Influence of preparation method on surface and bulk properties of sunlight-active Ti–W mixed oxide photocatalysts

Thermal and hydrothermal treatments were applied to two amorphous Ti–W mixed oxide precursors with varying W ∶ Ti atomic ratio to obtain nanostructured particles having different properties. The photocatalytic behavior of these Ti–W mixed oxides has been tested in the photoelimination of toluene using sunlight-type excitation. These materials were prepared by a microemulsion method and their physico-chemical properties were characterized by a multitechnique approach using X-ray diffraction and photoelectron spectroscopy, as well as Raman and UV-vis spectroscopies. All samples contain substitutionally disordered Ti–W mixed oxides with anatase-type structure. The hydrothermal treatment allows introduction of a larger quantity of W into the anatase structure and produces particles with lower particle size and a moderate surface W enrichment (dependent upon treatment conditions). A Fourier transform infrared (FTIR) investigation of the catalysts under conditions prevailing during the test photoreaction together with EPR experiments of oxygen activation and DMPO spin trapping were carried out in an attempt to clarify the different photoactivities of the samples. W insertion into the anatase structure appears to potentially affect photoactivity using sunlight-type excitation by three different factors, namely the increasing absorption power in the visible region, the presence of surface W centers enhancing oxygen activation and, finally, the creation of new surface centers with modified interaction with toluene. The dependence of these factors on the preparation method and conditions and their influence on the photocatalytic activity of Ti–W systems are analyzed.