EPR study of spin-trapped free radical intermediates formed in the heterogeneously-assisted photodecomposition of acetaldehyde †

Abstract

Electron paramagnetic resonance spectroscopy is used to detect radical adducts of PBN (α-phenyl N-tert-butyl nitrone) generated by exposure 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 irradiation of deoxygenated acetaldehyde generates PBN–acetyl adducts whereas direct sunlight exposure of oxygenated acetaldehyde produces PBN–acetoxyl adducts. High power UV irradiation of TiO2/acetaldehyde suspensions yields the same radical adduct generated when no TiO2 is present—this adduct (assigned to trapped formyl radicals or PBN degradation products) is produced irrespective of the state of oxygenation of solution. Direct sunlight irradiation of deoxygenated TiO2/acetaldehyde suspension results in the production of PBN–acetyl adducts as the primary species. In oxygenated TiO2/acetaldehyde suspension, PBN–acetyl adducts are again produced as the primary species, together with a weakly adducted 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 to the surface basicity of the metal oxide. Band gap mediated radical production 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 sunlight irradiation of both oxygenated and deoxygenated TiO2/acetaldehyde/ethanol suspensions.