Promotion versus Poisoning in Copper–Gallium-Based CO2-to-Methanol Hydrogenation Catalysts

Cu–Ga-based CO₂-to-methanol hydrogenation catalysts display a range of catalytic performance, depending on their preparation. In a previous study, we successfully prepared a highly active and selective Cu–Ga catalyst using surface organometallic chemistry. However, the specific location of active Ga has remained unknown because Ga was present as Ga^III sites dispersed on silica and Ga alloyed with copper in bimetallic nanoparticles. Here, we investigated how the Ga/Cu ratio and Ga speciation affect the catalytic activity. Using surface organometallic chemistry, we prepared a series of silica-supported 3–6 nm Cu_1–xGa_xO_y nanoparticles with a range of x_Ga. In these nanoparticles, Cu is always fully metallic, while Ga is partially alloyed with Cu in the core and partially oxidized on the surface. By minimizing the fraction of Ga^III species dispersed on silica, we could focus on the reactivity of the bimetallic nanoparticles. The thus-developed materials display a volcano-type activity behavior, where methanol formation is promoted when x_Ga < 0.13–0.18 and is suppressed at higher values, indicating a poisoning of the catalysts. Alternative materials containing bimetallic nanoparticles coexisting with Ga species dispersed on silica follow a similar trend when considering only the fraction of Ga in the nanoparticles. In situ X-ray absorption spectroscopy shows that GaO_x species over the promoted Cu_0.93Ga_0.07–SiO₂ catalyst are much more redox active than those over the poisoned Cu_0.77Ga_0.23–SiO₂. In situ infrared spectroscopy detected methoxy groups over the promoted Cu_0.93Ga_0.07–SiO₂ catalyst, while no formate or methoxy species could be observed over poisoned Cu_0.77Ga_0.23–SiO₂. The absence of formate and methoxy surface species and irreversible oxidation of GaO_x over poisoned catalysts suggest encapsulation of Cu by the GaO_x shell resulting in low activity.