Gas Phase Selective Propylene Epoxidation over La2O3-Supported Cubic Silver Nanoparticles
Herein, we present a systematic study on geometric and electronic effects exhibited by exposed Ag faceted nanoparticles and supports in the direct catalytic propylene epoxidation from O2 and propylene. Ag nanoparticles were first synthesised with well-defined morphologies (nanocubes or nanospheres) before they were placed on various supports, including conventional Al2O3 and a series of lanthanide oxides (Ln2O3). Results showed that faceted Ag nanoparticles and the oxide supports on their own are unable to deliver descent propylene oxide (PO) selectivity. However, their interfaces such as Ag nanocubes/La2O3 can dramatically enhance PO conversion (11.6%) and selectivity (51%) under dilute gas stream (3.33% C3H6 : 1.67% O2 : 95% He) at atmospheric pressure and 270 oC temperature. It is envisaged that exposed (001) polar face on La2O3 with a higher density of oxygen vacancies facilitates dioxygen dissociation and the dissociated active oxygen is transferred to Ag(100) surface at the interface where longer Ag-Ag interatomic distance prevents the removal of γ-H of propylene towards combustion products, thus accounting for the higher PO selectivity. These findings may lead to the design of new supported silver catalysts for environmentally-benign propylene oxide production.