Mechanism of Cs Precursor Effect on Cs-Ag/Al2O3: Tuning Oxygen Activation for Ethylene Epoxidation

Abstract

Cs is considered as an effective promoter in ethylene epoxidation to ethylene oxide (EO). However, the influence of different Cs precursors on the geometric and chemical properties affecting EO production remains unclear. In this work, Cs-Ag/Al₂O₃ catalysts were synthesized using three types of Cs precursors (CsNO₃, Cs₂CO₃, and CsOH) to investigate their effects on Ag particle size and the generation of active oxygen species. Catalytic tests demonstrate that the Cs-Ag/Al₂O₃ catalyst prepared with Cs₂CO₃ exhibits the highest EO selectivity and activity among all Cs-promoted and pure Ag/Al₂O₃ catalysts. Structural characterization reveals that Cs addition reduces Ag particle size, which increases the concentration of active oxygen species on Ag surface and enhances EO production. However, among the Cs-promoted catalysts, Ag particle size shows minimal influence on EO performance. H₂-TPR and O₂-TPD analyses indicate that Cs₂CO₃ promotes the formation of low-temperature O₂ adsorption species on Ag and weakens the Ag-O bond relative to other Cs precursors. In situ Raman spectroscopy corroborates the highest intensity of bands associated with peroxide (O₂^2-) adsorption on Ag surfaces, corresponding to the superior EO yield of the Cs₂CO₃-promoted catalyst. These results demonstrate that the nature of the Cs precursor modulates the Cs-Ag interaction, decreases Ag-O bond strength, and promotes active O₂ adsorption and activation, thereby enhancing EO selectivity.