XAFS method for the structural characterization of single atom catalysts

Single atom catalysts (SACs) have become a hot topic in catalysis research due to their 100% atomic utilization, unique coordination environment, and superior catalytic performance. To explain the special catalytic behavior of SACs and maximize the advantages of single atom configurations, understanding the structure-performance relationship of SACs at the atomic level is key to the research. X-ray absorption fine structure (XAFS) provides an irreplaceable method for characterizing SACs, which can be applied to almost all transition-metal SACs and can obtain rich structural information about atomistic structures and electronic properties. In this review, starting from the basic principles of XAFS technology, we discuss the application of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy in the study of the local coordination, symmetry, hybrid of orbitals, oxidation state, charge transfer and many other structural and electronic properties. After that, we introduce the emerging advanced X-ray spectroscopy methods that improve characterization capabilities and compensate for the shortcomings of traditional XAFS methods in SAC research, and then discuss in-situ and operando experimental techniques for SAC characterization to monitor the dynamic structure of the active sites in the presence of reactants under reaction conditions.