In Situ/Operando Techniques for Characterization of Single-Atom Catalysts

Abstract

In situ/Operando characterization techniques are powerful to provide fundamental information about molecular structure-activity/selectivity relationships for various catalytic systems under controlled condition. However, the lack of model catalyst, as the major obstacle for deeper understanding on the nature of active sites and reaction mechanisms, hinders the further advancements in catalysis. Fortunately, the rapid development of single-atom catalysts (SACs) offers us new opportunities for capturing the reaction intermediates, identifying the active sites, and even monitoring the dynamic behaviors of both the geometric structure and electronic environment of the catalytic sites at atomic scale. In this review, the recent advances on the in situ/operando characterization techniques including X-ray absorption spectroscopy, scanning tunneling microscopy, Fourier-transform infrared spectroscopy, and etc. for the characterization of SACs are thoroughly summarized. The results from these in situ/operando measurements reveal the crucial role of SACs as model systems for sharpening our understanding on the nature of catalytic sites. Furthermore, the challenges and outlooks in developing in situ/operando techniques for single atom catalysis are discussed.

Single‐Atom Catalysts in Catalytic Biomedicine

“…Theoretical calculation is an exceedingly efficient means to elucidate the possible reaction pathways and intermediates during the catalytic processes for biomedical use. The well‐established active sites of SACs endow them with pronounced superiority in identifying the catalytic mechanisms . Nevertheless, the possible reaction mechanisms have not definitely clarified for most SACs, especially in biomedical use.…”

“…Currently, the majority of catalytic processes in biomedical applications are monitored via ex situ spectroscopic and microscopic approaches . However, the actual active sites of catalytic processes in biomedical applications may be incongruity with that monitored by ex situ characterization strategies owing to some chemical or physical factors, such as pH, temperature, biological surroundings, and reaction substances . Consequently, developing direct and efficient in situ dynamic measurements, such as in situ STM, XAFS, STEM, and FTIR, is of prominent significance to fundamentally understand the in vitro and in vivo catalytic processes and reaction mechanism as well as rational design of high‐performance SACs for versatile biomedical applications.…”

“…In addition, single Pt atoms embedding resulted in the atomic rearrangement of Co atoms, which was further confirmed by FT‐EXAFS spectra (Figure d–g). Furthermore, Fourier transform infrared (FTIR) spectroscopy could be performed in situ or ex situ, playing a crucial role in detecting intermediate species in the catalytic processes by monitoring the variations in vibrational frequency and intensity of appropriate probe molecules . In addition, in situ FTIR provides an effective tool to identify the existence of active catalytic sites and exclude the presence of clusters/nanoparticles in supported SACs.…”

“…In addition, in situ FTIR provides an effective tool to identify the existence of active catalytic sites and exclude the presence of clusters/nanoparticles in supported SACs. For instance, in situ FTIR spectra of the absorbed CO molecule on the active sites were monitored to confirm the presence of isolated Pt active sites in Pt 1 /FeO x SAC …”

Oxygen Reduction Reactions on Single‐ or Few‐Atom Discrete Active Sites for Heterogeneous Catalysis

“…Although the physical origin of these two techniques are the same, they are sensitive for different properties, where XANES are more suited to reveal the coordination chemistry and oxidation state, and EXAFS can give information on coordination number, atomic bond distances and therefore are better suited to reveal the coordination environment of the metal atoms. These techniques are vital for the understanding of the catalytic activity of metal coordinated motifs, and we briefly refer to them in the following chapters; however we do not expand further on the interpretation of these techniques in this review, but refer to more specialized publications . In the case of iron‐containing species, we note however that 57Fe Mössbauer spectroscopy also can be particularly useful and aid to elucidate the coordination environment of the iron atom …”