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.