Single atoms or not? The limitation of EXAFS

Single atom catalysts (SACs) have recently attracted much attention. To identify the single atom materials, a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and synchrotron radiation-based extended x-ray absorption fine structure (EXAFS) is typically used. However, the present work reveals that EXAFS is not sensitive enough to identify single atoms in many cases, which may easily confuse the contributions from small clusters or nanoparticles due to the polydispersity and disorder effects. Also, HAADF-STEM measures very limited regions, which may easily miss other compositions. The real catalytic sites in various reactions could be significantly different from the single atom sites due to the complicated compositions in the so-called “SACs,” which may also lead to misunderstanding of the working mechanism.