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+(H2O)(3)/Pt(111), and O-2 + H+(H2O)(3) + e(-)/Pt(111). Starting from a parallel configuration where O-2 is 3.0 Angstrom 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+(H2O)(3) induces rapid formation of the protein-transfer intermediate H+-O2 (. . .) 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+(H2O)(3)/Pt(111) system, the presence of an electron in O-2 + H+(H2O)(3) + e(-)/ Pt(111) greatly enhances the lifetime of the proton-transfer intermediate H+-O-(OPt)-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.