Recently, Professor Xia Chuan of the School of Materials and Energy of the University of Electronic Science and Technology of China published a research paper entitled "General synthesis of single-atom catalysts with high metal loading using graphene quantum dots" in the internationally renowned journal Nature Chemistry as the first author and co-corresponding author. The study developed a set of universal synthesis strategies for high-capacity transition metal single atom materials, achieving a high transition metal atomic load of up to 40 wt.% or 3.8 at.%, which is several times or even dozens of times higher than the currently reported single atom load.
The work was done by the University of Electronic Science and Technology of China, Canada Light Source and Rice University in the United States. Professor Xia Chuan of the School of Materials and Energy is the first author and corresponding author of the paper, Professor Wang Yutian of Rice University in the United States and Professor Hu Yongfeng of Canadian Light Source are the corresponding authors of the paper. The collaborative team has established a solid foundation in the field of electrocatalytic materials research and electrochemical reactor design, and has achieved fruitful research results.
Figure 1
Transition metal single-atom materials exhibit excellent activity in a variety of electrocatalytic processes with their extremely high atomic utilization, unique electronic structure, and clear and adjustable coordination structure. However, the low density of metal atoms in conventional single-atom materials (usually less than 5 wt.% or 1 at.%) greatly limits their overall catalytic performance and industrial application prospects, so it is crucial to develop a universal synthesis strategy for high-capacity transition metal single-atom materials. Existing "top-down" and "bottom-up" processes have significant limitations on increasing the metal loading of synthetic single-atom materials (Figures 1, a-b). Taking single atoms loaded with carbon materials as an example, the existing "top-down" method works by creating defects on the surface of carbon material carriers and then stabilizing single atoms through defects. However, the inability to precisely regulate the size of the defect results in a greatly limited number of defect sites, and when the metal load is increased, it is easy to form clusters at large size defect sites. The "bottom-up" method uses metal and organic precursors (e.g., metal-organic frameworks, metal-porphyrin molecules, metal-organic small molecules) to obtain carbon materials supported by metal single atoms. When the metal load is too large, there will not be enough isolation space between the metal atoms to lead to the formation of clusters or particles during pyrolysis.
With this in mind, the team developed a single-atom catalytic material preparation method that differs from existing "top-down" and "bottom-up" processes (Figure 1c) to break through the limitations of single-atom loading. The team innovatively used graphene quantum dots that are larger than surfaces and have high thermal stability as carbon substrates, modifying them with -NH2 groups to have high coordination activity against metal ions. After the introduction of metal ions, a crosslinked network with metal ions as nodes and functional graphene quantum dots as structural units can be obtained, and finally pyrolysis can obtain a high-capacity metal single-atom material. Compared with the traditional "top-down" and "bottom-up" single-atom catalyst synthesis methods, the method reported in this study not only ensures the high dispersibility of the initial anchoring of high-content metal ions, but also effectively inhibits the agglomeration of metal atoms caused by the subsequent pyrolysis process of substrate sintering and reconstruction.
Figure 2. Structural guarantee of high-capacity Ir1-N-C single-atom catalytic materials
XAFS, HADDF-STEM and other characterization methods prove that the supported metal single-atom catalytic material obtained by this method can achieve a metal load that far exceeds the existing literature report level while ensuring the monodispersity of metal atoms. Using this method, the team successfully prepared an Ir single-atom catalytic material with a mass fraction of up to 41.6% (atomic fraction of 3.84%) (Figure 2), which was several times higher than the maximum load of Ir single atoms reported in the literature.
Figure 3. Structural characterization of high-capacity Pt single-atom and high-capacity Ni single-atom catalytic materials
In addition, this synthesis strategy is also universal and can be used to prepare high-capacity metal single-atom catalytic materials for other precious or non-precious metals. For example, on carbon-based substrate materials, Pt single atoms can be loaded up to 32.3 wt.%, ni single atom loads can reach 15 wt.% (Figure 3).
Xia Chuan, Professor of School of Materials and Energy, University of Electronic Science and Technology of China, National Young Talent. His research interests are electrocatalysis, electrosynthesis, and electrochemical biosynthesis based on new energy sources, and he is committed to achieving carbon balance in energy and material cycles. In the characteristic direction of "on-site synthesis of liquid fuels and basic chemicals", he has carried out in-depth and systematic research, and has achieved fruitful results in the field of reactor and catalyst design, published more than 50 academic papers, authorized 3 US patents, 34 H factors, and cited more than 5200 times. For nearly five years, as the first author/corresponding author, he has worked in Science, Nat. Energy、Nat. Catal.、Nat. Chem. and other high-level journals at home and abroad have published more than 20 papers, including 9 highly cited papers and 2 hot papers.