Cluster Size Controls Branching between Water and Hydrogen Peroxide Production in Electrochemical Oxygen Reduction at Ptn/ITO

Deposition of size-selected Pt-n clusters on indium tin oxide (ITO) films in ultrahigh vacuum was used to create electrodes with catalytic sites of controlled size. We report a study of the oxygen reduction reaction (ORR) in 0.1 M HClO4 at size-selected Pt-n/ITO electrodes that were prepared and characterized without exposure to laboratory air. It was found that the ORR onset potential was size-dependent, varying from similar to 0.66 V vs NHE for Pt-1/ITO to similar to 0.78 V vs NHE for Pt-n(n >= 10). The maximum ORR currents per gram of Pt were found to be about an order of magnitude higher than that for ITO with 5 nm Pt particles. The branching ratio between the production of water and hydrogen peroxide in ORR was found to be strongly size-dependent. For 5 nm Pt particles on ITO or for polycrystalline Pt, little H2O2 was produced, but as cluster size was decreased, the H2O2 branching became large, suggesting that small Pt clusters could be useful selective catalysts for H2O2 electrosynthesis. Because there was no obvious correlation of ORR activity with Pt, electronic properties, as probed by photoemission, the effect of size on branching is tentatively attributed to size of the available oxygen binding sites.