XPS and TPD studies of Cs-0 complexes on Ag surfaces: single crystal versus supported catalysts

The properties of Cs overlayers on silver supported catalysts for ethylene epoxidation and on Ag(1 11) single crystal have been studied by surface science techniques (XPS, TPD, TPRS) and by measurement of specific catalytic activity. It has been shown that depending on the mean size of the supported Ag particles and the treatment by reaction mixture three different types of surface complexes can be produced on the surface of supported catalysts. A comparison of their spectroscopic characteristics with those for Cs-0 overlayers on Ag(1 1 1) surface indicates that at least two of them are located on silver particles and are common both for supported and bulk silver. The properties of the Cs-0 complex characterized by the Cs3d,, line with BE = 724.2 eV enable us to assign it to surface Cs oxide. It does not take a direct part in the stage of ethylene oxide formation. Unlike Cs oxide, the Cs-0 complex characterized by the Cs3d,, line with BE = 725.0 eV seems to be involved in the surface centers active in ethylene epoxidation. However, additional studies are necessary to elucidate its structure. The influence of both Cs-0 surface complexes on silver catalytic action is discussed in the context of possible mechanisms for ethylene epoxidation.