Reactions of Surface Peroxides Contribute to Rates and Selectivities for C₂H₄ Epoxidation on Silver

Abstract

Partial oxidation of ethylene over silver catalysts produces more than 30 million metric tons of ethylene oxide (EO) annually. However, the form of active silver surfaces, reactive oxygen species, and dominant pathways of this chemical reaction remains controversial despite decades of research. Here, we use <i>operando</i> Raman spectroscopy and transient kinetic measurements to demonstrate that higher coverages of peroxide species, present only upon Ag oxide surfaces that form <i>in situ</i>, correlate with greater selectivities to EO. <i>Ab initio</i> calculations reveal that the reconstructed Ag oxides preferentially stabilize diatomic oxygen species (peroxide and superoxide) under relevant conditions, and these species contribute to the selective formation of EO. The dominant reaction pathways change with surface coverages; however, bound O₂ consistently activates by reaction with C₂H₄, and products form subsequently through peroxo- and oxometallacycle surface complexes. Taken together, density functional theory calculations and kinetic and transient experimental measurements show that the formation of peroxide intermediates requires oxidation of the Ag surface (via subsurface oxygen), and an increase in surface peroxides coincides with the highest EO selectivity for the unpromoted Ag catalyst. These findings suggest that the promoters ubiquitous for ethylene epoxidation (e.g., chlorine, transition metals, and alkali metals) may succeed by oxidation of Ag and increasing coverages of peroxides at industrial conditions.