Effects of the W precursor and amount on the photocatalytic activity of WO3-TiO2 mixed oxides

In order to overwhelm one of the limits affecting TiO2 as photocatalyst, i.e. the high recombination rate of photogenerated charge carriers, one of the most widely adopted strategy consists in producing TiO2-based mixed oxide photocatalysts. For that purpose, WO3-TiO2 mixed oxides were synthesized by a sol-gel method employing titanium tetra-isopropoxide (Ti(OC3H7)4) as titanium source and either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti molar ratios. Formic acid, undergoing direct photomineralisation without forming any stable intermediate species, was used as model substrate for the abatement of organic pollutants in water under UV-visible irradiation. The photocatalytic degradation of acetaldehyde, one of the main odour-causing gases in indoor air, was employed as photocatalytic test reaction in the gas phase. Furthermore, the so-obtained materials were characterized by XRPD, BET, UV-vis reflectance and XPS analyses, and also by photocurrent (IPCE) measurements. The synthesis of mixed WO3-TiO2 photocatalysts was successfully achieved starting from the organic precursors of the two metals. Materials with an intimate contact between the two oxides ensured an efficient photoproduced charge separation and a consequent enhanced photoactivity paralleled by increased IPCE values, with respect to both pure TiO2 prepared by the same route, and commercial benchmark P25 TiO2 from Degussa (Evonik). The particle sizes decrease observed with increasing the W content, the parallel beneficial increase of surface area and the persistence of the highly active crystalline anatase phase, even after calcination at 700 °C, also contribute to the higher photocatalytic performance of these materials. By contrast, a less performing photocatalyst was obtained when the inorganic precursor was employed in the sol-gel synthesis, due to its persistence in separate domains within the TiO2 structure. The optimal tungsten content was confirmed to correspond to a 3% W/Ti molar ratio, as in recent studies, as both the photocatalytic activity and the IPCE curves were negatively affected by the presence of larger tungsten amounts. Since WO3 is not a photocatalyst as effective as TiO2, the photocatalytic activity of the mixed oxides thus decreased as a consequence of overdoping, further proving that the photoactivity increase observed upon tungsten addition is to be mainly related to an improved electron-hole separation in TiO2 and it is not originated by the action of WO3 as photocatalyst.