Expanding the Reaction Network of Ethylene Epoxidation on Partially Oxidized Silver Catalysts

An extended microkinetic model (MKM) for the selective oxidation of ethylene to ethylene oxide (EO) is presented, based on an oxidic representation of the silver (Ag) surface, namely, the p(4 x 4) oxidic reconstruction of the Ag(111) phase to mimic the significant oxygen coverage under reaction conditions, as is evidenced by recent operando spectroscopic studies. The MKM features three pathways each for producing either ethylene oxide (EO) or carbon dioxide (CO2), including the common intermediate or oxometallacycle (OMC) pathway, an atomic oxygen pathway, as well as pathways centered around the role of a diatomic oxygen species occupying an oxygen vacancy (O2/O*). The MKM uses a composite set of experimental and density functional theory (DFT) kinetic parameters, which is further optimized and trained on experimental reaction data. A multistart ensemble approach was used to ensure a thorough sampling of the solution space, and a closer analysis was performed on the best-performing, physically meaningful solution. In agreement with published DFT data, the optimized MKM observed that the OMC pathway heavily favors the total combustion pathway and alone is insufficient in explaining the similar to 50% EO selectivity commonly reported. Furthermore, it confirmed the pivotal role of the O2/O* species in the flux-carrying pathways for EO production. The MKM additionally highlights the fluctuating nature of the catalyst surface, in that the proportion of metallic to oxidic phase changes according to the reaction conditions, accordingly resulting in kinetic implications.