11744
J. Phys. Chem. 1996, 100, 11744-11747
Water Dissociation on Pt(111) and (100) Anodes: Molecular Orbital Theory Seeyearl Seong and Alfred B. Anderson* Chemistry Department, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106-7078 ReceiVed: April 11, 1996X
Cluster models and the atom superposition and electron delocalization molecular orbital (ASED-MO) band shift technique have been used to study the electrochemical potential dependence of H2O(ads) decomposition to OH(ads) and H(ads) on Pt(111) and (100) anodes. The water molecule is found to bind most stably to the 1-fold site of the (111) surface and the 2-fold bridging site of the (100) surface. The transition state structure is closer to the 1-fold structure, which explains our result of lower activation energy over the Pt(111) surface. This finding provides an interpretation for results of electrochemical measurements in the literature.
Introduction The structures and reactions of water adsorbed on platinum surfaces continue to be subjects of studies in electrochemistry1-3 and heterogeneous catalysis.4 A variety of experimental techniques have provided data about adsorbed water, for example those described in the review of Thiel and Madey.5 Quantum mechanical calculations have favored the 1-fold adsorption site compared to the 4-fold site for an H2O molecule on Pt(100)6 and also favor the 1-fold over 2- and 3-fold sites on Pt(111).7 Water is a reactant in organic fuel cells, providing oxygen in the production of CO2 from the fuel at the anode. In the process of hydrocarbon oxidation, CO(ads) is formed on the surface and blocks the earlier reaction steps. The CO(ads) poison is removed by reaction with H2O:8
H2O + CO(ads) f CO2 + 2H+ + 2e-
(1)
It has long been believed that this reaction consists of two steps:9
H2O f OH(ads) + H+ + e-
(2)
CO(ads) + OH(ads) f CO2 + H+ + e-
(3)
Recent experimental10 and theoretical11 work supports the mechanism of reaction 2 over Pt and Ru/Pt alloy anodes, though over Sn/Pt anodes the experimental12 and theoretical13 evidence at low potentials suggests a different mechanism. There is evidence that OH(ads) may be forming at higher potentials on (100) Pt surfaces than on Pt(111). Cyclic voltammograms (50 mV s-1 sweep rate) of Pt(111) in 0.5 m perchloric acid show an asymmetric oxidation current commencing at about 0.6 V with a sharp peak at 0.785 V evidently superimposed on top of a broad peak centered at about 0.7 V. The broad peak has been ascribed to H oxidation from the surface as H+(aq) and the sharp one to surface oxidation.14 When CO is introduced, its oxidation peak at 0.75 V15,18 correlates with the onset of the surface oxidation peak, which implies that OH(ads) may be forming during surface oxidation. Cyclic voltammograms of Pt(100) show a peak around 1.0 V and a shoulder extending down to around 0.80 V.16 The oxidation of CO in this system produced a current onset at about 0.7 V followed by a peak at 0.75 V.16 The sharpness of the CO oxidation peaks suggests that CO covered surfaces are oxidized X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(96)01087-8 CCC: $12.00
suddenly and at potential less positive than the potentials at which the clean surfaces show large oxidation currents. If the onset of oxidation on the clean surface is due to OH(ads) formation by eq 2, and if CO(ads) is immediately oxidized by this OH, then CO(ads) is evidently catalyzing eq 2 to occur rapidly on both surfaces and at a significantly lower potential on the (100). As shown in the previous theoretical study,11 the activation of OH(ads) formation from H2O(ads), eq 2, as the potential is increased is a result of the increasing electron acceptor capability of the surface. The approach to the transition state for OH bond cleavage on the surface is characterized by strong σ donation from O and H, which becomes stronger as the potential is increased. Exposure17 of a Pt(111) surface to saturation coverages of water results in a reduction of the measured surface work function, Φ, by about 1.0 eV, that is, from ∼5.7 eV18 to about 0.0 V on the standard hydrogen electrode scale, for which the work function is about 4.6 eV. When the surface is initially nearly saturated with CO and then saturated with water, which does not displace the CO from the surface, this reduction in work function is about 0.5 eV, which would put it at about 0.5 V on the standard hydrogen scale. We propose that adsorbed CO increases the potential locally on the electrode surface and that at edges of CO islands the Pt atoms are better electron pair acceptors and therefore activate the formation of OH(ads) at a slightly lower voltage than in its absence. As OH(ads) forms it oxidizes the CO(ads) islands rapidly. The oxidation of CO in a CO-saturated solution and the oxidation of 0.1 M CH3OH both show a distinct sensitivity to Pt surface structure. On Pt(111) the onset for both occurs at about 0.5 eV and on Pt(100) the onset for both is about 0.6 eV19 (50 mV s-1 sweep rate; 0.1 m HClO4). This indicates that CH3OH is quickly oxidized to the CO(ads) poison on both surfaces. The sharp drops above 0.7 V for CO and CH3OH oxidation on both surfaces is indicative of the formation of a strongly held passivating OH(ads) overlayer. It has been suggested that OH(ads) might be forming at lower potentials than those mentioned above on Pt(100) in 0.1 M HClO4 and 0.1 M H2SO4. Peaks in cyclic voltammograms for these systems appeared at about 0.4 and 0.6 V, respectively, and were assigned to either water adsorption with so-called partial discharge or water adsorption followed by dissociation to OH(ads) and a proton and an electron.20 Similar peaks were evidently not observable for the (111) surface, implying that the (100) surface is more active for water dissociation. If OH(ads) is forming at these low potentials, it must be wondered why it is not oxidizing CO, as it would be expected to do, based © 1996 American Chemical Society
Water Dissociation on Pt(111) and (100) Anodes
J. Phys. Chem., Vol. 100, No. 28, 1996 11745
TABLE 1: Atomic Parameters Used in the Calculations: Principal Quantum Numbers, n, Diagonal Hamiltonian Matrix Elements, H (eV), Slater Orbital Exponents, ζ (au), and Linear Coefficients, c, for the Double-ζ d Function s
p
d
atom
n
Hs
ζ
n
Hp
ζ
n
-11.10
0.5619
ζ1
c2
ζ2
Pt O H
6 2 1
-10.50 -26.98 -12.10
2.554 2.146 1.200
6 2
-6.46 -12.12
2.254 2.127
5
-11.10
0.5619
6.013
0.6461
2.396
TABLE 2: Calculated H2O Adsorption Energies, Binding Energies (eV) at Various Sites on the Pt18 Model of the (111) Surface, and the Pt21 Model of the (100) Surface at 0, 1/2 and 1 V Potential with Results for the Pt49 and Pt61 Clusters in Parenthesesa potential 0 1/ 2 1 a
Figure 1. Two-layer Pt18 and three-layer Pt49 cluster models of Pt(111) and Pt21 and Pt61 models of Pt(100). Black dots stand for positions of third-layer atoms.
on past theoretical work.11 Consequently it may be concluded that theory favors the partial discharge of H2O hypothesis over the OH(ads) formation hypothesis on Pt(100) at these low potentials. Theoretical Method The atom superposition and electron delocalization molecular orbital (ASED-MO) theory was used in the same format as in recent related studies.13,21 An increasingly anodic surface potential, going from 0 to 1/2 to 1 V, was modeled with the band shift technique. This was done by decreasing the Pt s, p, and d diagonal Hamiltonian matrix elements by 1/2 and 1 eV. The 0 V parameters are in Table 1. We used Pt18 and Pt21 models for the (111) and (100) surfaces. These bulk superimposable clusters are assigned eight unpaired electrons so that each predominantly d band orbital is occupied by at least one electron. We checked the effects of model size by repeating the studies on three-layer thick Pt49 and Pt61 clusters for those surfaces. These clusters included nearest and next nearest neighbor atoms to the absorption sites and had 30 unpaired electrons, whereas the smaller ones included only the nearest neighbors. All cluster models are in Figure 1. All structures were optimized using an automatic energy minimization program. The band shift technique applied to the ASED-MO method is a model for potential dependence for the electrochemistry of adsorbed atoms and molecules. It allows the exploration of the effects of voltage changes on surface structures and reactions by varying the Fermi energy as a simple parameter. To systematically vary the Fermi level position using self-consistent methods would require treatment of the electrode surface atom charge and the double layer composition, charges, and structure, which has never been done and would be difficult. The band shift method used here gives results that are readily interpreted using perturbation theory to account for changes in donor and acceptor capabilities of surfaces as functions of changes in applied potential. A review of these ideas has been published22 and an extensive development is forthcoming.18 The ASED-
Pt(111) 1-fold
2-fold
Pt(100) 3-fold
1-fold
2-fold
1.31 (0.79) 1.05 (0.43) 1.17 (0.56) 1.24 (0.93) 1.35 (1.09) 1.79 (1.19) 1.62 (0.87) 1.17 (0.90) 1.73 (1.26) 1.97 (1.53) 2.37 (1.65) 2.15 (1.37) 1.67 (1.41) 2.37 (1.75) 2.56 (2.07)
4-fold b b b
The reference state for H2O is structurally optimized. b Unstable.
MO theory is based on partitioning a molecule’s electronic charge density distribution function into sums of superimposed atomic charge density functions and charge density delocalization functions. The electrostatic force on one nucleus of each atom pair is integrated as the atoms are superimposed. Each of these contributions to the Born-Oppenheimer potential energy is repulsive. The force due to the electron delocalization functions would give the electron delocalization component of the total Born-Oppenheimer potential energy if the functions were available for integration, but they are not. However, this component is simply and usefully estimated in terms of an a molecular orbital binding energy obtained from an extended Hu¨ckel form of Hamiltonian.23 The theory has been thoroughly reviewed recently.24 Results and Discussion As may be seen in Table 2, the H2O adsorption energies increase with increasing potential for all adsorption sites on all clusters except over the Pt(100) 4-fold site, which the calculations indicate does not attract H2O. The increases are caused by stronger lone-pair donation. As the Pt band is stabilized, its bottom begins to overlap the H2O lone-pair energy levels stabilizing them, and the antibonding counterparts become less occupied. Both results contribute to higher O-Pt bond strengths. Calculated H2O adsorption energies are uniformly less over the larger clusters compared to the same sites on the smaller clusters. This is because the larger clusters have wider occupied Pt valence bands, resulting in greater occupation of the O-Pt antibonding orbitals. H2O adsorption energies at 1-fold sites are within 0.14 eV for comparable cluster sizes at the 0, 1/2, and 1 V potentials. For the 2-fold sites this is no longer the case. On the (111) clusters the adsorption energies for the 1-fold sites are about 0.3 eV more stable than the 2-fold ones and on the (100) clusters the 1-fold site adsorption energies are about 0.2 eV less stable than the 2-fold ones. The 2-fold site was not tested in the extended Hu¨ckel study of ref 6 and another extended Hu¨ckel study25 claimed H2O did not bind at 2-fold sites at all on these surfaces, but this is probably not right. Our calculated most stable adsorbed H2O structures are in Table 3. In 1-fold sites the height decreases as the potential increases, reflecting enhanced σ donation. On the larger clusters these heights increase, for example by 0.10 and 0.08 Å at 0 V for the (111) and (100) surfaces. The correct H2O orientation, shown in Figure 2, is stable. In ref 25, the orientation is not
11746 J. Phys. Chem., Vol. 100, No. 28, 1996
Seong and Anderson
Figure 2. Most stable 2-fold and 1-fold H2O adsorption orientations on Pt(111) and (100) clusters.
TABLE 3: Optimized Structures of Water on Pt18 and Pt21 Cluster Models of the (111) and (100) Surfaces at 0, 1/2, and 1 V Potentials: Oxygen Height, hO (Å), OH Bond Lengths, ROH (Å), and HOH Angle (deg) (See Figure 2) hO sites
θHOH
ROH
potential
(111)
(100)
(111)
(100)
(111)
(100)
0
1.79 1.75 1.73 1.38 1.38 1.36 1.38 1.38 1.38
1.78 1.75 1.72 1.35 1.36 1.36
1.02 1.02 1.02 1.01 1.01 1.01 1.01 1.01 1.01
1.02 1.02 1.02 1.01 1.01 1.02
120 120 120 116 115 115 114 120 120
120 118 117 135 135 135
1-fold
1/ 2
1 0 1 /2 1 0 1 /2 1
2-fold 3-fold
Figure 3. Transition state structure for OH bond scission in H2O adsorbed on Pt(111) and Pt(100) cluster models.
TABLE 4: Potential (V) Dependence of Calculated Activation Energies, Ea (eV), for OH Bond Scission in H2O Bound to Pt18 and Pt21 Cluster Models of the (111) and (100) Surfaces with Results for the Larger Pt49 and Pt61 Clusters in Parenthesesa Ea potential
(111)
(100)
0 /2 1
0.44 (0.72) 0.33 (0.68) 0.17 (0.46)
0.44 (0.69) 0.49 (0.73) 0.32 (0.64)
1
a
The Pt18 (111) results are from ref 13.
divulged, but when we rotated H2O by 90°, it was unstable at this height, suggesting the stable structure was not used in ref 25. Our calculated activation energies for OH bond scission are in Table 4 and structures for the smaller clusters are in Table 5. On the (111) surface clusters the activation energies decrease as the potential increases, just as seen previously.11 On the (100) clusters at 0 V they are within 0.03 eV of the (111) results. However, instead of decreasing at 1/2 V they increase 0.05 eV for the small cluster and 0.04 eV for the large cluster. From 1/ to 1 V they then decrease 0.17 and 0.09 eV for these clusters. 2 The net result is that a potential of 1 V is needed for the (100) clusters to produce the activation energies calculated at 1/2 V for the (111) clusters. These activation energies for the (100) clusters are with respect to an initial state with H2O bound in the bridging sites as shown in Figure 2. To reach the transition state, the molecule must be reoriented so that an OH bond bridges two Pt atoms as shown in Figure 3. This is a small motion from the 1-fold site, also shown in Figure 2 but it is not a small motion from the
Figure 4. Calculated activation energies to reach OH bond scission transition states as in Figure 3 with respect to various 1-fold and 2-fold H2O adsorption states.
2-fold site. On Pt(100) the difference between 2-fold and 1-fold site adsorption energies is a reorientation energy that contributes in part to the activation energy for OH bond scission. Therefore, we calculated OH activation energies for the (100) clusters using the 1-fold sites as initial states for H2O to see if results similar to (111) clusters would be obtained. If so, the difference in initial state reorientation energy would be responsible for the relatively high activation energies on the (100) clusters. This in fact turns out to be true: as Figure 4 shows, the potential dependencies of activation energies beginning with H2O on 1-fold sites of all (100) and (111) cluster models look similar. The results for H2O on the more stable bridging (100) sites, also in the figure, have their different dependence illustrated and the deviation compared to 1-fold initial sites can be attributed to the greater binding energies in the 2-fold sites, that is, to the relative stability of the reactants, and not to variations in transition state stabilizations. The transition state structures follow trends consistent with greater donation bonding between the O lone-pair and OH σ orbitals and the surface as the potential increases. In Table 5 the decrease in oxygen and leaving hydrogen heights may be seen for the smaller clusters. For the larger clusters oxygen heights are 0.00-0.12 Å higher, the difference decreasing as the potential increases and the hydrogen heights are 0.10 Å higher to 0.04 Å lower. The extended Hu¨ckel study in ref 25 obtained activation energies for both surfaces that increased when the potential
TABLE 5: Potential (V) Dependence of Transition State Structures for OH Bond Scission in H2O Bound to Pt18 and Pt21 Cluster Models of the (111) and (100) Surfaces: Height of Oxygen, HO (Å), Displacement of Oxygen from 1-fold Site, dO (Å), Height of Leaving Hydrogen, hH (Å) (See Figure 3)a hO
dO
hH
potential
(111)
(100)
(111)
(100)
(111)
(100)
0 1/ 2 1
1.56 (1.68) 1.54 (1.60) 1.52 (1.51)
1.61 (1.65) 1.57 (1.65) 1.55 (1.55)
0.57 (0.53) 0.57 (0.50) 0.56 (0.49)
0.51 (0.61) 0.49 (0.49) 0.49 (0.49)
1.40 (1.43) 1.38 (1.35) 1.36 (1.32)
1.43 (1.53) 1.39 (1.39) 1.37 (1.37)
a The HOH angle was optimized to be 115° and the other OH distance was optimized to be 1.01 Å in each case. OH stretches are close to 0.2 Å. Results for the larger Pt49(111) and Pt61(100) clusters are in parentheses. Results on Pt49 differ in that the OHO angle is found to be 116° and the OH stretches are close to 0.3 Å.
Water Dissociation on Pt(111) and (100) Anodes increased. This is probably because the same initial and transition state structures were assumed for all calculations. Furthermore, activation barriers for OH bond scission in this study were higher for Pt(111) than for Pt(100), but if the proper bridging structures had been used, this might have changed. Conclusion Our calculations suggest that for OH bond cleavage in adsorbed water molecules a higher potential is needed over the Pt(100) surface than over the Pt(111) surface. The relative attractiveness of the (100) bridge sites for H2O is responsible because transition state structures are close to 1-fold site H2O orientations and it costs energy to promote them to these sites on Pt(100). On Pt(111) the initial H2O adsorption site is 1-fold, which for this surface is calculated to be more stable than 2-fold. This appears consistent with the onset of H2O oxidation observed at less anodic potentials over the (111) surface in perchloric acid solution.16 The oxidation of CO(ads) occurs rapidly at potentials where surface oxidation is just beginning, suggesting that the adsorption of CO may be catalyzing OH(ads) formation by displacing H2O from the surface so that the local work function increases. Sulfate16,20 and phosphate20 anions adsorb more strongly than perchlorate and push H2O oxidation of the surfaces to higher potentials. The CO oxidation peaks also shift in the anodic direction on these two surfaces, apparently because of the relationship between OH(ads) formation and CO(ads) oxidation. Acknowledgment. This work was funded by ARPA through ONR Contract No. N0014-92-J-1848. References and Notes (1) Wagner, F. T.; Moylan, T. E. Proc.-Electrochem. Soc. 1992, 9211, 25.
J. Phys. Chem., Vol. 100, No. 28, 1996 11747 (2) Kizhakevariam, N.; Stuve, E. M. Surf. Sci. 1992, 275, 223. (3) Ogasawara, H.; Yoshinobu, J.; Kawai, M. Chem. Phys. Lett. 1994, 231, 188. (4) Lee, T. R.; Whiteside, G. M. J. Am. Chem. Soc. 1991, 113, 2568. (5) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (6) Holloway, S.; Bennemann, K. H. Surf. Sci. 1980, 101, 327. (7) Anderson, A. B. Surf. Sci. 1981, 105, 159. (8) Gilman, S. J. Phys. Chem. 1964, 68, 70. (9) Bockris, J. O’M.; Wroblowa, H. J. Electroanal. Chem. 1964, 7, 428. (10) Gasteiger, H. A.; Marcovic, N.; Ross, Jr., P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617; 1993, 97, 12020. (11) Anderson, A. B.; Grantscharova, E. J. Phys. Chem. 1995, 99, 9143; 1995, 99, 9149. (12) Haner, A. N.; Ross, P. N. J. Phys. Chem. 1991, 95, 3740. (13) Anderson, A. B.; Grantscharova, E.; Shiller, P. J. Electrochem. Soc. 1995, 142, 1880. (14) Clavilier, J.; Rodes, A.; Achi, K. El; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291. (15) Lin, S. A. Ph.D. Thesis, Case Western Reserve University, 1991. (16) Morimoto, Y. Ph.D. Thesis, Case Western Reserve University, 1994. (17) Kizhakevariam, N.; Xudong, J.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (18) For a full discussion, see: Anderson, A. B. Quantum Chemical Modeling of Electrocatalytic Reactions, Including Potential Dependence. Beginning Stages, manuscript in preparation. (19) Lamy, C.; Leger, J. M.; Clavilier, J.; Parsons, R. J. Electroanal. Chem. 1983, 150, 71. (20) Herrero, E.; Franaszcuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074. (21) Shiller, P.; Anderson, A. B. J. Electroanal. Chem. 1992, 339, 201. (22) Anderson, A. B. J. Electroanal. Chem. 1990, 280, 37. (23) Anderson, A. B.; Hoffmann, R. J. Chem. Phys. 1974, 60, 4271. (24) Anderson, A. B. Int. J. Quantum Chem. 1994, 49, 581. (25) Estiu, G.; Maluendes, S. A.; Castro, E. A.; Arvia, A. J. J. Phys. Chem. 1988, 92, 2512.
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