Interaction of polycrystalline silver with oxygen, water, carbon dioxide, ethylene, and methanol: In situ Raman and catalytic studies

In situ, Raman spectroscopy was used to study the interaction of polycrystalline silver with O-16(2), O-18(2), H2O D2O, CO2, C2H4, and CH3OH at atmospheric pressure and temperatures between 25 and 500 degrees C. Raman bands at 956 and 800 cm(-1) were observed upon exposure of silver to O-16(2), and replacement of O-16(2) With O-18(2) shifted these bands to 931 and 778 cm(-1), respectively. The 956 cm(-1) Raman band was assigned to the stretching vibration of surface atomic oxygen with a Ag=O double bond, which sits on top of a Ag atom. This species is stable up to 250 degrees C and does not appear to interact with water, carbon dioxide, ethylene, or methanol. The 800 cm(-1) Raman band was assigned to the stretching vibration of surface atomic oxygen with bridging Ag-O-Ag bends, which sits on the interstices of three Ag atoms. This oxygen species is desorbed above 300 degrees C and interacts with water, ethylene, and methanol. Upon exposure of preoxidized silver to H2O and D2O, a new major Raman band at 866 cm(-1) was observed, which was also assigned to surface atomic oxygen species, but not surface OH because of the absence of a H/D isotopic effect. The selective oxidation of ethylene and methanol was determined to occur primarily on the surface atomic Ag-O-Ag oxygen species corresponding to the 800 cm(-1) Raman band.