IN-SITU RAMAN STUDIES OF THE SELECTIVE OXIDATION OF METHANOL TO FORMALDEHYDE AND ETHENE TO ETHYLENE-OXIDE ON A POLYCRYSTALLINE SILVER CATALYST

The combined techniques of in situ Raman microscopy and scanning elect ron microscopy (SEM) have been used to study the selective oxidation o f methanol to formaldehyde and the ethene epoxidation reaction over po lycrystalline silver catalysts. The nature of the oxygen species forme d on silver was found to depend critically upon the exact morphology o f the catalyst studied. Bands at 640, 780 and 960 cm(-1) were identifi ed only on silver catalysts containing a significant proportion of def ects. These peaks were assigned to subsurface oxygen species situated in the vicinity of surface dislocations, Ag-III-O sites formed on silv er atoms modified by the presence of subsurface oxygen and O-2(-) spec ies stabilized on subsurface oxygen-modified silver sites, respectivel y. The selective oxidation of methanol to formaldehyde was determined to occur at defect sites, where reaction of methanol with subsurface o xygen initially produced subsurface OH species (451 cm(-1)) and adsorb ed methoxy species. Two distinct forms of adsorbed ethene were identif ied on oxidised silver sites. One of these was created on silver sites modified by the interaction of subsurface oxygen species, and the oth er on silver crystal planes containing a surface coverage of atomic ox ygen species. The selective oxidation of ethene to ethylene oxide was achieved by the reaction between ethene adsorbed on modified silver si tes and electrophilic Ag-III-O species, whereas the combustion reactio n was perceived to take place by the reaction of adsorbed ethene with nucleophilic surface atomic oxygen species. Defects were determined to play a critical role in the epoxidation reaction, as these sites allo wed the rapid diffusion of oxygen into subsurface positions, and conse quently facilitated the formation of the catalytically active Ag-III-O sites.