EPR STUDY OF SPIN-TRAPPED FREE-RADICAL INTERMEDIATES FORMED IN THE HETEROGENEOUSLY-ASSISTED PHOTODECOMPOSITION OF ACETALDEHYDE

Electron paramagnetic resonance spectroscopy is used to detect radical adducts of PBN (alpha-phenyl N-tert-butyl nitrone) generated by expos ure of solutions and suspensions to ambient or high power UV at 300 K. Exposure of acetaldehyde to direct sunlight generates a different PBN radical adduct to high power UV irradiation. Direct sunlight irradiat ion of deoxygenated acetaldehyde generates PBN-acetyl adducts whereas direct sunlight exposure of oxygenated acetaldehyde produces PBN-aceto xyl adducts. High power UV irradiation of TiO2/acetaldehyde suspension s yields the same radical adduct generated when no TiO2 is present-thi s adduct (assigned to trapped formyl radicals or PBN degradation produ cts) is produced irrespective of the state of oxygenation of solution. Direct sunlight irradiation of deoxygenated TiO2/acetaldehyde suspens ion results in the production of PBN-acetyl adducts as the primary spe cies. In oxygenated TiO2/acetaldehyde suspension, PBN-acetyl adducts a re again produced as the primary species, together with a weakly adduc ted secondary species-assigned to PBN-acetoxyl adducts. TiO2 band gap transitions are observed to play no part in the production of radical intermediates in sunlight irradiated acetaldehyde/TiO2 suspension. The extent of non-band gap dependent processes is shown to be sensitive t o the surface basicity of the metal oxide. Band gap mediated radical p roduction is demonstrated to arise when acetaldehyde photoreduction is coupled to the concomitant photooxidation of ethanol. Ethanol derived PBN-ethoxy adducts are detected as the primary species arising from s unlight irradiation of both oxygenated and deoxygenated TiO2/acetaldeh yde/ethanol suspensions.