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Three-dimensionally ordered macroporous (3DOM) spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts were successfully prepared by a colloidal crystal template strategy and characterized by means of XRD, FE-SEM, BET, FT-IR, H2-TPR and O2-TPD. The 3DOM catalysts exhibited superior activity to their corresponding bulk counterparts, which were used as reference benchmarks in this study, due to the enlarged contact between the catalyst and the soot particle and improved mass transfer caused by the unique well-defined 3DOM structures. Meanwhile, the factors influencing the catalytic activity of the 3DOM catalysts in soot combustion were also optimized, such as metallic cations and feed compositions. Furthermore, the 3DOM catalysts displayed strong durability against structural collapse due to the robust 3DOM structure, demonstrating their promising potential in view of the practical working conditions of diesel engines.

Low temperature sol-gel auto combustion method is used to synthesize the spinel chromites of Co1-xMxCr2O4 (M = Zn, Mg {non?Jahn Teller (JT) ion}, Cu {JT ion}; x = 0.0, 0.5). Synchrotron, and lab x-ray diffraction pattern confirms the single-phase crystalline nature. Structural features from cubic (space group Fd3m) [CoCr2O4, Co0.5Mg0.5Cr2O4 and Co0.5Zn0.5Cr2O4] to tetragonal (space group I41/amd) [Co0.5Cu0.5Cr2O4] are reported. SEM micrograph of sintered samples results in less porosity with average particle size distribution of ～0.2–0.3 μm. Shifting of Raman active phonon modes is seen with doping and an additional Raman active mode is seen at 666.45 cm?1 for Co0.5Zn0.5Cr2O4. Dielectric behavior as a function of frequency reveals that dispersion in all these chromites is attributed to hopping mechanism. Higher value of dielectric constant (ε′) and minimum loss tangent (tan δ) for non-JT ion Co0.5Zn0.5Cr2O4 is measured inferring effective charge polarization in chromites as compare to doped JT ions. Both grains and grain boundaries are active in Co0.5Zn0.5Cr2O4 at lower frequencies as depicted from impedance analysis. Doping does not showed the presence of electric polarization in chromites.

Magnetic properties of cobalt chromite nanoparticles of size 812 nm synthesized through conventional coprecipitation route are reported. Magnetization versus temperature measurement plot reveals a transition from paramagnetic to superparamagnetic (SPM) phase in contrast with the transition from paramagnetic to long-range ferrimagnetic phase at Curie temperature, T c, reported in bulk. The blocking temperature, Tb, of SPM phase is found to be 5060 K. On cooling in the presence of 10 kOe field these nanoparticles show an enhancement in coercivity and shifting of loop at 10 K, which is absent at 50 K. While the later observation supports the blocking temperature of the SPM phase, the former one is attributed to a disordered spin configuration at the surfaces and the distribution of nanoparticle sizes.

In order to evaluate the compatibility between the SOFC (Solid Oxide Fuel Cells) interconnector material of the perovskite-type La0.7Ca0.3Cr1-yCoyO3 (y = 0.05, 0.1 and 0.2) and 8 mol%Y2O3-ZrO2 (YSZ), two types of experiments were carried out in air. One was by the reaction of powder mixtures. The reaction products were identified by X-ray analysis after heating at 1000-1300 °C. The other was the experiments using diffusion couples. The distribution of elements at the interface was observed by EPMA for the couples heated at 1300 °C and 1400 °C. The reaction progressed mainly by the dissolution of calcium ions from the perovskite phase into YSZ. In the perovskite phase, due to the loss of A-site Ca ion, the activity of B-site ions increased, resulting in the deposition of the spinel-type CoCr2O4. The excess amount of calcium ions incorporated in YSZ reacted ZrO2 to form CaZrO3. Increase of cobalt content enlarge the reactivity of the perovskite phase with YSZ. The parabolic rate constant of the reaction, kp, was determined by the thickness of CaZrO3 at the interface of the diffusion couples. Calculated kp were 2.9×10-12cm2sec-1 and 2.1×10-11cm2sec-1 at 1300 °C and 1400 °C, respectively. For the practical application as the SOFC interconnector material, the content of cobalt as a sintering accelerator in (La, Ca)CrO3 must be controlled at low level to keep the chemical interaction to a minimum.

Abstract: Two Co–Cr–O spinel catalysts with different stoichiometry were synthesized and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 physisorption and temperature-programmed reduction in H2. Excess Co in

Co3O4 with a spinel structure is a very active oxide catalyst for the oxidation of CO. In such catalysts, octahedrally coordinated Co3+ is considered to be the active site, while tetrahedrally coordinated Co2+ is assumed to be basically inactive. In this study, a highly ordered mesoporous CoO has been prepared by H2 reduction of nanocast Co3O4 at low temperature (250 °C). The as-prepared CoO material, which has a rock-salt structure with a single Co2+ octahedrally coordinated by lattice oxygen in Fm3ˉm symmetry, exhibited unexpectedly high activity for CO oxidation. Careful investigation of the catalytic behavior of mesoporous CoO catalyst led to the conclusion that the oxidation of surface Co2+ to Co3+ causes the high activity. Other mesoporous spinels (CuCo2O4, CoCr2O4, and CoFe2O4) with different Co species substituted with non/low-active metal ions were also synthesized to investigate the catalytically active site of cobalt-based catalysts. The results show that not only is the octahedrally coordinated Co3+ highly active but also the octahedrally coordinated Co2+ species in CoFe2O4 with an inverse spinel structure shows some activity. These results suggest that the octahedrally coordinated Co2+ species is easily oxidized and shows high catalytic activity for CO oxidation.

Single crystals of multiferroic cobalt chromite Co (Cr2-x Cox) O4 have been grown via several methods to have different Co3+ doping levels (x=0.0, 0.14, and 0.18). Under magnetic fields, all the crystals display electric polarization

Here we report on the preparation, structural and magnetic properties of solid solutions of Co(Cr1?xAlx)2O4. Substituted Al ions are found to occupy the B-site of the spinel structure. Enhancement in the magneti

A rational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis. Much work has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts, but little on intentionally promoting the surface in a controlled fashion. We now report controllable anodic leaching of Cr in CoCr2O4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr2O4 into a highly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies, exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. A novel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non-concerted proton-electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.

Nanocrystalline Cobalt chromite (CoCr2O4) ceramic has been synthesized under a mild condition, rather than by a high-temperature sintering (e.g. >1673 K, in general). A shifted hysteresis loop with an exchange-bias field of 35.7 kA/m and a high coercivity of 627.9 kA/m at 4.2 K was achieved under the cooling field of 2.39×106 A/m. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results reveal that a strong lattice distortion and a large amount of surface defects exist in CoCr2O4 nanocrystallites (NCs). The anomalous magnetic properties, such as bias field and large coercivity, are attributed not only to the nanosize effect but also to the lattice distortion and crystal defects.

Double complex compounds (DCC) of the [Cr(ur)6][Fe(CN)6]·4H2O (I), [Cr(ur)6][Co(CN)6]·4H2O (II), [Cr(ur)6][Fe(Ox)3]·2H2O (III), and [Cr(ur)6][Co(Ox)3].3.5H2O (IV) composition, where ur is CO(NH2)2, Ох – С2О42? have been synthesized and their crystal structures have been studied. The DCC I, II and IV are triclinic (Р-1); III is orthorhombic (С2221). The DCC thermal behavior in air and argon atmospheres was analyzed by the following parameters: the temperature interval of total decomposition and the nature of gaseous and solid products. The end solid products in air were FeO and FeCr2O4 for I, a mixture of СоCr2O4 + Со2CrO4 for II and IV and an X-ray amorphous phase with composition CrFeO2.5 for III. The gaseous products were HCN, NH3, HNCO, N2O, СО, СО2, N2, and urea. The temperatures of total decomposition in air were: I, II 400°С, III – 350°С, IV – 460°С. In argon, the solid residue contained, besides the oxides, 7–30% of carbon up to 1000°С.

Thermolysis of the double-metal complex [Co(NH3) 6][Cr(C2O4)3] was studied in air at 200, 350, and 500°C and in a hydrogen atmosphere at 200, 350, 500, 700, and 900°C, as well as the composition and properties of thermolysis products. Oxidative thermolysis produces mixed oxides CoCr2O4 and Co2CrO4; reductive thermolysis produces Co + Cr 2O3 mixture. Specific surface areas were measured for reductive thermolysis products; the maximal specific surface area and, therefore, maximal dispersion are reached at 500°C. The morphology of the reductive thermolysis products and the thermolysis chemism were studied in relation to the nature of the complex anion.

We have addressed problems associated with the measurement of zero-field-cooled (ZFC) magnetization and its interpretation in the light of negative magnetization reported in certain ferrimagnetic materials such as CoCr2O4. We demonstrate that a small negative trapped field in the sample space as well as large coercive fields are responsible for the observed negative magnetization. The problem is commonly encountered while working with magnetometers and a superconducting magnet where the sign of the trapped field can be positive or negative depending on the way the field is reduced to zero.

The magnetic permeability of CoCr2O4 and CoMn2O4 at high pressures has been measured in a piston-cylinder apparatus. The Curie temperature or the temperature of the Hopkinson peak of CoCr2O4 increases with pressure at a range of -0.1±0.1 to -0.3±0.2 K GPa-1 or -0.26±0.06 to -0.17±0.09 K GPa-1, respectively; namely, these are negative pressure derivatives and those of CoCr2O4 increase with pressure at a rate of 1.5±0.2 to 2.7±0.2 K GPa-1 or 1.37±0.08 to 1.9±0.4 K GPa-1, respectively.

We studied magnetic phase diagram and x-dependence of spiral spin correlation in magnetically ordered phase of (Co 1-xZn x)Cr 2O 4 by magnetization and neutron scattering experiments. The spiral correlation surv

Bulk (unsupported) bimetallic oxide catalysts and those supported on alumina and alumosilica (type 4Co-1Cr) have been investigated by thermoprogrammed reduction (TPR) and thermoprogrammed desorption (TPD) techniques. Influences of the support and the prep

CoCr2O4 nanocrystallites with and without covering of residues from the decomposed polymer at the surface have been fabricated, respectively. It has been revealed that the magnetic properties of the nanocrystallites covered with the

Fine particle metal chromites, MCr2O4 where M = Mg, Mn, Fe, Co, Ni, Cu and Zn have been obtained by the combustion of a redox mixture containing corresponding divalent metal nitrate, chromium (III) nitrate and tetraformal trisazine (TFTA). An aqueous saturated solution of the redox mixture when rapidly heated at 350°C boils, foams and ignites to yield voluminous, fine particle chromite powder. Formation of chromite was confirmed by X-ray diffraction pattern and IR spectra. Fine particle nature of chromites was studied using particle size analysis, surface area measurements, TEM and SEM. The particle size of chromites is below lum and the surface area ranges from 13 to 30 m2/g.

The catalytic activity of supported chromites MCr2O4/γ-Al2O3 (M = Cu, Co, Mn, Zn, Mg) in the oxidation of CO, C3H6, and o-xylene and NOx reduction was studied. The catalytic activity depends on the calcination temperature and cation nature. The features of the formation of the catalysts were studied by the UV-Vis diffuse reflectance and IR spectroscopies.

Three-dimensionally ordered macroporous CoCr2O4 (3DOM CoCr2O4) and its supported Au–Pd alloy (xAuPdy/3DOM CoCr2O4; x = 0.98 and 1.93 wt%; Pd/Au molar ratio (y) = 1.93–1.96) nanocatalysts were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected reduction methods, respectively. Physicochemical properties of the samples were characterized by means of a number of techniques, and their catalytic activities were evaluated for methane combustion. The 3DOM CoCr2O4 and xAuPdy/3DOM CoCr2O4 samples possessed a high-quality 3DOM structure and a surface area of 33–36 m2/g. The Au–Pd alloy nanoparticles (NPs) with an average size of 3.3 nm were uniformly dispersed on the surface of the samples. The 1.93AuPd1.95/3DOM CoCr2O4 sample showed the best catalytic performance: the T10%, T50%, and T90% (temperatures required for achieving methane conversion of 10, 50, and 90%, respectively) were 305, 353, and 394 °C at a space velocity (SV) of 20,000 mL/(g h). The effects of SV, water vapor, and sulfur dioxide on the catalytic activity of the 1.93AuPd1.95/3DOM CoCr2O4 sample were also examined. It is concluded that the excellent catalytic performance of 1.93AuPd1.95/3DOM CoCr2O4 was associated with the higher surface area and adsorbed oxygen species concentration, better low-temperature reducibility, and strong interaction between Au–Pd alloy NPs and 3DOM CoCr2O4.

Spinel-phase nanocrystalline CoCrxFe2-xO4 powders (0.0 ≤ x ≤ 2.0) were synthesized by citrate-gel precursor method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmissio

The 3d-metal chromites ACr2O4 where A is a magnetic ion, show the paramagnetic to ferrimagnetic phase transition at TC while for non-magnetic A-site ion, ACr2O4 show paramagnetic to antiferromagnetic phase transition at TN. In this report, we present the detailed study of magnetic and the magnetocaloric effect (MCE) of the 3d-metal chromites ACr2O4 (where A = Mn, Fe, Co, Ni, Cu, and Zn) near TC and TN. We find the magnitude of MCE (-ΔSM) decreases on decreasing the magnetic moment of A-site ion with an exception for CuCr2O4. Additionally, to know more about the order and nature of phase transition, we have made a scaling analysis of (-ΔSM) for all the chromites across the phase transition temperatures TC and TN.

NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater. 451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1? xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd3-m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θf or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θp, were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.

A comprehensive study of magnetic, optical, and magnetooptical properties was carried out for single crystals of the spinel-type ACr2X 4 (A = Mn, Fe, Co, Cu, Zn, and Cd; X = O, S, and Se). The optical reflectivity measurements for 0.1-30 eV revealed a wide variation in electronic structures on a large energy scale between oxides (X = O) and chalcogenides (X = S and Se). For A = Fe and Co, we observed the intra-atomic d-d transitions of A2+ ions with a tetrahedral coordination, and successfully deduced the crystal field splitting ΔE, the Racah parameter B, and the spin-orbit coupling constant ζ by analysis based on the ligand field theory. A comparison of these optical parameters between oxides and chalcogenides indicated the strong covalency effect in the chalcogenides. In A = Cu, the insulator-metal transition between X = O and Se was clearly demonstrated by optical conductivity spectra. Magnetic properties were discussed in relation to electronic structures. A compound with a small optical gap is typically a ferrimagnet with antiparallel arrangements of A2+ and Cr3+ spins, whereas a compound with a large optical gap undergoes first-order phase transition into spiral spin ordering at a low temperature. We found that the magnetic anisotropy constants K1 for ACr2S4 (A = Mn, Fe, and Co) are approximately scaled by the inverse of the intra-atomic d-d transition energies of A2+ ions in agreement with the second-order perturbation theory for single-ion anisotropy. The magnetooptical spectra in a wide energy range (0.2-4.5 eV) were measured for chalcogenides focusing on the d-d transition resonance. We observed gigantic magnetooptical signals up to 4.1° in the energy range of 4A2 → 4T2 and 4A2 → 4T1 transitions of Co2+ ions for CoCr 2S4, and analyzed them in the framework of the ligand field theory. We propose that the strong covalency of the ligand sulfur, as well as the local breakdown of inversion symmetry, in the tetrahedral site plays a crucial role in the enhancement of magnetooptical responses. 2008 The Physical Society of Japan.

Chromite Spinel materials were synthesized in this study by the citrate precursor method using four divalent cations (Ni2+, Co2+, Zn2+, and Cu2+). Citrate precursors consisting of mixed chromium citrates were first subjected to a thermogravimetric (TG) analysis for determining optimum temperatures for annealing. TGof coprecipitated chromium(III) citrate-zinc citrate gel has been carri ed out separately in N 2 and O2 atmospheres. In both the cases, dehydration is followed by a four-step decomposition. The TG data were subjected to kinetic/mechanistic analysis, and the values of activation energy and Arrhenius factor were approximated. TG curves ofvarious powders which were obtained on annealing at the two temperature s did exhibit thermal instability when carried out in N2 atmosphere. A large coercivity of 2701.01 Oe was observed for NiCr2O4 at 650 °C. On the basis of the results, 450 °C has been chosen for annealing treatment of the four gels. The samples were accordingly annealed at two different temperatures (450 and 650 °C) in a muffle furnace for 1 h in each case. The annealed powders were characterized using X-ray diffraction (XRD), SEM, and vibrating sample magnetometer (VSM). The XRD patterns show that annealing of CuCr2O4, NiCr 2O4, and CoCr2O4 at 450 °C yields very small crystallites with poor Bragg reflections, although ZnCr 2O4 samples show better peaks in XRD data. Annealing at 650 °C resulted in particle size range of 8-89 nm in the four cases. In the case of ZnCr2O4, the particle size was 8 nm.

The new vanadate Co5Cr2.667(VO4) 6 has been prepared by solid-phase synthesis at 670°C. The structural parameters of this compound were refined: space group Pnma, Z = 2, a = 4.9752(1) A?, b = 10.1543(2) A?, and c = 17.1255(3) A?, R wp = 7.94% (S = 2.03), Rp, = 5.95%, RB = 2.54%, and RF = 1.69%. The compound Co5Cr 2.667(VO4)6 is isostructural with the mineral lyonsite. The structure of Co5Cr2.667(VO4) 6 has three positions - M1, M2, and M3 - occupied by Co2+ and Cr3+ cations. The occupancy of the M3 position is 5/6; M1 and M2 are fully occupied. The M3O6 octahedra share faces to form infinite chains along the [100] direction. The M1O6 polyhedra are trigonal prisms that share edges and form infinite chains along the [100] direction. The M2O6 octahedra also share edges to form chains along the [100] direction. The chains of the M(2)O6 octahedra share vertices to form planes normal to the [001] direction. Two models of Co2+ and Cr 3+ distribution over M1-M3 positions are discussed.

In solid state emf measurements on sluggish systems, difficulties to achieve stable and reproducible values have been encountered. There are many indications that these problems are owing to the small, but obviously not negligible, transport of oxygen through the electrolyte. In the present work, an improved galvanic cell arrangement, here called the 'zero point' method, has been developed. The principles of this method are to approach zero emf for the sample cell, containing a sluggish system, by an adjustable reference gas phase, and determine the oxygen partial pressure in the reference gas phase, initially made up of a H2/CO2 mixture, by a reference cell, containing a fast reacting metal-metaloxide equilibrium system, which is unaffected by the oxygen permeation.

The decomposition of propan-2-ol has been investigated over copper, zinc and cobalt chromite catalysts in the vapour phase in a flow reactor.The decomposition of propan-2-ol to acetone follows a first order kinetics.The kinetic data obtained on zinc oxide are also included to show the better performance of the spinel compared to simple oxide.The order of reactivity for the decomposition shows the following order: cobalt chromite zinc chromite copper chromite.Zinc and cobalt chromites are more active when they are pretreated in oxygen than in hydrogen.Over zinc chromite the desorption of acetone is found to be the rate controlling step.However, over cobalt chromite the desorption of hydrogen is what determines the rate.

Decomposition of isopropanol has been studied over chromite spinels of the type MCr2O4 (M = Zn, Co and Cu) in the vapour phase in a flow reactor.The decomposition follows first order kinetics.Kinetic and thermodynamic parameters have been calculated for the reaction.The performances of these catalysts have been compared and explained in terms of the entropy of activation.A linear correlation is obtained between activation energy for electrical conduction and entropy of activation.