Size effects in ethylene oxidation on silver catalysts. Influence of support and Cs promoter Original Research Article

A new method for preparing supported silver catalysts with a narrow particle size distribution has been developed. For crystals with a diameter ranging from 100 to 1000 Å the dependencies of reaction rates on size have been studied for epoxidation and deep oxidation of ethylene. For particles 300–500 Å in size the reaction rate was found to increase sharply. The concentration of ethylene π-complexes with Ag+ cations increased proportionally to the reaction rate. These π-complexes are assumed to be the only intermediates directing the reaction towards epoxidation. The ratio of the reaction rate to the concentration of π-complexes was found to remain constant within a wide range of Ag crystal sizes including bulk silver samples. The influence of the support (α-Al2O3, SiO2) on the size effect relates to the secondary processes of ethylene oxide conversion. The factors that cause the catalytic size effect and the promoting effect of Cs are discussed.