Tungsten oxide reactivity versus CH4, CO and NO2 molecules studied by Raman spectroscopy

Reactivity of different nanometric tungsten oxide was measured by in situ Raman spectroscopy coupled with electrical measurements during the reaction with CH4, CO and NO2 molecules in the temperature range of 50–250°C. Sample with the smallest crystallite size (2 nm) exhibited a strong electrical and Raman response to the presence of these molecules. The formation of a very active form of carbon was observed already at 50°C on the surface of tungsten oxide during the catalytic reactions involving gases containing carbonaceous molecules. Switching the atmosphere from reducing to NO2 oxidizing gas leads to a complete and immediate disappearance of the carbon and the conductivity changes by a factor of 105. No such changes were observed for crystallite sizes of 35 nm. The results are discussed and interpretation based on quasi-chemical reactions is proposed.