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

Abstract.

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 π-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.