An important issue regarding the mechanism of heterogeneous epoxidation of ethylene on silver (Ag) is related to the formation of CÀO bonds. [1][2][3] There are two possible ways for the CÀO bonds to form: It is conceivable that the dominant pathway would proceed through either a concerted addition of surface oxygen to the C = C bond of ethylene or by sequential formation of the two CÀO bonds by a creation of an asymmetric surface intermediate (Scheme 1).It has been shown previously in temperature-programmed desorption (TPD) experiments that a surface intermediate formed on Ag(111) upon adsorption of ethylene oxide (EO) at about 250 K under ultrahigh vacuum (UHV) conditions reacts at about 310 K to reform EO plus small amounts of other products, including ethylene.[4] High-resolution electron energy loss spectroscopy (HREELS) was used to identify this surface intermediate as the asymmetric structure shown in Scheme 1, namely a surface oxametallacycle.[4] This work Scheme 1. Possible intermediates in ethylene epoxidation on Ag: a) in the concerted addition of ethylene to oxygen and b) in the sequential formation of CÀO bonds (surface oxametallacycle).
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