Comparison of the liquid and gas phase photocatalytic activity of flame-synthesized TiO2 catalysts: the role of surface quality

Abstract

A set of flame-hydrolytic TiO 2 samples was synthesized. Via systematically increasing the precursor (TiCl 4 ) feeding rate to the flame, nanocrystalline TiO 2 specimens with practically identical bulk structural properties (that is: particle size, anatase-to-rutile ratio, specific surface area) were obtained. However, it has been observed that with increasing precursor feed rate, the fraction of the polyhedral particles increased in the products at the expense of the spherical ones. In our previous work, we argued, that faceted particles are superior to spherical ones in phenol decomposition. In this work, the photocatalytic activities of these samples were determined in the gas phase using methanol and acetaldehyde as model compounds and they were compared with those obtained for an aqueous solution containing phenol. The photocatalytic activities were found to vary in parallel in the case of phenol and methanol: they increased with the precursor feed rate (and also with the frequency of the faceted nanocrystals) for both. However, for acetaldehyde, the photocatalytic activity did not change systematically within this series of experiments, and roughly remained constant. Our results indicate that the surface quality of TiO 2 is of primary importance in the selectivity of photocatalytic reactions.