Ethylene oxide is a base chemical product used for the production of antifreeze, polyesters, and polyurethanes. It is synthesized in quantities exceeding 10 million tons per year by partial oxidation of ethylene with O2 on silver catalysts:
The catalyst consists of alumina-supported Ag particles, with added Cs and Cl compounds. Ethylene oxide is formed with selectivities of more than 90%, thus almost completely preventing the total oxidation paths to give CO2. How the catalyst achieves this is still an unsolved question. Trying to clarify the reaction mechanism by typical surface science experiments, i.e., in ultra-high vacuum, fails, as then only CO2 is produced. The Ag-catalyzed epoxidation of ethylene is thus a classical example of the "pressure gap" problem in catalysis research. We investigate this reaction with a high-pressure STM, at temperatures and pressures close to the conditions of the industrial process. Ag single crystals serve as model catalysts.
Reaction experiment in the high-pressure STM cell. As the sample temperature (blue curve) was increased to 520K, ethylene oxide increased (red curve), evidencing that catalytic turnover over a Ag(111) single crystal can be measured in the STM cell. A blank experiment with a Au(111) sample was negative (grey curve). S. Böcklein, S. Günther, J. Wintterlin. Reproduced with permission from "High-pressure scanning tunneling microscopy of a silver surface during catalytic formation of ethylene oxide", S. Böcklein, S. Günther, J. Wintterlin, Angew. Chem. Int. Ed. 125, 5628 (2013). Copyright Wiley-VCH Verlag GmbH & Co. KGaA.