Hydrogen Energy Storage via CO2 Hydrogenation over Catalysts Prepared by Layered Double Hydroxide Precursor

Converting CO2 and green hydrogen into products such as methane and methanol not only has a negative carbon effect, but also stores renewable energy into energy chemicals. This represents a promising route for hydrogen energy storage technologies. The hydrogenation of CO2 to methane and methanol, which represent strongly exothermic reactions, are thermodynamically favored at low temperatures. However, the inherent inertness of CO2 makes it difficult to activate CO2 at low temperatures. Both reactions face the challenge of activating CO2 at low temperature, so catalysts exhibiting high activity under such conditions are a critical need. Layered double hydroxides (LDHs) have attracted considerable interest owing to their regular layered structure and uniform dispersion of multiple metallic components. However, there are few studies on the same effects of promoters over LDHs-derived catalysts. Here, we investigated the same effects of promoters on two LDHs-derived catalysts in different CO2 hydrogenation reactions to illustrate the effects of promoters on facilitating low-temperature CO2 activation in LDHs-derived catalysts. By adding promoters Fe and Mn to the catalysts NiAl-Fe and CuZnAl-Mn, the crystal lattices were expanded, surface areas were increased 38% and 25%, and the reduction temperatures were decreased to 97 °C and 10 °C, respectively. These promoters significantly enhanced the CO2 adsorption and activation of the catalysts NiAl-Fe and CuZnAl-Mn. The methanation catalyst NiAl-Fe achieved a CO2 conversion of 80.8% at 200 °C and 2 MPa, while the methanol synthesis catalyst CuZnAl-Mn exhibited a CO2 conversion of 21.3% and a methanol selectivity of 61.8% under the conditions of 250 °C and 3 MPa. The influence of the LDHs precursors’ structure and the addition of promoters Fe and Mn on the catalytic performance were studied by XRD, N2 adsorption–desorption, H2-TPR, H2-TPD, and CO2-TPD.