Characterization and catalytic activity of soft-templated NiO-CeO2 mixed oxides for CO and CO2 co-methanation

Nanosized NiO, CeO₂ and NiO-CeO₂ mixed oxides with different Ni/Ce molar ratios were prepared by the soft template method. All the samples were characterized by different techniques as to their chemical composition, structure, morphology and texture. On the catalysts submitted to the same reduction pretreatment adopted for the activity tests the surface basic properties and specific metal surface area were also determined. NiO and CeO₂ nanocrystals of about 4 nm in size were obtained, regardless of the Ni/Ce molar ratio. The Raman and X-ray photoelectron spectroscopy results proved the formation of defective sites at the NiO-CeO₂ interface, where Ni species are in strong interaction with the support. The microcalorimetric and Fourier transform infrared analyses of the reduced samples highlighted that, unlike metallic nickel, CeO₂ is able to effectively adsorb CO₂, forming carbonates and hydrogen carbonates. After reduction in H₂ at 400 °C for 1 h, the catalytic performance was studied in the CO and CO₂ co-methanation reaction. Catalytic tests were performed at atmospheric pressure and 300 °C, using CO/CO₂/H₂ molar compositions of 1/1/7 or 1/1/5, and space velocities equal to 72000 or 450000 cm³?h^–1?g_cat^–1. Whereas CO was almost completely hydrogenated in any investigated experimental conditions, CO₂ conversion was strongly affected by both the CO/CO₂/H₂ ratio and the space velocity. The faster and definitely preferred CO hydrogenation was explained in the light of the different mechanisms of CO and CO₂ methanation. On a selected sample, the influence of the reaction temperature and of a higher number of space velocity values, as well as the stability, were also studied. Provided that the Ni content is optimized, the NiCe system investigated was very promising, being highly active for the CO_x co-methanation reaction in a wide range of operating conditions and stable (up to 50 h) also when submitted to thermal stress.