Electrochemical Hydrogen Oxidation on Pt(110): A Combined Direct

Mar 1, 2010 - Catalytic Role of Minority Species and Minority Sites for Electrochemical Hydrogen Evolution on Metals: Surface Charging, Coverage, and ...
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J. Phys. Chem. C 2010, 114, 4995–5002

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Electrochemical Hydrogen Oxidation on Pt(110): A Combined Direct Molecular Dynamics/ Density Functional Theory Study Juan A. Santana, Juan J. Mateo, and Yasuyuki Ishikawa* Department of Chemistry, UniVersity of Puerto Rico, P.O. Box 23346, San Juan, Puerto Rico 00931-3346 ReceiVed: October 13, 2009; ReVised Manuscript ReceiVed: February 15, 2010

A first-principles direct molecular dynamics and density functional theory study of the electrooxidation of molecular hydrogen at the Pt(110)/water interface was conducted to gain an atomic-level understanding of the electrocatalytic processes and of adsorbed reaction intermediates. The H2 electrooxidation follows the Tafel-Volmer mechanismsa homolytic H-H bond cleavage and formation of adsorbed terminal hydrogen atoms atop the topmost platinum, followed by oxidation of the adsorbed reaction intermediates. Potentialdependent activation energies computed for the hydrogen redox reactions were employed to predict the Tafel plot for hydrogen electrooxidation. The theoretically predicted Tafel plot is in very good agreement with experiment in support of the Tafel-Volmer mechanism for the Pt(110) surface. 1. Introduction Understanding the mechanistic details of the electrochemical hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on various low-index crystal faces of Pt has been the focus of numerous experimental1-36 and theoretical investigations37-47 because of its fundamental importance in electrocatalysis. Although molecule-metal surface interactions at metal/electrolyte interfaces have been a subject of considerable interest for years, even the most elementary moleculesurface processes, such as the electrochemical HOR and HER, have not been fully understood. Moreover, the exact nature of the adsorbed species formed from the electrooxidation of hydrogen fuel has been the subject of extensive controversy for many decades. The recent renewed efforts31-35 to characterize the adsorbed reaction intermediates using surface spectroscopic techniques stem from hydrogen’s potential as a fuel for clean energy production. Characterizing the structure and properties of adsorbed reaction intermediates of the HOR is crucial in determining the underlying mechanism and reaction rates on Pt low-index singlecrystal surfaces. The inability to identify accurately the structure of the adsorbates led to the proposal of numerous mechanistic pathways for the electrooxidation reactions. With considerable advances in spectroscopic techniques16,24,32 as well as in firstprinciples simulation methodology,37-56 attempts have been made in recent years to characterize the elementary reaction steps and the adsorbed reaction intermediates involved. Surface structural and surface spectroscopic techniques6,16,24,30,32 have provided the most detailed look at the HOR/HER reactions on Pt polycrystalline and low-index single-crystal surfaces on the atomic scale. Of particular interest has been the quest to correlate the reaction kinetics with the adsorption of reaction intermediates.18,22,34,35 In an experimental study using in situ IR spectroscopy, Nichols and Bewick identified a Pt-H stretching vibration on a single-crystal Pt(111) electrode at potentials less than 110 mV.6 Ren et al. made similar observations using Raman spectroscopy.24 Kunimatsu et al.35 employed surface-enhanced IR absorption spectroscopy (SEIRAS) and electrochemical * To whom correspondence should be addressed. E-mail: yishikawa@ uprrp.edu.

kinetic analysis on polycrystalline Pt to establish that the H atom adsorbed at atop sites, Hads(top), is the reaction intermediate in HER, and the recombination of two Hads(top) atoms is the ratedetermining step. In a key mechanistic finding, our recent combined direct molecular dynamics (MD)/density-functional theory (DFT) study42,43 revealed that the HOR on Pt(111) follows the Heyrovsky-Volmer mechanism. The study also identified the adsorbed underpotential deposition (UPD) states of hydrogen, the reaction intermediate of the HOR on Pt(111), as well as the bridging hydrogen Hads(bridge) that interacts very little with ambient water molecules. Normal mode analysis clearly showed an absence of involvement of ambient water molecules in the Pt-Hads(bridge) stretch perpendicular to the surface.42 The Pt-H stretching frequencies of 1090 and 2054 cm-1 were predicted for an inert Hads(bridge) and an adjacent Hads(top), respectively, at 0.0 V versus RHE, in good agreement with the IR bands at ca. 1050 and 2090 cm-1.42 In a landmark study18 of the hydrogen electrochemistry on Pt low-index single-crystal surfaces, Markovic et al. detected, for the first time, a structural sensitivity of the kinetics of the HOR/HER to the crystal surface morphology in 0.05 M H2SO4 solution with activation energies that vary with the crystal surface. On the basis of detailed analysis of the kinetic parameters, they suggested the most probable reaction mechanisms for the electrochemical HER/HOR on the different Pt(hkl) surfaces. In particular, they concluded that, on Pt(110), the reaction follows the Tafel-Volmer mechanism, although the physical state of reaction intermediates was unclear. There is an inadequate understanding of what sites on the Pt(110) surface are critical to the hydrogen oxidation processes and how the surface controls binding energy and activation energy for the adsorbed intermediates at various steps of the electrooxidation. Whereas the experimental study18 identified the HOR on Pt(110) as the Tafel-Volmer process on the basis of the Tafel plots, the reaction intermediates of the electrochemical HOR on the surface have not yet been clearly identified,18 partly because the sensitivity of the reaction to the morphology of the Pt surface is unclear. In the present study, the mechanistic details of the electrochemical HOR on Pt(110) and the structure and properties of key reaction intermediates involved are examined via an atomic-level chemical dynamics simulation. A direct MD study

10.1021/jp909834q  2010 American Chemical Society Published on Web 03/01/2010

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Figure 1. Pt38(110) cluster model structure used in the calculations.

could reveal important details about reactive processes and the evolution of intermediates and products. We report here on a set of first-principles calculations intended to examine the energetics of H2 electrooxidation processes occurring on a Pt(110) nanocluster surface. This study is intended to provide insight into the mechanistic details of hydrogen oxidation at the Pt(110)/water interface. In particular, we address the catalytic activities of the HOR/HER at different electrode potentials by theoretical examinations of the energetics of H-H bond breaking associated with hydrogen electrooxidation and of the underpotential deposition state of Hads on the Pt(110) surface. The effects of electrode potential on HOR/HER reaction barriers are studied on the model cluster. The aim of this study is to conduct simulations of molecular activation and of dissociative chemisorptions and reaction pathways for molecular hydrogen on the Pt(110) surface at the Pt electrode/ electrolyte interface by means of first-principles direct MD based on the density functional theory. The study will provide vital insights into electronic processes, HOR pathways, and key adsorbed intermediates that control reaction kinetics that are not readily accessible to experimental studies. 2. Computational Method and Metal/Water Interface Modeling The Pt38(110) model electrocatalyst composed of two 19atom layers was employed throughout this study. Its geometric structure is displayed in Figure 1. The bulk Pt-Pt bond distance from experimental data (cell parameter a ) 3.92 Å) was initially adopted for the model clusters. In an earlier study, the binding energies of hydrogen in different adsorption sites on a twolayer Pt(111) cluster were presented.42,43 Our results obtained with the two-layer cluster model agree well with experiment and previous DFT slab calculations for the gas-solid interface.57,58 The effect of the slab/cluster thickness on the binding energies of hydrogen on Pt(110) is also expected to be small, well within 1 kcal mol-1. The Pt38(110) cluster model represents the lowindex face of metal particles employed to elucidate the HOR mechanism and the adsorbed intermediates on the (110) surface. The structure does not deform noticeably upon hydrogen adsorption, except for a slight displacement of topmost Pt atoms to which terminal hydrogens are adsorbed. The electrochemical HOR at the Pt(110)/water interface is a highly exothermic reaction with little dependence on the particle size because there is little change in binding energy of the terminal hydrogen when the cluster size is enlarged. Up to 22 water molecules were allowed to rain down on the Pt(110) cluster surface to create a solvated environment.42,43,49,54

Santana et al. This number leads to the density of water of ca. 0.97 g/cm3 in the solvent phase. All solvent water molecules were allowed to move freely during the structure optimizations. As a caveat, we note that the interface model does not explicitly include any electrolyte ions. The approximation is introduced because there is no coupling between hydrogen adsorption and anion adsorption in the potential region of 0.0-0.2 V versus RHE where the HOR is simulated.59 To control the effective electrode potential, the Pt cluster was charged (q represents the charge in the system) by adding (or removing) a given number of electrons to (or from) the system, in a manner similar to that discussed by other investigators, including Wang and Balbuena,52 Hartnig and Spohr,53 and Filhol and Neurock.54 As in previous studies,42,43,54,55 the electrode potential Uq was estimated from the ionization energy Øq relative to that of the standard hydrogen electrode ØSHE

Uq ) Øq - ØSHE

(1)

The geometries of the cluster-adsorbate structures at the Pt(110)/water interface were optimized in DFT calculations using DMol3 (Accelrys Inc.).60,61 The DNP basis sets60,61satomcentered numerical basis sets of double-ζ + polarization qualityswere used in the optimizations. Perdew-BurkeErnzerhof (PBE)62 exchange and correlation functionals were employed. An all-electron scalar relativistic algorithm was employed in conjunction with the density functionals in DMol3 to obtain the correct energetics.42,60,61 The DFT calculation employing the PBE functional and scalar relativistic algorithm has proven to provide accurate binding energies in a number of studies. In our earlier work on the electrochemical hydrogen oxidation and underpotential deposition of H on Pt(111),43 the theoretical binding energy of 55.6 kcal/mol for Hupd that we computed near the reversible potential is in good agreement with the experimental value (ca. 57.4 kcal/mol) obtained by Markovic et al.18 in alkali and acid solutions. The scalar relativistic DFT also provides an accurate binding energy for the PtH diatomic molecule. The calculated binding energy of 86.1 kcal/mol agrees with the experimental value of 83 ( 9 kcal/mol to well within the experimental error bar.63 In a large metal cluster, a number of low-lying unoccupied orbitals exist very close energetically to the ground state (∼0.1 eV). In our calculations, the fractional occupation number technique60 was employed, where electrons are “smeared” by an energy width of 0.1 eV over the orbitals around the Fermi energy. The resulting total energy may be viewed as an average over configurations lying energetically close to the ground state of the cluster. The HOR is a dynamical process, and the elucidation of its mechanism mandates direct MD.42-44,52,53,56 Dynamics studies have been conducted with the technique of direct ab initio MD developed and implemented in our group.42,43 Direct MD is a quantum-classical simulation in which quantum chemical electronic structure calculations are done at each time step to evaluate the energy gradient in the classical evolution of the positions of the atomic nuclei. Nonadiabatic quantum effects in the proton-coupled electron-transfer dynamics in the HOR are non-negligible.64 In the present study, however, all the nuclei involved are treated classically. The potential surface is generated “on the fly”, rather than fit to an analytic form beforehand. The artistry necessary to construct an accurate surface is avoided, but generating the surface on the fly restricts reactive system dynamics from being examined in as much detail as they can be on a fitted surface. For systems more complex than

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Figure 2. Tafel step in H2 oxidation: a sequence of atomic configurations in a sample MD trajectory at ca. +0.2 V vs RHE.

tetratomic, however, the direct MD is the more practical approach, and it has opened the door to accurate simulation of the dynamics of complex systems. The solution of the classical equations of motion in Newtonian form • d (miri) ) -∇riE, i ) 1,...,3N dt

Figure 3. Tafel-Volmer process in H2 oxidation at a submonolayer coverage of terminal hydrogen atoms. H2 is yellow and the terminal hydrogen atoms are white.

(2)

determines the nuclear trajectories. Here, N is the number of nuclei in the system and mi the mass of the ith nucleus. The Verlet explicit central difference method is employed to integrate the equations of motion. Energies and gradients were evaluated in the DFT calculations using DMol3.60,61 The Verlet algorithm with time steps of 0.5 fs was employed to integrate the classical equations of motion. H2 and solvent water molecules began each trajectory with vibrational, but no rotational, energy at any given electrode potential. No zero-point vibrational energy was given for the Pt38 cluster. The solvent water molecules, platinum, and hydrogen atoms were allowed to move freely during the direct MD simulation. The presence of the relatively large number of water molecules allowed the H+ oxidation product to fluctuate between a variety of structures, such as H5O2+ (Zundel ion) and H9O4+.65 Initial H2-metal surface separations of 2.5-3.5 Å were chosen. A total of eight trajectories were started, and all were dissociative. 3. Results and Discussion 3.1. Direct Dynamics Simulation of the Hydrogen Oxidation Reaction. Atomic-level chemical dynamics simulations of the reaction of H2 at the Pt(110)/water interface were conducted to examine the mechanism of the HOR and the adsorbed UPD states of hydrogen. Figure 2 illustrates a sequence of configurations in the H2-Pt(110) surface reaction at a positive electrode potential of ca. +0.2 V versus RHE on a clean Pt(110) surface. Figure 3 illustrates a sequence of configurations in the H2-Pt(110) surface reaction at a positive electrode potential at a higher submonolayer coverage of adsorbed hydrogen. The

trajectories exemplify all of the reactive surface processes in which the solvated H2 molecule makes side-on and end-on approaches to the Pt(110) surface at low and high coverage. The reaction mechanism was straightforward. At the beginning of the trajectory, a H2 molecule was placed along the bottom layer of solvating water molecules and was drawn to the metal surface by H2-Pt attraction (0-10 fs; Figures 2 and 3). Common to all trajectories was the approach of the hydrogen molecule to the nearest topmost Pt on the corrugated (110) surface and a subsequent rapid homolytic bond breaking, that is, a nonoxidative adsorption, of the surface-bound hydrogen via the Tafel pathway to form two terminal hydrogen Hads on the femtosecond time scale (0-20 fs; Figures 2 and 3). The homolytic bond breaking is highly exothermic. The exothermic energy is imparted to the kinetic energy of one of the terminal hydrogen atoms that undergoes a rapid site-to-site shift (30-50 fs) and to the excitations in the wagging vibrational mode of the second terminal hydrogen atom adsorbed to the topmost platinum (20-60 fs). Although the second terminal hydrogen atom is vibrationally excited, it remains in the same site, hydrogen bonded to a nearest ambient water molecule to be oxidized. By 70-80 fs, the hydrogen-bonded terminal hydrogen is oxidized via the Volmer step (Figure 3). None of the ambient water molecules are in close contact with the platinum surface because the surface is negatively polarized in the 0.0-0.2 V range, with the H ends of the water molecules pointing toward the metal surface. Oxidation of the terminal Hads takes place when one of the ambient water molecules in the contact layer is hydrogen bonded to the terminal hydrogen (Figure 3). The nonadiabatic effects must be included to accurately account for the rate of the electrochemical HOR or the

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Figure 4. (a) Diagram of relative energies of the reactant and product asymptotes at the reversible potential. The arrows indicate the distance, 1.93 Å, between the terminal Hads and the oxygen of the nearest water molecule and the 37° angle of the tilted Hads. (b) HOR activation energy curve as a function of H2O-Hads distance. The inset shows the geometry of the transition state, with the H2O-Hads distance of 1.3 Å. (c) Mulliken charge on Pt38 as a function of H2O-Hads distance near the reversible potential.

electrochemical proton-coupled electron transfer at metal-solution interfaces.64 The Volmer oxidation process is expected to be faster when the nonadiabatic effects on the rate of the electrochemical HOR are taken into account in the direct MD simulation. Because the simulation of the oxidation reaction is carried out in an open circuit in which the negative charges from the oxidation reactions remain on the electrode, the cluster electrode potential shifts to a more negative value, by as much as 0.035 V (see Figure 4), as the adsorbed hydrogen is oxidized to form a hydronium ion. The hydrogen adsorption/oxidation processes are highly sensitive to the single-crystal surface structure: The homolytic H-H bond cleavage, followed by the oxidation of the terminal Hads through the Tafel-Volmer process, is favored by the corrugated surface morphology of Pt(110). The HOR on Pt(110) is in stark contrast with one that takes place at the Pt(111)/ water interface: in the latter, the heterolytic H-H bond breaking and rapid formation of a hydronium ion occur on the fly via

the Heyrovsky-Volmer process and within 20-30 fs.42,43 The homolytic H-H fission reaction and the subsequent oxidation of terminal Hads to form a hydrated proton, as revealed in the MD trajectories, is consistent with the experimental study of Markovic et al.;18 these authors concluded that, for Pt(110), the homolytic (Tafel-Volmer) mechanism could be uniquely established. With the Pt-Hads axis tilted away from the surface normal by about a 37° angle (Figure 4a), every terminal hydrogen atom adsorbs only to the topmost platinum atoms on the corrugated metal surface. Thus, the terminal Hads tends to interact less strongly with the ambient water molecules than does the ontop Hads on Pt(111) through a hydrogen bond. The weaker interaction is revealed in the terminal Hads-water oxygen distance of 1.94 Å, which is noticeably greater than the distance between the ontop Hads and water oxygen (∼1.70 Å) on Pt(111). The Mulliken charge of +0.210 on the terminal hydrogen on Pt(110) is much less than that (i.e., +0.304) for the ontop Hads on

Electrochemical Hydrogen Oxidation on Pt(110) Pt(111), indicating a weaker hydrogen bonding and the reason that the Tafel stepsa homolytic H-H bond cleavage and formation of the two terminal Hadssprecedes the oxidation of the terminal Hads. The weaker interaction between the terminal Hads and the ambient water molecules is also manifested at the reversible potential: The calculated activation energy of 2.3 kcal/ mol for the terminal Hads oxidation is slightly higher than that of ontop Hads (∼1.5 kcal/mol) on Pt(111) at 0.0 V versus RHE.42,43 Near the reversible potential, at q ) -2, the reaction pathway for the terminal Hads oxidation via the Volmer process is mapped. Figure 4b illustrates the potential energy along the HOR reaction pathway. The transition state in HOR is approached, with 2.5 kcal/mol of activation energy, when the Pt-Hads distance elongates to 1.7 Å and the distance between the terminal Hads and an ambient water molecule hydrogen bonded to it contracts to 1.3 Å. The Mulliken charges on the metal clustersplotted along the reaction pathwaysindicate that there is a gradual polarization as the transition state is approached, until the distance between the terminal Hads and the nearest solvent H2O shortens to ca. 1.4 Å. There is then a sudden polarization at the transition state (Figure 4c). 3.2. Reversible Electrode Potential and the UPD States of Hydrogen. ReWersible Electrode Potential. Figure 4a displays a diagram of relative energies of reactant and product asymptotes on the H2/Pt38/water potential energy surface at a surface potential near q ) -2. The H2 dissociation at this surface potential is exothermic by as much as 29.5 kcal/mol and proceeds through a barrierless H-H bond breaking to form two terminal hydrogen atoms. At this surface potential, neither the oxidation of the terminal Hads nor the reduction of H+(aq) occurs spontaneously because there is an activation barrier between the two states (Figure 4a). The energy diagram indicates that the potential near q ) -2 comprises the reversible potential (0.0 V vs RHE) at low terminal hydrogen coverage because the two states are isothermal, with a matching activation energy (2.3 kcal/mol). Calculated activation energies for the HOR/HER at low and high terminal hydrogen coverages are shown, respectively, in Figure 5a,b as functions of the excess charge q added to or subtracted from the system. At low coverage, the activation energy for HER approaches zero between q ) -3 and -4, and the H+(aq) is spontaneously reduced via the Volmer process to form H adsorbed at terminal sites of the topmost Pt. The reversible potential, the potential at which the activation energies for the forward and reverse reactions are equal (the crossing point in Figure 5a), is achieved at ca. q ) -2. The calculated activation energy at the reversible potential is 2.3 kcal/mol, in excellent agreement with the experimental value (2.3 kcal/mol) obtained by Markovic et al.18 The results indicate that the terminal Hads atoms are stable up to ca. +0.1 V versus RHE and possess electronic properties characteristic of the HUPD on Pt(110).18 At higher terminal hydrogen coverage (Figure 5b), the reversible potential is achieved at ca. q ) -1.5. Whereas the activation energy for HER at low coverage vanishes near q ) -3, there is no sign of vanishing activation energy at ca. 1 ML coverage due to the repulsive interaction imparted by the adsorbed Hads atoms. Here, the calculated activation energy of 3.0 kcal/mol at the reversible potential is slightly higher. UPD State of H. The UPD states of the terminal hydrogen revealed in the HOR simulation prompted us to explore the potential energy surface of the H2-Pt(111)/water system near the reversible potential and examine the vibrational properties of the HUPD. Figure 6 illustrates the adsorbed UPD hydrogen

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Figure 5. (a) Activation energy curves for the Volmer step of the HOR and HER at low terminal Hads coverage as functions of electrode potential. (b) Activation energy curves for the Volmer step of the HOR and HER at nearly 1 ML of terminal Hads coverage as functions of electrode potential.

Figure 6. (a) Symmetric and (b) asymmetric stretching modes.

configuration at 6/19 ML of terminal hydrogen coverage obtained by structure optimization near the reversible electrode potential (-2 e q < -1). All structure optimizations that began with solvated H2 molecules in a variety of orientations to the Pt(110) surface invariably resulted in the dissociation into structurally equivalent terminal Hads atoms, with no barrier in the potential energy surface connecting the solvated H2 with its surface dissociation. For a gas-phase adsorption of H at Pt(110), chemisorbed H also occupies terminal sites at the topmost Pt atoms. The adsorbed UPD states of hydrogen on Pt(110) are in stark contrast with those on Pt(111). In the latter, strongly adsorbed UPD hydrogen atoms, each lying nearly flat in a bridging position, interact very little with ambient water molecules.42,43 Therefore, the activation energy of ∼5 kcal/mol for oxidation of bridging hydrogen does not decrease with an increasing electrode potential up to ca. 0.3 V versus RHE.66 In contrast, due to the interaction of terminal hydrogen with ambient water

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i ) A(U)exp[-Ea(U)/RT]

Figure 7. HUPD at a full coverage approaching one H per surface Pt (i.e., 1.0 ML) near the reversible potential. In panel b, all the water molecules are omitted for clarity.

(3)

where the preexponential factor A is proportional to a number of variables, such as the concentration of adsorbed hydrogens/ protons undergoing oxidation/reduction, the frequency factor, and the Faraday constant. T is the temperature of the system. Ea(U) is the activation energy at a potential U for the oxidation of a terminal Hads or reduction of a proton to form the Hads. In the low overpotential region, where oxidation and reduction reactions would contribute nearly equally, the total current density i may be given by the expression39

i ) |ioxi - ired | ) A(U)|exp[Eox a (U)/RT] via a hydrogen bond, the adsorbed UPD states of hydrogen on Pt(110) are unstable above +0.1 V versus RHE: The UPD hydrogens on Pt(110) are oxidized readily as the electrode potential exceeds +0.1 V. The results are in qualitative agreement with the voltammetric (current density) peak associated with the hydrogen desorption that appears in the 0.1-0.2 V range in the cyclic voltammogram of Pt(110) in 0.05 M H2SO4.18 Figure 6 also illustrates the collective stretching modes (symmetric and asymmetric) for two vibrating terminal Hads adsorbed to a topmost Pt computed at ca. the reversible potential. The IR intensity of the symmetric vibrational mode is about 1/2 that of the asymmetric vibrational mode. The calculated symmetric and asymmetric stretching frequencies, 2111 and 2083 cm-1, respectively, were experimentally observed as a broad band around 2080 cm-1 by Ogasawara and Ito.14 These authors14 reported a spectroscopic identification of electrochemically deposited hydrogen on Pt(110) at ca. 0.0 V and employed in situ IR to assign the broad band around 2080 cm-1 to a terminal hydrogen bound to a topmost Pt atom. The structures of UPD H at higher submonolayer coverages were also investigated near the reversible potential. Figure 7 displays the network of HUPD at 1 ML full coverage. The two structurally distinct adsorbed hydrogen intermediates on Pt(111)sthe bridging Hads identified as the UPD hydrogen and ontop Hads identified as the overpotentially deposited (OPD) hydrogen42sform a hexagonal 2D honeycomb network on the (111) surface.43 In contrast, all the adsorbed hydrogen atoms on Pt(110) occupy the structurally and energetically equivalent topmost Pt sites, forming the uniquely adsorbed H intermediates that act as both HUPD and HOPD. 3.3. Tafel Plots. Anderson and Cai39 employed quantum mechanically determined electrode-potential-dependent activation energies for H2 oxidation on Pt(100) to successfully predict the Tafel plot similar in shape to the experimental plot,18 providing strong theoretical support for the Heyrovsky-Volmer mechanism of H2 oxidation on the (100) surface. In the present study, we also employ the quantum mechanically determined potential-dependent activation energies for the HER/HOR on Pt(110) (Figure 5b) to predict the Tafel plot and compare it with experiment18 to provide further support for the Tafel-Volmer mechanism. For the HOR, the direct MD simulation clearly indicates the Tafel process as its first stepsthe rapid homolytic H-H cleavage to make two terminal hydrogen atoms Hads adsorbed at topmost Pt atoms. For the second step, the terminal Hads atoms are oxidized to form the protons in the rate-determining Volmer process. The kinetic current density i for the terminal-hydrogen oxidation at a given electrode potential U may be given by an Arrhenius equation

exp[Ered a (U)/RT]|

(4)

where iox and ired are, respectively, the oxidation and reduction kinetic current densities. The computed potential-dependent activation energies for the Volmer step in the HOR and HER in Figure 5b have been employed to predict the Tafel plot in the spirit of Anderson and Cai.39 Substitution of these activation energies into eq 4 gives rise to a smooth curve in the electrode potential versus log(current density i) plot for oxidation. When the constant value, 215, is chosen for the preexponential factor, the smooth curve fits the measured Tafel plot at low current densities (0.1 V) than the reversible potential. The reaction products are two structurally and energetically equivalent Hads terminally adsorbed to topmost Pt atomssthe uniquely adsorbed H intermediates identified as both HUPD and HOPD. The highly exothermic dissociation of molecular hydrogen imparts rapid site-to-site shifts of one of the terminal Hads along the metal surface, while the other Hads atom remains in the terminal site. The latter, vibrationally excited, is hydrogen bonded to an ambient water molecule and subsequently oxidized via the Volmer step. The oxidation of the terminal Hads atoms does not occur until the electrode potential exceeds +0.1 V versus RHE. With the ∼37° tilt to the surface normal, the hydrogen-bonding interaction between the terminal Hads and the ambient water molecules at the Pt(110)/water interface is slightly weaker than that of ontop Hads at the Pt(111)/water interface. Continued H2 bond cleavage via the Tafel step at the potentials below +0.1 V versus RHE leads to a higher submonolayer coverage, up to 1 ML, of the Pt(110) surface by the adsorbed terminal H atoms. Acknowledgment. The authors gratefully acknowledge financial support from the NASA-UPR Center for Advanced Nanoscale

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