Study of Cs-promoted, alpha-alumina-supported silver, ethylene epoxidation catalysts .3. Characterization of Cs-promoted and nonpromoted catalysts

A nonpromoted Ag/alpha-Al2O3 catalyst and a Ag/alpha-Al2O3 catalyst promoted with 420 ppm Cs used for ethylene epoxidation were studied using scanning electron microscopy (SEM), ion scattering spectroscopy (ISS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). SEM, ISS, XPS, and AES data indicate that the Cs-promoted catalyst consists of a thin film of Ag covering most of the support surface. Both a thin film and small Ag clusters exist on the nonpromoted catalyst, but the alumina is only partially covered. ISS and SIMS depth profiling data taken from the promoted catalyst indicate that most of the Cs lies beneath the surface although a small amount is present at the surface. The surface characterization data suggest that the Cs coats the support material during the preparation and acts as a binder between the Ag and the support resulting in a larger Ag coverage of the alpha-alumina. Total oxidation of ethylene and ethylene oxide occur primarily on the alumina surface. The enhancement in selectivity toward ethylene epoxidation may result from the fact that Cs addition results in the thin Ag film covering the alumina. (C) 1997 Academic Press.