Transformation of photons contained in solar radiation into chemical energy is a possible strategy to utilize our largest energy source, the sun. Solar light can promote electronic transitions in semiconductor materials which are commonly employed in photocatalytic reactions. Nevertheless, wide-bandgap semiconductors are not active in the full solar spectral region; therefore, utilization of ultraviolet light is required. In addition, bare mixed oxide semiconductors have low catalytic activities. In order to boost their efficiencies, photocatalysts are modified with co-catalyst materials (promoters).

In this Thesis, we studied the role of co-catalysts in the production of hydrogen from water splitting reaction and to generate chemical fuels (e.g. CH₄) from CO₂ photoreduction reaction. Spectroscopic techniques such as e.g. X-ray adsorption (XAS) and emission (XES) were used to gain deeper understanding on the electronic structures of photocatalytic materials, and diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) to learn about the reaction mechanisms. Furthermore, electrochemical methods were employed to get insights about the redox processes occurring during both photocatalytic reactions.