REACTION OF DICHLOROETHANE AND OXYGEN ON A ROUGH SILVER SURFACE

Electron energy loss spectroscopy (EELS) results indicate that dichloroethane physisorbs at 55 K on rough, coldly deposited silver. By contrast surface-enhanced Raman scattering (SERS) spectroscopy reveals that a large fraction of the absorbed dichloroethane decomposes into ethylene and chlorine on the type of surface at 55 K and above. With postadsorption of oxygen, dichloroethane or its decomposition products oxidize at temperatures equal to or greater than 140 K, forming products that include a species with a carbonyl group that is either bonded directly to chlorine, as in phosgene or oxalyl chloride, or to a metal atom with chlorine nearby. No hydrogen is bonded sufficiently closely to the CO group in the species to cause a deuterium shift. A second oxidation product is also detected containing an OH group. These reactions occur exclusively at the SERS-active sites and are undetected by EELS. We therefore conclude that SERS and EELS probe different surface sites. EELS seems to be the more globally sensitive technique, and SERS appears to be sensitive to fewer but chemically more reactive sites. This may be due either to the fact that these chemically reactive sites are located at "interior" portions of the surface, i.e., in valleys and pits located among surface features where, it has been suggested, the electromagnetic enhancement is unusually large, or to the extra enhancement that sometimes accompanies chemisorption resulting from the increase of the Raman polarizability of the surface complex, or perhaps to both effects. Whatever the reason, it appears that SERS is uniquely sensitive to those sites where the most interesting chemistry occurs on silver. These may also be the catalytically most important sites.