Ca10(CrVO4)6(CrVIO4), a disordered mixed‐valence chromium compound exhibiting inversion twinning

The structure analysis of so‐called 9CaO·4CrO₃·Cr₂O₃ proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr^VIO₄ tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr^VO₄ tetrahedra are in general positions and are ordered. The charge is balanced by Ca²⁺ cations, one of which is located on a threefold axis. The Ca²⁺ ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO–CrO_x system and its formation reduces the refractoriness of calcium‐rich compositions in an oxidizing atmosphere.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. II. Fixation of the Cu/Cr system on wood

The kinetic behavior of a Cr⁶⁺/Cu²⁺ system in its reaction with wood has been elucidated. The same reactions with carbohydrates and lignin of wood for Cr^VI alone are still present. Approximately 20% of the copper is fixed to the guaiacyl units of lignin in the form of copper chromate; the remaining copper forms complexes with cellulose and mainly with the guaiacyl units of lignin. The Cr^VI/lignin guaiacyl unit complexes already found for the treatment of wood with CrO₃ are still the main Cr^VI complexes formed. Copper chromate is present as [Cu(H₂O)₂(guaiacol)]CrO₄(guaiacol) complexes, and CuCrO₄ bridges between different guaiacyl units of the lignin network appear to be likely. Substantial interference of the Cu²⁺ ions in the second-zone reactions of Cr^VI with wood occurs. The amount of Cr^VI and Cr^III fixed onto lignin and cellulose compares well with experimental values. Rate constants, energies of activation, and Arrhenius equations of the various reactions have been obtained. The amount of Cu²⁺ interference in the reaction has also been calculated.     

Synthèse et propriétés magnétiques de solutions solides entre l'oxyde de chrome CrO2 et l'oxyhydroxyde de cobalt CoOOH

Solid solutions of general formula (1 ? x)CrO₂, xCoOOH have been prepared by hydrothermal synthesis under very high pressure conditions (80 kb). Cr⁶⁺O₃ and CoCr⁶⁺O₄ were used as starting materials. Homogeneous ferromagnetic phases were obtained when 0 ? x ? 0.5. X ray powder patterns clearly demonstrate the isotype with InOOH, an orthorhombic distorted rutile type structure. The results of the magnetic measures performed on samples with different compositions indicate that part of the Cr⁴⁺ cation have been reduced to Cr³⁺ and that the general formula of the solid solutions should be written Cr⁴⁺_1?(x+y)Cr³⁺_yCo²⁺_xO₂H_x+y.     

Spectroscopic and structural studies of chromate ions in zinc complexes with 2,2′-bipyridine. Analysis of the lowest triplet states in the CrO42− entity

Two novel complexes of Zn(II) chromate with 2,2′-bipyridine have been synthesised: [Zn(bpy)₃]CrO₄·7.5H₂O (1) and catena-(μ-CrO₄-O,O′)[Zn(bpy)(H₂O)₂]·2H₂O (2). Complex 1 has been characterised by a structural method. The [Zn(bpy)₃]CrO₄·7.5H₂O crystals have a monoclinic symmetry with space group C2/c and eight chemical units. The chromate ion is not coordinated to the zinc(II) ion. The O(3) and O(4) atoms of CrO₄²⁻ and O(8) of the water molecule statistically occupy their position with k=0.5, which means that the chromate ions execute reorientational motion between two equilibrium arrangements with equal probability. 4 K electronic spectra (1) revealed the vibrational fine structure in ν₃(F₂)=820 cm⁻¹ for the spin-forbidden ¹A₁→³T₁ transition. The pure electronic 0–0 transition in ¹A₁→¹T₁ was found at 20 270 cm⁻¹. In complex 2 the broad low intensity band at ca. 16 800 cm⁻¹ has been assigned to a forbidden ZnOCr transition in the bridge.     

Effect of Ammonium Ions on the Electroreduction of Anions at a Mercury Electrode

The effect of ammonium ions on the electroreduction of peroxodisulfate (S₂O^2- ₈) and perbromate (BrO^- ₄) anions is found to be commensurate with that of potassium ions. Ammonium ions accelerate the reduction of iodate (IO^- ₃), bromate (BrO^- ₃), and chromate (CrO^2- ₄) anions in nonbuffered solutions and does not, in buffered ones. In the former case, the effect is connected with the pH change in the near-electrode layer during the reaction and with the participation of ammonium ions in hydrolytic equilibriums, rather than with a simultaneous transfer of ammonium ions and electrons in an elementary act. The conclusions in the literature in favor of a simultaneous transfer of the proton and electron in the proton-consuming reactions of reduction of anions is shown to be ambiguous.     

CrOx/SiO2催化剂上丙烷在CO2气氛中脱氢反应的研究

采用XRD、UV-vis DRS、ESR和微分吸附量热等技术,考察了铬担载量分别为2.5、5和10wt%的CrO_x/SiO₂催化剂的结构、表面性质和氧化还原性能。结果表明,催化剂表面上存在多种Cr的氧化态和聚集形式。随着Cr担载量从2.5wt%到10wt%的逐渐增大,催化剂表面占主导地位的Cr物种由CrO₃单体转为多聚CrO₃和Cr₂O₃晶相。在CO₂气氛中催化剂对丙烷转化率和丙烯选择性的大小顺序为2.5wt%CrO_x/SiO₂>5wt%CrO_x/SiO₂>10wt%CrO_x/SiO₂,反应过程中的原位ESR和UV-visDRS测定结果表明,催化剂表面的反应活性中心为Cr⁵⁺,Cr⁵⁺可由催化剂预处理过程中Cr³⁺的氧化及丙烷反应过程中CrO₃单体的还原产生,在反应中CO₂可使Cr³⁺重新氧化为Cr⁵⁺.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

Schwingungsspektren und Kraftkonstanten von Chrom(VI)-Sauerstoff-Verbindungen

The infrared and RAMAN spectra of Cr₃O, CrO₄O and CrO₃ have been measured and assigned. The force constant of a modified valence force field have been calculated and compared with the force constants of CrO, Cr₂O, CrO₃F^?, CrO₃Cl^?, CrO₂F₂ and CrO₂Cl₂. The valence force constant f_CrO depends almost linearly on the distance d(CrO). The bending frequency δ(CrOCr) has to be reassigned. The unit cell parameters of α- and β-Cs₂Cr₃O₁₀ and Cs₂Cr₄O₁₃ have been determined by means of Guinier X-ray powder techniques.     

Bidentate reagents form cyclic organic-Cr(VI) molecules for aiding in the removal of Cr(VI) from water: density functional theory and experimental results

Hexavalent chromium, Cr(VI), in the form of chromate (CrO₄ ^2?) or dichromate (Cr₂O₇ ^2?) is a well-described carcinogen found in the drinking water in many parts of the country at levels deemed unsafe by the U.S. Environmental Protection Agency and the World Health Organization. We report on the ability of bidentate organic molecules containing diols or diamines to capture chromate ions from aqueous sources by forming cyclic organic-Cr(VI) carbonates or ureas. After their formation, the cyclic organic-Cr(VI) molecules are readily absorbed onto granulated activated charcoal to facilitate Cr(VI) removal. Using density functional theory, E ₀ values for the reactions of diols and diamines with chromate were calculated and correlated with the experimental findings of Cr(VI) removal.     

The influence of addition of silver and copper on the reducibility of CrAl<Subscript>3</Subscript>O<Subscript>6</Subscript> system

The influence of addition of silver and copper on the reduction behavior of binary oxide CrAl₃O₆ was investigated. The formation of copper chromite CuCr₂O₄ and silver chromate Ag₂CrO₄ during calcinations process was observed. The intermediate phase CrO was detected when copper and silver-copper systems were reduced at temperatures above 500°C. This intermediate is formed from Cr₂O₃, which is yielded by the initial reduction of Cr₂O₄²⁻ species.     

Light induced magnetic properties of spiropyrane tris(oxalato)chromate (III) single crystals

The effect of UV light on Weiss temperature and ESR spectra in 1-isopropyl-3, 3, 5′, 6′-tetramethylspiro[indolin-2,2′-[2H]pyrano[3,2-b]pyridinium] tris(oxalato)chromate (III) (Sp₃Cr(C₂O₄)₃) has been found. Additional line has been observed in the ESR spectra of irradiated samples in “strong” magnetic fields of ~15 kOe. The analysis of angular dependences of the ESR spectra allowed a contribution of Cr³⁺ ions to magnetic properties of Sp₃Cr(C₂O₄)₃ to be determined. The zero-field splitting parameters D=0.619 cm⁻¹, E=0.024 cm⁻¹ were derived from the experimental data. The parameters were typical for Cr³⁺ in the chromium oxalate. Weiss temperature changed sign from 25 to −25 K under UV irradiation. The value of Weiss temperature and its changing cannot be explained by exchange interaction, dipole-dipole interaction or the effect of crystal field. The existence of Weiss temperature is explained by the changes in amount and spin of paramagnetic particles. The change is due to thermoactivated redistribution of electrons between chromium ions and spiropyrane molecules. Light-induced transfer of electrons is also explaining the change in sign of Weiss temperature under UV irradiation.     

The mercury chromates Hg6Cr2O9 and Hg6Cr2O10—Preparation and crystal structures, and thermal behaviour of Hg6Cr2O9

The basic mercury(I) chromate(VI), Hg₆Cr₂O₉ (=2Hg₂CrO₄·Hg₂O), has been obtained under hydrothermal conditions (200 °C, 5 days) in the form of orange needles as a by-product from reacting elemental mercury and K₂Cr₂O₇. Hydrothermal treatment of microcrystalline Hg₆Cr₂O₉ in demineralised water at 200 °C for 3 days led to crystal growth of red crystals of the basic mercury(I, II) chromate(VI), Hg₆Cr₂O₁₀ (=2Hg₂CrO₄·2HgO). The crystal structures were solved and refined from single crystal X-ray data sets. Hg₆Cr₂O₉: space group P2₁2₁2₁, Z=4, a=7.3573(12), b=8.0336(13), , 3492 structure factors, 109 parameters, R[F²>2σ(F²)]=0.0371, wR(F² all)=0.0517; Hg₆Cr₂O₁₀: space group Pca2₁, Z=4, a=11.4745(15), b=9.4359(12), , 3249 structure factors, 114 parameters, R[F²>2σ(F²)]=0.0398, wR(F² all)=0.0625. Both crystal structures are made up of an intricate mercury-oxygen network, subdivided into single building blocks [O-Hg-Hg-O] for the mercurous compound, and [O-Hg-Hg-O] and [O-Hg-O] for the mixed-valent compound. Hg₆Cr₂O₉ contains three different Hg₂²⁺ dumbbells, whereas Hg₆Cr₂O₁₀ contains two different Hg₂²⁺ dumbbells and two Hg²⁺ cations. The Hg^I-Hg^I distances are characteristic and range between 2.5031(15) and 2.5286(9) Å. All Hg₂²⁺ groups exhibit an unsymmetrical oxygen environment. The oxygen coordination of the Hg²⁺ cations is nearly linear with two tightly bonded O atoms at distances around 2.07 Å. For both structures, the chromate(VI) anions reside in the vacancies of the Hg-O network and deviate only slightly from the ideal tetrahedral geometry with average Cr-O distances of ca. 1.66 Å. Upon heating at temperatures above 385 °C, Hg₆Cr₂O₉ decomposes in a four-step mechanism with Cr₂O₃ as the end-product at temperatures above 620 °C.     

Formation of Cr(VI) compounds during the thermal decomposition of amorphous chromium hydroxide

Chromium hydroxide (CH) was prepared by the reduction of purified sodium chromate using starch. CH was then used to prepare chromium oxide (Cr₂O₃). Results of thermogravimetric, X-ray diffraction, Fourier transform infrared spectroscopy, and chemical analyses suggested that a small amount of the initial Cr(VI) content speeds up the oxidation and reduction reactions, thereby promoting the purity and crystallization of Cr₂O₃. Cr(VI) in CH induced the evolution of CH and the formation of Cr(VI) containing compounds including CrO₃, NaCr(CrO₄)₂, Cr₃O₈, and Cr₅O₁₂ at low sintering temperature. Furthermore, homogeneous Cr₂O₃ nanoparticles with 99 % purity and particle size of 50 nm were obtained.     

The electrical conductivity of chromium trioxide and its suboxides

The electrical conductivity of CrO₃ and seven samples between CrO₃ and Cr₂O₃ is measured between room temperature and the temperature of preparation of the corresponding sample using an AC circuit. CrO₃ is found to be a semiconductor having a resistance of the order of 10⁶ Ω when completely dried by heating to 100°C under a vacuum of 10^?3mm Hg. Cr₂O₃ gives a resistance of the order of l0⁵–l0³ Ω in the temperature range 25–370°C. The samples in between possess values ranging between 10⁵ and 10⁴ Ω. The activation energies are interpreted as being due to the extrinsic range.     

Hydrothermal synthesis and structure of K6[(UO2)4(CrO4)7]·6H2O: A layered uranyl chromate with a new uranyl sheet topology

The layered uranyl chromate, K₆[(UO₂)₄(CrO₄)₇]·6H₂O (1), has been synthesized by reacting UO₃ with K₂Cr₂O₇ under mild hydrothermal conditions. The structure of 1 is formed from UO₂²⁺ cations that are bound by chromate anions to yield a pentagonal bipyramidal geometry around the uranium centers. These polyhedra are bridged by chromate anions to yield one-dimensional _∞¹[UO₂(CrO₄)₂]²⁻ chains. The chains are similar to the _∞¹[UO₂(CrO₄)₂(H₂O)]²⁻ chains found in the previously reported one-dimensional uranyl chromate, K₂[UO₂(CrO₄)₂(H₂O)]·3H₂O, a phase that forms concomitantly with 1. These chains are condensed with the loss of the coordinated water molecules into two-dimensional _∞²[(UO₂)₄(CrO₄)₇]⁶⁻ layers with additional one-dimensional _∞¹[(UO₂)₂(CrO₄)₃]²⁻ chains. These anionic layers form a new type of anionic sheet topology and are charge balanced by both intra- and interlayer K⁺ cations. Crystallographic data (193 K): 1, orthorhombic, space group P2₁2₁2, a=10.9583(5) Å, b=22.582(1) Å, c=7.9552(4) Å, Z=2, MoKα, λ=0.71073, R(F)=1.77% for 268 parameters with 4657 reflections with I>2σ(I).     

Preparation and crystal structure of a new bismuth chromate: Bi8(CrO4)O11

Single crystals of a new bismuth chromate, Bi₈(CrO₄)O₁₁, were prepared by hydrothermal reaction of NaBiO₃·nH₂O in K₂CrO₄ solution. The bismuth chromate crystallizes in the monoclinic space group P2₁/m with a=9.657(3), b=11.934(3), c=13.868(2)Å and β=104.14(1)°, Z=4 and the final R factors are R=0.038 and R_w=0.041 for 3541 unique reflections. The crystal structure has a slab built up by (CrO₄)²⁻ tetrahedra and distorted bismuth polyhedra which are five-fold pyramids, six-fold trigonal prisms and octahedra. The distance of lone pair from nucleus for bismuth atoms ranges from 0.29 to 1.12 Å, depending on the coordination environment. Bi₈(CrO₄)O₁₁ decomposes to Bi₁₄CrO₂₄ and a small amount of an unknown phase at 796 °C.     

Comparative study on energetic distortions of SO42− ions matrix-isolated in compounds with kröhnkite-type chains,K2Me(CrO4)2·2H2O and Na2Me(SeO4)2·2H2O (Me = Mg,Co, Ni,Zn, Cd)

Infrared spectra of compounds with kröhnkite-type infinite octahedral–tetrahedral chains, K₂Me(CrO₄)₂·2H₂O and Na₂Me(SeO₄)₂·2H₂O (Me = Mg, Co, Ni, Zn, Cd), as well as infrared spectra of the title double salts containing matrix-isolated SO₄^2? guest ions are presented and discussed in the regions of the X–O stretching modes.The SO₄^2? guest ions matrix-isolated in selenate and chromate matrices exhibit four infrared bands corresponding to the four site-group components of the stretching modes in good agreement with the low site symmetry of the host ions (C₁ site symmetry). The values of Δν₃ (site-group splitting) and Δν_max (the difference between the highest and the lowest wavenumbered components of the stretching modes) are used as an adequate measure for the extent of energetic distortion of the matrix-isolated SO₄^2? guest ions.The influence of different crystal-chemical parameters (Me²⁺–OXO₃ bond strengths, sizes of the Me²⁺ and Me⁺ ions, electronic configurations of the Me²⁺ ions, hydrogen bond strengths, and unit-cell volumes of the host compounds) on the extent of energetic distortion of both the host SeO₄^2? and CrO₄^2? ions, and the SO₄^2? guest ions is analyzed. Correlations between the values of Δν₃ and Δν_max of the guest ions and both the degree of covalency of the respective Me²⁺–OXO₃ bonds and the electronic configurations of the Me²⁺ ions have been found and will be discussed. For example, the energetic distortion of SO₄^2? ions included in the chromate lattices decreases in the order Zn > Cd > Mg as a result of the decreasing covalency of the respective Me²⁺–O bonds in the same order (Δν₃ have values of 73, 58 and 36 cm^?1, respectively). Furthermore, the values of Δν₃ and Δν_max are larger when the metal ions have CFSE ≠ 0 (crystal field stabilization energy, Co²⁺, Ni²⁺). These cations are more resistant to angular deformations of the MeO₆ octahedra (i.e. changes in the O–Me–O bond angles), thus facilitating the extent of distortion of the matrix-isolated SO₄^2? ions as compared to those having CFSE = 0 (Mg²⁺, Zn²⁺ and Cd²⁺). For example, Δν₃ and Δν_max of SO₄^2? ions matrix-isolated in K₂Zn(CrO₄)₂·2H₂O have values of 73 and 163 cm^?1, and 116 and 207 cm^?1 in Na₂Zn(SeO₄)₂·2H₂O, whereas in the respective nickel lattices Δν₃ and Δν_max have values of 88 and 173 cm^?1 (K₂Ni(CrO₄)₂·2H₂O) and 127 and 212 cm^?1 (Na₂Ni(SeO₄)₂·2H₂O).The SO₄^2? guest ions included in selenate matrices, Na₂Me(SeO₄)₂·2H₂O, are remarkably much distorted than in chromate ones, K₂Me(CrO₄)₂·2H₂O, as deduced from the values of Δν₃ and Δν_max owing to a stronger static field caused by the smaller Na⁺ ions as compared to that caused by the larger K⁺ ions. The smaller unit-cell volumes of the selenate host compounds, i.e. the higher repulsion potential at the lattice sites at which the guest ions are situated additionally favor the extent of energetic distortion of the sulfate guest ions in the selenate matrices.     

Enthalpy of solvation, ΔHsolv0(CrO42−) (g), of gaseous chromate ion as estimated from lattice energy calculations

The enthalpy of solvation of the gaseous chromate ion, ΔH_solv⁰(CrO₄^2?) (g) is estimated on the basis of recent lattice energy studies made in this laboratory, and a charge distribution assigned to the ion. On the basis of the assigned charge of ?0.57 proton units to the oxygen atoms of the CrO₄^2? unit, the total lattice potential energies are found to be: U_pot(Na₂CrO₄) = 1836 kJ mol^?1; U_pot(K₂CrO₄) = 1717 kJ mol^?1; U_pot(Rb₂CrO₄) = 1645 kJ mol^?1 and U_pot(Cs₂CrO₄) = 1598 kJ mol^?1. The corresponding value for ΔH_solv⁰(CrO₄^2?) (g) = ?1077 kJ mol^?1.     

Formation of Cr(II) Species in the H2/CrO3 System. Parameter control

The thermal behaviour of CrO₃ on heating up to 600°C in dynamic atmospheres of air, N₂ and H₂ was examined by thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy and diffuse reflectance spectroscopy (DRS). The results revealed three major thermal events, depending to different extents on the surrounding atmosphere: (i) melting of CrO₃ near 215°C (independent of the atmosphere), (ii) decomposition into Cr₂(CrO₄)₃ at 340–360°C (insignificantly dependent), and (iii) decomposition of the chromate into Cr₂O₃ at 415–490°C (significantly dependent). The decomposition CrO₃ → Cr₂(CrO₄)₃ is largely thermal and involves exothermic deoxygenation and polymerization reactions, whereas the decomposition Cr₂(CrO₄)₃ → Cr₂O₃ involves endothermic reductive deoxygenation reactions in air (or N₂) which are greatly accelerated and rendered exothermic in the presence of H₂. TG measurements as a function of heating rate (2–50°C min⁻¹) demonstrated the acceleratory role of H₂, which extended to the formation of Cr(II) species. This could sustain a mechanism whereby H₂ molecules are considered to chemisorb dissociatively, and then spillover to induce the reduction. DTA measurements as a function of the heating rate (2–50°C min⁻¹) helped in the derivation of non-isothermal kinetic parameters strongly supportive of the mechanism envisaged. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dipotassium tetra­chromate(VI), K2Cr4O13

The structure of dipotassium tetra­chromium(VI) trideca­oxide, K₂Cr₄O₁₃, has been determined from single‐crystal X‐ray data collected at 173 (2) K on a racemically twinned crystal with monoclinic Pc space‐group symmetry. The structure is composed of discrete [Cr₄O₁₃]²⁻ zigzag chains held together by the charge‐balancing potassium ions. The conformations adopted by the tetra­chromate anion in alkali metal salts and Cr₈O₂₁ are different and can be divided into three categories.