Catalytic consequences of reactive oxygen species during C3H6 oxidation on Ag clusters

Abstract

Rate measurements in the kinetically controlled regime, kinetic fittings, and isothermal C3H6 and O2 uptake experiments lead to a proposed mechanism for C3H6 epoxidation and combustion reactions on predominantly O∗ covered Ag cluster surfaces. Epoxidation occurs via kinetically relevant reactions between chemisorbed oxygen adatoms (O∗) and gas phase C3H6. In contrast, combustion occurs via kinetically relevant hydroperoxyl (OOH∗) formation, formed from H transfer from H2O∗ to O2∗. C3H6 oxidation reactions with C3H6-O2-H2O-H2O2 mixtures show that H2O2 derived OOH∗ species are more effective oxidants than O∗ for C3H6 combustion, as confirmed from H2O2/D2O2 kinetic isotope effects. Both C3H6∗ and O∗ coverages are near or at chemical equilibrium during steady-state reactions, as confirmed from in-situ chemical titrations. C3H6O and COx site-time-yields and COx selectivities increase with increasing Ag cluster diameters and Ag surface coordination, because O∗, O2∗, H2O∗, and OOH∗ oxidants remain more weakly bound and therefore much more reactive.