Oxidation mechanism of propylene on an Ag surface: Dipped adcluster model study

The mechanisms of the epoxidation and complete oxidation of propylene over a silver surface are studied by the dipped adcluster model (DAM) combined with the ab initio HF and MP2 methods. Several postulated reaction pathways involving both molecularly and atomically adsorbed oxygens are investigated. The energy diagrams of the epoxidation and complete oxidation of propylene on the silver surface are presented, and the detailed electronic mechanisms for the oxidation of propylene are clarified. The calculations show that the mechanism involving the single precursor intermediate common to both epoxidation and complete oxidation proposed in some literatures can be ruled out. Instead, the two competitive mechanisms are suggested: the epoxide formation is initiated by the reaction of the olefinic carbon with the adsorbed oxygen, while the combustion is initiated by the abstraction of the allylic hydrogen by the adsorbed oxygen. The former mechanism is essentially the same as that in the epoxidation of ethylene reported previously. The latter mechanism is energetically more favorable for propylene, which is a reason for the low selectivity of the epoxidation of propylene. The adsorbed allyl species, which is an intermediate in the latter mechanism, is demonstrated to exist in the form of chemisorbed anion. The mechanisms presented in this paper seem to be useful for explaining the epoxidation and complete oxidation of other olefins. A comparison with the previous studies is also presented.