Transition Metal Cations Research Articles

Catalytic Performance of Ti-MCM-22 Modified with Transition Metals (Cu, Fe, Mn) as NH3-SCR Catalysts

In the presented work, titanosilicate with the MWW structure (Ti-MWW) was hydrothermally synthesized using boron and titanium precursors, with piperidine as a structure-directing agent. The resulting layered zeolite precursor, with a Si/Ti molar ratio of 50, was treated in an HNO3 solution to remove extraframework Ti and B species. The acid-modified zeolite was functionalized with transition metal cations (Cu2+, Fe2+, Mn2+) and trinuclear oligocations (Fe(3) and Mn(3)). The application of this catalytic system is supported by the presence of titanium in the catalytic support structure—similar to a commercial system, V2O5–TiO2. The obtained samples were characterized with respect to their structure (P-XRD, DRIFT), textural parameters (low-temperature N2 sorption), surface acidity (NH3-TPD), transition metal content (ICP-OES) and form (UV–vis DRS) as well as catalyst’s reducibility (H2-TPR). Ti-MWW zeolite samples modified with transition metals were evaluated as catalysts for the selective catalytic reduction of NO with ammonia (NH3-SCR). The effective temperature range for the NO conversion varied depending on the type of active phase used to functionalize the porous support. The catalytic performance was influenced by transition metal content, its form, and accessibility for reactants as well as interactions between the active phase and titanium-containing support. Among the catalysts tested, the copper-modified Ti-MWW zeolite showed the most promising results, maintaining 90% NO conversion rates across a relatively broad temperature range from 200 to 325 °C. This catalyst meets the requirements of modern NH3-SCR installations, which aim to operate in the low-temperature region, below 250 °C.

Compositional heterogeneity of secondary minerals in mine waste rock: Origins and implications for water quality.

Secondary minerals in mine waste materials impose strong controls on water quality by scavenging solutes of concern. This study investigates the mineralogical and compositional characteristics of secondary Fe(oxy)hydroxides and Ca-sulfates, two globally ubiquitous secondary precipitates, in weathered mine waste rock. Bulk analyses show that Si, Ca, Fe, Al, and S-bearing primary phases were the most abundant in the entire samples, but up to a few wt% of secondary Fe(oxy)hydroxides and Ca-sulfates were present as well. In these secondary phases, trace metal impurities like V (37 mg/kg in Ca-sulfates), Cr (23 mg/kg in Fe-oxides and 21 mg/kg in Ca-sulfates), and Cd (up to 15 mg/kg in Fe-oxides and Ca-sulfates) could not be detected by bulk techniques (XRF and XRD), but their deportment to some extent characterized by automated mineralogy, and their spatial distribution assessed at high-resolution through microscale LA-ICP-MS analysis. Element mapping revealed that metal(loid)s were generally enriched at grain rims, reflective of peripheral sequestration through surface adsorption. An exception was V, which was uniformly distributed in the studied secondary Fe-oxides, suggestive of isomorphic substitution during co-precipitation. Factor analysis revealed distinct groups of elements co-associated within each secondary mineral (e.g., divalent transition metal cations versus oxyanionic metalloids), likely caused by similar primary mineral sourcing or a comparable sequestration mechanism. Our results demonstrate the importance of secondary mineral precipitates for scavenging (trace) elements and the insights that can be gained from complementary mineralogical and element analyses for the interpretation of trace element dynamics in waste rock.

Revisiting the Discrepancy between Experimental and Theoretical Predictions of the Adiabaticity of Ti+ + CH3OH.

We revisit the naked transition metal cation (Ti+) and methanol reaction and go beyond the standard Landau-Zener (LZ) picture when modeling the intersystem crossing (ISC) rate between the lowest doublet and quartet states. We use both (i) unconstrained Born-Oppenheimer molecular dynamics (BOMD) calculations with an approximate two-state method to estimate population transfer between spin diabats and (ii) constrained dynamics to explore energetically accessible portions of the NDOF - 1 crossing seam, where NDOF is the total number of internal degrees of freedom. Whereas previous LZ calculations (that necessarily relied on the Condon approximation to be valid) fell short and predicted much slower crossing probabilities than shown in experiment, we show that ISC can occur rapidly because the spin-orbit coupling (SOC) between the doublet and quartet surfaces can vary by 2 orders of magnitude (depending on where in the seam the crossing occurs during dynamics) and the crossing region is revisited multiple times during a dynamics run of a few hundred femtoseconds. We further isolate the two important nuclear coordinates that tune the SOC and modulate the transition, highlighting exactly how and why organometallic ISC can occur rapidly for small systems with floppy internal nuclear vibrational modes.

In Situ Quantification of Transition Metal Cation Leaching from a Pt-Alloy Cathode Catalyst in a PEM Fuel Cell

The application of Pt-alloy cathode catalysts for proton exchange membrane fuel cells (PEMFCs) is hampered by the leaching of the alloyed transition metal into the ionomer phase of the membrane electrode assembly (MEA). To date, accelerated stress tests used to assess the degradation of Pt-alloy catalysts lack non-destructive, facile diagnostics to quantify transition metal leaching in an operating PEMFC. Here, we present a method based on electrochemical impedance spectroscopy that exploits the high sensitivity of the high frequency resistance (HFR) at low relative humidity (RH) to quantify transition metal cation contamination. A series of model MEAs with varying fractions of protons intentionally exchanged by Co2+ cations was fabricated. Based on these, we identified the HFR at 30% RH and zero current (i.e., at a homogenous Co2+ distribution in the ionomer phase) as a robust measure of Co2+ contamination. A calibration curve that correlates the HFR at 30% RH to the fraction of protons displaced by Co2+ in the model MEAs could be established, which then allowed quantitative tracking of the Co leached from a PtxCo/C cathode catalyst, both at the beginning-of-test (∼6%) and after 100,000 voltage cycles (∼30%). Capabilities and limitations of this method for Pt-alloy catalyst testing are discussed.

Elevating photovoltaic efficiency: Cobalt doping approaches with sustainable transition metal cations in lead halide perovskite solar cells

Elevating photovoltaic efficiency: Cobalt doping approaches with sustainable transition metal cations in lead halide perovskite solar cells

Efficient and stable to coking catalysts of ethanol steam reforming comprised of Ni + Ru loaded on MgAl2O4 + LnFe0.7Ni0.3O3 (Ln = La, Pr) nanocomposites prepared via cost-effective procedure with Pluronic P123 copolymer

Abstract Mesoporous MgAl2O4 + LnFe0.7Ni0.3O3 (Ln = La, Pr) nanocomposites were prepared by a cost-effective one-pot procedure with the Pluronic P123 copolymer and Ni + Ru active components were supported on them by wet impregnation. The real structure of samples was studied by X-ray diffraction and transmission electron microscopy with energy-dispersive X-ray spectroscopy, surface properties were determined by Fourier transform infrared spectroscopy of adsorbed CO, reactivity was evaluated by temperature programmed reduction by H2, and catalytic activity was tested in ethanol steam reforming (ESR). Disordering of the real structure of nanocomposite supports due to incorporation of transition metal cations into MgAl2O4 results in the development of a metal–support interface and domination of single surface metal centers. This provides a high catalytic activity in the ESR reaction in the intermediate temperature range ∼550°C, close to that of the best known catalysts, and stability to coking. A higher activity for the Pr-containing catalyst is provided by the high reactivity of surface oxygen species bound with Pr cations.

Strong Magneto-Chiroptical Effects through Introducing Chiral Transition-Metal Complex Cations to Lead Halide.

The interplay between chirality with magnetism can break both the space and time inversion symmetry and have wide applications in information storage, photodetectors, multiferroics and spintronics. Herein, we report the chiral transition-metal complex cation-based lead halide, R-CDPB and S-CDPB. In contrast with the traditional chiral metal halides with organic cations, a novel strategy for chirality transfer from the transition-metal complex cation to the lead halide framework is developed. The chiral complex cations directly participate the band structure and introduce the d-d transitions and tunable magneto-chiroptical effects in both the ultraviolet and full visible range into R-CDPB and S-CDPB. Most importantly, the coupling between magnetic moment of the complex cation and chiroptical properties is confirmed by the magneto-chiral dichroism. For the band-edge transition, the unprecedented modulation of +514 % for S-CDPB and -474 % for R-CDPB was achieved at -1.3 Tesla. Our findings demonstrate a novel strategy to combine chirality with magnetic moment, and provide a versatile material platform towards magneto-chiroptical and chiro-spintronic applications.

CoFe Oxides-Coated 2D Black Phosphorus for Flame-Retardant Nanocoatings with Tunable Mechanical Strength and Efficient Elimination of Toxic Gases.

2D black phosphorus (BP) degrades irreversibly into phosphate compounds under ambient conditions, which limits its application in a variety of fields. In this study, by coating amorphous ferric-cobalt oxides (CoFeO)on BP nanosheets, a multifunctional CoFeO@2D BP is successfully developed that effectively inhibited combustion and catalyzed CO oxidation to eliminate toxic gases. Strong affinity between transition-metal cations and BP allowed the uniform growth of amorphous ferric‒cobalt oxides on the BP surface, which effectively prevented the spontaneous degradation of 2D BP. By combining CoFeO@2D BP with gelatin and kosmotropic salts, the as-obtained nanocoatings are used for surface treatment of flammable polyurethane foam (PU). Kosmotropic ions induced strong hydrophobic interactions and bundling within the gelatin chains which significantly enhanced the mechanical performance of the PU. BP accelerates the carbonization of gelatin to inhibit the combustion of PU, and CoFe oxides, which act as true active centers to accelerate the oxidation of CO, effectively inhibiting the production of harmful gas. The release rate of CO decreases by 73% and the limiting oxygen index (LOI) increases from 17% to ≈32% during PU combustion. The developed novel 2D material opens the way for multifunctional coatings with integrated durability, flame retardancy, and high smoke suppression efficiency.

(Invited) Structure and Reactivity of Cobalt-Containing Nanocomposites for Electrocatalytic Oxygen Evolution in Acid Medium

Electrochemical water splitting involves two heterogeneous multi-step half-reactions, which are referred to as the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER). It should be noted that the OER under acidic conditions has the following two advantages compared to the process in neutral and alkaline solutions. The kinetics of the OER in acidic media could be much faster due to the higher proton transfer rate between anode and cathode. The proton exchange membrane (PEM) is an acidic solid polymer electrolyte membrane characterized by good proton conductivity, excellent electrochemical durability, and high mechanical strength.Polynuclear iron(III) hexacyanoferrate(II), or Prussian blue, and its analogues possess unique open framework structures with the general chemical formula of AxMy[Fe-(CN)6]z·nH2O (where A = alkali metal cation, M = transition metal cation). The possibility to accommodate different transition metal cations within the coordination framework renders them with appealing electrochemical, ion-exchange, sensing, or photomagnetic properties, which have been the subject of intense research for decades. Despite their rigid structure, these mixed-metal coordination polymers are usually nonstoichiometric. This chemical variety makes them a versatile type of molecule-based materials. Prussian Blue type cobalt hexacyanoferrates seem to activate water molecules during photoelectrochemical oxidations. Here we report the water oxidation catalytic activity found in Prussian blue-type cobalt hexacyanoferrate modified electrodes both under conventional electrochermcal and photoelectrochemical conditions (using WO3 n-type semiconductor) in acid medium. It is noteworthy that some of the hybrid cobalt-ruthenium hexacyanoferrate compositions showed remarkable catalytic ability towards the oxygen evolution reaction in acid electrolyte. The enhanced catalytic activities of the as-synthesized electrodes should also be attributed to such features as high population of hydroxyl groups and high Broensted acidity (due to presence of Ru or W oxo sites) and related fast electron transfers coupled to unimpeded proton displacements. The possibility of metal-metal interactions between nanosized metals (Co and Ru or Co and W) cannot be excludedFor comparison, we also show that mixed-metal oxide nanocomposites combining the favorable catalytic properties of Co3O4 and CeO2, nanocomposites (with different phase distribution and Co3O4 loading) can modify the cobalt-oxo-species and enhance its intrinsic oxygen evolution reaction activity. Synthesis of Co3O4-modified CeO2, which was addressed through three different sol-gel based routes, each with 10.4 wt% Co3O4 loading, yielded three different nanocomposite morphologies: CeO2-supported Co3O4 layers, intermixed oxides, and homogeneously dispersed Co. It is reasonable to expect that the local bonding environment of Co3O4 can be modified after the introduction of nanocrystalline CeO2, which allows the CoIII species to be easily oxidized into catalytically active CoIV species, thus avoiding the undesirable surface reconstruction process. Here, ceria is likely to regulate the surface oxygen concentration, due to its oxygen buffer (storage/release) capacity, associated with the fast CeIV/CeIII redox transition. Our research addresses the problems related to the fabrication of efficient earth-abundant catalysts for oxygen evolution reaction, and it provides strategies for designing more active and stable catalytic systems.

Investigating the Effect of Copper Ion Impurities on the Structural Evolution and Electrocatalytic Activity of Catalysts for HER

Seawater splitting has garnered significant research interest recently as a method to produce green hydrogen at reduced costs1. One major limitation is the effect of naturally present impurities on all parts of an electrolyzer2– from the membranes to the catalysts. Transition metal cations with reduction potentials in the range of the hydrogen evolution reaction (HER) could interfere with the electrocatalytic reaction. Although this effect has been investigated in the context of underpotential deposition3, there is limited understanding of how competitive redox processes involving impurity ions can influence HER on electrocatalytic electrodes. Here, we study the effect of copper as an impurity ion in the presence of electrocatalytic (platinum) and non-electrocatalytic (gold) electrodes to understand the dynamic competition between electrodeposition and HER. We find that copper deposition competes with HER on both platinum and gold surfaces, leading to a significant decrease in current density with increasing copper concentration. The electrode surfaces are characterized with scanning electron microscopy and atomic force microscopy to understand morphological evolution of the growing copper metal, and X-ray photoelectron spectroscopy is used to quantify the nanoscale thickness of deposited copper. At relatively high dissolved Cu2+ concentrations, copper metal is observed to electrodeposit uniformly on gold electrodes, whereas platinum features nonuniform copper growth with pits and channels likely due to the influence of hydrogen evolution. In situ atomic force microscopy is also used to gain insight into the nucleation and growth behavior of copper on these different electrodes. Together, the results clearly show that redox competition of copper impurities can occur even at the relatively low concentrations of Cu2+ found in seawater (~1 μM)4, highlighting the importance of studying the effect of different impurities on the catalyst under consideration. This work is intended to eventually lead to robust electrolyzers that can be used to efficiently produce hydrogen under impure input stream conditions, potentially lowering costs.REFERENCES: Tong, W., et al., Nature Energy (2020) 5 (5), 367Becker, H., et al., Sustainable Energy & Fuels (2023) 7 (7), 1565Cadle, S. H., and Bruckenstein, S., Analytical Chemistry (1971) 43 (13), 1858Leal, P. P., et al., Sci Rep (2018) 8 (1), 14763

(Invited) Mn-Based High-Energy Cathodes for Lithium-Ion Batteries

Cation-disordered Li-excess rocksalts (DRX) are promising cathode materials for next-generation lithium-ion batteries utilizing sustainable resources. [1] These compounds are typically Co- and Ni-free, and they offer exceptionally large charge storage capacities by activating the redox reactions of both cationic transition-metals and anionic oxygen in the lattice. [2-4] Recent studies showed that significant performance improvement can be achieved by introducing fluorine substitution. Fluorine was found to improve local structural stability and chemomechanical properties, as well as reduce oxygen gas release, impedance rise and capacity fade. [5-7] However, achieving adequate cycle life remains a formidable barrier in DRX cathodes.In this presentation, we will show our recent effort in developing stable DRX oxyfluorides based on Mn-redox chemistry. Mn-rich DRX oxyfluoride cathodes consisting of earth-abundant elements will be presented. These materials experience unique local structural transformations upon charge and discharge, leading to dynamic evolution in voltage profile and significant capacity rise during early cycles. Stable cycling can be achieved after the initial activation process. Our understanding in how chemistry can impact local and long-range structures and their evolution during electrochemical cycling will also be presented, as well as the perspectives on future directions in cathode development. References Chen D.; Ahn J.; Chen, G. An Overview of Cation-Disordered Lithium-Excess Rocksalt Cathodes. ACS Energy Lett. 2021, 6, Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G. Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries. Science 2014, 343, 519.Yabuuchi, N.; Takeuchi, M.; Nakayama, M.; Shiiba, H.; Ogawa, M.; Nakayama, K.; Ohta, T.; Endo, D.; Ozaki, T.; Inamasu, T.; Sato, K.; Komaba, S., High-Capacity Electrode Materials for Rechargeable Lithium Batteries: Li3NbO4-based System with Cation-Disordered Rocksalt Structure. Natl. Acad. Sci. 2015, 112, 7650.Chen, D.; Kan, W. H.; Chen, G. Understanding Performance Degradation in Cation-Disordered Rock-Salt Oxide Cathodes. Energy Mater. 2019, 9, 1901255.Lee, J.; Papp, J. K.; Clément, R. J.; Sallis, S.; Kwon, D.-H.; Shi, T.; Yang, W.; McCloskey, B. D.; Ceder, G. Mitigating oxygen loss to improve the cycling performance of high capacity cation-disordered cathode materials. Commun. 2017, 8, 981.Lun, Z.; Ouyang, B.; Kitchaev, D. A.; Clément, R. J.; Papp, J. K.; Balasubramanian, M.; Tian, Y.; Lei, T.; Shi, T.; McCloskey, B. D.; Lee, J.; Ceder, G. Improved Cycling Performance of Li-Excess Cation-Disordered Cathode Materials upon Fluorine Substitution. Energy Mater. 2018, 9,1802959.Ahn, J.; Chen, D.; Chen, G.. A Fluorination Method for Improving Cation-Disordered Rocksalt Cathode Performance. Energy Mater. 2020, 10, 2001671.

Transition Metal Ions in Solution: Complexed Ion Speciation at the Air-Liquid Interface.

We have studied the interfacial properties of Zn2+ vs selected transition metal cations such as Fe2+, Cu2+, and Cr3+ in a water-ethanol mixture using field-induced droplet ionization. This was in particular inspired by the specific surface activity of Zn2+ that has been observed on several occasions and a desire to clarify the root cause for this behavior. We have found that Zn2+, due to its unique electronic configuration and atomic size, is the only ion of those under study that gives rise to specific speciation at the air-liquid interface with three ethanol molecules attached to the central atom for optimal polarity.

Diffusive ex situ galvanic growth of metal nanocrystals on transmission electron microscopy grid

The uniform modification of commercial and standardized objects features considerable potential, as in the case of dip-pen lithography, biosensor kits, and disposable electrodes. In this study, we devised a dense nanoparticle array formation using the metallic mesh of a transmission electron microscopy (TEM) grid, which was formerly solely employed as a sampling stage. Incompletely oxidized cationic species generated from the galvanic replacement reaction of the metallic support mesh at the opposite side of the carbon layer diffused to the exposed top side and led to ex situ nanostar formation. A uniform nanoparticle array was obtained by immersion in an aqueous solution of replacing metal cations without reducing agents or surface energy-controlling compounds. Moreover, the array density was exclusively regulated by the incubation time. The straightforward nanoparticle array formation was successfully extended to various metal mesh TEM grids and different transition metal cations capable of spontaneous redox reactions. Cu mesh TEM grid and Mo grids could be utilized as sacrificial templates, and the growth of Au, Pd, Pt, Ag, and Rh nanostructures was confirmed.

Computational Exploration of the Impact of Low‐Spin and High‐Spin Ground State on the Chelating Ability of Dimethylglyoxime Ligand on Dihalo Transition Metal: A QTAIM, EDA, and CDA Analysis

ABSTRACTWe have explored the chelation of dimethylglyoxime ligand to divalent (ndx: x = 6, 7, 8) transition metal (TM) cations in two media (gas phase and water) at the B3LYP//LANL2DZ/6–311+G (d,p) and B3LYP/def2‐TZVP level at lower multiplicity and higher multiplicity states. Majority of the 18 optimized halide (chloride and bromide) complexes prefer square planar configuration. The correlations discerned between the experimental structural data and their estimated counterparts demonstrate a good credibility for complexes at lower multiplicity state. The basis set superposition errors (BSSEs) estimated is very small which reflects the fact that the choice of different basis sets (B3LYP//LANL2DZ/6–311+G (d,p)) introduces a slight bias in the calculation of energies. The ADMP (atom‐centered density matrix propagation) simulations in water on chloride complexes indicate the irreversible nature of these M—N dissociation in trajectory simulation process. This fact explains our exclusive focus on the examination of the [glyoxime ligand]…[MX2] interactions. In addition, the solvation of (3d and 4d) transition metal chloride complexes causes a sensitive augmentation of the metal ion affinity (MIA) with an average of 0.29 and 0.24 kcal/mol. In both multiplicity states, the topological parameters have illustrated that the M—N and M—X bonds are typical metal–ligand in both media. The average ΔEorb/ΔEsteric ratio equal to 0.45 and 0.11 in gas phase and water, respectively, reveals the predominance of the contributions from non‐covalent bonding interactions (NCI) compared to those of covalent bonding. But, the maximal value equal to 6.760 is obtained for bromide rhodium complex in water. NBO analysis in both media highlights the fact that a more pronounced ionic character is observed for the majority of the chloride complexes at both spin multiplicity states because of their higher retained charges on the metal atom. For [dimethylglyoxime]…[MX2] interaction (X = Cl and Br), the charge decomposition analysis demonstrates that the lowest value of the d/b ratio is found for the chloride platinum complex at lower multiplicity state in water. This is a proof of its strong relativistic effects.

Transition Metals Coordination by Bis-imidazole-calix[4]arene Ligands with and Without Pyrene Units Grafted at the Large Rim.

Herein, the presented results show that previously studied DNA/RNA-interacting bis-imidazole-calix[4]arene systems can, in aqueous solutions, efficiently bind a series of biorelevant transition metal cations by coordination with the two imidazole arms at the small rim of their macrocyclic basket. The SCXRD and NMR results structurally characterised the complexes formed by referent bis-imidazole-calix[4]arene with Cu2+ and Zn2+. In solid-state (crystal), the bis-anilino derivative/Cu2+ complex, only upon exposure to the air, undergoes intramolecular dehydrogenative coupling of two neighbouring aniline units, yielding an azo bridge at the large rim of the calix[4]arene basket. In the biorelevant aqueous solution, the comparison of fluorometric titrations of referent calix[4]arene, with its analogues having one or two pyrene units grafted at the opposite (large) rim, revealed moderate-to-strong affinity towards transition metal cations, and, more importantly, a strong impact of pyrene on the binding affinity towards some cations. The pyrene arm(s) significantly diminished the affinity of the calix[4]arene-imidazole ligand towards Cu+ and strongly increased the affinity towards divalent Co2+ and Cd2+ cations. Moreover, the fluorometric response of some studied derivatives was strappingly sensitive to cation type. Since the counter-anion plays only a marginal role, such a change in selectivity is attributed to the intramolecular interaction of pyrene(s) with the calix[4]arene-imidazole system, sterically controlling the metal cation binding site.

Toward a Circular Lithium Economy with Electrodialysis: Upcycling Spent Battery Leachates with Selective and Bipolar Ion-Exchange Membranes.

Recycling spent lithium-ion batteries offers a sustainable solution to reduce ecological degradation from mining and mitigate raw material shortages and price volatility. This study investigates using electrodialysis with selective and bipolar ion-exchange membranes to establish a circular economy for lithium-ion batteries. An experimental data set of over 1700 ion concentration measurements across five current densities, two solution compositions, and three pH levels supports the techno-economic analysis. Selective electrodialysis (SED) isolates lithium ions from battery leachates, yielding a 99% Li-pure retentate with 68.8% lithium retention, achieving relative ionic fluxes up to 2.41 for Li+ over transition metal cations and a selectivity of 5.64 over monovalent cations. Bipolar membrane electrodialysis (BMED) converts LiCl into high-purity LiOH and HCl, essential for battery remanufacturing and reducing acid consumption via acid recycling. High current densities reduce ion leakage, achieving lithium leakage as low as 0.03%, though hydronium and hydroxide leakage in BMED remains high at 11-20%. Our analysis projects LiOH production costs between USD 1.1 and 3.6 per kilogram, significantly lower than current prices. Optimal SED and BMED conditions are identified, emphasizing the need to control proton transport in BMED and improve cobalt-lithium separation in SED to enhance cost efficiency.

Internal Electric Fields and Structural Instabilities in Insulating Transition Metal Compounds: Influence on Optical Properties

AbstractThis work reviews new ideas developed in the last two decades which play a key role for understanding the optical properties of insulating materials containing transition metal (TM) cations. Initially, this review deals with compounds involving d4 and d9 ions where the local structure of the involved MX6 complexes (M=dn cation, X=ligand) is never cubic but distorted, a fact widely ascribed to the Jahn‐Teller (JT) effect. Nevertheless, that assumption is often wrong as the JT coupling requires an orbitally degenerate ground state in the initial geometry a condition not fulfilled even if the lattice is tetragonal. For this reason, the equilibrium geometry of d4 and d9 complexes in low symmetry lattices, is influenced by two factors: (i) The effects, usually ignored, of the internal electric field, ER, due to the rest of lattice ions on the active electrons localized in the MX6 unit. (ii) The existence of structural instabilities driven by vibronic interactions that lead to negative force constants. As first examples of these ideas, we show that the equilibrium structure, electronic ground state of KZnF3:Cu2+, K2ZnF4:Cu2+ and K2CuF4 obey to different causes and only in KZnF3:Cu2+ the JT effect takes place. These ideas also explain the local structure and optical properties of CuF2, CrF2 or KAlCuF6 compounds where the JT effect is symmetry forbidden and those of layered copper chloroperovskites where the orthorhombic instability explains the red shift of one d−d transition under pressure. In a second step, this review explores stable systems involving d3, d5 or d9 cations, where the internal electric field, ER, is responsible for some puzzling phenomena. This is the case of ruby and emerald that surprisingly exhibit a different color despite the Cr3+‐O2− distance is the same. A similar situation holds when comparing the normal (KMgF3) and the inverted (LiBaF3) perovskites doped with Mn2+ having the same Mn2+‐F distance but clearly different optical spectra. The role of ER is particularly remarkable looking for the origin of the color in the historical Egyptian Blue pigment based on CaCuSi4O10.

Click Chemistry Derived Hexa-ferrocenylated 1,3,5-Triphenylbenzene for the Detection of Divalent Transition Metal Cations.

The 1,3-dipolar cycloaddition reaction (click chemistry approach) was employed to create a hexa-ferrocenylated 1,3,5-triphenylbenzene derivative. Leveraging the presence of metal-chelating sites associated with 1,2,3-triazole moieties and 1,4-dinitrogen systems (ethylenediamine-like), as well as tridentate chelating sites (1,4,7-trinitrogen, diethylene triamine-like) systems, the application of this molecule as a chemosensor for divalent transition metal cations was investigated. The interactions were probed voltammetrically and spectrofluorimetrically against seven selected cations: iron(II) (Fe2+), cobalt(II) (Co2+), nickel(II) (Ni2+), copper(II) (Cu2+), zinc(II) (Zn2+), cadmium(II) (Cd2+), and manganese(II) (Mn2+). Electrochemical assays revealed good detection properties, with very low limits of detection (LOD), for Co2+, Cu2+, and Cd2+ in aqueous solution (0.03-0.09 μM). Emission spectroscopy experiments demonstrated that the title compound exhibited versatile detection properties in solution, specifically turn-off fluorescence behavior upon the addition of each tested transition metal cation. The systems were characterized by satisfactory Stern-Volmer constant values (105-106 M-1) and low LOD, especially for Zn2+ and Co2+ (at the nanomolar concentration level).

Exploring electrochemical kinetic behaviors and interfacial charge transfer of pure and Ni-doped ZnFe2O4 nanoparticles-based sensing nanoplatform for ultra-sensitive detection of chloramphenicol

ZnFe2O4 (ZFO) nanomaterial was doped with a divalent transition metal cation of Ni2+ (NixZn1-xFe2O4, x=0, 0.2, 0.4, and 0.8) and characterized by various analytical techniques. Powder X-ray diffraction revealed the formation of a single-phase cubic spinel structure, while the stabilization of crystal structure for Ni2+-doped samples was observed. The average crystalline size, d-spacing, and lattice parameters increased with increasing in Ni2+ concentration within NixZn1-xFe2O4, due to differences in the ionic radius, the cation distribution at A-B sites, and the creation of surface oxygen vacancies within ZFO structure. From electrochemical measurements, NixZn1-xFe2O4-based electrodes showed excellent enhancements in charge transfer ability and conductivity with the highest rate constant (0.018 ms−1), the lowest peak-to-peak separation (206 mV), the lowest Rct (118 Ω), and the largest electrochemical active area (0.248 cm2), compared to that of bare SPE. Among them, Ni0.8Zn0.2Fe2O4/SPE provided outstanding electrochemical behaviors and achieved the best sensing performance with the widened concentration linear range from 0.25 to 50 μM and a rather low detection limit of 0.2 μM for chloramphenicol detection. The most important reason for this positive advance comes from the unique synergistic effects of Ni doping into the ZFO host structure. The excellent enhancements in adsorption capacity (Г) (1.4 times higher), number of oxygen vacancies, charge transfer rate constant (approximately 1.15 times higher), and catalytic rate constant (30 times greater) were recorded at Ni-doped ZFO-based electrodes, compared to pure ZFO-based electrode. Furthermore, the detailed hypotheses and possible mechanisms explaining these impressive enhancements were explored. Our work provides insight into the correlation between the Ni-doping and electrochemical characteristics, which has implications for tailoring the electrochemical performance of spinel ferrites across diverse applications and the design of novel spinel ferrite nanomaterials.

Revealing the degradation pathways of layered Li-rich oxide cathodes.

Layered lithium-rich transition metal oxides are promising cathode candidates for high-energy-density lithium batteries due to the redox contributions from transition metal cations and oxygen anions. However, their practical application is hindered by gradual capacity fading and voltage decay. Although oxygen loss and phase transformation are recognized as primary factors, the structural deterioration, chemical rearrangement, kinetic and thermodynamic effects remain unclear. Here we integrate analysis of morphological, structural and oxidation state evolution from individual atoms to secondary particles. By performing nanoscale to microscale characterizations, distinct structural change pathways associated with intraparticle heterogeneous reactions are identified. The high level of oxygen defects formed throughout the particle by slow electrochemical activation triggers progressive phase transformation and the formation of nanovoids. Ultrafast lithium (de)intercalation leads to oxygen-distortion-dominated lattice displacement, transition metal ion dissolution and lithium site variation. These inhomogeneous and irreversible structural changes are responsible for the low initial Coulombic efficiency, and ongoing particle cracking and expansion in the subsequent cycles.