New Mechanism for the Enhancement of the Oxygen Reduction Reaction on Stepped Platinum and Platinum–Iron Surfaces

It has long been recognized that the oxygen reduction reaction occurs more readily on Pt(111) surfaces that include steps, both (111) and (100), than on near-perfect Pt(111). Theoretical models were developed involving the water structure in the electric double layer and its interactions with adsorbed OH, with the actual O2 reduction occurring on the (111) terraces adjacent to the steps. However, the present density functional theory (DFT) calculations confirms that O2 adsorbs strongly at the steps and can undergo dissociation aided by adjacent water molecules to produce adsorbed OH. OH produced at the steps can move to the (111) terraces, where it can be more readily reduced to H2O and desorbed. This model avoids the scaling relation, which predicts that all oxygen-containing reactants and intermediates are proportional to each other on any given surface, i.e., strong O2 adsorption at steps compared with water ensures that the reaction can proceed. Efforts to develop new O2 reduction catalysts have been hampered by the assumption that the reaction rate can be increased by decreasing OH adsorption strength, even though decreased OH adsorption strength is accompanied by decreased O2 adsorption strength on any given crystallographic facet. This proposed model can explain the experimental results on stepped surfaces as well as nanoparticle catalysts, particularly the higher ORR activity on alloys such as PtFe, but with the obligatory presence of steps. The results may also be important for the development of Pt nanoparticle catalysts.