Methanation of CO2 using ultrafine NixFe3−xO4

Ultrafine Ni(II)-ferrites (NiχFe3−χO4, χ = 0.14−1.0; UNFs) with a high reactivity for CO2 methanation have been synthesized by the hydrolysis of Ni2+, Fe2+ and Fe3+ ions at 60–90°C, followed by heating of the co-precipitates to 300°C. The crystallite sizes of the UNFs ranged from 14 to 17 nm. The ferrite particles, size-controlled in the nanoscale range, improved the methanation reactivity. In an H2/CO2 mixed gas-flow system at 300°C, the CH4 yield with UNF was 1.6 times as large as that using Ni(II)-ferrite with a crystallite size of 100–200 nm prepared by the conventional wet method. The maximum yield (40%) and selectivity (95%) for CH4 were obtained at the Ni2+-substitution of χ = 0.36 in UNF. The XRD analysis showed that only the oxygen-deficient Ni(II)-ferrite phase was formed in the UNF with χ = 0.36 during methanation. For χ > 0.36, the carbide phase was formed in the UNF during methanation. The formation of the carbide phase deactivated the ultrafine oxygen-deficient Ni(II)-ferrite.