  We report here results of our density functional theory based computational
studies of the electronic structure of the Pd-Co alloy electrocatalysts and
energetics of the oxygen reduction reaction (ORR) on their surfaces. The
calculations have been performed for the (111) surfaces of pure Pd,
Pd0.75Co0.25 and Pd0.5Co0.5 alloys, as well as of the surface segregated
Pd/Pd0.75Co0.25 alloy. We find the hybridization of dPd and dCo electronic
states to be the main factor controlling the electrocatalytic properties of
Pd/Pd0.75Co0.25. Namely the dPd - dCo hybridization causes low energy shift of
the surface Pd d-band with respect to that for Pd(111). This shift weakens
chemical bonds between the ORR intermediates and the Pd/Pd0.75Co0.25 surface,
which is favorable for the reaction. Non-segregated Pd0.75Co0.25 and Pd0.5Co0.5
surfaces are found to be too reactive for ORR due to bonding of the
intermediates to the surface Co atoms. Analysis of the ORR free energy
diagrams, built for the Pd and Pd/Pd0.75Co0.25, shows that the co-adsorption of
the ORR intermediates and water changes the ORR energetics significantly and
makes ORR more favorable. We find the onset ORR potential estimated for the
configurations with the O - OH and OH - OH co-adsorption to be in very good
agreement with experiment. The relevance of this finding to the real reaction
environment is discussed.
