Highly active NiO-CeO2 catalysts for synthetic natural gas production by CO2 methanation

NiO-CeO2 mixed oxides were prepared by the soft-template method with nominal Ni/Ce molar ratios between 0.3 and 4.0. NiO and CeO2 pure oxides were also prepared through the same procedure. A second series of NiO-CeO2 mixed oxides (nominal Ni/Ce values = 0.3 1.5 mol mol-1) was synthesized by using the soft-templated ceria as the support and depositing Ni through incipient wetness impregnation. All samples were characterized by different techniques as to their chemical composition, structure, morphology, texture, and redox features. NiO nanocrystals of about 4 nm in size were obtained in the case of the soft templated samples, regardless of the Ni loading. Larger NiO nanocrystals (up to 20 nm in size) were formed by depositing Ni through the impregnation procedure. The catalytic performance was investigated in the CO2 methanation reaction after mild reduction pretreatment (H2 at 400 °C for 1 h). Catalytic testing was performed under atmospheric pressure, 300 °C, 72000 cm3 h-1 gcat-1, and stoichiometric H2/CO2 molar ratio. Pure NiO as well as the two NiO CeO2 mixed oxides series showed high CO2 conversions (up to 87 mol%) together with CH4 selectivity values close to 100 mol%. Despite the significantly different size (6-8 and 15-35 nm, respectively) of the Ni0 nanocrystals originated by NiO reduction, comparable CO2 conversion values were observed for the soft-templated and impregnated catalysts. The catalyst stability as well as the effect of reaction temperature, space velocity, and H2/CO2 molar ratio were investigated on selected samples. The catalytic results were explained by taking into account that CO2 and H2 are activated on different phases.