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 in situ 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.