| The role of support in selective oxidation of ethylene over supported silver catalysts has been investigated by studying the effects of silver crystallite size on the catalytic activity of silver supported on various oxides with low to high BET surface area in ethylene oxidation reactions, and by measuring the catalytic activity of pure and modified supports in ethylene oxide isomerization/oxidation reactions. The specific activity as well as selectivity of the supported silver catalysts have been found to be strong functions of the silver crystallite size. The size effects observed are different depending on the supports used. The different effects have been explained in terms of surface oxide formation, distribution of various crystallographic planes, and metal-support interaction. Except for (alpha)-Al(,2)O(,3), all the supports used in the preparation of supported silver catalysts in this work have been found to catalyze the isomerization and oxidation of ethylene oxide. The non-selective nature of silver supported on these supports is mainly due to the further reaction of ethylene oxide to acetaldehyde and carbon dioxide on the support surfaces. Alkali impregnation of the support effectively suppressed ethylene oxide isomerization. Oxidation of ethylene oxide, however, increased initially with alkali impregnation but decreased with increase in alkali concentration. The concept of acid-base bifunctional catalysis has been invoked to explain these results. Alkali impregnation of supported silver catalysts was not found to be beneficial to ethylene epoxidation due to alkali poisoning of active silver sites. |
