Variable Oxidation States Research Articles

Effect of preparation conditions on the ordering of structural elements in vanadium pentoxide xerogels

Vanadium(V) oxide xerogels with an ordered laminar structure are of considerable interest in modern inorganic chemistry [1]. They combine the ease of intercalation of inorganic ions [2] or organic molecules [3] into the interlayer space and the presence of a transition metal ions with variable oxidation states, giving rise to nonlinear electrical and, in some cases, magnetic characteristics. Hybrid inorganic‐organic materials based on xerogels of vanadium(V) oxide and its nanotubular forms [4] attract considerable attention due to their unique physical and unusual physicochemical and rheological properties, which enable their use as cathode materials for lithium batteries [5], chemical sensors [6], electrochromic materials [7], and catalysts [8]. Vanadium(V) oxide xerogels can be prepared by hydrolysis of organic derivatives of vanadic acid, by polycondensation of vanadates in an aqueous solution in an acid medium, and by decomposition of peroxovanadium(V) compounds upon dissolution of the crystalline oxide in hydrogen peroxide [1, 9, 11]. The mechanisms of these processes are essentially different; therefore, it is important to elucidate the structural features of the xerogels as a function of the preparation method. Small-angle X-ray scattering, which is a standard method for measuring the kinetics of gel condensation in solutions [10], can also be used rather efficiently to study the structural features of xerogels. This study reveals the correlation between the chemical prehistory and the structural organization of samples obtained. Complex analysis of small-angle X-ray scattering (SAXS) patterns of xerogel films synthesized by different methods was carried out for the first time and possible models for description of their structure were considered. Colloid solutions of vanadium(V) oxide for the formation of xerogel films were prepared by three procedures (table). According to method A, analytical grade crystalline V 2 O 5 (0.5 g) was dissolved in 15% H 2 O 2 (30 mL) by slow addition of V 2 O 5 powder to the hydrogen peroxide solution [11]. After the oxygen evolution ceased, the solution became yellow. Heating the solution to ~40°C gave a dark red solution, which was kept under ambient conditions for 7 days until a xerogel film formed. According to method B, the gel was obtained by hydrolysis of tert -butyl metavanadate VO(OC(CH 3 ) 3 ) 3 . The ester was synthesized [11] by refluxing a mixture of calcined vanadium(V) oxide with excess tert -butyl alcohol in the presence of a desiccant (silica gel) for 7 h. The resulting mixture was separated by distillation at 81°C . Hydrolysis was carried out by mixing the ester with an equal volume of water (an excess) for the preparation of bulk samples or under ambient conditions. In method C, polycondensation of V took place in an aqueous solution of sodium vanadate on acidification. A colloid solution of V 2 O 5 · n H 2 O was obtained by

Precipitative Growth Templated by a Fluid Jet

Tubular growth by chemical precipitation at the interface between two fluids, a jet and its surroundings, underlies the development of such important structures as chimneys at hydrothermal vents. This growth is associated with strong thermal and/or solute gradients localized at those interfaces, and these gradients, in turn, often produce radial compositional stratification of the resulting tube wall. A fundamental question underlying these processes is how the interplay between diffusion, advection, and precipitation determines the elongation rate of the tubes. Here we report experimental and theoretical results that reveal a regime in which there exists a new scaling law for tube growth. The model system studied consists of a jet of aqueous ammonia injected into a ferrous sulfate solution, precipitating iron hydroxides with varying oxidation states at the jet boundary. Despite the complex chemistry and dynamics underlying the precipitation, the tube growth exhibits a strikingly simple scaling form, with characteristic lengths and times increasing linearly with the mean velocity of the jet. These observations follow from a kinetic model of advection-dominated flows.

Rheological phase synthesis and electrochemical performances of LiVMoO6 as a high-capacity anode material for lithium ion batteries

Research for new anode materials has been stimulated by the development of lithium ion batteries since the introduction of carbon as a negative material in 1990 by Sony Energetics Inc. Some metal oxides and metalbased composite oxides [1, 2], nitrides [3, 4] and intermetallics [5, 6], which capacities are comparable or superior to those of graphitized carbon (theoretical maximum capacity of 372 mA hr g−1 [7, 8]), were considered as possible candidates for next generation of anode materials. Recently, several vanadium composite oxides exhibited their high-capacity and high-rate performance as negative electrode materials due to their open structure and interesting characteristics from a standpoint of the variety of oxidation states [9–12]. LiMoVO6 crystallizes with the ThTi2O6 branneritetype structure [13] and its electrochemical properties as a positive electrode material for lithium secondary batteries have been investigated [14–16]. But the performance of LiMoVO6 as negative electrode material was seldom reported. In this work, brannerite-type LiMoVO6 powder was obtained by rheological phase reaction method [17]. The result of electrochemical tests showed that the obtained LiMoVO6 as a novel anode material was also feasible and demonstrated very high energy density. The precursor was prepared by rheological phase reaction method. Analytical reagent grade chemicals, LiOH·H2O, NH4VO3 and (NH4)6Mo7O24·4H2O, were used as the starting materials and fully mixed by grinding in a 1:1:1 molar ratio. A proper amount of water was added to get a rheological body and the mixture was sealed in a closed container for 8 hr at 80 ◦C. After drying under vacuum at 80 ◦C for several hours, the precursor was pyrolyzed at 550 ◦C for 10 hr in air, and soft yellow powder LiMoVO6 was obtained. Thermogravimetry and differential thermal analysis (TG/DTA) of the precursor were performed by the Netzsch STA 449 thermal analysis system at a heating rate of 10 ◦C/min in air. The X-ray diffraction (XRD) measurements of the sample were carried out by Shimadzu XRD-6000 diffractometer with CuKα1 radiation (λ = 1.54056 A). The particle sizes and morphological features were observed by a scanning electron microscope (Hitachi SEM X-650).

Magnetism and electronic transport in Sr4−xLaxRu2−xMnxO9: Interplay of Mn and Ru redox chemistry

Sr 4 Ru 2 O 9 has a P6¯2C space group and differs from the Ruddelsden–Popper series of the strontium ruthenate family where Ru is distinctly present in Ru+5 state with no Jahn–Teller activity. A significant increase in magnetic moment and coercivity is observed along with lowering of resistivity, especially around a critical concentration of x=0.2 in Sr4−xLaxRu2−xMnxO9. Such an unusual effect is correlated to the mixed magnetic pair effect arising out from the variable oxidation states of Ru as suggested by x-ray photoelectron spectroscopic studies. Our results substantiate the long-range ferromagnetism observed for Ruthenium-substituted three-dimensional manganites.

Sorption of the herbicide dichlobenil and the metabolite 2,6-dichlorobenzamide on soils and aquifer sediments.

The worldwide used herbicide dichlobenil (2,6-dichlorobenzonitrile) has resulted in widespread presence of its metabolite 2,6-dichlorobenzamide (BAM) in pore- and groundwater. To evaluate the transport of these compounds we studied the sorption of dichlobenil and BAM in 22 sediment samples of clayey till, sand, and limestone including sediments exhibiting varying oxidation states. Dichlobenil sorbed to all investigated sediments, with a high sorption in topsoils (Kd = 7.4-17.4 L kg(-1)) and clayey till sediments (Kd = 2.7-126 L kg(-1)). The sorption of the polar metabolite BAM was much lower than the sorption of dichlobenil but followed the same tendency with the highest sorption in the topsoils (Kd = 0.24-0.66 L kg(-1)) and in the clayey till sediments (Kd = 0.10-0.93 L kg(-1)). The sorption of both compounds was significantly higher (2-47 times) in the unoxidized (reduced) clayey till than in the weathered (oxidized) clayey till. Such a difference in sorption capacity could neither be explained by a higher organic carbon content, sorption to clay minerals, differences in clay mineralogy, nor by blocking of reactive surface sites on clay minerals by iron oxides. However, by removing an average of 81% of the organic carbon from the reduced clayey till with H2O2, the sorption decreased on average 50%. Therefore, most of the sorption capacity in the reduced clayey till was related to organic carbon, which indicates that sorption processes are affected by changes in organic compound composition due to weathering.

Near-infrared reflectance spectroscopy of Ca-rich clinopyroxenes and prospects for remote spectral characterization of planetary surfaces

We present results of laboratory near-infrared reflectance studies of a set of calcic pyroxenes with comparable calcium contents (Wo 45–50) but variable iron content and oxidation states. This new dataset complements earlier published data (Cloutis and Gaffey, 1991, J. Geophys. Res. 96, 22809–28826, and references therein). In particular, our new spectra extend the scarce available spectral data on chemically analyzed Fe-rich high-Ca clinopyroxenes. We attempted to interpret the spectral behavior of our samples in terms of chemistry and coordination site occupancies. Tentatively, we conclude that Fe-rich calcic pyroxenes have very low contents of Fe 2+ in the M2 sites and belong to the spectral type A lacking the 2-μm band. This may be due to high Ca and Mn contents in these pyroxenes. Fe-poor high-Ca pyroxenes are more spectrally variable. In general, they tend to belong to the spectral type B with two major bands near 1 and 2 μm, unless the samples have high Fe 3+/Fe 2+ ratios or are rich in Mn and Ca. Some of them (including unusual meteorite Angra dos Reis) are of type B despite very high Ca contents. We applied the Modified Gaussian Model (MGM) to characterize three major Fe 2+ absorption bands in the 1-μm region of the spectra of Ca-rich pyroxenes. Only the band due to Fe 2+ in the M1 coordination site near 1.15 μm may be potentially useful to estimate the Fe content in calcic pyroxenes on remotely-sensed surfaces of Solar System bodies. The spectral variability of basaltic meteorites (angrites) that are rich in calcic pyroxenes is also discussed. The presence of spectral type A calcic pyroxenes in these meteorites complicates unambiguous identification of olivine in asteroid spectra.

The effect of doping LiMn 2O 4 spinel on its use as a cathode in Li-ion batteries: neutron diffraction and electrochemical studies

A series of compounds Li 1+ y Mn 2− x M′ xO 4 ( x≤0.1; y≤0.02), have been synthesised by doping the parent LiMn 2O 4 spinel with various metal ions of variable oxidation state. Powder neutron diffraction data has been collected on these samples alongside a series of electrochemical experiments in order to elucidate the relationship between structure on the performance of these systems as Li batteries. Doping the LiMn 2O 4 spinel with a small amount of metal ions has a remarkable effect on the electrochemical properties. Whereas the capacity of the spinels doped with trivalent ions is much greater, the cycling fading properties are much enhanced with using divalent ions as dopants. The underlying reasons for this are discussed, and it is suggested that the occupancy of the tetrahedral site with divalent ions to form a more compact structure offers an improved structural stability to support greater Li insertion/extraction, but which ultimately prevents the free movement of Li also sited on the tetrahedral site of the lattice.

Involvement of surface cysteines in activity and multimer formation of thimet oligopeptidase.

Thimet oligopeptidase is a metalloenzyme involved in regulating neuropeptide processing. Three cysteine residues (246, 248, 253) are known to be involved in thiol activation of the enzyme. In contrast to the wild-type enzyme, the triple mutant (C246S/C248S/C253S) displays increased activity in the absence of dithiothreitol. Dimers, purportedly formed through cysteines 246, 248 and 253, have been thought to be inactive. However, analysis of the triple mutant by native gel electrophoresis reveals the existence of dimers and multimers, implying that oligomer formation is mediated by other cysteines, probably on the surface, and that some of these forms are enzymatically active. Isolation and characterization of iodoacetate-modified monomers and dimers of the triple mutant revealed that, indeed, certain dimeric forms of the enzyme are still fully active, whereas others show reduced activity. Cysteine residues potentially involved in dimerization were identified by modeling of thimet oliogopeptidase to its homolog, neurolysin. Five mutants were constructed; all contained the triple mutation C246S/C248S/C253S and additional substitutions. Substitutions at C46 or C682 and C687 prevented multimer formation and inhibited dimer formation. The C46S mutant had enzymatic activity comparable to the parent triple mutant, whereas that of C682S/C687S was reduced. Thus, the location of intermolecular disulfide bonds, rather than their existence per se, is relevant to activity. Dimerization close to the N-terminus is detrimental to activity, whereas dimerization near the C-terminus has little effect. Altering disulfide bond formation is a potential regulatory factor in the cell owing to the varying oxidation states in subcellular compartments and the different compartmental locations and functions of the enzyme.

The bioinorganic chemistry of iron in oxygenases and supramolecular assemblies.

The bioinorganic chemistry of iron is central to life processes. Organisms must recruit iron from their environment, control iron storage and trafficking within cells, assemble the complex, iron-containing redox cofactors of metalloproteins, and manage a myriad of biochemical transformations by those enzymes. The coordination chemistry and the variable oxidation states of iron provide the essential mechanistic machinery of this metabolism. Our current understanding of several aspects of the chemistry of iron in biology are discussed with an emphasis on the oxygen activation and transfer reactions mediated by heme and nonheme iron proteins and the interactions of amphiphilic iron siderophores with lipid membranes.

Physical properties of ZnF 2–As 2O 3–TeO 2 glasses doped with Cr 3+ ions

Clear glasses of the system, ZnF 2–As 2O 3–TeO 2, containing different concentrations of Cr 2O 3 (ranging from 0 to 0.6 wt%) were prepared. A number of studies viz., differential thermal analysis, infrared spectra, optical absorption, ESR, magnetic susceptibility, thermoluminescence and dielectric properties (constant ε, loss tan δ , AC conductivity σ AC over a range of frequency and temperature and dielectric breakdown strength), on these glasses were carried out as a function of chromium ion concentration. Results have been analysed in the light of varying oxidation states of chromium ion; the analysis indicates a gradual conversion of chromium ions from Cr 3+ state to CrO 4 2− structural units (that take part in network forming positions) in the glass network, when Cr 2O 3 is present at higher concentrations (>0.3 wt%) in the glass matrix.

Synthesis and crystal structure of novel Ruddlesden–Popper strontium niobium oxynitrides

The family (SrO)(SrNbO 2-xN) n is the first example of Ruddlesden-Popper strontium niobium oxynitrides and provides layered structures to obtain mixed valence niobium compounds with a variety of oxidation states and transport properties. We have prepared members with n=1 (Sr 2NbO 3N) and n=2 (Sr 3Nb 2O 5N 2) by solid state reaction of Nb 2O 5 and SrCO 3 at 900–1050 °C under ammonia flow. These compounds crystallize in the I4/mmm space group and show a range of anionic stoichiometry that together with the N/O ratio determines the formal oxidation state for niobium and therefore the resultant physical properties. Samples of the n=1 member show mixed valence Nb 5+/Nb 4+ and are paramagnetic and very resistive at room temperature. This behavior may be understood by considering electron localization due to N/O disorder.

Black body radiation induced hydrogen formation in hydrated vanadium cations V+(H2O)n

Hydrated vanadium cations V+(H2O)n, n = 5–30, are stored in the collision-free environment of an FT-ICR mass spectrometer and their reactions due to absorption of black body radiation are studied. Besides the loss of water ligands, the clusters show two different intracluster redox reactions, whose branching ratios are strongly size-dependent. Oxidation to the +II state results in V(OH)+(H2O)n ions, and a concurrent release of atomic hydrogen. Alternatively V(OH)2+(H2O)n clusters can form leaving vanadium in the +III state, common in aqueous solutions, and simultaneously molecular H2 evaporates from the cluster. This behavior reflects the properties of transition metals, and the ability of vanadium to form stable compounds in a variety of oxidation states, and differs from the previously studied intracluster reactions involving the hydrated monovalent main group metals Mg+ and Al+. These only react to their preferred oxidation states, MgOH+ and Al(OH)2+, respectively.

Micro X-ray absorption near edge structure at the sulfur and iron K-edges in natural silicate glasses

Both sulfur and iron chemical environments were studied in natural compounds; sulfides, sulfates and silicate basaltic glasses which are supposed to have variable oxidation states, using XANES (X-ray absorption near edge structure) spectroscopy. Although X-ray absorption methods are well suited for such investigations, the size of natural glass inclusions trapped in volcanic minerals, ranging from a few micrometers to several tenths of micrometers, makes micro-spectroscopy necessary. Hence, we present the μ-XANES spectra at the sulfur and iron K-edges of olivine-hosted glass inclusions containing from 1000 to 1650 ppm S, and between 5.5 and 7.8 wt.% Fe. By combining both high energy and spatial resolutions, we demonstrate the ability of the μ-XANES to produce quantitative determination of sulfur and iron valence states. We have identified various species for sulfur, specifically S II−, S VI and possibly S IV, in basaltic glass inclusions hosted in olivine grains. We propose a method to calibrate the proportion of Fe dissolved as Fe 3+ in basaltic glasses for which the Fe 3+/ΣFe ratio varies between 0.05 and 0.48 with a relative precision of less than 10%.

Recent advances in synthesis and applications of transition metal containing mesoporous molecular sieves.

Work in mesoporous silica-based materials began in the early 1990s with work by Mobil. These materials had pore sizes from 20-500 A and surface areas of up to 1500 m(2) g(-1) and were synthesized by a novel liquid crystal templating approach. Researchers subsequently extended this strategy to the synthesis of mesoporous transition metal oxides, a class of materials useful in catalysis, electronic, and magnetic applications because of variable oxidation states, and populated d-bands-features not found in silicates. These materials are already showing promise in electronic and optical applications hinging on the semiconducting properties of transition metal oxides and their potential to act as electron acceptors, an important feature in the design of cathodic materials. This is the first general review of non-silicate mesoporous materials and will focus on recent advances in this area, emphasizing materials possessing unique electronic, magnetic, or optical properties. Also covered are advances in the synthesis and applications of mesostructured sulfides as well as a new class of template-synthesized platinum-based materials that show promise in heterogeneous catalysis.

Synthesis of α, ω-dichloropermethyloligosilanes by reactions of polydimethylsilane with metal chlorides

The reactions of high-molecular-weight polydimethylsilane with metal chlorides in variable oxidation states at high temperature in the absence of a solvent afford mixtures of α,ω-dichloropermethyloligosilanes Cl(Me2Si) m Cl (m= 2—9). The influence of the reaction conditions (temperature, reaction time, and the reagent ratio) on the composition and yields of the reaction products was examined.

Synthesis and investigation of plutonium oxide cluster ions: Pu xO y+ ( x≤18)

The plutonium–oxygen system is fundamentally and technologically important. The primary goal of the present study was to prepare and carry out initial chemical studies on gas-phase plutonium oxide cluster ions, which would contribute to the general understanding of PuO x chemistry. In this effort, several plutonium oxide, oxide-hydroxide and hydroxide cluster ions, Pu x O y (OH) z +, were synthesized by pulsed laser ablation of hydrated plutonium oxalate in vacuum. Cerium was studied for comparison, and a chemically less diversified oxide cluster system was observed. The large variety of Pu x O y (OH) z + compositions found is attributed to the accessibility of multiple oxidation states for Pu, and the results demonstrate the feasibility of preparing clusters with diverse stoichiometries and variable plutonium oxidation states. These clusters may be studied by various gas-phase techniques to illuminate the behavior of plutonium as a function of key variables, such as oxidation state, structure/coordination and bonding. In addition, some gas-phase reaction studies were carried out and at least one significant cluster reaction product with dimethylether was identified. This result demonstrates the desirability of studying plutonium cluster chemistry in ion traps where longer residence times, and more ion–molecule collisions will result in greater reaction efficiencies. The largest primary oxide cluster identified here comprised six Pu atoms, but two distinctive ‘magic number’ clusters were also produced, which incorporated 16 and 18 Pu atoms, respectively. These ‘magic number’ clusters suggest that special stabilities obtain for these species and that it may be possible to prepare plutonium oxide ‘nanocrystals’ in the gas phase for examination by techniques complementary to those applicable to bulk materials. The plutonium cluster compositions and abundances observed in this work are discussed in the context of the oxidation states, structures, and bonding exhibited by plutonium.

A Polymer-Supported Nickel(II) Catalyst for Room Temperature Tamao–Kumada–Corriu Coupling Reactions

A nickel(II) co-ordination complex, covalently immobilised on a Merrifield resin, is an effective recyclable heterogeneous catalyst in a room temperature coupling reaction between a Grignard reagent and an organobromide. The immobilised catalyst does not leach metal into solution. Catalyst stability has been rationalised in terms of the combination of hard and soft donor atoms around the variable oxidation state nickel.

Carrier Mobilities in a Regioregular Poly(3-octylthiophene) Film at Various Oxidation Stages

Abstract Apparent mobilities of positive charge carriers in a regioregular poly(3-octylthiophene) film in varying oxidation states were successfully evaluated by using electrochemical techniques. Electrochemical ClO4− doping caused a decrease of the mobility by about one order of magnitude until the doping level reached about 2%. Further electrochemical doping resulted in the drastic increase of the mobilities up to 0.5 cm2 V−1 s−1 at a maximum doping level of 23%.

The catalytic oxidation of ammonia: influence of water and sulfur on selectivity to nitrogen over promoted copper oxide/alumina catalysts

The CuO/Al 2O 3 system is active for ammonia oxidation to nitrogen and water. The principal by-products are nitrous oxide and nitric oxide. Nitrous oxide levels increase with the addition of various metal oxides to the basic copper oxide/alumina system. Addition of sulfur dioxide to the reaction stream sharply reduces the level of ammonia conversion, but has a beneficial effect on selectivity to nitrogen. Added water vapour has a lesser effect on activity but is equally beneficial in terms of selectivity to nitrogen. The CuO/Al 2O 3 is also active for the selective catalytic reduction of nitric oxide by ammonia, but this reaction is not effected by sulfur dioxide addition. A mechanism for ammonia oxidation to nitrogen is proposed wherein part of the ammonia fed to the catalyst is converted into nitric oxide. A pool of monoatomic surface nitrogen species of varying oxidation states is established. N 2 or N 2O are formed depending upon the average oxidation state of this pool. An abundance of labile lattice oxygen species on the catalyst surface leads to overoxidation and to N 2O formation. On the other hand, reduced lability of surface lattice oxygen species favours a lower average oxidation state for the monoatomic surface nitrogen pool and leads to N 2 formation.

Chemisorption and Reaction of NO and N2O on Oxidized and Reduced Ceria Surfaces Studied by Soft X-Ray Photoemission Spectroscopy and Desorption Spectroscopy

We have examined the chemisorption and reaction of NO and N2O on ceria surfaces by soft X-ray photoelectron spectroscopy (SXPS), X-ray absorption spectroscopy, and isothermal and temperature programmed desorption (TPD) spectroscopy. Samples include highly oriented CeO2(100) and CeO2(111) thin films with surfaces of variable oxidation states. CeO2(111) thin films were grown in situ allowing control over the initial oxidation state of the ceria surface, quantitatively determined by valence band, Ce 4d, and Ce 3d photoemission. Following NO exposure, various N-containing surface species were observed by N 1s photoemission, and the distribution of these species depended upon surface oxidation state, exposure, and adsorption temperature. These species included N2O, nitrite, NO−, and three states believed to be associated with atomic or anionic forms of N. Nitrite and N2O are seen on a fully oxidized surface while NO−, N2O, and dissociation products are observed on a reduced surface. The primary reaction of NO with reduced ceria is reoxidation of the ceria by the NO, both by NO− formation and by NO dissociation, leading to immediate and thermally induced N2 desorption. Adsorption of NO at 150 K is predominantly molecular while exposure to NO at 400 K leads to a thermally activated nitride which desorbs at temperatures above 500 K. Dissociative adsorption of NO leads to the displacement of N3− at high exposures.