One step hydrothermal synthesis of TiO₂ with variable HCl concentration: Detailed characterization and photocatalytic activity in propene oxidation

TiO₂ materials have been prepared by a novel one-step hydrothermal synthesis method at mild conditions (low temperature and time) using titanium tetraisopropoxide as precursor and HCl solutions of different concentration, ranging from 0.5 to 12 M. A detailed physico-chemical characterization has been done, putting especial attention to the quantification of crystallinity and surface OH content. All the obtained samples have high surface areas (100–135 m²/g), small anatase crystal size (8–11 nm) and high crystallinity (around 80%). The concentration of the HCl used in the synthesis has a significant effect on the two first mentioned parameters and on the proportion of crystalline phases (anatase-brookite-rutile) developed. There is a good correlation between the amount of hydroxyl surface groups and the specific surface area (S_BET), but regarding surface density of OH groups there are, as well, some differences between samples (the samples with higher OH surface densities are those prepared with less concentrated HCl solutions). Most of the prepared photocatalysts are more active than commercial TiO₂-P25 in the gas phase oxidation of propene at low concentration. The best photocatalytic performance is found for samples prepared with 0.5 and 0.8 M HCl, what can be explained by a suitable combination of properties: high surface area and developed porosity, high surface OH groups’ content and density and large proportion of anatase with small crystal size.