Effect of Rhenium Loading Sequence on Selectivity of Ag–Cs Catalyst for Ethylene Epoxidation

Abstract

Rhenium is an important promotor for silver catalyst, and very little work has been focused on the effect of rhenium loading sequence on the selectivity of ethylene oxide. Re was introduced to the Ag–Cs catalyst supported on α-Al2O3 in three different ways: co-impregnation (Ag–Cs–Re), step-impregnation with Re loaded first (Re/Ag–Cs), and step-impregnation with Re loaded last (Ag–Cs/Re). The selectivity of Re/Ag–Cs and Ag–Cs–Re are about 20% higher than that of Ag–Cs/Re, which indicates that the performance of the catalyst is influenced by the synergistic effect of Re and Cs. Rhenium oxides with the valence of + 4 to + 7 are distributed around the Ag particles. The binding energy of Re is increased by about 0.2 eV by the addition of Cs in the Ag–Cs–Re catalyst, and the presence of Re with high valence around Ag will cause the electrons to shift toward the Re atom, facilitating the formation of weakly adsorbed oxygen and improving the selectivity of the Ag–Cs catalyst. On the Ag–Cs/Re catalyst, Re is mostly loaded on the external Ag surface, and the binding energies of Ag 3d5/2 and Cs 3d5/2 increase by 0.2 and 0.4 eV, respectively, from the Ag–Cs/Re catalyst to the Ag–Cs–Re and Re/Ag–Cs catalysts. The higher binding energy of Ag leads to the stronger oxygen adsorption, which contributes to the complete oxidation reaction, thereby resulting in lower selectivity.