Cluster Size Controls Branching between Water and Hydrogen Peroxide Production in Electrochemical Oxygen Reduction at Pt n /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 HClO 4 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 ∼0.66 V vs NHE for Pt 1 /ITO 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 H 2 O 2 was produced, but as cluster size was decreased, the H 2 O 2 branching became large, suggesting that small Pt clusters could be useful selective catalysts for H 2 O 2 electrosynthesis. Because there was no obvious correlation of ORR activity with Pt n electronic properties, as probed by photoemission, the effect of size on branching is tentatively attributed to size of the available oxygen binding sites.