Insight into the impacts of MnO2 crystal phases on CO2 selective hydrogenation to CH4 on Ni/MnOx catalysts

Ni-based catalysts using ε-, α- and β-MnO2 crystal phases supports were prepared and applied to CO2 hydrogenation. The samples were characterized by XRD, H2-TPR, BET, SEM, (HR)TEM, CO2-TPD, Raman, quasi in situ XPS, and in situ DRIFTS. Ni/MnOx-NS (ε-MnO2) was found to exhibit stable activity (with 69.0% CO2 conversion) and selectivity to CH4 (99.0%) at 350 ℃, much higher than Ni/MnOx-R (α-MnO2) and Ni/MnOx-B (β-MnO2). Partially reduced Mn3+ and enriched defects facilitated CO2 and intermediates adsorption/activation, enabling unpaired electrons trapped in defect oxide surface to accelerate the cleavage of C-O to form CH4 assisted by active H, whereas CO production via reverse water-gas reaction was strongly suppressed. This work highlights the key role of synergistic interaction between Ni and MnOx-NS and CO2 activation abilities by various MnO2, which directed the selective CO2 hydrogenation performance.