Investigation of Single Atom Catalysts by X‐Ray Absorption Spectroscopy

Recently, Pt‐free and highly efficient, nanostructured M–N–C catalysts employing earth‐abundant metals (M = Fe, Co, Ni, Mn, Cu, etc.) have been recognized as a very promising alternative to noble metal (in particular platinum)‐ based oxygen reduction reaction materials. Structural engineering of the M–N–C catalysts is expected to further boost their catalytic performance, and thus, intense efforts are currently ongoing. Herein, a report is prepared on X‐ray absorption fine structure (EXAFS) studies of different carbon‐based catalyst materials. EXAFS with its intrinsic sensitivity toward the local atomic arrangement around the X‐ray absorbing element allows to characterize isolated, active single‐atom transition metal centers and their atomic environments. In the case of Co and Fe, the analysis suggests in particular asymmetric, penta‐coordinated Co‐N5 and dual‐coordinated Fe–N–P sites. In both cases, a symmetric environment of the metals with only four nitrogen atoms appears to be unfavorable for the catalytic activity. In addition, for Fe–N4 centers embedded in a graphene host matrix, the EXAFS analysis allows to precisely determine the atomic arrangement around the iron atom, and thereby the geometry of the catalytic site. A discussion will be done on how EXAFS investigations can support the further development of single‐atom catalysts.