Modification of Ni/Sepiolite with Ultrathin MnO_<i>x</i> 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 (MnO_<i>x</i>) 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 MnO_<i>x</i> nanosheets facilitated CO₂ activation. Moreover, the incorporation of MnO_<i>x</i> 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 MnO_<i>x</i> 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.