The interaction of light with semiconducting materials becomes the center of a wide range of technologies, such as photocatalysis. This technology has recently attracted increasing attention due to its prospective uses in green energy and environmental remediation. The characterization of the electronic structure of the semiconductors is essential to a deep understanding of the photocatalytic process since they influence and govern the photocatalytic activity by the formation of reactive radical species. Electron paramagnetic resonance (EPR) spectroscopy is a unique analytical tool that can be employed to monitor the photoinduced phenomena occurring in the solid and liquid phases and provides precise insights into the dynamic and reactivity of the photocatalyst under different experimental conditions. This review focus on the application of EPR in the observation of paramagnetic centers formed upon irradiation of titanium dioxide and niobium oxide photocatalysts. TiO₂ and Nb₂O₅ are very well-known semiconductors that have been widely used for photocatalytic applications. A large number of experimental results on both materials offer a reliable platform to illustrate the contribution of the EPR studies on heterogeneous photocatalysis, particularly in monitoring the photogenerated charge carriers, trap states, and surface charge transfer steps. A detailed overview of EPR-spin trapping techniques in mechanistic studies to follow the nature of the photogenerated species in suspension during the photocatalytic process is presented. The role of the electron donors or the electron acceptors and their effect on the photocatalytic process in the solid or the liquid phase are highlighted.

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H₂ burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as photocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO₂ by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO₂-based semiconductors is presented.