Revisit the Role of Metal Dopants in Enhancing the Selectivity of Ag-Catalyzed Ethylene Epoxidation: Optimizing Oxophilicity of Reaction Site via Cocatalytic Mechanism

Abstract

The pristine transition metals (TMs) offer limited choices for high-performance heterogeneous catalysts because they are usually restricted in less optimal regions on the reactivity volcano map. Using ethylene epoxidation as a probe reaction, it is found that the reaction pathway selection is determined by the binding affinity of catalytic sites toward oxygen and carbon. By introducing extrinsic metal dopants to engineer the oxygen affinity of reactive sites of host Ag catalyst, the optimal (peak) region on the selectivity volcano map can be reached after optimizing the adsorption energies of atomic oxygen. The oxidation formation enthalpy (ΔHmetal-oxide) is employed as a descriptor to screen metal dopants, and the promising metal elements are identified as either single or dual dopants to achieve higher catalytic selectivity than pristine Ag catalysts, validated by the available experimental data. Our investigation demonstrated that, in addition to the traditional promoter mechanism, metal dopants also realize catalytic selectivity improvement of Ag-catalyzed ethylene epoxidation through a cocatalytic mechanism. By acting as cocatalytic sites to participate in the reaction, metal dopants modify the element affinity of the reaction center, such as oxophilicity, and disproportionally alter the adsorption strength of the key intermediates to enhance the reactivity of pristine TM catalysts, which may be applied in heterogeneous reactions beyond ethylene epoxidation.