TiO₂ and Nitrogen Doped TiO₂ Prepared by Different Methods; on the (Micro)structure and Photocatalytic Activity in CO₂ Reduction and N₂O Decomposition

TiO₂ as nanostructured powders were prepared by (1) sol–gel process and (2) hydrothermal method in combination with (A) the processing by pressurized hot water and methanol or (B) calcination. The subsequent synthesis step was the modification of prepared nanostructured TiO₂ with nitrogen using commercial urea. Textural, structural, surface and optical properties of prepared TiO₂ and N/TiO₂ were characterized by nitrogen physisorption, powder X-ray diffraction, X-ray photoelectron spectroscopy and DR UV-vis spectroscopy. It was revealed that TiO₂ and N/TiO₂ processed by pressurized fluids showed the highest surface areas. Furthermore, all prepared materials were the mixtures of major anatase phase and minor brookite phase, which was in nanocrystalline or amorphous (as nuclei) form depending on the applied preparation method. All the N/TiO₂ materials exhibited enhanced crystallinity with a larger anatase crystallite-size than undoped parent TiO₂. The photocatalytic activity of the prepared TiO₂ and N/TiO₂ was tested in the photocatalytic reduction of CO₂ and the photocatalytic decomposition of N₂O. The key parameters influencing the photocatalytic activity was the ratio of anatase-to-brookite and character of brookite. The optimum ratio of anatase-to-brookite for the CO₂ photocatalytic reduction was determined to be about 83 wt.% of anatase and 17 wt.% of brookite (amorphous-like) (TiO₂-SG-C). The presence of nitrogen decreased a bit the photocatalytic activity of tested materials. On the other hand, TiO₂-SG-C was the least active in the N₂O photocatalytic decomposition. In the case of N₂O photocatalytic decomposition, the modification of TiO₂ crystallites surface by nitrogen increased the photocatalytic activity of all investigated materials. The maximum N₂O conversion (about 63 % after 18 h of illumination) in inert gas was reached over all N/TiO₂.