WO3–TiO2 vs. TiO2 photocatalysts: effect of the W precursor and amount on the photocatalytic activity of mixed oxides

Aiming at producing TiO2-based photocatalytic materials with reduced charge carriers recombination, WO3–TiO2 mixed oxides were synthesized by a sol–gel method employing either an inorganic salt, Na2WO4, or an organic alkoxide, W(OC2H5)6, as tungsten precursor, with different W/Ti ratios. The so-obtained materials were characterized by XRPD, BET, UV–vis reflectance, XPS and EDX analyses and their photoactivity was tested under UV–visible irradiation in both the mineralization of formic acid in aqueous suspension and the gas phase oxidation of acetaldehyde. Both photoactivity results and photocurrent measurements point to a superior performance of photocatalysts obtained from the organic precursor with an optimal tungsten content (3%). The formation of an intimately mixed oxide, as revealed by XRPD analysis, results in photoactivity higher than that of pure TiO2, and also of benchmark P25 TiO2, consequent to a better charge separation due to the migration of photoproduced holes from WO3 domains to TiO2 and of photopromoted electrons in the opposite direction. The persistence of pure anatase phase in W-containing photocatalysts also after calcination at 700 °C and their higher surface area with respect to pure TiO2 also contribute in increasing the photocatalytic activity of the WO3–TiO2 mixed oxides.