The rapid development of technology and industrialization leads to higher consumption of energy. Energy shortage and environmental problems caused by fossil fuels force the development of renewable energy.

The hydrogenation of CO₂ into fuels and valuable chemicals has emerged as an effective strategy to mitigate the greenhouse effect and achieve a sustainable carbon cycle. CO₂ hydrogenation also serves as an efficient approach to store hydrogen which derives from electrolytic production.

Prof. ZENG Jie's group at University of Science and Technology of China of Chinese Academy of Sciences has investigated the active phase of cobalt (Co)-based catalysts in CO₂ hydrogenation. They 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 in Nature Energy.

The chemical transformation of CO₂ has been achieved in industry, and the reaction requires high pressure (50-100 bar) and elevated temperature (200-300^oC) 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 in its infancy.

In this study, 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^oC, 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. The adsorbed H₂O* was found to activate the amido-hydrogen atoms via the interaction of hydrogen bonds, which facilitated the hydrogenation process.

The study not only provided a way to engineer efficient, low-cost catalysts for CO₂ hydrogenation via the incorporation of N atoms, but also advanced the understanding of active phase of Co-based catalysts.

This work was supported by the Ministry of Science and Technology (MOST) of China, National Basic Research Program of China and the National Natural Science Foundation of China.

Figure: Illustration of CO₂ hydrogenation over Co₄N catalysts (Image by ZENG Jie)