metal interface

Current Opinion in Solid State and Materials Science 9 (2005) 49–65

Theoretical insights into the structure and reactivity of the aqueous/metal interface Christopher D. Taylor b

a,1

, Matthew Neurock

b,*

a Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22904-4745, United States Departments of Chemical Engineering and Chemistry, University of Virginia, Charlottesville, VA 22904-4745, United States

Abstract A review describing recent developments in the theoretical description of molecular and atomic interactions occurring at the interface between an aqueous solution and a metal surface. The effect of solvent upon reaction kinetics and thermodynamics is the result of steric, as well as electrostatic participation. We describe theoretical developments in the description of isolated, film, cluster and condensed phase models of water molecules on metal surfaces, electron and ion transfer between adsorbed and solution particles and the metal surface, chemical transformations of water on metal surfaces, and theoretical investigations of catalytic and electrocatalytic processes at the metal/water interface.  2006 Elsevier Ltd. All rights reserved. Keywords: Electrochemistry; Electron transfer; Ion transfer; Molecular dynamics simulations; Electrocatalysis; Density functional theory; Metal/water interactions; Interfaces; Methanol fuel cell; Oxygen reduction; Ab initio quantum mechanics

1. Introduction The interface that forms between an aqueous solution and a metal surface creates a unique reaction environment that can markedly influence the reactivity of molecules within the interface. A collective understanding of the synergy between the effects of solution, applied potential and electronic structure of the metal would be invaluable for understanding elementary processes that govern the electrochemical and electrostatic phenomena. Recent progress has been made in the elucidation of some of these effects, such as the dynamics of interfacial water [1–5], the electrochemical potential of the interface [6–13], the effects of solvation/desolvation along the reaction coordinate [2,5,14–16], and the stabilization of intermediates having unusual oxidation states and partial charges by a combination of solvation and chemisorption to the metal [5,17–21]. *

Corresponding author. Tel.: +1 434 924 6248; fax: +1 434 982 2658. E-mail addresses: [email protected] (C.D. Taylor), mn4n@virginia. edu (M. Neurock). 1 Tel.: +1 434 924 4038; fax: +1 434 982 2658. 1359-0286/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2006.03.007

The reactivity of the metal is, to some extent, controlled by the availability of delocalized and partially localized electrons at the Fermi level. For transition metals, the reactivity can often be rationalized in terms of the center and the width of the d-band for metal atoms at the surface [22,23]. In an aqueous environment, electrons readily reorganize with the dynamic fluctuation of the dipolar water molecules. The electrons can partake in covalent interactions with solvated moieties including water and can also transfer to ions separated by one or two solvation shells from the metal surface. The solution phase can significantly alter the surface reactivity by changing the electronic structure of the metal, solvating charged reactive states, or participating directly in chemical reactions by providing proton or ion shuttling path. The three-dimensional structure of the hydrogen-bonding network that forms in protic solution is interrupted at the metal surface, thus resulting in the adoption of particular orientations by the interfacial water molecules so as to maintain an optimal set of interactions with the remaining solution layer. This interaction is of similar strength to the binding of water molecules to the metal, and, depending on its strength may lead to the

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dissociation of water and the oxidation of the metal substrate. The complex interplay of metal–water interactions, water–water interactions and electrochemical properties at the interface obscures an understanding of the fundamental features that control its reactivity. This has spurred significant theoretical interest over the past three decades. The metal/aqueous interface also offers a unique reaction environment for molecules other than H2O, by providing a reaction center and environment that combines features that are important for both homogeneous and heterogeneous catalyzed reactions. In homogeneous catalytic reaction systems, a metal–ion surrounded by an appropriate ligand sphere facilitates a concerted reaction mechanism through the adoption of different oxidation states while providing a templated coordination environment that brings the participating species into close contact. The ligands bound to the metal center can influence the electronic structure, provide steric constraints, assist in bonding as well as participate in the actual chemistry. In heterogeneous catalytic systems, the metal surface provides sites for the adsorption of reactant molecules and the activation of specific chemical bonds within the adsorbate through dissociation or rearrangement reactions in addition with other molecules. The ‘‘ligand field’’ is significantly limited and typically comprised of only neighboring metal surface atoms or next-nearest neighbor coadsorbates. While the neighboring metal atoms provide additional adsorption sites and can aid in coordinating and directing molecular transformations of intermediates into products, they are typically more difficult to control and act as poor ‘‘ligands’’. The chemistry occurs at the surface and so the reaction environment is two-dimensional unlike the 3D constructs that can be formed in homogeneous systems. It is typically easier to control the activity and selectivity of homogeneous catalysts since their structures are welldefined and can be tuned by changing the ligand sphere. The drawback, however, is that they are difficult to separate and may be less robust than supported catalysts. The presence of solution and applied electric fields, however, can begin to provide some of the flexibility offered by homogenous catalysts by altering the geometric as well as the electronic structure of the local reaction environment. While supported catalysts have been used to carry out solution phase chemical processes for some time, there is little fundamental understanding of the controlling reaction mechanisms that occur at the solution/metal interface due to the complexity of the structure and properties of the interface. While theoretical methods can help complement experimental efforts, the size necessary to capture the local reaction environment has historically been prohibitive. Advances in both computational resources as well as the development of improved theoretical methods over the past decade, however, have enabled more advanced ab initio treatments of more realistic surface models, and have thus begun to provide insights into the surface chemistry at the aqueous/metal interface. The ability to directly simulate the local reaction center and the surrounding solvent

molecules has been found to be critical since the solvent molecules can, in many cases, directly or indirectly participate in the reactions occurring on the substrate surface. In this review, we consider the status of contemporary molecular reaction models for simulating the aqueous/ metal interface, drawing particularly from some of our own recent publications. We consider attempts to characterize the structure and dominant interactions between water itself and the metal substrate, under both equilibrium and non-equilibrium conditions; the existing models for both ion and electron transfer between the metal and solution phases; the chemical reactivity of water (as a solvent) on a metal surface; and the influences of solvent and electrochemical potential on the performance of other chemical reactions on the surface. We shall show, in particular, that the chemistry at the metal/aqueous interface is determined by the steric and electrostatic effects of the solvent, as well as the reactivity of water on the metal, and the modification of chemical reactivity according to the electrochemical potential. We describe the application of ab initio quantum mechanical methods to gain insights into experimentally inaccessible conditions and reactions occurring at the interface between the metallic and solution phases. 2. Thermodynamic and statistical nature of H2O in contact with a metal substrate Ab initio calculations as well as molecular simulations have been used to provide a fundamental understanding of the geometric as well as the electronic structure and reactivity of water at the metal/solution interface. We present some of the salient features associated with the interface by first examining the adsorption and reactivity of isolated water molecules, molecular water clusters, wetting water layers and condensed aqueous phase ensembles on metal surfaces as well as metallic clusters. The smallest and simplest unit of the metal/aqueous interface involves a single metal atom in contact with a single molecule of water. The electronic structure of a single metal atom in isolation, however, is comprised of discrete electronic states with an energy gap between the highest occupied and lowest unoccupied states. This is significantly different from that of a metal atom at the surface of a bulk metal particle or slab. The electronic character of molecular water in isolation, however, is not very different from that of water contained in a condensed phase. For this reason, much of our initial understanding of the adsorption and reactivity of water on a metal surface has been derived from ab initio calculations for a single water molecule adsorbed to the surface of either a metal cluster [24–26], or a periodic metal slab [27–29]. Michaelides et al., for example, provided a fairly comprehensive study on the adsorption of molecular water on various close-packed (1 1 1) transition and noble metal surfaces [27]. They found that water was typically oriented nearly flat with respect to the metal surface, with the OH bonds only very slightly tilted up out of the plane parallel to the metal surface.

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The 3a1 and 1b1 orbitals are the frontier orbitals of water that interact with the unfilled dz2 states on the metal surface by donating electrons into unfilled or partially filled dz2 states. If, on the other hand, the dz2 state is occupied, a four-electron bond to the surface can form, whereby electrons that would otherwise occupy anti-bonding states are emptied into the Fermi level. Michaelides et al. determined that the optimized metal–oxygen bond lengths for ˚ adsorbed water on different metals varied between 2.25 A ˚ for Cu on up to 3.02 A for Au. The calculated adsorption energies for water, DEads, over the different transition and noble metals considered were all in the range of weak interactions, with reported binding energies between 0.13 eV for Au and 0.42 eV for Rh. The balance between the water– water hydrogen-bond strengths (EHB, 0.2–0.4 eV) and the water–metal interaction energies dictates whether or not water can form ordered structures over these hexagonal metal surfaces. Meng, Wang and Gao developed this comparison to establish a hydrophilicity (or wettability) parameter to describe what water will do on the surface [30]: w ¼ EHB =DEads

ð1Þ

For values of w 6 1, the water–metal interactions tend to dominate. This suggests that the water molecules wet the metal surface. Values of w  1, on the other hand, indicate that water–water interactions dominate and thus lead to cluster formation, rather than surface wetting. The calculated ratio w on Au(1 1 1) is close to 3, whereas on Pt(1 1 1) the ratio is approximately 1. As such, wetting structures for water on Pt(1 1 1) have been observed using low-energy electron diffraction (LEED) and conform to ordered hexagonal overlayers [31,32]. The presence of an electric field or an applied potential can significantly alter the nature of the water–metal bonding and subsequently influence the strength of the water– water interactions. In the presence of an applied electric field, water molecules tend to rotate away from this

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almost-flat orientation with respect to the surface. The response of water molecules to such a field was simulated by Sanchez [33]. Water molecules possess a dipole, due to the difference in electronegativity between the oxygen and hydrogen atoms, and therefore a positive electric field will result in the standing orientation of water such that the oxygen binds to the metal whereas hydrogen atoms are directed away from the surface. The presence of a negative electric field, however, tends to invert the direction of this structure, such that water engages with the metal through its two protons. The calculations by Sanchez indicate that complete polarization to such a standing structure is achieved on Ag(1 1 1) by surface charge densities of ±15 lC/cm2. This phenomenon is referred to in the electrochemistry literature as dielectric saturation, as it leads to a reduced dielectric constant at the surface, and therefore a stronger electrostatic effect between species across the metal/aqueous interface. As shall be seen, much of the effort to elucidate the structure and chemistry of the metal/aqueous interface has been performed by electrochemists, in an effort to elucidate the kinetics of electrochemical processes. We have recently demonstrated that periodic quantum mechanical calculations, which provide a more realistic model of the electronic structure of the metal, can be used in order to establish the potential dependence on the overall reaction energies. This will be discussed in one of the following sections. Hexagonal water structures are prominent in studies of water/close-packed transition metal surfaces. For example, the wetting layer discovered on Ru(0 0 0 1) by Doering and Madey [34] was interpreted in terms of a bilayer structure, whereby water orients in a hexagonal arrangement with two water layers that appear closely coordinated with the ˚ ngstrom surface and show only a small separation of an A or less between the two planes of the puckered hexagonal configuration. The top view of the bilayer is shown in Fig. 1a, and the side view in Fig. 1b. A number of theoretical

Fig. 1. (a) Top view of the ideal hexagonal water bilayer over a (1 1 1) metal surface. (b) Side view of the outwardly oriented water bilayer, (c) side view of the inwardly oriented water bilayer.

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studies of bilayer structures on transition and noble metal surfaces have appeared recently in the literature [35–40]. The networked hydrogen-bonding structure suggests that proton transport over the metal surface and also between the layers parallel to the surface is facile, and can occur in a coordinated fashion via the hydrogen bonding network. The connectedness of the hydrogen-bonded network, suggests that there will be a significant barrier to the approach of ions and other reactive species to the surface, as they would have to interrupt the network and subsequently result in substantial molecular reorganization. A number of variations on the bilayer structure have been observed in ultra-high vacuum (UHV) conditions, such as the dissociated water bilayer [41,42] on Ru(0 0 0 1), and various two-dimensional hexagonal ice-patterns on Pd(1 1 1) at sub-monolayer coverage [43]. The exact form of the bilayer structure is sensitive to both thermodynamic preference as well as the kinetics allowed under experimental conditions. For metals with a weak metal/water interaction, there is little correlation between the atomic structure of the metal and the molecular ordering of H2O. The particular activity of the metal towards molecular water (in terms of Meng’s wettability parameter, for example) can therefore guide the structure of the interfacial water. This is expected to have consequences on the metal’s ability to carry out specific reactions. To begin to develop a more quantitative understanding of the flexibility of such a water network Taylor et al. [44] considered the dynamic properties of water bilayers, by focusing on a network of two such bilayers over a Cu(1 1 1) surface. The barrier to switching a bilayer from an orientation that pointed away from the surface, to one that points towards the surface, was found to be between 0.16 and 0.3 eV. The barrier is lowest when water is coordinated to the metal substrate, but not to the surrounding water matrix, since the metal stabilizes the inversion transition state by engaging with both the oxygen and hydrogen atoms of the H2O in the inversion, whereas the severage of hydrogen bonds to nearby water molecules would increase the energy of the transition state. They also found that bilayers, formed on Cu(1 1 1), comprised of three water orientations (up, down and flat dipolar orientations) were more stable than bilayers consisting of only two (either up/down and flat). This is due to the synergy between the charge enrichment occurring in bonds that form between hydrogen and the metal, and the charge depletion that occurs in areas of bonding between oxygen and the metal. A more detailed analysis of the bilayer adsorption energetics showed that, surprisingly, a stronger adhesion was found for water bound through its hydrogen atoms rather that through the oxygen. The structure that forms (the inwardly oriented bilayer in Fig. 1c) contains more steric strain between the water molecules. The combination of water molecules that can adopt a variety of orientations, thus allows for the optimization of both adhesive and steric effects. This illustrates the importance of both geometric (steric) and electronic (adhesive) effects in directing the

adsorption of water at the metal/aqueous interface. This will impact the ability for reactive species to access the metal surface and interact with the surface electronic environment. While rigorous ab initio calculations can begin to provide reliably accurate information for bond breaking and making processes at the metal/solution interface important in electrochemistry, they are typically limited to welldefined model systems and very short (tens of picoseconds) dynamic simulations. More approximate simulations of the condensed phase metal/aqueous interface, consisting of large numbers of water molecules in contact with some approximation to a metal surface, have been performed since the late 1970s. Ensemble-sampling methods, such as Monte Carlo [45–52], or molecular dynamics, as well as integral equation techniques [53–59] have been used to estimate the structure and electrical properties of the metal/ aqueous interface. A general finding from these studies is that the configuration of the water molecules near the metal surface is ice-like, in that the water network is somewhat rigid, developing a time-averaged layer-like structure with a number of hexagonal networks. Any such structure, however, is clearly in flux, as the simulations have demonstrated [1,4,60–71], although it has been shown that there is a tendency for the formation of four-, five- and six-membered rings of water near the metal surface to persist for up to 12.5 ps [4,65]. The majority of the reported molecular dynamics and Monte Carlo simulations have adopted an empirical point-charge type model for H2O, and have used various degrees of approximation to treat the metallic character of the solid surface. It has been shown that the metallic properties of the surface can have significant effects, depending on how the surface is modeled, yet even simulations of water in contact with a non-polarizable, rigid wall [50,51,68,69,71] (either with Lennard-Jones metal/water potentials, or simply a hard metal–water interaction cutoff) show a particular water structure at the interface, indicating that the mere fact that the water network is unavoidably terminated at the interface imposes a significant structural requirement on H2O and can lead to ice-like boundary conditions. In these simulations, a vertical structure is typically observed to persist in terms of an oscillating profile for the density of water molecules near the wall, with two main peaks indicating an adsorbed inner-layer, and a secondary solvation sheath for the wall. The first ab initio molecular dynamics simulations of the metal/aqueous interface were performed by Halley, Price and co-workers [63,64,72,73]. Building from a history of calculations at progressively more advanced levels of theory (earlier studies invoked a continuum dielectric, or spherical neon atoms with artificial ‘water’ dipoles, over a jellium surface [74–76]), these theoretical results include a reasonable description of the electronic interactions between the electrode and the solution. The oxygen density distributions, derived at potentials both positive and negative of the potential of zero charge, compared favorably

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 American Institute of Physics 1995

C.D. Taylor, M. Neurock / Current Opinion in Solid State and Materials Science 9 (2005) 49–65

Fig. 2. Hydrogen and oxygen correlation functions for (a) r < 0, and (b) r > 0 as a function of distance from the metallic slab. r is the surface charge density. (Reused with permission from David L. Price, Journal of Chemical Physics, 102, 6603 (1995).)

with Toney’s X-ray scattering experiments [77]. At cathodic potentials (negative surface charge, r), the water molecules were shown to be repelled from the surface (Fig. 2), and at positive potentials (positive surface charge, r), the first distribution peak is closer to the electrode. These interactions arise due to electrostatic effects between the electric field at the interface, and the dipole of the bonding H2O. Direct ab initio molecular dynamics simulations of the metal/aqueous interface are computationally expensive, and have only been performed for a few systems in the literature [1,4,78]. The method is quite useful in that it provides the dynamics, albeit on very short time scales, with appropriate electronic description of the water and the metal substrate, which can be very useful in modeling reactive systems. These simulations have been invaluable in identifying not only how solution can externally influence the surface chemistry, but how it can also actually directly participate in the chemistry. Desai and Neurock, for example, used ab initio molecular dynamics to simulate dynamics of water over a Pt/Ru bimetallic surface [2,3], and showed that water was adsorbed and activated at the Ru sites to form a surface hydroxyl intermediate and a proton which migrated into the solution phase via proton transfer through the local hydrogen bonding networks from the water molecules at the metal/water interface. The hydroxyl surface intermediates that formed at the ruthenium sites rapidly transferred across the surface via subsequent proton transfer processes that occurred across the bilayer network at the surface (Fig. 3). The presence of the surrounding water matrix, therefore, assists in the elementary surface diffusion processes by providing direct proton transfer paths. This is characteristically different than vapor-phase heterogeneous catalytic systems in that the solution not only changes the energetics but provides novel pathways that are not existent in vapor phase systems. The heterolytic activation of water at the interface to form the surface hydroxyl species and release a proton to solution was found to have an activation energy that is 60 kJ/mol lower than that found for the homolytic surface reaction

Fig. 3. Water dissociation initially occurs at the more reactive Ru site. This is subsequently followed by proton transfer through the aqueous phase, and consequently the exchange of a proton between the surface H2O species, such that the OH moiety is found on a Pt site (reproduced from Desai and Neurock [2]).

(involving a metal atom insertion into the O–H bond). The energy of the charge-separated transition state that forms is lowered by stabilizing interactions with local water molecules in solution. The hydrogen ion that forms upon dissociation was shown to exist primarily as a Zundel þ H5 Oþ 2 state, and also as H9 O4 . The metal-mediated dissociation of H2O on ruthenium was stabilized in solution as the surrounding water molecules stabilized the partial charge on the transition state by about 15 kJ/mol. 3. Electron transfer reactions between a metal surface and species in solution While questions concerning surface structure are yet to be resolved, advances in surface spectroscopy and theory have begun to provide a wealth of information that help to offer insights into structure as well as reactivity at the aqueous/metal interface. This is important since the structure and reactivity at the interface ultimately dictate its electrochemical and electrocatalytic behavior. The electrochemical behavior at the metal/solution interface is controlled thermodynamically by the Nernst equation [79]:

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DGðU Þ ¼ DGð0Þ þ nFU

ð2Þ

and kinetically by the Tafel equation [80], or its more generalized form, the Butler–Volmer [81,82] equation:

O2 þ Hþ þ e ! OOH OOH þ Hþ þ e ! H2 O2

ð6Þ ð7Þ

H2 O2 þ Hþ þ e ! OH þ H2 O

ð8Þ

þ

RT logðiÞ g ¼ a  2:303 bF  ð1bÞF g=RT  i ¼ i0 e  ebF g=RT

OH þ H þ e ! H2 O ð3Þ ð4Þ

Here DG(U) is the reaction free energy at potential U, DG(0) is the free energy at 0 V, n is the number of electrons transferred, b is the transfer coefficient, g is the overpotential, and F is Faraday’s constant. A thermodynamic description of a particular half-cell reaction, therefore, can be attained by calculating the overall reaction Gibbs free energy, DG, from some appropriate summation of internal energies, entropies and pressure-volume terms, and applying equation [1]. Most of these terms can be determined by carrying out quantum mechanical calculations. The results can be coupled with the statistical mechanics contributions that result due to the thermal motion of the constituents on relevant chemical systems. Similarly, the transfer coefficient, b, can be derived from the over-potential dependence of the activation energy [83] for the rate-limiting step occurring in the half-cell reaction: b ¼ DEz =DU

ð5Þ

The electrode surface can also support the direct decomposition of OOH into O* and OH*, provided there are two adjacent catalytic sites available for the product states: OOH ! O þ OH 

þ

ð10Þ 

O þ H þ e ! OH 2½OH þ Hþ þ e ! H2 O

ð11Þ ð12Þ

Ab initio calculated reactant and product energies were used to determine equilibrium potentials for each step, and thus determine the overpotential that could be achieved. The solvation of surface intermediates was modeled by incorporating one to three water molecules into this ‘local reaction center’ (one such local reaction center complex, involving the platinum dimer as a model of the electrode surface, is shown in Fig. 4a). The influence of the electrode was modeled using a single Pt atom or a Pt2 dimer to coordinate with the adsorbed species, leading to shifts in the relative energies, and therefore changes in the resulting equilibrium potential, relative to the reaction steps performed in the absence of the model electrode. In both cases, it appears that the first reduction step, which involves the opening of the oxygen double bond to form the hydrogen peroxy radical, is rate-limiting. Anderson and coworkers have also recently developed models using a periodic slab model of the electrode surface to consider oxygen reduction over platinum/chromium [88,89] and platinum/ cobalt [10,90] alloys. Similar models have been utilized by others [91–101] to study the effects of alloying on oxygen reduction. The ‘local reaction center’ model has also been used to determine activation energies for electrochemical systems as a function of potential. Anderson et al., for example, implemented a constrained search along various reaction coordinates to find the lowest energy pathway corresponding to a given electrochemical potential [11,13,33,85– 87,102]. The electrochemical potential was determined for

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whether the reaction involves ion solvation/desolvation, bond-breaking/formation or adsorption to the electrode. Both the kinetics and the thermodynamics, therefore, are ultimately controlled by atomistic processes that occur at the interface, and can be probed directly using ab initio quantum mechanical methods. Theory can be used to calculate the state internal energies, vibrational terms, and estimate important terms for calculating entropy. In addition, direct molecular dynamics simulations and/or statistical mechanics can be used to help determine the overall reaction as well as the activation entropies. The direct determination of thermodynamic parameters necessary for the Nernst equation was made by Anderson [13,84–87] for the reduction of oxygen to water via the following reaction pathways:

ð9Þ

Fig. 4. Local reaction center models developed by (a) Anderson using the cluster approximation (Reprinted from Journal of Electroanalytical Chemistry, 580, Anderson AB, p. 17–22 [103]), in which either the ionization energy or the electron affinity can be related to the electrochemical potential required for oxidation or reduction, respectively, and (b) Nørskov [6] using the periodic slab model, in which reaction energies were determined via the Nernst equation using 1/2H2/H+ as an internal reference, and (c) Neurock et al. [7–9] to represent the electrochemical interface.

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oxidation reactions by calculating the ionization energy, and for reduction reactions by calculating the electron affinity. The electrochemical potential relative to the standard hydrogen reference electrode was then determined from the equation: U ¼ ðI:P: or E:A:Þ=e  4:6 V

ð13Þ

Ab initio calculated activation energies can subsequently be used to determine the parameters for the Tafel equation directly from an atomistic model of the interface. A different ‘local reaction center’ model was established by Nørskov and co-workers and applied to the oxygen reduction reaction [6], using a periodic model for a Pt(1 1 1) surface. The equilibrium potentials were calculated by referencing the energy of the proton released in the above reaction steps to the energy of the standard hydrogen electrode, via the equilibrium that exists between H+ + e and 1/2 H2. The influence of water was simulated by Nørskov et al. by recalculating the adsorption and overall reaction energies of the reacting species and intermediates on Pt(1 1 1) in the presence of a water bilayer (Fig. 4b). The Nernst equation was used to determine the overall reaction energies for each of the steps illustrated in the ORR mechanism given in the equations above. In doing so, Nørskov and co-workers identified the formation of a strongly bound oxygen intermediate and its subsequent reduction to a surface hydroxide to be a leading cause of the ORR overpotential. At the total equilibrium potential of 1.23 V, it was found that the intermediate step of atomic reduction to OH is considerably endothermic. At 0.78 V (an overpotential of 0.45 V) the intermediate step becomes thermoneutral. The importance of understanding the binding of intermediates has also been described theoretically by others [100,104,105]. The changes in the reaction energy due to potential induced changes in the adsorbate geometries and adsorption strengths have been explored by Hyman and Medlin [106]. The Nernst equation appears to hold quite well in these cases, as the adsorbate interaction is not strongly dependent upon the electric field at the surface. In other cases, however, the structure of the adsorbed intermediates may vary strongly with the applied electrochemical potential, and thus require a second order correction to the Nernst equation: DGðU Þ ¼ DGð0Þ þ nFU þ 1=2½DCFU 2

ð14Þ

where DC is the change in the capacitance of the electrochemical interface between the reactant and product states. Such variations in adsorbate structure and energies with the applied field have been reported for cluster and slab calculations. To determine the dependence of the free energy of reaction upon the electrochemical potential, Filhol and Neurock applied a charge/counter-charge model in order to polarize the metal/water interface for condensed water between metal slab surfaces [9]. The energy was constrained such that its derivative with respect to the applied charge corresponded to the electrochemical potential [8,9]. The free energy of the interface, Gq,U, charged by an

55

amount q, and corresponding to a potential, U, is determined from the internal electronic structure energy by the equation: Z q hV ðQÞi dQ  qU ð15Þ Gq;U ¼ E þ 0

The electrochemical potential U was referenced to the standard hydrogen electrode by relating the work function of the slab with a representative of the model aqueous/metal interfacial system to the electrochemical potential of the interface via Trasatti’s relationship [107,108]: U ¼ U=e  4:6 V

ð16Þ

where the value 4.6 eV is taken to be the work function of the standard hydrogen electrode. In this way, Filhol and Neurock were able to use ab initio methods to directly calculate the structural and energetic response of the adsorbed water molecules at the metal/water interface to changes in the applied potential. As such they were able to map out the phase diagram for the electrochemical activation of water to form surface hydroxyl species (via oxidation) and surface hydrides (via a reduction process). The reaction center model used in these simulations consists of not only the electrode slab and the reacting water molecules, but also a more expansive model of the surrounding water matrix (Fig. 4c). This last feature was shown to be important for the transport of protons between the environment and the reaction center, and for polarizing adsorbates at the metal surface. The phase diagrams for water and the structural response for water over the Pd(1 1 1) surface observed by Filhol and Neurock are presented in Section 5. A more detailed comparison to phase diagrams derived using electric field methods, and the Nernst equation for water activation over Pt(1 1 1) is the subject of a separate communication. Electron transfer kinetics have been successfully treated using the theory developed by Marcus [109–111], in which the reactant and product states are separated by a generalized reaction coordinate, representing the change in the solvation of the reduced/oxidized and reactant species. Following the harmonic dielectric response approximation for the aqueous environment, the energy is assumed to be parabolic with respect to this reaction coordinate and from this assumption the activation energy for the non-adiabatic electron transfer can be determined. The electron transfer rate can be determined quantum mechanically via calculating the coupling constant, K, for electron transition between the reactant and product states, as derived by Levich and Dogonadze [112] from Fermi’s golden rule: rate ¼

2p 2 hjKðfRgÞj d½V i ðfRgÞ  V f ðfRgÞi h

ð17Þ

in which the delta function maintains conservation of energy, V, between the initial and final states, at some reaction coordinate, R. The electron transfer may alternatively follow the adiabatic pathway, whereby the pre- and posttransfer states are quantum mechanically mixed, such that

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a diagonalization of the Hamiltonian is necessary to find the intermediate partially charged states. The rates may be calculated in the same way as Eq. (17). For a more indepth discussion, we refer the interested reader to the tight-binding molecular dynamics theoretical studies by Halley et al. [72,73] and the combined quantum-mechanical and molecular mechanics approach by Illas et al. [113]. In both cases the molecular effects of solvent reorganization and the approach of the ion to the electrode were shown to have a significant impact on the overall transfer kinetics. 4. Ion transfer – chemisorption of ions at the metal/water interface Ion transfer involves the passage of ionic species to the electrode where they adsorb with discharge. The exact nature of the discharge however has been debated. The ion transfer of iodide to form iodine at a model electrode surface, for example, was considered separately by Xia and Berkowitz [114], and by Pecina and Schmickler [115] using molecular dynamics. Xia and Berkowitz proposed that discharge occurred in discrete steps, following the adsorption of iodide on the electrode, the release of an electron, and the desorption of the neutral atom, vis-a`-vis Marcus theory. The calculations by Pecina and Schmickler, however, support a gradual discharge process, in that the solvated iodide ion gradually donates charge to the electrode upon the approach to the electrode and desolvation. Its valence shell then overlaps with the surface states of the metal. The overall ion and electron transfer was suggested to occur via the reaction sequence: d d d 0 I ðaq;bulkÞ ! I ðaq;psÞ ! I ðadsÞ ! I ðaq;psÞ ! I ðaq;bulkÞ

ð18Þ

d Hþ ðaqÞ þ ð1 þ dÞe ! HðadsÞ

ð19Þ

By determining the charge on the atom using an electron partitioning scheme, Lorenz and Kuznetsov proposed the value of d to be close to 0.5. Crispin et al. demonstrated the existence of small charge transfer effects between an organic molecule (acrylonitrile) and a nickel electrode, which, in this case were tunable depending upon the potential applied [119]. For example, the increased charge transfer from the nickel electrode to acrylonitrile could be obtained by making the electrode potential more anodic. The reorganization of water is a necessary consideration when considering ion transport to the metal surface. Pecina

 Elsevier B.V. 1995

where ps indicates partial-solvation as the ion penetrates the inner layer of water coating the electrode, and d- indicates some partial and unquantified charge on the ion. In this case the rate-limiting step involves the displacement of water from the metal surface and the concomitant par-

tial desolvation of the transferring ion. Using an extended Anderson–Newns model, Schmickler was able to determine two-dimensional solvent-coordinate/partial charge potential energy surfaces to describe the iodide/iodine transfer reaction as well as the Zn2+/Zn couple [19,20]. Chemisorbed states are likely to involve a partial charge on the adsorbate [116], due to the electronegativity differences between the surface and the adsorbate (the electrosorption valence [117]), and as the result of the statistical distribution of electrons between the variously charged adsorbates. Partial charge transfer in a potentiostatic process has also been rationalized as the charge that is necessary to correct the change in work function/electrode potential induced by an adsorption event. A partial charge on an adsorbate develops with the broadening of the HOMO and LUMO states of the adsorbate as they begin to interact with the continuum of states of the electrode surface, leading to a Fermi distribution of the occupiable states in the adsorbate-metal bond (Fig. 5). Accordingly, partial charge transfer has been implicated in a number of electrochemical reactions, including the hydrogen evolution reaction, in which there is postulated to be an adsorbed hydrogen intermediate [17,18,118]. Lorenz and co-workers postulated that hydrogen adsorbs on the Cu(1 0 0) and Cu(1 1 1) electrode via the reaction:

Fig. 5. A rationalization of partial charging of an adsorbate using the frontier interaction picture of the highest occupied molecular orbital (HOMO) of iodine with the Fermi level of the metal. Broadening of the HOMO density of states, and emptying of electron density into the Fermi level of the metal leads to a partial occupation of the highest occupied orbital of iodine, and thus an electronic structure which is intermediate to that of I0 and I (Reprinted from Journal of Electroanalytical Chemistry, 394, Pecino O, p. 29–34).

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and Schmickler have suggested that the rate-determining step for proton transfer/hydrogen chemisorption is the reorientation of the water molecule belonging to both the first solvation shell of the hydronium ion bearing the proton, and the first adsorbed water layer of the metal [21]. The peculiar orientation to be adopted requires a large negative surface charge on the electrode, and thus explains the rather high cathodic overpotentials necessary for hydrogen evolution on non-catalytic metals, such as silver and gold. A solvent reorganization barrier was also observed for the specific adsorption of acetate on Pd(1 1 1). Desai and Neurock determined that the acetate anion is stabilized by 57 kJ/mol more when it is completely solvated by water molecules (non-specifically adsorbed), than when it is directly adsorbed on the surface [5]. Ab initio molecular dynamics calculations performed on the specifically

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adsorbed acetate indicated that the acetate anion is not displaced by H2O when carried out at low temperature. The acetate–metal interaction is strong, at around 114 kJ/ mol. While the desorption of acetate from the surface is thermodynamically favorable, the simulations suggest that it requires an activated process. For simulations run at 300 K, acetate recombined with protons to form acetic acid but the acetic acid remained bound to the surface. At 350 K, however, the acetic acid that forms is readily displaced by water. It subsequently dissociates in solution thus resulting in solvated acetate anions and protons. The displacement of acetic acid and the transfer of protons required overcoming a barrier to rearrange water molecules near the surface (Fig. 6a). This is consistent with results by Stuve and co-workers who determined the desorption of perchlorate in the presence of H2O. Evidence of specific

Fig. 6. (a) Schematic depiction of acetate desorption from Pd(1 1 1); and (b) the product state of acetic acid dissociation over Pd(1 1 1) in solution [5].

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and nonspecific adsorption of perchlorate (CTO4) on silver (1 1 0) [135]. The elucidation of potential-dependent displacement energies is important for understanding how the explicit electrochemical conditions dictate what is ultimately adsorbed on the surface. Neurock and Desai also found that stabilization of the acetate ion and a proton in solution led to a reduced reaction energy for acetic acid dissociation over Pd(1 1 1), an important step in the synthesis of vinyl acetate. The dissociation energy was lowered from +1483 kJ/mol for the heterolytic dissociation reaction to +37 kJ/mol by virtue of the interaction of the acetate ion and the surface and the proton which forms with surrounding water molecules in the aqueous phase (Fig. 6b). In summary, the adsorption of ions and neutral species on an electrode surface may lead to the partial transfer of charge to the surface, thus activating both the surface as well as the adsorbate. The ability for both the metal surface and the surrounding H2O network to stabilize partial charges aids reactions that pass through charged intermediates or transition states. The presence of water, however, can affect the ability of the reacting species to adsorb (desorb), in that the hydrogen-bonded network must reorganize to accommodate the arriving (leaving) species. Entropic considerations can also become important. Ab initio calculations have helped to shed light on the basic mechanisms of these processes, and can continue to be used to explore new mechanisms for the many processes in electrochemistry and aqueous-phase catalysis. 5. Theoretical studies of water reactivity and electrochemistry on metal surfaces Chemical reactions at the metal/water interface have been modeled with varying degrees of sophistication [120–126]. The dissociation of H2O at the surface into products presents an important first probe reaction: H2 OðadsÞ þ e ! HðadsÞ þ OH ðaqÞ H2 OðadsÞ !

Hþ ðaqÞ

þ OHðadsÞ þ e

OHðadsÞ ! OðadsÞ þ Hþ þ e

ð20Þ ð21Þ ð22Þ

Various cluster and periodic slab quantum mechanical calculations have been performed to determine the adsorption structure and energy for water and its dissociation products over different metal surfaces. Anderson and coworkers for example presented some of the first studies using the semiempirical ASED-MO method [120], to model the homolytic dissociation of H2O on Pt(1 1 1) and Pt(1 0 0), at potentials of 0 V, 0.5 V and 1.5 V relative to the potential of zero charge [121]. While the calculated energies were extremely high (1.31 eV for H2O), they show the general trend for stronger adsorption at more anodic potentials. In addition, they provide a basis for this shift in terms of the electronic structure: anodic potentials lower the d-band of Pt such that there is greater overlap between the d-orbitals at the surface and the lone-pair orbitals of H2O which strengthens the overall bonding

interaction and reduces the occupation of the antibonding states. This description coincides with the four electron attractive bond model described by Hoffman [127], in which anti-bonding states that would otherwise be filled in a frontier orbital interaction, are instead emptied into the lower energy state provided by the Fermi level of the metal. Earlier studies by Anderson considered iron clusters bonded to single molecules of water, hydronium and hydroxyl species [122–124]. These theoretical calculations captured the influence of the band structure on the stability of each of the adsorbates. The shifts in the dband structure of surface Fe that result from the adsorption of OH destabilize the surface FeOH complex. This weakens the Fe–Fe contacts and eventual leads to the dissolution of Fe as FeOH. These are some of the initial processes involved in corrosion. Later extensions of this ‘local reaction center’ have been successful in estimating the transfer coefficients and equilibrium potentials for water dissociation and oxygen reduction, as discussed previously [11,13,33,85–87,102]. Other studies using cluster systems to model the structure and adsorption of water and its dissociation products upon isolated metal clusters have used direct charging (creating a charged cluster system), or an electric field across the cluster surface plane to model the effect of an applied potential. For example, Patrito et al. noted elongation of Pt–OH adsorption bond lengths for the hydrated OH species, as well as a reorientation of the water dipoles, in the presence of increasingly cathodic electric fields [125,126]. Partial charge transfer was found to be influential in the adsorption of OH, indicating that approximately 0.3 electrons are transferred to the metal surface, and therefore that the simplistic view of electron transfer exhibited in Eqs. (20)–(22) may in fact be more complicated, and require the transfer of a partial number of electrons, while some electron density is retained by the local adsorption environment. By directly applying an excess or deficit surface charge to a periodic slab model of the Pd(1 1 1)/H2O interface, Neurock and Filhol directly modeled the electrochemical response of water and its activation products to changes in the electrochemical potential [9]. By changing the number of electrons available to the metal surface, the attraction of water to the slab could be tuned, such that at negative surface charge densities, water was repelled and assumed the inward orientation (hydrogen atoms directed towards the metal), and at positive surface charge densities, water was attracted and assumed an outward orientation (hydrogen atoms directed towards the aqueous phase), as shown in Fig. 7a. Further polarization of the interface in the anodic direction leads to the dissociation of water to form an adsorbed hydroxide, whereas polarization in the cathodic direction leads to the dissociation of water to form surface hydrogen. The energies of each of these phases can be determined following the decoupling scheme introduced above, and described in the papers by Filhol and Taylor [8,9]. The phase diagram derived by Neurock

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Fig. 7. (a) Structural response of water to changes in the applied electrochemical potential. (b) Energetic response of water to changes in the applied potential, with phase diagrams indicated for hydroxyl and hydrogen formation (from Ref. [9]).

and Filhol for water and its dissociation products over Pd(1 1 1) is shown in Fig. 7b. A similar procedure has been used to examine the potential dependent reactivity of water on Cu(1 1 1), and the same general features of dissociation and dipole rotation are observed. Furthermore, the extremes of anodic polarization were shown to dissociate the hydroxide to form an oxide, which is then active in corroding the metal surface via the

place exchange of oxygen with the metal substrate [128]. Calculations for water and its reactive intermediates at the Ni(1 1 1)/water interface showed that the range of potentials for which water is inert over Pd(1 1 1) (see Fig. 7b) is non-existent on Ni(1 1 1) [44]. The varying activity of metals for the dissociation of H2O can be projected from the studies of bilayer wetting layers by Michaelides and co-workers [36], who calculated the energy to dissociate the hexagonal

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wetting layer of water into layers containing hydroxyl and atomic hydrogen on the surface. To summarize, the activation of water over metal surfaces requires that one follow the potential-dependent behavior of adsorbed water as well as its activation products which include adsorbed hydrogen, hydroxyl and oxygen. For some metals such as nickel, water is reactive over all electrochemical potentials. The surface of these metals contain either adsorbed hydroxyl or hydrogen species, and in some cases, both. This can then impact other surface reactions due to changes in the surface coverage as well as the formation of local acidic or basic intermediates at the metal/aqueous interface. 6. Catalysis and electrocatalysis at the metal/water interface Recent interest in proton-exchange membrane (PEM) fuel cells has solicited a great deal of interest in modeling electrocatalytic systems. We have previously discussed the models proposed by Anderson and Nørskov for the oxygen reduction reaction occurring at the cathode. In this section we discuss in detail recent calculations concerning oxidation at the anode of the PEM fuel cell. More specifically we examine the dehydrogenation of methanol to CO over Pt(1 1 1) because of its importance in the oxidation of methanol at the anode of the direct methanol fuel cell. The dehydrogenation of methanol and subsequent desorption of CO over Pt as well as other metals has been extensively explored using theoretical methods under ideal conditions. Mattson and Paddison [78], for example, carried out ab initio dynamics simulations to examine the adsorption and reactivity of methanol at the aqueous/ metal interface, by modeling the Pt(1 1 1) surface as a 2 · 3 periodic unit. Nineteen water molecules were used in the unit cell to simulate the aqueous region and one molecule of methanol was chosen to follow the surface reaction chemistry. In this way the methanol molecule was solvated by up to 10 water molecules. These simulations of the neutral metal/water interface were performed at the open circuit potential, and were commenced with methanol starting at three distinct positions: specifically adsorbed to the metal surface, non-specifically adsorbed (i.e. separated only by its solvation sphere from the metal surface), and embedded as far as possible from the metal surface within the aqueous phase. Although methanol in the vapor phase prefers to bind to Pt(1 1 1) through the oxygen atom, after about 3 ps into the simulation methanol twists through 90 orienting its CH bond toward the surface. The driving force for this rotation appears to be the maximization of CH3OH–H2O interactions, in preference to the interaction of CH3OH with the surface. There is a competition between the Pt metal and water molecules in solution to interact with the OH group on methanol. In this new orientation, the C–H bond is weakened via interactions between Pt and the hydrogen which is favorable for the dehydrogenation mechanism on this surface, a feature which is also observed in the AIMD simulations. Wasileski

and Neurock [7] simulated the dehydrogenation pathways of methanol in the presence of a vapor phase over Pt as well an aqueous solution phase over Pt. In addition, they followed the changes in the reaction chemistry over a range of different applied potentials in order to simulate more realistic electrochemical conditions. They demonstrated that the preference of platinum for the –CH or –OH ends of the methanol molecule depends on the electrochemical potential of the aqueous/metal interface similar to that discussed above for water. The potentials required for the dehydrogenation and oxidation of methanol are typically significantly greater than that which is present for the open circuit. At the potentials relevant for fuel cell conditions, methanol predominantly binds to the surface through its OH group. Okamata and co-workers [129] have also considered the dehydrogenation p of methanol at the Pt(1 1 1)/aqueous p interface, using a 7 · 7 model of the platinum surface and one methanol molecule per 22 water molecules. Molecular dynamics simulations of the model system were then quenched, and either a CO or a CHxOHy species was substituted for a surface water molecule, in order to simulate the various stages of methanol dehydrogenation over Pt(1 1 1). Excess hydrogen atoms resulting from the dissociation were placed on the Pt(1 1 1) surface thus resulting in a metal surface with a high hydrogen surface coverage, interacting with water and the methanol decomposition products. Wasileski and Neurock [7] adopted an alternative procedure choosing to release a protons into solution (a heterolytic dissociation). They calculated that this path was significantly lower in energy than that for forming the surface hydride. This is largely due to the fact that aqueous water molecules solvate the released H+ species and significantly stabilizing the reaction and activation energies. Okamato’s work indicates that the first step is exothermic [129], in contradiction to the results obtained by Mattson and Paddison’s simulations [78]. In contrast, they find that the second step is rate-determining in solution. The net exothermicity of the total methanol dehydrogenation path is also reduced by 0.7 eV, compared to the vapor phase calculations. This is possibly due to the fact that they retain H on the surface in these simulations, or due to a loss of hydrogen bonds. Wasileski and Neurock [7] modeled the methanol dehydrogenation pathway in both the vapor as well as the aqueous phase and in addition followed the influence of an applied potential. They found that the dehydrogenation of methanol at each step leads to a change in the electrochemical potential of the electrode. More importantly, they showed that most of the previously reported calculations reported in the literature for methanol over Pt in the aqueous phase were carried out at potentials that were somewhere between 0.5 and 0.8 V whereas the experimental results were typically carried out at potentials of +0.3 to +0.7 V. Therefore, in order to compare with experiments performed under potentiostatic conditions, they

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adopted the method of Filhol and Neurock [9] to determine the dehydrogenation thermodynamics as a function of potential. Starting from a flat CH3OH orientation (in which the primary interaction of methanol with the surface is through its OH group), they modeled all O–H and C–H activation steps thus resulting in the dehydrogenation of methanol to CO. The choice of two different paths at each stage leads to the possibility for a ‘dual pathway’ at higher potentials, with a major path that is thermodynamically preferred, and a minor path, which is somewhat less favorable and requires the activation of an OH bond early in the reaction mechanism. There is evidence for a dual path from the experiments by Iwasita who showed the formation of appreciable amounts of formaldehyde in solution and speculated that formaldehyde was formed via a minor path which went through an adsorbed methoxy as an intermediates [130–134]. Wieckowski subsequently carried out combined chronoamperometry and fast cyclic voltammetry to show that a dual path begins to occur at about 0.35 V NHE over Pt(1 1 1) [7]. Wasileski and Neurock’s results support this interpretation, in that more positive potentials tend to enhance the O–H activation and thus significantly reduce the endothermicity of this reaction, similar to that found for the activation of water described earlier from the work by Filhol and Neurock [9]. The activation of the O–H bond in methanol becomes exothermic at 0.45 V, and therefore allows thermodynamic access to the dual path. At the higher potential of 1.2 V NHE, Wasileski and Neurock found that the two pathways became of similarly thermodynamic impetus. The dual path nature can be seen in the overall reaction energies reported for methanol dehydrogenation over Pt(1 1 1) held at a constant potential of 0.5 V NHE as is shown in Fig. 8 in that both

Fig. 8. Dual pathway for methanol dehydrogenation held at a constant potential of +0.5 V NHE. Reaction energies in kJ/mol are given next to the arrows, indicating the direct of methanol dehydrogenation. The primary path is shown with bold black arrows whereas the secondary path is shown with bold blue arrows.

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the initial C–H and O–H bond activation steps are exothermic. 7. Summary The making and breaking of chemical bonds at the aqueous/metal interface is controlled by the strength of the adsorbate bonds to the surface as well as by the influence of the reaction environment. We have focused much of this review on the chemistry of water over ideal metal surfaces in order to establish fundamental concepts. The adsorption of a single water molecule to the metal provides initial insights into the electronic interactions which govern the adsorption and reactivity of water. It is clear, however, that the chemistry of water under realistic conditions is much more complex. Theory has effectively shown that the electronic interactions between water and the metal surface are highly dependent upon the nature of the metal as well as the orientation adopted by H2O. In certain configurations, there is a synergy between these effects that can lead to improved metal/water interactions. The metal–oxygen bond that forms when a single water molecule is adsorbed alone is weakened by its interaction with a condensed solution phase environment due to the competition between metal/water and water/water interactions, and the thermal motion of water molecules at most temperatures of interest. The majority of molecular simulations performed under such conditions, however, indicate that sometimesensitive ordering remains on the surface, with local hexagonal networks of water persisting for up to 12 ps on the surface, and the approximate ordering of water molecules near the electrode into at least two discrete layers near the surface. The semi-rigidity of the water layer near interface imposes a barrier to the transport of ions and reactive species to the metal surface.The same hydrogen bonds that may hinder transport, however, can also aid reactivity by: (1) localizing a surface species close to the metal, (2) stabilizing partial charges incurred in the transition or product states, and (3) providing pathways for proton shuttling in protic solutions. This has been particularly noted for heterolytic bond activation processes. In addition, the interaction between the metal surface and solution can stabilize the formation of partially charged surface intermediates that may have a reactivity which is different than either of the neutral or ionic forms. This could result in unique paths such as those present for the hydrogen evolution reaction. As such, ab initio methods have been used to calculate reaction energies and, in addition, determine the equilibrium potentials for various reaction mechanisms relevant for electrochemistry and electrocatalysis. Local reaction center models have been devised to model the electrochemical processes in a simple fashion. In such cases the energies of the surface states have been considered to be independent of the electrochemical potential and the electron transfer energies shifted linearly according to the potential. This is in contrast to the picture of homogeneous catalysis,

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in which changes in the formal charge of the metal center allow differences in coordination and binding strength. Recent models developed by Filhol et al. [7–9] however allow one to follow the influence of the potential on the structure and reactivity at the electrochemical aqueous/ metal interface. The presence of solution allows one to begin to stabilize adsorbed species and charged intermediates which are not possible in the absence of solution. In addition, the presence of an applied potential can facilitate specific oxidation and reduction reactions and, as such, be used to tune particular reactions in order to control activity and selectivity. The environment offered by electrocatalytic systems therefore enables heterogeneous catalytic systems the ability to adopt some of the benefits of homogeneous catalytic systems. We have shown herein that the electronic state of the metal (whether charge rich or poor) along with the environment offered by the solution phase have a significant impact on the reactivity and electrochemical behavior at the interface [7,9]. Challenges in modeling electrochemical and electrocatalytic systems remain, in the ability to directly incorporate important factors, such as the influence of local ions, the dynamics within the double layer, and the appropriate statistical sampling of the many possible different configurations within the double layer. Acknowledgements We would like to thank R.G. Kelly, J.S. Filhol. S.A. Wasileski and M.J. Janik for their helpful discussions. In addition we thank the Army Research Office for financial support through the MURI grant (DAAD19-03-1-0169). A portion of the research described in this paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. References [1] Vassilev P, van Santen RA, Koper MTM. Ab initio studies of a water layer at transition metal surfaces. J Chem Phys 2005;122:54701–12. [2] Desai S, Neurock M. First-principles study of the role of solvent in the dissociation of water over a Pt–Ru alloy. Phys Rev B 2003;68: 75420–6. [3] Desai S, Neurock M. A first principles analysis of CO oxidation over Pt and Pt66.7Ru33.3 (1 1 1) surfaces. Electrochim Acta 2003;48: 3759–73. [4] Izvekov S, Mazzolo A, VanOpdorp K, Voth GA. Ab initio molecular dynamics simulation of the Cu(1 1 0)–water interface. J Chem Phys 2001;114:3248–57. [5] Desai SK, Pallassana V, Neurock M. A periodic density functional theory analysis of the effect of water molecules on deprotonation of acetic acid over Pd(1 1 1). J Phys Chem B 2001;105:9171–82. [6] Nørskov JK, Rossmeisl J, Logadottir A, Lindqvist L, Kitchin JR, Bligaard T, et al. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J Phys Chem B 2004;108:17886–92.

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