Design of active sites in Ni/CeO2 catalysts for the methanation of CO2: tailoring the Ni-CeO2 contact

The role of different active sites on Ni/CeO2 catalysts in the CO2 methanation reaction has been studied. Two types of active sites have been identified: CO2 chemisorption and dissociation sites at the NiO-ceria interface and H2 dissociation sites on Ni0 entities. Additionally, the proportion of these active sites has been optimized to maximize the activity. For these purposes, Ni/CeO2 catalysts with different proportion of active sites have been prepared varying the Ni-incorporation method and controlling the structure of the ceria support in order to modify the NiO-Ceria interaction. Three dimensionally ordered macroporous (3DOM) catalysts have been synthesized, and catalysts with uncontrolled structure. The formation of NiO-CeO2 mixed oxides improves nickel dispersion and NiO-CeO2 interaction, increasing the proportion of active sites for CO2 dissociation but limiting the number of centres for H2 dissociation. On the contrary, the impregnation of a nickel salt on the previously synthesized CeO2 support promotes the formation of active centres for H2 dissociation but hinders the formation of active sites for CO2 dissociation. An optimal proportion of both sites is required to achieve the maximum conversion of CO2 to CH4, with 25% of Ni0 for H2 dissociation and 75% of NiO-ceria for CO2 dissociation. In situ DRIFTS experiments showed that the catalyst with the optimum ratio of active sites keeps the catalyst surface clean of carbon intermediates under reaction conditions, while surface bicarbonates are accumulated in a catalyst with an excess of active sites for CO2 chemisorption and dissociation. Isotopic experiments with 13C18O2 confirmed this different behavior of surface carbon intermediates depending on the ratio of active sites for CO2 and H2 dissociation, and also evidenced the participation of catalyst oxygen in the methanation mechanism, since all water molecules emitted as reaction product contains catalyst oxygen