Investigation of the oxygen coverage of propylene epoxidation on Ag(111) surfaces from DFT

The selectivity of PO decreased with the increase of oxygen coverage, which was mainly due to the influence of the primary reaction while the secondary reaction had little influence. (defined ΔE a(primary) = E a(AHS) - E a(OMMP) , ΔE a(secondary) = E a(AC) - E a(PO) ) • the stepwise absorption energy is adopted to investigate the saturated coverage of oxygen on Ag(111). • the elementary reaction of two competing paths for propylene epoxidation at different oxygen coverage is studied. • the potential energy of key species involved in elementary reaction is obtained. • The selectivity of PO formation is analyzed based on the competitive results of the two reaction pathways. The direct epoxidation of propylene on Ag(111) under different oxygen coverage conditions was systemically studied on the basis of density functional theory calculation and the general trends of oxygen coverage on the two parallel competitive reaction paths in propylene oxide (PO) formation: α -H stripping versus epoxidation and the generation of PO versus acetone (AC) were explored. Then, the PO formation selectivity was investigated. Results showed that increase in oxygen coverage enhanced the adsorption strength of adsorbed species, except the O* species. In the primary reaction of propylene epoxidation, high oxygen coverage facilitates the first α -H stripping process, and opposite trend was observed in epoxidation process. As for the secondary chemistry in propylene epoxidation, increase in oxygen coverage benefits PO and AC formation. The coverage of O* has considerable influences on the primary reaction but small influence on the secondary reaction. Although a high oxygen coverage benefits the first α -H stripping, the second α -H stripping has higher barrier than that in secondary chemistry, and thus the PO formation would be better than that of acrolein formation at higher oxygen coverage.