Oxygen Reduction Reaction on Cobalt-(6)Pyrrole Cluster: Density Functional Theory Study

We investigate the potential energy surface profile for various water formation reaction schemes on an unsupported cobalt-(6) pyrrole [Co-(6)Ppy] cluster in the vacuum state by density functional theory (DFT) calculations. We find that in the Co-(6) Ppy cluster, the formation of H2O2 is energetically not favorable. Instead of forming H2O2ad, the HO2ad + H reaction forms 2OH(ad) or O-ad + H2O immediately. The adsorption of H2O2 on the Co-(6) Ppy cluster is possible only if the H2O2 molecule comes from or forms outside of the cluster. The formation of two OH molecules instead of H2O2 on the Co-(6) Ppy cluster suggests that the oxygen reduction reaction (ORR) mechanism on the unsupported Co-(6) Ppy cluster in the vacuum state prefers the direct four-electron reduction to water.