Restructuring of Palladium Nanoparticles during Oxidation by Molecular Oxygen.

An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time-resolved X-ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided. XAS analysis revealed the restructuring of the fcc-like palladium surface into the 4-coordinated structure of palladium oxide upon absorption of oxygen from the gas phase and formation of core@shell Pd@PdO structures. The reconstruction starts from the low-coordinated sites at the edges of palladium nanoparticles. Formation of the PdO shell does not affect the average Pd‒Pd coordination numbers, since the decrease of the size of the metallic core is compensated by a more spherical shape of the oxidized nanoparticles due to a weaker interaction with the support. The metallic core is preserved below 200 °C even after continuous exposure to oxygen, with its size decreasing insignificantly upon increasing the temperature, while above 200 °C, bulk oxidation proceeds. The Pd‒Pd distances in the metallic phase progressively decrease upon increasing the fraction of the Pd oxide due to the alignment of the cell parameters of the two phases.

X-ray Spectroscopy at the SuperXAS and Debye Beamlines: from in situ to Operando.

Understanding structure-performance relationships are essential for the rational design of new functional materials or in the further optimization of (catalytic) processes. Due to the high penetration depth of the radiation used, synchrotron-based hard X-ray techniques (with energy > 4.5 keV) allow the study of materials under realistic conditions (in situ and operando) and thus play an important role in uncovering structure-performance relationships. X-ray absorption and emission spectroscopies (XAS and XES) give insight into the electronic structure (oxidation state, spin state) and local geometric structure (type and number of nearest neighbor atoms, bond distances, disorder) up to ~5 Å around the element of interest. In this mini review, we will give an overview of the in situ and operando capabilities of the SuperXAS beamline, a facility for hard X-ray spectroscopy, through recent examples from studies of heterogeneous catalysts, electrochemical systems, and photoinduced processes. The possibilities for time-resolved experiments in the time range from ns to seconds and longer are illustrated. The extension of X-ray spectroscopy at the new Debye beamline combined with operando X-ray scattering and diffraction and further developments of time-resolved XES at SuperXAS will open new possibilities after the Swiss Light Source upgrade mid 2025.

Droplet-Based Microfluidics Reveals Insights into Cross-Coupling Mechanisms over Single-Atom Heterogeneous Catalysts.

Single-atom heterogeneous catalysts (SACs) hold promise as sustainable alternatives to metal complexes in organic transformations. However, their working structure and dynamics remain poorly understood, hindering advances in their design. Exploiting the unique features of droplet-based microfluidics, we present the first in-situ assessment of a palladium SAC based on exfoliated carbon nitride in Suzuki-Miyaura cross-coupling using X-ray absorption spectroscopy. Our results confirm a surface-catalyzed mechanism, revealing the distinct electronic structure of active Pd centers compared to homogeneous systems, and providing insights into the stabilizing role of ligands and bases. This study establishes a valuable framework for advancing mechanistic understanding of organic syntheses catalyzed by SACs.

Quantitative Structural Description of Zeolites by Machine Learning Analysis of Infrared Spectra.

Application of machine learning (ML) algorithms to spectroscopic data has a great potential for obtaining hidden correlations between structural information and spectral features. Here, we apply ML algorithms to theoretically simulated infrared (IR) spectra to establish the structure-spectrum correlations in zeolites. Two hundred thirty different types of zeolite frameworks were considered in the study whose theoretical IR spectra were used as the training ML set. A classification problem was solved to predict the presence or absence of possible tilings and secondary building units (SBUs). Several natural tilings and SBUs were also predicted with an accuracy above 89%. The set of continuous descriptors was also suggested, and the regression problem was also solved using the ExtraTrees algorithm. For the latter problem, additional IR spectra were computed for the structures with artificially modified cell parameters, expanding the database to 470 different spectra of zeolites. The resulting prediction quality above or close to 90% was obtained for the average Si-O distances, Si-O-Si angles, and volume of TO4 tetrahedra. The obtained results provide new possibilities for utilization of infrared spectra as a quantitative tool for characterization of zeolites.

Rational Functionalization of UiO-66 with Pd Nanoparticles: Synthesis and In Situ Fourier-Transform Infrared Monitoring.

Functionalization of metal-organic frameworks (MOFs) with noble metal nanoparticles (NPs) is a challenging task. Conventional impregnation by metals often leads to agglomerates on the surface of MOF crystals. Functional groups on linkers interact with metal precursors and promote the homogeneous distribution of NPs in the pores of MOFs, but their uncontrolled localization can block channels and thus hinder mass transport. To overcome this problem, we created nucleation centers only in the defective pores of the UiO-66 MOF via the postsynthesis exchange. First, we have introduced defects into UiO-66 using benzoic acid as a modulator. Second, the modulator was exchanged for amino-benzoic acid. As a result, amino groups have decorated mainly the defective pores and attracted the Pd precursor after impregnation. The interaction of the metal precursor with amino groups and the growth of NPs were monitored by in situ infrared spectroscopy. Three processes were distinguished: the gaseous HCl release, NH2 reactivation, and growth of extended Pd surfaces. Uniform Pd NPs were located in the pores because of the homogeneous distribution of the precursor and pore diffusion-limited nucleation rate. Our work demonstrates an alternative approach of controlled Pd incorporation into UiO-66 that is of great importance for the rational design of heterogeneous catalysts.

Hydrogenation of ethylene over palladium: evolution of the catalyst structure by operando synchrotron-based techniques.

Palladium-based catalysts are exploited on an industrial scale for the selective hydrogenation of hydrocarbons. The formation of palladium carbide and hydride phases under reaction conditions changes the catalytic properties of the material, which points to the importance of operando characterization for determining the relation between the relative fractions of the two phases and the catalyst performance. We present a combined time-resolved characterization by X-ray absorption spectroscopy (in both near-edge and extended regions) and X-ray diffraction of a working palladium-based catalyst during the hydrogenation of ethylene in a wide range of partial pressures of ethylene and hydrogen. Synergistic coupling of multiple techniques allowed us to follow the structural evolution of the palladium lattice as well as the transitions between the metallic, hydride and carbide phases of palladium. The nanometric dimensions of the particles resulted in the considerable contribution of both surface and bulk carbides to the X-ray absorption spectra. During the reaction, palladium carbide is formed, which does not lead to a loss of activity. Unusual contraction of the unit cell parameter of the palladium lattice in the spent catalyst was observed upon increasing hydrogen partial pressure.

Partial and Complete Substitution of the 1,4-Benzenedicarboxylate Linker in UiO-66 with 1,4-Naphthalenedicarboxylate: Synthesis, Characterization, and H2-Adsorption Properties.

We describe the synthesis and corresponding full characterization of the set of UiO-66 metal-organic frameworks (MOFs) with 1,4-benzenedicarboxylate (C6H4(COOH)2, hereafter H2BDC) and 1,4-naphthalenedicarboxylate (C10H6(COOH)2, hereafter H2NDC) mixed linkers with NDC contents of 0, 25, 50, and 100%. Their structural (powder X-ray diffraction, PXRD), adsorptive (N2, H2, and CO2), vibrational (IR/Raman), and thermal stability (thermogravimetric analysis, TGA) properties quantitatively correlate with the NDC content in the material. The UiO-66 phase topology is conserved at all relative fractions of BDC/NDC. The comparison between the synchrotron radiation PXRD and 77 K N2-adsorption isotherms obtained on the 50:50 BDC/NDC sample and on a mechanical mixture of the pure BDC and NDC samples univocally proves that in the mixed linkers of the MOFs the BDC and NDC linkers are shared in each MOF crystal, discarding the hypothesis of two independent phases, where each crystal contains only BDC or NDC linkers. The careful tuning of the NDC content opens a way for controlled alteration of the sorption properties of the resulting material as testified by the H2-adsorption experiments, showing that the relative ranking of the materials in H2 adsorption is different in different equilibrium-pressure ranges: at low pressures, 100NDC is the most efficient sample, while with increasing pressure, its relative performance progressively declines; at high pressures, the ranking follows the BDC content, reflecting the larger internal pore volume available in the MOFs with a higher fraction of smaller linkers. The H2-adsorption isotherms normalized by the sample Brunauer-Emmett-Teller specific surface area show, in the whole pressure range, that the surface-area-specific H2-adsorption capabilities in UiO-66 MOFs increase progressively with increasing NDC content. Density functional theory calculations, using the hybrid B3LYP exchange correlation functional and quadruple-ζ with four polarization functions (QZ4P) basis set, show that the interaction of H2 with the H2NDC linker results in an adsorption energy larger by about 15% with respect to that calculated for adsorption on the H2BDC linker.

Exact Stoichiometry of Ce xZr6- x Cornerstones in Mixed-Metal UiO-66 Metal-Organic Frameworks Revealed by Extended X-ray Absorption Fine Structure Spectroscopy.

Bimetallic Ce/Zr-UiO-66 metal-organic frameworks (MOFs) proved to be promising materials for various catalytic redox applications, representing, together with other bimetallic MOFs, a new generation of porous structures. However, no direct proof for the presence of both metals in a single cornerstone of UiO-type MOFs was reported so far. Employing element-selective X-ray absorption spectroscopy techniques herein, we demonstrate, for the first time, that our synthesis route allows obtaining Ce/Zr-UiO-66 MOFs with desired Ce content and bimetallic CeZr5 cornerstones. Performing multiple-edge extended X-ray absorption fine structure analysis, we determine the exact stoichiometry of the cornerstones, which explains the dependence of thermal and chemical stability of the materials on Ce content.

Fluorescence-detected XAS with sub-second time resolution reveals new details about the redox activity of Pt/CeO2 catalyst.

A setup for fluorescence-detected X-ray absorption spectroscopy (XAS) with sub-second time resolution has been developed. This technique allows chemical speciation of low-concentrated materials embedded in highly absorbing matrices, which cannot be studied using transmission XAS. Using this setup, the reactivity of 1.5 wt% Pt/CeO2 catalyst was studied with 100 ms resolution during periodic cycling in CO- and oxygen-containing atmospheres in a plug-flow reactor. Measurements were performed at the Pt L3- and Ce L3-edges. The reactivity of platinum and cerium demonstrated a strong correlation. The oxidation of the catalyst starts on the ceria support helping the oxidation of platinum nanoparticles. The new time-resolved XAS setup can be applied to various systems, capable of reproducible cycling between different states triggered by gas atmosphere, light, temperature, etc. It opens up new perspectives for mechanistic studies on automotive catalysts, selective oxidation catalysts and photocatalysts.

Unravelling the Redox-catalytic Behavior of Ce4+ Metal-Organic Frameworks by X-ray Absorption Spectroscopy.

The introduction of Ce4+ as a structural cation has been shown to be a promising route to redox active metal-organic frameworks (MOFs). However, the mechanism by which these MOFs act as redox catalysts remains unclear. Herein, we present a detailed study of the active site in [Ce6 O4 (OH)4 ]-based MOFs such as Ce-UiO-66, involved in the aerobic oxidation of benzyl alcohol, chosen as a model redox reaction. X-ray absorption spectroscopy (XAS) data confirm the reduction of up to one Ce4+ ion per Ce6 cluster with a corresponding outwards radial shift due to the larger radius of the Ce3+ cation, while not compromising the structural integrity of the framework, as evidenced by powder X-ray diffraction. This unambiguously demonstrates the involvement of the metal node in the catalytic cycle and explains the need for 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a redox mediator to bridge the gap between the one-electron oxidation of the Ce4+ /Ce3+ couple and the two-electron alcohol oxidation. Finally, an improved catalytic system with Ce-MOF-808 and TEMPO was developed which outperformed all other tested Ce4+ -MOFs.

Time-resolved operando studies of carbon supported Pd nanoparticles under hydrogenation reactions by X-ray diffraction and absorption.

The formation of palladium hydride and carbide phases in palladium-based catalysts is a critical process that changes the catalytic performance and selectivity of the catalysts in important industrial reactions, such as the selective hydrogenation of alkynes or alkadienes. We present a comprehensive study of a 5 wt% carbon supported Pd nanoparticle (NP) catalyst in various environments by using in situ and operando X-ray absorption spectroscopy and diffraction, to determine the structure and evolution of palladium hydride and carbide phases, and their distribution throughout the NPs. We demonstrate how the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) spectra and X-ray powder diffraction (XRPD) patterns allows discrimination between the inner "core" and outer "shell" regions of the NP during hydride phase formation at different temperatures and under different hydrogen pressures, indicating that the amount of hydrogen in the shell region of the NP is lower than that in the core. For palladium carbide, advanced analysis of X-ray absorption near-edge structure (XANES) spectra allows the detection of Pd-C bonds with carbon-containing molecules adsorbed at the surface of the NPs. In addition, H/Pd and C/Pd stoichiometries of PdHx and PdCy phases were obtained by using theoretical modelling and fitting of XANES spectra. Finally, the collection of operando time-resolved XRPD patterns (with a time resolution of 5 s) allowed the detection, during the ethylene hydrogenation reaction, of periodic oscillations in the NPs core lattice parameter, which were in phase with the MS signal of ethane (product) and in antiphase with the MS signal of H2 (reactant), highlighting an interesting direct structure-reactivity relationship. The presented studies show how a careful combination of X-ray absorption and diffraction can differentiate the structure of the core, shell and surface of the palladium NPs under working conditions and prove their relevant roles in catalysis.

Effect of Molecular Guest Binding on the d-d Transitions of Ni2+ of CPO-27-Ni: A Combined UV-Vis, Resonant-Valence-to-Core X-ray Emission Spectroscopy, and Theoretical Study.

We used Ni K-edge resonant-valence-to-core X-ray emission spectroscopy (RVtC-XES, also referred to as direct RIXS), an element-selective bulk-sensitive synchrotron-based technique, to investigate the electronic structure of the CPO-27-Ni metal-organic framework (MOF) upon molecular adsorption of significant molecular probes: H2O, CO, H2S, and NO. We compare RVtC-XES with UV-vis spectroscopy, and we show that the element selectivity of RVtC-XES is of strategic significance to observe the full set of d-d excitations in Ni2+, which are partially overshadowed by the low-energy π-π* transitions of the Ni ligands in standard diffuse-reflectance UV-vis experiments. Our combined RVtC-XES/UV-vis approach provides access to the whole set of d-d excitations, allowing us a complete discussion of the changes undergone by the electronic configuration of the Ni2+ sites hosted within the MOF upon molecular adsorption. The experimental data have been interpreted by multiplet ligand-field theory calculations based on Wannier orbitals. This study represents a step further in understanding the ability of the CPO-27-Ni MOFs in molecular sorption and separation applications.

Overcoming Pd Catalyst Deactivation in the C-H Coupling of Tryptophan Residues in Water Using Air as the Oxidant.

The Pd-catalyzed C-H activation of natural tryptophan residues has emerged as a promising approach for their direct synthetic modification. While using water as the solvent and harnessing air as the oxidant is enticing, these conditions induce catalyst deactivation by promoting the formation of inactive Pd(0) clusters. In this work, we have studied optimized Pd-based catalytic systems via nonsteady state kinetic analysis and in situ X-ray absorption spectroscopy (XAS) to overcome catalyst deactivation, which enables the effective use of lower Pd loadings.

Machine Learning for Quantitative Structural Information from Infrared Spectra: The Case of Palladium Hydride.

Infrared spectroscopy (IR) is a widely used technique enabling to identify specific functional groups in the molecule of interest based on their characteristic vibrational modes or the presence of a specific adsorption site based on the characteristic vibrational mode of an adsorbed probe molecule. The interpretation of an IR spectrum is generally carried out within a fingerprint paradigm by comparing the observed spectral features with the features of known references or theoretical calculations. This work demonstrates a method for extracting quantitative structural information beyond this approach by application of machine learning (ML) algorithms. Taking palladium hydride formation as an example, Pd-H pressure-composition isotherms are reconstructed using IR data collected in situ in diffuse reflectance using CO molecule as a probe. To the best of the knowledge, this is the first example of the determination of continuous structural descriptors (such as interatomic distance and stoichiometric coefficient) from the fine structure of vibrational spectra, which opens new possibilities of using IR spectra for structural analysis.

Dual ligand approach increases functional group tolerance in the Pd-catalysed C-H arylation of N-heterocyclic pharmaceuticals.

The excellent functional group tolerance of the Suzuki-Miyaura cross-coupling reactions has been decisive for their success in the pharmaceutical industry. Highly diversified (hetero)aromatic scaffolds can be effectively coupled in the final step(s) of a convergent synthetic route. In contrast, electrophilic Pd catalysts for non-directed C-H activation are particularly sensitive to inhibition by coordinating groups in pharmaceutical precursors. While C-H arylation enables the direct conversion of (hetero)aromatics without preinstalled functional or directing groups, its functional group tolerance should be increased to be viable in late-stage cross-couplings. In this work, we report on a dual ligand approach that combines a strongly coordinating phosphine ligand with a chelating 2-hydroxypyridine for the highly robust C-H coupling of bicyclic N-heteroaromatics with aryl bromide scaffolds. The catalyst speciation was studied via in situ XAS measurements, confirming the coordination of both ligands under the reaction conditions. The C-H activation catalyst was shown to be tolerant to a wide range of pharmaceutically relevant scaffolds, including examples of late-stage functionalization of known drug molecules.