Gas phase selective propylene epoxidation over La2O3-supported cubic silver nanoparticles

Abstract

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 decent 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 °C temperature. It is envisaged that the exposed (001) polar face on La₂O₃ with a higher density of oxygen vacancies facilitates dioxygen dissociation and the dissociated active oxygen is transferred to the Ag(100) surface at the interface where longer Ag–Ag interatomic distances prevent the removal of γ-H of propylene towards combustion, thus accounting for higher PO selectivity. These findings may lead to the design of new supported silver catalysts for environmentally-benign propylene oxide production.