Selectivity maximization of ethylene epoxidation via NEMCA with zirconia and β″-Al2O3 solid electrolytes

The silver-catalyzed epoxidation of ethylene is a reaction of great technological importance and also represents one of the most challenging and thoroughly studied catalytic systems. It was found that the catalytic activity and selectivity of polycrystalline Ag for the epoxidation and complete oxidation of ethylene can be affected in a pronounced and reversible manner by electrochemically supplying or removing oxygen ions O^2- or Na⁺ to or from the silver catalyst surface in ZrO₂ (8 mol%Y₂O₃) or β″-Al₂O₃ solid electrolyte cell reactors and in the presence or absence of traces of chlorinated hydrocarbons in the gas phase. The steady-state changes in catalytic rates of formation of C₂H₄O and CO₂ are typically 10 to 100 times larger than the corresponding rate of ion transport to or from the catalyst surface, i.e., the reaction exhibits the effect of Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) or “Electrochemical Promotion”. The selectivity to C₂H₄O can be very significantly altered, relative to open circuit conditions. Under fuel rich conditions, temperatures near 250°C and in the presence of traces of 1,2-C₂H₄Cl₂ in the gas phase selectivity values as high as 88% can be obtained, well above the ones reported in the open literature. The observed phenomena are discussed and interpreted within the framework of previous NEMCA studies and the currently prevailing ideas regarding the mechanism of ethylene epoxidation. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11 – 18 Sept. 1994