Oxygen Reduction on Pd0.75Co0.25 (111) and Pt0.75Co0.25 (111) Surfaces:  An ab Initio Comparative Study

Density functional theory studies of adsorption of oxygen electroreduction intermediates and free energy profiles are used to discuss possible reaction mechanisms:  one leading directly to H2O production and another having H2O2 as an intermediate, on (111) surfaces of pure Pt, pure Pd, and bimetallic systems Pd0.75Co0.25 and Pt0.75Co0.25. It is found that the calculated affinities toward the different ORR intermediates in the studied surfaces follow the Hammer-Norskov d-band model predictions. The calculated free energy profiles and the magnitude of the barriers in both mechanisms seem to favor the hypothesis that both the direct and series O2 reduction mechanisms might be operating in parallel. The highest thermodynamic barriers at (1)/4 of a monolayer atomic oxygen coverage and without solvent are located in the first hydrogenation reaction for both mechanisms.