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 mmol_MeOH mol_Ni ^-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.