Interdependence Between the Extent of Ga Promotion, the Nature of Active Sites, and the Reaction Mechanism Over Cu Catalysts for CO₂ Hydrogenation to Methanol

The effect of low Ga contents on Cu/SiO2 catalysts was studied for the CO2 hydrogenation reaction. Catalysts were synthesized with different Ga2O3 contents by incipient wetness impregnation, resulting in similar average Cu nanoparticle sizes, between 6.0 and 6.6 nm. Characterization techniques such as IR-CO, quasi in situ XPS and in situ XAS, kinetic tests, in situ/operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and steady state isotopic transient kinetic analysis (SSITKA) were combined to disclose the promoting effect of Ga and its relationship with the nature of active sites and the reaction mechanism. It was found that a Cu+ site was formed at the Cu–Ga2O3 interface with the addition of a promoter, and it was demonstrated that this site allows the direct dissociation of CO2. The intrinsic rate of methanol formation on promoted catalysts increases by approximately 1 order of magnitude without significantly changing the intrinsic rate of CO formation. The same was observed for the apparent activation energies, which were constant for CO and decreasing for methanol, further depicting a change in the nature of active sites for the latter. On the unpromoted Cu catalyst, methanol is formed mainly on Cu0 through the formate pathway, while over Cu–Ga2O3 the main active site shifts to the Cu+ species generated at the interface, which moreover favored the reverse water–gas shift followed by the hydrogenation of carbonyl intermediates (RWGS + CO–Hydro). SSITKA experiments confirmed that Ga contributed positively to the formation and stabilization of additional active sites where methanol is formed; the amount of these sites increased with the loading of promoter. This was reflected as an increase in the measured number of intermediates that lead to methanol, while the coverage of intermediates that form CO remained constant, irrespective of Ga loading. It was further verified that the intrinsic reactivity (TOFITK) of the Cu+ site at the Cu–Ga2O3 interface is lower than that of the Cu monometallic catalyst, but this is balanced by the contribution of a greater number of these active sites. On the other hand, the CO formation rate was not modified. Therefore, it is concluded that Ga does not change the nature of the active sites involved in the CO formation. By combining state of the art characterization techniques, rational kinetic measurements, operando spectroscopy and isotopic labeling, this work clearly stablishes a relationship between Ga2O3 promotion, nature of active sites, and reaction mechanism over Cu catalyst.