Different net effect of TiO2 sintering temperature on the photocatalytic removal rates of 4-chlorophenol, 4-chlorobenzoic acid and dichloroacetic acid in water

Our purpose was to show that the sintering temperature of TiO2 can have a different net effect (thought to arise from a decrease in surface area against a decrease in recombination rate of charge carriers) on the photocatalytic removal rate of various organic pollutants in water. For that, we have chosen four chlorinated pollutants, viz. 4-chlorophenol (4-CP), 2,5-dichlorophenol (2,5-DCP), 4-chlorobenzoic acid (4-CBA) and dichloroacetic acid (DCAA). Their photocatalytic removal was studied over four TiO2 samples (from Millennium Chemicals or affiliate) all obtained identically by TiOSO4 thermohydrolysis with subsequent calcination at various temperatures, TiO2 Degussa P25 was used for comparison. At equal TiO2 mass in the slurry photoreactor, the pseudo-first-order removal rate constant k increased with the calcination temperatures for the three aromatic pollutants, whereas it was the opposite for the aliphatic acid. Results obtained with P25 were consistent with the reasoning based on the combined effects of surface area and charge recombination rate. Similar k values for 4-CP and 2,5-DCP, irrespective of the TiO2, further illustrate the importance of the molecular structure. For 4-CBA, the possibility of decarboxylation in addition to an attack on the ring, as well as a much higher extent of adsorption, can explain a higher k with respect to the chlorophenols. The implication of these results is that the hole attack mechanism for carboxylic acids is much more sensitive to surface area variation than would be the (diffusible) OH radical mechanism for aromatics which could react in the near-surface solution-phase.