Insights into Cl effect for propylene epoxidation over Ag2O(111) surface: A periodic density functional theory study

Abstract

As an environmentally friendly way, propylene epoxidation forming propylene oxide (PO) catalyzed by Ag-based catalyst had received considerable attentions, which was important in the chemical industry. The experimental results exhibited that the products of propylene epoxidation catalyzed by Ag₂O were PO and carbon dioxide. In this work, the spin-polarized density functional theory (DFT) calculations combined with a Hubbard U correction were performed to investigate propylene epoxidation on Ag₂O(111) and Cl−Ag₂O(111) surfaces, and reaction micro-mechanism of propylene epoxidation was discussed in detail. The micro-mechanism mainly included two pathways: the allylic hydrogen stripping (AHS) pathway and the intermediary propylene oxametallacycles (OMMP) pathway. In the AHS pathway, the allyl radical can be generated, which was considered as a precursor for acrolein formation, and completed combustion yielding CO₂. In the OMMP pathway, PO, propanal and acetone can be created through the propylene oxametallacycle intermediates. Our calculated results indicated that the O_suf site on the Ag₂O(111) surface has a stronger basicity than the O_suf site on the Cl−Ag₂O(111) surface, the stronger basicity was beneficial for the AHS pathway, and carbon dioxide can be regarded as the main product for propylene epoxidation. It was also found that PO became the main product with the effect of Cl doping on the Ag₂O(111) surface, and the electrostatic effect of Cl−Ag_cus can improve the adsorption ability between the Ag_cus site and the absorbate. Moreover, energetic span model analysis were carried out and found that the TOF or the orders of selectivity are: acrolein > acetone > propanal ≅ PO on clean surface, PO > acetone > acrolein > propanal on the Cl doped surface, and acrolein, as a precursor, was easily completely burned to CO₂, the results confirmed that the selectivity of PO can be enhanced by the effect of subsurface Cl^- doping. The present study aimed to help workers to find high selectivity and activity catalyst for propylene epoxidation.