Recently, the research group of Prof. Zeng has investigated the active phase of cobalt (Co)-based catalysts in CO₂ hydrogenation. They have incorporated N atoms into Co catalysts to fabricate Co₄N catalysts. Based on in-situ mechanistic studies, they found that Co4NH_x served as the active phase during CO₂ hydrogenation. This work has been published on Nature Energy with the title of “Incorporating nitrogen atoms into cobalt nanosheets as a strategy to boost catalytic activity toward CO₂ hydrogenation” [Nature Energy 2017, DOI: 10.1038/s41560-017-0015-x].

Illustration of CO₂ hydrogenation over Co₄N catalysts

With the rapid development of technology and industrialization, the demand for energy has been increasing year by year. Because of the heavy use of fossil fuels, the energy and environmental crisis have compelled human society to seek for  renewable energy sources. The hydrogenation of CO₂ into fuels and valuable chemicals has emerged as an effective strategy to both mitigate the green house effect and achieve a sustainable carbon cycle. CO₂ hydrogenation also serves as an efficient approach to store hydrogen which derives from electrolytic production.

Although the chemical transformation of CO₂ has been achieved in industry, the reaction requires high pressure (50-100 bar) and elevated temperature (200-300 ℃) because of the chemical inertness of CO₂. The past decades have witnessed the development of strategies to improve the activity of non-noble metal catalysts towards CO₂ hydrogenation. Up to now, the investigation of the active phase of non-noble catalysts in CO2 hydrogenation is still at the rudimentary stage.

The researchers incorporated N atoms into Co catalysts to synthesize Co4N catalysts. In CO₂ hydrogenation, Co₄N catalysts exhibited a turnover frequency (TOF) of 25.6 h-1 under 32 bar at 150 ℃, which was 64 times as high as that of Co catalysts. The activation energy for Co₄N catalysts was 43.3 kJ mol-1, less than half of that (91.4 kJ mol-1) for Co catalysts. Further mechanistic studies revealed that Co₄NH_x was formed by adsorbing H atoms on N atoms in Co₄N nanosheets in the presence of H₂. The amido-hydrogen atoms in Co₄NH_x directly added to CO₂ to form HCOO* as the intermediates. In addition, the adsorbed H₂O* was found to activate the amido-hydrogen atoms via the interaction of hydrogen bonds, which facilitated the hydrogenation process. This work not only provides a way to engineer efficient, low-cost catalysts for CO₂ hydrogenation via the incorporation of N atoms, but also advances our understanding of active phase of Co-based catalysts.

This work was supported by MOST of China, and the National Natural Science Foundation of China.

Source: https://www.nature.com/articles/s41560-017-0015-x.