Mechanism and structural sensitivity of the synthesis of methanol from CO2.

Abstract

Methanol is considered an effective liquid chemical for hydrogen storage, being easier to handle than solid or gaseous materials. It is considered a transition molecule from fossil fuels to renewable energy. Currently, the technology of CO2 hydrogenation to methanol presents challenges that lie in the search for catalysts that are capable of increasing the reaction rate at low temperature and pressure, and improving the selectivity towards methanol. The scientific community has carried out strong theoretical and experimental work that has served as the basis for the various CO2 hydrogenation mechanisms that have been proposed up to now. Achieving the design of effective catalysts for the CO2 hydrogenation to methanol has generated the need to study and understand the important role that promoters have in the active site and how this is finally reflected in the reaction mechanisms. Therefore, this study aims to understand and explain the effect of the Ga and Zn promoters on the mechanism and selectivity for the CO2 hydrogenation to methanol. For the study, several Ga- and Znpromoted Cu catalysts were synthesized by the incipient wetness impregnation method with similar Cu nanoparticle sizes between them and compared to non-promoted Cu/SiO2. Characterization techniques, kinetic tests, operando diffuse reflectance infrared Fourier transform spectroscopy studies (operando-DRIFTS) and steady state isotopic transient kinetic analysis (SSITKA) were combined. The intrinsic rate of methanol formation on Cu/Ga2O3 catalysts was found to increase by approximately one order of magnitude compared to Cu without significantly changing the intrinsic rate of CO formation. The IR studies suggested that on Cu catalyst, methanol is formed mainly over Cu0 by the formate pathway, while in Cu/Ga2O3 new interfacial Cu+ active sites were generated that favored the carboxylic intermediate pathway and the reverse water gas shift reaction (RWGS+COHydro). On the other hand, with the incorporation of increasing amounts of Zn, two different groups of catalysts were generated: at low Zn content, catalysts with multiple types of surface sites (Cu0 , Cu+ and CuZnX alloy) with strong affinity for CO, while, with high Zn loading the catalysts were more homogeneous (Cu-ZnOX interface). The partial coverage of the copper nanoparticles with zinc oxide was essential to stabilize the formates and reduce the apparent activation energy of the CO2 hydrogenation to methanol from 54.0 (for Cu) to 24.7 kJ mol-1 (for CuZn(0.20)). In addition, the SSITKA-DRIFTS experiments revealed the formation of sites five times more active for CO on CuZn(0.01) catalysts than on Cu-only catalysts. Meanwhile, over the CuZn(0.20) catalyst, the increase in new methanol sites coincided with a reduction in CO formation sites, meaning that these sites formed from Cu, and the new sites Cu-ZnOX promoted the formation of reactive formates, in addition to methoxyls. IR studies suggest that the methanol pathway proceeds on catalysts with low Zn content via the RWGS+CO-Hydro pathway, whereas, with high Zn contents, the formate pathway predominated. It was found that Zn is directly involved in the newly generated methanol formation sites, and that Cu is directly responsible for CO formation. Finally, some kinetic models were proposed that can describe the formation of methanol and CO in each of the types of surfaces that occur on catalysts of Cu-Zn nature.