NixMg1-xO Solid Solution Catalysts for Improving CO2 Methanation Performance

Abstract

CO2 methanation has been studied as a part of a power-to-gas (P2G) strategy to compensate for the intermittence of renewable energy. Since CO2 methanation is an exothermic reaction, enhancing the low-temperature catalytic activity is a major challenge. In this study, NixMg1-xO solid solutions synthesized by the co-precipitation method were used as CO2 methanation catalysts. The solid solution catalysts exhibited enhanced low-temperature activity compared to those prepared by the conventional wet impregnation method. This is attributed to the uniform distribution of Ni, which promotes the efficient formation of Ni-O-Mg solid solution sites. Subsequently, the optimal synthesis conditions for maximizing catalytic performance were investigated by adjusting the calcination temperature and the Ni:Mg ratio. For the calcination temperature, it was confirmed that optimal performance was achieved at 600 oC, where NiO and MgO form a complete solid solution, the nitrate in the precursor is fully thermally decomposed, and the surface area is maintained at a moderate level. For the Ni ratio, the solid solution catalysts exhibited enhanced catalytic performance as the Ni ratio increased, and particularly at 225 oC, the low-temperature activity of the Ni0.86 and Ni0.96 catalysts was superior to other catalysts. Based on the characterization results, phase segregation was confirmed: Ni0 is produced by the reduction treatment at the solid solution with a high Ni ratio, and the presence of Ni0 plays a key role in improving the low-temperature activity of the catalyst. According to the results of CO2-TPD and H2-TPD, the NixMg1-xO solid solution exhibited an improved CO2 adsorption capacity compared to pure MgO, and Ni0 formed by phase segregation shows an excellent H2 dissociation and spillover ability. These two properties exhibit a synergistic effect, rapidly hydrogenating adsorbed CO2 to CH4, resulting in the superior catalytic performance even at low temperature. In addition, CO2-TPSR and in situ DRIFTS results have verified that for the solid solution with a high Ni ratio, methanation reaction occurs via both the CO pathway and the formate pathway, and each intermediate is rapidly decomposed to produce CH4 at low temperature. Therefore, this study establishes a new foundation for the strategies for developing CO2 methanation catalysts by providing a new breakthrough in promoting the low-temperature activity of the CO2 methanation catalysts.