Density Functional Theory Study of Anode Reactions on Pt-Based

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J. Phys. Chem. C 2007, 111, 272-279

Density Functional Theory Study of Anode Reactions on Pt-Based Alloy Electrodes Yoshiki Shimodaira,† Toshitaka Tanaka,‡ Toshiko Miura,‡ Akihiko Kudo,† and Hisayoshi Kobayashi*,‡ Department of Applied Chemistry, Faculty of Science, Science UniVersity of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan, and Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyoku, Kyoto 606-8585, Japan ReceiVed: July 19, 2006; In Final Form: October 16, 2006

DFT calculations have been applied to investigate reaction mechanisms occurring on anode catalysts of polymer electrolyte fuel cells. This Article consists of three parts. First, CO and H2 adsorption was investigated using Pt-based alloy electrodes modeled by Pt5M5 (M ) V, Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Sn, W, and Au) clusters. The most durable catalysts for CO poisoning were searched using two criteria: (1) adsorption of CO should be weakened as compared to pure Pt, and (2) the transition state (TS) energy for H-H bond fission of adsorbed H2 should not be higher than that for pure Pt. Pt-Ru alloy and a few candidates were found. Second, periodic DFT calculations were carried out to explain the surface segregation tendency of one component element over the other for the nine alloys, Pt-Fe, Pt-Co, Pt-Ni, Pt-Mo, Pt-Ru, Pt-Rh, PtPd, Pt-Ag, and Pt-Au. For example, for Pt-Ru alloy, Pt atom was shown to have higher potential for surface segregation, but for Pt-Ag alloy, Ag atom had higher potential. Finally, in relation to the CO elimination mechanism, the energetics of four typical reactions were investigated using extended cluster models and including the solvation effects: (1) COad + H2Oad + H2O f COOHad + H3O+, (2) H2Oad + H2O f OHad + H3O+, (3) COOHad + OHad f (CO2)ad + H2Oad, and (4) COad + OHad + H2O f COOHad + H2O f (CO2)ad + H3O+. Individual reactions were compared among Pt metal and Pt and Ru sites of Pt-Ru alloy. Reaction 1 is most preferable on Pt sites of Pt-Ru alloy, whereas the disproportionation of water (reaction 2) is most favorable for pure Pt and then Ru sites of the alloy, which is consistent with the so-called bifunctional mechanism. Reactions 3 and 4 proceed without the activation energy, and the difference in metals was less significant.

1. Introduction From the viewpoint of global energy and environmental issues, research on fuel cells spans a wide region from basic laboratory experiments to demonstration in industrial plants. Among an enormous number of reports, Pt metal and Pt-Ru alloy have been studied most intensively because Pt-Ru alloy is known to be the most promising electrode to cope with the CO poisoning problem. These are many papers on CO adsorption and oxidation on Pt1-8 and Pt-Ru,4,9-13 but only a few papers are available for OH species treating adsorption on Pt (111) in ultrahigh vacuum or gas-phase conditions.14,15 Alloying effects are so far interpreted in terms of the ligand and bifunctional effects.4,10 For methanol decomposition reaction, Watanabe and co-workers gave a clear understanding that dehydrogenation occurs at Pt sites and oxidation of CO proceeds at Ru sites with easily formed OH species. Since pioneering works by Anderson and Grandscharova,16 more elaborate calculations based on the DFT methods have been reported.17-24 Michaelides and Hu reported on OH and H2O mixed adspecies on Pt (111).17 Koper and co-workers examined CO and OH adsorption on Pt, Ru, and their alloys.18 CO adsorption on Pt-Ru alloys was studied by the periodic models19 and cluster models.20 Gong and co-workers reported * Corresponding author. Fax: +81-75724-7580. E-mail: kobayashi@ chem.kit.ac.jp. † Science University of Tokyo. ‡ Kyoto Institute of Technology.

CO oxidation by water molecules on Pt (111),21 and Dunietz et al. also treated CO oxidation with cluster models introducing electric field.22 Karlberg et al.23 and Desai and Neurock24 examined the surface species after water dissociation on Pt (111) and Pt-Ru alloy, respectively. In this paper, adsorption energies of CO and H2 are investigated using 13 Pt-based binary alloys (Pt-M, M ) V, Cr, Fe, Co, Ni, Mo, Ru, Rh, Pd, Ag, Sn, W, and Au) as well as three metals, Ni, Pd, and Pt. Most CO durable anode catalysts are searched according to the two criteria: (1) CO adsorption energy must be smaller than that of pure Pt metal. (2) The activation energy for H-H bond fission must not be higher than that of pure Pt metal. The local minimum for CO adsorption and the TS for H-H bond fission are optimized. In real alloys, one element is highly segregated in the surface layer more than an averaged atomic ratio of the sample. Two-dimensional fourlayer slab model calculations are carried out to examine the relative stability of surface-bulk localization for Pt-Mo, PtFe, Pt-Co, Pt-Ni, Pt-Ru, Pt-Rh, Pt-Pd, Pt-Ag, and PtAu alloys. Finally, CO oxidation and elimination mechanisms on electrode surfaces are investigated with the four typical reactions: (1) COad + H2Oad + H2O f COOHad + H3O+, (2) H2Oad + H2O f OHad + H3O+, (3) COOHad + OHad f (CO2)ad + H2Oad, and (4) COad + OHad + H2O f COOHad + H2O f (CO2)ad + H3O+. The energetics are compared between Pt metal and Pt-Ru alloy, and between gas-phase and solvation models.

10.1021/jp064563u CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2006

DFT Study of Anode Reactions

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TABLE 1: Nearest-Neighbor Distances for Alloys and Pure Metals Employed in the Present Calculationa components N-N distance components N-N distance

Pt-V 2.698 Pt-Rh 2.729

Pt-Cr 2.633 Pt-Pd 2.761

Pt-Fe 2.629 Pd 2.751

Pt-Co 2.641 Pt-Ag 2.832

Pt-Ni 2.630 Pt-Sn 2.791

Ni 2.489 Pt-W 2.754

Pt-Mo 2.750 Pt-Au 2.885

Pt-Ru 2.741 Pt 2.772

a Nearest-neighbor distances for metals are taken from: Kittel, C. Introduction of Solid State Physics, 8th ed.; John Wiley & Sons, Inc.: New York, 2005.

Figure 1. Optimized structures for CO and H2 adsorbed on Pt5M5 and M5Pt5 clusters. Two types of clusters are employed per each alloy. The number of total models including three pure metals is 29.

2. Models and Methods of Calculation Pt10 and Pt15 cluster models are used to represent the local structure of Pt (111) surface. They consist of 7 or 10 atoms in the first layer and 3 or 5 atoms in the second layer. For models of alloys, the clusters with a similar shape, that is, the same local symmetries are employed by changing the nearest-neighbor distances. The nearest-neighbor distances are set to the averaged values of those of component metals. Between the metals belonging to the fcc (face-centered cubic) and hcp (hexagonal close packed) structures, the local structure is almost the same up to the second layer. Even if one element belongs to the bcc (body-centered cubic) structure (for Pt-V, Pt-Cr, Pt-Fe, PtMo, and Pt-W) or to the diamond structure (for Pt-Sn), those atoms are thought to be incorporated into the fcc lattice. The nearest-neighbor distances for pure metals and alloys adopted in this work are shown in Table 1. Atomic arrangements of M5N5 cluster with the configurations of CO and H2 adsorption are shown in Figure 1. Either M or N atom is Pt except for pure metal Ni10, Pd10, and Pt10 clusters. On M5N5 cluster, CO and H2 molecules are adsorbed to the on-top site on the central atom with the end-on and side-on configurations, respectively. For each alloy composed of metals M and N, adsorption is examined at both metal atom sites by exchanging the atom position. Hereafter in our notation, the metal atom to which adsorbate binds is specified first, and so M5N5 and N5M5 clusters mean that the molecule is adsorbed to the M and N atoms, respectively. All of the cluster calculations are carried out with the Gaussian 03 program package.25 The Becke three-parameter Lee-Yang-Parr hybrid functional is employed.26-29 The Los Alamos model core potential basis sets are used.30,31 CO and H2 adsorption is calculated using bare cluster models in the gas phase. As for the four reactions relating CO elimination, the structures are optimized in the gas phase. The solvation effects are considered in an indirect manner adopting the self-consistent reaction field method and at the level of polarized continuum model (PCM). In all calculations, the metal cluster moieties are fixed. For characterization of the TS, the standard technique with the second energy derivatives is used, and the single imaginary frequency is confirmed. In figures drawing the TS structures, the leading components of internal coordinates are

Figure 2. Unit cells consisting of four layers of Pt2M2M2Pt2 and M2Pt2Pt2M2 used for the two-dimensional slab model calculations. The sizes of two atoms are exaggerated.

indicated in parentheses to show that the structure is the proper TS on the reaction path. Although individual TS structures corresponding to H-H bond fission are not illustrated, it is confirmed that the leading component characterizing the TS is the H-H bond length. We use the terms of adsorption energy or stabilization energy to indicate energy lowering of the local minima and intermediates during the reaction. A sum of energies for a free alloy cluster and a free stable molecule such as CO, H2, H2O, or CO2 is taken as the reference, that is, zero energy. TS energies are also estimated by the same reference. The activation barrier is the TS energy measured from the local minimum located just before. In some cases, the term of relative energy is used. For example, the TS and product energies are represented with respect to the energy of reactant. Slab calculations are carried out by the plane wave-based program Castep32 with the ultra-soft core potentials.33 The fourlayer slab model is adopted, as shown in Figure 2. For each Pt-M alloy, two types of unit cells, Pt2M2M2Pt2 and M2Pt2Pt2M2 (M ) Fe, Co, Ni, Ru, Rh, Pd, Ag, and Au), are employed. The outer layers and inner layers represent the “surface” and “bulk” of these alloy materials. The averaged lattice constants are adopted similar to the cluster model case. In the plane wavebased method, three-dimensional periodic boundary condition is imposed even for the two-dimensional slab. For the direction perpendicular to the slab surface, slab structures are repeated with a vacuum region of 10 Å. The kinetic energy cutoff is set to 240 eV. Because this cutoff energy is rather coarse, only the Pt-Ru slab is recalculated with a higher cutoff energy of 340 eV. 3. Results and Discussion 3.1. CO and H2 Adsorption onto Binary Alloy Clusters. Figure 3shows the stabilization energies for CO and the TS

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Shimodaira et al. TABLE 2: Difference Energy (∆E)a for Plane Wave DFT Slab Calculations with Experimental Surface Tension Values (σ)b and Their Differences (∆σ)c M Pt Fe Ru Rud Mo Co Rh Ni Pd Ag Au

∆E (kJ/mol)

σ (dyn/cm)

∆σ

-342.3 -331.7 -321.6 -230.5 -161.1 -157.5 -150.2 -39.4 38.1 106.1

1865 1880 2250 2250 2250 1880 2000 1725 1500 921 1130

-15 -385 -385 -385 -15 -135 140 365 945 735

a ∆E is defined by subtracting total energy for M2Pt2Pt2M from total energy for Pt2M2M2Pt2. b CRC Handbook of Chemistry and Physics, 66th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1986; pp F20-24. c ∆σ is defined by subtracting surface tension for Pt from surface tension for the metal M. d Higher level calculation where the kinetic energy of cutoff is increased to 340 eV.

Figure 3. CO and H2 adsorption energy adsorbed to Pt5M5 and M5Pt5 clusters, as well as Ni10, Pd10, and Pt10 clusters. Two horizontal lines show the CO and H2 energies for Pt10 cluster. The “b” represents the energy for local minima of CO, and the “O” represents the TS energy for H-H bond fission. Calculation of H2 (TS) is performed only for the systems where the adsorption energy of CO is lower than that for Pt10.

energies for H-H bond fission of H2 adsorbed on the 26 Pt5M5 and M5Pt5 alloy clusters and three pure Ni10, Pd10, and Pt10 clusters. The abscissa represents the alloy composition and adsorption site. For example, Ru5Pt5 represents the model where a molecule is adsorbed to the Ru site. The adsorption energies for CO (local minimum) and H2 (TS) on Pt10 cluster as described by the two horizontal lines are taken as the standard. Two criteria are adopted to select CO durable catalyst: (1) CO adsorption energy must be smaller than that for pure Pt, and (2) the TS energy for H-H fission must not be higher than that for pure Pt. The first criterion is satisfied by the 14 alloy systems, for which the TS structure and energy are evaluated. The second criterion is satisfied by the four systems, Co-Pt, Pt-Ni, PtRu, and Pt-Au. The second criterion is not strictly satisfied for Pt-Fe and Pt-Mo, but they are thought to be on the border. The Pt-Ru alloy has been known to be the best CO durable catalyst by experimental works so far. Our calculation also reproduces this experimental finding and suggests the other three or five candidates. For other alloy systems, available experimental data are few. Watanabe et al. examined CO oxidation on Pt-Fe alloy electrode and reported that saturated coverage of CO is lower than that of pure Pt.34 Furthermore, Watanabe and co-workers investigated H2 oxidation in the presence of CO with several non-precious metal alloys.35 They reported that Pt-Fe, Pt-Ni, Pt-Co, and Pt-Mo alloys exhibit excellent CO tolerance in H2 oxidation similar to that of Pt-Ru alloy. Except for Pt-Au, the Co-Pt, Pt-Ni, Pt-Fe, and Pt-Mo alloys are also selected in our calculation as promising candidates. Thus, the present computational approach including the two criteria is thought to be useful to search CO durable catalysts.

3.2. Slab Model Calculation on Surface Concentration. Four-layer slab model calculations are carried out by the plane wave DFT method. Table 2 shows the difference energies (∆E) defined as a difference between the total energies for Pt2M2M2Pt2 and M2Pt2Pt2M2. Although the total energies (not shown) were different by ca. 5 eV between the lower and higher cutoff energies for Pt-Ru slab, the difference in ∆E is only on the order of 10 kJ. So the results with the lower cutoff energy seem to be qualitatively correct. The ∆E is negative for Pt-Fe, Pt-Ru, Pt-Mo, Pt-Co, PtRh, Pt-Ni, and Pt-Pd alloys. It means that the alloy configuration is more stabilized when Pt atoms are located at the surface rather than bulk. For Pt-Ru alloy, for example, Pt atoms are localized at higher concentration in the surface layers than the bulk. Therefore, the local structure of Pt5Ru5 will have a higher probability of appearance than that of Ru5Pt5. This result is consistent with what has been suggested by Gasteiger et al.36 Watanabe and co-workers also referred that, for Pt-Fe alloy, Pt skin is formed during the repetition of potential cycling.34 These are consistent with our results. Among the other five alloy systems that have passed the two criteria, the local structures of Co5Pt5 and Pt5Au5 seem to appear with smaller probability because Pt atom in Pt-Co alloy and Au atom in Pt-Au alloy have a higher tendency of surface deposition, respectively, whereas Pt5Fe5, Pt5Mo5, and Pt5Ni5 models are consistent with the result expected by the ∆E. To test the usefulness of this approach, the ∆E is compared to some experimental value. Surface free energy is an adequate quantity, whereas it is less available in data books. We assume that surface free energy is equivalent to surface tension for the present purpose. Table 2 also shows the surface tension of metals (σ) and the difference from Pt, that is, ∆σ ≡ σPt - σM. Except for Pt-Ni and Pt-Pd alloys, the signs of ∆E and ∆σ are the same. It means that metals with the smaller surface free energy tend to be localized at the surface layer. The discrepancy occurs for the alloys with the negative sign and the smallest absolute values in ∆E. The present comparison is based on a rough slab calculation with a single averaged value of interatomic distances. Optimization of such parameters will be necessary for more quantitative comparison. 3.3. Reaction between CO and H2O. Adsorbed CO is assumed to be removed through interactions with H2O or OH. If the alloying effects work to promote CO desorption, the activation barrier is expected to be lower than that for pure Pt.


Figure 4. Optimized structures for the reactant, TS, and product for COOH formation from CO and H2O on Pt5Ru5 alloy cluster. Configurations are Pt5Ru5-CO(H2O), Pt5Ru5-CO(H2O)2, and Pt5Ru5-COOH, respectively. For energetics, H2O and H3O+ are added to reactant and product, respectively. For the TS, the leading components of internal coordinates are indicated in parentheses.

Figure 5. Energy profile for COOH formation. Relative energy for TS and product is defined as the difference from the reactant. The relative energy is compared among Pt10 cluster, Pt5Ru5 (Pt site), and Ru5Pt5 (Ru site) in the gas-phase or PCM calculations.

Reaction mechanisms for the formation of COOH and H3O from CO and two H2O molecules are investigated and compared between pure Pt and Pt-Ru alloy, employing Pt10, Pt5Ru5, and Ru5Pt5 clusters. Figure 4 shows the optimized structures for the reactant, TS, and product on Pt5Ru5 cluster obtained in gasphase calculations. The atomic configurations are Pt5Ru5CO(H2O), Pt5Ru5-CO(H2O)2, and Pt5Ru5-COOH, respectively. The reactant is a coadsorbed state of CO and H2O. COOH adspecies is formed in the product. Similar structures are also obtained for Pt10 and Ru5Pt5 clusters. These structures obtained in the gas phase are employed for the PCM calculations without further optimization to evaluate the total energy in solution. Because the total energy should be compared within the same number of atoms along the reaction, the energies of isolated H2O and H3O+ (with PCM calculation) are added to those of the reactant and product, respectively. Figure 5 shows the relative energies for TS and product with respect to the reactant with/without the solvation effects. The activation energies for the gas phase measured from the corresponding reactants are 74, 85, and 134 kJ/mol for Pt metal, and Pt and Ru sites on these alloys. For PCM calculation, they are 126, 122, and 168 kJ/mol, respectively. The activation energy is larger for the PCM calculation. This result is rather normal because the TS structure is weakly polarized as compared to the reactant, and the solvation effects stabilize the reactant more. However, a major difference is found for the product. In the gas phase, the energy is raised to 661, 577, and 665 kJ/mol with respect to the corresponding reactants because the charges are separated to [M5N5-COOH]- and H3O+, whereas the energy only increases by 63, 77, and 145 kJ/mol with the solvation effects. Thus, for the systems composed of anion and cation, the PCM calculation improves the energetics essentially.


Figure 6. Optimized structures for the reactant and TS for water disproportionation reaction on Ru5Pt5 cluster. Configurations are Ru5Pt5-(H2O)2. For the TS, the leading components of internal coordinates are indicated in parentheses.

Activation energies for the TS are compared among the Pt surface, Pt, and Ru sites on Pt-Ru alloy with the solvation effects. Between Pt10 and Pt5Ru5 clusters, the energies are almost the same (126 and 122 kJ/mol). The superiority of Pt5Ru5 to Pt10 is recognized slightly. This is not a surprising result because the so-called ligand effects by alloying are generally known to be of secondary importance, as reported by experimental works.10 However, the activation energy for Ru5Pt5 (168 kJ/ mol) is much larger. This means that CO adsorption at the Ru atom site is not favorable to process the shift-type reaction. It is also consistent with the larger adsorption energy of CO to the Ru site of Pt-Ru cluster as shown in Figure 3. 3.4. Disproportionation of H2O. Another important reaction relating removal of CO by oxidation is formation of oxygencontaining species such as OH. Figure 6 shows optimized structures for the reactant and TS for H2O disproportionation on Ru5Pt5 cluster. Similar structures are obtained on Pt10 cluster. Both configurations are Pt5Ru5(H2O)2 and Pt10(H2O)2. Activation energies for the TS measured from the corresponding reactant are evaluated to be 64 and 91 kJ/mol, respectively. (Similar to the previous case, the structures are optimized in the gas phase, and then the total energies are reevaluated with PCM.) The smaller activation energy is consistent with facile formation of OH species at the Ru site on Pt-Ru alloy. Because the structures are optimized in the gas phase, that is, without the solvent effects, the oxonium ion is not stabilized in the product. It is reasonable that at the TS structure, one H2O molecule accepts a H atom and simultaneously releases the other H atom to the surface metal atom. Feasibility of the formation of OH species is furthermore examined by separate calculations. In Figure 7, adsorption energies of H2O and OH- species are calculated for the three cluster models. For example, the sum of energy of Ru5Pt5-OH2 and H2O as for the reactant is compared to the sum of energy of [Ru5Pt5-OH]- and H3O+ as for the product. The relative energies of the product with respect to the reactant are shown in Figure 8, for the Ru and Pt sites of alloy and Pt metal without/ with the solvent effects. In the gas phase, the energies of product are destabilized by ca. 600 kJ/mol as compared to the reactant, whereas the destabilization or endothermicity is reduced to the order of 15-90 kJ/mol with the PCM calculation. The endothermicity increases in the order of pure Pt metal, Ru site, and Pt site on the Pt-Ru alloy. The lowest endothermicity for Pt is unexpected a little. Between the Ru and Pt sites on Pt-Ru alloy, the Ru site is found to be more favorable for disproportionation of water molecule. This result is in conformity with the bifunctional mechanism presented by experimental workers.4,10


Shimodaira et al.

Figure 7. Optimized structures for H2O and OH adsorption on Ru5Pt5 cluster.

Figure 8. Energy profile for water disproportionation reaction. Relative energy is defined as the difference in the sum of energies of [M-OH]and [H3O]+ and the sum of energies of M-OH2 and H2O. The energy is compared among Pt10 cluster, Pt5Ru5 (Pt site), and Ru5Pt5 (Ru site) in the gas-phase or PCM calculations.

Figure 9. Pt8Ru7 cluster model for the reaction, COOH + OH f CO2 + H2O. The system adsorbed by COOH and OH is regarded as a double anionic system. Two oxonium ions are put beneath the cluster to cancel the negative charges brought in by COOH and OH.

3.5. Reaction between COOH and OH. The third reaction is CO2 formation from adsorbed COOH and OH species, which have been formed by the previous two reactions on the alloy surface. From the reason that a Pt5Ru5 cluster is too small to simulate this reaction, a Pt8Ru7 cluster is employed. We put a

Figure 10. Structure change on Pt8Ru7 cluster along the reaction, [PtsCOOH]- + [Rus-OH]- f [Pts-CO2]2- + [Rus-OH2]. The reference energy is taken as the sum of energies of free Pt8Ru7 cluster, free CO2, and H2O molecules. Subscript “s” means the site in the cluster.

criterion that only CO and H2O are neutral species, and then regard adsorbed COOH and OH as anionic species. To simulate the reaction [M-COOH]- + [M-OH]- f [M-CO2]2- +



Figure 11. Energy change along the reaction, [Pts-COOH]- + [RusOH]- f [Pts-CO2]2- + [Rus-OH2]. The reference energy is taken as the sum of energies of free Pt8Ru7 cluster, free CO2, and H2O molecules. The structures are optimized with two oxonium ions (b). Those structures are used for the single point energy calculation without oxonium ions. “O” and “0” mean the energies for neutral and dianion clusters without oxonium ions. Subscript “s” means the site in the cluster.

M-OH2 on the electrode surface, the employment of double anionic cluster is a straightforward way. However, in anionic clusters, even the orbital energies for the occupied valence molecular orbitals are raised to positive values. Therefore, it is better to use neutral clusters. So, two H3O+ cations are put at 10 Å beneath the cluster as shown in Figure 9. These positive charges cancel the negative charges brought in by COOH- and OH-. For the first simulation, COOH is adsorbed to a Pt atom through the C atom and OH is adsorbed to a Ru atom. Structure change along the reaction is shown in Figure 10, where two H3O+ cations are not shown for clarity. The corresponding energy is shown in Figure 11. The reaction proceeds without the activation energy. The H atom in COOH is smoothly transferred to OH, and CO2 and H2O are formed. Figure 11 also shows the two sets of energies evaluated without two H3O+ cations for the neutral and dianionic charge states, where the structures optimized with two H3O+ cations are used without further optimization. The three energy profiles show similar downhill reactions. Therefore, it means that the neutralization of cluster does not affect the energetics significantly. For the second simulation, the reaction is reexamined by exchanging the role of Pt and Ru. As shown in Figure 12, OH is adsorbed to a Pt atom and COOH is adsorbed to a Ru atom at the reactant. Two H3O+ cations are employed to neutralize the cluster, but they are not drawn in Figure 12. The reaction proceeds without barrier again. However, the stabilization energy of the product (172 kJ/mol) is much larger than that for the previous simulation (108 kJ/mol). This is due to the strong interaction between the O atom in CO2 and peripheral Ru atoms. It is expected that CO2 is bound very strongly, and greater activation energy is necessary to desorb CO2 from the electrode surface. 3.6. Reaction between CO and OH. The forth reaction is CO2 formation from adsorbed CO and OH via surface COOH species. Although this reaction has been dealt with in several computational works,18,21,22 the present work is the first case using the PCM calculation. The Pt8Ru7 cluster is employed. CO and OH are adsorbed to Pt and Ru atoms at the reactant. Optimized structures for the reactant, TS, and product are shown in Figure 13. One H3O+ cation is added to the cluster for the reactant, TS, and product. Furthermore, the product cluster is calculated with a single negative charge to compensate the positive charge by desorbed H3O+. To compare the energy along

Figure 12. Structure change on Pt8Ru7 cluster along the reaction, [RusCOOH]- + [Pts-OH]- f [Rus-CO2]2- + [Pts-OH2]. The reference energy is taken as the sum of energies of free Pt8Ru7 cluster, free CO2, and H2O molecules.

the reaction, the energies of H2O and H3O+ are added to those of the reactant and product, respectively, for equalization of the number of atoms. Figure 14 shows relative energies with respect to the reactant in the gas phase and the PCM calculations. The reaction proceeds with slight activation barrier for the PCM, whereas the product is destabilized for the gas-phase calculation.


Figure 13. Structure change on Pt8Ru7 cluster along the reaction, PtsCO + [Rus-OH]- + H2O f [PtsRus-COOH-OH2]- f [PtsRusCO2]2- + H3O+. To neutralize the system, one H3O+ is added to reactant, TS, and product. Furthermore, mono negative charge state is used for product. For energetics, the energies of isolated H2O and H3O+ are added to reactant and product. For the TS, the leading components of internal coordinates are indicated in parentheses.

Shimodaira et al. first scheme of the reaction, CO reacts with H2O to eliminate H+ as H3O+, and then the resulting COOH species reacts with OH to produce CO2 and H2O. This reaction was treated in subsections 3.3 and 3.5. Our calculation showed that the activation barriers of more than 120 kJ/mol were encountered for the former half, but the reaction proceeded downhill for the latter half. In the second scheme of the reaction, CO reacts with OH first, and then the resulting COOH species reacts with H2O to produce CO2 and H3O+. As discussed in subsection 3.6, this reaction proceeded with almost no activation barrier. Thus, the present work revealed fundamental aspects on the Pt-based alloy electrodes and the reaction mechanisms proceeding on them, such as the ligand effects, by functional mechanism, and surface atomic compositions. Acknowledgment. This research has been conducted as the project “Research and Development of Polymer Electrolyte Fuel Cell Systems” under an entrustment contract from the New Energy and Industrial Technology Development Organization (NEDO). We are grateful to Toyota Motor Corp. for financial support. We express our appreciation to Prof. Yasushi Murakami and Prof. Takehiro Matsuse of Shinshu University for their helpful advice and discussion. References and Notes

Figure 14. Energy profile for the reaction, Pts-CO + [Rus-OH]- + H2O f [PtsRus-COOH-OH2]- f [PtsRus-CO2]2- + H3O+ in the gas-phase or PCM calculations. Relative energy for TS and product is defined as the difference from the reactant.

It is again concluded that the solvent effects are essentially important when the charge separation occurs. 4. Conclusion In this Article, several important reactions occurring on the anode of fuel cell were investigated by the DFT-based calculations. From the viewpoint of the two criteria presented by the authors, CO durable electrode catalysts were searched employing the M5N5 cluster models. Among the 13 binary alloys, the six systems, that is, Co-Pt, Pt-Ni, Pt-Au, Pt-Fe, and Pt-Mo, as well as well-known Pt-Ru, were selected as the candidates. Periodic slab calculations were carried out to examine the relative stability of surface-bulk localization between the two alloy elements. The calculated results were in relatively good agreement with what was expected from the difference in experimental surface tension data. Especially, the calculation reproduced that Pt atoms were enriched in the surface layer of Pt-Ru alloy, which has been believed by experimental workers. CO oxidation was decomposed into the four typical reactions. The energetics was compared among the Pt metal, Pt, and Ru sites on the alloy using Pt10, Pt5Ru5, Ru5Pt5, and Pt8Ru7 clusters. The PCM calculation was found to be essentially important for the energetics when charges were separated within the system. The activation energy for COOH formation was almost the same for Pt metal and Pt site of the alloy and a little higher for Ru site of the alloy. On the other hand, OH formation was most favorable for Pt metal, next favorable for Ru site of the alloy, and the least favorable for Pt site of the alloy. Therefore, it was concluded that these two tendencies reflect the bifunctional mechanism. Furthermore, CO2 formation process was simulated through the two sets of reactions using the Pt8Ru7 cluster in part. In the

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Density Functional Theory Study of Anode Reactions on Pt-Based

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