X-ray absorption spectroscopy in coordination chemistry

Open-source electrochemical cell for in situ X-ray absorption spectroscopy in transmission and fluorescence modes

X-ray spectroscopy is a valuable technique for the study of many materials systems. Characterizing reactions in situ and operando can reveal complex reaction kinetics, which is crucial to understanding active site composition and reaction mechanisms. In this project, the design, fabrication and testing of an open-source and easy-to-fabricate electrochemical cell for in situ electrochemistry compatible with X-ray absorption spectroscopy in both transmission and fluorescence modes are accomplished via windows with large opening angles on both the upstream and downstream sides of the cell. Using a hobbyist computer numerical control machine and free 3D CAD software, anyone can make a reliable electrochemical cell using this design. Onion-like carbon nanoparticles, with a 1:3 iron-to-cobalt ratio, were drop-coated onto carbon paper for testing in situ X-ray absorption spectroscopy. Cyclic voltammetry of the carbon paper showed the expected behavior, with no increased ohmic drop, even in sandwiched cells. Chronoamperometry was used to apply 0.4 V versus reversible hydrogen electrode, with and without 15 min of oxygen purging to ensure that the electrochemical cell does not provide any artefacts due to gas purging. The XANES and EXAFS spectra showed no differences with and without oxygen, as expected at 0.4 V, without any artefacts due to gas purging. The development of this open-source electrochemical cell design allows for improved collection of in situ X-ray absorption spectroscopy data and enables researchers to perform both transmission and fluorescence simultaneously. It additionally addresses key practical considerations including gas purging, reduced ionic resistance and leak prevention.

Atomic Design of Alkyne Semihydrogenation Catalysts via Active Learning

Alkyne hydrogenation on palladium-based catalysts modified with silver is currently used in industry to eliminate trace amounts of alkynes in alkenes produced from steam cracking and alkane dehydrogenation processes. Intensive efforts have been devoted to designing an alternative catalyst for improvement, especially in terms of selectivity and catalyst cost, which is still far away from that as expected. Here, we describe an atomic design of a high-performance Ni-based intermetallic catalyst aided by active machine learning combined with density functional theory calculations. The engineered NiIn catalyst exhibits >97% selectivity to ethylene and propylene at the full conversion of acetylene and propyne at mild temperature, outperforming the reported Ni-based catalysts and even noble Pd-based ones. Detailed mechanistic studies using theoretical calculations and advanced characterizations elucidate that the atomic-level defined coordination environment of Ni sites and well-designed hybridization of Ni 3d with In 5p orbital determine the semihydrogenation pathway.

Understanding Copper(I)-Ethylene Bonding Using Cu K-Edge X-ray Absorption Spectroscopy

Copper plays many important roles in ethylene chemistry, thus generating significant interest in understanding the structures, bonding, and properties of copper(I)-ethylene complexes. In this work, the ethylene binding characteristics of a series of isolable Cu(I)-ethylene compounds supported by a systematic set of fluorinated and nonfluorinated bis- and tris(pyrazolyl)borate and the related bis(pyrazolyl)methane ligands have been investigated. Through a combination of X-ray absorption spectroscopy and quantum chemical calculations, we characterize their geometric and electronic structures and the role that fluorinated ligands play in lowering the electron density at Cu sites. Such ligands increase the ethylene-to-Cu σ-donor interaction and, correspondingly, decrease the Cu-to-ethylene π back-bonding. This latter interaction leads to a partial vacancy in the Cu 3d level, which manifests experimentally as a low-energy feature in the Cu K pre-edge, allowing for its direct observation and comparison within a series of Cu(I) compounds. The pre-edge feature is reproduced by TD-DFT calculations, and its energy position and total intensity are used to quantitatively probe Cu-ethylene bonding. The variations in the Cu electronic structure influence the stability and overall ethylene bonding strength of these compounds, ultimately showing how substituents on the supporting ligands have a notable effect on their physical and chemical properties.

Symmetry-Breaking p-Block Antimony Single Atoms Trigger N-Bridged Titanium Sites for Electrocatalytic Nitrogen Reduction with High Efficiency

The electrochemical nitrogen reduction reaction (eNRR) under mild conditions emerges as a promising approach to produce ammonia (NH3) compared to the typical Haber-Bosch process. Herein, we design an asymmetrically coordinated p-block antimony single-atom catalyst immobilized on nitrogen-doped Ti3C2Tx (Sb SA/N-Ti3C2Tx) for eNRR, which exhibits ultrahigh NH3 yield (108.3 μg h-1 mgcat-1) and excellent Faradaic efficiency (41.2%) at -0.3 V vs RHE. Complementary in situ spectroscopies with theoretical calculations reveal that the nitrogen-bridged two titanium atoms triggered by an adjacent asymmetrical Sb-N1C2 moiety act as the active sites for facilitating the protonation of the rate-determining step from *N2 to *N2H and the kinetic conversion of key intermediates during eNRR. Moreover, the introduction of Sb-N1C2 promotes the formation of oxygen vacancies to expose more titanium sites. This work presents a strategy for single-atom-decorated ultrathin two-dimensional materials with the aim of simultaneously enhancing NH3 yield and Faradaic efficiency for electrocatalytic nitrogen reduction.

Dysprosium site occupancy in SrZnO2 nanophosphors probed through XANES

Doping-assisted lattice site engineering is widely practiced to obtain a tailor made response, which subsequently poses a need for an efficient probe of the local electronic structure of the system. This study presents a detailed analysis of the local electronic structure around the host cations (Zn2+ and Sr2+) and dopant (Dy3+) through combined experimental and simulated X-ray absorption near edge structure. The real space full multiple scattering-based simulations of the Zn K-edge are done by substituting Dy at cationic sites in the second coordination shell around Zn, in various combinations along with and/or without oxygen vacancies in the system. The results revealed that Dy tends to substitute the less symmetric Sr2+ site at low doping concentration, whereas it starts substituting the relatively more symmetric Zn2+ lattice site with an increase in doping concentration, consequently affirming the origin of cold white emission upon charge transfer in the system (Manju, M. Jain, P. Vashishtha, G. Gupta, A. Sharma, S. O. Won, A. Vij and A. Thakur, J. Phys.: Condens. Matter, 2020, 33, 035703). The effect of Zn site occupancy is seen as bifurcation of the single peaked Dy L3 absorption edge, which is usually reported as the sole indication of the existence of a mixed valence state. Thus, the combined analyses decipher the effect of lattice site occupancy on the local electronic structure of host as well as dopant atoms.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Fractional-Electron and Transition-Potential Methods for Core-to-Valence Excitation Energies Using Density Functional Theory

Methods for computing X-ray absorption spectra based on a constrained core hole (possibly containing a fractional electron) are examined. These methods are based on Slater's transition concept and its generalizations, wherein core-to-valence excitation energies are determined using Kohn-Sham orbital energies. Methods examined here avoid promoting electrons beyond the lowest unoccupied molecular orbital, facilitating robust convergence. Variants of these ideas are systematically tested, revealing a best-case accuracy of 0.3-0.4 eV (with respect to experiment) for K-edge transition energies. Absolute errors are much larger for higher-lying near-edge transitions but can be reduced below 1 eV by introducing an empirical shift based on a charge-neutral transition-potential method, in conjunction with functionals such as SCAN, SCAN0, or B3LYP. This procedure affords an entire excitation spectrum from a single fractional-electron calculation, at the cost of ground-state density functional theory and without the need for state-by-state calculations. This shifted transition-potential approach may be especially useful for simulating transient spectroscopies or in complex systems where excited-state Kohn-Sham calculations are challenging.

Intermetallic Single-Atom Alloy In-Pd Bimetallene for Neutral Electrosynthesis of Ammonia from Nitrate

Harvesting recyclable ammonia (NH₃) from the electrocatalytic reduction of nitrate (NO₃RR) offers a sustainable strategy to close the ecological nitrogen cycle from nitration contamination in an energy-efficient and environmentally friendly manner. The emerging intermetallic single-atom alloys (ISAAs) are recognized to achieve the highest site density of single atoms by isolating contiguous metal atoms into single sites stabilized by another metal within the intermetallic structure, which holds promise to couple the catalytic benefits from intermetallic nanocrystals and single-atom catalysts for promoting NO₃RR. Herein, ISAA In-Pd bimetallene, in which the Pd single atoms are isolated by surrounding In atoms, is reported to boost neutral NO₃RR with a NH₃ Faradaic efficiency (FE) of 87.2%, a yield rate of 28.06 mg h^-1 mg_Pd^-1, and an exceptional electrocatalytic stability with increased activity/selectivity over 100 h and 20 cycles. The ISAA structure induces substantially diminished overlap of Pd d-orbitals and narrowed p-d hybridization of In-p and Pd-d states around the Fermi level, resulting in a stronger NO₃^- adsorption and a depressed energy barrier of the potential-determining step for NO₃RR. Further integrating the NO₃RR catalyst into a Zn-NO₃^- flow battery as the cathode delivers a power density of 12.64 mW cm^-2 and a FE of 93.4% for NH₃ production.

Extraction of Se(iv) and Se(vi) from aqueous HCl solution by using a diamide-containing tertiary amine

Here, we investigated the mechanism underlying the extraction of Se(iv) and Se(vi) from aqueous HCl solutions by N-2-ethylhexyl-bis(N-di-2-ethylhexyl-ethylamide)amine (EHBAA). In addition to examining extraction behavior, we also elucidated structural properties of the dominant Se species in solution. Two types of aqueous HCl solutions were prepared by dissolving a Se^IV oxide or a Se^VI salt. X-ray absorption near edge structure analyses revealed that Se(vi) was reduced to Se(iv) in 8 M HCl. Using 0.5 M EHBAA, ∼50% of Se(vi) was extracted from 0.5 M HCl. In contrast, Se(iv) was hardly extracted from 0.5 to 5 M HCl; however, at molar concentrations above 5 M, the extraction efficiency of Se(iv) increased drastically, reaching ∼85%. Slope analyses for the distribution ratios of Se(iv) in 8 M HCl and Se(vi) in 0.5 M HCl showed that apparent stoichiometries of Se(iv) or Se(vi) to EHBAA were 1 : 1 and 1 : 2, respectively. Extended X-ray absorption fine structure measurements revealed that the inner-sphere of the Se(iv) and Se(vi) complexes extracted with EHBAA was [SeOCl₂] and [SeO₄]^2-, respectively. Together, these results indicate that Se(iv) is extracted from 8 M HCl with EHBAA via a solvation-type reaction, whereas Se(vi) is extracted from 0.5 M HCl via an anion-exchange-type reaction.

Lattice Strain and Schottky Junction Dual Regulation Boosts Ultrafine Ruthenium Nanoparticles Anchored on a N-Modified Carbon Catalyst for H2 Production

Ruthenium-based materials are considered great promising candidates to replace Pt-based catalysts for hydrogen production in alkaline conditions. Herein, we adopt a facile method to rationally design a neoteric Schottky catalyst in which uniform ultrafine ruthenium nanoparticles featuring lattice compressive stress are supported on nitrogen-modified carbon nanosheets (Ru NPs/NC) for efficient hydrogen evolution reaction (HER). Lattice strain and Schottky junction dual regulation ensures that the Ru NPs/NC catalyst with an appropriate nitrogen content displays superb H₂ evolution in alkaline media. Particularly, Ru NPs/NC-900 with 1.3% lattice compressive strain displays attractive activity and durability for the HER with a low overpotential of 19 mV at 10 mA cm^-2 in 1.0 M KOH electrolyte. The in situ X-ray absorption fine structure measurements indicate that the low-valence Ru nanoparticle with shrinking Ru-Ru bond acts as catalytic active site during the HER process. Furthermore, multiple spectroscopy analysis and density functional theory calculations demonstrate that the lattice strain and Schottky junction dual regulation tunes the electron density and hydrogen adsorption of the active center, thus enhancing the HER activity. This strategy provides a novel concept for the design of advanced electrocatalysts for H₂ production.

Mechanism driven design of trimer Ni1Sb2 site delivering superior hydrogenation selectivity to ethylene

Mechanism driven catalyst design with atomically uniform ensemble sites is an important yet challenging issue in heterogeneous catalysis associated with breaking the activity-selectivity trade-off. Herein, a trimer Ni₁Sb₂ site in NiSb intermetallic featuring superior selectivity is elaborated for acetylene semi-hydrogenation via a theoretical guidance with a precise synthesis strategy. The trimer Ni₁Sb₂ site in NiSb intermetallic is predicted to endow acetylene reactant with an adequately but not excessively strong σ-adsorption mode while ethylene product with a weak π-adsorption one, where such compromise delivers higher ethylene formation rate. An in-situ trapping of molten Sb by Ni strategy is developed to realize the construction of Ni₁Sb₂ site in the intermetallic P6₃/mmc NiSb catalysts. Such catalyst exhibits ethylene selectivity up to 93.2% at 100% of acetylene conversion, significantly prevailing over the referred Ni catalyst. These insights shed new lights on rational catalyst design by taming active sites to energetically match targeted reaction pathway.

Designing atomically uniform ensemble sites for matching targeted reaction pathway is important yet challenging in heterogeneous catalysis. Here, the authors fabricate a trimer Ni₁Sb₂ site featuring superior selectivity for acetylene semi-hydrogenation via a mechanism-driven design strategy.

Calculation of Core-Excited and Core-Ionized States Using Variational Quantum Deflation Method and Applications to Photocatalyst Modeling

The possibility of performing quantum-chemical calculations using quantum computers has attracted much interest. Variational quantum deflation (VQD) is a quantum-classical hybrid algorithm for the calculation of excited states with noisy intermediate-scale quantum devices. Although the validity of this method has been demonstrated, there have been few practical applications, primarily because of the uncertain effect of calculation conditions on the results. In the present study, calculations of the core-excited and core-ionized states for common molecules based on the VQD method were examined using a classical computer, focusing on the effects of the weighting coefficients applied in the penalty terms of the cost function. Adopting a simplified procedure for estimating the weighting coefficients based on molecular orbital levels allowed these core-level states to be successfully calculated. The O 1s core-ionized state for a water molecule was calculated with various weighting coefficients, and the resulting ansatz states were systematically examined. The application of this technique to functional materials was demonstrated by calculating the core-level states for titanium dioxide (TiO₂) and nitrogen-doped TiO₂ models. The results demonstrate that VQD calculations employing an appropriate cost function can be applied to the analysis of functional materials in conjunction with an experimental approach.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

A complementary characterisation technique for spin crossover materials; the application of X-ray photoelectron spectroscopy for future device applications

Spin crossover (SCO) materials have long been studied for their inherent electronic switchability, which has been well investigated for potential application in electronic and switching devices. As the technologies for the fabrication of thin films and monolayers continue to develop at an exceedingly rapid pace, an emerging challenge for the SCO community has become the characterisation of spin transitions in the surface layers of a material, as well as understanding the origins of discrepancies observed between SCO in thin films and that of the bulk material. For the manufacture of such devices to become a reality, it is crucial to understand how spin crossover is affected by interactions with the substrate material and within thin films. As such, detailed analysis of the surface layers without interference from the substrate material emerged as a critical area of characterisation for future developments in SCO devices. In this regard, X-ray Photoelectron Spectroscopy (XPS) has emerged as a complementary technique for the analysis of SCO in the surface layers of a material, becoming an essential part of a multi-technique protocol that is driving advances in the field. Here we describe the complementary application of XPS to a variety of SCO materials, review major developments and provide illustrative examples of innovations made through surface analysis with XPS.

Comparative analysis of XANES and EXAFS for local structural characterization of disordered metal oxides

In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.

Controlling Radical-Type Single-Electron Elementary Steps in Catalysis with Redox-Active Ligands and Substrates

Advances in (spectroscopic) characterization of the unusual electronic structures of open-shell cobalt complexes bearing redox-active ligands, combined with detailed mapping of their reactivity, have uncovered several new catalytic radical-type protocols that make efficient use of the synergistic properties of redox-active ligands, redox-active substrates, and the metal to which they coordinate. In this perspective, we discuss the tools available to study, induce, and control catalytic radical-type reactions with redox-active ligands and/or substrates, contemplating recent developments in the field, including some noteworthy tools, methods, and reactions developed in our own group. The main topics covered are (i) tools to characterize redox-active ligands; (ii) novel synthetic applications of catalytic reactions that make use of redox-active carbene and nitrene substrates at open-shell cobalt-porphyrins; (iii) development of catalytic reactions that take advantage of purely ligand- and substrate-based redox processes, coupled to cobalt-centered spin-changing events in a synergistic manner; and (iv) utilization of redox-active ligands to influence the spin state of the metal. Redox-active ligands have emerged as useful tools to generate and control reactive metal-coordinated radicals, which give access to new synthetic methodologies and intricate (electronic) structures, some of which are yet to be exposed.

Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film

Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.

MAP2XANES: a Jupyter interactive notebook for elemental mapping and XANES speciation

MAP2XANES is an intuitive Jupyter notebook that automatizes the analysis of synchrotron X-ray fluorescence imaging and X-ray absorption spectroscopy for the characterization of complex and heterogeneous samples. The notebook uses basic modules and functions from Numpy, Scipy, Pandas, iPywidgets and Matplotlib libraries for a powerful data reduction process that, in just a few clicks, guides the user through the visualization of elemental maps, space-resolved absorption spectra and their automatized analysis. In particular, by means of linear combination fit of the XANES spectra, the notebook determines the chemical species distribution in the sample under investigation. The direct output of the analysis process is the correlation between the different elemental distributions and the spatial localization of the chemical species detected. An application to mineralogy is thus presented, analyzing the Mn²⁺, Mn³⁺ and Mn⁴⁺ distribution in a mineral sample of hausmannite (Mn²⁺Mn₂³⁺O₄), courtesy of the Museum of Natural Science of Barcelona.

Latent Representation Learning for Structural Characterization of Catalysts

Supervised machine learning-enabled mapping of the X-ray absorption near edge structure (XANES) spectra to local structural descriptors offers new methods for understanding the structure and function of working nanocatalysts. We briefly summarize a status of XANES analysis approaches by supervised machine learning methods. We present an example of an autoencoder-based, unsupervised machine learning approach for latent representation learning of XANES spectra. This new approach produces a lower-dimensional latent representation, which retains a spectrum-structure relationship that can be eventually mapped to physicochemical properties. The latent space of the autoencoder also provides a pathway to interpret the information content "hidden" in the X-ray absorption coefficient. Our approach (that we named latent space analysis of spectra, or LSAS) is demonstrated for the supported Pd nanoparticle catalyst studied during the formation of Pd hydride. By employing the low-dimensional representation of Pd K-edge XANES, the LSAS method was able to isolate the key factors responsible for the observed spectral changes.

Revealing differences in the chemical form of zinc in brain tissue using K-edge X-ray absorption near-edge structure spectroscopy

Zinc is a prominent trace metal required for normal memory function. Memory loss and cognitive decline during natural ageing and neurodegenerative disease have been associated with altered brain-Zn homeostasis. Yet, the exact chemical pathways through which Zn influences memory function during health, natural ageing, or neurodegenerative disease remain unknown. The gap in the literature may in part be due to the difficulty to simultaneously image, and therefore, study the different chemical forms of Zn within the brain (or biological samples in general). To this extent, we have begun developing and optimising protocols that incorporate X-ray absorption near-edge structure (XANES) spectroscopic analysis of tissue at the Zn K-edge as an analytical tool to study Zn speciation in the brain. XANES is ideally suited for this task as all chemical forms of Zn are detected, the technique requires minimal sample preparation that may otherwise redistribute or alter the chemical form of Zn, and the Zn K-edge has known sensitivity to coordination geometry and ligand type. Herein, we report our initial results where we fit K-edge spectra collected from micro-dissected flash-frozen brain tissue, to a spectral library prepared from standard solutions, to demonstrate differences in the chemical form of Zn that exist between two brain regions, the hippocampus and cerebellum. Lastly, we have used an X-ray microprobe to demonstrate differences in Zn speciation within sub-regions of thin air-dried sections of the murine hippocampus; but, the corresponding results highlight that the chemical form of Zn is easily perturbed by sample preparation such as tissue sectioning or air-drying, which must be a critical consideration for future work.

Spectral Decomposition of X-ray Absorption Spectroscopy Datasets: Methods and Applications

X-ray absorption spectroscopy (XAS) today represents a widespread and powerful technique, able to monitor complex systems under in situ and operando conditions, while external variables, such us sampling time, sample temperature or even beam position over the analysed sample, are varied. X-ray absorption spectroscopy is an element-selective but bulk-averaging technique. Each measured XAS spectrum can be seen as an average signal arising from all the absorber-containing species/configurations present in the sample under study. The acquired XAS data are thus represented by a spectroscopic mixture composed of superimposed spectral profiles associated to well-defined components, characterised by concentration values evolving in the course of the experiment. The decomposition of an experimental XAS dataset in a set of pure spectral and concentration values is a typical example of an inverse problem and it goes, usually, under the name of multivariate curve resolution (MCR). In the present work, we present an overview on the major techniques developed to realize the MCR decomposition together with a selection of related results, with an emphasis on applications in catalysis. Therein, we will highlight the great potential of these methods which are imposing as an essential tool for quantitative analysis of large XAS datasets as well as the directions for further development in synergy with the continuous instrumental progresses at synchrotron sources.

Characterizing electronic and atomic structures for amorphous and molecular metal oxide catalysts at functional interfaces by combining soft X-ray spectroscopy and high-energy X-ray scattering

Amorphous thin film materials and heterogenized molecular catalysts supported on electrode and other functional interfaces are widely investigated as promising catalyst formats for applications in solar and electrochemical fuels catalysis. However the amorphous character of these catalysts and the complexity of the interfacial architectures that merge charge transport properties of electrode and semiconductor supports with discrete sites for multi-step catalysis poses challenges for probing mechanisms that activate and tune sites for catalysis. This minireview discusses advances in soft X-ray spectroscopy and high-energy X-ray scattering that provide opportunities to resolve interfacial electronic and atomic structures, respectively, that are linked to catalysis. This review discusses how these techniques can be partnered with advances in nanostructured interface synthesis for combined soft X-ray spectroscopy and high-energy X-ray scattering analyses of thin film and heterogenized molecular catalysts. These combined approaches enable opportunities for the characterization of both electronic and atomic structures underlying fundamental catalytic function, and that can be applied under conditions relevant to device applications.

Engineering a metal-organic framework derived Mn-N4-C x S y atomic interface for highly efficient oxygen reduction reaction

Atomic interface engineering is an effective pathway to regulate the performance of single metal atom catalysts for electrochemical reactions in energy applications. Herein, we construct a sulfur modified Mn-N-C single atom catalyst through a metal-organic framework derived atomic interface strategy, which exhibits outstanding ORR activity with a half-wave potential of 0.916 V vs. RHE in alkaline media. Moreover, operando X-ray absorption spectroscopy analysis indicates that the isolated bond-length extending the low-valence Mn-N4-C x S y moiety serves as an active site during the ORR process. These findings suggest a promising method for the advancement of single atom catalysis.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.

Structural Characterization of a Series of N5-Ligated MnIV -Oxo Species

Analysis of extended X-ray absorption fine structure (EXAFS) data for the MnIV -oxo complexes [MnIV (O)(DMM N4py)]2+ , [MnIV (O)(2pyN2B)]2+ , and [MnIV (O)(2pyN2Q)]2+ (DMM N4py=N,N-bis(4-methoxy-3,5-dimethyl-2-pyridylmethyl)-N-bis(2-pyridyl)methylamine; 2pyN2B=(N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine, and 2pyN2Q=N,N-bis(2-pyridyl)-N,N-bis(2-quinolylmethyl)methanamine) afforded Mn=O and Mn-N bond lengths. The Mn=O distances for [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ are 1.72 and 1.70 Å, respectively. In contrast, the Mn=O distance for [MnIV (O)(2pyN2Q)]2+ was significantly longer (1.76 Å). We attribute this long distance to sample heterogeneity, which is reasonable given the reduced stability of [MnIV (O)(2pyN2Q)]2+ . The Mn=O distances for [MnIV (O)(DMM N4py)]2+ and [MnIV (O)(2pyN2B)]2+ could only be well-reproduced using DFT-derived models that included strong hydrogen-bonds between second-sphere solvent 2,2,2-trifluoroethanol molecules and the oxo ligand. These results suggest an important role for the 2,2,2-trifluoroethanol solvent in stabilizing MnIV -oxo adducts. The DFT methods were extended to investigate the structure of the putative [MnIV (O)(N4py)]2+ ⋅(HOTf)2 adduct. These computations suggest that a MnIV -hydroxo species is most consistent with the available experimental data.

Additional Lithium Storage on Dynamic Electrode Surface by Charge Redistribution in Inactive Ru Metal

Beyond a traditional view that metal nanoparticles formed upon electrochemical reaction are inactive against lithium, recently their electrochemical participations are manifested and elucidated as catalytic and interfacial effects. Here, ruthenium metal composed of ≈5 nm nanoparticles is prepared and the pure ruthenium as a lithium-ion battery anode for complete understanding on anomalous lithium storage reaction mechanism is designed. In particular, the pure metal electrode is intended for eliminating the electrochemical reaction-derived Li₂ O phase accompanied by catalytic Li₂ O decomposition and the interfacial lithium storage at Ru/Li₂ O phase boundary, and thereby focusing on the ruthenium itself in exploring its electrochemical reactivity. Intriguingly, unusual lithium storage not involving redox reactions with electron transfer but leading to lattice expansion is identified in the ruthenium electrode. Size-dependent charge redistribution at surface enables additional lithium adsorption to occur on the inactive but more environmentally sensitive nanoparticles, providing innovative insight into dynamic electrode environments in rechargeable lithium chemistry.

Evaluation of cobalt complexes with tripod ligands for zinc finger targeting

We report the ability of Co^II and Co^III complexes of tri(2-pyridylmethyl)amine and N,N-di(2-pyridylmethyl)glycinate to disrupt zinc fingers.

Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene

We report the first Fe─CPh₃ complex, and show that the long Fe─C bond can be disrupted by neutral π-acceptor ligands (benzophenone and phenylacetylene) to release the triphenylmethyl radical. The products are formally iron(I) complexes, but X-ray absorption spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description of all the complexes is iron(II). In the formally iron(I) complexes, this does not imply that the π-acceptor ligand has radical character, because the iron(II) description arises from doubly-occupied frontier molecular orbitals that are shared equitably by the iron and the π-acceptor ligand, and the unpaired electrons lie on the metal. Despite the lack of substantial radical character on the ligands, alkyne and ketone fragments can couple to form a high-spin iron(III) complex with a cyclized metalladihydrofuran core.

Probing Conformation Change and Binding Mode of Metal Ion-Carboxyl Coordination Complex through Resonant Surface-Enhanced Raman Spectroscopy and Density Functional Theory

Understanding carboxyl-metal ligand interaction has great significance in analytical chemistry. Herein, we use resonant surface-enhanced Raman scattering (SERS) to probe the physiochemical interaction and conformation change in several metal ion-carboxyl coordination complex systems adsorbed on the surface of plasmonically resonant metal nanostructures. Our SERS results and density function theory calculations jointly reveal that low-valence metal ions (such as K⁺ and Pb²⁺) tend to bind to the carboxyl active site of a Raman tag molecule, 4-mercaptobenzoic acid (4-MBA), in a unidentate binding mode of low binding energy whereas high-valence metal ions (such as Fe³⁺) favor a bidentate binding mode of relatively high binding energy. Particularly, Pb²⁺-ion concentration-dependent SERS suggests a repulsive interaction among the coordination complex leading to a tilted configuration of 4-MBA on the metal surface. This work indicates the resonant SERS approach is suitable not only for studying the carboxyl-metal ligand interaction but also for detecting various types of heavy metal ions at low concentrations.

H/D Isotope Effects Reveal Factors Controlling Catalytic Activity in Co-Based Oxides for Water Oxidation

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while ¹⁶O-to-¹⁸O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled Co^II/III and Co^III/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent Co^IV atoms and a rate-determining step that involves oxygen-oxygen bond formation and compare it with models proposed in the literature.

The influence of ZnII coordination sphere and chemical structure over the reactivity of metallo-β-lactamase model compounds

The first coordination sphere influences the reactivity of metallo-β-lactamase monozinc model complexes.

Synthesis, characterization, and evaluation of iron nanoparticles as hydrogenation catalysts in alcohols and tetraalkylphosphonium ionic liquids: do solvents matter?

Best recyclability seen for Fe nanoparticles in alcohol solvents in which Fe-core, Fe oxide shell nanoparticles are formed.

Advanced X-ray Spectroscopic Methods for Studying Iron-Sulfur-Containing Proteins and Model Complexes

In this chapter, a brief overview of X-ray spectroscopic methods that may be utilized to obtain insight into the geometric and electronic structure of iron-sulfur proteins is provided. These methods include conventional methods, such as metal and ligand K-edge X-ray absorption, as well as more advanced methods including nonresonant and resonant X-ray emission. In each section, the basic information content of the spectra is highlighted and important experimental considerations are discussed. Throughout the chapter, recent applications to iron-sulfur-containing models and proteins are highlighted.

Comparison of the Extractabilities of Tetrachloro- and Tetrabromopalladate(II) Ions with a Thiodiglycolamide Compound

Using N,N,N',N'-tetra-2-ethylhexyl-thiodiglycolamide (TEHTDGA) in n-dodecane as the extractant, we compared the percentages of Pd(II) extracted from HCl and HBr solutions, and analyzed the structures of the Pd(II)-extractant complexes. For comparison, similar experiments were performed with di-n-hexyl sulfide (DHS), a well-known sulfide-type extractant. TEHTDGA extracted Pd(II) from both HCl and HBr solutions much faster than DHS. The Pd(II)/(TEHTDGA or DHS) stoichiometry in the organic phase was 1:2. For TEHTDGA, the extractability of Pd(II) from HBr solution was inferior to that from HCl solution, whereas the opposite was true for DHS. However, FT-IR spectroscopy and EXAFS measurements indicated that the inner-sphere structure of Pd(II) in the TEHTDGA complex was almost the same as that in the DHS system: in both cases, two of the halide ions in the tetrachloro- or tetrabromopalladate(II) ion were replaced by the sulfur atoms of two extractant molecules.

First-Principles Fe L2,3-Edge and O K-Edge XANES and XMCD Spectra for Iron Oxides

X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) spectroscopies are tools in widespread use for providing detailed local atomic structure, oxidation state, and magnetic structure information for materials and organometallic complexes. The analysis of these spectra for transition-metal L-edges is routinely performed on the basis of ligand-field multiplet theory because one- and two-particle mean-field ab initio methods typically cannot describe the multiplet structure. Here we show that multireference configuration interaction (MRCI) calculations can satisfactorily reproduce measured XANES spectra for a range of complex iron oxide materials including hematite and magnetite. MRCI Fe L_2,3-edge XANES and XMCD spectra of Fe(II)O₆, Fe(III)O₆, and Fe(III)O₄ in magnetite are found to be in very good qualitative agreement with experiment and multiplet calculations. Point-charge embedding and small distortions of the first-shell oxygen ligands have only small effects. Oxygen K-edge XANES/XMCD spectra for magnetite investigated by a real-space Green's function approach complete the very good qualitative agreement with experiment. Material-specific differences in local coordination and site symmetry are well reproduced, making the approach useful for assigning spectral features to specific oxidation states and coordination environments.

Design of ultrathin Pt-Mo-Ni nanowire catalysts for ethanol electrooxidation

Developing cost-effective, active, and durable electrocatalysts is one of the most important issues for the commercialization of fuel cells. Ultrathin Pt-Mo-Ni nanowires (NWs) with a diameter of ~2.5 nm and lengths of up to several micrometers were synthesized via a H2-assisted solution route (HASR). This catalyst was designed on the basis of the following three points: (i) ultrathin NWs with high numbers of surface atoms can increase the atomic efficiency of Pt and thus decrease the catalyst cost; (ii) the incorporation of Ni can isolate Pt atoms on the surface and produce surface defects, leading to high catalytic activity (the unique structure and superior activity were confirmed by spherical aberration-corrected electron microscopy measurements and ethanol oxidation tests, respectively); and (iii) the incorporation of Mo can stabilize both Ni and Pt atoms, leading to high catalytic stability, which was confirmed by experiments and density functional theory calculations. Furthermore, the developed HASR strategy can be extended to synthesize a series of Pt-Mo-M (M = Fe, Co, Mn, Ru, etc.) NWs. These multimetallic NWs would open up new opportunities for practical fuel cell applications.

High performance platinum single atom electrocatalyst for oxygen reduction reaction

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm⁻² at 80 °C with a low platinum loading of 0.09 mg_Pt cm⁻², corresponding to a platinum utilization of 0.13 g_Pt kW⁻¹ in the fuel cell. Good fuel cell durability is also observed. Theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction.

High-performance electrocatalysts for the oxygen reduction reaction (ORR) typically use platinum (Pt), however its high cost is a hindrance to commercial scale up. Here, the authors report a cost-effective, efficient and durable Pt single-atom electrocatalyst for ORR with a Pt utilization of 0.13 g_Pt kW⁻¹ in a fuel cell.