Enhanced direct methanation of CO2 using Ni-based catalysts supported on ZrO2, CeO2-ZrO2, and La2O3-ZrO2: The effect of support material on the reducible NiO-interacted species and catalytic activity

The lack of reducible NiO-interacted species in ZrO2-supported Ni catalyst at reaction temperature (350 degrees C) renders it inactive towards CO2 conversion. However, the addition of 2.5 wt.% Ce to the catalyst (Ni/Ce-ZrO2) promotes the formation of reducible NiO-interacted species at around 350 degrees C, resulting in 23-18.5% CO2 conversion, 60% CH4 selectivity, and 40% CO selectivity during 13 h of time on stream. The lanthana-zirconia support features size-optimized stable Ni sites, the highest population of oxygen vacancy/interacted CO2 species and oxide diminution, leading to constant 38-36% CO2 conversion with 100% CH4 selectivity up to 13 h of time on stream through the direct methanation route (without involving the CO intermediate step). In the longer time on stream (11-13 h), the formate species destabilizes over Ni/CeO2 catalyst, CO2 conversion drops (from 33 to 24%), and CO selectivity increases (from 0.5% to 5.3%) at the expense of CH4 selectivity (from 99.5% to 94.7%).