Roles of Proton and Electric Field in the Electroreduction of O₂ on Pt(111) Surfaces:  Results of an Ab-Initio Molecular Dynamics Study

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 2 O) 3 /Pt(111), and O 2 + H + (H 2 O) 3 + e - / Pt(111). Starting from a parallel configuration where O 2 is 3.0 Å 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 2 O) 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−Pt n, 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 2 O) 3 /Pt(111) system, the presence of an electron in O 2 + H + (H 2 O) 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 .