Photocatalytic oxidation of organics in water using pure and silver-modified titanium dioxide particles

The simultaneous photocatalytic degradation of organic compounds and reduction of silver ions in titanium dioxide suspensions at pH 3.0-3.5 has been studied. The organic compounds of interest were sucrose and salicylic acid. The presence of silver ions in TiO2 suspensions was found to enhance the photooxidation of high loadings (2000 mug C) of sucrose. However, for low sucrose loadings (100 mug C), pure TiO2 particles performed as well as modified TiO2 particles. An optimum silver ion loading of 2.0 at.% Ag+ was observed for the mineralisation of 2000 mug C sucrose. At this silver ion loading, the mineralisation of 2000 mug C sucrose was enhanced by a factor of approximately 4.0 (based on 90% overall oxidation rates). In contrast, the addition of silver ions to TiO2 suspensions did not have any significant effect on the photocatalytic mineralisation of salicylic acid to carbon dioxide, for both low and high loadings of salicylic acid in the suspension. It was also observed that pure TiO2 particles performed as well as silver-modified TiO2 particles for the degradation of 2000 mug C salicylic acid. The higher activity of silver ion-modified titanium dioxide suspensions for sucrose mineralisation is predominantly due to the presence of small silver particles on the titania surface, rather than due to the trapping of electrons during the reduction of silver ions. Approximately 50% of the initial mass of silver ions added to TiO2 suspensions were reduced to metallic silver deposits in the presence of sucrose and salicylic acid mineralisation at the 2.0 at.% Ag+ loading. Nanosize silver deposits on TiO2 particles act as sites of electron accumulation where the reduction of adsorbed species such as oxygen occur. The enhanced reduction of oxygen through better electron-hole separation in Ag/TiO2 particles compared to pure TiO2 particles increases the rate of sucrose mineralisation. Therefore, it is proposed that the rate-limiting step in the sucrose photooxidation reaction is the transfer of electrons to dissolved oxygen molecules, whereas in the case of salicylic acid degradation and mineralisation, the rate-limiting step is the attack of salicylic acid molecules and its degradation intermediate products by holes and hydroxyl radicals. Hence silver deposits on TiO2 particles are not beneficial for the photocatalytic degradation and mineralisation of salicylic acid but are advantageous for the mineralisation of sucrose. (C) 2002 Elsevier Science B.V. All rights reserved.