Výzkum fotokatalyticky aktivních materiálů pro fotokatalytickou redukci oxidu uhličitého

Abstract

This presented doctoral thesis focuses on the synthesis, characterization, and activity study of defective TiO₂-based photocatalysts for photocatalytic carbon dioxide (CO₂) reduction. Four sets of defective TiO₂ photocatalysts were investigated. In the first two sets, the conditions of chemical reduction using sodium borohydride (NaBH₄) as the reducing agent were systematically varied, specifically the amount of NaBH₄ and the reduction temperature. The third set of samples was prepared using a Ti-based salt synthesized under alkaline conditions (pH = 10). The final set of defective TiO₂ photocatalysts was prepared exclusively by chemical reduction with NaBH₄, using three different commercially available TiO₂ nanomaterials as precursors – P25, (Start)TiO₂_(PL), and KC7050. The synthesized defective TiO₂ samples were characterized using analytical techniques such as X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and N₂ physisorption. These techniques enabled a detailed description of the physicochemical properties of the investigated photocatalyst samples, which were then correlated with the results obtained from the photocatalytic experiments. The photocatalytic activity of the defective TiO₂ samples was tested in batch reactors, where the samples were dispersed in a 0.2 M aqueous sodium hydroxide (NaOH) solution. Carbon dioxide (purity = 4.8) was used as the reactant. The irradiation source was an 8 W Hg pen-ray lamp (λmax = 254 nm). The main products of the photocatalytic experiments were methane, carbon monoxide, and hydrogen, the latter being the major product of the competing water-splitting reaction. The products were identified and quantified using gas chromatography with a barrier ionization detector (GC-BID). The highest photocatalytic activity was observed for the defective TiO₂ sample reduced at 350 °C using 1.5 g of NaBH₄. The enhanced performance of this sample can be mainly attributed to the successful introduction of surface defects in the form of oxygen vacancies and Ti³⁺ active sites. These features are responsible not only for improved reactant adsorption but also for its subsequent activation on the sample surface. The results presented in this dissertation further highlight the positive effect of nitrogen doping. This conclusion is supported by the increased photocatalytic performance of a defective TiO₂ sample whose Ti-based precursor was synthesized under alkaline conditions (pH = 10) and subsequently reduced at 500 °C. A detailed analysis revealed that the enhanced activity of this sample results from the synergistic effect of nitrogen doping, oxygen vacancies, and Ti³⁺ centers, which together significantly contribute to improved photocatalytic performance. Finally, this work also demonstrates the potential of using commercially available TiO₂ nanomaterials for the synthesis of efficient defective TiO₂ photocatalysts. This conclusion is based especially on the results obtained with the reduced KC7050-based sample, which exhibited not only significantly higher photocatalytic activity compared to its pristine form, but also to the commonly used benchmark – TiO₂ Evonik P25. The enhanced performance of this sample is mainly attributed to its rich defect structure and preserved crystallinity.