Understanding of Adsorption and Catalytic Properties of Bimetallic Pt

Publication Date (Web): March 30, 2010 ... that the surface compositions of Pt−Co alloy surfaces were tunable by careful control of preparation cond...
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J. Phys. Chem. C 2010, 114, 7141–7152

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Understanding of Adsorption and Catalytic Properties of Bimetallic Pt-Co Alloy Surfaces from First Principles: Insight from Disordered Alloy Surfaces Wai-Leung Yim*,†,‡ and Thorsten Klu¨ner† Department of Theoretical Chemistry, UniVersity of Oldenburg, Institute of Pure and Applied Chemistry, Carl-Von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany, and Institute of High Performance Computing, 1 Fusionopolis Way, No. 16-16 Connexis, Singapore 138632 ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: March 17, 2010

Recent experimental studies revealed that the surface compositions of Pt-Co alloy surfaces were tunable by careful control of preparation conditions. This controlled variation of surface composition can be used to modify the chemical activity of the corresponding surfaces. We performed a systematic study on the adsorption and catalytic properties of Pt-Co alloy surfaces based on density functional theory (DFT). We established a protocol to sample the Pt-Co surfaces, in which 4-layer slabs with varying Pt/Co compositions were placed in a hexagonal supercell, exhibiting a (2 × 2) Pt(111) orientation. The characterization of surface properties of these sampled surfaces by DFT calculations was comparable to the available experimental findings. The current work forms a solid basis for extracting complicated chemical information from disordered alloy surfaces, which is essential for the understanding of new materials of increasing complexity. 1. Introduction Heterogeneous catalysis has been an extremely active area of experimental and theoretical research for several decades. In particular, an extensive number of investigations have been carried out to examine fundamental aspects of heterogeneous catalysis in general.1-3 The ultimate goal of these studies is a rational design of a catalyst based on a combination of experimental and theoretical data. To understand fundamental aspects of heterogeneous catalysis and to pave the way to the construction of new high-performance catalysts, some simple bonding models were constructed to facilitate the identification of catalytic active surfaces. For instance, the d-band model as suggested by Hammer et al.4 was used to explain the reactivity of Pt-Fe and Pt-Co alloy surfaces with respect to O2 dissociation.5 The Pt-Co alloy system has been used for multiple purposes, because of its interesting magnetic and catalytic behavior.6-9 On the one hand, Pt-Co exhibits a large magnetic anisotropy that can be used for the development of novel data storage media.10 On the other hand, Pt-Co has been found to be an active catalyst in the Fischer-Tropsch process and lowtemperature oxidation and reduction reactions, respectively.8,9 Nowadays, Pt-Co alloy structures of different sizes and shapes are accessible, e.g., as nanowires, core-shell structures, and nanoparticles, respectively.11 Several preparation parameters, including preparation temperature, are found to be crucial to determine the size and shape of the Pt-Co alloys.10,12-14 It is of fundamental importance to systematically examine the relationship between preparation parameters, product structures, and their physiochemical properties. Unfortunately, the structure-property relationships are still poorly understood from a fundamental point of view, partly due to structural floppiness. For many bimetallic alloy surfaces, the surface structures are * To whom correspondence should be addressed. E-mail: yimwl@ ihpc.a-star.edu.sg. Phone: +65 64191247. Fax: +65 64632536. † University of Oldenburg. ‡ Institute of High Performance Computing.

not well-defined because of the disorder of metastable phases which can hardly be mimicked by simple cluster or periodic surface slab models.15-17 For instance, Ru atoms aggregate together and form particles on cobalt surfaces, while Pd and Pt are highly dispersed into the substrate forming Pd-Co and Pt-Co surface alloys, respectively.18 As a consequence, the predictive value of theoretical model studies has been quite limited for those cases. The experimentalists are also restricted concerning an accurate interpretation of their spectroscopic data.19 Examinations of compositional effects on the adsorption and catalytic properties are nontrivial and rare.19-22 Most of recent model studies focused on a simplified description of catalytically active sites. The complexity arising from a variety of compositions of alloy surfaces was usually underestimated. In our previous combined experimental and theoretical study, we found that the Pt-Co alloy surface system exhibited metastable phases at different temperatures.19 Originally, several layers of cobalt atoms were deposited on top of a Pt(111) surface. Pt atoms were segregated to the outer surface at elevated temperature. In the Pt-Co alloy surface prepared at 370 °C, our XPS results suggested that the surface consisted of 25% Pt in the first and second layer.19 By STM experiments, Gauthier et al. reported that the Pt-Co alloy surface prepared at 425 °C contained about 85% of Pt in the first layer. By LEED experiments, the Pt-Co surface annealed at 470 °C was found to exhibit a Pt-Co ratio of 60:40 in the first ten layers.13,14 For further details, we refer to the results reported by Tsay and Shern12 and Baudoing et al.13,14 In addition, carbon monoxide (CO) was used as a probe molecule in STM and IRAS experiments. On the PtCo surfaces prepared at 430 °C, the CO-molecule adsorbed on the Pt-sites exclusively. The CO-molecule exhibited preferential adsorption on the Co-sites of the alloy surfaces which were prepared at 370 °C.19,23 These novel findings need further investigations to unravel the underlying physical mechanism. For this purpose,

10.1021/jp101602b  2010 American Chemical Society Published on Web 03/30/2010

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quantum chemical calculations based on density functional theory (DFT) are a very powerful tool to extract relevant information. On the basis of the previous experimental observations mentioned above, the preparation temperature/structure relationship provides the experimental basis for subsequent theoretical studies. We established a systematic protocol to investigate the chemistry of a series of representative and chemically nonequivalent alloy surfaces (cf. Computational Details section and the Supporting Information). Only this set of representative surfaces permits us to obtain a meaningful trend of adsorption and catalytic properties. In this work, our aim is to explore the surface composition effects on CO adsorption characteristics. We will show later that our computed results agree very well with available experimental data. Moreover, the compositional effect on two selected key steps of catalytic reactions;CO activation and CO oxidation;has been studied. Not only for the CO adsorption behavior, compositional effect plays a crucial role in influencing reaction profiles of key steps of catalyses on flat surfaces. Our protocol is not limited to the flat PtCo system, but turns out to be applicable to any other bimetallic transition metal surface of different morphology. 2. Computational Details 2.1. Electronic Structure Calculations. We used the Vienna Ab Initio Simulation Package (VASP) to perform calculations within DFT.24-27 Throughout the study, the PBE exchangecorrelation functional was used.28 Projected augmented wave (PAW) pseudopotentials were adopted for Pt, Co, C, and O atoms.29 The planewave energy cutoff and the augmentation charge cutoff were set to 400 and 645 eV, respectively. Geometry optimizations were carried out by using the conjugate gradient minimization scheme in VASP. The convergence threshold for electronic structure calculations and geometry optimizations were set to 1 × 10-4 eV. We used a 5 × 5 × 1 and a 6 × 6 × 1 Monkhorst-Pack (MP) grid for the optimal alloy searches and CO adsorption energy calculations, respectively,30 and both of them used Gaussian smearing with a smearing parameter of 0.1 eV. To obtain the correlations between vibrational frequency and adsorption energy, we obtained the optimized structures and the corresponding CO-vibrational frequency by using a 3 × 3 × 1 MP grid and a Gaussian smearing parameter of 0.2 eV.31,32 Furthermore, the Climbing Nudged Elastic Band scheme was used in order to locate the transition structures of CO dissociation and CO + O combination.33,34 2.2. Surface Alloy Models. Surface slab models were used to simulate surface properties within a periodic supercell approach. We have performed convergence tests with respect to slab thickness (from one to seven atomic layers), studying the CO chemisorption energies on Pt(111), Co(111), and Co(0001) surfaces. The quantum confinement effect was strong when the surface slabs had only three or fewer atomic layers. A 4-layer slab model turned out to be sufficiently thick to give reasonable results for CO chemisorption energies and CO vibrational frequencies, respectively. Thus, we considered a 4 atomic layer Pt-Co surface slab that contained 16 transition metal atoms in a hexagonal supercell, exhibiting a (2 × 2) Pt(111) orientation. All atoms and cell dimensions were allowed to relax. The separation between two next neighboring slab images was set to 13 Å to avoid spurious interactions between adjacent slab images. The layer composition can be evaluated experimentally by XPS analysis;19 however, the absolute configuration is not yet

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Figure 1. Selected distance indices. A (4 × 4) supercell, i.e., four periodic unit cells, is displayed. Atoms inside the periodic unit cell are highlighted. rPt-Co is the distance between Pt and Co; rPt-Pt is the distance between Pt and Pt; rCo-Co is the distance between Co and Co.

TABLE 1: Relative Energies of the Optimized Pt-Co Alloysa Pt content per layer (%)

rel energy (eV)

(0,100) (25,75) (50,50) (75,25) (100,0)

0.00 –1.18 –2.12 –2.50 –2.64

a Four atomic-layer slabs were used. (m,n) denotes m % Pt and n % Pt in the first and second layer, respectively. Both the 3rd and 4th layers have a fixed Pt:Co ratio of 1:1. Energy of (0,100) PtCo-alloy is set as the reference.

detectable because of the disorder of the Pt-Co structure. To compare with the experimentally observed adsorption and catalytic behavior of the alloy surfaces, the alloy surfaces have to be sampled carefully. In our sampling method, we used the 4-layer slab models, in which the composition of the first two atomic layers was allowed to vary while the two bottom layers had a constant composition characterized by a Pt:Co ratio of 1:1. The Pt:Co ratio at the two bottom-most layers was suggested by previous XPS experiments.19 Flat surfaces were selected according to the previous experimental observations. We denote the surface layer composition by (m,n), where m and n are the percentage of Pt content in the first and second layers, respectively, while each of the third and fourth layers is implied to have 50% of Pt. For a set of defined compositions (m,n), there exists a number of possible configurations. For example, within a Pt-Co alloy surface with a Pt:Co ratio of 1:1, four atomic sites are occupied by Pt atoms in the first two layers in a supercell. The number of possible configurations can be calculated, resulting in overall 70 possible configurations. Symmetry is taken into account to reduce the number of these configurations. To reduce the redundancy, we designed a set of distance parameters, ∑rPt-Pt, ∑rPt-Co, and ∑rCo-Co, as shown in Figure 1. The summation covered the supercell images within a radius of 30 Å. Configurations are considered to be chemically equivalent if they have the same set of distance parameters. We used the nonoptimized atomic positions of a 4 ML-(2 × 2)-Pt(111) slab, thus the lattice symmetry is kept. From this, we obtained only 25 nonequivalent structures for models 1-5, as shown in Table 1. All nonequivalent structures have been optimized. Only the most stable one, i.e., of lowest total energy for a particular surface composition, was selected for further characterization studies. 2.3. CO Adsorption Energy and Vibrational Frequency. The adsorption energy is defined as follows: Ead ) E(chemisorbed CO) - E(free CO) - E(bare surface). The vibrational

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Figure 2. (a-c) CO adsorption energy on Pt-Co alloy surfaces. Ead ) E(surface + CO) - E(bare surface) - E(free CO).

frequency of CO was calculated by fitting the potential energy curve to the functional form of a Morse potential.31,32 The Morse potential is expressed as:

V(r) ) hcDe(1 - exp(-ar))2

(1)

where h is the Planck constant and c is the speed of light. De and a are obtained by curve fitting. All atoms were fixed in their optimized positions, while the O atom was moved along the C-O bond direction with the C-O distance ranging from 0.85 to 1.45 Å, sampled at every 0.02 Å. The harmonic frequency, νh, is expressed as:

1 νh ) 2πc



2Dea2 µ

(2)

where µ is the reduced mass of CO. The anharmonic frequency, νah, can be obtained by:

νah ) νh(1 - 2χe)

(3)

where

νhχe )

a 2h 2µ

(4)

To compare absolute values of vibrational frequencies, we introduced a factor to scale the calculated anharmonic frequency of free CO to the experimental value of 2143 cm-1. Note that the selected vibrational frequency calculation method is superior to the direct force constant approach.35 For the direct force constant approach, finite differentiation of forces may result in uncertainty, which will prohibit a comparison of vibrational frequency of chemisorbed-CO on different Pt-Co alloy surfaces. 2.4. Chemical Bonding Analyses. To corroborate our computational findings, we employed a chemical analyses by two approaches: atoms-in-molecules (AIM) analysis and the electron localization function.36,37 By using these methods, we can quantify the strength of the chemical bonds. The AIM method characterizes the chemical bond according to its electron density at a bond critical point (BCP). The electron density at BCP loosely indicates the strength of the bond.36,38 A typical covalent

bond is characterized by a FBCP larger than 0.1 au. Moreover, the corresponding Laplacian at the BCP reveals the covalency of the bond, where positive/negative Laplacian indicates an ionic/covalent character, respectively. For more detail, we refer to the literature.36,38-40 3. Results and Discussion Relative energies of the fully optimized surface structures with a Pt:Co ratio of 1:1 are summarized in Table 1. Note that the relative energies refer to the structure of lowest energy at a particular chemical composition. A strong segregation trend of Pt to Co surfaces is observed, which agrees with the previous experimental and theoretical findings.10,13,14,23,41 The first and second Pt atom segregations lower the surface energy by -1.18 and -0.94 eV subsequently. The energy release by the third and forth Pt atom segregations are largely reduced to -0.38 and -0.14 eV. This indicates that the segregation tendency is proportional to the concentration gradient at the interfaces. Using the same method, we have obtained a set of optimized Pt-Co surface structures of particular compositions, at which the CO adsorption behavior and chemical reactions have been studied. 3.1. Adsorption Site Preferences. The former conclusions by experiments are briefly described here. Gauthier et al. reported an exclusive CO adsorption on Pt-sites on the Pt-Co alloy surfaces prepared at 425 °C which exhibited a Pt:Co ratio of 0.85:0.15 in the first layer.23 Recently, Fenske et al. observed that CO adsorbed preferentially on Co-sites of the Pt-Co surfaces prepared at 370 °C, which was characterized by a Pt: Co ratio of 0.25:0.75 in the first two layers as measured by XPS.19 The origin of the adsorption site preference was corroborated by our DFT calculations as illustrated in Figure 2. We investigated the CO adsorption on Pt-atop and Co-atop sites, which was observed in the previous IRAS experiments.19 The most stable configuration, when two or more atop sites of the same element are available in the periodic unit cell, is selected and its result is plotted in Figure 2. The sampled surfaces cover a wide composition range of Pt:Co from 0.5:1 to 2.2:1. For the sake of readability, we plot the CO adsorption energies in Figure 2a-c according to the first layer Pt contents. Panels a-c in Figure 2 show that the overall Pt:Co ratio controls the adsorption site preference: (1) in the Co-rich region (region 1), where Pt:Co ratios range from 0.5:1 to 1:1, CO adsorbs preferentially on Co-sites; (2) in a borderline region (region 2), where Pt:Co ratios are about 1.3:1, no clear adsorption site preference can be observed; and (3) in the Pt-

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rich region (region 3), where Pt:Co ratios range between 1.7:1 and 2.2:1, CO adsorbs preferentially on Pt sites. It should be noted that our calculations cannot sample the surface compositions continuously due to the finite size of the supercell. Apart from this, our predicted trends agree very well with both experimental findings on Pt-rich and Co-rich surfaces, respectively.19,23 In this respect, the Pt-rich surface prepared at 470 °C has a stoichiometric ratio of Pt60Co40 (or Pt:Co ) 1.5: 1) within the first ten layers, as measured by LEED.13,14,23 Such a Pt:Co ratio falls in between region 2 and region 3, so the prediction of preferential adsorption of CO on the Pt-sites of the Pt-Co surfaces is corroborated. On the other hand, the Corich surface prepared at 370 °C contained 25% Pt on both the first and second layers, and the overall composition falls in region 1.19 In agreement with our calculated results, it was observed that CO adsorbed preferentially on Co-sites at low CO coverage. The ligand effect as proposed by the previous theoretical studies is also observed in our results (cf. Figure 2).23 It agrees with the findings of Gauthier et al. that CO is more likely to adsorb on the Pt atop sites which have more next-neighboring cobalt atoms. This can be rationalized by comparing the adsorption energy of chemisorbed CO molecules on the Pt-sites of (m,0) Pt-Co alloy surfaces, where the CO adsorption energy is arranged in descending order: CO-(25,0) > CO-(50,0) > CO(75,0). Additionally, our calculations can further evaluate the second layer effect on the adsorption energy, which was not accessible by previous STM experiments.23 In general, the Pt-CO binding becomes stronger with increasing number of Pt atoms in the second layer. However, the situation for Co-CO binding is altogether different, where a nonmonotonous dependence of the adsorption energy with respect to the number of Pt in the second layer is observed. At first glance, this second layer effect is the main reason for the change in CO-adsorption site preference for different preparation conditions. To explain the dependence of adsorption site preference on the surface compositions, we investigated the contributions of surface distortion, CO distortion, and the interaction energy between the two distorted species.

E(surface distortion) ) E(distorted surface) E(bare surface)

(5)

E(CO distortion) ) E(distorted CO) - E(free CO)

(6) E(interaction) ) E(chemisorption) E(surface distortion) - E(CO distortion)

(7)

The results are shown in Table 2. Among all the studied cases, the deformation energy of CO is very small ( (25,25) > (75,25). For high Pt content in the second layer, the energy contribution is in descending order: (25,100) > (50,100) > (75,100). Despite this complexity, the energy variation is not as large as that for CO deformation. So, we can conclude that the apparent barrier is larger for higher Pt content in the first layer, which is predominately due to the energetic contribution caused by CO deformation. Concerning the second layer effect, the apparent CO dissociation barrier decreases when n increases in the (25,n), (50,n),

and (75,n) series as shown in Table 4. However, the underlying stabilization forces are different. For the (25,n) and (50,n) surfaces, CO shifts to a later TS with increasing n because of a larger ∆E(CO def.), i.e., the C-O distance increases. The penalty paid by CO deformation can be compensated by a strong CO-surface interaction. In addition, with larger n, the (25,n) and (50,n) surfaces are less distorted. As a result, by summing up all three factors, the barriers are smaller for larger n. For the (75,n) series, the surface is less distorted and ∆E(surf. def.) varies in a small range. Furthermore CO exhibits an earlier transition state with larger n. At the same time, the CO-surface interaction strength also increases. In the present study, we studied physical/chemical properties on flat Pt-Co surfaces. However, low coordination sites and defects would make interpretation more difficult in reality. In the presence of low coordination site, the Fischer-Tropsch process can happen differently on cobalt nanoparticles.50 The Fischer-Tropsch process is complex and keeps attracting a lot of interest. It was demonstrated by Bezemer et al. that C5+ selectivity can be tailored by controlling particle sizes of cobalt.50 The study of detailed mechanisms of the Fischer-Tropsch process is fundamentally important but yet to be understood. To understand the particle size effects, we speculate that a combination of nonstandard computational methods will be required, which is beyond our up-to-date technology. Despite the complexity, our protocol is still applicable for other surface slabs of different morphology. And a good computational sample is inevitable for obtaining meaningful results in any cases. 3.4.2. CO Oxidation on Pt-Co Alloy Surfaces. The Pt-Co alloy is a potential candidate for catalytic CO oxidation, which is the most important technique nowadays to solve the CO poisoning problem in fuel cells.5,51 However, the origin of its activity is not clear. Furthermore, PtCo alloys have been found to be active catalysts in the oxygen reduction reaction (ORR), which is a key step taking place at fuel cell electrodes. The active role of PtCo and PtFe toward O2 activation was investigated by Xu et al. using crystalline surface alloy models.5 They suggested that the reactivity of the Pt skin (a single and pure Pt layer supported by a Pt-M alloy surface) might be due to the alleviation of oxygen poisoning on PtCo surfaces. In our recent study, we suggested strategies to acquire an active surface toward O2 activation.49 In this subsection, the underlying reason for the reactivity of PtCo alloy surfaces toward CO oxidation is presented. At a first glance, CO is less probable to poison the PtCo surfaces when

Figure 10. Energy decomposition analyses for the transition states of CO dissociations on Pt-Co alloy surfaces: (a) contribution by surface deformation; (b) contribution by CO deformation; and (c) contribution by interaction energy.

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TABLE 6: Coadsorption of CO and O on PtCo Alloy Surfacesf surface

∆ECOa,b (eV)

∆EOc (eV)

∆ECO+Oa,d (eV)

Eae (eV)

(25,25) (25,50) (25,75) (25,100)

–1.42 –1.53 –1.67 –1.84

–2.48 –2.55 –2.62 –2.60

–3.72 –3.91 –4.14 –4.34

–0.15 –0.27 –0.33

(50,25) (50,50) (50,75) (50,100)

–1.38 –1.38 –1.65 –1.69

–1.58 –1.82 –1.68 –1.95

–2.88 –3.09 –3.27 –3.60

–0.61 –0.38

(75,25) (75,50) (75,75) (75,100)

–1.20 –1.29 –1.46 –1.52

–0.97 –0.95 –1.10 –1.42

–2.13 –2.27 –2.52 –2.95

–0.39 –0.82 –0.76 –0.58

a CO adsorbs on Pt site. b ∆ECO ) E(CO · · · surface) - E(free CO) - E(bare surface). c ∆EO ) E(O · · · surface) - 1/2E(free O2) E(bare surface). d ∆ECO+O ) E(CO · · · O · · · surface) - E(free CO) 1 /2E(free O2) - E(bare surface). e Ea ) E(TS) - E(free CO) 1 /2E(free O2) - E(bare surface). f CO and O adsorb on the Pt-atop site and the surface hollow site, respectively.

the first atomic layer contains a higher Pt content (see Figure 2b,c). As a reference, the CO adsorption energy on pure Pt is found to be -1.64 eV. So, the CO molecules exhibit a weaker interaction with the Pt-atop sites for both (50,n) and (75,n) series as compared to CO-Pt(111). This implies that the (75,n) series, which can be prepared at higher temperature (over 400 °C), might not be poisoned by the chemisorption of CO. Recently, we have investigated oxygen dissociation (the initial step of CO oxidation)49 and revealed that this process is barrierless on our selected Pt-Co alloy surfaces. Consequently, the rate-determining step of CO oxidation on the Pt-Co alloy surfaces is the combination of the chemisorbed CO-molecule and the oxygen adatom.44 We summarized the results of the CO-O recombination reaction in Table 6 and Figure 11, respectively. In all calculations, the optimal atop-CO geometries on Pt or Co adsorption sites have been determined which correspond to the configurations displayed in Figure 2. Subsequently, geometry optimizations of preferential adsorption sites for oxygen adatoms have been performed. Therefore, for the coadsorption of CO and O, CO molecules will adsorb on the Pt-/Co-atop sites while the O adatoms adsorb on the hollow sites. In Figure 11a, we illustrate the coadsorption of CO and O on PtCo surfaces, where CO adsorbs on the Pt-atop sites. In Figure 11b, the coadsorption results are displayed with CO adsorbing on the Co-atop sites. As shown in Figure 11, the chemisorption energies of CO and [CO + O] are loosely correlated, except for the (25,n) series of surfaces. Nevertheless, we can still observe stronger CO and stronger CO + O interactions with alloy surfaces when the surfaces have a larger Co content in the first layer and a larger Pt content in the second layer. The (25,n) surfaces are poisoned by coadsorption of CO and O, which is predominately due the strong O-surface interaction. As shown in Table 6, we observe that ∆EO is larger than ∆ECO on the (25,n) surfaces, while ∆EO is comparable to ∆ECO on the (50,n) and (75,n) surfaces. Thereby, to alleviate CO and O poisoning, a higher Pt content in the first layer and a higher Co content in the second layer would be preferable. Combining CO and O on the surfaces would form CO2. Taking the energy of CO and 1/2O2 in the gas phase as a reference, the relative energy of free CO2 is at -3.24 eV, which is represented by dotted lines as shown in Figure 11 (∆ECO+O

Figure 11. Energetics of chemisorption of CO and coadsorption of CO and O on PtCo alloy surfaces: (a) CO molecules adsorb on Pt-atop sites and (b) CO molecules adsorb on Co-atop sites.

Figure 12. Energy landscape for the CO + O combination process. The energy states for the (25,n), (50,n), and (75,n) series are displayed as dotted, black, and gray lines, respectively. The energy of [CO(g) + 1 /2O2(g)] is referenced to 0 eV.

) -3.24 eV). The energy states of the adsorbed states of CO(ad) and O(ad) on the (25,n) PtCo alloy surfaces are lower than that of free CO2 (Figure 11a,b). So, the equilibrium would be driven to the precursor states and the surfaces would be poisoned. The (75,n) series are not poisoned because their adsorbed states are potentially less stable than free CO2. To further estimate the feasibility of CO oxidation, we select some models to demonstrate the effect of surface composition on the combination

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Figure 13. (a-c) Reaction energy profiles of CO + O combination on (a) (25,25) surface, (b) (25,50) surface, and (c) (25,75) surface.

barrier. Since the coadsorption of CO and O is energetically more favorable when CO adsorbs on the Pt-atop sites instead of Co-atop sites, we select the models displayed in Figure 11a. The energy profile along the CO + O reaction coordinate is illustrated in Figure 12. The combination barriers range from -0.15 to -0.82 eV, relative to the reference state (free CO + 1 /2O2 + bare surface). When CO approaches a preoxidized surface, the combination of CO and O is preferred rather than CO dissociation, since the former reaction experiences a negative apparent barrier while the latter overcomes a positive apparent barrier. Among our studied models, the (75,n) series is found to be most active toward CO oxidation, rather than the (25,n) series. This can be rationalized since the former series are not likely to be poisoned and exhibit smaller combination barriers forming CO2(g). Both factors contribute to the facile CO oxidation on the Pt-rich alloy surfaces. The (25,n) surfaces, however, have poor catalytic performance toward CO oxidation. From a mechanistic point of view, as observed in Figure 12, the alleviation of CO and O poisoning at the precursor state is the major factor favoring CO oxidation, as the range of coadsorption energy is as large as 2 eV, while the differences between the energy of the transition states are within 1 eV. Such a conclusion is in agreement with Xu’s findings.5 For a low Pt content in the first layer, we observed that CO and O would combine and form a chemisorbed CO2 intermediate on the Pt-Co alloy surfaces, which will then escape from the

surface with a small second barrier of 0.25 eV on the (25,25) surface. The reaction profiles for the (25,25), (25,50), and (25,75) surfaces are given in Figure 13a-c. With higher Pt content in the first layer (Figure 13c), however, the chemisorbed CO2 intermediate is less favorable and becomes less obvious in the reaction profile. This is in excellent agreement with recent theoretical work by Dupont et al.44 It is noteworthy that the catalytic oxidation on Pt-Co surfaces can be facilitated by a wide range of Pt-Co compositions, but the CO activation only occurs in a narrow window of Pt-Co composition including the (25,75) and (25,100) series, respectively. Together with the fact that the Pt-Co alloy surfaces have a larger Pt content in the upper layer after thermal annealing as evidenced by Gauthier et al.,23 the Pt-CO surfaces have a higher application potential for CO oxidation and ORR. Furthermore, a higher CH4 selectivity in the Fischer-Tropsch process will be expected according to the recently proposed mechanism by Chen and Liu.46 4. Conclusions In summary, we have established a systematic protocol to sample the bimetallic Co-Pt alloy surfaces of various compositions. We have characterized the adsorption and catalytic properties of these surfaces and the surface composition effects have been elucidated. As a main result, the adsorption site

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preference of CO on the alloy surfaces depends on the overall Pt:Co ratio of the Pt-Co slabs: the CO-molecule adsorbs preferentially on the Pt-sites (Co-sites) of the Pt-rich (Co-rich) alloy surfaces. Concerning the catalytic performance, on the one hand, the (25,n) series of PtCo alloy surfaces lower the CO dissociation barrier, as compared to pure Co(0001). On the other hand, the (75,n) series of PtCo alloy surfaces are effective for the combination process of CO and the O atom. From the viewpoint of PtCo preparation, this surface alloy is more suitable for ORR and CO oxidation, because it would exhibit a high Pt content at the outer surface after thermal annealing. Acknowledgment. We thank the Alexander von Humboldt Foundation (W.L.Y.), the Hanse Wissenschaftskolleg (W.L.Y.), and the EWE AG (T.K.) for financial support. The simulations were performed on the national supercomputer NEC SX-8 at the High Performance Computing Center Stuttgart (HLRS) under grant no. WLYIM. Supporting Information Available: Optimized structures of bare surface slabs, CO/PtCo, and precursors of CO + O combinations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Conner, W. C.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (2) Ge, Q. F.; Neurock, M. J. Phys. Chem. B 2006, 110, 15368. (3) Liu, Z. P.; Jenkins, S. J.; King, D. A. J. Am. Chem. Soc. 2003, 125, 14660. (4) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. ReV. Lett. 1996, 76, 2141. (5) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717. (6) Park, J. I.; Kim, M. G.; Jun, Y. W.; Lee, J. S.; Lee, W. R.; Cheon, J. J. Am. Chem. Soc. 2004, 126, 9072. (7) Zhang, Z. T.; Blom, D. A.; Gai, Z.; Thompson, J. R.; Shen, J.; Dai, S. J. Am. Chem. Soc. 2003, 125, 7528. (8) Qian, Y. D.; Wen, W.; Adcock, P. A.; Jiang, Z.; Hakim, N.; Saha, M. S.; Mukerjee, S. J. Phys. Chem. C 2008, 112, 1146. (9) Sotelo, J. C.; Seminario, J. M. J. Chem. Phys. 2007, 127, 244706. (10) De Santis, M.; Baudoing-Savois, R.; Dolle, P.; Saint-Lager, M. C. Phys. ReV. B 2002, 66, 085412. (11) Penuelas, J.; Andreazza, P.; Andreazza-Vignolle, C.; Tolentino, H. C. N.; De Santis, M.; Mottet, C. Phys. ReV. Lett. 2008, 100, 115502. (12) Tsay, J. S.; Shern, C. S. Surf. Sci. 1998, 396, 313. (13) Baudoing-Savois, R.; Dolle, P.; Gauthier, Y.; Saint-Lager, M. C.; De Santis, M.; Jahns, V. J. Phys.: Condens. Matter 1999, 11, 8355. (14) Saint-Lager, M. C.; Baudoing-Savois, R.; De Santis, M.; Dolle, P.; Gauthier, Y. Surf. Sci. 1998, 418, 485. (15) Gonzalez, S.; Sousa, C.; Fernandez-Garcia, M.; Bertin, V.; Illas, F. J. Phys. Chem. B 2002, 106, 7839.

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