Relationship between Structural Properties of the Unsupported Ni₅Ga₃ Catalyst and Methanol Synthesis Activity

Here, the unsupported Ni₅Ga₃ catalyst was prepared at different reduction temperatures in H₂ for methanol synthesis via CO₂ hydrogenation. The structural variation incurred by changing the reduction temperature was characterized carefully (including extended X-ray absorption fine structure (EXAFS)) and related to the activity and reaction mechanism (by in situ diffuse reflectance Fourier transform spectroscopy (DRIFTS)). The findings show the significant influence of H₂ activation on the structural properties, activity, and selectivity of the catalysts. Specifically, Ni₅Ga₃-500 (i.e., reduced at 500 °C) encourages the copresence of Ga₂O₃ and NiGa alloys, which is key to achieve the appropriate interaction strength with the CO₂-related intermediates, thus giving comparatively highest CO₂ conversion and methanol yield. Longevity tests (100 h) of Ni₅Ga₃-500 showed slightly declined performance at the lower reaction temperature (viz., 220 and 260 °C), which was due to the further reduction of Ga₂O₃ and NiGa alloying during the catalysis.