Unveiling the Role of Surface Oxygen Coverage on Product Selectivity in Ethylene Epoxidation over Silver Catalysts via Operando Raman Spectroscopy

An important challenge in heterogeneous catalysis is quantitatively decoupling how the surface coverage of reactive intermediates alters their intrinsic electronic character versus their statistical geometric availability under operating reaction conditions. This challenge is particularly critical in selective oxidation reactions, such as silver (Ag)-catalyzed ethylene epoxidation to ethylene oxide (EO). Despite decades of research, the precise mechanisms governing product selectivity and the long-standing debate over selective (“electrophilic”) versus unselective (“nucleophilic”) surface oxygen species have remained unresolved due to the difficulty of quantifying individual intermediate configurations on working catalysts. Here, we develop an operando surface-enhanced Raman spectroscopy (SERS) framework that allows us to measure the chemical character and surface concentration of intermediates on working solid catalysts. We used this approach to study how surface oxygen coverage impacts and controls ethylene epoxidation selectivity on supported silver (Ag) nanoparticle catalysts. We show that EO selectivity is governed by two competing mechanisms: an electronic effect, where higher oxygen coverage lowers the relative activation barrier for EO formation, and a geometric effect, where high oxygen coverage increases the statistical likelihood of multioxygen collisions with ethylene that drive deep oxidation to CO2. Kinetic Monte Carlo-inspired simulations were used to verify the experimental findings. These results establish surface oxygen coverage as a predictive descriptor of EO selectivity and provide a general strategy for analyzing the mechanistic origins of product selectivity in heterogeneous catalysis.