Structural Analysis of Single-Atom Catalysts by X-ray Absorption Spectroscopy.

ConspectusMetal nanoparticles (NPs) are one of the most frequently used heterogeneous catalysts. However, only the surface atoms in the NPs can participate in catalytic reactions. To maximize the atomic efficiency, the active sites can be reduced to single atoms. Generally, catalysts that have isolated metal atoms on the surface of a support are called single-atom catalysts (SACs). Many techniques have been developed and applied to probe the structures of SACs. Nevertheless, the structural characterization of SACs is still challenging as it requires the analysis of their structure and properties with atomic and sometimes even subatomic resolution. X-ray absorption spectroscopy (XAS) is a powerful tool in investigating the local coordination environment of SACs since it is element-specific and can provide accurate structural information at the subatomic level (∼0.01 Å).In this Account, we present our perspectives on the structural analysis of SACs from some unique features in the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We first highlight the importance of the XANES peak features in the sensitive analysis of SAC structures. Such analysis is illustrated to be even more useful in the joint applications of experimental and theoretical XAS. The inspection of the metal-metal (M-M) peak in Fourier transformed EXAFS (FT-EXAFS) spectra is a widely used method to identify the single-atom structure, but this method is not always reliable. Thus, we point out the importance of fitting EXAFS and the thorough interpretation of structural parameters such as coordination numbers (CNs, the number of neighboring atoms next to a chosen atom), bond distances, and the Debye-Waller factor (σ). The small FT-EXAFS peak for the M-M shell is often ignored in the structural analysis of SACs. Here, it is demonstrated that a careful analysis of these small peaks could help more reliably analyze the SAC structure, and it would be particularly useful in the analysis of a single-atom alloy (SAA). Next, the usefulness of bond distance and σ analysis is highlighted, and such analysis is shown to be particularly helpful for the analysis of SAAs, which is rarely discussed in the literature. Given the advantage that XAS data can be collected under various conditions, we show that in situ XAS can provide important information about the catalytic mechanism of the SAC catalyst. In particular, we emphasize the significance of using an advanced in situ technique to extract detailed structural information that is difficult to obtain from regular XAS experiments. Finally, we highlight the importance of jointly using XAS with other complementary methods in a more complete understanding of the structure and properties of SACs. It is anticipated that with further development of XAS techniques and improved data analysis, XAS will become even more powerful in providing insights into the structure-property relationships of SACs, which can advance their practical applications.