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.