Computational and Experimental Insights into Reactive Forms of Oxygen Species on Dynamic Ag Surfaces Under Ethylene Epoxidation Conditions

Ethylene epoxidation, the selective oxidation of ethylene to ethylene oxide (EO), is one of the most important industrial processes because EO is a crucial intermediate for many desired chemicals. This reaction is catalyzed by supported Ag particles and has two competing reaction pathways: epoxidation to EO, or undesired complete combustion to CO2 and H2O. The selectivity toward EO is around 45% on unpromoted Ag, while it can go upwards of 85% with the inclusion of promoters. Despite years of study, the reactive intermediates, mechanism, and active phase of the catalyst are still heavily debated.

In this study, we use a combination of density functional theory (DFT) calculations and Raman Spectroscopy to investigate the dynamics and structure of the Ag surface and intermediates under reaction conditions. Our findings suggest that the active phase of the catalyst consists of a several nanometers thick Ag2O-like film on Ag bulk. This implies that under industrial conditions, EO formation is on a highly oxidized and reconstructed surface which cannot be fully represented by well-defined and low-coverage Ag surface models (e.g., Ag(111), Ag(110), Ag(100) ). We propose that the structure, dynamics, and prevailing reaction pathways upon Ag-based EO catalysts depend strongly on oxygen coverage and speciation.