Study of Cs-promoted, alpha-alumina-supported silver, ethylene-epoxidation catalysts .4. Model study

In an attempt to gain a better understanding of the structure of ethylene epoxidation catalysts and the role of the Cs promotor, a model study using Al and Ag foils was performed. Air-exposed Al and Ag foils were dipped into a 400 ppm CsOH solution, rinsed with distilled H2O, and characterized using X-ray photoelectron spectroscopy (XPS), The data indicate that Cs readily adsorbs at the Al2O3 surface on the Al foil, Very little Cs adsorbs to the Ag foil which contains a thin oxide film at the surface. A sputter-cleaned Ag foil (oxide free), however, adsorbs significant quantities of CsOH indicating that the Cs bonds to a Ag metal surface but not to Ag oxide, Characterization studies were also performed on two Al foils: one which was dipped into a heated, 85% lactic acid solution containing dissolved Ag2O and the other which was dipped into an identical solution but containing CsOH as well, XPS and ion scattering spectroscopy data show that the Al foil does not adsorb Ag from the solution unless the CsOH is present. Furthermore, the XPS data indicate that there is a chemical interaction (electronic effect) between the Al, Cs, and Ag. These results support the conclusion of the previous part of this study that one of the roles of the Cs is to act as a binder between the Ag and alumina support in ethylene epoxidation catalysts. (C) 1997 Academic Press.