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 O2 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 O2 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 O2 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 Co0, 0.8 V for CoII in the form of Co(OH)2, 0.7 V for CoII in the form H2OCo(OH)2, and 0.7 V for CoIII 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 O2 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+.