Catalytic activities of unsupported and supported vanadium oxides for ethylene oxidation were investigated in relation to the catalyst structures. From results of reaction rates and steady-state catalyst structures at various O2 concentrations, the active oxygen species was found to be the surface V=O species. The specific activity of the surface V=O species, or the turnover frequency, on unsupported V2O5 catalyst did not change with the treatment of the catalyst: Neither fusion nor reduction-oxidation treatment affected the turnover frequency. This indicates that ethylene oxidation on vanadium oxide catalyst is a structure-insensitive reaction. The turnover frequency for V2O5/TiO2 (anatase) was considerably higher than that for V2O5/TiO2 (rutile) or unsupported V2O5 at any V2O5 content. This indicates the promoting effect of TiO2 (anatase) support on the activity of vanadium oxide and provides experimental evidence for the validity of previous inference in solid-state chemistry.
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