First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 106

In addition to enhancing surface reactions, water can also facilitate surface transport
processes. First-principles ab initio molecular dynamics simulations of the aqueous/
metal interface for Rh(111) [Vassilev et al., 2002] and PtRu(0001) alloy [Desai et al.,
2003b] surfaces showed that the aqueous interface enhanced the apparent transport or
diffusion of OH intermediates across the metal surface. Adsorbed OH and H 2 O mol-
ecules engage in fast proton transfer, such that OH appears to diffuse across the sur-
face. The oxygen atoms, however, remained fixed at the same positions, and it is
only the proton that transfers. Transport occurs via the symmetric reaction

OH (ad)[site1] þ H 2 O (ad)[site2] ! H 2 O (ad)[site1] þ OH (ad)[site2] (4 : 7)

where site 1 and site 2 are neighboring adsorption sites on the metal surface. In
Reaction (4.7), proton transfer in one direction appears as OH diffusion in the opposite
direction in a Grotthus-type mechanism.
These studies on the aqueous/metal interface provided a wealth of information on
the influence of water that had not been fully understood, and they have been directly
connected with recent spectroscopic characterizations of surface intermediates and
elementary reactions at the aqueous/metal interface. In addition, they provided the
basic foundation by which we began to explore chemical reactivity at the electrified
aqueous/metal interface. By adopting the double-reference method described
above, we found that, under anodic potentials, there is an increase in the demand of
the metal for electron density—a demand that was manifested by increasingly
strong metal - oxygen bonding at the metal - water interface. This additional bonding,
however, weakens the bonding between the oxygen atom and its covalently attached
hydrogen atoms, to the point at which dissociation may occur, which is apparent in the
polarization study of water bound to the copper electrode in Fig. 4.7. Electron density
diminishes on the copper atom, correlating with reduced metal - oxygen bond lengths
and water dissociation (indicated by the points marked “b” and “c” in Fig. 4.7). The
aqueous environment provides a “sea” of acceptors for the detached protons, and a
rich set of pathways for the distribution of the excess charge produced in solution
[Desai et al., 2003b]. The metal conducts the excess electron away through the external
circuit, or to a cathodic site elsewhere on the surface. The potential at which this occurs
controls the thermodynamic stability of this reaction, in addition to the environmental
effects (i.e., electrical fields) that will hinder or stabilize the products in solution.
The aforementioned DFT model has been used to investigate the thermodynamic
stability of water over a number of other metals [Filhol and Neurock, 2006;
Rossmeisl et al., 2006; Taylor and Neurock, 2005; Taylor et al., 2006a- c, 2007a- c].
Detailed DFT analysis of the interplay between hydrogen bonding to solution, surface
charging, and electric fields has shown that the metal - adsorbate bonding is influenced
by a shift in the d-band of the metal, which is accompanied by a shift of the local orbi-
tals of the adsorbate resulting from a change in the local electrochemical potential
[Taylor et al., 2007a]. This effect leads to a change in the total binding energy of
the adsorbate to the electrode. The magnitude of this effect upon the dissociation of
water, however, has been shown to be rather small compared with the larger thermo-
dynamic shifts induced in the energy of the conducting electron when the potential is