Comparative Study of Oxygen Reduction Reaction Mechanisms on the Pd(111) and Pt(111) Surfaces in Acid Medium by DFT

DFT geometry optimization and minimum energy path calculations were used to investigate the mechanisms of oxygen reduction reaction (ORR) on the Pd(111) and Pt(111) surfaces, including the adsorption and dissociation of O 2 molecule and the protonation of dissociated adsorbates. The results indicated that in the presence of a hydrated proton ORR on the Pd(111) surface proceeded through the adsorption and dissociation of O 2 molecule, whereas ORR on the Pt(111) surface may involve in parallel the adsorption and dissociation of O 2 molecule as well as the formation and dissociation of OOH species. During the entire four-electron ORR, the protonation of adsorbed O atom to form OH is the rate-determining step (rds) on both of the Pd(111) and Pt(111) surfaces. Such a finding about the rds of ORR can well explain why Pt- and Pd-based catalysts that more weakly bind atomic oxygen have better ORR activity. Comparison of the ORR mechanisms on the Pd(111) and Pt(111) surfaces revealed that the adsorption and dissociation processes of O 2 molecule more easily occurred on the Pt(111) surface and that the serial protonation of the dissociative product to form H 2 O molecule also more easily occurred on the Pt(111) surface than on the Pd(111) surface. Therefore, the difference between the catalytic activities for ORR between both metals was large, explaining why Pt can serve as ORR electrocatalysts and the inexpensive Pd cannot.