XPS AND TPD STUDIES OF CS-O COMPLEXES ON AG SURFACES - SINGLE-CRYSTALVERSUS SUPPORTED CATALYSTS

The properties of Cs overlayers on silver supported catalysts far ethy lene epoxidation and on Ag(111) single crystal have been studied by su rface science techniques (XPS, TPD, TPRS) and by measurement of specif ic catalytic activity. It has been shown that depending on the mean si ze of the supported Ag particles and the treatment by reaction mixture three different types of surface complexes can be produced on the sur face of supported catalysts. A comparison of their spectroscopic chara cteristics with those for Cs-O overlayers on Ag(111) surface indicates that at least two of them are located on silver particles and are com mon both for supported and bulk silver. The properties of the Cs-O com plex characterized by the Cs3d(5/2) line with BE = 724.2 eV enable us to assign it to surface Cs oxide. It does not take a direct part in th e stage of ethylene oxide formation. Unlike Cs oxide, the Cs-O complex characterized by the Cs3d(5/2) line with BE = 725.0 eV seems to be in volved in the surface centers active in ethylene epoxidation. However, additional studies are necessary to elucidate its structure. The infl uence of both Cs-O surface complexes on silver catalytic action is dis cussed in the context of possible mechanisms for ethylene epoxidation.