Photocatalytic activation of CO2 and water has potential for producing fuels by conversion of photon energy. However, the low productivity still limits practical application. In this study, the goal was to gain more fundamental insight in CO2 activation, and to provide guidelines for rational design of improved photocatalytic systems. In-situ IR spectroscopy (DRIFTS) was used to explore the surface chemistry in converting CO2 and H2O of a Cu(I)/TiO2 catalyst, reported quite effective for CO2 conversion in the literature. Carbon residue, not completely removed by calcination, was found to influence the evaluation of genuine activity of CO2 activation. It is also demonstrated that 13CO2 reduced into 13CO over titania-based catalysts. Besides, carbonate decomposition leads to CO formation. Increasing the surface content of carbonate, by changing the concentration of the Na2CO3 solution used for impregnation, showed increasing 12CO quantities, further evidence for a role of carbonates in CO formation over titania-based catalysts. A parallel photoreactor system is presented, applied in gas phase photocatalytic CO2 reduction. The apparatus was designed as a platform for the comparison of photocatalytic activity under identical conditions. Catalysts-screening showed that dispersed titania in a porous silica support, Ti-SBA-15, is the most active catalyst in CH4 production. A mechanistic study of photocatalytic CO2 reduction over Ti-SBA-15 is reported. Formaldehyde was found very reactive over the catalytic system. Based on the experimental activity results, a mechanism for photocatalytic CO2 reduction is proposed involving formation of CO in the initial stages, followed by consecutive formation of formaldehyde, which converts to CH4, C2H4, and C2H6. The half reaction of water oxidation was also investigated by applying photo-activation of the Ru(bpy)32+ photosensitizer complex and a sacrificial electron acceptor (S2O82-). 4nm Co3O4 particles dispersed in SBA-15, showed higher O2 evolution rates than 7 nm Co3O4 particles. A similar trend but lower activity was observed for Co3O4 embedded in MCM-41. The effect of the silica scaffold was demonstrated to be the consequence of the higher surface area of MCM-41 and the consequently higher content of the [Ru(bpy)3]2+ photosensitizer complex adsorbed on the silica walls, which renders this inactive for Co3O4 oxidation.