How Does the Ni-Ga Alloy Structure Tune Methanol Productivity and Selectivity?

In this work, we assess how the structure of SiO2-supported, Ni-Ga alloys determines their activity and selectivity for the hydrogenation of CO2 to methanol. Using a hydrothermal deposition-precipitation approach followed by activation at 700 °C in H2, we synthesize catalysts containing α-Ni, α-Ni9Ga, α'-Ni3Ga, or δ-Ni5Ga3 phases supported on amorphous SiO2. Operando X-ray pair distribution function analysis and X-ray absorption spectroscopy confirm unequivocally the structure of all phases and their stability under reaction conditions; additionally, all catalysts contain GaO x species in varying amounts. We observe that the catalysts α'-Ni3Ga/SiO2 and δ-Ni5Ga3/SiO2 exhibit high methanol formation rates (∼0.8 mmolMeOH molNi -1 s-1), which are 27 times greater than those of α-Ni9Ga/SiO2 and α-Ni/SiO2. Notably, α'-Ni3Ga/SiO2 shows the highest selectivity for methanol at 71%, compared to 55% for δ-Ni5Ga3/SiO2 and 11% for α-Ni9Ga/SiO2, which challenges the conventional view of α'-Ni3Ga being a poor catalyst for methanol synthesis. To explain the high methanol selectivity and productivity of α'-Ni3Ga/SiO2 compared to the other alloy phases, DFT calculations were performed. It was found that the Ni-rich step sites in α'-Ni3Ga effectively stabilize key reaction intermediates (HCOO* and CH3O*) for the formation of methanol. However, such Ni-rich step sites in α'-Ni3Ga also favor CO* dissociation, which could facilitate methane formation, yet the presence of GaO x decreases the stability of CO* on α'-Ni3Ga, explaining ultimately the promotion of HCOO* formation. This study highlights the importance of Ga species (both metallic and oxidic) in modulating the electronic properties of heterogeneous catalysts, providing a versatile toolbox to stabilize key reaction intermediates, leading ultimately to high product selectivity.