Supported-Single Nickel Atom Catalysts for the Methanation of Carbon Dioxide

Synthesis of twenty-seven bimetallic catalysts consisting of nickel and one of nine different dopants (B, Co, Cu, Fe, Mg, Mn, Sn, V, and Zn) supported on three different metal oxides (Al₂O₃, CeO₂, and SiO₂) is carried out via organometallic grafting. The catalysts are evaluated for their activity and selectivity for the CO₂ methanation reaction at a feed ratio of H₂/CO₂ of 4 at 300 °C in a high-throughput flow reactor system. After <i>in situ</i> pre-activation (500 °C in H₂), Ni/Co/CeO₂ exhibited high conversion (84.3%) and selectivity for methane (99.6%). Ni/Co/CeO₂ was characterized by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction, H₂-temperature-programmed reduction (H₂-TPR), and CO₂-temperature-programmed desorption (CO₂-TPD). HRTEM showed the presence of single Ni and Co atoms on ceria after pre-reduction at 500 °C and after the methanation reaction at 300 °C for 15 h. XPS determined that the strong interaction between Ni, Co, and ceria increased after the reduction, leading to a charge transfer between Ni and Ce that created oxygen vacancies in ceria. Nickel was found to be Ni²⁺ in the as-prepared material and was partially reduced in the presence of cobalt and after the activation in H₂ at 500 °C. The DFT results show that both nickel and cerium exhibit lower Bader charges in the Ni/Co/CeO₂ system, confirming that the presence of cobalt enhances the reduction of both Ni and Ce through electronic interactions. This indicates that single cationic Ni atoms are highly effective for the methanation reaction. The organometallic grafting technique is found to be efficient for synthesizing catalysts with highly homogeneous dispersed species at low metal loadings (0.16 wt % Ni-0.15 wt % Co), which leads to high turnover frequency (up to 248.7 h^-1) and durability for methanation.