified Au surface is superior and more active than pure Cu toward CO oxidation. Analogous to the Au−Cu bimetallic structure model, a Ag−Cu slab consisting of a top monolayer of Cu and three layers beneath Ag was considered for comparison. A similar pronounced effect regarding O2 adsorption and an easy CO oxidation reaction was revealed. The results suggest that Cu bearing Au and Ag surfaces appear to be good candidates for low-temperature CO oxidation.

1. INTRODUCTION Auafter Haruta et al.1 discovered that its nanoparticles (2−5 nm) were exceptionally active for CO oxidationhas remained an important subject of research and debate among scientists. However, recently, Liu et al.2 have demonstrated that Au−Cu alloy nanoparticles have even superior performance than their monometallic counterparts. Owing to the better performance of alloys, these are attracting a great deal of attention not only from academic but also industrial viewpoints. In general, synergy between the two components of a bimetallic catalyst can be understood in terms of ensemble and ligand effect, i.e., one component may serve as a spacer to isolate active sites3,4 or as an electronic modifier to the other component.4,5 Some examples where bimetallic catalysts have shown superior performance are discussed here. Pd−Au bimetallic systems are widely used as selective hydrogenation catalyst.6,7 Au−Ag alloy catalysts were prepared by a one-spot synthesis route, and a strong synergy was observed between Au and Ag for CO oxidation.8,9 Au−Cu alloy nanoparticles, highly dispersed in the channels of SBA-15 support were synthesized. These nano© 2013 American Chemical Society

particles showed high resistance against sintering even after high temperature treatment and were found to be highly active for CO oxidation in the presence of H2.10 In practical situations, a number of factors influence the catalytic performance of the bimetallic nanoparticles. For instance, nanostructure, surface composition, particle shape, and size are important factors responsible for optimized accomplishment. A bimetallic nanoparticle can be a core shell structure, a random alloy or just mixed monometallic particles. Therefore, tunable nanostructures and surface composition are key features to differentiate between bimetallic and monometallic catalysts. This has already been demonstrated in an experimental study by Alayoglu et al. in which Ru@Pt core− shell architecture gave significantly better results than Pt−Ru nanoalloys and a mixture of monometallic nanoparticles in a PrOx reaction.11 Similarly, surface composition of a bimetallic Received: November 12, 2012 Revised: February 4, 2013 Published: February 11, 2013 5084

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different Au−Cu compositions for O2 adsorption and decomposition were computed. Then on the optimized bimetallic surface CO, CO+O coadsorption and activation barriers for CO2 formation were calculated. Finally, the calculations were repeated on pure Cu slabs for comparison and to understand the role of Cu toward enhanced activity.

nanoparticle is also a key factor to effect catalytic performance because surface segregation occurrence may alter the surface composition of a bimetallic catalyst markedly. Theoretical investigations on copper−gold nanostructures/ surfaces are very rare. However, these calculations have given substantial insight into the structures and properties of Au−Cu nanoparticles. Ferrando et al.12 have recently published a review article that aims to establish the structure of copper−gold alloy particles, and the distribution of gold and copper throughout the nanoparticles. The conclusion of most of these studies is that, to some extent, copper segregates into the core and gold segregates toward the outer part of the particle, i.e., the Au−Cu alloy is not homogeneously mixed.13 Toai et al.14 have examined the structure of copper−gold clusters with three different compositions, Cu3Au, CuAu, and CuAu3, using simulations of their growth starting from different seeds. It was found that the structure of the Au−Cu particle was size dependent. Some segregation of copper and gold was also observed in the particles. Nevertheless, Meitzner et al.15 conducted extended X-ray absorption fine structure (EXAFS) studies on silica supported silver−copper and gold−copper bimetallic clusters to obtain information on their structures. They observed that Cu atoms deposited onto an Au147 core did not migrate into the core. This information is very applicable for catalytic reactions in general and CO oxidation in particular. It is anticipated that these theoretical studies can be informative with respect to catalysis with nanoalloys of Au−Cu. CO oxidation is an exceptionally exceedingly studied reaction due to its applications in important reactions such as removal of CO from fuel cell feed stocks, as a model reaction for methanol production and the water gas shift reaction, and so on. During the past two decades, CO oxidation over small Au nanoparticles remained a prominent subject of research. Controversy over its high activity toward oxidation reactions and active sites still exists.16 Presently, gold-based bimetallic nanoparticles are growing in a category of efficient catalysts in a diversity of important reactions including low-temperature CO oxidation.2,17,18 For example, addition of Cu in a series of Au/ TiO2 nanotube catalysts showed higher activity and 100% CO conversion at 70 °C, which was at 170 °C in the case of Au/ TiO2 catalyst without the presence of Cu.19 The monometallic copper catalyst is also active for CO oxidation, but less than the Cu−Au catalyst. The Cu−Au catalyst containing 0.1% Au is significantly more active than the 0.1% Au/TiO2 catalyst, and more active than the corresponding Cu/TiO2 catalyst. This was ascribed to a synergistic interaction between copper and gold, which enhances the catalytic activity. It is not clear whether both copper and gold are active in this catalyst (i.e., the increase in activity is due to one metal aiding the dispersion of the second metal), or whether one metal is acting as a promoter for the other.13 Computational efforts could be employed to have more clarifications on these issues. Computational methods have become an important tool in providing data of atomic and molecular clusters that give valuable insight into the mechanisms of molecule surface binding and the structures of the adsorption complexes. Calculations based on density functional theory (DFT) and empirical force fields have been used successfully in the prediction of gas solid interactions and the calculation of binding energies on the surfaces of carbon-based adsorbents.20,21 The present study aims to systematically explore the mechanism and the reason why Au−Cu alloy catalyst causes an enhancement of the activity for CO oxidation. First of all,

2. COMPUTATIONAL DETAILS We used the Vienna ab initio simulation package (VASP),22 which performs an iterative solution of the Kohn−Sham equations in a plane-wave basis set. Plane-waves with a kinetic energy below or equal to 400 eV were included in the calculations. The exchange-correlation energy was calculated within the generalized gradient approximation (GGA) proposed by Perdew and Wang (PW91).23 The electron-ion interactions for C, O, Ag, and Au atoms were described by the projector-augmented wave (PAW) method developed by Blöchl.24 This is essentially a scheme combining the accuracy of the all-electron methods and the computational simplicity of the pseudo potential approach.25 The relative positions of the Au metal atoms were initially fixed as those in the bulk, with an optimized lattice parameter of 4.18 Å (the experimental value is 4.08 Å).26 The computed optimized lattice constant of Ag was 4.16 Å versus the experimentally determined value of 4.09 Å. The optimized lattice parameter was calculated using the face-centered cubic (fcc) unit cell, and its reciprocal space was sampled with a (15 × 15 × 15) k-point grid generated automatically using the Monkhorst−Pack method.27 A first-order Methfessel−Paxton smearing-function with a width ≤0.1 eV was used to account for fractional occupancies.28 Partial geometry optimizations were performed including the RMM-DIIS algorithm.29 Geometry optimizations were stopped when all the forces were smaller than 0.05 eV/Å. Vibrational frequencies for transition states (TSs) were calculated within the harmonic approximation. The adsorbate−surface coupling was neglected, and only the Hessian matrix of the adsorbate was calculated.30 The climbing-image nudged elastic band (cNEB) method31 and constraint minimization technique were used in this study to determine minimum-energy paths. Closed-shell CO and CO2 molecules were optimized at the Γ point by nonspin-polarized calculations using a 10 × 10 × 10 Å3 cubic unit cell. Spinpolarized calculations in a 10 × 12 × 14 Å3 orthorhombic unit cell were performed for the open-shell species O and O2. The Au/Cu(100) and Ag/Cu(100) surfaces were represented with slab models using four-metal layers of which the top 2 were relaxed with a vacuum gap of >10 Å for both the surfaces to separate the periodic slabs. For these models I used a p(3 × 3) unit cell with the reciprocal space sampled with (3 × 3 × 1) k-point meshes. 3. RESULTS AND DISCUSSION 3.1. Adsorption and Dissociation of O2. For CO oxidation on gold catalysts, O2 adsorption and activation is regarded as the rate-limiting step.10 We performed quite extensive DFT calculations to discover a Au surface capable of adsorbing and dissociating O2 in our previous studies.32 However, we realized that Au surfaces were unable to clearly chemisorb and decompose O2 to provide active oxygen for CO oxidation except Au38 cluster, which showed unambiguous potential for the process.32 Alternatively, one can alloy gold with a second metal on which O2 can easily be adsorbed and 5085

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Figure 1. Molecular O2 adsorption on the different positions of various Au−Cu bimetallic and pure Cu surfaces. The substitution of Cu atoms in the top row has been increased until the replacement of the whole top Au layer by Cu atoms. Panels f−h correspond to the adsorption on Cu add atoms on the four layers Au slab. Panels n−p represent the O2 adsorption upon pure Cu slab models. The adsorption energy and bond length of O2 has been given in each case below the corresponding panel. Panel q shows the most stable configuration for the dissociated coadsorbed O atoms. The adsorption energies reported are calculated as Eads(O2) = Eslab‑O2 − Eslab − EO2(gas).

activated.10 Therefore, in this study the main focus was to investigate such a combination of materials, which should be capable of adsorption and decomposition of O2 and be representative of a low activation barrier for CO2 formation. CO adsorption energies are quite in the favorable range on the Au surfaces, which contain coordinatively unsaturated Au atoms to bind CO.33 Therefore, the counterpart material should be capable of adsorbing and splitting O2 easily in order to experience synergic effect of alloying for CO oxidation process. Cu is the material, which can split O2 easily.34 At around 170 K, the molecular states dissociate, and above 170 K only atomic oxygen is found on Cu(111).35 An important task is to optimize the proportion of Au and Cu to make a Au−Cu alloy/ catalytic surface that can yield the best results for CO oxidation. As a first step, different Au−Cu slabs, where the amount of Cu was varied from a single atom until the replacement of a full top layer, were tested by adsorbing O2. For a detailed view, see Figure 1a−q. It can be seen that O2 has significant interaction with the surface containing Cu atoms, whereas the Au(100) surface only yields Eads O2 = −0.12 eV for the most stable configuration if computed using GGA = PW91.32 When two Cu atoms are doped in the top layer of the unit cell, the most stable configuration is top-bridge-top (hereafter referred to as tbt) configuration with adsorption energy of −1.08 eV, as shown in Figure 1a. However, if one oxygen atom of O2 binds with Au and other one binds with Cu in tbt mode, as shown in Figure 1b, the adsorption energy of the molecule drops significantly to −0.66 eV, shortening dO−O slightly. On

replacement of another Cu atom in the top layer, the Eads O2 increases to −1.16 eV in the tbt pattern (Figure 1d), which is substantially higher than the same alloy composition except with the O2 axis placed diagonally depicted in Figure 1c. When four Cu atoms are replaced by Au atoms in the top layer making a 4-fold hollow configuration (see Figure 1e), a gigantic increase in Eads (−1.97 eV) of O2 was observed. This higher value of adsorption energy also accompanies the O−O bond activation significantly from 1.24 Å (gas phase computed value) to 1.53 Å. The O2 molecule occupies a symmetric position, being a distance of 1.98 Å away from all the 4 Cu atoms making this hollow arrangement. The O2 adsorption was checked on the structure prepared by adding Cu atoms on the four-layer slab of Au as shown in Figure 1f,g. Again, the highest interaction of O2 has been noticed at the 4-fold hollow configuration. The value of Eads is similar to that obtained for the similar pattern shown in Figure 1e. Another Au−Cu alloy surface composed of Au and Cu in a ratio of 3:1 has been investigated. The top layer of a four-layer slab of Au has been replaced by Cu atoms, keeping the lattice parameter the same as that of the Au. The O2 adsorption was examined at different sites as shown in Figure 1i−k. The highest binding energy yielded is −2.20 eV, resulting in a O−O bond scission during the optimization procedure. The resulting O−O bond distance has been measured to be 1.86 Å, which is 0.62 Å greater than the O2 gas phase computed bond length. This 3 to 1 ratio among the Au and Cu gives the best synergetic effect regarding O2 adsorption and dissociation. The inves5086

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Figure 2. Adsorption of CO, coadsorption of CO and O and O−CO complex formation on different Au−Cu and Cu surfaces. Adsorption energy, geometrical parameters, and reaction energy (in parts k and q) have also been indicated.

tigations taking two layers each of Au and Cu were also carried out, but the resulting O2 adsorption energy and bond activation were significantly less imperative (Figure 1l,m). For more clarity on the comparison between clean Cu and Au−Cu mixed surfaces meant for O2 activation, calculations were carried out on a pure Cu slab using its experimentally determined lattice parameter 3.61 Å.36 Three different configurations were explored (Figure 1n−p). It was determined that, like Au−Cu surfaces, the O2 adsorption energy was different depending upon the adsorption site and mode of adsorption. The highest interaction has been noted when O2

adsorbs on the 4-fold hollow location in the tbt pattern. The Eads O2 value −1.93 eV is close to that determined for other similar configurations of Au−Cu structures, but considerably lower than the value computed for the configuration shown in Figure 1j. Additionally, the O−O bond activation is also substantially less, as it can be compared in Figure 1j,n. In panel q of Figure 1, dissociated coadsorbed oxygen atoms are shown, which occupy the 4-fold hollow configuration. This model reflects the highest computed Eads of −9.97 eV for a coadsorbed oxygen atoms pair. It can be seen that the nearest neighbor top layer Cu atoms shrink to hold these atoms. An earlier study also 5087

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suggests that surface composition of a bimetallic catalyst can be different from the bulk, and surface segregation is a common phenomenon caused by intrinsic properties such as bond length surface energy and is affected by external factors such as atmosphere and temperature during pretreatment or the reaction process.37 From the above discussion it can be concluded that O2 preferably chemisorbs on the Au−Cu alloy catalytic surface when O2 lies on Cu atoms having its axis parallel to the surface. The flat lying of O2 on the surface is in agreement with a recently reported GGA = PW91 study.38 The highest interaction occurs on 4-fold hollow configurations comprising Cu atoms forming the top layer, and particularly bond scission takes place when the top layer of Au slab model is replaced by an atomic thin layer of Cu atoms. Hence, the main hurdle regarding O2 dissociation on Au surfaces is resolved by adding Cu atoms, which facilitates CO oxidation. The adsorption energy of O2 on Cu surfaces has been reported up to −1.19 eV along with a bond activation of 1.52 Å,34 but no direct comparison can be made with the previous investigations. Liu et al.10 performed experimental studies to optimize the particle size and performance for CO oxidation. Interestingly, upon alloying Au and Cu, the particle size becomes much smaller than that of gold (2.9 nm for Au:Cu = 3:1 and 3.0 nm for Au:Cu = 1:1), consistent with X-ray diffraction (XRD) results. For this specific ratio, Au−Cu alloy performed very well for CO oxidation. We conducted Bader charge analysis to quantify the amount of charge transfer from Au−Cu and Cu surfaces toward O2 and between Au and Cu atoms.39 This charge transfer is one of the major reasons for superiority of the Au−Cu surface compare to Cu and Au monometallic surfaces for O2 dissociation and CO oxidation. Addition of the Cu atoms on the Au surface results in oxidation of Cu and, in turn, reduction of Au. Nevertheless, the amount of charge transfer depends upon the number of added Cu atoms. As the amount of charge transfer increases from metallic surface to the anti-bonding molecular orbital of O2, the binding energy of O2 with the surface and O−O bond activation increases. The maximum charge gained by the Au atoms of 0.15 to 0.28 e is observed in the case of models depicted in Figure 1i−k, resulting in simultaneous oxidation of nearby Cu atoms up to 0.25 e in the absence of O2. However, in the presence of O2 on the surface shown in Figure 1j, O2 is reduced to 0.62 and 0.74 e, which is the maximum observed for Au−Cu surface composition. Each Cu atom constituting this hollow configuration is oxidized to the same amount of 0.50 e. The rest of the Cu atoms of the top layer oxidize by 0.24 e. Additionally, the Au atoms beneath this Cu layer gain a maximum charge up to 0.31 e. The analysis carried out on pure Cu showed less charge transfer from Cu to O2. The most stable hollow configuration shown in Figure 1n represented the quantity of charge transfer to the O2 of 0.53 and 0.59 e accompanied by surface oxidation up to 0.31 e. This explains why Au atoms cause Cu atoms to lose more charge, which is gained by O2, resulting in strong binding and O−O bond scission. The charge transfer analysis is investigated for O2 adsorption on Ag−Cu bimetallic surface too, which is reported later in this paper. Nevertheless, less charge transfer as compared to Au− Cu alloy surface occurs in the absence of O2. A maximum reduction in Ag atoms of the third layer up to 0.11 e and corresponding oxidation in Cu atoms up to 0.11 e are observed. On the contrary, for the most active hollow sites for O2

dissociative adsorption, the charge transferred to O2 is 0.97 e (maximum observed in this study), in agreement with the higher heat of adsorption and larger dissociated bond length. In the case of O2 adsorption depicted in Figure 4(b,c), maximum charge received by O2 is 0.46 and 0.57 e, respectively; accompanying less Eads and bond activation. Thus the more the back-donation from the surface, the higher the Eads and bond activation of O2. 3.2. CO Adsorption, CO + O Coadsorption, and CO2 Formation on Au−Cu Alloyed Surface. Carbon monoxide adsorption has been investigated on various locations of the (100) surface of Au−Cu and Cu slabs alone. In the previous section, it has been established that the O2 splitting takes place instantaneously on the model comprising top Cu layer atoms and three layers beneath Au atoms; therefore, the rest of the CO oxidation process has been studied on this surface. On the other hand, for comparison, a pure Cu slab has also been considered to carry out CO oxidation calculations. CO adsorbs with reasonable adsorption energy on the three sites available on the surface under study. However, the highest Eads of −1.12 eV has been observed for on-top CO (see Figure 2a). Comparison of this value with pure Au(100) surface demonstrates that on a Cu-modified surface, the Eads of CO drastically increases from −0.46 eV to −1.12 eV for linear coordination, from −0.55 eV to −0.96 eV for bridge binding, and from −0.07 to −0.81 eV in the case of hollow position (Figure 2a−e) when compared with our previous results.33 Investigations for CO + O coadsorption were carried out to elaborate the CO2 formation. Initially, carbon monoxide was positioned on its favorite linear coordinated position, and atomic oxygen was positioned on the nearest and next nearest neighboring top, bridge, and hollow sites. After placement of both CO and O on nearby top sites, the system was substantially destabilized, leading the configuration to be least favored. The system yielded the highest coadsorption energy (−6.09 eV) when O was on a hollow position (keeping CO on top), resulting in rearrangement of Cu surface atoms in agreement with previous investigations37 (Figure 2g). In this pattern, the coadsorbents almost feel neither attraction nor repulsion if compared with the circumstances of placing CO and O in two separate unit cells on the same sites. However, this configuration was not found to be the most favorable one for CO2 formation. The O atom is symmetrically 1.95 Å away from the nearest Cu atoms, but 3.2 Å away from the Cu atom adsorbing CO. Nevertheless, freezing the O atom in the center (equidistant from the four Cu atoms, one of them adsorbs CO) destabilizes the system very minutely by an amount of energy of −0.05 eV. The most favorite coadsorption energy (−6.11 eV) has been computed by adjusting both the CO and the O on hollow sites initially, but during the optimization process CO is repelled to the next bridge position (see Figure 2i). Interestingly, this is the most favorable route for CO2 formation as well. It can be concluded that when O occupies its favorite 4-fold hollow, the movement of CO on different positions causes the coadsorption energy to change only marginally. Occupation of O on this highly coordinated site is in agreement with previous DFT calculations.38 It was mentioned earlier that keeping O on the hollow position and CO on the top or bridge positions, the coadsorption energy remains almost invariable. The reaction energy path has been computed from the optimized configuration depicted in Figure 2i. The coadsorption energy 5088

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Figure 3. CO oxidation process on the two Au−Cu (A) and Cu (B) surfaces. Activation energies, TSs, O−CO complex, and CO2 formation are depicted energetically. Zero level corresponds to gas phase values of CO and atomic O.

of this pair is −6.11 eV, which verifies that coadsorbents feel no repulsion. When CO is moved from a hollow location toward O for CO2 formation using the constraint minimization method, initially the coadsorbents feel repulsion and the O atom starts advancing toward the bridge position until the transition state is reached. In the transition state, CO is slightly away from the bridge site and has unsymmetric distances of 1.95 and 2.28 Å from the Cu atoms constituting the bridge. Similarly, O has also displaced unevenly with distances 2.27 and 2.38 Å from these Cu atoms, but maintains the equidistance of 1.93 Å from the next two Cu atoms (see Figure 3A(b)). This distance is larger than the Cu−O−Cu bond lengths (1.82 Å) when O is adsorbed on the bridge position in a separate unit cell. The activation energy required to cross the barrier is 0.64 eV. The TS has been confirmed by a unique imaginary frequency of 139 cm−1. The reaction is thermodynamically favorable with an exothermicity of −0.25 eV. Crossing TS, CO continues to advance toward the already pushed away O, which does not move further. Finally, the C atom of the tilted CO advances and creates a bond with O, creating the O−CO complex shown in Figure 3A(c). In this state, the newly formed O−C bond length is 1.34 Å, while the old C−O bond is of 1.23 Å span, substantially stretched in comparison with its gas phase configuration (1.14 Å). CO2 formation from this configuration needs negligible activation energy (no transition state exists), leading CO2 more than 3 Å away from the surface. The process is slightly exothermic (−0.15 eV). Here small exothermicity makes such surfaces a potential candidate to study CO2 reduction. For confirmation, NEB calculations were employed. Interestingly, an activation barrier of 0.60 eV was observed, very close to the already determined activation energy of 0.64 eV computed by the constraint minimization technique. In transition state (Figure 3A(b)), the C of CO has asymmetric distances of 2.15 and 1.96 Å from Cu atoms forming bridge, while the O atom is nearly symmetric being 2.43 Å away from these atoms but 1.90 Å away from the next Cu atoms. After crossing the TS, the C atom of CO advances forward and the O

moves back toward the hollow position to make a O−C bond forming a O−CO complex, which eventually flies away from the surface. 3.3. CO Adsorption, CO + O Coadsorption and CO2 Formation on Cu(100). Carbon dioxide formation from CO and O has been investigated on a pure Cu slab for comparison. As mentioned earlier, the O2 splitting is more favorable on a Au−Cu composed surface than on a pure Cu system. The CO Eads is highest (−0.97 eV) on the bridge site, in contradiction to the Au−Cu catalytic surface where linear coordination is favored, and here the value is lower in comparison. Eads of CO on top (−0.92) and hollow (−0.89 eV) sites is slightly lower, suggesting that CO can swap over different positions easily (see Figure 2m−o). Atomic O occupies the hollow position in agreement with previous investigations38 for its strongest value of binding energy −5.21 eV. The difference found for bridge (−4.44 eV) and top (−3.32 eV) sites is large, presenting difficult diffusion. Our value of Eads of O on Cu(100) is higher than the previously examined values of −4.29 and −4.5 eV on Cu(111) and Cu(211) surfaces, respectively, using GGA = PW91.34 To find the lowest activation energy path for CO2 formation, different coadsorption configurations have been examined. It is evident that CO slightly depends on the adsorption site, but O to a very large extent; the highest coadsorption (O at hollow and CO at bridge) value of adsorption energy has been found to be −6.17 eV, which shows that coadsorbents neither feel attraction nor repulsion (Figure 3B(e)). Carbon monoxide and O coadsorbents do not adsorb on the consecutive hollow positions, and CO moves away due to repulsion unless constrained. Advancement of CO until the hollow site from its favored bridge site (see Figure 2m) requires an energy of 0.40 eV. Further movement of CO toward O pushes away the O toward the next bridge like it happened on the Au−Cu surface. The repulsion rises further until the TS state, confirmed by a unique imaginary frequency of 114 cm−1 (shown in Figure 3B(f)), is crossed over. An activation barrier of 0.41 eV is required to produce a reaction of CO with O. 5089

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Figure 4. The a part in the upper panel of the figure shows dissociative adsorption of O2, whereas the other two parts b and c represent two different modes of O2 adsorption on the bridge position of the surface. In the lower panel, the initial to final states including the TS for O−CO complex formation are depicted. The O−CO desorption in the form of CO2 is given in parts g−i.

with an enthalpy of −0.33 eV and stays at a distance >3 Å from the surface, as shown in Figure 3B(h). From the above discussion, it is evident that both of the surfaces can go for CO oxidation, but the process is more favorable and least hurdled on the Au−Cu surface when the Au surface bears an atomic (top) layer of Cu atoms. The cNEB method was employed for reconfirmation of minimum energy path. The activation energy found was the same as that of previous findings. Nevertheless, CO passes over the bridge lopsidedly, having Cu−C distances of 1.86 and 2.43 Å. To create a bond with O, slight rearrangement of the top layer of the Cu slab occurs, as shown in Figure 3B(f). The O atom has its distances 1.90, 1.85, 2.28, and 2.58 Å from the Cu atoms. TS is characterized by a unique imaginary frequency of 130 cm−1. The activation barrier found is 0.80 eV and is higher than that on the Au−Cu model. Additionally, the reaction is thermodynamically neutral. On both the surfaces processed for CO oxidation, the minimum energy paths were also explored from other coadsorbed configurations, but these were found to be more resistive to create CO2.

Thus a total activation energy of 0.81 eV is necessary for O− CO complex formation, which is 0.21 eV higher than that on a Au−Cu surface for a similar reaction path. This relatively higher activation barrier might be the result of stronger bonding of O than on the Au−Cu surface. In the TS, both CO and O hold the asymmetric positions, having the O at distances of 1.86 Å and 2.86 Å from Cu atoms, while the Cu−C bond length is 2.04 Å. O is slightly distanced if compared to its bridge position in a separate unit cell (1.83 Å). In the semifinal position depicted in Figure 3g, the new O−C bond length is 1.34 Å, while the old C−O stretched bond distance is 1.22 Å. Atomic O also moves to join the C of the CO (to make O−CO) complex having symmetric distances of 2.08 Å from Cu atoms forming the bridge, substantially larger than the 1.83 Å found in the TS. CO also moves further, being equidistant (2.16 Å) from Cu atoms constructing the CO holding bridge. However, unlike the Au−Cu catalytic surface, the system is almost thermodynamically neutral, explaining the role of Au (see Figure 3). The vibrational analysis of the state confirms that the O−CO complex formed is a stable one. The CO2 formation from this state has to surmount an activation barrier of 0.20 eV. The formation of CO2 is a thermodynamically favorable process 5090

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Figure 5. The reaction profile for O−CO formation from the CO and atomic O coadsorbants. Relatively loosely bonded CO moves toward O and makes the O−CO complex intermediate for CO2.

In the final state, the CO is repelled back toward the bridge, but the O almost occupies the hollow due to stronger bonding. As a consequence of repulsion, the C atom of CO in the eventual coadsorption location has distances 2.01, 2.04, 2.34, and 2.64 Å from the Cu atoms constituting the hollow site. On the other hand, the position symmetry of O is hardly disturbed during the optimization process, where distances from neighboring Cu atoms are 2.0, 2.0, 2.04, and 2.05 Å. This arrangement has been used as the initial state for the minimum energy path search for CO2 creation. From the coadsorption with CO at the bridge site with neighbor hollow-positioned O, ends up in O−CO complex formation through small exothermicity as shown in Figure 4f. The configuration also serves as final state for TS location. The CO oxidation from the situation given in Figure 4d is too easy in the Ag−Cu system under study. Pushing CO toward strongly bonded O has to face an activation barrier of 0.36 eV before the C of CO interacts with O and creates the O−CO complex (an intermediate stable state for CO2 formation). In the TS shown in Figure 4e, the C of CO is almost symmetrically 2.01, 2.03 Å away from bridge forming Cu atoms, while O is pushed away with distances from neighbor atoms of 1.91, 1.92, 2.36, and 2.43 Å. In the final state, the C of CO is distant 2.11, 2.12 from Cu, and the O−C bond is 1.35 Å long while the C−O bond stretches to 1.23 Å. Finally, O has 2.05 and 2.06 Å, almost the symmetric distances from nearest Cu atoms. The overall reaction is thermodynamically exothermic by −0.27 eV. The TS has been confirmed by unique imaginary frequency of 38 cm−1. The complete reaction profile is shown in Figure 5. The resulting O−CO intermediate does not automatically fly away from the surface but needs to surmount an activation barrier of 0.33 eV for desorption. The transition (top and side view) and final states are shown in Figure 4g−i.

3.4. Dissociative Adsorption of O2 on Ag/Cu(100). For comparison, we used another coin metal Ag having similar lattice constant as of Au and replaced top layer of silver atoms with Cu atoms to make a configuration similar to that of Au− Cu surface composition, which served the best for CO oxidation. Based on the energetic data, the surface exhibited an even superior effect toward O2 activation and CO2 formation. During optimization, the hollow configuration as on the Au−Cu bimetallic surface activates O2 into two O atoms, which eventually occupy the bridge positions being 3.72 Å apart (vs 1.24 Å initial state), as depicted in Figure 4a with a heat of adsorption of −3.47 eV. Thus O2 activation is highly exothermic. It is well-known that copper oxidizes easier than silver and also that it is way more energetically favorable for oxygen to adsorp to Cu(100) than Ag(100). Dissociative adsorption of O2 on the surface causes the Cu atoms to rearrange (segregation), as indicated by the uneven distances marked on the upper panel of Figure 4. It is also expected that Cu, as an admetal in Ag, segregates to the surface rather than forming an alloy (because of the size mismatch). Low index surfaces of the Ag−Cu alloy in equilibrium40 were investigated by means of ab initio atomistic thermodynamics with an oxygen atmosphere, accounting for the effects of temperature and pressure. For all the surface orientations considered in that work, the presence of oxygen on the surface leads to copper segregation, due to the strength of the O−Cu bond relative to the O−Ag one. Other modes and positions (see Figure 4b,c) also present the tendency toward O2 adsorption and activation, but the difference is very significant in comparison with the previously mentioned hollow configuration. Adsorption of O2 on Ag surfaces, resulting in structural changes and metal-to-O2 charge transfer, is reported in reference 41, where trends agree with my results; however, no direct comparison with literature can be made due to the nonavailability of such reports. 3.5. CO Adsorption, CO + O Coadsorption, and CO2 Formation on Ag/Cu(100). The adsorption of CO occurs on all three sites available on the surface with clear preference for top or bridge. The adsorption energy of CO (−1.0 on top and −0.98 eV on bridge) is quite favorable for catalytic reaction. The CO adsorption energies reported in ref 42 on Ag(111) using the PW91 functional are −0.29 eV for linear bonding at 1/4 ML coverage. Experimental values reported in this study are also very small. Therefore, it appears that analogous to that in the Au−Cu case, Cu addition on Ag has improved CO interaction substantially. For CO2 formation, different CO + O coadsorption configurations were examined. The configuration used for the TS search converges as shown in Figure 4d with an adsorption energy of −6.23 eV, where before the simulation starts both the CO and the O were positioned at consecutive hollow positions.

4. DISCUSSIONS The present work aims to explore the cause of synergetism, which enhances the rate of reaction for low-temperature CO oxidation on alloying Cu with Au as observed experimentally10 and an optimal bimetallic ratio between Au and Cu, which serves as the best possible alloy composition for the desired purpose. On the basis of the extensive computational effort, it was concluded that a monoatomic layer of Cu on the Au slab performed very well for O2 adsorption/activation and CO2 formation from coadsorbed CO and O. Additionally, an analogous model for comparison was used, taking Ag substrate where the top layer of Ag atoms was substituted with Cu atoms. In both cases (Au/Cu or Ag/Cu), the bimetallic surface composition in 3:1 performs wonderfully well in line with experimental observations.10 Nevertheless, these models might not be a true representative of the experimentally synthesized 5091

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A systematic DFT study using the GGA = PBE functional on Pd−Au bimetallic catalysts was carried out for low-temperature CO oxidation by Wu and co-workers.47 In a Pd(111)-p(2 × 2) unit cell, the authors varied the number of Au atoms from 1 to 4 in the top layer and discussed the influence of strain effect (due to lattice mismatch) and electronic effect caused by Au on catalytic activity of bimetallic system in detail. It was proposed that a Pd core/Au shell structure will have the best catalytic activity. Activation barriers computed on stretched Pd systems for CO + O → CO2 reaction were slightly higher than those on unstrained Pd systems, implying that the strain effect caused by Au decreases the catalytic activity of Pd, though not substantially. Additionally, the active ligand effect of Au was investigated by checking the adsorption energy of O. The authors found remarkable decrease in adsorption energy of O (−4.63 to −3.08 eV: very important for catalytic activity) with the increase in concentration of Au in the top layer until the full top layer was replaced. The activation barrier for CO2 formation was drastically reduced to 0.21 eV versus 0.91 on pure Pd. It was also ascertained that this effect was limited to the top layer only, in partial agreement with our observations. It is noteworthy that Pd16Au4 with lattice constant of unstrained Pd(111) offered a slightly lower (0.19 eV) barrier due to the absence of Au strain effect. On the basis of these results, it was suggested that Au can electronically effect the catalytic properties of Pd, and the ligand effect of Au is predominant compared to the strain for the Au involving a Pd(111) slab with a Au-rich surface. The above discussion leads one to conclude that the increased catalytic activity (synergetism) of bimetallic catalysts results from the electronic charge transfer between the metals: between metal and adsorbents, change in adsorption energy, and optimum ratio of the metals concentration in the surface composition. The strain caused by the lattice mismatch also affects a catalyst performance. In the calculations reported here, a significant decrease in adsorption energy of O on Au−Cu bimetallic surface has been noted if compared with that on the monometallic Cu. In addition, substantial increase in O2 adsorption energy and bond activation as a consequence of charge donation from metal surfaces to O2 antibonding molecular orbital as discussed in previous sections is observed.

bimetallic catalysts. However, I am still convinced that sites with such surface composition on an alloy might be present, which actively participate in a catalytic reaction, and consequently the reaction rate is increased. In the following text, some bimetallic studies are discussed that support the mechanism presented in this paper. The effects of lattice mismatch, electronic charge transfer, and adsorption strength on the catalytic properties are given. Wang et al.43 synthesized a Au−Ag alloy catalyst supported on mesoporous aluminosilicate Au−Ag@MCM by a one-pot synthesis procedure characterized using different techniques such as XRD, ultraviolet−visible spectrospcopy (UV−vis), Xray photoelectron spectroscopy (XPS), and EXAFS for lowtemperature CO oxidation. It was elucidated that structure and surface composition change during calcination and the reduction process. XPS and EXAFS data demonstrated that calcination led to complete phase segregation of the Au−Ag alloy and after other treatments (reduction), the final surface composition Au/Ag was 3:1 despite the catalyst being more enriched with Ag. Its intensity change correlates well with the trend of catalytic activity. The enhanced activity for lowtemperature CO oxidation caused by alloying Au with Ag was attributed to the modification of electronic properties of both Au and Ag resulting in a stronger tendency to lose electrons to the adsorbates. In CO oxidation, an electron transfer from the metal to the antibonding orbital of the O2 molecule would help weaken the O−O bonding, thus improving the oxygen activation. This is very important and helpful for the CO oxidation reaction.43 These authors performed DFT calculation using GGA = PW91 for confirmation and reported that O2 adsorption and CO and O2 coadsorption on neighboring sites on the Au−Ag alloy particle (Au25Ag30) was stronger than that on either Au55 or Ag55 having the same structure. A recent spinpolarized DFT study44 at the GGA = RPBE level carried out on a Ag12Cu cluster with Ih symmetry for CO oxidation revealed similar facts. These calculations helped to realize that the Cudoped Ag clusters were relatively stable, and CO and/or O2 molecules were easily adsorbed on Ag12Cu clusters. The charging of Ag12Cu can improve not only O2 adsorption, but also CO + O2 coadsorption and reduce reaction barriers. Excess charge does improve CO oxidations under most CO + O2 coadsorptions.44 Recently in situ scanning tunneling microscopy (STM) and DFT-GGA studies of CO oxidation on Auenriched Ag(110) surfaces were presented by Chou et al.,45 which resulted in reaction enhancement of bimetallic catalysts. Their reactivity (CO adsorption and oxidation) was dependent on the local configuration of Au and Ag. In particular, the presence of Au enhanced CO adsorption and oxidation. It is worth mentioning here that both the metals have similar lattice parameters (no strain effects), but insertion of Au in a particular surface composition has enhanced the CO adsorption resulting in enhanced CO oxidation rate. Wang et al.46 prepared Au−Ag alloy nanoparticles on a mesoporous support by a novel onespot method. Although thus formed alloy nanoparticles were relatively large, they exhibited exceptionally high activity for CO oxidation at low temperatures compared with monometallic Au or Ag catalysts. In this alloy system, Ag plays a key role in oxygen activation, and Au sites adsorb CO while assisting the activation of oxygen by decreasing the activation energy for molecular adsorption of oxygen, showing a strongly synergistic effect between Au and Ag. The best-performance Au−Ag alloy catalyst is the one with a nominal Au/Ag ratio of 3:1.

5. CONCLUSIONS DFT calculations have been carried out on plenty of Au−Cu composed and Cu(100) surfaces to investigate CO oxidation. It has been elaborated that O2 molecules interact very strongly with the surfaces where Cu atoms are present on the surface. However, the strength of binding varies with the surface composition. The adsorption energy of O2 varies between −0.61 and −2.20 eV, and the highest value corresponds to the system comprising a top layer of Cu atoms and three layers of Au atoms beneath. This surface performs the best for CO oxidation. O−O bond scission takes place during the optimization procedure, and split atoms are 1.86 Å far-off. Carbon monoxide adsorbs preferably on the top configuration of Cu−Au(100) catalytic surface with an Eads of −1.12 eV. This adsorption energy is more than two times that on Au(100) (−0.46 eV). The atomic O adsorption is strongly site specific, the 4-fold hollow being the most favorable. Therefore, diffusion of O on the surface is very difficult in comparison with CO, which is virtually mobile on the surface. A coincident activation barrier of 0.60 eV has been found employing constraint minimization and the cNEB method for CO2 formation 5092

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initiating from CO and O coadsorbents on bridge and hollow sites, respectively. Initially, a stable O−CO complex is formed, which leads to CO2 without the expenditure of any reportable energy. Both O−CO complex formation and its conversion to CO2 are thermodynamically favorable processes. The process has been repeated on a pure Cu(100) surface to distinguish the role of Au. CO on bridge yields the highest adsorption energy (−0.97 eV). Nevertheless, the difference on the other sites is feeble, thus CO diffuses easily, and from the bridge position C of CO moves toward O at hollow (Eads −5.21 eV) creating bond to make CO2. The minimum energy path follows the similar route to CO2 as on the Au−Cu surface with higher activation energy and no exothermicity. This CO2 created desorbs by surmounting a small barrier through the exothermic process. Comparison suggests that the Au−Cu surface is superior and more active than pure Cu toward CO oxidation. It offers less activation barrier and higher thermodynamic favor, which serves as a deriving force for the reaction on the catalytic surface. The Cu-modified Ag(100) surface was also investigated for CO oxidation reaction to compare with. The surface showed dominant potential toward dissociative O2 adsorption and CO2 formation. The calculations presented suggest that Cu-modified Au and Ag catalytic surfaces appear to be good candidates for low-temperature CO oxidation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +92-51-22072317 Ext. 4330; Fax: +92-51-9248808. Notes

The authors declare no competing financial interest.

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REFERENCES

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