Bifunctional Property of Size-Selected Platinum Cluster Bound to Silicon Substrate in Low-Temperature Electron-Donation Catalysis for CO Oxidation

Electron-donation catalysis of size-selected platinum clusters bound to a silicon substrate surface, Pt N /Si (N = 20 and 30; cluster size), and its cluster-size dependence were investigated by measuring a turnover rate of CO oxidation in combination with bistability kinetic analysis. Pt N /Si were prepared by impact of size-selected Pt N + onto the substrate; cluster-impact deposition. The turnover rate from O2 + CO to CO2 proceeding on Pt N /Si in a gas-mixture flow of O2 and CO was measured by means of mass analysis as a function of the temperature and the partial pressures of O2 and CO. Hysteresis was observed: The turnover rate is different in heating and cooling processes even at the same temperature and the same partial pressures. This feature was explained by bistability of surface adsorption states of O and CO on Pt N /Si. In comparison with disappearance of the bistability for small nanoparticles supported on metal oxides, it has been concluded that the bistability of Pt N /Si originates from efficient dissociative activation of O2 and rich O adsorption on Pt N /Si. The bistability window of Pt30/Si with respect to the CO fraction in the reactant gas mixture is wider than that of Pt20/Si. This size dependence leads to importance of an appropriate ratio between the reactor sizes for the bifunctional property: the electron-accumulating periphery of Pt N /Si for the O2 activation and the center zone for the Langmuir–Hinshelwood reaction between O and CO.