Natural Kaolin-Based Ni Catalysts for CO(2) Methanation: On the Effect of Ce Enhancement and Microwave-Assisted Hydrothermal Synthesis

[Image: see text] Natural kaolin-based Ni catalysts have been developed for low-temperature CO(2) methanation. The catalysts were prepared via a one-step co-impregnation of Ni and Ce onto a natural kaolin-derived metakaolin using a microwave-assisted hydrothermal method as an acid-/base-free synthesis method. The influences of microwave irradiation and Ce promotion on the catalytic enhancement including the CO(2) conversion, CH(4) selectivity, and CH(4) yield were experimentally investigated by a catalytic test of as-prepared catalysts in a fixed-bed tubular reactor. The relationship between the catalyst properties and its methanation activities was revealed by various characterization techniques including X-ray fluorescence, X-ray diffraction, Brunauer–Emmett–Teller, scanning electron microscopy, selected area electron diffraction, transmission electron microscopy, elemental mapping, H(2) temperature-programmed reduction, and X-ray absorption near-edge structure analyses. Among the two enhancement methods, microwave and Ce promotion, the microwave-assisted synthesis could produce a catalyst containing highly dispersed Ni particles with a smaller Ni crystallite size and higher catalyst reducibility, resulting in a higher CO(2) conversion from 1.6 to 7.5% and a better CH(4) selectivity from 76.3 to 79.9% at 300 °C. Meanwhile, the enhancement by Ce addition exhibited a great improvement on the catalyst activities. It was experimentally found that the CO(2) conversion increased approximately 7-fold from 7.5 to 52.9%, while the CH(4) selectivity significantly improved from 79.9 to 98.0% at 300 °C. Though the microwave-assisted synthesis could further improve the catalyst activities of Ce-promoted catalysts, the Ce addition exhibited a more prominent impact than the microwave enhancement. Cerium oxide (CeO(2)) improved the catalyst activities through mechanisms of higher CO(2) adsorption capacity with its basic sites and the unique structure of CeO(2) with a reversible valence change of Ce(4+) and Ce(3+) and high oxygen vacancies. However, it was found that the catalyst prepared by microwave-assisted synthesis and Ce promotion proved to be the optimum catalyst in this study. Therefore, the present work demonstrated the potential to synthesize a nickel-based catalyst with improved catalytic activities by adding a small amount of Ce as a catalytic promoter and employing microwave irradiation for improving the Ni dispersion.