Origin of enhanced ethylene oxide selectivity by Cs-promoted silver catalyst

Abstract

Cesium (Cs) is a key promoter to improve the selectivity of Ag catalyst for ethylene epoxidation. However, there is still no consensus in the literature about the origin of enhanced ethylene oxide (EO) electivity by Cs-promoted Ag catalysts. In this paper, the role of Cs as a promoter for ethylene epoxidation on Ag catalyst is studied by both experiments conducted under industrially relevant conditions as well as density functional theory (DFT) calculations. Ag/α-Al2O3 and Ag-Cs/α-Al2O3 catalyst are prepared by excessive impregnation and evaluated for ethylene epoxidation to determine the mechanism of selectivity enhancement for Ag catalysts that contain Cs. With the optimal loading of Cs, Ag particles of uniform size are evenly distributed on the support and EO selectivity increases from approximately 72.5%–82.0% compared with the Ag/α-Al2O3. O2-TPD results indicate that Cs decreases the adsorption strength of oxygen on Ag and XPS analyses show that Cs shifts the Ag 3d5/2 binding energy to facilitate the desorption of the EO precursor to form gas EO. DFT calculations demonstrate that Cs-promotor can weaken the adsorption of atomic oxygen. Electron density of atom oxygen absorbed on silver decreases with the addition of Cs on Ag(111) thereby enhancing the electrophilic attack by atomic oxygen at the electron-rich CC bond of C2H4. Therefore, Cs lowers the activation energy for the formation of the oxometallacycle (OMC) and EO, while increasing the barrier for acetaldehyde (AA) formation, thereby leading to higher EO selectivity.