Revisiting the Sr–Cr(IV)–O system at high pressure and temperature with special reference to Sr3Cr2O7

High pressure and temperature are used to synthesise perovskite related phases in the Sr–Cr(IV)–O system. The n = 1, 2 and ∞ members of the Sr_n+1Cr_nO_3n+1 family have been obtained. Another new member, n = 3 as well as an hexagonal layered perovskite have also been observed by transmission electron microscopy and electron diffraction. The average structure of Sr₃Cr₂O₇ as determined by XRD has space group I4/mmm, whereas its microstructure includes a large amount of defects both in the layer stacking and within the layers. 2D magnetism and a large electrical resistance in Sr₃Cr₂O₇ are observed as opposed to the nonlocalized electronic behaviour of SrCrO₃.     

A WO3 nanorod-Cr2O3 nanoparticle composite for selective gas sensing of 2-butanone

The sensor based on WO₃-Cr₂O₃ nanocomposites show good selectivity to 2-butanone.     

Structural and Mössbauer characterization of the ball milled Fex(Cr2O3)1−x system

The Fe_x(Cr₂O₃)_1?x system, with 0.10 ≤ X ≤ 0.80, was mechanically processed for 24 h in a high-energy ball-mill. In order to examine the possible formation of iron–chromium oxides and alloys, the milled samples were, later, thermally annealed in inert (argon) and reducing (hydrogen) atmospheres. The as-milled and annealed products were characterized by X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy and magnetization. The as-milled samples showed the formation of an Fe_1+YCr_2?YO_4?δ nanostructured and disordered spinel phase, the α₁-Fe(Cr) and α₂-Cr(Fe) solid solutions and the presence of non-exhausted precursors. For the samples annealed in inert atmosphere, the chromite (FeCr₂O₄) formation and the recrystallization of the precursors were verified. The hydrogen treated samples revealed the reduction of the spinel phase, with the phase separation of the chromia phase and retention of the Fe–Cr solid solutions. All the samples, either as-milled or annealed, presented the magnetization versus applied field curves typical for superparamagnetic systems.     

Kinetics and mechanism of the chromic oxidation of 3-O-methyl-d-glucopyranose

The oxidation of 3-O-methyl-d-glucopyranose (Glc3Me) by Cr^VI in acid medium yields Cr^III, formic acid and 2-O-methyl-d-arabinose as final products when a 50-times or higher excess of Glc3Me over Cr^VI is used. The redox reaction takes place through the combination of Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III pathways. Intermediacy of free radicals and Cr^II in the reaction was demonstrated by the observation of induced polymerization of acrylamide and detection of CrO₂²⁺ formed by reaction of Cr^II with O₂. Intermediate oxo-Cr^V–Glc3Me species were detected by EPR spectroscopy. In 0.3–0.5 mol/L HClO₄, intermediate Cr^V rapidly decompose to the reaction products, while, at pH 5.5–7.5, where the redox processes are very slow, five-coordinate Cr^V bis-chelates of the pyranose and furanose forms of Glc3Me remain more than 15 h in solution. The C1–C2 bond cleavage of Glc3Me upon reaction with Cr^VI distinguishes this derivative from glucose, which is oxidized to gluconic acid.     

Characterization,EPR and photoluminescence studies of LiAl5O8:Cr phosphors

Red emitting Cr³⁺ doped LiAl₅O₈ powder phosphor was prepared by combustion route using corresponding metal nitrates and urea in a single step. The prepared powder was characterized by X-ray diffraction and surface area measurements were carried out by Brunauer–Emmet–Teller adsorption isotherms. The electron paramagnetic resonance spectrum in the low field regions is typical for isolated Cr³⁺ ions whereas the resonance signal in the high field region with g = 1.95 is due to exchange coupled Cr³⁺–Cr³⁺ pairs. The optical studies show two broad and intense bands characteristic of Cr³⁺ ions in distorted octahedral symmetry. The photoluminescence spectrum gives a narrow red emission at 710 nm corresponding to ²E_g → ⁴A_2g transition upon excitation of 562 nm. The crystal field parameter (Dq), Racah inter-electronic repulsion parameters (B and C) and nephelauxetic parameters have been evaluated and discussed.     

Stable free-standing aramid resin film containing vanadium pentoxide and new colour electrochromism of the film by electrodeposition of gold

Vanadium pentoxide (V₂O₅) was electrodeposited on a poly(p-phenylene terephtalamide) (PPTA)-film coated electrode. The cyclic voltammogram of the film had a reversible redox current peak. The film was dark green in the reduced state and yellow in the oxidized state. To obtain new colour, gold was further electrodeposited on the film. Not only the redox current peak but also a new redox current shoulder appeared in the cyclic voltammogram of the obtained film, and it exhibited a multicoloured electrochromism: blackish green ↔ dark green ↔ green ↔  bright red. The red colour in the oxidized state was first obtained for the V₂O₅ film. The new redox current shoulder and the colour were probably due to Au_yV₂O₅ partially formed during electrodeposition of the gold. The redox of the Au_yV₂O₅ was accompanied by egress and ingress of Li⁺ ions and the new colour change.     

Effect of anion substitution onto structural and magnetic properties of chromium chalcogenides

We investigated experimentally and theoretically the effect of the substitution of Te by Se onto the structural, magnetic and electronic properties of ferromagnetic Cr_5±xTe₈ as parent material. Whereas Cr₅Te₈ is dimorphic crystallizing in a monoclinic and trigonal modification, Se substituted samples crystallize in two different trigonal modifications depending on the synthesis conditions. One of the modifications can be viewed as self-intercalated dichalcogenides Cr_1+xQ₂ (Q = Te, Se) and the other is a superstructure which is isostructural to one of the Cr_5±xTe₈ modifications. For the Se richest samples (Te:Se = 1:7) a new modification is identified which was formerly reported for Cr_3+xSe₄. For a distinct Cr content the replacement of Se by Te induces a reduction of the unit cell volumes, of the Cr-Cr and Cr-Te/Se distances. Increasing the Cr content for a constant Te:Se ratio has the opposite effect. The results also suggest that the homogeneity range extends to more Cr rich compounds with decreasing Te content. For a given Cr content the substitution of Te by Se weakens the ferromagnetic exchange interactions and strengthens the antiferromagnetic exchange. With increasing Cr content and a fixed Te:Se ratio ferromagnetic properties become more pronounced. The low temperature magnetic behavior is characterized by spin-glass, spin-glass like or cluster-glass properties depending on the Cr content and the Te:Se ratio. Electronic structure calculations done within the framework of LSDA (local spin density approximation) gave a detailed insight into the electronic and magnetic properties of the investigated systems supporting the interpretation of the achieved experimental results. This applies in particular for the calculated exchange coupling constants that provided the necessary input for Monte Carlo simulations used for theoretical investigations on the magnetic properties at finite temperatures.     

Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

The thermal expansion and phase transition of solid solutions Yb_2?xCr_xMo₃O₁₂ have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb_2?xCr_xMo₃O₁₂ indicate that the solid solution limit was in the composition range of 0.0 ≤ x ≤ 0.4 and 1.7 ≤ x ≤ 2.0. Yb_2?xCr_xMo₃O₁₂ (0.0 ≤ x ≤ 0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200 °C and 800 °C. Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) present positive thermal expansion. Orthorhombic Yb_2?xCr_xMo₃O₁₂ exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb³⁺ for Cr³⁺ exhibits depressed monoclinic to orthorhombic phase transition.     

Synthesis and characterization of Co3O4 nanoparticles by a simple method

Using solid complex molecular precursor [bis(salicylaldehyde)ethylenediiminecobalt(II)], [Co(salen)], a simple and surfactant-free method to synthesize Co₃O₄ nanoparticles was proposed. Cubic-phase Co₃O₄ nanoparticles of size 30–50-nm could be produced by thermal treatment of the Co(salen) in the air at 500 °C for 5 h. The as-prepared samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The optical absorption spectrum indicates that the direct band gaps of Co₃O₄ nanoparticles are 1.53 and 2.02 eV. The optical property test indicates that the absorption peak of the nanoparticles shifts towards short wavelengths, and the blue shift phenomenon might be ascribed to the quantum effect. The hysteresis loops of the obtained samples reveal their ferromagnetic behavior, an enhanced coercivity (H_c) and a decreased saturation magnetization (M_s) as compared to their respective bulk materials.     

Crystal structure and thermal analysis of the tetramethylammonium dichromate and trichromate

The structures of the tetramethylammonium dichromate, [(CH₃)₄N]₂Cr₂O₇ and trichromate, [(CH₃)₄N]₂Cr₃O₁₀, were determined from single-crystal X-ray diffraction data. These compounds crystallize in the orthorhombic system (space group Pnma, with Z=4 and a=17.192(1) Å, b=8.55(1) Å, c=10.637(1) Å), for the dichromate and in the monoclinic system (space group P2₁/n, with Z=4 and a=11.366(2) Å, b=8.493(2) Å, c=20.187(4) Å, β=103.98(3)° for the trichromate. The structures consist of discrete dichromate anions (Cr₂O₇)^2– or trichromate anions (Cr₃O₁₀)^2–, respectively, stabilized by quaternary ammonium [(CH₃)₄N]⁺. Phase transitions in [(CH₃)₄N]₂Cr₂O₇ have been evidenced by differential scanning calorimetry as well as a new allotropic variety of [(CH₃)₄N]₂Cr₂O₇ which was characterized by X-ray powder diffraction. It crystallizes in an orthorhombic system with the unit cell parameters a=24.49(1) Å, b=8.85(1) Å, c=8.705(8) Å.     

Ethylbenzene to chemicals: Catalytic conversion of ethylbenzene into styrene over metal-containing MCM-41

Isomorphously substituted (MeDM) and impregnated metal-containing MCM-41 (MeO_x/IM) catalysts, in which Me = Co, Cu, Cr, Fe or Ni, have been prepared. Structural and textural characterizations of the catalysts were performed by means of X-ray diffraction (XRD), chemical analysis, Raman spectroscopy, electron paramagnetic resonance (EPR), N₂ adsorption isotherms and temperature programmed reduction (TPR). Cu²⁺, Co²⁺, and Cr⁴⁺/Cr³⁺ species were found over the catalysts as cations incorporated in the MCM-41 structure (MeDM) or highly dispersed oxides on the surface (MeO_x/IM). The MeDM catalysts exhibited a good performance in the dehydrogenation of ethylbenzene with CO₂. However, MeO_x/IM catalysts had a low performance in styrene production (activity less than 15 × 10^?3 mmol h^?1 and selectivity for styrene less than 80%) due to the high reducibility of the metals species. However, Ni²⁺ or Fe³⁺ coordinated with the MCM-41 framework, as well as NiO_x and Fe₂O₃ extra-framework species, is continuously oxidized by the CO₂ to maintain the active sites for dehydrogenating ethylbenzene. Deactivation studies on the FeDM sample showed that Fe³⁺ species produced active sp² carbon compounds, which are removed by CO₂; the referred sample is catalytically selective for styrene and stable over 24 h of reaction. In contrast, highly active Ni²⁺ and Ni⁰ species produced a large amount of polyaromatic carbonaceous deposits from styrene, as identified by TPO, TG and Raman spectroscopy. An acid–base mechanism is proposed to operate to adsorb ethylbenzene and abstract the β-hydrogen. CO₂ plays a role in furnishing the lattice oxygen to maintain the Fe³⁺ active sites in the dehydrogenation of ethylbenzene to form styrene.     

Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal

The effects of impregnated Cr³⁺ on sulfur removal during pyrolysis and hydropyrolysis of coal were investigated by loading CrCl₃ into raw, demineralized and pyrite removed coal, respectively. The results indicate that Cr has no effect on the removal of pyrite. Cr affects the removal of total sulfur by forming Cr₇S₈ and affecting the removal of organic sulfur. Cr acts as the sulfur removing agent by promoting the decomposition of the unstable organic sulfur at low temperature. However, it behaves to be sulfur fixing agent between 400 and 700 °C so as to inhibit the evolution of H₂S, even in hydropyrolysis. With the increase of temperature from 700 to 1050 °C, a certain ratio of Cr₇S₈ is converted into organic sulfur during pyrolysis; however, almost all the Cr₇S₈ is reduced into Cr at 1050 °C during hydropyrolysis. And Cr significantly promotes the removal of organic sulfur at high temperature within reducing atmosphere. The XPS results indicate that the sulfur is enriched on coke surface by Cr, which is attributable to the formation of Cr₇S₈ as well as the transfer of organic sulfur from bulk to surface during pyrolysis and hydropyrolysis.     

Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4

High-temperature heat capacity measurements were obtained for Cr₂O₃, FeCr₂O₄, ZnCr₂O₄, and CoCr₂O₄ using a differential scanning calorimeter. These data were combined with previously available, overlapping heat capacity data at temperatures up to 400 K and fitted to 5-parameter Maier–Kelley C_p(T) equations. Expressions for molar entropy were then derived by suitable integration of the Maier–Kelley equations in combination with recent S^∘(298) evaluations. Finally, a database of high-temperature equilibrium measurements on the formation of these oxides was constructed and critically evaluated. Gibbs free energies of Cr₂O₃, FeCr₂O₄, and CoCr₂O₄ were referenced by averaging the most reliable results at reference temperatures of (1100, 1400, and 1373) K, respectively, while Gibbs free energies for ZnCr₂O₄ were referenced to the results of Jacob [K.T. Jacob, Thermochim. Acta 15 (1976) 79–87] at T = 1100 K. Thermodynamic extrapolations from the high-temperature reference points to T = 298.15 K by application of the heat capacity correlations gave Δ_fG^∘(298) = (−1049.96, −1339.40, −1428.35, and −1326.75) kJ · mol⁻¹ for Cr₂O₃, FeCr₂O₄, ZnCr₂O₄, and CoCr₂O₄, respectively.     

Solvothermal synthesis,structure, and magnetic property for nano-‘chromic wheels’ [Cr10(μ-O2CMe)10(μ-OMe)10(μ-OEt)10]

Nano-decanuclear cyclic Cr(III) complex, [Cr₁₀(μ-O₂CMe)₁₀(μ-OMe)₁₀(μ-OEt)₁₀] (1), has been synthesized in high yield by solvothermal technique using trinuclear basic chromium acetate [Cr₃O(CO₂Me)₆(MeOH)₃]Cl in EtOH. Complex 1 crystallizes in the monoclinic space group P2₁/n. The Cr₁₀ rings are close to planar, with each pair of neighboring Cr(III) ions bridged by one μ-acetate, one μ-ethoxide, and one μ-methoxide groups. Magnetic susceptibility studies reveal very weak antiferromagnetic Cr⋯Cr exchange of J = 0.37 cm⁻¹ .     

Fabrication of solid thin film electrolyte and LiCoO2 by aerosol flame pyrolysis deposition

Aerosol flame pyrolysis deposition method was applied to deposit the oxide glass electrolyte film and LiCoO₂ cathode for thin film type Li-ion secondary battery. The thicknesses of as-deposited porous LiCoO₂ and Li₂O–B₂O₃–P₂O₅ electrolyte film were about 6 μm and 15 μm, respectively. The deposited LiCoO₂ was sintered for 2 min at 700 °C to make partially densified cathode layer, and the deposited Li₂O–P₂O₅–B₂O₃ glass film completely densified by the sintering at 700 °C for 1 h. After solid state sintering process the thicknesses were reduced to approximately 4 μm and 6 μm, respectively. The cathode and electrolyte layers were deposited by continuous deposition process and integrated into a layer by co-sintering. It was demonstrated that Aerosol flame deposition is one of the good candidates for the fabrication of thin film battery.     

Nanocomposite of a chromium Prussian blue with TiO2. Redox reactions and the synthesis of Prussian blue molecule-based magnets

The reaction of [NEt₄]₃[Cr(CN)₆] with titanium(III) p-toluenesulfonate at a pH of 2 affords a gray solid whose metal content and spectroscopic and magnetic properties are fully consistent with it being a Prussian blue material of stoichiometry “Ti^III[Cr^III(CN)₆] · H₂O”. The carbon, nitrogen, and hydrogen content, however, are not consistent with this stoichiometry, and further investigation showed that the gray material has a powder X-ray diffraction profile, infrared spectrum, and magnetic properties very similar to those of the “all-chromium” Prussian blue Cr^II[Cr^III(CN)₆]_0.67 · 6H₂O. All data, including the C, H, and N weight percentages, are consistent with the conclusion that the material isolated is a nanocomposite of Cr^II[Cr^III(CN)₆]_0.67 · xH₂O and TiO₂ in the ratio of 1–1.6. These results suggest that Ti^III reduces some of the [Cr^III(CN)₆]³⁻ ions to generate Ti^IV and Cr^II; the former hydrolyzes to amorphous TiO₂ · 2H₂O, the latter loses its bound CN ligands and reacts with unreacted [Cr^III(CN)₆]³⁻ ions to generate the crystalline all-chromium PB species. The electrochemical potentials suggest that the [Cr^III(CN)₆]³⁻ ion should not be reduced by Ti^III; evidently, this unfavorable reaction is driven by the insolubility of the reaction products. The results constitute a cautionary tale in two respects: first, that the characterization of Prussian blue materials must be conducted with care and, second, that the insolubility of Prussian blue analogues can sometimes drive reactions that in solution are thermodynamically unfavorable.     

High performance cathode-supported SOFC with perovskite anode operating in weakly humidified hydrogen and methane

A high performance cathode-supported solid oxide fuel cell (SOFC), suitable for operating in weakly humidified hydrogen and methane, has been developed. The SOFC is essentially made up by a YSZ/LSM composite supporting cathode, a thin YSZ film electrolyte, and a GDC-impregnated La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) anode. A gas tight thin YSZ film (∼27 μm) was formed during the co-sintering of cathode/electrolyte bi-layer at 1200 °C. The cathode-supported SOFC developed in this study showed encouraging performance with maximum power density of 0.182, 0.419, 0.628 and 0.818 W cm⁻² in air/3% H₂O–97% H₂ (and 0.06, 0.158, 0.221 and 0.352 W cm⁻² in air/3% H₂O–97% CH₄) at 750, 800, 850 and 900 °C, respectively. Such performance is close to that of the cathode-supported cell (0.42 W cm⁻² vs. 0.455 W cm⁻² in humidified H₂ at 800 °C) developed by Yamahara et al. [Solid State Ionics 176 (2005) 451–456] with a Co-infiltrated supporting LSM-YSZ cathode, a (Sc₂O₃)_0.1(Y₂O₃)_0.01(ZrO₂)_0.89 (SYSZ) electrolyte of 15 μm in thickness and a SYSZ/Ni anode, indicating that the performance of the GDC-impregnated LSCM anode is comparable to that made of Ni cermet while stable in weakly humidified methane fuel.     

Structural and electrochemical properties of a porous nanostructured SnO2 film electrode for lithium-ion batteries

A B₂O₃-doped SnO₂ thin film was prepared by a novel experimental procedure combining the electrodeposition and the hydrothermal treatment, and its structure and electrochemical properties were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, energy dispersive X-ray (EDX) spectroscopy and galvanostatic charge–discharge tests. It was found that the as-prepared modified SnO₂ film shows a porous network structure with large specific surface area and high crystallinity. The results of electrochemical tests showed that the modified SnO₂ electrode presents the largest reversible capacity of 676 mAh g^?1 at the fourth cycle, close to the theoretical capacity of SnO₂ (790 mAh g^?1); and it still delivers a reversible Li storage capacity of 524 mAh g^?1 after 50 cycles. The reasons that the modified SnO₂ film electrode shows excellent electrochemical properties were also discussed.     

Sensing characteristics of mixed-potential-type zirconia-based sensor using laminated-oxide sensing electrode

A planar sensor was fabricated using a yttria-stabilized zirconia (YSZ) plate and laminated-oxide sensing electrode (SE) aiming for selective detection of NO₂ at high temperature. The NO₂ sensing characteristics as well as the cross-sensitivity to various gases were examined in the temperature range of 700–900 °C under the wet condition (5 vol.% H₂O). It was found that the sensor attached with the laminated hetero-oxide layer (Cr₂O₃/NiO (+WO₃)) could detect NO₂ selectively at 895 °C. It was speculated that the additional oxide layer (Cr₂O₃) placed on the NiO (+WO₃) layer acted as a catalyst for the oxidation of CO, hydrocarbons and NO, and then led to better NO₂ selectivity.     

Adsorption of some metal complexes derived from acetyl acetone on activated carbon and purolite S-930

A new Schiff base (HL) derived from condensation of p-anisidine and acetyl acetone has been prepared and used as a chelating ligand to prepare Cr(III), Mn(II), Co(II), Ni(II) and Cu(II) complexes. The study of the nature of these complexes formed in ethanol solution following the mole ratio method (2:1, L:M) gave results which were compared successfully with these obtained from isolated solid state studies. These studies revealed that the complexes having square planner geometry of the type (ML₂), M = Co(II), Ni(II) and Cu(II), and octahedral geometry of the type [Cr^IIIL₂(H₂O)₂]Cl and [MN^IIL₂(H₂O)₂]. The adsorption studies of three complexes Cr(III), Mn(II), and Co(II) on activated carbon, H and Na-forms of purolite S-930 resin show high adsorption percentage for Cr(III) on purolite S-930 due to ion exchange interaction compared with high adsorption of neutral Mn(II), Co(II) complexes on activated charcoal. Linear plot of log Q_e versus log C_e showed that the adsorption isotherm of these three complexes on activated carbon, H and Na-forms of purolite S-930 surface obeys Freundlich isotherm and was similar to S-curve type according to Giles classification which investigates heterogeneous adsorption. The regression values indicate that the adsorption data for these complexes fitted well within the Freundlich isothermal plots for the concentration studied. The accuracy and precision of the concentration measurements of these complexes were determined by preparing standard laboratory samples, the results show relative error ranging from ±1.08 to 5.31, ±1.04 to 4.82 and ±0.28 to 3.09 and the relative standard deviation did not exceed ±6.23, ±2.77 and ±4.38% for A1, A2 and A3 complexes, respectively.