Selective ethylene oxidation to ethylene oxide (EO) is one of the most important chemical processes that is primarily used for production of ethylene glycol (antifreeze). The industrial supported Ag/α-Al2O3 catalyst is promoted by Cl, Cs and Re in order to inhibit total oxidation and increase EO selectivity. The nature of the active surface sites on promoted Ag catalysts have been extensively debated in the literature over the years because of the scarcity of molecular level measurements and theoretical computational calculations. HAADF-STEM images exhibit that the Cs and Re promoters are located both on Ag and the α-Al2O3 support. The number of active sites on the supported Ag/α-Al2O3 catalysts is strongly dependent on the promoters. Ethylene Oxidation-Temperature Programmed Surface Reaction (C2=+O2-TPSR) on unpromoted and promoted supported Ag/α-Al2O3 catalysts indicates that the Cl, Cs and Re oxide promoters decrease the ethylene oxidation activity (increase in Tp temperature: Ag &lt; Cl &lt; Re &lt;&lt; Cs). With the aid of surface enhancement Raman spectroscopy (SERS), the nature of Cs and Re species is resolved at ppm level. Furthermore, the experimental results show strong synergies between the Cl, Cs and Re promoters that affect the number of active sites and the surface kinetics. The present findings provide for the first time a complete understanding of the active surface sites of the promoted supported Ag/α-Al2O3 catalysts and their surface kinetics. This study reveals the role of each promoter in the ethylene epoxidation reaction, thus, providing fundamental insights that can guide the rational design of improved catalysts for selective oxidation of ethylene by supported Ag/α-Al2O3 catalysts.

Revealing the nature of active surface sites on promoted Ag/α-Al2O3 catalysts for selective epoxidation of ethylene

ACS Spring 2022

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