Octahedral Species Research Articles

Co(II) Substitution Enhances the Esterase Activity of a de Novo Designed Zn(II) Carbonic Anhydrase.

Carbonic Anhydrases (CAs) have been a target for de novo protein designers due to the simplicity of the active site and rapid rate of the reaction. The first reported mimic contained a Zn(II) bound to three histidine imidazole nitrogens and an exogenous water molecule, hence closely mimicking the native enzymes' first coordination sphere. Co(II) has served as an alternative metal to interrogate CAs due to its d7 electronic configuration for more detailed solution characterization. We present here the Co(II) substituted [Co(II)(H2O/OH-)]N(TRIL2WL23H)3 n+ that behaves similarly to native Co(II) substituted human-CAs. Like the Zn(II) analogue, the cobalt-derivative at slightly basic pH is incapable of hydrolyzing p-nitrophenylacetate (pNPA); however, as the pH is increased a significant activity develops, which at pH values above 10 eventually yields a catalytic efficiency that exceeds that of the [Zn(II)(OH-)]N(TRIL2WL23H)3 + peptide complex. X-ray absorption analysis is consistent with an octahedral species at pH 7.5 that converts to a 5-coordinate species by pH 11. UV-vis spectroscopy can monitor this transition, giving a pKa for the conversion of 10.3. We assign this conversion to the formation of a 5-coordinate Co(II)(Nimid)3(OH)(H2O) species. The pH dependent kinetic analysis indicates the maximal rate (kcat), and thus the catalytic efficiency (kcat/Km), follow the same pH profile as the spectroscopic conversion to the pentacoordinate species. This correlation suggests that the chemically irreversible ester hydrolysis corresponds to the rate determining process.

Transition metal complexes of the (2,2,2-trifluoroethyl)phosphinate NOTA analogue as potential contrast agents for 19F magnetic resonance imaging.

A new hexadentate 1,4,7-triazacyclononane-based ligand bearing three coordinating methylene-(2,2,2-trifluoroethyl)phosphinate pendant arms was synthesized and its coordination behaviour towards selected divalent (Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) and trivalent (Cr3+, Fe3+, Co3+) transition metal ions was studied. The ligand forms stable complexes with late divalent transition metal ions (from Co2+ to Zn2+) and the complexes of these metal ions are formed above pH ∼3. A number of complexes with divalent metal ions were structurally characterized by means of single-crystal X-ray diffraction. The complex of the larger Mn2+ ion adopts a twisted trigonally antiprismatic geometry with a larger coordination cavity and smaller torsion of the pendant arms, whereas the smaller ions Ni2+, Cu2+ and Zn2+ form octahedral species with a smaller cavity and larger pendant arm torsion. In the case of the Co2+ complexes, both coordination arrangements were observed. The complexes with paramagnetic metal ions were studied from the point of view of potential utilization in 19F magnetic resonance imaging. A significant shortening of the 19F NMR longitudinal relaxation times was observed: a sub-millisecond range for complexes of Cr3+, Mn2+ and Fe3+ with symmetric electronic states (t2g3 and HS-d5), the millisecond range for the Ni2+ and Cu2+ complexes and tens of milliseconds for the Co2+ complex. Such short relaxation times are consistent with a short distance between the paramagnetic metal ion and the fluorine atoms (∼5.5-6.5 Å). Among the redox-active complexes (Mn3+/Mn2+, Fe3+/Fe2+, Co3+/Co2+, Cu2+/Cu+), the cobalt complexes show sufficient stability and a paramagnetic-diamagnetic changeover with the redox potential lying in a physiologically relevant range. Thus, the Co3+/Co2+ complex pair can be potentially used as a smart redox-responsive contrast agent for 19F MRI.

Synthesis, characterization and sensing mechanism of a novel fluorescence probe for Fe(III) in semi-aqueous solution based on a Schiff base hexadentate receptor

A new acyclic Schiff base chemosensor L was synthesized by the one pot condensation reaction of 2-[3-(formyl phenoxy)2-hydroxypropoxy]benzaldehyde and 2-aminophenol in a 1:2 molar ratio and was characterized by elemental analysis, FTIR, 1H- and 13C NMR, and fluorescence spectroscopies. These studies were complemented with a thorough conformational study at the molecular mechanics and density functional theory (DFT) levels of theory to further elucidate the structure of the compound in solution. The chemosensor L displays high sensitivity and selectivity for Fe3+ in semi-aqueous (H2O-DMF, 1:1) solution, except in the presence of a significant amount of Ni2+, with the presence of Fe3+ being signaled through the total fluorescence quenching of the fluorophore when Fe3+ binds to the recognition unit. The synthesized ligand also shows high selectivity for Fe3+ compared to the metal ions Cu2+, Zn2+, Mg2+, Mn2+, Pb2+, Hg2+, Na+, Ba2+ and Cd2+, and reasonable selectivity in the presence of Ag+, Co2+ and Cr3+. The stoichiometry and structure of the complex formed between Fe3+ and the probe L were determined from a Job’s plot and DFT calculations, respectively. The complex was characterized as a high-spin 1:1 octahedral species, in which the ligand coordinates to the metal through the two ether oxygen atoms, two nitrogen atoms and two terminal hydroxyl groups. Time dependent (TD-DFT) calculations were performed to provide information on the type of mechanism causing the quenching of the fluorescence in the presence of Fe3+.

Local structure and dynamics of ions in LiCl-GdCl3, KCl-GdCl3 and LiCl-GdCl3-Gd2O3 melts: Ab initio molecular dynamics simulations and Raman spectroscopy

Currently, the novel technologies of gadolinium electrochemical production from raw oxide materials using molten oxide-salt media are being developed. To understand the phenomena, data on the structure of gadolinium-containing oxide-salt melts are required. In this study, using ab initio molecular dynamics (AIMD) and in situ Raman spectroscopy, we bring insight on local structure details and ion dynamics in these systems by the examples of LiCl–GdCl3, KCl–GdCl3 chloride melt and LiCl–GdCl3–Gd2O3 oxide-chloride melt with a Gd2O3 concentration up to 2 mol %. The Raman spectroscopy reveals the octahedral species [GdCl6]. According to the AIMD, the Gd-based groupings are joined by various numbers of shared chlorine anions. Gadolinium oxide in the chloride melt dissociates to form [Gd2OLi] groups, which are incorporated into the melt structure. The calculated self-diffusion coefficients of ions and the lifetimes of ion pairs showed that the local structure of the potassium containing melt is more stable as compared to that of the lithium containing melt, which is closely concurring with the divergence in conductivities of these melts. LiGdO2 and GdOCl phases were found in the solidified oxide-chloride melt LiCl–GdCl3–Gd2O3. It has been suggested that the complex oxide LiGdO2 is formed by the reaction Gd2O3 + LiCl → GdOCl + LiGdO2. Based on the obtained data, it was concluded that lithium-containing chloride melt should be considered not only as a solvent for gadolinium oxide, but also as an active reaction medium. Here, lithium is involved in ongoing chemical reactions and participate in reaction product.

Exceptional catalytic activity of oxygen evolution reaction via two-dimensional graphene multilayer confined metal-organic frameworks

Oxygen evolution reaction (OER) plays a key role in many renewable energy technologies such as water splitting and metal-air batteries. Metal-organic frameworks (MOFs) are appealing to design efficient OER electrocatalysts, however, their intrinsic poor conductivity strongly hinders the activity. Here, we show a strategy to boost the OER activity of poor-conductive MOFs by confining them between graphene multilayers. The resultant NiFe-MOF//G gives a record-low overpotential of 106 mV to reach 10 mA cm−2 and retains the activity over 150 h, which is in significant contrast to 399 mV of the pristine NiFe-MOF. We use X-ray absorption spectroscopy (XAS) and computations to demonstrate that the nanoconfinement from graphene multilayers not only forms highly reactive NiO6-FeO5 distorted octahedral species in MOF structure but also lowers limiting potential for water oxidation reaction. We also demonstrate that the strategy is applicable to other MOFs of different structures to largely enhance their electrocatalytic activities.

Synthesis, characterisation and coordination properties of 5-methyl-3-formylpyrazole 4 N-benzylthiosemicarbazone (HMPzNB) Cobalt(III), nickel(II) and copper(II) complexes with a NNS donor system

The title ligand, 5-methyl-3-formylpyrazole 4 N-benzylthiosemicarbazone (abbreviated as HMPzNB) has been synthesised and physicochemically characterised by elemental analysis, IR, PMR and mass spectrum. The ligational behaviour of the pyrazole-derived thiosemicarbazone, HMPzNB, has been examined by isolation and characterisation of its complexes with Co III , Ni II and Cu II with different counterions. Elemental analyses, magnetic and spectral features indicate [Co(MPzNB) 2 ]X (X = Cl - , Br - , NO - 3 , ClO - 4 or BF - 4 ) as the diamagnetic octahedral species with the ligand in the deprotonated thiol form, while [Ni(HMPzNB) 2 ]X 2 (X = Cl - , Br-, NO - 3 or ClO - 4 ) is proposed as octahedral complex with the ligand in the neutral thione form. However, in the neutral condition (pH ∼6) nickel(II) salt affords octahedral bis-nickel(II) complex, Ni(MPzNB) 2 , while copper(II) salt gives [Cu(MPzNB)]X (X = Cl - , Br - , NO - 3 or ClO - 4 ), with the deprotonated ligand. 5-Methyl-3-formylpyrazole 4N-benzylthiosemicarbazone possesses in its structure, the tertiary nitrogen atom ( 2 N) of the pyrazole ring, the azomethine nitrogen and the sulphur atom as potential binding sites; thus the title ligand is expected to behave as a potential NNS tridentate during complex formation with a metal ion.

A New Member of the Growing Family of Interconvertible {RuNO}6,7,8 Species. Redox and Acid‐Base Characterization of [Ru((CH2py)2Me[9]aneN3)(NO)]n+

AbstractThe preparation and characterization of a new ruthenium nitrosyl based on the pentadentate ligand (CH2py)2Me[9]aneN3 (1‐methyl‐4,7‐bis(pyridin‐2‐ylmethyl)‐1,4,7‐triazacyclononane) is reported. The octahedral species contains a {RuNO}6 fragment and can be isolated in the solid state as a PF6− salt. In acetonitrile, this platform allows exploring the reduction processes in 1‐electron steps from {RuNO}6 to {RuNO}8, with associated E° values of 0.421 V and −0.628 V (vs. Ag/AgCl, 3 M NaCl), respectively. The {RuNO}7 species is paramagnetic, its EPR spectrum in vitrified acetonitrile at 90 K is consistent with an S= center with g=(2.0095, 1.9992, 1.8785) coupled to a 14N nucleus, with A=(6.9, 30.24, 1.85)×10−4 cm−1. In acidified aqueous solution, the first reduction process at 0.101 V leads to the formation of {RuNO}7, as in aprotic medium (acetonitrile). However, the incorporation of a second electron is coupled to a protonation process of the nitroxyl group, generating a coordinated azanone (HNO) compound with pKa(HNO)=11.0. Spectroscopic information obtained via spectroelectrochemistry and electronic structure calculations assist in the rationalization of results. Overall, this new compound enhances the library of nitrosylated Ru species with combined redox, acid‐base, and spectroscopic characterization in water and confirms experimental correlations found in related species.

Acetonitrile Combustion over Copper-Based Nanocatalysts: A Structure-Performance Relationship Study

In this paper, the relationship between activity and structure of Cu2+ in different chemical environments of Cu-BETA, La2CuO4, and CuO nanocatalysts was systematically investigated for acetonitrile combustion. The study revealed that exchanged and octahedral species of Cu2+ coexist in Cu-BETA, while octahedral species are dominant in CuO and La2CuO4. All nanocatalysts achieved high conversion rates of acetonitrile, which rapidly increased with temperature. CuO and La2CuO4 led to the formation of undesired products such as N2O and NO. On the other hand, Cu-BETA showed high acetonitrile conversion along with a high N2 yield. The excellent performance of Cu-BETA can be attributed to the easy reducibility of the highly dispersed Cu-species and the small crystallite size. Cu-BETA also exhibited exceptional stability. Therefore, the high conversion rate and the high N2 yield make Cu-BETA a promising catalyst for acetonitrile combustion.

Intrinsic reactivity and dissolution characteristics of tetracalcium aluminoferrite

Tetracalcium aluminoferrite is one of the main phases of Portland cement, the dissolution characteristics of which however have not been fully revealed. This work adopted first-principles calculations and Mössbauer spectroscopy as well as X-ray diffraction analysis to clarify the intrinsic reactivity of different species of tetracalcium aluminoferrite crystals. Ca ions are the most reactive species of tetracalcium aluminoferrite and Al ions are slightly more reactive than Fe ions. Moreover, the tetrahedral Al and Fe ions are more reactive than the octahedral Al and Fe ions. These indicate dissolution characteristics of tetracalcium aluminoferrite crystals, namely, Ca ions dissolve first and the tetrahedral species dissolve faster than the octahedral species. After the initial dissolution, the dissolved ions nucleate and grow on the defective surface of C4AF and the C4AF residues are fully hydroxylated. The further hydration thereafter becomes the complex dissolution with the nucleation and growth of hydration products.

Highly Absorbing Lead-Free Semiconductor Cu2AgBiI6 for Photovoltaic Applications from the Quaternary CuI-AgI-BiI3 Phase Space.

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu2AgBiI6: a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 105 cm–1 near the absorption onset, several times that of CH3NH3PbI3. Solution-processed Cu2AgBiI6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm2 V–1 s–1), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI3. The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs2AgBiI6 thin films have not been synthesized to date, Cu2AgBiI6 is a valuable example of a stable Ag+/Bi3+ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

Probing Ni2+ and Co2+ speciation in carboxylic acid based deep eutectic solvents using UV/Vis and FT-IR spectroscopy

In this work, the coordination behaviour of Ni2+ and Co2+ in choline chloride–carboxylic acid (malonic, succinic, glutaric, glycolic and lactic acid) deep eutectic solvents (DES's) using UV/Vis and FT-IR spectroscopy is reported. In all systems Co2+ was observed to form the [CoCl4]2−. This species was also observed for solutions of CoCl2 and CoO in a choline chloride–lactic acid deep eutectic solvent. Ni2+ showed more complex chemistry, with speciation being dictated by whether the carboxylic acid was mono- or di-acidic. The [NiCl4]2− species was dominant in DES's containing dicarboxylic acids, but in DES containing monocarboxylic acids, an octahedral species was present alongside the tetrachloronickelate ion. FT-IR confirms that the acid is involved in coordinating to the metal centre. This is the first time this behaviour has been reported in the literature, and sheds light on the relative strength of the interactions between metals and solvent molecules in DES. The research presented here, in relation to previously reported works, highlights the importance of process conditions on metal speciation in DES. Of particular importance is the presence of water, as this can effect Co2+ and Ni2+ speciation even when the same DES is used. Our data show no difference in Co2+ speciation when using either chloride or oxide as the metal ion source.

Ab initio molecular dynamics simulations and Raman spectra of the YbCl3 - KCl and Yb2O3 - YbCl3 - KCl ionic melts

A direct reduction of solid ytterbium oxides into the metal in molten salts is a promising environmental friendly process. To understand the phenomena, which occur during the electrochemical ytterbium production, data on the structure of ytterbium-containing oxide-salt melts are required. In this study, using ab initio molecular dynamics and in situ Raman spectroscopy, we bring insight on the local structure of these systems by the examples of YbCl3–KCl chloride melt and Yb2O3–YbCl3–KCl oxide-chloride melt with a Yb2O3 concentration up to 2 mol%. We show that there are octahedral species [YbCl6], which, with an increase in YbCl3 concentration, can combine into a weakly bonded network through the bridging chlorine anions. Ytterbium oxide in the chloride melt dissociates to form [OYb3] ionic groups, which are incorporated into the melt structure.

Synthesis and Crystal Chemistry of Octahedral Rhodium(III) Chloroamines

Rhodium(III) octahedral complexes with amine and chloride ligands are the most common starting compounds for preparing catalytically active rhodium(I) and rhodium(III) species. Despite intensive study during the last 100 years, synthesis and crystal structures of rhodium(III) complexes were described only briefly. Some [RhClx(NH3)6-x] compounds are still unknown. In this study, available information about synthetic protocols and the crystal structures of possible [RhClx(NH3)6−x] octahedral species are summarized and critically analyzed. Unknown crystal structures of (NH4)2[Rh(NH3)Cl5], trans–[Rh(NH3)4Cl2]Cl⋅H2O, and cis–[Rh(NH3)4Cl2]Cl are reported based on high quality single crystal X-ray diffraction data. The crystal structure of [Rh(NH3)5Cl]Cl2 was redetermined. All available crystal structures with octahedral complexes [RhClx(NH3)6-x] were analyzed in terms of their packings and pseudo-translational sublattices. Pseudo-translation lattices suggest face-centered cubic and hexagonal closed-packed sub-cells, where Rh atoms occupy nearly ideal lattices.

Optimization of Co-precipitation Condition for Preparing Molybdenum-Based Sulfur-Resistant Methanation Catalysts

In this study, the effects of ZrO2 carrier precursors, MoO3 loading, and washing treatment on the catalytic performance of MoO3/ZrO2 toward sulfur-resistant methanation were investigated. All the catalysts were prepared by co-precipitation method and further characterized by N2 adsorption–desorption, H2-temperature-programmed reduction, X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The prepared MoO3/ZrO2 catalysts were tested in a continuous-flow pressurized fixed bed reactor for CO methanation. The results revealed that the carrier precursors, MoO3 loading, and washing treatment affected not only the crystalline phase of Mo species but also the grain size of ZrO2 carrier and consequently influenced the MoO3/ZrO2 activity toward sulfur-resistant methanation. The 25 wt% MoO3/ZrO2 catalyst prepared using Zr(NO3)4·5H2O as the precursor and treated by water washing displayed the best activity for sulfur-resistant methanation due to its greater number of octahedral Mo species and smaller ZrO2 grain size.

Bis phenylene flattened 13-membered tetraamide macrocyclic ligand (TAML) for square planar cobalt(III)

The preparation, characterization, and evaluation of a cobalt(III) complex with 13-membered tetraamide macrocyclic ligand (TAML) is described. This is a square-planar (X-ray) S = 1 paramagnetic (1H NMR) compound, which becomes an S = 0 diamagnetic octahedral species in excess d5-pyridine. Its one-electron oxidation at an electrode is fully reversible with the lowest E½ value (0.66 V vs SCE) among all investigated CoIII TAML complexes. The oxidation results in a neutral blue species which is consistent with a CoIII/radical-cation ligand. The ease of oxidation is likely due to the two benzene rings incorporated in the ligand structure (whereas there is just one in many other CoIII TAMLs). The oxidized neutral species are unexpectedly EPR silent, presumably due to the π-stacking aggregation. However, they display eight-line hyperfine patterns in the presence of excess of 4-tert-butylpyridine or 4-tert-butyl isonitrile. The EPR spectra are more consistent with the CoIII/radical-cation ligand formulation rather than with a CoIV complex. Attempts to synthesize a similar vanadium complex under the same conditions as for cobalt using [VVO(OCHMe2)3] were not successful. TAML-free decavanadate was isolated instead.

Deep oxidative desulfurization with simultaneous oxidative denitrogenation of diesel fuel and straight run gas oil

High levels of sulfur organic compounds (SOCs) and nitrogen organic compounds (NOCs), which are contaminants present in diesel fuel and straight run gas oil (SRGO), were removed using tungsten/zirconia catalysts and hydrogen peroxide by oxidative desulfurization and oxidative denitrogenation reactions. Two tungsten sources were used: tungstic acid and ammonium metatungstate. The catalysts obtained by anionic exchange at low pH with tungstic acid showed remarkable homogeneity and high W dispersion attributed to tetrahedral W species and full dibenzothiophene (DBT) oxidation was achieved in 5 min of reaction. On the other hand, the catalysts obtained by impregnation with ammonium metatungstate developed mainly octahedral species with less DBT oxidation. It was also found that tetrahedral species generate more acidity than octahedral species. A density functional theory computational study confirmed the observed DBT oxidation trend shown by tetrahedral and octahedral W species on the peroxide-activated catalysts. Diesel fuel and SRGO were desulfurized and denitrogenated to a high extent: 97% in diesel fuel and 70% in SRGO for sulfur compounds and 96% in diesel fuel and 89% in SRGO for nitrogen compounds, whereas C1–C3 dibenzothiophenes, 4-methyl dibenzothiophene, and 4,6-dimethyl dibenzothiophene were fully removed. The elimination of SOCs and NOCs present in fuels can be effectively carried out in a single process to obtain clean fuels.

β-Diketonate Titanium Compounds Exhibiting High In Vitro Activity and Specific DNA Base Binding

Herein, we report 30 new β-diketonate titanium compounds of the type [Ti(O,O)2X2], whereby O,O=asymmetric or symmetric β-diketonate ligand and X=Cl, Br, OEt or OiPr. Thirteen new crystal structures are discussed and show that these octahedral species all adopt cis geometries in the solid state. These compounds have been tested for their cytotoxicity using SRB and MTT assays, showing several of the compounds are as potent as cisplatin against a range of tumor cell lines. Results also show the [Ti(O,O)2Br2] complexes are more potent than [Ti(O,O)2Cl2], [Ti(O,O)2(OEt)2] and [Ti(O,O)2(OiPr)2]. Using a simple symmetrical heptane-3,5-dione (O,O) ligand bound to titanium, we observed more than a 50-fold increase in potency with the [Ti(O,O)2Br2] (28) when compared to [Ti(O,O)2Cl2] (27). One of the more potent compounds (6) has been added to three different sixmers of DNA, in order to analyse the potential DNA binding of the compound. NMR studies have been carried out on the compounds, in order to understand the structural properties and the species formed in solution during the in vitro cell assays.

Fluoride Addition Effects on Voltammograms and UV-Vis Spectra of Neodymium Cation in Molten Chlorides

Neodymium is the one of the rare earth fission products and was used as the simulant of some trans uranium elements due to its similarity of electrochemical behavior. Although molten salt should be relatively constructed by a simple structural model due to the predominant ionic species in liquid phase, electrochemical behavior has not been well explained microscopically. For a recent few years, the electrochemical behavior of rare earths including neodymium in the molten chlorides with small amount of fluorides has been focused, and structural elucidation by extended absorption fine structure and UV-vis spectroscopy of neodymium in molten salts has been performed. In this study, fluoride concentration dependence on the voltammograms of neodymium in molten LiCl-KCl and spectra of neodymium in molten NaCl-2CsCl was remeasured and the structural variation is discussed by some parameters derived from the Judd-Ofelt analysis of UV-vis spectra. All the electrochemical experiments using molten salts have been performed in the electric furnace which is built inside a glove box filled with an argon atmosphere in high purity. Cyclic voltammetry, and differential pulsed voltammetry have been performed by using the electrodes as follows: working electrode: tungsten, counter electrode: pyrocarbon and reference electrode: silver wire dipped in molten LiCl-KCl eutectic + AgCl (1 mol%) inside the borosilicate tube, respectively. Silica glass tube was used as a crucible. To observe the fluoride addition effect, 0 to 20 times amount of LiF to the concentration of neodymium was added into the molten salt. All measurements have been performed at 773 K. UV-vis spectroscopy of neodymium in molten NaCl-CsCl-NaF at 953 K has been carried out by using the spectrophotometer. A quartz cell with 10 mm of light path was used for the molten salt container. For the calculation of oscillator strength of the hypersensitive transition and derivation of Ω 2,4,6 parameters by the Judd-Ofelt analysis, density was assumed to be the additivity of molar volume of each component, and refractive index was temporary used from the value of KCl due to the non-availability of the data of CsCl. According to the differential pulsed voltammograms of cathodic sweeps which exhibit mainly neodymium reduction peaks, the potential of neodymium reduction to metal was not shifted positively, however, drastic negative shift depending on fluoride addition did not occur as well. More strikingly, with increasing fluoride amount, the potenital gap between Nd3+/Nd2+ and Nd2+/Nd decreased, thus disproportionation reaction may be restricted with increasing concentration of fluoride. The variation of oscillator strength of hypersensitive transition absorption of neodymium at ca. 589 nm depending on fluoride addition has been evaluated. This absorption peak has been considered to the indication of symmetric coordination around neodymium cation, i.e. 6 coordinated octahedral species, and the smaller value relates to perfect symmetry. One striking feature is that with increasing fluoride amount until ca. F/Nd = 2 in molten NaCl-CsCl-NaF, the value of oscillator strength once increases until F/Nd = 2, and decreases rapidly. While in molten LiCl-KCl-LiF, the value of oscillator strength decreases almost linearly. This study is performed under the projects of 26P-1, 25P1-10, and 24P2-3 at Research Reactor Institute, Kyoto University.

Understanding Cation-Disordered Cathode Materials Based on Percolation Theory and Ligand Field Theory

Throughout the history of Li-ion batteries, cathode materials with high energy density have been sought from well-ordered oxide compounds in which lithium and other cations occupy distinct sites within an oxygen FCC framework.[1,2] In contrast, cation-disordered materials have received only a limited attention as cathodes because lithium diffusion tends to be limited by their structure, resulting in poor cycling performance.[3] However, recent studies have shown that cation-disordered materials can in fact deliver higher energy densities than the ordered materials once enough excess lithium (x > 0.1 in Li1+xTM1-xO2, TM: transition metal) is introduced to their structure, opening a new search space for high energy density Li-ion cathodes.[4,5] The understanding of disordered cathodes was first made with percolation theory which links the composition of a compound to the population of Li diffusion channels with low barriers.[4,5] In rocksalt-type oxides, Li diffusion takes place between two octahedral sites through a face-sharing tetrahedral site. Li+ ion in this tetrahedral site in the activate state in diffusion, whose electrostatic energy largely determines the diffusion barrier. Hence, (i) the oxidation states of species in the face-sharing octahedral sites and (ii) the tetrahedron height, along which the activated Li+ ion relaxes away the strong electrostatic repulsion from face-sharing octahedral species, largely determine the activity of a Li diffusion channel. [4] In the disordered rocksalt structure with small tetrahedron heights, it was found that only the diffusion channel through which an activated Li+ ion shares faces with no transition metal ions (0-TM channels) has low Li diffusion barriers. However, for the 0-TM channel to dominate macroscopic Li diffusion, the channel must be percolating in a crystal structure, such that every Li hopping occurs through the channel. Percolation theory predicts that Li excess introduces such 0-TM percolation, and hence Li-excess disordered cathodes should allow for facile Li diffusion.(Fig. 1a)[4] Consistent to the theory, recently developed cation-disordered Li-excess cathodes (e.g. Li1.211Mo0.467Cr0.3O2, Li1.3Mn0.4Nb0.3O2) deliver high capacity and energy density with facile Li diffusion.[4,6] However, percolation theory alone cannot completely guide the design of high capacity disordered cathodes because it does not take redox process into account. While percolation theory predicts higher Li-excess contents should lead to better disordered cathodes, higher Li excess necessarily leads to lower transition metal contents hence their redox capacity.[7] Therefore, unless transition metals can exchange multiple electrons or oxygen redox can reversibly occur, the electron-storage capacity should decrease with Li excess, which is unwanted. In this presentation, we explain how lithium excess can affect both Li diffusion and redox process in disordered cathodes using percolation theory and ligand field theory, respectively.(Fig. 1b, 1c)[4,8] Based on this understanding, we will show that Li diffusion and redox process in the materials are highly correlated hence need to be considered simultaneously. We further demonstrate how such complete understanding can be used to explain the performance of recently developed high capacity disordered cathodes (e.g. Li-Ni-Ti-Mo oxides) and to design improved disordered cathode materials.[7,8] References [1] K. Kang, Y. S. Meng, J. Bréger, C. P. Grey, G. Ceder, Science 311, 977–980 (2006). [2] M. M. Thackeray, P. J. Johnson, L. A. De Picciotto, P. G. Bruce, J. B. Goodenough, Mater. Res. Bull.19, 179–187 (1984). [3] M. N. Obrovac, O. Mao, J. R. Dahn, Solid State Ion.112, 9–19 (1998). [4] J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Science 343, 519–522 (2014). [5] A. Urban, J. Lee, G. Ceder, Adv. Energy Mater.4, 1400478 (2014). [6] N. Yabuuchi et al., PNAS 112, 7650–7655 (2015). [7] J. Lee, D.-H. Seo, M. Balasubramania, N. Twu, X. Li, G. Ceder, Energy Environ. Sci.8, 3255 (2015).[8] D.-H. Seo‡, J. Lee‡, A. Urban, R. Malik, SY. Kang, G. Ceder, Nature Chem., in press (2016) (‡ equal contribution) Figure 1

Monolayer Tantalum Oxide on Mesoporous Silica Substrate

Monolayer Ta2O5 was fabricated on mesoporous silica (SBA-15) substrate by the reaction of Ta(OEt)5 with surface hydroxyl groups on silica. The monolayer was defined at the saturation amount of Ta(OEt)5 anchored on SBA-15 surface in a series of increment of the feed amount the precursor, which was confirmed by elemental (inductively coupled plasma-atomic emission, ICP-AE) analysis of the product (Ta2O5/SBA-15). Ta/Si ratio of Ta2O5/SBA-15 with saturated amount of Ta was 0.11–0.12. X-ray photoelectron spectroscopy (XPS) analysis of Ta/Si ratio of a series of samples by using the Ta4 f and Si2p signals met agreement with the gradual increase and the saturation at about 0.11–0.12, as well as the local elemental analysis by energy dispersive X-ray spectrometry (EDS) using transmission electron microscopy (TEM). The absorption edge of UV-vis. diffuse reflectance spectra of a series of samples appeared very sharp, decreasing the excitation energy from 5.1 to 4.7 eV by increasing the Ta/Si ratio from 0.01 to 0.11−0.12. X-ray absorption near edge fine structure (EXAFS) indicated the inhomogeneous (amorphous) feature of Ta2O5 phase on SBA-15 with Ta octahedral species. The unique surface property of monolayer Ta2O5/SBA-15 was characterized by infrared (IR) spectroscopy and catalytic reactions.