CW-EPR investigation on the formation, stability and reactivity of oxygen centred radicals on TiO2

Electron Paramagnetic Resonance (EPR) spectroscopy has been used to study and identify a number of defects (eg., Ti3+, O) and transient oxygen centred radicals (eg., HO2*, ROO*, etc) formed under different co-adsorption conditions over the surface of polycrystalline Ti02 (P25). Paramagnetic surface Ti3+ centres were formed by thermal treatment of Ti02 under vacuum. This system provides a good model for probing electron transfer (ET) sites at the surface of TiC>2 that may be involved in catalysis. Upon O2 addition to the reduced surface, superoxide (O2 ) anions are formed and stabilised. Different sites for O2" stabilisation have been identified notably at oxygen vacancy sites and at a non- vacancy, high co-ordinated Ti4+ sites. ET from the precursor Ti3+ centres to O2 (forming O2") was prevented at each respective site through selective blocking using specific probe molecules (CO2 or Ar). Carbon dioxide specifically adsorbed at the non-vacancy sites, whereas Ar preferentially adsorbed at vacancy sites, thereby preventing ET. The decomposition of acetonitrile, methanol and toluene occurs under dark conditions over TiC>2 (P25) containing adsorbed O2" radicals. The reaction of the organic substrates with O2" resulted in the production of thermally unstable O2 ...substrate associated surface complexes which decayed at T > 240K. Under photochemical conditions, a series of thermally unstable alkylperoxy radicals were identified following UV irradiation of a series of co-adsorbed organic: O2 mixtures. The formation of oxygen centred radicals (CV and ROO*) and defects (O) on the dehydrated, partially and fully hydrated TiC>2 surface under photochemical conditions was investigated. The O2 radicals were formed on the dehydrated and partially hydrated surfaces. At low temperatures O2" radicals were accompanied by O defects. These O' species were thermally unstable and decayed at T > 200K, whereas 02 was stable at room temperature. The population of O2 and O" on the partially hydrated surface decreased as the water pressure increased this was caused by site blocking via adsorbed H2O and OH groups. On fully hydrated surfaces, no 02 radicals were observed. The peroxy type radical, ROO*, was also formed on the Ti02 surface under all of the investigated hydration conditions