Unpredictable photocatalytic ability of H2-reduced rutile-TiO2 xerogel in the degradation of dye-pollutants under UV and visible light irradiation

A series of H2-reduced TiO2 xerogels of low specific surface area was prepared by the sol–gel process. The gels were dried, calcined in air at various temperatures (400, 500, 700 °C) and then reduced at 400 °C under hydrogen flow (H2-HTR treatment). The materials were characterized by X-ray diffraction, transmission electron microscopy (TEM) and UV/Visible diffuse reflectance spectroscopy, and their texture was determined by nitrogen adsorption–desorption. IR spectroscopy was used to study to what extent samples were reduced. The effects of the calcination/H2-HTR treatments on the adsorption of methylene blue (MB) in aqueous solution and on the photocatalytic degradation of MB and crystal violet (CV) under UV and visible light irradiation were also evaluated. Results showed predictable modifications in the physicochemical properties caused by the annealing of TiO2 xerogel at high temperature (700 °C), such as a total anatase-to-rutile phase transition and a considerable loss of specific surface area from 260 to 2 m2 g−1. However, the higher degree of reduction exhibited by the rutile-TiO2 lattice led to unpredictable photocatalytic activity for the dye conversion under UV and visible light irradiation. The loss of specific surface area of the rutile-TiO2 sample was compensated by the increase in the affinity of this sample for the dye. Under UV light, the rutile-TiO2 xerogel obtained at 700 °C showed a similar level of photoactivity as the one obtained with anatase-TiO2 xerogels obtained by calcination at 400 and 500 °C. On the other hand, under visible light, unlike anatase-TiO2 xerogels, the rutile-TiO2 xerogel showed a dye photoconversion rate per external surface area that was up to 40 times higher than the one obtained with commercial Degussa P25 TiO2.