EPR investigation of stable and transient oxygen centred radicals over polycristalline titanium dioxide

Electron Paramagnetic Resonance Spectroscopy (EPR) has been used to identify and characterise the nature of several oxygen centred radicals over the surface of noncrystalline TiC2. The observed radicals exhibit varying stabilities over the different polymorphs of TiO2 samples studied, namely mixed-phase P25, pure phase anatase and a pure rutile material. Paramagnetic Ti centres were formed by thermal treatment of Ti02 under vacuum. Following the addition of oxygen, the superoxide anion (O2') was formed and exhibited a distribution of sites on the surface. In particular, O2' preferentially stabilises at oxygen vacancies on the surface. However, the degree of site occupancy was found to be temperature dependent. Pronounced activity for vacancy stabilised anions under the influence of thermal, photochemical and chemical treatment was identified compared to non vacancy sites. Co-adsorption of a series of aliphatic and aromatic ketones and oxygen over P25, followed by low temperature (77K) UV irradiation led to the formation of a series of unstable (transient) alkylperoxy and peroxyacyl radicals. The sequential reaction of acetone with surface adsorbed superoxide anions was also found to result in the formation of an jacetone-02 (a) surface complex which was unstable at temperatures above 250K. Thermally produced Ti centres reacted with acetone to form an unstable organic intermediate. This species subsequently dissociated and underwent further reaction with oxygen to generate peroxy and peroxacyl radicals. The identities of these oxygen centred radicals were confirmed using isotopically enriched O2. Finally, results are presented on the work involved in the development of an Ultra-High Vacuum EPR spectrometer for investigation of paramagnetic species on single crystal oxide surfaces. Samples of Cu(acac)2 supported on quartz and TiC2 (l 10) were examined by EPR and XPS. The two techniques were combined to identify paramagnetic centres on the single crystals and to provide proof of concept in the operation of the spectrometer