An emerging and high-performance sepiolite-supported Ni catalyst for low-temperature CO2 methanation: The critical role of hydroxyl groups

Abstract

Natural clay minerals compensate for the high cost, complex synthesis and environmental unfriendliness of conventional catalytic supports. To obtain highly-effective and stable clay mineral-supported Ni catalysts for CO2 methanation, 10 wt% Ni was loaded on five common clay minerals (sepiolite (Sep), attapulgite (ATP), montmorillonite (Mnt), diatomite (Dia) and halloysite (Hly)), SiO2 and Al2O3 with a simple impregnation method. 10 wt% Ni/Sep displayed the best catalytic activity, the highest turnover frequency (TOF) and excellent stability, due to the smallest Ni nanoparticles, the strongest weak and moderate basicity, the most abundant surface hydroxyl groups, electron transfer from Sep to Ni, and good resistance to sintering and carbon deposition. Meanwhile, correlation results indicated that hydroxyl groups of clay mineral catalysts are critical for CO2 methanation. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis revealed that magnesium in Sep demonstrates an outstanding ability to activate CO2 at room temperature and the reaction follows both CO and formate routes over 10 wt% Ni/Sep, where CO2 reacted with abundant hydroxyl groups to form bicarbonate; Then, the bicarbonate was further hydrogenated into formate or decomposed into CO, and finally the generation of CH4, which also explains the excellent catalytic performance of 10 wt% Ni/Sep, and deepens the understanding of CO2 methanation mechanism over clay mineral catalysts. This work provides a novel perspective regarding the natural character of clay minerals on CO2 methanation and facilitates the industrial application of high-performance sepiolite-supported Ni catalyst in CO2 methanation.