Preparation of CuCr<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders using two different chromium sources

CuCr₂O₄ spinel powders were synthesized starting from different chromium sources, namely (i) chromium oxide (α-Cr₂O₃) and (ii) ammonium dichromate ((NH₄)₂Cr₂O₇). The copper source was a Cu(II) carboxylate-type complex. The Cu(II) carboxylate complex was obtained by the redox reaction between Cu(NO₃)₂·3H₂O and 1,3-propanediol (1,3PG) at 130 °C. In the first case (i), we have started from a mixture of α-Cr₂O₃, Cu(NO₃)₂·3H₂O and 1,3PG that upon heating formed the copper malonate complex, which decomposed around 220 °C forming an oxide mixture (CuO + α-Cr₂O₃). In the second case (ii), (NH₄)₂Cr₂O₇, Cu(NO₃)₂·3H₂O and 1,3PG were homogenously mixed. Heating this mixture at 130 °C resulted, in situ, in the Cu(II) complex. On controlled temperature increase, the violent decomposition of (NH₄)₂Cr₂O₇ took place at 180 °C along with the decomposition of the Cu(II) complex, leading to an amorphous oxide mixture of Cr₂O_3+x and CuO. By annealing the samples in the temperature range 400–1000 °C, the spinel phase (CuCr₂O₄) was obtained in both cases: (i) at 800 °C and (ii) at 600 °C as a result of the interactions between the precursors used, when the oxide system was amorphous and highly reactive. The presence of CuCr₂O₄ was highlighted by XRD and FTIR analyses.     

The first chromate containing both Cr<Subscript>2</Subscript>O<Subscript>7</Subscript> and Cr<Subscript>3</Subscript>O<Subscript>10</Subscript> groups,bis(1,3-bis(4-piperidinium)propane) dichromate trichromate monohydrate: Synthesis and crystal structure

A new organic-inorganic hybrid material (C₁₃H₂₈N₂)₂[Cr₂O₇][Cr₃O₁₀]·H₂O (1) was synthesized by slow solvent evaporation at room temperature, and its crystal structure was determined by single crystal X-ray diffraction. This compound crystallizes in the monoclinic space group P2₁/c. The asymmetric unit contains two crystallographically independent 1,3-bis(4-piperidinium)propane, (H₂bppp)²⁺ cations (A and B), one trichromate Cr₃O ₁₀ ^2? anion, one dichromate Cr₂O ₇ ^2? anion, and one water molecule. All these entities are interconnected into a complicated two-dimensional hydrogen bonded network via N–H?O and O–H?O hydrogen bonds. Furthermore, this structure is stabilized by a large number of C–H?O interactions, thus establishing a three-dimensional network structure. This compound appears to be the first example of chromates containing both Cr₂O₇ and Cr₃O₁₀ groups.     

Thermal expansion of ternary oxides in the M<Stack><Subscript>2</Subscript><Superscript>I</Superscript></Stack>O-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> system

Compounds formed in the M₂^IO-Cr₂O₃-TiO₂. system were synthesized by solid-state reactions. These compounds crystallize in hollandite- and spinel-type structures. The features of themal decomposition of the compounds with the compositions M₂^ICr₂Ti₆O₁₆(M^I = Na, K, Cs) and LiCrTiO₄ were revealed, and their thermal expansion coefficients were determined with the use of high-temperature X-ray diffraction analysis.     

Nonstoichiometric Zn Ferrite and ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Composite Spheres: Preparation,Magnetic Properties,and Chromium Removal

Monodisperse and porous nonstoichiometric Zn ferrite can be prepared by a solvothermal method. Such non-Zn ferrite was used to be the precursor for synthesis of ZnFe₂O₄/Fe₂O₃ composite via calcination at 600°C for 3 h in air. X-ray powder diffractometer (XRD) and Energy Dispersive Spectrometer (EDS) proved the nonstoichiometry of Zn ferrite synthesized by solvothermal method and the formation of ZnFe₂O₄/Fe₂O₃ composite via calcination. TEM image showed that non-Zn ferrite spheres with wormlike nanopore structure were made of primary nanocrystals. BET surface area of non-Zn ferrite was much higher than that of ZnFe₂O₄/Fe₂O₃ composite. Saturation magnetization of non-Zn ferrites was significantly higher than that of ZnFe₂O₄/Fe₂O₃ composites. Calcination of non-Zn ferrite resulted in the formation of large amount of non-magnetic Fe₂O₃,which caused a low magnetization of composite. Because of higher BET surface area and higher saturation magnetization, non-Zn ferrite presented better Cr⁶⁺ adsorption property than ZnFe₂O₄/Fe₂O₃ composites.     

Catalytic activity and physicochemical properties of Ni-Au/Al<Subscript>3</Subscript>CrO<Subscript>6</Subscript> system for partial oxidation of methane to synthesis gas

This work is focused on the role of gold and Al₃CrO₆ support for physicochemical properties, and catalytic activity of supported nickel catalysts in partial oxidation of methane (POM). Catalysts, containing 5% Ni and 5% Ni-2% Au active phases dispersed on mono- (Al₂O₃, Cr₂O₃) and bi-oxide Al₃CrO₆ support, were investigated by TPR, BET and XRD methods, and the activity tests in POM reaction were carried out. Bimetallic Ni-Au catalysts dispersed on Al₃CrO₆ support remained highly stable and active. The amorphous binary oxide Al₃CrO₆ can stabilize considerable amount of Cr⁴⁺, Cr⁵⁺, and Cr⁶⁺ species in Ni-Au/Al₃CrO₆ catalyst network during its calcination in the air. Nickel supported on binary oxide Ni/Al₃CrO₆ can form Ni(III)CrO₃ bi-oxide phase in reductive conditions. During TPR H2 reduction of Ni-Au/Al₃CrO₆ catalyst chromium(II) oxide Cr(II)O phase is observed. After POM reaction the existence of bimetallic Au-Ni alloy was experimentally confirmed on mono-oxide Al₂O₃ support surface, but its formation was not identified on bioxide Al₃CrO₆ support. Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 1, pp. 149–156. The article is published in the original. Based on a report at the VII Russ. Conf. on Mechanisms of Catalytic Reactions (with international participation), St. Petersburg, July 2–8, 2006.     

Changes in the heat capacity of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> solid solutions near the point of antiferromagnetic phase transition in Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>

Al₂O₃-Cr₂O₃ solid solutions with 0, 4, 7, 10 and 20 mol% of corundum were synthesized using a high-pressure/high-temperature apparatus and characterized by X-ray powder diffraction. Calorimetric measurements were carried out using DSC-111 (Setaram). Heat capacity was measured by the enthalpy method in a temperature range of 260–340 K, near magnetic phase transition in pure Cr₂O₃ (305 K). Magnetic contribution into the heat capacity was derived and found to change irregularly with the composition.     

Adsorption behavior of <Superscript>99</Superscript>Tc on Fe,Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Summary The adsorption of ⁹⁹Tc on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ was studied by batch experiments under aerobic and anoxic conditions. The effects of pH and CO₃^2- concentration of the simulated ground water on the adsorption ratios were also investigated, and the valences of Tc in solution after the adsorption equilibrium were studied by solvent extraction. The adsorption isotherms of TcO₄- on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ were determined. Experimental results have shown that the adsorption ratio of Tc on Fe decreases with the increase of pH in the range of 5-12 and increases with the decrease of the CO₃^2- concentration in the range of 10^-8M-10^-2M. Under aerobic conditions, the adsorption ratios of ⁹⁹Tc on Fe₂O₃ and Fe₃O₄ were not influenced by pH and CO₃^2-concentration. When Fe was used as adsorbent, Tc existed mainly in the form of Tc(IV) after equilibrium and in the form of Tc(VII) when the adsorbent was Fe₂O₃ or Fe₃O₄ under aerobic conditions. The adsorption ratios of Tc on Fe, Fe₂O₃ and Fe₃O₄ decreased with the increase of pH in the range of 5-12 and increased with the decrease of the CO₃^2- concentration in the range of 10^-8M-10^-2M under anoxic conditions. Tc existed mainly in the form of Tc(IV) after equilibrium when Fe, Fe₂O₃ and Fe₃O₄ was the adsorbent under anoxic conditions. The adsorption isotherms of TcO_4- on the adsorbers Fe, Fe₂O₃ and Fe₃O₄ are fairly in agreement with the Freundlich’s equation under both aerobic and anoxic conditions.     

Novel bimetallic Cr<Subscript>3</Subscript>Yb<Subscript>3</Subscript> mixed salt containing a 3<Emphasis Type="Italic">d</Emphasis>-4<Emphasis Type="Italic">f</Emphasis> heterometallic Cr<Subscript>2</Subscript>Yb<Subscript>3</Subscript> cluster

A novel bimetallic Cr₃Yb₃ coordination compound containing a 3d-4f heterometallic Cr₂Yb₃ cationic cluster has been synthesized and structurally characterized. The crystal structure was determined by X-ray analysis. Results denote that the complex consists of an original [Cr ₂ ^III Yb ₃ ^III ]³⁺ moiety with a trigonal-bipyramidal topology of the [Cr₂Yb₃(μ-OOCCH₃)₆(μ-OH)₆(H₂O)₆]³⁺ core, an isolated [Cr^III(CN)₆]^3? anion, and four molecular neutral 4,4′-bipyridene (Bipy) ligands, namely, [Cr₂Yb₃(μ-OOCCH₃)₆(μ-OH)₆(H₂O)₆][Cr(CN)₆] · 4Bipy · 13H₂O.     

Phase equilibria in the system LaMnO<Subscript>3</Subscript>-SrMnO<Subscript>3</Subscript>-SrCrO<Subscript>4</Subscript>-LaCrO<Subscript>3</Subscript>

A continuous solid solution LaMn_1?y Cr _y O₃ with an orthorhombic structure is found to exist in the range of 0.0 ≤ y ≤ 1.0. An orthorhombic solid solution La_1?x Sr _x CrO₃ exists in the range of 0.0 ≤ x ≤ 0.1. The stability boundaries are determined for the perovskite phase La_1?x Sr _x Mn_1?y Cr _y O₃. An isobaric-isothermal section LaMnO₃-SrMnO₃-SrCrO₄-LaCrO₃ of the system La₂O₃-SrO-Mn₃O₄-Cr₂O₃ in air at 1100°C is designed.     

Preparation and characterization of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>composite particles

Using Fe₃O₄ nano-particles as seeds, a new type of Fe₃O₄/Au composite particles with core/shell structure and diameter of about 170 nm was prepared by reduction of Au³⁺ with hydroxylamine in an aqueous solution. Particle size analyzer and transmission electron microscope were used to analyze the size distribution and microstructure of the particles in different conditions. The result showed that the magnetically responsive property and suspension stability of Fe₃O₄ seeds as well as reduction conditions of Au³⁺to Au⁰are the main factors which are crucial for obtaining a colloid of the Fe₃O₄/Au composite particles with uniform particle dispersion, excellent stability, homogeneity in particle sizes, and effective response to an external magnet in aqueous suspension solutions. UV-Vis analysis revealed that there is a characteristic peak of Fe₃O₄/Au fluid. For particles with d(0.5)=168 nm, the λmax is 625 nm.     

Crystal Structure of New One-Dimensional Triple Molybdate Na<Subscript>2</Subscript>K<Subscript>2</Subscript>Cu(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>

The title compound (disodium dipotassium copper(II) tris-[molybdate (VI)]) is prepared by form melt and characterized by single crystal X-ray diffraction and UV-vis spectroscopy. It crystallizes in the triclinic space group P-1 with a = 7.4946(8) Å, b = 9.3428(9) Å, c = 9.3619(9) Å, α = 92.591(7)°, β = 105.247(9)°, γ = 105.496(9)°, V = 604.7 ų, and Z = 2. Its structure is isotypic with that of Na₄Mn(MoO₄)₃. It is formed by Cu₂O₁₀ distorted bi-octahedral dimers linked by two bridging bidentate Mo₂O₄ tetrahedra and, additionally, two monodentate Mo₁O₄ tetrahedra to form Cu₂Mo₄O₂₀ units. These units are linked by the insertion of Mo₃O₄ tetrahedra to build infinite ribbons disposed along the c axis. All of these ribbons form a one-dimensional framework. Both K1 and K3 cations are located in the inversion center, and all the other atoms are at general positions. The structure model is supported by the bond valence sum (BVS) and charge distribution CHARDI methods. The Cu²⁺ cations adopt the [4+2] CuO₆ Jahn-Teller distortion giving rise to an intense d–d transition in the UV-vis absorption spectra.     

Structure and real composition of undoped and Cr- and Ni-doped Sr<Subscript>0.61</Subscript>Ba<Subscript>0.39</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> single crystals

Strontium barium niobate crystals with congruent melting composition Sr_0.61Ba_0.39Nb₂O₆ (SBN-61), both nominally pure and doped with Cr³⁺ и Ni³⁺ ions, have been investigated by neutron diffraction. Different strontium and barium contents as well as their different distribution over the Sr1, of Sr2 and Ba2 crystallographic sites of SBN-61 structure, caused by introduction of dopants, have been revealed. Coordination polyhedra of cations have been established based on the analysis of cation–anion internuclear distances together with the calculation of bond-valence sums for cations, which are equal to their formal charge. It was found that the Nb1 and Nb2 atoms are located in distorted octahedra with quadfurcated (the Nb1O₆ polyhedron) or bifurcated (the Nb2O₆ polyhedron) vertices, and the Sr1 atoms are located in a cuboctahedron with bifurcated vertices in the base plane. Different polyhedra have been revealed for the Sr2 and Ba2 atoms: Sr2 atoms are coordinated by 15 oxygen atoms to form a highly distorted five-capped pentagonal prism, whereas Ba2 atoms are located in a highly distorted three-capped trigonal prism with a coordination number 9. Comparison of interatomic and internuclear distances, determined by X-ray and neutron diffraction analyses, respectively, allowed to reveal a highly pronounced shift of electron density in Nb1 and Sr2 polyhedra, responsible for the covalent bond and properties of crystals. Location of Cr³⁺ и Ni³⁺ dopant ions in the SBN-61 structure as well as their formal charges has been discussed.     

Studies on the synthesis of higher alcohol over modified Cu/ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

Higher alcohol has been considered as a potential fuel additive. Higher alcohol, including C₂–C₄ alcohol was synthesized by catalytic conversion of syngas (with a ratio of CO/H₂?=?1) derived from natural gas over modified Cu/ZnO/Al₂O₃ catalyst. Modified Cu/ZnO/Al₂O₃ catalysts promoted by alkali metal (Li) for higher alcohol synthesis (HAS) were prepared at different pH (6, 6.5, 7, 8, and 9) by co-precipitation to control Cu surface area and characterized by N₂ physisorption, XRD, SEM, H₂-TPR and TPD. The HAS reaction was carried out under a pressure of 45 bar, GHSV of 4000 h^?1, ratio of H₂/CO?=?1, and temperature ranges of 240 and 280 °C. It was found that the malachite phase of copper causes the size of copper to be small, which is suitable for methanol synthesis. Methanol and HAS share a common catalytic active site and intermediate. It was also found that the productivity to higher alcohol was correlated with Cu surface area.     

Influence of annealing temperature on material properties of red emitting ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>: Cr<Superscript>3+</Superscript> nanostructures

Zinc gallate (ZnGa₂O₄) nanopowders doped with Cr³⁺ (1?mo%) were synthesized by the citric acid assisted sol–gel method. The influence of annealing temperature, structural, morphological, and optical properties of ZnGa₂O₄: Cr³⁺ (1?mol%) nanosized particles were investigated. The X-ray diffraction (XRD) spectra indicated that the nanoparticles are cubic in structure and the annealing temperature did not influence any c in structure. The average crystallite size of ZnGa₂O₄: Cr³⁺ nanoparticles were observed to increase from 11.85 to 30.88?nm as the annealing temperature increased from 600 to 1000?°C. The scanning electron microscopy (SEM) showed nearly spherical nanostructures that change in size with annealing temperature. The high resolution transmission electron microscope (HR-TEM) images show well resolved lattice fringes which is an indications of highly crystalline samples. Ultraviolet–visible (UV–Vis) measurement show decrease in reflectance in visible region and energy band gap was found to decrease with annealing temperature. The photoluminescence (PL) intensity was found to be maximum for sample annealed at high temperature (1000?°C) and least with sample annealed at low temperature (600?°C). An increase in annealing temperature leads significantly increment in PL intensity. The degree of crystallinity also increased with annealing temperature from XRD, SEM, and HR-TEM analysis. The photoluminescence lifetimes, particle size, and emission spectra are comparable with reports on bioimaging applications.

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Study on the oxygen exchange capacities of Ce<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript> pyrochlore

Ce₂Sn₂O₇ pyrochlore was synthesized by a hydrothermal method. X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the composition and valence state of the sample. The oxygen exchange property of the Ce₂Sn₂O₇ phase was measured by an oxidation reaction in sealed air atmosphere and a followed reduction reaction in 5% H₂-95% N₂ atmosphere. Gas chromatography (GC) was used to analyze the oxygen change in the reaction. The results show that Ce₂Sn₂O₇ sample has excellent oxygen absorption capacity at 250°C as Ce³⁺ ions are oxidized to Ce⁴⁺ ions. The oxidized sample can be reduced by 5% H₂-95% N₂. The refreshed sample remains the capacity of oxygen absorption, while the oxygen exchange capacity degrades with the reduction times.     

Mechanism and kinetics of formation of La<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript> and Nb<Subscript>2</Subscript>SrFe<Subscript>2</Subscript>O<Subscript>7</Subscript>

Phase formation processes in the systems Ln₂O₃-SrO-Fe₂O₃ (Ln = La, Nd) in air in the temperature range 1200–1500°C were studied. The synthesis of the complex ferrites La₂SrFe₂O₇ and Nb₂SrFe₂O₇ involves the formation of the intermediate compounds LnFeO₃ and LnSrFeO₄ and occurs by the same mechanism as the synthesis of the corresponding aluminates, but much faster.     

Kinetic studies on the hydrogenation of nitrate in water using Rh/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Rh-Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts

Liquid-phase reduction NO ₃ ⁻ using monometallic and bimetallic catalysts (5% Rh/Al₂O₃, 5% Rh-0.5% Cu/Al₂O₃, 5% Rh-1.5% Cu/Al₂O₃, 5% Rh-5% Cu/Al₂O₃ and a physical mixture of 5% Rh/Al₂O₃ and 1.5% Cu/Al₂O₃) was studied in a slurry reactor operating at atmospheric pressure. Kinetic measurements were performed for a low concentration of nitrate (0.4 × 10⁻³−3.2 × 10⁻³ mol dm⁻³) and the temperature range 293–313 K. From the experimental data, it was found that the reduction of nitrate is first order with respect to nitrate. On the basis of the rate constants, the apparent activation energy was established using a graphic method. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 881–886. This article was submitted by the authors in English.     

Promoted porous Co<Subscript>3</Subscript>O<Subscript>4</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for ammonia decomposition

Transition metal catalysts have been considerably used for NH₃ decomposition because of the potential application in CO_x-free H₂ generation for fuel cells. However, most transition metal catalysts prepared via traditional synthetic approaches performed the inferior stability due to the agglomeration of active components. Here, we adopted an efficient method, aerosol-assisted self-assembly approach (AASA), to prepare the optimized cobalt-alumina (Co₃O₄-Al₂O₃) catalysts. The Co₃O₄-Al₂O₃ catalysts exhibited excellent catalytic performance in the NH3 decomposition reaction, which can reach 100% conversion at 600 °C and maintain stable for 72 h at a gaseous hourly space velocity (GHSV) of 18000 cm³ g_cat^?1 h^?1. The catalysts were characterized by various techniques including transmission electron microscope (TEM), scanning electron microscope (SEM), nitrogen sorption, temperature-programmed reduction by hydrogen (H₂-TPR), ex-situ/in-situ Raman and ex-situ/in-situ X-ray diffraction (XRD) to obtain the information about the structure and property of the catalysts. H₂-TPR and in-situ XRD results show that there is strong interaction between the cobalt and alumina species, which influences the redox properties of the catalysts. It is found that even a low content of alumina (10 at%) is able to stabilize the catalysts due to the adequate dispersion and rational interaction between different components, which ensures the high activity and superior stability of the cobalt-alumina catalysts.     

A New Molybdophosphate Constructed From {Mo<Stack><Subscript>2</Subscript><Superscript>V</Superscript></Stack>O<Subscript>4</Subscript>(H<Subscript>2</Subscript>O)<Subscript>6</Subscript>}<Superscript>2+</Superscript> and 1-Hydroxyethylidenediphosphonate

A new molybdophosphate (NH₄)₈{Mo₂^VO₄[(Mo₂^VIO₆)CH₃C(O)(PO₃)₂]₂}·14H₂O (1), has been synthesized by the reaction of {Mo₂^VO₄(H₂O)₆}²⁺ fragments with 1-hydroxyethylidenediphosphonate (hedp HOC(CH₃)(PO₃H₂)₂), and it is characterized by ³¹P NMR, IR, UV, element analysis, TG and single-crystal X-ray analysis. The structure analysis reveals that the polyoxoanion can be described as two {(Mo₂^VIO₆)(CH₃C(O)(PO₃)₂} units connected by a {Mo₂^VO₄}²⁺ moiety. In the structure, the six Mo atoms are arranged into a new “W-shaped” structure, which represents a new kind of molybdophosphate.     

Synthesis,structure and thermal decomposition of [Ni(NH<Subscript>3</Subscript>)<Subscript>6</Subscript>][VO(O<Subscript>2</Subscript>)<Subscript>2</Subscript>(NH<Subscript>3</Subscript>)]<Subscript>2</Subscript>

The compound [Ni(NH₃)₆][VO(O₂)₂(NH₃)]₂ was prepared and characterized by elemental analysis and vibrational spectra. The single crystal X-ray study revealed that the structure consists of [Ni(NH₃)₆]²⁺ and [VO(O₂)₂(NH₃)]⁻ ions. As a result of weak interionic interactions V′···O_p (O_p-peroxo oxygen), ([VO(O₂)₂(NH₃)]⁻)₂ dimers are formed in the solid-state. The thermal decomposition of [Ni(NH₃)₆][VO(O₂)₂(NH₃)]₂ is a multi-step process with overlapped individual steps; no defined intermediates were obtained. The final solid products of thermal decomposition up to 600°C were Ni₂V₂O₇ and V₂O₅.