Photocatalytic Oxidation of Toluene on Nanoscale TiO2 Catalysts: Studies of Deactivation and Regeneration

Nanoscale TiO2 catalysts prepared using a sol–gel method exhibit higher initial activity than commercially available P-25 TiO2 for the photocatalytic oxidation of toluene. Unlike P-25 TiO2, nonporous, nanoscale TiO2 catalysts are composed mainly of mesopores with pore sizes in the range of 35–44 nm. Calcination at 420°C leads to agglomeration of nanoscale TiO2 particles, formation of rutile, a decrease in pore capacity, and an enlargement of the mesopores. Catalysts treated at such a temperature display relatively low activity. Results of competitive adsorption of water and toluene on TiO2 samples confirm that TiO2 has a highly hydrophilic surface, which intrinsically suppresses the oxidation rate of toluene at high water content in the feed stream. Severe deactivation of TiO2 catalysts is due to the accumulation of partially oxidized intermediates, such as benzaldehyde and benzoic acid, on active sites. Complete recovery of catalytic activity requires a regeneration temperature above 420°C. Using platinum loaded on TiO2 results in lower oxidation rates of toluene, but facilitates the removal of poisonous intermediates from the deactivated TiO2 surface. Kinetic studies of the deactivation process indicate that the adsorption of poisonous intermediates in the initial stage of the photocatalytic reaction is almost irreversible. The initial oxidation rates on the catalysts are proportional to their surface areas. The surface concentration of illuminated active sites on TiO2 catalysts is estimated to be 0.85–0.96 μmol/m2.