Isotopic and in situ DRIFTS study of the CO2 methanation mechanism using Ni/CeO2 and Ni/Al2O3 catalysts

The CO2 methanation mechanism was studied for Ni/CeO2 and Ni/Al2O3 catalysts. The higher methanation activity and selectivity of Ni/CeO2 is attributed to: i) Ni/CeO2 combines two types of active sites efficient for CO2 dissociation at the NiO-Ceria interface and for H2 dissociation on Ni0 particles; ii) water desorption is the slowest mechanism step, and, due to the high oxygen mobility throughout the ceria lattice, water is not necessarily formed on the same active sites that chemisorb CO2, i.e., the CO2 chemisorption sites are not blocked by water molecules; iii) the Ni/CeO2 surface does not accumulate carbon-containing species under reaction conditions, which allows faster chemisorption and dissociation of CO2. The Ni/Al2O3 catalyst handicaps are that all the steps of the mechanism take place on the same active sites, and the slow release of water and the accumulation of surface formates on these sites delay the chemisorption of further CO2 molecules.