Operando XAS/SAXS: Guiding Design of Single‐Atom and Subnanocluster Catalysts

Abstract

Single‐atom and subnanocluster catalysts (SSCs) represent a highly promising class of low‐cost materials with high catalytic activity and high atom‐utilization efficiency. However, SSCs are susceptible to undergo restructuring during the reactions. Exploring the active sites of catalysts through in situ characterization techniques plays a critical role in studying reaction mechanism and guiding the design of optimum catalysts. In situ X‐ray absorption spectroscopy/small‐angle X‐ray scattering (XAS/SAXS) is promising and widely used for monitoring electronic structure, atomic configuration, and size changes of SSCs during real‐time working conditions. Unfortunately, there is no detailed summary of XAS/SAXS characterization results of SSCs. The recent advances in applying in situ XAS/SAXS to SSCs are thoroughly summarized in this review, including the atomic structure and oxidation state variations under open circuit and realistic reaction conditions. Furthermore, the reversible transformation of single‐atom catalysts (SACs) to subnanoclusters/nanoparticles and the application of in situ XAS/SAXS in subnanoclusters are discussed. Finally, the outlooks in modulating the SSCs and developing operando XAS/SAXS for SSCs are highlighted.

Mechanistic understanding and design of non-noble metal-based single-atom catalysts supported on two-dimensional materials for CO2 electroreduction

“…An XANES spectrum can be used to determine the chemical valence state and electronic conguration of the target element, while pre-edge features can offer information such as bond angles. 122,123 EXAFS can be used to probe the local structure and give us valuable spatial insight into the geometric structure and coordination environment. 122 Specically, tting the results from Fourier-transform EXAFS (FT-EXAFS) can help to infer the interatomic distances between the absorber and scattering atoms and coordination numbers.…”

“…For instance, in the work done by Li and coworkers, the doublets (D1 and D2) tted in the Mössbauer spectra are in accordance with Fe-N x moieties wherein the metal center is in low-spin (LS) and intermediumspin (MS) states, respectively. 123 Fontecave and colleagues exploited a series of Fe-N-C catalysts for the CO 2 RR and utilized 57 Fe Mössbauer spectroscopy to distinguish its components, such as a-Fe and iron carbide. 131 In composition analysis by Mössbauer spectroscopy, the isolated Fe-N 4 was proven to be the active site for CO 2 -to-CO conversion.…”

Synchrotron-radiation spectroscopic identification towards diverse local environments of single-atom catalysts

“…164 Thus it is convincing that SAXS can play a vital part in identifying real sites in the processes of reduction and clustering of SACs. 165,166 However, the applications of XES, XRD and SAXS are still scarce in SACs to the best of our knowledge. It is worth noting that the applications of SACs have been extended to the whole courses of energy-storage and conversion.…”

Electrocatalytic acidic oxygen evolution reaction: From nanocrystals to single atoms

“…Moreover, in situ SAXS provides an ideal way to visually observe the morphology of subnanometer clusters. [ 144 ] Anomalous SAXS (A‐SAXS) is a technique that allows one to obtain contrast variation in a multicomponent system. A‐SAXS curves provide information about the operando evolution of the samples’ morphology and particle size.…”