Effect of Cu/Au for propylene epoxidation over the Ag₂O(111) surface: a DFT study

Catalysts belonging to the IB group for propylene epoxidation have garnered significant attention and have been extensively developed in the chemical industry. In this study, spin-polarized density functional theory (DFT) calculations combined with a U correction were performed to study propylene epoxidation on the Ag₂O(111) surface with and without Cu or Au doping. Our calculations revealed a dual propylene epoxidation mechanism: the allylic hydrogen stripping (AHS) route and the intermediary propylene oxametallacycle (OMMP) route. The doped Cu or Au sites on the Ag₂O(111) surface exhibited superior adsorbate activity, which also influenced the activity of adjacent surface O_suf sites. On the Cu-Ag₂O(111) surface, the O_suf site exhibited the lowest basicity, favoring the OMMP route. Conversely, on the Au-Ag₂O(111) surface, the O_suf site had relatively stronger basicity, which favored the AHS route. Furthermore, energetic span model analysis was carried out and showed that product selectivity followed different patterns depending on the doping: acrolein > acetone > propanal ≅ PO on the pure surface, acrolein > PO > propanal ≅ acetone on the Cu-doped surface, and acrolein > acetone > propanal > PO on the Au-doped surface. Notably, acrolein was prone to complete combustion to carbon dioxide, which became the primary product on both doped and undoped Ag₂O(111) surfaces. While the selectivity of PO can be enhanced slightly by the doping of Cu, unfortunately, the selectivity of PO can be reduced by the doping of Au. This study aims to provide insights into the nature of IB group catalysts for propylene epoxidation, which mainly regulate the lower basicity of lattice oxygen through doping promoters and optimize the industrial yield of PO.