An N,N′-dialkyl-4,4′-bipyridinium-modified titanium-dioxide photocatalyst for water remediation -: observation and application of supramolecular effects in photocatalytic degradation of π-donor organic compounds

The photocatalytic activity of TiO2 (Degussa P-25) modified with a 4,4'-bipyridinium monolayer (V2+-TiO2) has been compared with that of conventional TiO2-P-25 by investigating the efficiency of degradation of a series of four organic model compounds with increasing pi -donor capacity (2,4-xylidine, 2,4-dimethylphenol, hydroquinone, and dimethylhydroquinone). As far as the mechanism of the first oxidation reaction is concerned, evidence for the formation of supramolecular donor-acceptor complexes with the bipyridinium units at the semiconductor surface was obtained by comparison of the Langmuir-adsorption characteristics and the efficiencies of photodegradation of the different substrates. Furthermore, the main intermediates of the photocatalytic degradation of 2,4-xylidine were identified, and the presence of 2,4-dimethylphenol indicates that the main pathway of substrate oxidation proceeds via electron transfer from the adsorbed organic substrate to the "holes" within the valence band of the photoexcited semiconductors V2+-TiO2 and TiO2. The efficiencies of photocatalytic degradation by both V2+-TiO2 and TiO2 were limited by the trapping efficiency of the conduction band electrons by molecular oxygen.