Non-steady state validation of kinetic models for ethylene epoxidation over silver catalysts

Abstract

Kinetic modelling has been key to developing a mechanistic understanding of the epoxidation of ethylene to ethylene oxide over silver catalysts. However, models of varying active site and mechanistic complexity have all been able to recreate steady state activity and selectivity, leading to ambiguity about the exact mechanism and nature of the active site. Herein, we validate three leading kinetic models for ethylene epoxidation over metallic silver catalysts by numerically recreating non-steady state temporal analysis of products experiments. We find all of the models are able to very generally recreate the trends observed in the pulse experiments, but that only a two-site model modified to mimic the presence of a subsurface oxygen reservoir is able to accurately recreate the trends observed in a state-altering experiment over oxidised silver. Specific to this model is the inclusion of a electrophilic oxygen species adsorbed on top of the surface oxide which acts as the active site for the selective oxidation of ethylene. This work exemplifies that while simplified single-site models for ethylene epoxidation are useful tools for broad screening, more complex models are required to capture the precise activity of the catalyst.