The synthesis of formates and formic acid by hydrogenation of carbon dioxide offers a promising path to valorise carbon dioxide and an appealing alternative to the commercial processes. However, such reaction is not thermodynamically favourable in gas phase. Thus, the use of basic additives together with homogeneous catalysts under batch conditions represent the state-of-the-art in this field. The scarce research on this reaction over heterogeneous catalysts may have hindered its further development at industrial scale. In this work, the continuous synthesis

of formates from CO2 and H2 is demonstrated over heterogeneous catalysts, while taking advantage of high-pressure conditions. On one hand, in the presence of methanol, the reaction product is methyl formate, which can be a potential intermediate in the continuous synthesis of formic acid through its hydrolysis. New insights are provided regarding the reactivity of coinage metals, the roles of metal oxides as supports, and the reactive intermediates and reaction mechanisms. On the other hand, the cofeeding of water or triethylamine over a heterogenized molecular catalyst promotes the formation of formic acid. The potential of using a covalent triazine framework to immobilize an iridium molecular complex, as well as of the unique properties of the resulting material in CO2 hydrogenation reactions is explored. This thesis highlights the importance of employing combined methodologies to verify the location and nature of active sites, which can facilitate rational catalyst design by tuning active metal and support materials.