Modification of Ni/Sepiolite with Ultrathin MnOx Nanosheets Driving Superior CO2 Methanation: Synergistic Potentiation of Surface Alkalinity and Oxygen Vacancies

Modification of monometallic Ni-based catalysts by transition metals may open a new window to compensate for their poor low-temperature activity and high-temperature deactivation in the CO2 methanation reaction. Here, some transition metals were incorporated into Ni/sepiolite (Sep) by a facile one-step coimpregnation process, where the ultrathin (1.5-2.0 nm) manganese oxides (MnOx) nanosheet-modified Ni/Sep with a Ni/Mn molar ratio of 9 (Ni9Mn/Sep) displayed superior CO2 methanation performance compared to the other elements. Over 85.9% CO2 conversion and 99.6% CH4 selectivity in a 100 h stability test at 350 °C were detected on this sample, which exceeded most of the reported Ni-based catalysts. Molecular dynamic calculations proved the superiority of Sep as a CO2 methanation support and explained the exceptional activity of Ni9Mn/Sep. The enriched hydroxyl groups on the Sep surface and readily formed oxygen vacancies derived from ultrathin MnOx nanosheets facilitated CO2 activation. Moreover, the incorporation of MnOx nanosheets maximized the Ni dispersion, surface alkalinity, NiO reducibility, and metal-support interactions, thus optimizing the catalytic activity and stability. In situ diffuse reflectance infrared Fourier transform spectroscopy also witnessed that the enhanced surface alkalinity and generated oxygen vacancies (due to MnOx nanosheet assembly) were essential for the formation of more active intermediates, ultimately promoting the formate and CO* routes over Ni9Mn/Sep. This work provides an in-depth and comprehensive insight into the nature of Mn modification of Ni-based catalysts for the first time and reveals the potential industrialization prospect of the Sep-supported Ni-Mn bimetallic catalyst for CO2 methanation.