Interdependence\nBetween the Extent of Ga Promotion,\nthe Nature of Active Sites, and the Reaction Mechanism Over Cu Catalysts\nfor CO<sub>2</sub> Hydrogenation to Methanol

The effect of low Ga contents on Cu/SiO<sub>2</sub> catalysts\nwas\nstudied for the CO<sub>2</sub> hydrogenation reaction. Catalysts were\nsynthesized with different Ga<sub>2</sub>O<sub>3</sub> contents by\n incipient wetness impregnation, resulting in similar average Cu nanoparticle\nsizes, between 6.0 and 6.6 nm. Characterization techniques such as\nIR-CO, <i>quasi in situ</i> XPS and <i>in situ</i> XAS, kinetic tests, <i>in situ/operando</i> diffuse reflectance\ninfrared Fourier transform spectroscopy (DRIFTS), and steady state\nisotopic transient kinetic analysis (SSITKA) were combined to disclose\nthe promoting effect of Ga and its relationship with the nature of\nactive sites and the reaction mechanism. It was found that a Cu<sup>+</sup> site was formed at the Cu–Ga<sub>2</sub>O<sub>3</sub> interface with the addition of a promoter, and it was demonstrated\nthat this site allows the direct dissociation of CO<sub>2</sub>. The\nintrinsic rate of methanol formation on promoted catalysts increases\nby approximately 1 order of magnitude without significantly changing\nthe intrinsic rate of CO formation. The same was observed for the\napparent activation energies, which were constant for CO and decreasing\nfor methanol, further depicting a change in the nature of active sites\nfor the latter. On the unpromoted Cu catalyst, methanol is formed\nmainly on Cu<sup>0</sup> through the formate pathway, while over Cu–Ga<sub>2</sub>O<sub>3</sub> the main active site shifts to the Cu<sup>+</sup> species generated at the interface, which moreover favored the reverse\nwater–gas shift followed by the hydrogenation of carbonyl intermediates\n(RWGS + CO–Hydro). SSITKA experiments confirmed that Ga contributed\npositively to the formation and stabilization of additional active\nsites where methanol is formed; the amount of these sites increased\nwith the loading of promoter. This was reflected as an increase in\nthe measured number of intermediates that lead to methanol, while\nthe coverage of intermediates that form CO remained constant, irrespective\nof Ga loading. It was further verified that the intrinsic reactivity\n(<i><u>TOF</u></i><sup>ITK</sup>) of the\nCu<sup>+</sup> site at the Cu–Ga<sub>2</sub>O<sub>3</sub> interface\nis lower than that of the Cu monometallic catalyst, but this is balanced\nby the contribution of a greater number of these active sites. On\nthe other hand, the CO formation rate was not modified. Therefore,\nit is concluded that Ga does not change the nature of the active sites\ninvolved in the CO formation. By combining state of the art characterization\ntechniques, rational kinetic measurements, <i>operando</i> spectroscopy and isotopic labeling, this work clearly stablishes\na relationship between Ga<sub>2</sub>O<sub>3</sub> promotion, nature\nof active sites, and reaction mechanism over Cu catalyst.