Local Coordination Environment Research Articles

A Noncanonical Tryptophan Analogue Reveals an Active Site Hydrogen Bond Controlling Ferryl Reactivity in a Heme Peroxidase.

Nature employs high-energy metal-oxo intermediates embedded within enzyme active sites to perform challenging oxidative transformations with remarkable selectivity. Understanding how different local metal-oxo coordination environments control intermediate reactivity and catalytic function is a long-standing objective. However, conducting structure–activity relationships directly in active sites has proven challenging due to the limited range of amino acid substitutions achievable within the constraints of the genetic code. Here, we use an expanded genetic code to examine the impact of hydrogen bonding interactions on ferryl heme structure and reactivity, by replacing the N–H group of the active site Trp51 of cytochrome c peroxidase by an S atom. Removal of a single hydrogen bond stabilizes the porphyrin π-cation radical state of CcP W191F compound I. In contrast, this modification leads to more basic and reactive neutral ferryl heme states, as found in CcP W191F compound II and the wild-type ferryl heme-Trp191 radical pair of compound I. This increased reactivity manifests in a >60-fold activity increase toward phenolic substrates but remarkably has negligible effects on oxidation of the biological redox partner cytc. Our data highlight how Trp51 tunes the lifetimes of key ferryl intermediates and works in synergy with the redox active Trp191 and a well-defined substrate binding site to regulate catalytic function. More broadly, this work shows how noncanonical substitutions can advance our understanding of active site features governing metal-oxo structure and reactivity.

Deciphering the Phillips Catalyst by Orbital Analysis and Supervised Machine Learning from Cr Pre-edge XANES of Molecular Libraries.

Unveiling the nature and the distribution of surface sites in heterogeneous catalysts, and for the Phillips catalyst (CrO3/SiO2) in particular, is still a grand challenge despite more than 60 years of research. Commonly used references in Cr K-edge XANES spectral analysis rely on bulk materials (Cr-foil, Cr2O3) or molecules (CrCl3) that significantly differ from actual surface sites. In this work, we built a library of Cr K-edge XANES spectra for a series of tailored molecular Cr complexes, varying in oxidation state, local coordination environment, and ligand strength. Quantitative analysis of the pre-edge region revealed the origin of the pre-edge shape and intensity distribution. In particular, the characteristic pre-edge splitting observed for Cr(III) and Cr(IV) molecular complexes is directly related to the electronic exchange interactions in the frontier orbitals (spin-up and -down transitions). The series of experimental references was extended by theoretical spectra for potential active site structures and used for training the Extra Trees machine learning algorithm. The most informative features of the spectra (descriptors) were selected for the prediction of Cr oxidation states, mean interatomic distances in the first coordination sphere, and type of ligands. This set of descriptors was applied to uncover the site distribution in the Phillips catalyst at three different stages of the process. The freshly calcined catalyst consists of mainly Cr(VI) sites. The CO-exposed catalyst contains mainly Cr(II) silicates with a minor fraction of Cr(III) sites. The Phillips catalyst exposed to ethylene contains mainly highly coordinated Cr(III) silicates along with unreduced Cr(VI) sites.

Ab initio random structure searching for battery cathode materials.

Cathodes are critical components of rechargeable batteries. Conventionally, the search for cathode materials relies on experimental trial-and-error and a traversing of existing computational/experimental databases. While these methods have led to the discovery of several commercially viable cathode materials, the chemical space explored so far is limited and many phases will have been overlooked, in particular, those that are metastable. We describe a computational framework for battery cathode exploration based on ab initio random structure searching (AIRSS), an approach that samples local minima on the potential energy surface to identify new crystal structures. We show that by delimiting the search space using a number of constraints, including chemically aware minimum interatomic separations, cell volumes, and space group symmetries, AIRSS can efficiently predict both thermodynamically stable and metastable cathode materials. Specifically, we investigate LiCoO2, LiFePO4, and LixCuyFz to demonstrate the efficiency of the method by rediscovering the known crystal structures of these cathode materials. The effect of parameters, such as minimum separations and symmetries, on the efficiency of the sampling is discussed in detail. The adaptation of the minimum interatomic distances on a species-pair basis, from low-energy optimized structures to efficiently capture the local coordination environment of atoms, is explored. A family of novel cathode materials based on the transition-metal oxalates is proposed. They demonstrate superb energy density, oxygen-redox stability, and lithium diffusion properties. This article serves both as an introduction to the computational framework and as a guide to battery cathode material discovery using AIRSS.

An efficient combination strategy for high-performance asymmetric-electrolyte metal–air batteries

Asymmetric-electrolyte metal–air batteries (AMABs) are the most promising technologies for realizing efficient energy conversion. However, the issues in cathode and separator parts seriously impede their practical applications. This article previews the latest finding on high-voltage AMABs with long cycle stability. Impressively, the generality of the developed approach is demonstrated by the AZnABs, ASiABs and ASnABs, exhibiting the enhanced operating voltage, power densities, and excellent rate performance.

High-Pressure O2 Annealing Enhances the Crystallinity of Ultrawide-Band-Gap Sesquioxides Combined with Graphene for Vacuum-Ultraviolet Photovoltaic Detection

(AlxGa1-x)2O3 is emerging as a promising wide-band-gap sesquioxide for vacuum-ultraviolet (VUV, 10-200 nm) photodetectors and high-power field-effect transistors. However, how the key parameters such as the band gap and crystalline phase of the (AlxGa1-x)2O3-based device vary with stoichiometry has not been explicitly defined, which is due to the unclear underlying mechanism of the Al local coordination environment. In this work, a high-pressure O2 (20 atm) annealing (HPOA) strategy that can significantly improve the crystallinity of β-(AlxGa1-x)2O3 and achieve a tunable optical band gap was proposed, facilitating the revelation of the local structure of Al3+ varying with Al content and the kinetic mechanism of Al3+ diffusion. By combining the as-HPOA-treated single-crystalline β-(Al0.69Ga0.31)2O3 films with p-type graphene (p-Gr), which serves as a transparent conductor, a VUV photovoltaic detector is fabricated, showing an improved photovoltage (0.80 V) and fast temporal response (2.1 μs). All of these findings provide a rewarding and important strategy for enhancing the band-gap tunability of sesquioxides, as well as the flexibility of zero-power-consumption photodetectors.

Preparation and characterization of Gd3(ScAl)2Al3O12:Ce3+ garnet phosphors towards high-color-rendering white-light illumination

The design and development of phosphors have received widespread attention with the increasing demand for white light-emitting diodes (WLEDs). In this work, a series of Ce3+-doped gadolinium scandium aluminum garnet (GSAG) phosphors were successfully synthesized by the sol-gel method, and the relevant structural, optical, and lighting properties were investigated in detail. The optimal sintering process was optimized to 1873 K for 5 h under an Ar/H2 reduction atmosphere, and the structural analysis showed that the lattice parameters were gradually increased with an increase in Sc/Al substitution. The cationic substitution of GSAG adjusted the local coordination environments of Ce3+ ions effectively, and the increasing Sc3+-doped concentrations gave rise to a blue-shift of photoluminescence spectra and a decline of related fluorescence lifetimes. Furthermore, the doping concentration of Ce3+ ions was optimized and the concentration quenching mechanism was also clarified. Based on the temperature-dependent spectral survey, the thermally induced fluorescence quenching behavior of GSAG:Ce3+ (x = 1 at.%, y = 0.65) was measured, and the relevant activation energy was evaluated to 0.28 eV. Moreover, the effects of different Sc/Al substitution ratios on the thermal stability of GSAG:Ce3+ samples were also tested. Benefited from the tunable emission of GSAG:Ce3+ phosphors, the WLEDs device was fabricated by combining with a blue LED, yielding the Ra value of 82.1 and the CCT value of 3981 K. This study indicates the potential application of GSAG:Ce3+ phosphors in white lighting and also provides some guiding inspirations in the exploration of new garnet phosphors.

Isolated copper\u2013tin atomic interfaces tuning electrocatalytic CO2 conversion

Direct experimental observations of the interface structure can provide vital insights into heterogeneous catalysis. Examples of interface design based on single atom and surface science are, however, extremely rare. Here, we report Cu–Sn single-atom surface alloys, where isolated Sn sites with high surface densities (up to 8%) are anchored on the Cu host, for efficient electrocatalytic CO2 reduction. The unique geometric and electronic structure of the Cu–Sn surface alloys (Cu97Sn3 and Cu99Sn1) enables distinct catalytic selectivity from pure Cu100 and Cu70Sn30 bulk alloy. The Cu97Sn3 catalyst achieves a CO Faradaic efficiency of 98% at a tiny overpotential of 30 mV in an alkaline flow cell, where a high CO current density of 100 mA cm−2 is obtained at an overpotential of 340 mV. Density functional theory simulation reveals that it is not only the elemental composition that dictates the electrocatalytic reactivity of Cu–Sn alloys; the local coordination environment of atomically dispersed, isolated Cu–Sn bonding plays the most critical role.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Engineering local coordination environment of atomically dispersed platinum catalyst via lattice distortion of support for efficient hydrogen evolution reaction

Engineering the local coordination environment in atomically dispersed catalyst has proven to be a prospective route to boost catalyst performance for hydrogen evolution reaction (HER). Herein, we present the utilization of local lattice distortion of TiO2 support to regulate the local coordination environment and electronic structures of atomically dispersed Pt catalysts, resulting in the enhanced performance toward HER. Spectral analysis uncovers an elongated Pt–O bond length, lower Pt–O coordination numbers, as well as less electron holes residing in Pt 5d orbitals for Pt species on distorted TiO2. Density functional theory (DFT) calculation reveal that the variation might weaken the hydrogen adsorption on Pt sites and cause the optimized ΔG value of H∗. As a result, the atomically dispersed Pt catalyst displays superior HER mass activity (62.34 A mg−1 Pt) and the highest turnover frequency (TOF) (56.1H2·s−1) at the 50 mV overpotential in an acid media that are 18.7 and 5.56 times higher than commercial Pt/C. The work should create a new avenue in manipulating the local coordination environment of catalysts via the lattice distortion of the support, and in pursuing desired catalytic performance.

Carbon-Supported Single Metal Site Catalysts for Electrochemical CO2 Reduction to CO and Beyond.

The electrochemical CO2 reduction reaction (CO2 RR) is a promising strategy to achieve electrical-to-chemical energy storage while closing the global carbon cycle. The carbon-supported single-atom catalysts (SACs) have great potential for electrochemical CO2 RR due to their high efficiency and low cost. The metal centers' performance is related to the local coordination environment and the long-range electronic intercalation from the carbon substrates. This review summarizes the recent progress on the synthesis of carbon-supported SACs and their application toward electrocatalytic CO2 reduction to CO and other C1 and C2 products. Several SACs are involved, including MNx catalysts, heterogeneous molecular catalysts, and the covalent organic framework (COF) based SACs. The controllable synthesis methods for anchoring single-atom sites on different carbon supports are introduced, focusing on the influence that precursors and synthetic conditions have on the final structure of SACs. For the CO2 RR performance, the intrinsic activity difference of various metal centers and the corresponding activity enhancement strategies via the modulation of the metal centers' electronic structure are systematically summarized, which may help promote the rational design of active and selective SACs for CO2 reduction to CO and beyond.

Unraveling the valence states of manganese ions and the effects of composition variation and post-processing in YGG1-x-LuGGx:Mn garnet optical sensor

Transition metal manganese ions possess multiple valence states, and the change of local coordination environments would seriously influence their energy-level schemes and luminescence properties. Herein, YGG1-x-LuGGx:Mn garnet phosphors were proposed, the valence states of manganese ions and the effects of composition variation and post-processing were investigated systematically. Based on the temperature-dependent photoluminescence (PL) spectral analysis and XPS survey, the existing manganese ions are verified as Mn3+ and Mn4+. And both co-doping of Zr4+ ions and reduction processing can enhance and weaken the emissions of Mn3+ and Mn4+, respectively. Besides, introducing lutetium ions causes a PL blueshift of the two kinds of manganese ions and raises the resistance to thermally induced fluorescence quenching of Mn3+. Moreover, as for LuGG:Mn, the reduction treatment gives rise to a new garnet phase with a larger lattice parameter than that of the LuGG phase because of the existence of the LuGa antisite-defects. Finally, the combination of Mn3+ and Mn4+ ions demonstrates outstanding temperature sensing performance, further verifying the application potential of YGG1-x-LuGGx:Mn phosphors in optical thermometry.

Engendering Unprecedented Activation of Oxygen Evolution via Rational Pinning of Ni Oxidation State in Prototypical Perovskite: Close Juxtaposition of Synthetic Approach and Theoretical Conception

Rational optimization of the OER activity of catalysts based on LaNiO3 oxide is achieved by maximizing the presence of trivalent Ni in the surface structure. DFT investigations of the LaNiO3 catalyst and surface structures related to it predict an improvement in the OER activity for these materials to levels comparable with the top of the OER volcano if the La content is minimized while the oxidation state of Ni is maintained. These theoretically predicted structures of high intrinsic OER activity can be prepared by a templated spray-freeze freeze-drying synthesis followed by a simple postsynthesis exfoliation-like treatment in acidic media. These nanocrystalline LaNiO3-related materials confirm the theoretical predictions, showing a dramatic improvement in OER activity. The exfoliated surfaces remain stable in OER catalysis, as shown by an in-operando ICP-OES study. The unprecedented OER activation of the synthesized LaNiO3-based materials is related to a close juxtaposition of the theoretical conception of ideal structural motifs and the ability to engender such motifs using a unique synthetic procedure, both principally related to stabilization and pinning of the Ni oxidation state within the local coordination environment of the perovskite structure. © 2021 American Chemical Society. All rights reserved.

Approaches to achieve surface sensitivity in the in situ XAS of electrocatalysts

In situ and operando techniques providing information regarding adsorbate bonding and atomic arrangements on the electrode surface along with pure electrochemical measurements are needed to more fully understand the detailed mechanism of electrocatalytic reactions on high surface areas/nanoparticle electrocatalysts. X-ray adsorption spectroscopy (XAS) is a powerful tool to interrogate the electronic structure and local coordination environment of such electrocatalysts under working conditions, but it should be acknowledged that standard XAS methods are not intrinsically surface sensitive. This review will present recent in situ XAS studies on single-atom, metal, and metal oxide electrocatalysts, highlighting the approaches taken to achieve surface sensitivity by careful designing of the sample under investigation.

Slow magnetic relaxation in distorted tetrahedral Dy(III) aryloxide complexes.

Three distorted tetrahedral Dy(iii) aryloxide complexes, [Na(THF)6][Dy(OArAd2tBu)2Cl2] (1) (OArAd2tBu = OC6H2Adamantyl2-2,6-tBu-4) and [Na(THF)6][Dy(OMes*)3X] (X = Cl, 2; BH4, 3), (OMes* = OC6H2tBu3-2,4,6) exhibit easy axis magnetic anisotropy and slow magnetic relaxation at zero field, with relaxation rates 1 < 2 < 3.

Dual-supramolecular contacts induce extreme Hofmann framework distortion and multi-stepped spin-crossover.

An extended nitro-functionalised 1,2,4-triazole ligand has been used to induce considerable lattice distortion in a 2-D Hofmann framework material via competing supramolecular interactions. Single crystal X-ray diffraction analyses on [Fe3(N-cintrz)6(Pd(CN)4)3]·6H2O (N-cintrz: (E)-3-(2-nitrophenyl)acrylaldehyde) reveal a substantial deviation from a regular Hofmann structure, in particular as the intra- and inter-layer contacts are dominated by hydrogen-bonding interactions rather than the typical π-stacking arrays. Also, the 2-D Hofmann layers show an assortment of ligand conformations and local FeII coordination environments driven by the optimisation of competing supramolecular contacts. Temperature-dependent magnetic susceptibility measurements reveal a two-step spin crossover (SCO) transition. Variable temperature structural analyses show that the two crystallographically distinct FeII centres, which are arranged in stripes (2 : 1 ratio) within each Hofmann layer, undergo a cooperative HS ↔ HS/LS ↔ LS (HS = high spin, LS = low spin) transition without periodic spin-state ordering. The mismatch between crystallographic (2 : 1) and spin-state (1 : 1) periodicity at the HS : LS step provides key insight into the competition (frustration) between elastic interactions and crystallographically driven order.

Oxygen Coordination on Fe-N-C to Boost Oxygen Reduction Catalysis.

The coordination environments of iron (Fe) in Fe-N-C catalysts determine their intrinsic activities toward oxygen reduction reactions (ORR). The precise atomic-level regulation of the local coordination environments is thus of critical importance yet quite challenging to achieve. Here, atomically dispersed Fe-N-C catalyst with O-Fe-N2C2 moieties is thoroughly studied for ORR catalysis. Advanced synchrotron X-ray characterizations, along with theoretical modeling, explicitly unraveled the penta-coordinated nature of the Fe center in the catalytic domain and the energetically optimized ORR pathways on the well-tailored O-Fe-N2C2 moieties. The combined structure identification from both experiments and theory provides an opportunity to understand the role of the coordination environments in directing the catalytic activity of single-atom or single-site catalysts; not only the center metal atom but also the whole coordinating atoms participate in the catalytic cycle.

Ionic Liquid in Li Salt Electrolyte: Modifying the Li+ Transport Mechanism by Coordination to an Asymmetric Anion

In binary ionic liquid/Li salt electrolytes for lithium‐ion batteries the use of asymmetric anions reduces the tendency for crystallization and enables liquid systems with high Li concentration. The ionic liquid composed of the N‐(methoxyethyl)‐N‐methylpyrrolidinium (Pyr12O1) cation and the (fluorosulfonyl)(trifluoromethanesulfonyl)imide (FTFSI) asymmetric anion at molar Li FTFSI fractions up to 0.6 are investigated by 19F and 7Li chemical shifts, 1H, 19F, and 7Li electrophoretic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations. Thereby, the local coordination environment of Li is elucidated and correlated with the Li+ ion transport properties. At low Li salt fraction, the preferred Li+ coordination is to the trifluoromethanesulfonyl side of the anion, resulting in vehicular Li+ transport in stable, net negatively charged Li‐anion clusters causing negative Li transference numbers. The Li coordination is, however, shifting to the fluorosulfonyl group at salt fractions &gt;0.4, as consistently evidenced by DFT and 19F NMR. Herein, Li+ mobilities give evidence of an increasing relevance of structural Li+ ion transport, which is key toward developing efficient ionic liquid–based batteries. This knowledge will serve further tailored design of cations and anions, which reduces crystallization and promote structural transport in ionic liquids for safe and high‐power batteries.

Redox Chemistry and the Role of Trapped Molecular O2 in Li-Rich Disordered Rocksalt Oxyfluoride Cathodes.

In the search for high energy density cathodes for next-generation lithium-ion batteries, the disordered rocksalt oxyfluorides are receiving significant attention due to their high capacity and lower voltage hysteresis compared with ordered Li-rich layered compounds. However, a deep understanding of these phenomena and their redox chemistry remains incomplete. Using the archetypal oxyfluoride, Li2MnO2F, we show that the oxygen redox process in such materials involves the formation of molecular O2 trapped in the bulk structure of the charged cathode, which is reduced on discharge. The molecular O2 is trapped rigidly within vacancy clusters and exhibits minimal mobility unlike free gaseous O2, making it more characteristic of a solid-like environment. The Mn redox process occurs between octahedral Mn3+ and Mn4+ with no evidence of tetrahedral Mn5+ or Mn7+. We furthermore derive the relationship between local coordination environment and redox potential; this gives rise to the observed overlap in Mn and O redox couples and reveals that the onset potential of oxide ion oxidation is determined by the degree of ionicity around oxygen, which extends models based on linear Li–O–Li configurations. This study advances our fundamental understanding of redox mechanisms in disordered rocksalt oxyfluorides, highlighting their promise as high capacity cathodes.

Na-substitution induced oxygen vacancy achieving high transition metal capacity in commercial Li-rich cathode

Abstract High-capacity and low-cost Li-rich layered Mn-based oxides (LLMOs) hold the great promise for next-generation lithium ion battery cathode but LLMOs still encounter grand challenges in voltage decay and gas release. Here, we proposed a simple but effective as well as scalable approach of creating surface oxygen vacancies (OVs) and simultaneously enhancing structural stability. A series of Li1.2–2xNaxMn0.56Ni0.16Co0.08O2 (x = 0, 0.05, 0.1 and 0.2) cathode materials are synthesized, based on Na-pre-embedded precursor and nonstoichiometric lithiation processes, to render the OVs confirmed by synchrotron radiation analysis. First-principles calculations suggest that the architecture induced by surface OVs obviously affects the local Mn coordination environments and enhances the structural stability. Meanwhile, enlarged Li layer spacing by Na doping enables increased Li diffusion, decreased voltage polarization, and enhanced structural stability. Accordingly, the optimized Na0.1-LLMO cathode delivers highly initial coulombic efficiency of 84.2% compared to the pristine one (79.9%) and remarkable electrochemical behaviors in terms of cycling stability, voltage retention and rate performance. Pouch cell investigation further verifies the practical applicability of Na-doped LLMO cathode materials to scale up.

Utilizing X-Ray Absorption Spectroscopy to Investigate Solute-Solvent Interactions in Molten Salt Environments

Molten Salt Reactors (MSRs) are the leading candidates out of the six-generation IV advanced nuclear power reactor designs chosen for deployment for nuclear energy in US. MSR technology is particularly attractive due to better passive safety, operation at atmospheric pressure, high thermal efficiency, lower spent fuel per unit energy and increased solubility of fission products in molten salts.To facilitate robust and economical design of such systems, accurate knowledge and fundamental understanding of the structure and speciation of salts and metals in and near molten salt environments is necessary. Speciation of solutes is important because it determines how chemical behavior in solution and complex speciation from fission and corrosion products impact kinetic, thermodynamic, and transport properties of the molten salt systems. Investigating effects of radiation on metallic species and radiation-induced nucleation and growth of metallic nanoparticles under ionizing radiation is crucial for predicting the stability and reactivity of molten salts.Molten Salts in Extreme Environments (MSEE) EFRC aims to investigate speciation of metals and radiation-induced reactions in molten salt systems by utilizing synchrotron based XAS methods. In our work, Extended X-ray Absorption Fine Structure (EXAFS) and X-Ray Absorption Near Edge structure (XANES) are used to investigate local coordination environment and chemical structure of metal species such as Nickel in Zinc Chloride based molten salt systems. In addition, the effect of metal concentration and temperature on changes in local and chemical structure of metal is studied. XAS studies are complemented by optical ultra-violet spectroscopy studies of molten salts, enabling a direct correspondence between UV-Vis peak shape and coordination geometry and number, determined by EXAFS. Such knowledge of speciation of metals and radiation-induced nanoparticles in molten salt environments will provide critical understanding needed to predict and control the physical and chemical properties of molten salts and corrosion mechanisms in molten salt systems. This work was supported as part of the Molten Salts in Extreme Environments, Energy Frontier Research Center, funded by the U.S. Department of Energy Office of Science.