Electronic structure and quadrupole interactions in triple molybdates Li<Subscript>2</Subscript>M<Subscript>3</Subscript>Al(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>, M = Cs,Rb

Within the density functional theory the electronic structure of triple molybdates Li₂M₃Al(MoO₄)₄, where M = Cs, Rb, is studied for the first time. It is found that all molybdates studied belong to wide band insulators with a band gap of ~4 eV. Quadrupole frequencies and asymmetry parameters of the electric field gradient near magnetic ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are calculated and experimental NMR spectra are interpreted.     

Isotope exchange of strontium and molybdate ions in strontium polymolybdates

The heterogeneous isotopic exchange reactions in strontium polymolybdates of Sr²⁺ and MoO₄ ^2- ions in the strontium nitrate and sodium molybdate solutions have been studied using ⁹⁰Sr and ⁹⁹Mo as tracers. Electrometric methods have been used to study the compositions of strontium molybdates obtained by adding strontium chloride to a progressively acidified solution of sodium molybdate. It has been found that the exchange fraction increases with increasing chain length of strontium polymolybdate. The exchange equilibrium constant (K _ex) has been calculated between 298 and 348 K as well as DG°, DH° and DS°. The results indicate that Sr²⁺ cations have a much higher affinity for exchangers than MoO₄ ^2- anions. By fitting the data to the Dubinin-Radushkevich (D-R) isotherm it has been shown that the exchange capacity (X _m ) for both ions is affected by the ion adsorption process at low temperatures and by the ion exchange process at high temperatures. At high concentrations, the recrystallization process contributes to on the cation exchange but is ineffective on the anion exchange mechanism.     

Investigation on the thermal decomposition and flame retardancy of wood treated with a series of molybdates by TG–MS

A flame-retardant wood was prepared using a series of insoluble molybdates through the double bath technique. The flame retardancy of the wood samples was studied with the limiting oxygen index (LOI) method. The relationships between the flame-retardant performance and the thermal property of wood were studied by the thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA), scanning electron microscopy (SEM), and the thermogravimetry–mass spectrometry (TG–MS) analysis methods. The results showed that the insoluble molybdates, which were precipitated into the wood by the double bath technique, can obviously improve the flame retardancy of wood. Similarly, the transition metal molybdates showed higher flame-retardant efficiency than the main group metal molybdates do, which probably due to the thermal barrier effect that Fe₂(MoO₄)₃ acts during the combustion of the samples. At the same time, Fe₂(MoO₄)₃ catalyzed the dehydration and carbonization reactions of wood, and caused an increase in the amount of char produced, and an improvement of the stability of the char residue. Moreover, the mass spectrometry results indicated that the excess transition metal ions speed up the deep decomposition of the char residue, and resulting in the smoldering of wood.     

Comparative study of the synthesis of layered transition metal molybdates

Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T-Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT₂(OH)(MoO₄)₂·H₂O, where A=NH₄⁺, Na⁺ or K⁺. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu-Al and Zn-Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products.     

Potassium magnesium and potassium cobalt molybdates: Synthesis and ion conductivity

Potassium magnesium and potassium cobalt molybdates of composition K₂M ₂ ^II (MoO₄)₃ (M = Mg, Co) and the products of their heterovalent doping by scandium(III) and vanadium(V) ions have been studied by impedance spectroscopy, powder X-ray diffraction, and electron microscopy. The compounds have high ion conductivity. Partial heterovalent substitutions of scandium for magnesium or vanadium for molybdenum additionally increases ion conductivity. The high conductivity of potassium magnesium and potassium cobalt molybdates allows us to consider them as promising solid electrolytes with potassium conductivity.     

Electrical Properties of Molybdates in the Systems M2MoO4-AMoO4-Zr(MoO4)2

The temperature dependence of conductivity of molybdates in the systems M₂MoO₄-AMoO₄-Zr(MoO₄)₂ was studied. The starting molybdates and molybdates of 5 : 1 : 3 and 1 : 1 : 1 compositions, formed in the systems, exhibit mixed conduction, turning ionic at high temperatures. The sharp bends in linear portions of the temperature dependence of conductivity coincide with the temperatures of polymorphic transitions in the corresponding molybdates.     

立方Nd2O3上过氧物种光诱导生成的18O同位素示踪考察

采用原位显微Raman光谱和¹⁸O同位素示踪技术,以325 nm激光为激发光源,对立方Nd₂O₃上过氧物种的光诱导生成过程进行了详细表征,进一步证实过氧源于分子氧对晶格氧的氧化反应. 结果还表明,325 nm激光在室温下即可诱导过氧的生成,在实验条件下,生成的过氧物种可与Nd₂O₃的晶格氧发生快速的氧交换反应,位于Nd₂O₃体相的晶格氧也可迁移至样品表层进而参与过氧的生成. 325 nm激光照射有助于促进晶格氧的迁移以及晶格氧与分子氧之间的氧交换反应.     

Silver magnesium molybdate and silver cobalt molybdate: Synthesis and ionic conductivity

Impedance spectroscopy, X-ray powder diffraction, and electron microscopy are used to study silver magnesium and silver cobalt molybdates of composition Ag₂A ₂ ^II (MoO₄)₃ (A = Mg, Co) and the products of their aliovalent doping by scandium(III) and vanadium(V). The double molybdates have high ionic conductivities at temperatures above 600 K. A partial aliovalent substitution of scandium for magnesium or vanadium for molybdenum increases the ionic conductivity of the molybdates below 473 K. The defect mobility and the enthalpy of defect formation in the Ag₂Mg₂(MoO₄)₃ structure are estimated proceeding from the experimental data.     

TG and DTA study of oxygen depletion and reoxidation of bismuth molybdates (2∶1;2∶3)

Oxygen depletion and reoxidation of bismuth molybdates (2∶1; 2∶3) have been studied by means of isothermal thermogravimetry and DTA measurements. From the isothermal curves, Arrhenius energies were obtained between 411 and 683 K. The activation energies for oxygen depletion from Bi₂O₃·MoO₃ were lower than those for Bi₂O₃·3MoO₃. Two kinetically different types of oxygen release were identified for both molybdates. Arrhenius plots were also obtained from reoxidation experiments: Bi₂O₃·MoO₃ was more easily reoxidable than Bi₂O₃·3MoO₃. The substantial closeness of the respective activation energies suggests that depletion and reoxidation follow the same mechanistic steps. Some DTA measurements confirm the existence of at least two types of reoxidation sites for both oxysalts.     

AMoO4 (A = Mg,Ni) molybdates: Phase stabilities,electronic structures and chemical bonding properties from first principles

Stability of different phases of AMoO₄ (A = Mg, Ni) molybdates versus A–O bonding and the corresponding electronic structures are examined from first principles. The energy-volume equations of state for three forms (β, α, ω), characterized by decreasing volumes in the sequence of Mg and Ni molybdates are established. While NiMoO₄ is energy stabilized in the sequence β → α → ω, an opposite behavior is identified for the Mg molybdate. Charge analysis characterizing ionic Mg²⁺ versus covalent Ni^+1.2 behaviors can explain the trend. Electronic band structure also shows large differences: MgMoO₄ is insulating with a ～2 eV band gap while in a magnetic state, NiMoO₄ is a small gap (～0.2 eV) semi-conductor. Chemical bonding properties show weak Mg and strong Ni bonding with oxygen, while identifying the Mo–O interaction as prevailing.     

Oxygen surface exchange kinetics of erbia-stabilized bismuth oxide

The surface oxygen exchange kinetics of bismuth oxide stabilized with 25 mol% erbia (BE25) has been studied in the temperature and pO₂ ranges 773–1,023 K and 0.1–0.95 atm, respectively, using pulse-response ¹⁸O–¹⁶O isotope exchange measurements. The results indicate that BE25 exhibits a comparatively high exchange rate, which is rate determined by the dissociative adsorption of oxygen. Defect chemical considerations and the observed pO₂^1/2 p{{\hbox{O}}_2}^{1/2} dependence of the rate of dissociative oxygen adsorption suggest electron transfer to intermediate superoxide ions as the rate determining step in surface oxygen exchange on BE25.     

Direct hydrothermal synthesis of metal intercalated hexagonal molybdates, M + x Mo6? x /3O18? x (OH)x.yH2O (M = Li, Rb, Cs, NH4)

Here we report direct hydrothermal synthesis of a few hexagonal molybdates with composition, M _x ^/+ Mo _6−x ^{Emphasis>/3/6+} O_18−x (OH) _x .yH₂O (M = Li, Rb, Cs, NH₄). The molybdates crystallize in the space groupP6 ₃ /m with a r∼ 10.5 andc r∼ 3.7 ? . Unlike previous studies, our work suggests that hexagonal molybdates could be stabilized in the presence of monovalent cations with varying ionic size (smaller lithium to larger cesium) under hydrothermal condition. The phases showed exceptional thermal stability till 550°C. Dedicated to Professor C N R Rao on his 70th birthday     

The kinetics of the chlorine isotopic exchange between chloride ion at O,O-diarylphosphorochloridates or O,O-diarylphosphorochloridothionates

The kinetics of the chlorine isotopic exchange reaction between tetraethylammonium chloride-³⁶Cl and O,O-diarylphosphorochloridates (p-RC₆H₄O)₂POCl or O,O-diarylphosphorochloridothionates (p-RC₆H₄O)₂PSCl has been studied in acetonitrile solution. Good Hammett's correlations of the rate constants with Taft's σ⁰ constants were obtained. The values of the reaction constants ρ were found identical for phosphoryl and thiophosphoryl compounds. In comparison with oxygen in the phosphoryl group, the sulfur atom exhibits an electron-donating effect (Δσ⁰ ～ 0.80). No correlation has been found for the enthalpy and entropy of activation. The effect of the substituents aryloxy groups, oxygen, or sulfur atoms in the phosphoryl group on the kinetics of the S_N2-P reaction is discussed. The reactivity of the investigated compounds is determined by the extent of the positive charge localized on the phosphorus atom. The positive charge is formed by the direct interactions of the substituents with the reaction center and the indirect–intramolecular interactions revealed in the structure of the compound.     

C?H Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Metal–superoxo species are ubiquitous in metalloenzymes and bioinorganic chemistry and are known for their high reactivity and their ability to activate inert C H bonds. The comparative oxidative abilities of M–O₂^.− species (M=Cr^III, Mn^III, Fe^III, and Cu^II) towards C H bond activation reaction are presented. These superoxo species generated by oxygen activation are found to be aggressive oxidants compared to their high‐valent metal–oxo counterparts generated by O⋅⋅⋅O bond cleavage. Our calculations illustrate the superior oxidative abilities of Fe^III– and Mn^III–superoxo species compared to the others and suggest that the reactivity may be correlated to the magnetic exchange parameter.     

C?H Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Metal–superoxo species are ubiquitous in metalloenzymes and bioinorganic chemistry and are known for their high reactivity and their ability to activate inert C? H bonds. The comparative oxidative abilities of M–O₂^.? species (M=Cr^III, Mn^III, Fe^III, and Cu^II) towards C? H bond activation reaction are presented. These superoxo species generated by oxygen activation are found to be aggressive oxidants compared to their high‐valent metal–oxo counterparts generated by O???O bond cleavage. Our calculations illustrate the superior oxidative abilities of Fe^III– and Mn^III–superoxo species compared to the others and suggest that the reactivity may be correlated to the magnetic exchange parameter.     

Crystal structures of some crystal hydrates of binary molybdates

The structures of a series of binary molybdates with the lithium cation LiM(MoO₄) · H₂O (M = K⁺, Na⁺, Rb⁺, NH ₄ ⁺ ) are analyzed and compared. Except for LiNa(MoO₄) · 2H₂O, in all other compounds the lithium cations have a tetrahedral coordination formed by the oxygen atoms of the water molecules and molybdate groups. The structure of LiNa(MoO₄) · 2H₂O was found to contain a unique coordination polyhedron of lithium, i.e., a trigonal bipyramid formed by the O atoms of the water molecules and oxo anions.     

1,2‐Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

The reaction thermodynamics of the 1,2‐dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li?O₂ rechargeable batteries, has been studied by first‐principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O₂^? in oxygen‐poor or oxygen‐rich environments is studied by density functional calculations. Solvation effects are considered by employing a self‐consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen‐poor environments and methyl oxalate, methyl formate, 1‐formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen‐rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li?O₂ cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O₂^?/O₂ molecules. These findings pave the way for a rational development of new solvent molecules for Li?O₂ electrolytes.     

Controlled Reactivity Tuning of Metal‐Functionalized Vanadium Oxide Clusters

Controlling the assembly and functionalization of molecular metal oxides [M_xO_y]^n? (M=Mo, W, V) allows the targeted design of functional molecular materials. While general methods exist that enable the predetermined functionalization of tungstates and molybdates, no such routes are available for molecular vanadium oxides. Controlled design of polyoxovanadates, however, would provide highly active materials for energy conversion, (photo‐) catalysis, molecular magnetism, and materials science. To this end, a new approach has been developed that allows the reactivity tuning of vanadium oxide clusters by selective metal functionalization. Organic, hydrogen‐bonding cations, for example, dimethylammonium are used as molecular placeholders to block metal binding sites within vanadate cluster shells. Stepwise replacement of the placeholder cations with reactive metal cations gives mono‐ and difunctionalized clusters. Initial reactivity studies illustrate the tunability of the magnetic, redox, and catalytic activity.     

Detailed Description of Pulse Isotopic Exchange Method for Analyzing Oxygen Surface Exchange Behavior on Oxide Ion Conductors

A novel pulse ¹⁸O-¹⁶O isotopic exchange (PIE) technique for measurement of the rate of oxygen surface exchange of oxide ion conductors was presented. The technique employs a continuous flow packed-bed micro-reactor loaded with the oxide powder. The isothermal response to an ¹⁸O-enriched pulse passing through the reactor, thereby maintaining chemical equilibrium, is measured by on-line mass spectrometry. Evaluation of the apparent exchange rate follows from the uptake of ¹⁸O by the oxide at given reactor residence time and surface area available for exchange. The developed PIE technique is rapid, simple and highly suitable for screening and systematic studies. No rapid heating/quenching steps are required to facilitate ¹⁸O tracer anneal or analysis, as in other commonly used techniques based upon oxygen isotopic exchange. Moreover, the relative distribution of the oxygen isotopologues ¹⁸O₂, ¹⁶O¹⁸O, and ¹⁶O₂ in the effluent pulse provides insight into the mechanism of the oxygen exchange reaction. The PIE technique has been demonstrated by measuring the exchange rate of selected oxides with enhanced oxide ionic conductivity in the range of 350?900 oC. Analysis of the experimental data in terms of a model with two consecutive, lumped steps for the isotopic exchange reaction shows that for mixed conductors Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ(BSCF) and La₂NiO_4+δ the reaction is limited by the apparent rate of dissociative adsorption of O₂ molecules at the oxide surface. For yttria-stabilized zirconia (YSZ), a change-over takes place, from rate-limitations by oxygen incorporation below ∽800 oC to rate-limitations by O₂ dissociative adsorption above this temperature. Good agreement is obtained with exchange rates reported for these materials in literature.     

Oxygen-ionic conductors based on substituted bismuth molybdates with column-type structural fragments

The possibility of synthesizing oxygen-ionic conductors from substituted bismuth molybdates containing [Bi₁₂O₁₄] _n ⁸ⁿ⁺ columns, MoO₄ tetrahedra, and isolated Bi ions in their structure was studied. The specifics of their structure and electric conductivity were investigated. The general formula of the solid solutions can be recorded as Bi₁₃Mo_{5 ? x} Me _x O_{34 ? δ}, where Me is the fouror five-valent d metal (Ti, Zr, V, Nb). The electric conductivity of doped bismuth molybdates considerably increased compared with that of the matrix compound. The electric conductivity reached 5.5 × 10^?3 S cm^?1 at 700°C and 1.8 × 10^?4 S cm^?1 at 350°C for the zirconium-doped compound with x = 0.4. The porosity of the ceramics was less than 5%; the thermal expansion coefficient was of the order of 14 × 10^?6 K^?1. Based on the set of their characteristics, these compounds are recommended as materials for membranes of electrochemical devices.