A COMBINED ENVIRONMENTAL SCANNING ELECTRON-MICROSCOPY AND RAMAN MICROSCOPY STUDY OF METHANOL OXIDATION ON SILVER(I) OXIDE

The techniques of environmental scanning electron microscopy (ESEM) an d Raman microscopy have been used to respectively elucidate the morpho logical changes and nature of the adsorbed species on silver(I) oxide powder, during methanol oxidation conditions. Heating Ag2O in either w ater vapour or oxygen resulted firstly in the decomposition of silver( I) oxide to polycrystalline silver at 578 K followed by sintering of t he particles at higher temperature. Raman spectroscopy revealed the pr esence of subsurface oxygen and hydroxyl species in addition to surfac e hydroxyl groups after interaction with water vapour. Similar species were identified following exposure to oxygen in an ambient atmosphere . This behaviour indicated that the polycrystalline silver formed from Ag2O decomposition was substantially more reactive than silver produc ed by electrochemical methods. The interaction of water at elevated te mperatures subsequent to heating silver(I) oxide in oxygen resulted in a significantly enhanced concentration of subsurface hydroxyl species . The reaction of methanol with Ag2O at high temperatures was interest ing in that an inhibition in silver grain growth was noted. Substantia l structural modification of the silver(I) oxide material was induced by catalytic etching in a methanol/air mixture. In particular, ''pin-h ole'' formation was observed to occur at temperatures in excess of 773 K, and it was also recorded that these ''pin-holes'' coalesced to for m large-scale defects under typical industrial reaction conditions. Ra man spectroscopy revealed that the working surface consisted mainly of subsurface oxygen and surface Ag=O species. The relative lack of subs urface hydroxyl species suggested that it was the desorption of such m oieties which was the cause of the ''pin-hole'' formation.