Natural Kaolin-Based Ni Catalysts for CO2 Methanation: On the Effect of Ce Enhancement and Microwave-Assisted Hydrothermal Synthesis

Natural kaolin-based Ni catalysts have been developed for low-temperature CO2 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 CO2 conversion, CH4 selectivity, and CH4 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 CO2 conversion from 1.6 to 7.5% and a better CH4 selectivity from 76.3 to 79.9% at 300 degrees C. Meanwhile, the enhancement by Ce addition exhibited a great improvement on the catalyst activities. It was experimentally found that the CO2 conversion increased approximately 7-fold from 7.5 to 52.9%, while the CH4 selectivity significantly improved from 79.9 to 98.0% at 300 degrees 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 (CeO2) improved the catalyst activities through mechanisms of higher CO2 adsorption capacity with its basic sites and the unique structure of CeO2 with a reversible valence change of Ce4+ and Ce3+ 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.

Automated summary

The study developed nickel catalysts based on natural kaolin for low-temperature CO2 methanation. Microwave-assisted synthesis produced a catalyst with highly dispersed nickel particles and enhanced catalytic activities, while cerium promotion significantly improved CO2 conversion and CH4 selectivity. The addition of cerium oxide improved catalyst activities through increased CO2 adsorption capacity and unique CeO2 structure properties. The study concluded that the catalyst prepared by microwave-assisted synthesis and cerium promotion was the optimal catalyst for improved catalytic activities.