Recently, Our Center in collaboration with Prof. Jong-Beom Baek’s group from Ulsan National Institute of Science and Technology (UNIST), South Korea, realized the precise characterization of mechanochemically prepared single atom iron catalyst (Fig. 1) by using Mössbauer spectroscopy technique.

This work was published in Nature Nanotechnology on Feb. 7.

In comparison with traditional methods, the mechanochemical method has advantages:(1) the bulk metal is applied as precursor, rather than frequently used metal salts; (2) no solvent is adopted, even no water; (3) there are no any byproducts nor wastes. However, in the prepared single atom catalyst, there still exists occasionally formed metal clusters. Since these clusters are very less and only short-range ordered, thus it is difficult to be found, and even the high resolution extended X-ray absorption fine structure spectroscopy(EXAFS) is hard to quantitatively and qualitatively identify them.

As shown in Fig. 2, although the High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM, Fig. 2a) and EXAFS (Fig. 2b) determined that the Fe featured single atom states, but the coordination environment of Fe and whether all the Fe are single atoms are still unclear.

Fig. 1 | Scheme of preparing single atom catalyst. The surface of bulk metal (blue color) can generate many high-energy defects during mechanical movements. Those defects have low coordination number, and subsequently are easily stripped from the bulk body. The defective substrate materials (yellow color) can facilely capture those active atoms, and finally the single atoms catalyst is formed.

In this study, we resorted to ⁵⁷Fe Mössbauer spectroscopy (Fig. 2c,d) to successfully reveal the coordination environment of Fe (mainly composed of Fe₄N species), and a small amount of Fe clusters identified as Fe₃C (S1) were detected, which are difficult to be found in other characterizations. After heat-treatment purification, even the content of Fe₃C was only 4%, it can also be easily detected by Mössbauer spectroscopy. Our studies demonstrate that Mössbauer spectroscopy is one of the most important techniques to characterizethe purity of single atom Fe.

Fig. 2 | Characterizations of Fe–N–Csingleatomcatalysts. a, HAADF-STEM image of the original sample without heat treatment. The white dot in the red circle is attributed to a typical single iron atom. Scale bar, 1 nm. b, The RDF for Fe–N–Csingleatomcatalystsbefore and after heat treatment. The RDFs were Fourier transformed (FT) from EXAFS. c,d, Mössbauer spectroscopy for Fe–N–C single atom catalysts before (c) and after (d) heat treatment.

This work was supported by National Key Research and Development Program of China, National Natural Science Foundation of China, the International Partnership Program of CAS.

Link: https://www.nature.com/articles/s41565-022-01075-7