Boundary Conditions for Promotion versus Poisoning in Copper-Gallium-based CO2–to–Methanol Hydrogenation Catalysts

Cu-Ga-based CO2-to-methanol hydrogenation catalysts are known to display a range of catalytic performance depending on their preparation. Here, using surface organometallic chemistry, we have prepared a series of silica-supported 3-6 nm Cu1-xGaxOy nanoparticles with a range of xGa to establish how the concentration of Ga and alloy formation affect the activity. Cu is always fully metallic in this series, while Ga is partially alloyed with Cu in the core and partially oxidized on the surface. These materials display a volcano-type activity behavior, where methanol formation is promoted when xGa < 0.13-0.18 and is suppressed at higher values, indicating a poisoning of the catalysts. In situ X-ray absorption spectroscopy shows that GaOx species over promoted Cu0.93Ga0.07-SiO2 catalyst are much more redox active than those over the poisoned Cu0.77Ga0.23-SiO2. In situ infrared spectroscopy detected methoxy intermediates over the promoted Cu0.93Ga0.07-SiO2 catalyst, while no formate or methoxy species could be observed over the poisoned Cu0.77Ga0.23-SiO2. The absence of reactive intermediates and irreversible oxidation of GaOx over poisoned catalyst suggests encapsulation of Cu by GaOx shell resulting in low activity.