The formation of O
- radical over various metal oxides, its reactivity, and catalytic oxidations using O
- as an oxidant have been described. It is confirmed by ESR, XPS, TPD etc. that O
- radical can be generated through decomposition or charge transfer processes of O
2, O
2-, OH
-, or N
2O. Respective characters of O
- on various metal oxides, that is, g-values of ESR spectra and desorption temperatures etc., were summarized. Among oxygen species such as O
-, O
-2 and O
-3, O
- radical shows the highest activity for stoichiometric oxidations of hydrocarbons and the order of reactivities has been clarified to be O
->O
-3>O
-2 independent of metal oxides. Reaction schemes of O
- radical with hydrocarbons have been proposed on the basis of the ESR, IR, and TPD observations and also been compared with those of O
-2 and O
-3. In the last section, catalytic oxidations of hydrocarbons by nitrous oxide, which decomposes readily and selectively to yield an O
- radical on metal oxide surfaces, have been outlined, including new types of catalytic oxidations such as one-step conversions of methane into methanol, of alkanes into alkenes, and of benzene into phenol.
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