Transformation of Various Oxygen Species on the Surface of Electrolytic Silver Characterized by in Situ Raman Spectroscopy

Transformation of various oxygen species adsorbed on the electrolytic silver surface pretreated by pure oxygen at 423–973 K under ambient pressure was observed for the first time by in situ confocal microprobe Raman spectroscopy. Ag-O2 species can transform into Ag[O-O]− species slowly at temperatures below 423 K. Once the temperature rises to 423 K, the transformation of Ag[O-O]− to Ag-O(α) species takes place. With further increase in temperature, the Ag-O(α) species first transforms to Ag-O-O-Ag and then to Ag-O(γ) species, which is the most stable oxygen species on the silver surface and can be maintained even above 973 K. As for the practical reaction system, the adsorbed molecular oxygen was found to be present on the silver surface at low temperature. It transforms to Ag-O(α) species under similar conditions to those for the partial epoxidation of ethylene, and then transforms to Ag-O(γ) oxygen species under conditions similar to those for selective oxidation of methanol to formaldehyde. A possible mechanism for the detailed transformation of oxygen species was also proposed.