Surrounded Ni@Al₂O₃ as a Robust Catalyst for CO Methanation: The Size Effect of Ni Nanoparticles

Ni-based catalysts encounter significant obstacles, such as carbon deposition and sintering in the CO methanation reactions. To date, although there have been many reports concerning the influence of Ni particle size on the stability and resistance to carbon deposition in CO methanation, there remains significant diversity and even contradiction regarding the optimal Ni particle size. Furthermore, systematic regulation of Ni particle size and elucidation of its specific impact mechanisms on anticoking performance have been far less explored. Herein, we designed and prepared a series of Ni@Al2O3-n catalysts, confining Ni nanoparticles with gradually increasing sizes within the same γ-Al2O3 cages, to systematically investigate the influence of Ni particle size on the stability and carbon deposition resistance during CO methanation. An interesting volcano-shaped curve relationship between the amount of carbon deposition and the average Ni particle sizes in the catalysts was discovered. Among them, the Ni@Al2O3-3 catalyst with nickel nanoparticles of moderate size (average size ∼7.6 nm) achieved high activity with complete conversion of CO at 260 °C and exhibited superior resistance to carbon deposition, delivering remarkable stability performance over 300 h of continuous operation at 260 °C. Further density functional theory results demonstrated that for medium-sized Ni nanoparticles, the transformation rate of C* to CH* was significantly enhanced compared to the generation rate of C*. This rapid conversion of generated C* species prevented their accumulation on the catalyst surface, leading to remarkable resistance to carbon deposition. This study provides valuable insights and strategies for the design and fabrication of high-performance catalysts for CO methanation.