Probing multiple effects of TiO2 sintering temperature on photocatalytic activity in water by use of a series of organic pollutant molecules

Our goal was to determine whether changes induced in TiO2 by sintering have a net effect on the photocatalytic removal rate of trace organics in water that depends on the organic. To that end, we have used (i) commercialized TiO2 samples prepared by varying the sintering temperature of the parent TiO2; (ii) several probe molecules: phenol, anisole, 4-chlorophenol, 2,5-dichlorophenol, 4-chlorobenzoic acid, pyridine and dichloroacetic acid. For aromatics, except pyridine, the removal rates increased with sintering temperature, whereas the opposite was true for pyridine (with one exception) and dichloroacetic acid. These results can be interpreted on the basis of the following hypotheses: decarboxylation requires direct pollutant–TiO2 contact; pyridine can react by formation of a N-centered radical cation; photocatalytic hydroxylation can occur within the near-surface solution layers. This last hypothesis was checked by comparing the removal rates of some of the probes over powdered TiO2 to rates over TiO2 coatings where the accessibility to the TiO2 surface was restricted by a SiO2 binder. In practice, the consequences are that one test pollutant does not suffice to quantitatively compare photocatalytic activities, and the selection of an optimal TiO2 sintering temperature for photocatalytic water treatment depends on the target pollutant(s).