The oxidation of copper catalysts during ethylene epoxidation was characterized using in situ photoemission spectroscopy and electron microscopy. Gas chromatography, proton-transfer reaction mass spectrometry and electron-ionization mass spectrometry were used to characterize the catalytic properties of the oxidized copper. We find that copper corrodes during epoxidation in a 1:1 mixture of oxygen and ethylene. The catalyst corrosion passes through several stages, beginning with the formation of an O-terminated surface, followed by the formation of Cu<inf>2</inf>O scale and eventually a CuO scale. The oxidized catalyst exhibits measurable activity for ethylene epoxidation, but with a low selectivity of <3%. Tests on pure Cu<inf>2</inf>O and CuO powders confirm that the oxides intrinsically exhibit partial-oxidation activity. Cu<inf>2</inf>O was found to form acetaldehyde and ethylene epoxide in roughly equal amounts (1.0% and 1.2% respectively), while CuO was found to form much less ethyl aldehyde than ethylene epoxide (0.1% and 1.0%, respectively). Metallic copper catalysts were examined in extreme dilute-O<inf>2</inf> epoxidation conditions to try and keep the catalyst from oxidizing during the reaction. It was found that in feed of 1 part O<inf>2</inf> to 2500 parts C<inf>2</inf>H<inf>4</inf> (PO<inf>2</inf> = 1.2 × 10<sup>-4</sup> mbar) the copper surface becomes O-terminated. The O-terminated surface was found to exhibit partial-oxidation selectivity similar to that of Cu<inf>2</inf>O. With increasing O<inf>2</inf> concentration (>8/2500) Cu<inf>2</inf>O forms and eventually covers the surface.
Greiner, M. T., Jones, T. E., Johnson, B. E., Rocha, T. C. R., Wang, Z. J., Armbrüster, M., … Schlögl, R. (2015). The oxidation of copper catalysts during ethylene epoxidation. Physical Chemistry Chemical Physics, 17(38), 25073–25089. https://doi.org/10.1039/c5cp03722k