Reactions of Surface\nPeroxides Contribute to Rates\nand Selectivities for C<sub>2</sub>H<sub>4</sub> Epoxidation on Silver

Partial oxidation\nof ethylene over silver catalysts produces more\nthan 30 million metric tons of ethylene oxide (EO) annually. However,\nthe form of active silver surfaces, reactive oxygen species, and dominant\npathways of this chemical reaction remains controversial despite decades\nof research. Here, we use <i>operando</i> Raman spectroscopy\nand transient kinetic measurements to demonstrate that higher coverages\nof 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\noxides preferentially stabilize diatomic oxygen species (peroxide\nand superoxide) under relevant conditions, and these species contribute\nto the selective formation of EO. The dominant reaction pathways change\nwith surface coverages; however, bound O<sub>2</sub> consistently\nactivates by reaction with C<sub>2</sub>H<sub>4</sub>, and products\nform subsequently through peroxo- and oxometallacycle surface complexes.\nTaken together, density functional theory calculations and kinetic\nand transient experimental measurements show that the formation of\nperoxide intermediates requires oxidation of the Ag surface (via subsurface\noxygen), and an increase in surface peroxides coincides with the highest\nEO selectivity for the unpromoted Ag catalyst. These findings suggest\nthat the promoters ubiquitous for ethylene epoxidation (e.g., chlorine,\ntransition metals, and alkali metals) may succeed by oxidation of\nAg and increasing coverages of peroxides at industrial conditions.