Interaction of Polycrystalline Silver with Oxygen, Water, Carbon Dioxide, Ethylene, and Methanol: In Situ Raman and Catalytic Studies

Abstract

In situ Raman spectroscopy was used to study the interaction of polycrystalline silver with 16O2, 18O2, H2O, D2O, CO2, C2H4, and CH3OH at atmospheric pressure and temperatures between 25 and 500 °C. Raman bands at 956 and 800 cm-1 were observed upon exposure of silver to 16O2, and replacement of 16O2 with 18O2 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 AgO double bond, which sits on top of a Ag atom. This species is stable up to 250 °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 bonds, which sits on the interstices of three Ag atoms. This oxygen species is desorbed above 300 °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.