Ambient-Pressure CO₂ Hydrogenation to Methanol over Ni₅Ga₃ Catalysts Prepared via a Solvent-Free Route: The Influence of Structure and Composition

Ni5Ga3-based catalysts hold promise for CO2 reduction into methanol because the operating pressure is significantly lower than that required for commercial catalysts. While the synergy between Ni5Ga3 and Ga2O3 is crucial in methanol synthesis, the controllable formation of Ga2O3 on Ni5Ga3 nanoparticles and their structure-related interactions under varied preparation conditions remain underexplored. This study introduces a series of Ni5Ga3 catalysts derived from the reduction of oxide precursors prepared via a combustion method with different fuels like oxalic acid, citric acid, and urea. Besides the combustion processes that were studied, various characterization techniques disclosed that the components and physicochemical properties of these catalysts can be strongly influenced by the choice of fuel, affecting the Ni5Ga3 nanoparticle size along with the content of Ga2O3 and, consequently, the catalytic performance toward CO2 reduction. The catalysts prepared with oxalic acid were composed of the smallest Ni5Ga3 nanoparticles and the most abundant Ga2O3 species and thus exhibited the best performance compared to its counterpart. In addition, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis was performed to provide insights into reaction intermediates and mechanisms. This study highlights the critical role of Ga2O3 in catalytic efficiency and underscores the influence of the synthesis parameters on the performance of Ni5Ga3 catalysts in CO2 hydrogenation to methanol.