Theoretical predictions concerning oxygen reduction on nitrided graphite edges and a cobalt center bonded to them

Density functional theory and a linear Gibbs free energy relationship are employed in a theoretical investigation of catalytic properties of cobalt-graphite-nitride systems for O-2 reduction to hydrogen peroxide and water. Nitrided graphite edges, with N atoms substituting one or two CH groups, are modeled to establish some of the effects of N on edges with and without Co added. The calculations show that a bare graphite edge with one N atom, which the calculations indicate is not hydrogenated at potentials greater than 0.3 V, is not active for O-2 reduction because OOH bonds too weakly. At potentials lower than 0.3 V, for which N is hydrogenated, making it a radical center, the NH edge is not active for O-2 reduction because OOH bonds too strongly, resulting in a high overpotential for its reduction to H2O2 on this site. Over a Co site bridging two N substituting for CH on an edge, the onset formation potential for OOH(ads) is about 0.4 V for Co-0, 0.8 V for Co-II in the form of Co(OH)(2), 0.7 V for Co-II in the form H2OCo(OH)(2), and 0.7 V for Co-III as Co(OH)(3). Later steps have higher predicted reversible potentials. A water molecule bonds to each of the Co centers but most weakly in the case of H2OCo(OH)(2), which means this cobalt center is least likely to be blocked against O-2 interaction with it. All of the cobalt complexes are predicted to bond weakly, 1.5 eV and less, to the graphite edge N atoms, which means that the catalyst is not expected to be stable due to cobalt dissolution as soluble Co2+.