First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 112

the Pt(111) surface indicate that lower electric fields (corresponding to more negative
potentials on the NHE scale) cause stronger binding of O 2 through increased donation
of electron density to the 2p orbital. The importance of back-donation in oxygen
binding may lead to a noticeable difference in binding energy relative to water as
the electrode potential is varied.
Figure 4.10 plots the potential-dependent displacement energy associated with
replacing an adsorbed water molecule with an oxygen molecule on the Pt(111) sur-
face. There is a 1.5 eV preference to bind O 2 compared with H 2 Oat0V/NHE.
This value exceeds the UHV binding energy preference (about 0.25 eV) and is the
result of the difference in solvation energy between O 2 and H 2 O. However, as the
potential is increased, water binding becomes relatively stronger, until, at a potential
of 1.36 V/NHE, it becomes favorable to adsorb water rather than molecular
oxygen. This potential, of course, is well past that of interest for the ORR, and well
past the potential at which hydroxyl species would replace water molecules at the sur-
face, changing the species with which oxygen must compete. In addition, the results
here describe only the overall thermodynamic preference. In an aqueous medium,
there may be an activation barrier for oxygen to actually displace water from the sur-
face. The adsorption of O 2 includes electron transfer, to a varying extent depending
on electrode potential, for which Hartnig and Koper suggest, based on molecular
dynamics simulations, that solvent reorganization can have a dominant contribution
to the overall rate [Hartnig and Koper, 2002].
The potential of relevance for competitive adsorption of water may shift upon
alloying Pt. The calculated ratio of water to oxygen binding energies at the UHV
Figure 4.10 Energy of displacing water with molecular oxygen at the Pt(111) surface as a
function of electrode potential. At potentials below 1.36 V, it is favorable to adsorb molecular
oxygen, while at potentials above 1.36 V, water is more stable on the surface.