Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light ...stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal–organic frameworks and (ii) light induced dynamics of spin crossover in metal–organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code Phys. Rev. B, 2001, 63, 125120 we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni–CO configuration was found to be linear, while Ni–NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous Fe(bpy)32+ with a 100 ps delay we identified the Fe–N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.
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•Different theoretical methods in the simulation of the XANES spectra are discussed.•An extended list of available codes for XANES spectra simulation is provided.•The potential of ...operando XANES in catalysis is described with relevant examples.•Chemometric methods in the treatment of operando XANES spectra is discussed.•Machine learning approaches are used to provide structural determination from XANES.
In the last decade the appearance of progressively more sophisticated codes, together with the increased computational capabilities, has made XANES a spectroscopic technique able to quantitatively confirm (or discard) a structural model, thus becoming a new fundamental diagnostic tool in catalysis, where the active species are often diluted metal centers supported on a matrix. After providing a brief historical introduction and the basic insights on the technique, in this review article, we provide a selection of four examples where operando XANES technique has been able to provide capital information on the structure of the active site in catalysts of industrial relevance: (i) Phillips catalyst for ethylene polymerization reaction; (ii) TS-1 catalyst for selective hydrogenation reactions; (iii) carbon supported Pd nanoparticles for hydrogenation reactions; (iv) Cu-CHA zeolite for NH3-assisted selective reduction of NOx and for partial oxidation of methane to methanol. The last example testifies how the multivariate curve resolution supported by the alternating least-squares algorithm applied to a high number of XANES spectra collected under operando conditions allows to quantitatively determine different species in mutual transformation. This approach is particularly powerful in the analysis of experiments where a large number of spectra has been collected, typical of time- or space-resolved experiments. Finally, machine learning approaches (both indirect and direct) have been applied to determine, from the XANES spectra, the structure of CO, CO2 and NO adsorbed on Ni2+ sites of activated CPO-27-Ni metal-organic framework.
The design of active acidic oxygen evolution reaction (OER) catalysts is of paramount importance to achieve efficient large-current-density industrial hydrogen fuel production via water electrolysis. ...Herein, we develop a Pt-based catalyst with high electrochemical activity for the OER in acidic conditions under a large current. We achieve this by modulating the electronic structure of Pt into a high-valence, electron-accessible Pt1(2.4+δ)+ (δ = 0–0.7) state during the reaction. This electron-accessible Pt1(2.4+δ)+ single-site catalyst can effectively maintain a large OER current density of 120 mA cm−2 for more than 12 h in 0.5 M H2SO4 at a low overpotential of 405 mV, and it shows a high mass activity of ∼3350 A gmetal−1 at 10 mA cm−2 current density and 232 mV overpotential. Using in situ synchrotron radiation infrared and X-ray absorption spectroscopies, we directly observe in an experiment that a key (∗O)–Pt1–C2N2 intermediate is produced by the potential-driven structural optimization of square pyramidal Pt1–C2N2 moieties; this highly favors the dissociation of H2O over Pt1(2.4+δ)+ sites and prevents over-oxidation and dissolution of the active sites.
The valence self-modulation Pt(2.4+δ)+ (δ = 0–0.7) single-atom catalyst delivers an efficient acidic OER activity at a large current density. Simultaneously, in-situ synchrotron IR and XAFS techniques originally identified the generation of key (O∗)-Pt1-C2N2 intermediate over the potential-driven energetic tetrahedral Pt1–C2N2 moieties, which promises a high-efficiency 4e− OER process. Display omitted
•The Pt1–C2N2 moieties were strongly interfacial chemically coupled in the 3D conductive carbon substrate for improved stability.•A self-modulating valence state of Pt1(2.4+δ)+ (δ = 0–0.7) sites is for effective accessibility of oxo-containing species.•The Pt1–C2N2 SAC maintains 120 mA/cm2 at overpotential of 405 mV and delivers a high mass activity of 3350 A g−1 at 232 mV.•In situ techniques directly detected a self-modulating Pt(2.4+δ)+ site to produce the ∗O for an efficient 4e− OER process.
Accurately manipulating the electronic structure of metal active sites under working conditions is central to developing efficient and stable electrocatalysts in industrial water‐alkali ...electrolyzers. However, the lack of an intuitive means to capture the evolution of metal sites during the reaction state inhibits the manipulation of its electronic structure. Here, atomically dispersed Ru single‐sites on cobalt nanoparticles confined onto macro‐microporous frameworks (M‐Co NPs@Ru SAs/NC) with tunable electron coupling effect for efficient catalysis of alkaline hydrogen evolution reaction (HER) are constructed. Using operando X‐ray absorption and infrared spectroscopies, a dynamic CoRu bond shrinkage with strong electron coupling effect under working conditions is identified, which significantly promotes the adsorption of water molecules and then accelerates its dissociation to form the key H* over Ru sites for high HER activity. The well‐designed M‐Co NPs@Ru SAs/NC delivers efficient HER performance with a small overpotential of 34 mV at 10 mA cm−2 and a high turnover frequency of ≈4284 H2 h−1 at −0.05 V, 40 times higher than that of the benchmark Pt/C. This work provides a new point of view to manipulate the electronic structure of the metal active sites for highly effective electrocatalysis processes.
Atomically‐dispersed Ru coupled on cobalt nanoparticles within macro‐micro porous nitrogen‐doped carbon are prepared by a carbonization and galvanic replacement strategy. The electronic structure of Ru sites can be accurately manipulated by the dynamic CoRu bond shrinkage under working conditions, which significantly promotes the adsorption and dissociation of the water to form the key H* over Ru sites.
Synthesis of the MIL-100 metal-organic framework particles was carried out by hydrothermal (HT) and microwave (MW)-assisted methods. Transmission electron microscopy showed formation of ...microparticles in the course of hydrothermal synthesis and nanoparticles for microwave-assisted synthesis. Powder X-ray diffraction confirmed formation of larger crystallites for hydrothermal synthesis. Particle aggregation in aqueous solution was observed by dynamic light scattering. However, the stability of both samples could be improved in acetic acid solution. Nitrogen sorption isotherms showed high porosity of the particles. ᶫ-leucine molecule was used as a model molecule for loading in the porous micro- and nanoparticles. Loading was estimated by FTIR spectroscopy and thermogravimetric analysis. UV-VIS spectroscopy quantified ᶫ-leucine release from the particles in aqueous solution. Cytotoxicity studies using the HeLa cell model showed that the original particles were somewhat toxic, but ᶫ-leucine loading ameliorated the toxic effects, likely due to signaling properties of the amino acid.
Innovations often play an essential role in the acceleration of the new functional materials discovery. The success and applicability of the synthesis results with new chemical compounds and ...materials largely depend on the previous experience of the researcher himself and the modernity of the equipment used in the laboratory. Artificial intelligence (AI) technologies are the next step in developing the solution for practical problems in science, including the development of new materials. Those technologies go broadly beyond the borders of a computer science branch and give new insights and practical possibilities within the far areas of expertise and chemistry applications. One of the attractive challenges is an automated new functional material synthesis driven by AI. However, while having many years of hands-on experience, chemistry specialists have a vague picture of AI. To strengthen and underline AI's role in materials discovery, a short introduction is given to the essential technologies, and the machine learning process is explained. After this review, this review summarizes the recent studies of new strategies that help automate and accelerate the development of new functional materials. Moreover, automatized laboratories' self-driving cycle could benefit from using AI algorithms to optimize new functional nanomaterials' synthetic routes. Despite the fact that such technologies will shape material science in the nearest future, we note the intelligent use of algorithms and automation is required for novel discoveries.
A series of 73 ligands and 73 of their Cu+2 and Cu+1 copper complexes with different geometries, oxidation states of the metal, and redox activities were synthesized and characterized. The aim of the ...study was to establish the structure–activity relationship within a series of analogues with different substituents at the N(3) position, which govern the redox potentials of the Cu+2/Cu+1 redox couples, ROS generation ability, and intracellular accumulation. Possible cytotoxicity mechanisms, such as DNA damage, DNA intercalation, telomerase inhibition, and apoptosis induction, have been investigated. ROS formation in MCF-7 cells and three-dimensional (3D) spheroids was proven using the Pt-nanoelectrode. Drug accumulation and ROS formation at 40–60 μm spheroid depths were found to be the key factors for the drug efficacy in the 3D tumor model, governed by the Cu+2/Cu+1 redox potential. A nontoxic in vivo single-dose evaluation for two binuclear mixed-valence Cu+1/Cu+2 redox-active coordination compounds, 72k and 61k, was conducted.
Herein, we report a novel one-step solvothermal synthesis of magnetite nanoclusters (MNCs). In this report, we discuss the synthesis, structure, and properties of MNCs and contrast enhancement in T ...2-weighted MR images using magnetite nanoclusters. The effect of different organic acids, used as surfactants, on the size and shape of MNCs was investigated. The structure and properties of samples were determined by magnetic measurements, TGA, TEM, HRTEM, XRD, FTIR, and MRI. Magnetic measurements show that obtained MNCs have relatively high saturation magnetization values (65.1–81.5 emu/g) and dependence of the coercive force on the average size of MNCs was established. MNCs were transferred into an aqueous medium by Pluronic F-127, and T 2-relaxivity values were determined. T 2-Weighted MR phantom images clearly demonstrated that such magnetite nanoclusters can be used as contrast agents for MRI.
The oxygen reduction reaction (ORR) catalyzed by transition-metal single-atom catalysts (SACs) is promising for practical applications in energy-conversion devices, but great challenges still remain ...due to the sluggish kinetics of O═O cleavage. Herein, a kind of high-density iron network-like sites catalysts are constructed with optimized intermetallic distances on an amino-functionalized carbon matrix (Fe-HDNSs). Quasi-in situ soft X-ray absorption spectroscopy and in situ synchrotron infrared characterizations demonstrate that the optimized intermetallic distances in Fe-HDNSs can in situ activate the molecular oxygen by fast electron compensation through the hybridized Fe 3d-O 2p, which efficiently facilitates the cleavage of the O═O bond to *O species and highly suppresses the side reactions for an accelerated kinetics of the 4e
ORR. As a result, the well-designed Fe-HDNSs catalysts exhibit superior performances with a half-wave potential of 0.89 V versus reversible hydrogen electrode (RHE) and a kinetic current density of 72 mA cm
@0.80 V versus RHE, exceeding most of the noble-metal-free ORR catalysts. This work offers some new insights into the understanding of 4e
ORR kinetics and reaction pathways to boost electrochemical performances of SACs.
Functionalization of metal-organic frameworks with metal nanoparticles (NPs) is a promising way for producing advanced materials for catalytic applications. We present the synthesis and in situ ...characterization of palladium NPs encapsulated inside a functionalized UiO-67 metal-organic framework. The initial structure was synthesized with 10% of PdCl2bpydc moieties with grafted Pd ions replacing standard 4,4'-biphenyldicarboxylate linkers. This material exhibits the same high crystallinity and thermal stability of standard UiO-67. Formation of palladium NPs was initiated by sample activation in hydrogen and monitored by in situ X-ray powder diffraction and X-ray absorption spectroscopy (XAS). The reduction of PdII ions to Pd0 occurs above 200 °C in 6% H2/He flow. The formed palladium NPs have an average size of 2.1 nm as limited by the cavities of UiO-67 structure. The resulting material showed high activity towards ethylene hydrogenation. Under reaction conditions, palladium was found to form a carbide structure indicated by operando XAS, while formation of ethane was monitored by mass spectroscopy and infra-red spectroscopy.
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