First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 109

4.4.3 Competitive Adsorption at the Aqueous, Electrified
Metal Interface

The previous section addressed how ab initio methods can be used to examine the struc-
ture and state of water on the electrode surface as a function of electrochemical poten-
tial. The structure and binding interactions of water can affect the binding energies of
reactant or product species at the electrocatalytic interface. The use of DFT to calculate
the binding energies of species to metal surfaces is well established as a standard step in
modeling heterogeneous catalytic systems. Binding energies can be used to determine
the coverage of a single adsorbate as a function of gas-phase pressure and temperature,
examine the competitive adsorption between gas phase species, compare various solid
surfaces for the affinity to adsorption, or predict coverages of intermediate/product
species developed during a catalytic reaction. The calculation of adsorption energies
at the aqueous, electrified surface is less straightforward, as an adsorption process
includes the displacement of another molecule from the surface and an exchange
with a molecule in solution. While ab initio methods have been used to calculate rela-
tive binding energies of adsorbates on various metals and generate predictions of elec-
trocatalytic activity [Greeley and Nørskov, 2005; Zhang et al., 2005a, b], they have not
been commonly used to examine the competitive adsorption between the various
species present in an electrocatalytic environment. Most simulations do not consider
the energetics of supplying the reactant to the surface or the desorption of product
species from within the inner layer when examining electrocatalytic pathways.
However, the relative binding preference of water, electrolyte ions, or reactant/product
species may directly affect the rates of electrocatalytic processes. Furthermore, the rela-
tive binding energies of these species will be a function of electrode potential, and
thereby may affect the potential dependence of electrocatalytic rates and reaction
paths. For these reasons, we felt that it was important to develop a theory of substitution
chemistry at the electrode/electrolyte interface. In this section, we illustrate how the
double-reference method may be used to consider the relative binding energies of
adsorbates at the aqueous, electrified interface.
Let us reconsider the situation in which water on the surface is replaced with an OH
species. This situation is analogous to the water dissociation reaction, discussed pre-
viously. We will use this as a model system to demonstrate how the methods used
in examining water dissociation can be re-applied to consider competitive adsorption
at the electrocatalytic interface. At a given pH, some concentration of hydroxyl ions is
present in solution. The adsorption of a hydroxyl ion from solution to the surface
requires moving the ion from the diffuse layer (outside the interfacial field region)
to the surface, and results in the transfer of the negative charge (or partial negative
charge) to the electrode. Thus, the water dissociation event could be just as easily
cast as a H 2 O/OH substitution event. In the aqueous environment, therefore, we
assume that, rather than finding a bare metal site to adsorb to, the ion must displace
a water molecule previously adsorbed, returning the water molecule to the diffuse
layer (although the assumption of a one-for-one exchange is not obvious for any
given adsorbate). The hydroxyl adsorption event can now be written as:

OH (aq) þ H 2 O ! H 2 O (aq) þ OH þ e (4 : 8)