Green photo-oxidation of styrene over W–Ti composite catalysts

A series of WO3–TiO2 composite catalysts with variable quantities of tungsten was prepared by a single-pot microemulsion procedure and evaluated in the gas-phase selective photo-oxidation of the aromatic hydrocarbon styrene. Samples improve the performance of TiO2 by a factor ca. 3 and increase significantly the selectivity to valuable products (as styrene oxide) upon UV and, more importantly, sunlight-type excitation. Optimum performance in terms of both activity and selectivity was achieved under a sunlight-type renewable energy source. A complete structural (bulk and surface) and electronic characterization using Transmission Microscopy, X-ray diffraction (XRD), X-ray Photoelectron (XPS), Raman, and UV–visible spectroscopies and calorimetry with the help of the ammonia probe molecule was carried out. The study was able to detect and quantify the presence of different W entities as a function of the tungsten content of the catalysts. Three species were found to be present throughout the series; oligomeric W species and WO3 nanoparticles in close contact with anatase-TiO2 and non-contacting WO3 platelets. From a spectro-kinetic analysis carried out with the help of electron paramagnetic resonance (EPR), we provide evidence that these W-containing species play different roles in reaction. Activity and selectivity improvements with respect to the bare titania are intimately related with presence of oligomeric W species at anatase surfaces.