Theoretical Study of Metal and Metal-oxide Catalysts by Molecular Orbital Method

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The mechanism of ethylene epoxidation on Ag surfaces has been investigated using the density functional method and Ag_n clusters (n=3 to 10) modeling the Ag(111) surface. Adsorption energy of 0_2 to the Ag clusters was strongly dependent on the HOMO level of the cluster, and the clusters with higher HOMO level afforded larger 0_2 adsorption energies. The energetics was investigated for both the molecular and atomic oxygen epoxidation mechanisms. For the atomic oxygen mechanism, the epoxidation was found to proceed ithout the activation energy, whereas a small amount of activation energy (ca 5 kcal/mol) was calculated for the molecular oxygen mechanism. Since the activation energy is not so large, this reaction seems to occur under usual reaction conditions above the room temperatures. Irrespective of the molecular oxygen mechanism, our calculation showed that the epoxidation could proceed through the atomic oxygen mechanism. The selectivity higher than 6/7 is possible if side reactions are supressed effectively.