Nanostructured Ni/CeO₂-ZrO₂ Catalysts for CO₂ Conversion into Synthetic Natural Gas

NiO-CeO₂-ZrO₂ mixed oxides, with Ni/(Ce + Zr) = 1 mol/mol and different Ce/Zr molar ratios, were prepared by the soft-template method. The chemical composition, texture, structure, and redox features of the synthesized systems were investigated by different techniques. All samples were nanocrystalline (NiO nanocrystal average size 4 nm) and had high surface area and quite an ordered mesoporous system. The catalytic performances in the CO₂ conversion into methane were studied at atmospheric pressure, 300 °C, and stoichiometric H₂/CO₂ molar ratio. Prior to reaction the catalysts were submitted to a mild reduction pretreatment (H₂ at 400 °C for 1 h). XRD analysis of the samples after pretreatment showed the presence of small Ni crystals (4-7 nm) on all the samples as well as of some unreduced NiO nanocrystals on the systems with high Zr content, in accordance with H₂-TPR experiments, which indicated that NiO reduction is promoted by CeO₂ but hindered by ZrO₂. The catalytic tests were performed at two different space velocities (72000 and 900000 cm³ h^-1 g^-1_cat) on a series of Ni-based catalysts supported on CeO₂-ZrO₂ systems with different Ce/Zr ratios, including the two pure oxides. CO₂ conversion and selectivity to CH₄ (which was always close to 100 mol%) were constant throughout the 6-hour runs. CO₂ conversion resulted to increase with CeO₂ content in the catalyst, thus indicating the role of the CeO₂ component of the support in activating CO₂, whereas H₂ is activated on the Ni nanoparticles.