SIMS, AES, AND TPD OF SILVER MODEL CATALYSTS (MECS, MODEL CATALYSTS, SILVER)

Silver model catalysts have been studied using SIMS (Secondary Ion Mass Spectrometry), AES (Auger Electron Spectroscopy), and acetylene TPD (Temperature Programmed Desorption) at low temperatures and very low pressures (\u3c 5 x 10(\u27-9) torr). Uniform size (in the gas phase) silver metal particles generated by the MECS (Multiple Expansion Cluster Source) technique were deposited on thin carbon and silica films to produce Ag/Carbon and Ag/SiO(,2) model catalysts with a wide range of silver loadings. Analogous Ag/Carbon TEM (Transmission Electron Microscopy) samples were made simultaneously in the MECS for comparison to the metal backed Ag/Carbon and Ag/SiO(,2) samples. The physical characterization of Ag/Carbon and Ag/SiO(,2) by SIMS showed that the Ag(\u27+) yields were large from these silver model catalysts compared to silver foil. These silver ion yields increased to a limiting value as silver loading increased, as did the AES measured silver intensity, indicating that both SIMS and AES were sensitive to changes in silver coverage. The range of silver surface coverages studied was 5-40%, and AES also detected relatively large amounts of contaminants (eg. C, S, Cl) on the model catalysts. Acetylene TPD after oxygen and acetylene dosing at 153K showed a single acetylene desorption peak (160-175K) from the silver model catalysts, which was assigned to physically bound acetylene. Chemical characterization of the acetylene/oxygen reaction on silver with SIMS has shown (Ag(,2)-(\u2713)C(,2)H)(\u27+) ions at 153K and (Ag(,2)-(\u2713)C)(\u27+) ions at 253K from silver foil and from Ag/SiO(,2) (high silver loading). These ions are indicative of the stable surface intermediates (\u2713)C(,2)H(a) and (\u2713)C(,2)(a), known to exist on Ag(110) at these temperatures. Ag/Carbon model catalysts with lower silver loading also produced (Ag(,2)-(\u2713)C(,2)H)(\u27+) ions at 153K, but SIMS analysis at successively higher temperatures up to 253K showed no (Ag(,2)-(\u2713)C(,2))(\u27+) ions. These results further illustrate the use of SIMS as a probe of surface reactions and extend applicability of the technique to supported metals