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

用密度泛函方法和模拟Ag(111)表面的Agn团簇(n=3~10)研究了乙烯在Ag表面的环氧化反应机理。O_2在银团簇上的吸附能强烈依赖于团簇的HOMO能级，HOMO能级越高，O_2吸附能越大。对分子氧和原子氧的环氧化反应机理进行了能量学研究。对于原子氧机理，没有活化能的环氧化反应进行，而对于分子氧机理，只计算了少量的活化能(约5千卡/摩尔)。由于活化能不是很大，这个反应似乎是在常温以上的反应条件下进行的。不考虑分子氧机理，我们的计算表明，环氧化反应可以通过原子氧机理进行。如果有效地抑制副反应，选择性高于6/7是可能的。

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.