Metal-containing crystalline porous materials (CPMs) are gaining popularity in heterogeneous catalysis because of their highly crystalline and porous systems, and their excellent chemical tunability. Modification of the metal species and framework structure permits them to have greater activity, selectivity, and stability over other materials. An in-depth understanding of the complex nature of metal active sites in CPMs is essential for revealing the structure-performance relationships and directing the rational design of such catalysts. Compared to conventional characterization techniques, the rapid development of X-ray absorption spectroscopy (XAS) has provided element- and site-specific deep insights into the electronic and structural information of metal species in CPMs. As such, this review begins by summarizing novel XAS techniques and analysis methods in accurately obtaining such data. Next, the combination of XAS with other high-level characterization methods into disclosing the configuration of active sites in metal-containing CPMs is presented. Then, the utilization of theory-assisted XAS data analysis in examining complex metal-containing CPM catalysts is discussed. Afterwards, advanced in-situ/operando XAS studies into revealing the working sites in metal-containing CPMs under catalytic conditions are highlighted. We conclude by outlining the future challenges and prospects of XAS measurements, data analyses, and in-situ/operando setups in advancing the study of metal-in-CPM catalysts.

Introduction

Crystalline porous materials (CPMs), such as zeolites, metal–organic frameworks (MOFs), covalent organic frameworks (COFs), and zeolitic imidazolate frameworks (ZIFs), are of great interest due to their high crystallinity, superior porosity, and excellent chemical tunability, making them ideal materials in the design of heterogeneous catalysts.¹^–³ Implanting active metal species into CPMs can endow them with tremendous catalytic potential in heterogeneous catalysis, including thermocatalysis, electrocatalysis, and photocatalysis.⁴^–⁶ Owing to the different forms of metal species (nanoparticles, nanoclusters, single-atoms, cations, etc.) and the diversity in framework structure of CPMs, such catalysts can exhibit activity, selectivity, and stability in heterogeneous catalysis.⁷^–¹² Despite the promising aspects of metal-containing CPMs, understanding the complex configurations of metal active sites in CPMs remains challenging.

The interaction between the metal species and the long-range ordered frameworks of CPMs modulates the unique geometric and electronic structures of the metal species.⁵ Therefore, the determination of the precise configuration of metal species in CPMs plays a crucial role in designing and optimizing such catalysts. However, the routine use of conventional characterization methods such as X-ray diffraction, infrared spectroscopy, ultraviolet–visible spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron microscopy, can provide fundamental identification of particle sizes, species, and valence states for metal active sites in CPMs.¹³^–¹⁵ To dig out atomic level structural and dynamical information from metal-in-CPM catalysts, more powerful characterizations techniques are still required.

X-ray absorption spectroscopy (XAS) is a characterization technique that element-selectively examines the bulk geometries, electronic properties, and atomic structures of metal active sites in multielement CPMs.¹⁶ Typically, an XAS spectrum can be divided into two regions (Figure 1): X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES region, occurring approximately 50 eV before and after the absorption edge, exhibits sensitivity towards the valence states and electronic structures of the detected elements. This is because the core electron energy is influenced by the electron distribution in the valence state.¹⁷ Notably, for 3d transition metals, the main absorption edge pertains to transitions from the 1s to 4p states while the corresponding pre-edge, on the other hand, covers transitions from the 1s to 3d states, which is useful in probing the geometries of metal clusters.¹⁸ The EXAFS region starts from about 50 eV and extends to roughly 1000 eV beyond the absorption edge, which is dominated by photoelectron absorption. The EXAFS region provides limited electronic structure details due to the high density of available electronic states and the high energy of the photoelectrons; yet, it is capable of providing information on coordination numbers (CNs) and bond distances around the absorber, as it is a sensitive probe of spatial arrangements of nearest neighbors around the absorbing atom(s).¹⁹ The rapid development of XAS, both as an X-ray technique [e.g., high energy resolution fluorescence detection (HERFD)] and as an analysis method [e.g., wavelet transformation (WT) for EXAFS, linear combination fitting (LCF) for XANES] has drawn significant attention since more detailed information on the atomic structures, electronic properties, and species determination of the metal active sites in CPMs can be obtained.²⁰^–²²

Figure 1 | Illustration of an XAS spectrum and the advantages of XAS in metal-in-CPMs’ characterization.

Additionally, when XAS data is coupled with experimental results obtained using complementary advanced techniques such as scanning transmission electron microscope (STEM), CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), electron paramagnetic resonance (EPR) spectroscopy, Mössbauer spectroscopy, and XPS, more integrated characterization for metal-in-CPM catalysts can be achieved.²³^–²⁵ Moreover, the successful adoption of computational theory [e.g., XANES/EXAFS simulation, multivariate curve resolution-alternating least squares analysis (MCR-ALS), new XAS analysis program, etc.] is helping to unravel the complex nature of metal active sites in CPMs.²⁶^–²⁸

In recent years, in-situ/operando XAS is becoming more commonly used in a broad range of metal-in-CPM materials (ordered, disordered, nanostructured, etc.) under different working conditions, which have facilitated the capture of oxidation states and local geometric coordination changes, thus promoting the elucidation of catalysis mechanisms.²⁹^–³¹ Furthermore, thanks to the development of in-situ/operando XAS, particularly in time-resolved measurements, simultaneous multispectra measurements, and machine-learning (ML) assisted analyses, such advanced characterization techniques allow deeper insight of the evolution of coordination environments of active metal sites in CPMs under catalytic conditions.³²^–³⁴

XAS shows many advantages in the characterization of metal-in-CPMs (Figure 1) and is becoming increasingly important in the design and optimization of metal-in-CPM catalysts. Therefore, in this minireview, we will summarize the progress of some representative works about advanced XAS analyses for metal-containing CPMs in heterogeneous catalysis over the last 5 years. We will first introduce the novel X-ray techniques and analysis methods which have been used to accurately identify the chemical states and atomic structures of metal sites in CPMs. Then, we will outline the combined use of XAS and other advanced characterization techniques to disclose the configuration of metal active centers in CPMs. Next, we will discuss theory-assisted XAS analyses which greatly deepen the understanding on complex metal-in-CPM systems. In-situ/operando XAS, which is divided into thermocatalysis, electrocatalysis/photocatalysis, and advanced in-situ/operando XAS techniques, will also be introduced, thus shedding light on the structure-performance relationships of metal-containing CPM catalysts. Finally, we will highlight the current challenges and prospects of XAS measurements, data analyses, and in-situ/operando setups for metal-in-CPM catalysts.

Advanced XAS Analysis for Metal-Containing CPM Catalysts

The cutting-edge XAS techniques

Conventional XAS has been beneficial in understanding the oxidation states and atomic structures for metal active sites in CPMs, however its resolution (especially in XANES) is limited by core-hole lifetime broadening.³⁵^,³⁶ The development of X-ray techniques such as HERFD mode, which reduces the effective core-hole lifetime, opens the door to disclosing the complex nature of metal species in CPMs.²⁰^,³⁷ For instance, HERFD-XANES is a technique that can be used to investigate the pre-edge features of 3d transition metals, which correspond to the 1s to 3d electronic transitions and reflect the oxidation states and coordination geometries of these metals, which was done by Zhang et al.³⁷ using Fe K-edge HERFD-XANES to identify the iron species in different Fe-containing zeolites (Figure 2a,b). By comparing the pre-edge peak information of [Fe]zeolite catalysts and reference samples in their HERFD-XANES spectra (Figure 2a), the iron species were shown to be isomorphously substituted into the zeolitic framework (FeCHA100 and FeMFI100), which were directly hydrothermally synthesized in the form of tetrahedral coordination (Fe³⁺ T_d). In contrast, the octahedral coordinated (Fe³⁺ O_h) iron species, like Fe₂O₃ nanoparticles and Fe-oxo clusters, were mainly present in impregnated [Fe]zeolite catalysts (FeCHA100-imp and FeMFI100-imp). The HERFD-XANES spectroscopy results were combined with those from X-ray emission spectroscopy, which further confirmed the iron species in zeolite. Therefore, compared to the impregnation method, the hydrothermal synthesis was proven to be beneficial for the dispersion of Fe atoms in the zeolitic framework. Consequently, isomorphously substituted [Fe] zeolite catalysts can exhibit enhanced catalytic performance in the nonoxidative coupling of methane.

Figure 2 | (a) Pre-edge region of Fe K-edge HERFD-XANES, and (b) the related pre-edge peak analysis of various [Fe]zeolite catalysts. Reprinted with permission from ref 37. Copyright 2023 Wiley-VCH. (c) Dynamic changes in the average oxidation states of cobalt according to the edge position of Co K-edge XANES spectra of the reference Co@Y sample and Co@Y after the successive feeding of O₂ and C₃H₆ at 500 °C. Reprinted with permission from ref 38. Copyright 2022 American Chemical Society. (d) LCF of the Zn K-edge spectra of Zn₁AlO_x/AlPO-18. Reprinted with permission from ref 39. Copyright 2023 American Chemical Society. 2D plots of WT-EXAFS for (e) CuO and (f) Cu-exchanged FAU zeolite. Reprinted with permission from ref 40. Copyright 2020 Royal Society of Chemistry.

With the development of new analysis methods for XANES spectra, quantitative valence state analysis for metal centers in CPMs (cast into a single energy value) can be achieved by establishing a linear correlation between the edge energy and the valence state of reference samples.⁴¹ For example, Li et al.³⁸ revealed that the average oxidation state was +2.11 for cobalt in Co@Y according to the Co K-edge absorption edge positions (Figure 2c). In addition, the valence state of Co was around +2 (+2.11, +2.21, +2.00) during redox cycles of propylene aerobic epoxidation, indicating its stability at high temperature (500 °C). Despite the valence state determination for metal-containing zeolite, such analysis methods have also been used to resolve the oxidation states of some metal active centers (e.g., Ni, Ru) in ZIF and MOF catalysts.⁴²^,⁴³

In addition to valence state analysis, LCF of XANES can be used to quantitatively distinguish metal species from a mixture by comparing the obtained spectra with well-known reference materials.²¹^,³⁹^,⁴⁴ An example of the use of LCF for XANES was shown by Su et al.,³⁹ who determined the active zinc species in the AlPO-18 zeolite bifunctional catalyst during syngas conversion. Due to the existence of different Zn²⁺ species, a direct comparison of XANES spectra was unable to characterize the catalytic sites in the bifunctional catalyst. Upon comparison with reference spectra, it was found that the XANES curve of Zn₁AlO_x in the bifunctional catalyst exhibited characteristics of both ZnO and ZnAl₂O₄ phases. Therefore, LCF of the XANES was conducted to unveil the concentrations of ZnO (56 mol %) and the ZnAl₂O₄ spinel (44 mol %) in the bifunctional catalyst (Figure 2d), which were close to the theoretical value (50:50 in mol ratio) of the actual feeding. Based on this result, the ZnAl₂O₄ spinel was revealed as the active species in syngas conversion from the mixture.

Other than XANES analysis, WT-EXAFS analysis can aid in the chemical and structural identification for metal active sites in CPMs.⁴⁰^,⁴⁵^,⁴⁶ As WT has both radial distance and k-space resolution, which can detect backscattering in low k-regions by lighter elements (unlike traditional Fourier transform (FT)-EXAFS analysis), it can provide higher resolution for EXAFS (especially for directly distinguishing different types of neighbors).⁴⁶ As an example, by comparing the Pt L₃- and Ge K-edge 3D contour maps with the 2D projection of the Pt@Ge-UTL zeolite catalyst, Ma et al.⁴⁵ confirmed the bonding environments of Ge-zeolite confined sub-nanometric Pt clusters. Moreover, by carefully studying the WT-EXAFS of a catalyst structure with known reference structures, better understanding of test samples can be obtained as done Sushkevich et al.⁴⁰ for Cu-exchanged faujasite (FAU) zeolite. The WT-EXAFS spectrum of CuO (Figure 2e) helped distinguish the different chemical bonds in CuO, including Cu-O_short, Cu-O_long, and Cu-Cu_oxide. Based on these results, the WT-EXAFS spectrum of Cu-exchanged FAU zeolite (Figure 2f) was employed to reveal the bonding environment of the Cu-exchanged FAU zeolite, including Cu-O, Cu-Al-Cu (superposition), and Cu-Cu_oxide.

XAS-guided multitechniques

Since particle sizes can be estimated from a first shell analysis (Figure 3a,b), XAS can be used for the determination of the sizes, shapes, and distributions of metal nanoparticles/nanoclusters, but the information obtained from XAS is averaged.⁴⁷ More accurate structural results can be achieved by combining XAS with other advanced characterization techniques. Notably, STEM is regarded as a direct visualization tool, but only offers local information for the size/spatial distribution and shape determination of metal species in CPMs. Therefore, by combining the XAS and STEM, the size distribution of metal nanoparticles/nanoclusters in CPMs can be accurately determined. For example, Sun et al.⁴⁸ proved that Pt@S-1-C (calcination in air then reduction under H₂) with a CN_Pt-Pt near 11 contained large Pt nanoparticles, whereas Pt@S-1-H (ligand-protected direct reduction in H₂) with a CN_Pt-Pt below 2 exhibited subnanometer Pt₃ clusters, revealing the advantage of pure H₂ reduction method in reducing the size of Pt species. Moreover, Cs-corrected STEM-high-angle annular dark-field (HAADF) images (Figure 3c,d) confirmed the Pt particle sizes of Pt@S-1-C (3.1 nm) and Pt@S-1-H (<1 nm) and that the Pt nanoparticles/nanoclusters were confined into zeolite channels. Similar studies into Mo₃S₄@ZSM-5, MOF-confined PdZn intermetallic nanoparticles, and zeolite-confined PtSn clusters have also been reported.⁵²^–⁵⁴

Figure 3 | (a) Comparison of first-shell CNs based on the results of conventional EXAFS analysis and theory-assisted XAS analysis for Pt nanoparticles on γ-Al₂O₃; (b) corresponding possible 3D models of Pt nanoparticles. Reprinted with permission from ref 47. Copyright 2017 American Chemical Society. Cs-corrected STEM-HAADF images of (c) Pt@S-1-C and (d) Pt@S-1-H. Reprinted with permission from ref 48. Copyright 2020 Wiley-VCH. (e) Fourier transform of k²-weighted EXAFS spectra of Rh foil and various zeolite-encaged Rh zeolite catalysts at Rh K-edge; (f) In-situ CO-DRIFTS of the Rh zeolite catalysts. Reprinted with permission from ref 49. Copyright 2019 Wiley-VCH. (g) EPR spectra and (h) XANES spectra of auto-reduced (black), CH₄-reacted (green), and N₂O-activated (pink) Cu-MOR. Reprinted with permission from ref 50. Copyright 2022 American Chemical Society. (i) Fe K-edge XANES spectra of Fe zeolites and reference samples; ⁵⁷Fe Mössbauer spectra for (j) Fe-Z5-C, (k) Fe-Z5-TF, (l) Fe-HZ5-C, and (m) Fe-HZ5-TF. ISO Fe_FW, ESSR Fe_FW, and Fe_EFW refer to isolated framework Fe, framework Fe with the ESSR effect, and extra-framework Fe species, respectively. Reprinted with permission from ref 51. Copyright 2023 American Chemical Society.

The interaction between metal species and CPMs permit single-atom/single-site metals to be confined or immobilized in CPMs, leading to significantly enhanced catalytic properties in a variety of reactions. The combined use of XAS and CO-DRIFTS (using CO as probe molecules) can aid the identification of isolated single atoms in CPMs.⁴⁹^,⁵⁵^–⁵⁷ For example, Sun et al.⁴⁹ reported the structural characterization of Rh species in S-1 zeolite by using XAS and in-situ CO-DRIFTS. As shown in FT-EXAFS spectra (Figure 3e), it was found that there was a sole Rh-O bond in Rh@S-1-H with no peaks due to Rh-Rh or metal oxides, indicating the oxygen-stabilized single Rh atoms are formed in the zeolite framework. Moreover, as depicted in Figure 3f, two peaks at 2092 and 2023 cm⁻¹ in the CO-DRIFTS of Rh@S-1-H were ascribed to the symmetrical and asymmetrical stretching of CO from the isolated mononuclear RhI(CO)₂ species. In addition, the peak at 2048 cm⁻¹ due to linear-adsorbed CO on Rh NPs is absent in Rh@S-1-H, further revealing the atomic dispersion of Rh species in Rh@S-1-H. Apart from zeolite-encaged metal single-atom catalysts, XAS and CO-DRIFTS coupling has been used to resolve the atomic structures for MOF-immobilized single-atom/site catalysts in multimetal systems.⁵⁵^,⁵⁶

Since XAS is element-specific characterization technique, the combined use of multielements’ XAS for one metal-in-CPM catalyst can provide more detailed structure information from different perspectives. For instance, Ma et al.⁵⁷ revealed the unique Ni-S coordination of Ni₁-S/MOF by jointly using conventional K-edge XAS (for Ni active sites) and soft K-edge XAS (for coordinated S atoms). In addition, Sun et al.⁴⁸ extracted structure information for subnanometer PtZn alloy from Pt L₃-edge and Zn K-edge XAS, confirming the configuration of bimetallic PtZn₄ clusters in S-1 zeolite.

In addition to atomic structure investigation, the joint use of XANES and EPR can accurately and quantitatively (in some cases) unveil the oxidation states for transition metal species in CPMs.⁵⁰^,⁵⁸^,⁵⁹ For instance, Heyer et al.⁵⁰ reported the use of XANES and EPR spectra for quantitatively analyzing the Cu species in mordenite (MOR) zeolite (Figure 3g,h). By conducting the spin quantification of EPR spectra and LCF of XANES spectra, changes in the Cu(I)/Cu(II) ratio during auto-reduced, CH₄-reaction and N₂O activation conditions were resolved, which allows for better understanding of the Cu speciation in MOR zeolite in the conversion of methane to methanol. Likewise, by combining XPS and XANES, valence state analysis for metal species in CPMs, such as investigating electron transferring phenomenon in multimetal-containing CPM systems, can be achieved.⁴³^,⁶⁰^,⁶¹

Coupling XANES spectra with Mössbauer spectroscopy can provide more comprehensive and quantitative information of the chemical states, spin states, and coordination symmetries of specific elements such as Fe and Sn.⁵¹^,⁶²^,⁶³ One of this coupling was exhibited by Cheng et al.⁵¹ who studied the Fe species in zeolite socony mobil-5 (ZSM-5) zeolite. According to Figure 3i, the XANES spectra for Fe-Z5-TF and Fe-Z5-C revealed the appearance of a pre-edge peak at 7114 eV. Notably, this pre-edge peak was rarely observed in Fe³⁺ with centrosymmetric octahedral coordination (as in the Fe₂O₃ reference). That is to say, such a peak suggested the formation of Fe³⁺ within the tetrahedral zeolite framework and indicated a significant interaction between the 3d and 4p metal orbitals caused by electric-quadrupole coupling. Moreover, the intensity of the pre-edge peak detected in Fe-HZ5-C, which was assigned to the partial conversion of framework Fe to octa-coordinated extraframework Fe, was comparatively lower than that of Fe-Z5-C. In the XANES spectrum of Fe-HZ5-TF, the remarkably weak pre-edge peak indicated complete migration of framework Fe. Furthermore, more accurate analysis for Fe species in different states was investigated by ⁵⁷Fe Mössbauer spectroscopy (Figure 3j–m). Such a quantitative study disclosed that most of the Fe (∼93%) in Fe-Z5-C was enhanced spin–spin relaxation framework Fe (ESSR Fe_FW) whereas most of the Fe (∼65%) in Fe-Z5-TF was isolated framework Fe (ISO Fe_FW) (Figure 3j,k). In contrast, a doublet component was observed in the analysis of Fe-HZ5-C and Fe-HZ5-TF (Figure 3l,m), which was assigned to extraframework Fe³⁺ species (denoted as Fe_EFW). The results indicated that during the conversion from Fe-Z5-C to Fe-HZ5-C, approximately 48% of the framework Fe underwent transformation into extraframework Fe. Conversely, when transforming Fe-Z5-TF to Fe-HZ5-TF, nearly 84% of the framework Fe was converted. As a whole, the remarkable differences between the Fe-ZSM-5 samples synthesized with and without the organic template were identified.

Theory-assisted XAS analysis

Theory-guided XAS spectra simulation

The well-known structure of CPMs make it suitable to employ computational methods in the analysis of XAS data, which can greatly extend the power for investigating the electronic properties and atomic structures of metal active sites in CPMs.⁶⁴^–⁶⁸ As an example, Signorile et al.⁶⁷ constructed a series of Ti cluster structural models for titanium silicalite-1 (TS-1) zeolite using a computational approach (B3LYP-D2). Then, the simulated spectra were obtained via finite difference method (for XANES) and the density functional theory (DFT)-based method (for the pre-edge region). The structural differences between two different TS-1 samples were distinguished by comparing their measured spectra with the simulated XANES spectra of different cluster models in TS-1 zeolites while the configuration of Ti in perfect tetracoordinated TS-1 and defective hexacoordinated TS-1 was well-resolved. This study more accurately depicted the catalytic role of defects in TS-1 in partial oxidation reactions.

Another work employing this approach was done by Lee et al.⁶⁸ who used both XANES and EXAFS simulations to determine the structure of Cu species in MOR zeolite with assistance from DFT-based calculations. Figure 4a shows snapshots of the simulated structures. Figure 4b indicates that the calculated k²χ(k) EXAFS spectrum of the Cu₂AlO₃ species agreed with the experimental spectrum. Based on the Cu K-edge (Figure 4c) and Cu L₃-edge (Figure 4d) XANES simulated spectra, the presence of a [Cu₂AlO₃]²⁺ cluster exchanged on MOR Al pairs, which showed the ability to oxidize up to two CH₄ molecules per cluster at ambient pressure, was confirmed.

Figure 4 | (a) Annealed structure of Cu clusters; (b) compared k²-weighted EXAFS (k²χ(k)) from calculation (blue) and measurement (red) for Cu clusters; (c) Cu K-edge XANES and (d) Cu L₃-edge XANES spectra of the Cu trimer and Cu₂AlO₃. (c) and (d) Compare theoretical spectra (blue) with experimental ones (red) along with red vertical guiding lines and energies, respectively. Reprinted with permission from ref 68. Copyright 2021 American Chemical Society.

Theory-led extraction of structural parameters from EXAFS

To gain an atomistic picture of metal-containing CPMs, an interpretation of the EXAFS data is essential. For the conventional analysis, the EXAFS data are typically compared to a model, and the suitability of the fit is evaluated using a goodness of fit parameter known as the R factor (a lower R factor indicates a better fit). The fitting process of the EXAFS data can often be challenging and demanding. Recently, applying DFT-computed structures as a beginning for EXAFS explanation has been proven to be superior to traditional analyses, particularly in a complicated multishell fitting process.⁶⁹^–⁷² As an example, Zhang and coworkers⁷⁰ used the DFT-optimized MOF structure as the fitting model to do multishell fitting for the MOF-808-Zn catalyst during the hydrogenation of CO₂ to methanol (Figure 5a–d). Fitting of the Zn K-edge EXAFS data of MOF-808-Zn-BR (as-synthesized, BR stands for before catalytic reaction) and MOF-808-Zn-AR (AR stands for after catalytic reaction) showed single-site Zn²⁺ centers instead of ZnO nanoparticles or clusters. Moreover, the multishell EXAFS fittings of MOF-808-BR and MOF-808-AR disclosed the existence of Zn-Zr bonds (3.41 and 3.01 Å, respectively).

Figure 5 | Local structures of (a) MOF-808-Zn-BR and (b) MOF-808-Zn-AR. R-space EXAFS fitting plot of (c) MOF-808-Zn-BR and (d) MOF-808-Zn-AR. The sample before catalysis was not exposed to air for the EXAFS analysis, so there is still an ethyl group on the Zn center. Reprinted with permission from ref 70. Copyright 2021 American Chemical Society. Local environment of platinum in Pt/ZSM-5: interpretation of EXAFS spectra. (e) R-factors representative of EXAFS fits of each of 301 DFT-optimized structures of 2Al-Pt and 1Al-PtOH in ZSM-5. Vertical white gridlines mark increments of 20 structures. (f) Dependence of relative energy on the zeolite Al-Al distance of 2Al-Pt structures with relative energies ≤2.6 eV. (g) Magnitudes (fit, blue; experiment, black) and imaginary parts (fit, green; experiment, gray) of Fourier-transformed EXAFS of Pt/ZSM-5 after exposure to O₂ at 700 °C with fitting based on a 2Al-Pt structure. (h) Visual representation of the 2Al-Pt structure corresponding to fit shown in (g). Red, O; yellow, Si; pink, Al; gray, Pt; white, H. (i) Magnitudes (fit, blue; experiment, black) and imaginary parts (fit, green; experiment, gray) of Fourier-transformed EXAFS of Pt/ZSM-5 after exposure to O₂ at 700 °C with fitting based on the lowest R-factor 1Al-PtOH structure. (j) Visual representation of the 1Al-PtOH structure from (i). Reprinted with permission from ref 28. Copyright 2022 American Chemical Society. (k) Illustration of the MFI intersection region and the confined Pt_xSn_yO_z clusters. (l) The minimum distance between metal atom and zeolite oxygen (O_zeo) for different Pt_xSn_yO_z clusters. (m) The probability of Pt facing the channels (P) and the Pt concentration (cPt) in the most stable PtSn alloy clusters. Reprinted with permission from ref 73. Copyright 2022 Springer Nature.

Another example of DFT-guided multishell fitting for EXAFS spectra was done by Felvey et al.²⁸ who used a newly developed analysis method called theory-guided QuantEXAFS to analyze the platinum atomic structure in ZSM-5 zeolite. According to the DFT calculation, a total of 301 unique geometrically arranged aluminum structures were optimized for 2Al-Pt and 1Al-PtOH frameworks. As shown in Figure 5e,f, such calculated structures were used to fit the experimental EXAFS data and the R factor (Figure 5e) was employed to quantify the accuracy of each fit. The 2Al-Pt structures were resolved from the 118 EXAFS fits with the lowest R factors. In Figure 5f, the zeolite framework Al-Al distance was plotted against the relative energies determined by DFT. The relative energies of all 2Al-Pt structures were ≤2.6 eV, with the most stable structures having Al-Al distances of approximately 5–6 Å. Based on these findings, it was determined that the most favorable structural model involves platinum being coordinated to four oxygen atoms of a six-membered ring (6-MR) which includes two next-next-nearest neighbor aluminum atoms (Figure 5h). Moreover, the theoretical FT-EXAFS spectrum of this structure closely matched the experimental spectrum characterizing calcined Pt/ZSM-5 (Figure 5g). Notably, this structure (Figure 5j) had the second lowest energy of all the 2Al-Pt structures found and included platinum at a 6-MR paired aluminum site. However, this structure was deemed improbable and subsequently rejected due to limited access to the site, which is obstructed by 5-MR openings that are not typically penetrated by platinum. Nevertheless, the area of the FT-EXAFS spectrum, which is indicative of Pt-O bonding, in the range of approximately 1–2 Å was well-matched with the experimental results (Figure 5i). In contrast, the region beyond 2 Å, which signifies the existence of Si, Al, and O atoms at higher distances from platinum, did not fit the experimental results.

Furthermore, owing to the strength of ML-based atomic simulations, Ma and Liu⁷³ scanned the thermodynamically stable MFI zeolite-confined PtSn alloy catalysts from millions of possible structures (Figure 5k) by carefully comparing the Pt-Sn CN and bond distance (Figure 5l,m) between the experimental EXAFS fitting results and the computational structures. They determined that the small PtSn alloy and Sn₄O₄ clusters were major components in MFI channels, which are beneficial for the pyruvate dehydrogenation reaction. Thus, stabilizing and increasing the concentration of these clusters should be a key goal.

In-situ/operando XAS analysis

Using in-situ/operando XAS techniques to determine the relationships between the dynamic electronic and atomic structure of the catalysts and their catalytic performance under actual reaction conditions is essential to gain a deeper understanding of reaction mechanisms and reveal the configuration of working catalytic sites in CPMs.¹⁴^,⁷⁴^–⁷⁶ Recently, in-situ/operando XAS techniques have been broadly adopted in a variety of catalytic systems, such as thermocatalysis, photocatalysis, and electrocatalysis (Table 1).

**Table 1 | Selected In-Situ/Operando XAS Study for Metal-Containing CPMs Catalysts**
Catalysts	Targeted Edge	Application	In-Situ/Operando Condition	Main Findings	Ref.
Cu-zeolite (CHA)	Cu K-edge	SCR of NO_x	Temperature up to 400 °C;Atmosphere: O₂/N₂, H₂/He	Determine the correlations between the SCR performance and Cu-speciation behavior of the SO₂-poisoned catalyst	77
Cu-zeolite (CHA)	Cu K-edge	SCR of NO_x	Temperature at 200 °C;Atmosphere: O₂/NO/NH₃/CO₂/H₂O/N₂	Verify the faster rates of CuI oxidation relative to CuII reduction at higher framework Al densities (>1 Al center per d6r)	29
Ta-zeolite (MFI)	Ta L₃-edge	Methanol-to-olefin	Temperature up to 370 °C;Atmosphere: CH₃OH/N₂;Quick XAS: 4s/spectrum	Reveal the high stability of the framework Ta(V) sites under reaction condition	32
Ni-zeolite (BEA)	Ni K-edge	Alkane dehydrogenation	Temperature at 300 °C;Atmosphere: O₂/He, iC₄H₁₀/He, H₂	Confirm the dehydrogenation active sites are isolated, four-coordinate Ni(2+) in beta zeolite	78
Fe-MIL-100	Fe K-edge	Intermediate study	Temperature at 250 °C; Atmosphere: N₂O/CO, N₂O/O₂	Prove the active oxygen intermediate as an iron-oxyl (Fe³⁺–O⁻) during NO₂-mediated CO oxidation reaction	79
Cu-zeolite (MOR)	Cu K-edge	Methane oxidation to methanol	Temperature at 210 °C;Atmosphere: O₂, CH₄;Quick XAS: 1s/spectrum	Mono-μ-oxo dicopper and Cu(OH)⁺ monomers with extraframework oxygen in Cu-MOR zeolite are active sites in methane oxidation reaction	80
Pt-zeolite (MFI)	Pt L₃-edge	Intermediate study	Temperature up to 700 °C;Atmosphere: O₂/He, H₂/He, CO/He	Upon exposure of platinum clusters to O₂ at 700 °C, oxidative fragmentation occurs, and Pt²⁺ ions are stabilized at 6-MRs in the zeolite that contain paired aluminum sites. When exposed to CO under mild conditions, these Pt²⁺ ions form highly uniform platinum gem-dicarbonyls, which can be converted in H₂ to Pt^δ+ monocarbonyls	28
TS-1 (MFI)	Ti K-edge	Intermediate study	The addition of H₂O₂ aqueous	Six-coordinate Ti species is the active site for the vinyl-to-carbonyl reaction pathway	81
Ru-zeolite (BEA)	Ru K-edge	Amination of fatty alcohol	Temperature up to 450 °C;Atmosphere: O₂/He, H₂/He, NH₃/He, 1-hexanol vapor	The N-containing species are present at the surface of Ru nanoparticles in beta zeolite	26
Pt-NU-1000	Pt L₃-edge	Ethylene hydrogenation	Temperature at 150 °C;Atmosphere: H₂, H₂/He, H₂/C₂H₆	The Pt nanoclusters in NU-1000 are sinter-resistance under the strongly exothermic hydrogenation reaction of ethylene	82
Cu-UiO-66	Cu K-edge	CO oxidation	Temperature up to 250 °C;Atmosphere: CO/N₂, H₂/N₂, CO/O₂/N₂	The mean Cu oxidation state (Cu²⁺–Cu⁺) can vary significantly with the reaction temperature (120–250 °C)	23
CoNi-MOF-74	Co K-edgeNi K-edge	Electrocatalytic oxygen evolution	XAS signal is recorded at different potentials (0–1.5 V) in 1 M KOH	Ni_0.5Co_0.5OOH_0.75 with abundant oxygen vacancies and high oxidation states forms in NiCo-MOF-74 during OER	30
LaNi-COF-5	Ni K-edgeLa L₃-edge	Photocatalytic CO₂ reduction	XAS signal is recorded at Ar or CO₂-saturated aqueous solution with xenon lamp	Ni atoms are the active centers for the CO₂ reduction, while La atoms are not only the optically active center but also the catalytically active center for CO₂ adsorption and activation	31

In-situ/operando XAS analysis in thermocatalysis

One of the most reported applications for in-situ/operando XAS (Figure 6a: the in-situ cell) for metal-in-CPM catalysts is studying under thermocatalysis conditions.⁸³^,⁸⁴ As an example, Qi et al.⁸⁴ used in-situ XANES spectra to monitor the evolution of Pt species in dealuminated beta zeolite (DeAlBEA) under thermal conditions. As shown in Figure 6b, the Pt L₃-edge XANES spectra of the 0.04Pt0.36Zn-DeAlBEA catalyst were measured during propane dehydrogenation pretreatment process, with the temperature gradually increasing from 25 °C to 550 °C in a flowing He environment. In the spectra, the presence of three isosbestic points was noted, signifying that there was no accumulation of intermediates throughout the treatment process. This phenomenon would not be anticipated if the initially isolated Pt atoms were converted into Pt and/or PtZn alloy nanoparticles, which could have a variety of compositions and sizes. This further implies that the Pt atoms were in an isolated state both prior to and following the pretreatment.

Figure 6 | (a) Schematic view of the heatable/coolable in situ fluorescence/transmission EXAFS cell. Reprinted with permission from ref 83. Copyright 2007 International Union of Crystallography. (b) Isosbestic points of Pt L₃-edge XANES spectra of 0.04Pt0.36Zn-DeAlBEA recorded during treatment in flowing He as temperature was ramped from 25 °C to 550 °C. Reprinted with permission from ref 84. Copyright 2021 American Chemical Society. (c) In-situ Pd K-edge XANES spectra and (d) corresponding Pd K-edge ΔXANES spectra during the reaction of Pd+CHA under 4% NO/He flow at 600 °C and O₂-treated Pd_imp/CHA(H₂_NO) under He flow at 100 °C. The data for the ΔXANES spectra were obtained by subtraction with the spectrum taken at 0 min. Reprinted with permission from ref 85. Copyright 2021 American Chemical Society. (e) Theoretical “pure” Cu K-edge HERFD-XANES spectra of PC1, PC2, PC3, PC4, and PC5 from MCR analysis. (f) Corresponding temperature-dependent concentration profiles of 0.18Cu-HMOR(7)-He-act., 0.18Cu-HMOR(7)-O₂-act., 0.36Cu-HMOR(11)-He-act., and 0.36Cu-HMOR(11)-O₂-act. Reprinted with permission from ref 86. Copyright 2018 American Chemical Society. Time evolution of Cu-O (g) CN and (h) bond distance based on the EXAFS data recorded during CO oxidation (1% CO, 1% O₂, N₂ balance-30 NmL min⁻¹) at 250 °C after the H250 pretreatment. Reprinted with permission from ref 87. Copyright 2019 American Chemical Society.

Another technique used to clearly investigate changes of in-situ XANES spectra is the ΔXANES analysis.²⁶^,⁶⁷^,⁸⁵ For example, Yasumura et al.⁸⁵ studied Pd species evolution in chabazite (CHA) zeolite (Pd+CHA) by the in-situ ΔXANES analysis. In order to shed light on the structural changes of Pd species during atomic dispersion, in-situ XAS measurements were conducted on Pd+CHA during the nitrogen oxide (NO) treatment process in 4% NO/He atmosphere at 600 °C. As evidenced by the Pd XANES spectra acquired at t = 0, 2, 20, 60, 120, and 145 min (Figure 6c), the changes were distinctly noticeable in the Pd K-edge ΔXANES spectra (Figure 6d). According to this spectra difference analysis, the intensity of the peak originating from Pd⁰ species sharply declined, concurrently with the initial emergence of a Pd²⁺ peak (t = 0–20 min) that consistently diminished until t = 120 min. These findings corroborated the atomic dispersion of Pd black through the oxidation of Pd⁰ species to Pd²⁺.

As previously discussed, it is feasible to separate different contributions using XANES LCF analysis. However, the XANES study of some Cu-containing CPMs presents a unique and often hard-to-replicate coordination environment (e.g. cluster geometry and oxidation state). This complexity makes the LCF analysis of synthetic model compounds challenging. To overcome this problem, the MCR-ALS algorithm is frequently employed. This approach involves the iterative refinement of concentration profiles and pure spectra through an alternating least squares routine, all the while maintaining specific constraints.⁸⁸ By using MCR-ALS analysis, Pappas et al.⁸⁶ compared HERFD-XANES spectra of two most different Cu-MOR materials (Cu/Al = 0.18 and Si/Al = 7; Cu/Al = 0.36 and Si/Al = 11) to quantitatively resolve the Cu speciation (active/inactive species) during the conversion of methane to methanol in the Cu K-edge XANES region (Figure 6e,f). Specifically, the activation in helium notably encouraged the creation of Cu^I via auto-reduction. This could potentially be due to the carbonaceous residues left within the materials after synthesis. During aerobic activation, the auto-reduction process that produced Cu^I was practically nonexistent at higher temperatures and is only briefly noticeable at lower temperatures. Moreover, varying activation conditions shifted the equilibrium between two types of Cu^II species. These species originated from a four-coordinate Cu^II intermediate species (PC4), and they exhibited markedly different tendencies to resist auto-reduction (as indicated in Figure 6e,f as PC3 and PC5). When the accumulated evidence was linked with reactivity studies, it suggested that PC5 was the Cu^II species that showed the highest resistance to auto-reduction during anaerobic activation. Therefore, the PC5 Cu species drove the activity of Cu/MOR in the selective oxidation of methane to methanol.

Recently, Krishna et al.²⁹ conducted another operando XAS study on Cu-containing zeolite catalysts (Table 1). They presented the steady-state selective catalytic reduction (SCR) mechanistic explanations for Cu-CHA zeolites with different compositions at 1, 10, and 60 kPa O₂. The XAS data offered conclusive proof that the oxidation rates of Cu(I) were quicker compared to the reduction rates of Cu(II) at elevated framework Al densities (>1 Al center per d6r).

Other than in-situ XANES analysis, in-situ EXAFS analysis can be conducted as shown by Abdel-Mageed et al. for the Cu/UiO-66 catalyst.²³^,⁸⁷ The EXAFS data, measured over a period of 375 min, revealed minimal alterations in both the Cu-O CN (Figure 6g) and the bond distance (Figure 6h) of the Cu species during CO oxidation, after just a few minutes into the reaction. Also, these results suggested high degree of stability in coordination environment surrounding the Cu atoms during CO oxidation. Altogether, these findings emphasized the existence of individual Cu atoms, each coordinated to two oxygen atoms, in an atomically dispersed state within the defect site of UiO-66. This arrangement was distinct from those found in Cu₂O or CuO clusters/nanoparticles.

Operando EXAFS can also be analyzed in WT; as an example, Negri et al.⁸⁹ utilized in-situ Cu K-edge WT-EXAFS for the analysis of Cu species in NH₃-mediated SCR (NH₃-SCR) of NO_x. The operando WT-EXAFS analysis provided a more precise and in-depth differentiation of the scattering contributions surrounding the absorber atom when compared to traditional FT-EXAFS analysis. Through a comprehensive investigation of operando WT-EXAFS spectra, the species evolution of solvated Cu-pairs in Cu-CHA catalysts during the NH₃-SCR reaction cycle can be obtained.

In-situ/operando XAS analysis in electro and photo-catalysis

Recently, with the rapid development of metal-containing MOF electrocatalysts, in-situ/operando XAS characterization (the in-situ cell showed in Figure 7a) in electrocatalysis has become increasingly important.²⁹^,³²^,⁹⁰ An example is done by Zhao et al.³⁰ who used operando cell for XAS measurement under electrocatalysis condition (Table 1). The variations in the electronic and atomic structures of the metal sites inside NiCo-MOF-74 throughout the entire electrocatalytic cycle were monitored. As a result, the high oxygen evolution reaction (OER) activity observed can be attributed to the in-situ formation of Ni_0.5Co_0.5OOH_0.75 with abundant oxygen vacancies and elevated oxidation states. Likewise, another in-situ XAS study for metal-containing MOFs in electrocatalysis was reported by Xu et al.⁹¹ During the electrocatalysis process (potential drop from 0.3 to −0.9 V), the catalytic center for the nitrate-to-ammonia reduction reaction is formed by the in situ clustering of single-atom Cu, resulting in MOF-confined ultrasmall Cu nanoclusters, as depicted in in-situ XANES and FT-EXAFS spectra (Figure 7b,c).

Figure 7 | (a) In situ XAS setup for electrocatalysis, where CE, RE, and WE stand for counter, reference, and working electrodes, respectively. Reprinted with permission from ref 90. Copyright 2013 American Chemical Society. In-situ (b) XANES and (c) FT-EXAFS at the Cu K-edge for Cu-MOF during the electrocatalytic nitrate-to-ammonia reduction reaction. Reprinted with permission from ref 91. Copyright 2022 American Chemical Society. (d) Schematic of the photocatalytic cell for in-situ XAS study. Reprinted with permission from ref 92. Copyright 2023 Multidisciplinary Digital Publishing Institute. (e) Ni K-edge XANES spectra of LaNi-Phen/COF-5 in CO₂ photoreduction reduction reaction at room temperature and 1 atm of Ar or CO₂-saturated aqueous solution (inset: the enlarged Ni K-edge XANES spectra). Reprinted with permission from ref 31. Copyright 2023 Springer Nature. (f) FT-EXAFS spectra of Co-Ru-UiO-67(bpy) before illumination and the intermediate species formed after induction period ends. Reprinted with permission from ref 93. Copyright 2018 American Chemical Society.

As with electrocatalysis conditions, in-situ/operando XAS measurements can also be conducted for the characterization of metal-in-MOFs under photocatalysis conditions (the in-situ cell showed in Figure 7d).⁹² For instance, Zhou et al.³¹ reported the use of Ni K-edge and La L₃-edge operando XAS measurements under photocatalytic CO₂ reduction conditions to dynamically monitor the oxidation state and gain insights into the atomic-scale functionality of active sites for LaNi-Phen/COF-5 (Figure 7e). According to the Ni K-edge XANES spectra of LaNi-Phen/COF-5, the white-line intensity was slightly higher in a CO₂-saturated CH₃CN solution than that in an Ar-saturated CH₃CN solution, implying the increase of valence state of Ni, which can be attributed to the transfer of electrons from the active Ni center to CO₂. In contrast, based on the change of XANES spectra under xenon lamp irradiation, the active Ni center’s oxidation state was gradual recovery during CO₂ reduction. Additionally, the alteration in oxidation state was also noticed in the La L₃-edge XANES spectra. To summarize, the in-situ XAFS analysis revealed that Ni atoms played a crucial role as active centers in the CO₂ photoreduction, while La atoms served as both the optically active center and the catalytically active center in the process of CO₂ adsorption and activation. In addition to in-situ XANES analysis for oxidation state, the in-situ EXAFS study under photocatalysis condition for Co-Ru-UiO-67(bpy) was done by Yang et al.⁹³ As shown in Figure 7f, by quantitatively analyzing the coordination environment of a long-lived Co^I intermediate, they identified the rate-limiting step in the light-driven H₂ production, shedding light on the catalysis mechanism of these single-site Co-Ru-MOF photocatalysts.

Other advanced in-situ/operando XAS techniques

Typically, it takes several minutes or more to acquire a complete scan for in-situ XAS spectrum in conventional beamline, which poses challenges for time-resolve XAS studies on metal-in-CPM catalysts. To overcome such difficulty, by continuously moving the monochromator (which is conventionally moved stepwise) for swift XAS measurements, high quality XAS data can be collected in several tenths of a second in some advanced beamlines.²⁵^,³²^,³⁷^,⁹⁴

An example employing these measurements was shown by Petrov et al.⁹⁴ in assessing the redox properties of palladium in MOR zeolites (Pd/Na-MOR and Pd/H-MOR) of in a quick XAS experiment at the time resolution of 1 s (Figure 8a–d, respectively, 90 min at 400 °C). O₂ was removed from the feed to reduce the catalyst using CH₄ and then reintroduced to allow palladium to be oxidized again under reaction conditions. The XANES LCF analysis confirmed that both Pd/H-MOR and Pd/Na-MOR exhibited similar evolution behavior of Pd species during reduction and reoxidation. However, the fraction of Pd²⁺ in the steady state under reaction conditions was considerably higher in Pd/Na-MOR. The observed phenomenon can be attributed to the accelerated rate of reoxidation from Pd⁰ to Pd²⁺ in the presence of sodium, which was four times faster (as evidenced by the initial rates in Figure 8a,b). This can be explained by the smaller size of the metal nanoparticles in Pd/Na-MOR (as indicated by the smaller CN_Pd–Pd in Figure 8c,d) and their larger specific surface areas, which generally enhanced reactivity towards oxygen.

Figure 8 | Transient operando EXAFS experiment with oxygen cut-off. (a) and (b) Fraction of oxidized palladium in Pd/Na-MOR and Pd/H-MOR and corresponding catalytic activity in 1 vol % CH₄, 4 vol % O₂, 5 vol % H₂O, bal. N₂, gas hourly space velocity (GHSV) = 350,000 h⁻¹ at 350 °C. Oxygen was removed from the feed to perform reduction of palladium by methane and then added for subsequent reoxidation in reaction conditions. (c) and (d) Fourier transforms of in situ Pd K-edge EXAFS spectra (nonphase shift corrected) of Pd/Na-MOR and Pd/H-MOR upon averaging 15 spectra at 1, 90, 210, 280, and 550 s of the experiment. Initial rates were obtained from fitting the linear function over the first 10 points during reoxidation. Reprinted with permission from ref 94. Copyright 2018 Springer Nature. (e) Design of the in-situ XAS experiment; (f) The combined in-situ XANES-MS experimental setup at beamline P65, Petra III. Reprinted with permission from ref 33. Copyright 2023 American Chemical Society. (g) Molecular models for the structural components included in the ML-assisted EXAFS fitting model: 1 [Cu^I(NH₃)₂]⁺, 2 “planar”, and 3 “bent” motifs for μ-η2,η2-peroxo diamino dicopper(II); when relevant, characteristic EXAFS-derived ranges for Cu-Cu interatomic distances are reported, in Å. (h–j) Comparison between magnitudes of experimental (colored circles) and best fit (thick lines) in-situ FT-EXAFS spectra at the oxidation condition for (h) 0.1_5, (i) 0.5_15, and (j) 0.6_29 Cu-CHA zeolites. The scaled components for Cu-species 1, 2/2′, 3 are also reported, vertically translated for the sake of clarity, together with percentages of each component over total Cu refined by ML-EXAFS fitting. Reprinted with permission from ref 27. Copyright 2022 American Chemical Society.

The combined use of XAS and other characterization techniques has also been conducted under in-situ conditions. For example, by a combination of Sr K-edge XANES-mass spectroscopy (MS), Liutkova et al.³³ investigated the evolution of Sr species in ZSM-5 zeolite under the methanol to hydrocarbon (MTH) reaction conditions (Figure 8e,f). Firstly, they confirmed the high conversion of methanol (indicated by a low m/z = 31 signal) and the formation of propylene (highlighted by an intense m/z = 41 signal), which demonstrated the catalyst’s activity in the MTH reaction and also indicated the development of a functional hydrocarbon pool within the pores of the zeolite. In addition, they carefully analyzed the simultaneous recorded operando XAS data. The operando ΔXANES analysis revealed the interaction between Sr cations in the ZSM-5 catalyst and adsorbates. Despite a decrease in coverage at higher temperatures, there were still residual adsorbates present at 450 °C. Moreover, the operando EXAFS analysis revealed that the Sr CN elevated in the presence of adsorbates and stayed high when the feed was switched to He, suggesting that the Sr moieties can possess a strong ability to retain adsorbates and thereby influenced the local environment under MTH reaction conditions.

With the advancement of artificial intelligence technology, ML was also applied to assist the in-situ XAS analysis, which in theory enables the refinement of any structure beginning from an initially estimated molecular complex, even in cases where there are notable disparities in interatomic distances and angles. An example of this approach was performed by Martini et al.²⁷ to understand the emerging structural Cu complex in CHA zeolite (Figure 8g–j) under SCR conditions. First, by comparing the in-situ XANES features, they identified the formation of [Cu₂(NH₃)₄O₂]²⁺ motif in the Al rich 0.1_5 catalyst, which differs from those observed for the 0.5_15 and 0.6_29 catalysts. Additionally, in the high-k range characteristic of Cu–Cu scattering (WT maps), two local maxima were observed along the R direction at approximately 2.5 Å (low-R) and 3.2 Å (high-R), pointing to an EXAFS resolvable bimodal distribution of Cu–Cu interatomic distances. However, the conventional refinement method (solely based on one DFT-optimized model) is insufficient to extract the real structure information from these in-situ EXAFS data. Because ML-assisted fitting can start from models with significantly different interatomic distances and angles, such an approach was employed to resolve in-situ EXAFS data and accurately analyze the subtle differences observed in the XANES spectral shape and the high-R portion of WT-EXAFS. As a result, the low-R/high-R features were well-matched with the optimized Cu–Cu distances corresponding to the planar and bent μ-η2,η2-peroxo diamino dicopper(II) structures. Hence, the potential of in-situ XAS combined with integrated ML-assisted analysis should be highlighted.

A newly discovered XAS-related characterization method, X-ray adsorbate quantification (XAQ), have been used to generate thermogravimetric analysis-, temperature-programmed desorption-, and temperature-programmed reduction-like information from raw XAS data under in-situ conditions. The XAQ data can be derived from transmission XAS spectra collected simultaneously, without requiring any adjustments of the experimental procedure or additional equipment or software. Moreover, combining it with XANES/EXAFS does not result in an increase in the measurement time because the data in the XAQ region is usually collected as a necessary step for normalizing XANES and EXAFS spectra. Such an example was reported by Lomachenko et al. for quantitatively investigating the absorption behavior of Cu-CHA zeolite under in-situ conditions.⁹⁵^,⁹⁶

Summary and Outlook

This minireview summarizes the most recent progresses in advanced XAS studies for metal-containing CPMs in heterogenous catalysis. Depending on the high sensitivity in electronic behavior and atomic resolution in local structure required, different catalysis model systems can be investigated by such element-selective characterization techniques. Pre-edge fingerprints and XANES region have been used for the qualitative and even quantitative identification of different metal species in CPMs. EXAFS analysis has shown strength in resolving the atomic structure for metal active sites in CPMs. Furthermore, with the help of other advanced characterization techniques and theories, the power of XAS has been significantly enhanced. In particular, in-situ/operando XAS has been highlighted for the structure-performance relationship studies of metal-in-CPM due to the unparalleled advantage in determining working sites in CPMs under real catalytic conditions.

Although there have been many breakthroughs in the investigation of metal-containing CPMs in heterogeneous catalysis, ongoing challenges with advanced X-ray spectroscopic approaches remain. The quality of XAS data is greatly influenced by sample preparation and the synchrotron light source’s beamline, which can introduce data artifacts like self-absorption and pinhole effects, and so on. Therefore, X-ray sources with higher flux, stronger coherence, and better time resolution need to be developed. Also, combining XAS with other advanced characterization techniques adopts time-consuming alternating signal collection, which make them inappropriate for time-resolved studies. Thus, the development of simultaneously measured XAS-guided multitechniques is needed to more precisely unveil the catalytic sites in CPMs. Moreover, researchers who possess moderate XAS training can only achieve fundamental details regarding catalyst oxidation states and its overall structure from the absorption edge and the main peaks in the FT-EXAFS spectra. However, such qualitative information is sometimes insufficient for obtaining crucial insights into the electronic and atomic structures of metal active sites in CPMs. Fitting of the FT-EXAFS data is usually complicated and time-consuming, and it is easy to get erroneous results when the fitting method or chosen model is not reliable. Consequently, the availability of ab initio codes and user-friendly data analysis packages must be developed to easily achieve fully resolved and accurate results for the in-depth understanding of metal-sites in CPMs. Additionally, in-situ/operando XAS studies have been demonstrated as indispensable tools to analyze the dynamic electronic behavior and local structural changes of metal active sites in CPMs, which have revealed the underlying reaction mechanisms and the structure-performance relationships of these catalysts. To guarantee signal collection efficiency and high signal-to-noise ratio in the collected data, the in-situ cells need to be rationally designed to closely mimic the true reaction environment. Also, advanced in-situ/operando XAS studies with higher spatial, energy, and time resolution can produce more useful information than currently available. As such, it is important to give special attention to the state-of-the-art analysis technologies such as artificial intelligence, machine learning, and so on.

Overall, the vast quantity of information extracted from XAS data concerning the electronic/atomic structure, composition, and dynamics under reaction conditions makes it well-suited for in-depth analysis of the metal active sites in CPMs. In the near future, we foresee the following advances in the use of XAS in heterogeneous catalysis with metal-containing CPMs: (1) With HERFD-XANES and EXAFS becoming more readily available, deeper insights on oxidation state and coordination geometry of metal active sites will be discovered; (2) Thanks to the newly established X-ray free electron laser source with ultra high brightness and extreme short duration, XAS studies will be greatly improved, especially in time-resolves experiments; (3) In-situ/operando XAS studies will rapidly increase. In the meantime, in-situ/operando equipment will become more versatile and efficient; (4) With the development of theoretical codes, the strength of XANSES simulation will become increasingly important. It can be used in conjunction with EXAFS refinement to resolve the real atomic structure of metal active sites in CPMs. While many opportunities and challenges arising from recent advanced XAS studies in metal-containing CPMs remain, XAS has been demonstrated to be a major tool for the characterization of metal-in-CPM catalysts and should be expected to play an increasingly crucial role in advancing characterization techniques in this field.

Conflict of Interest

The authors declare no competing financial interests.

Acknowledgments

T.Z. acknowledges the funding from the National Natural Science Foundation of China (grant no. 22301057), the financial support by the Natural Science Foundation of Hebei Province (grant no. B2023201065), and Hebei University High-level Talent Research Program (grant no. 521100223025). Y.L. thanks the funding from the National Natural Science Foundation of China (grant no. 22305060), and Hebei University High-level Talent Research Program (grant no. 521100222060). P.Z. acknowledges the financial support from an Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant. A.G.W. thanks financial support from an NSERC Canada Graduate Scholarships - Doctoral Program (CGS-D) scholarship.

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Heterogeneous Catalysis with Metal-Containing Crystalline Porous Materials: Element- and Site-Specific Insights