(258f) Effects of the Types and Coverages of Reactive Oxygen on the Catalytic Fate of Propylene during Epoxidation on Silver Clusters

Catalytic epoxidation of propene with di-atomic oxygen is the desired process for propylene oxide production, because this step utilizes oxygen to generate reactive oxygen species that insert into the C=C bond of propylene to form propylene oxide. The yields of this process, in contrast to using ethylene as the feedstock, are low and currently not viable for commercialization. Here, we report the mechanistic pathways that lead to the formation of C3H6O and COx during propylene epoxidation on Ag clusters. We address the catalytic roles of the reactive oxygen species, as reactive chemisorbed oxygen atoms or hydroperoxyl species, in directing the reaction pathways for C3H6O, CO, and CO2 formation, based on rate measurements in the kinetically controlled regime and active site titrations with O2 and C3H6. The reactive collision frequencies for propylene oxide formation increase proportionally with chemisorbed oxygen atoms. In contrast, the reactive collision frequencies for COx formation are insensitive to C3H6 pressure but increase by five-fold at 443 K as the oxygen coverages vary within a narrow range. This kinetic dependence may indicate that hydroperoxyl species, generated only at higher O2 pressures, from reactions of H2O and O2 on Ag clusters mostly covered with reactive oxygen species, are the reactive intermediates for the undesired combustion reactions. We confirm the catalytic role of H2O in generating the OOH* species by incorporating H2O into the C3H6-O2 feed mixtures. H2O addition leads to larger OOH* concentrations that promote the C3H6 combustion while reduces the number of reactive O* centers available for the desired epoxidation reactions. In summary, we show that the relative amounts of O*, O2* and OOH* species play a defining role on the catalytic fate of propylene during epoxidation reactions.