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 degrees C, and stoichiometric H-2/CO2 molar ratio. Prior to reaction the catalysts were submitted to a mild reduction pretreatment (H-2 at 400 degrees 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 H-2-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 cm(3) h(-1) g(cat)(-1)) 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 H-2 is activated on the Ni nanoparticles.