Estudo dos mecanismos da síntese de metanol via reação de hidrogenação do CO2 sobre catalisadores à base de cobre

Abstract

The use of CO2 as a carbon source in the production of more complex molecules with high added value is a topic that has been gaining ground in academia as the relationship between climate change and the increase in the concentration of CO2 in the atmosphere is confirmed. In this sense, one of the processes widely investigated is the hydrogenation of CO2 via heterogeneous catalysis. One of the most interesting products that can be produced via heterogeneous catalysis is methanol (H3COH). The present work aims to critically review studies that contributed to the development of the field of catalytic hydrogenation of CO2 to methanol on copper-based materials, pointing out the main advances, challenges and promising directions to be followed. It is agreed that the two main mechanisms of methanol synthesis via exclusive hydrogenation of CO2 over copper-based catalysts are (i) the formate route and (ii) the RWGS+CO-hydro route. The development of these routes is strongly related to the interaction between the surface and the intermediate species formed during the synthesis. In the formate route, the first intermediate formed is the formate itself (HCOO*). The formation of this intermediate occurs through the interaction of CO2(g) with a hydride adsorbed to the surface. After this first step, the following hydrogenations are observed: HCOO*→H2COO*→H2COOH→H3CO*→H3COH*→H3COH(g). In the RWGS+CO-hydro route, the first intermediate formed is the carboxyl (COOH*). This species is formed after the hydrogenation of one of the oxygen atoms of the adsorbed CO2. Carboxyl, however, is an unstable species on copper-based catalysts and dissociates quickly, leading to the formation of CO* and OH*. Although the first intermediate in this route is carboxyl, it is a consensus that the key intermediate for methanol production via the RWGS+CO hydro route is CO*. After its formation, CO* can be hydrogenated, leading to methanol synthesis, or desorbed, leading to the formation of CO(g). Pure copper surfaces are excellent RWGS catalysts, however, in these catalysts the CO* formed is easily desorbed, therefore methanol synthesis via RWGS+COhydro is negligible over pure copper. One of the strategies that can be adopted to increase methanol production via the RWGS+CO-hydro route over copper catalysts is the addition of dopants that act to stabilize CO* and the HCO* species, formed in the first hydrogenation of CO*. After the production of CO*, the hydrogenations follow: CO*→HCO*→H2CO→H3CO* → H3COH* → H3COH(g). Recently, studies have shown that the presence of water in small amounts promotes global methanol production. The main results presented in the literature indicate that the water molecule can act as a hydrogen donor species, reducing the activation energy of the processes of formation of O-H bonds and the breaking of the C-O(H) bond. However, in addition to hydrogenating the intermediates of this synthesis, the presence of water in the reaction medium may be related to surface changes, such as increased surface basicity and the formation of oxides.