First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 119

method inherently requires that the number of electrons in the system remain constant
along the reaction path, and, therefore, within a double-reference model system, the elec-
trode potential may change along the reaction coordinate. However, for a nonredox
chemical reaction, the potential change between reactants and products is small, and
may be assumed negligible such that the constant charge activation barrier can be
assigned to the potential of the reactant state.
The barrier to CO þ OH coupling was found to be potential-dependent. As the sur-
face of the electrode becomes increasingly positively charged, the activation barrier to
coupling decreases from 0.55 to 0.30 eV [Janik and Neurock, 2006]. Although the
complexity of the double-reference model system complicates assigning this trend
to a specific interaction, changes in the solvation structure about the transition state
suggests that the more positive surface charge facilitates greater solvation of the tran-
sition state. With increasing positive charge, the hydrogen bond distance between the
hydroxyl species and a nearby water molecule decreases from 1.91 ˚ at the transition
state at - 0.53 V to 1.64 ˚ at 1.58 V. The potential dependence of the activation barrier
appears to be a consequence of the dependence of the interfacial water structure on
the electric field. The increasing rate of CO oxidation appears to be the result of
both a more favorable reaction free energy to activate water and the decreasing
coupling barrier.
4.4.6 Ion Transport through the Double Layer: Reduction of O 2
For elementary electrochemical steps, water plays an intrinsic role in the transport of
ions across the electrochemical double layer. In Section 4.3.4, we discussed in detail
the thermodynamics involved in the oxidation of methanol and generation of protons
and electrons. The resulting proton product that formed in the activation of methanol
remained in the model system, bound to an adjacent water molecule, forming a hydro-
nium ion. The completion of the electrochemical step requires the transport of the ion
across the electrochemical double layer, which, together with electron conduction
across the electrochemical cell, maintains the constant potential drop across the inter-
face. In the case of methanol oxidation, these processes are relatively rapid with
respect to the breaking of C22H and O22H bonds. However, in the case of oxygen
reduction, both experiment [Damjanovic and Brusic, 1967; Damjanovic and Sepa,
1990; Grgur et al., 1997] and theory [Anderson and Albu, 1999, 2000; Anderson
et al., 2005] have led to speculation that the initial reduction of adsorbed molecular
oxygen is the rate-limiting step. This reduction process is used as an initial example
of the application of the double-reference method to examine the kinetics of elemen-
tary redox reactions. It should be noted that our work does not address whether
electron transfer during the adsorption of O 2 itself should be referred to as the first
reduction step; rather, we adopt the terminology of Anderson in referring to the
electron transfer coinciding with the formation of OOH as the first reduction step.
Adsorption of O 2 occurs, with a net transfer of electron density from the electrode
surface to the adsorbate, and this extent of electron transfer, as discussed above
and by others [Hyman and Medlin, 2005; Panchenko et al., 2004], varies with the
electrode potential.