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

The photocatalytic activity of TiO2 (Degussa P-25) modified with a 4,4'-bip yridinium monolayer (V2+-TiO2) has been compared with that of conventional TiO2-P-25 by investigating the efficiency of degradation of a series of fou r 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 f ormation of supramolecular donor-acceptor complexes with the bipyridinium u nits at the semiconductor surface was obtained by comparison of the Langmui r-adsorption characteristics and the efficiencies of photodegradation of th e different substrates. Furthermore, the main intermediates of the photocat alytic degradation of 2,4-xylidine were identified, and the presence of 2,4 -dimethylphenol indicates that the main pathway of substrate oxidation proc eeds via electron transfer from the adsorbed organic substrate to the "hole s" within the valence band of the photoexcited semiconductors V2+-TiO2 and TiO2. The efficiencies of photocatalytic degradation by both V2+-TiO2 and T iO2 were limited by the trapping efficiency of the conduction band electron s by molecular oxygen.