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 16 O 2, 18 O 2, H 2 O, D 2 O, CO 2, C 2 H 4, and CH 3 OH 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 16 O 2, and replacement of 16 O 2 with 18 O 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 °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 H 2 O and D 2 O, 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.