Roles of Proton and Electric Field in the Electroreduction of O_2 on Pt(111) Surfaces: Results of an Ab-Initio Molecular Dynamics Study

摘要

The electroreduction of O_2 on the Pt(111) surface is studied with Car-Parrinello molecular dynamics simulations of O_2/Pt(111), O_2 + H~+(H_2O)_3/Pt(111), and O_2 + H~+(H_2O)_3 + e~-/ Pt(111). Starting from a parallel configuration where O_2 is 3.0 A over the Pt(111) surface along a bridge site, stepwise adsorptions of two oxygen atoms are observed, leading to a one-end chemisorption precursor (Pauling model of adsorption). The formation of the precursor has a very low barrier (0.08 eV), whereas no clear barrier was found for its decomposition. Addition of H~+(H_2O)_3 induces rapid formation of the proton-transfer intermediate H~+-O_2···Pt(111), followed by an electron transfer from the Pt slab that yields a chemisorbed precursor H-O-O-Ptn, in which the hydroxyl end is considerably more separated than the oxygen end from the Pt(111) surface. Compared with the O_2 + H~+(H_2O)_3/Pt(111) system, the presence of an electron in O_2 + H~+(H_2O)_3 + e~-/ Pt(111) greatly enhances the lifetime of the proton-transfer intermediate H~+-O-O···Pt(111), and consequently it delays the subsequent electron transfer, which yields the formation of the one-end chemisorbed precursor H-O-O-Pt_n. Evolutions of the Kohn-Sham energy for the last two cases show that the formation of H-O-O-Pt_n has a much higher activation barrier than its dissociation (0.4 vs 0.1 eV), indicating that the formation of chemisorbed H-O_2 species is the rate-determining step for the first electron transfer of the electroreduction of O_2.