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1.
The “114” YBaCo 4O 7 cobaltite undergoes structural transition just beyond room temperature at TS∼310 K. Correspondingly, its signature in the physical properties is detected by T-dependent measurements of electrical resistivity, magnetic susceptibility and thermoelectric power. It is found that low-level substitutions of divalent ( M=Zn 2+) or trivalent ( M=Ga 3+, Al 3+) cations for cobalt according to the YBaCo 4−xMxO 7 formula with x?0.4 have a strong impact upon this transition. On the one hand, Zn 2+ substitutions preserve the transition but with TS decreasing as x increases. On the other hand, for x=0.2 Ga 3+ or Al 3+, the transition is suppressed, i.e., for only 5% trivalent foreign cation substituted for cobalt. Though at first, this contrasted behaviour between divalent and trivalent substituting cations appears to be linked to the opposite evolution of hole carriers “Co 3+” concentration with x, a possible destabilization of 3Co 2+: 1Co 3+ charge ordering induced by the M3+ cations is considered. 相似文献
2.
(Pr 0.7Sm 0.3) 0.7Ca 0.3CoO 3 belongs to a class of cobalt oxides undergoing a first-order transition ( T* ≈90 K) associated to a coupled change in the valence and spin-state degrees of freedom. The Curie–Weiss regime present around room temperature ( T >> T*) was analyzed in detail to address the controversial issue of the cobalt spin states above the transition. This magnetic investigation indicates that the Co 4+ are in an intermediate spin-state, while the Co 3+ are in a mixed state combining low-spin and high-spin states. These results are discussed with respect to the literature on related compounds and recent results of X-ray absorption spectroscopy. 相似文献
3.
Paracrystalline array of defect clusters ca. five times the lattice spacing of the average Co 3−δO 4 spinel structure occurred more or less in a relaxed manner when the sintered Co 1−xO polycrystals were air-quenched below the Co 1−xO/Co 3−δO 4 transition temperature to activate oxy-precipitation of cube-like Co 3−δO 4 at dislocations. The same paracrystalline spacing was obtained for Co 3−δO 4 when formed via oxidizing/sintering the Co 1−xO powders at 800°C in air, suggesting a nearly constant δ value for Co 3−δO 4 in the T- PO2 conditions encountered. The extra cobalt vacancies and Co 3+ interstitials, as a result of δ value, may form additional 4:1-derived defect clusters for further paracrystalline distribution in the spinel lattice. The nanosize defect clusters self-assembled by columbic interactions and lattice relaxation in ionic crystal may have potential applications as step-wise sensor of oxygen partial pressure at high temperatures. 相似文献
4.
The effects of doping cobalt oxides with different amounts of ZrO 2 and ThO 2 (1.5–9 mol%) on the thermal stability of Co 3O 4 and the re-oxidation of CoO by O 2 to Co 3O 4 were investigated. The techniques employed were DTA, with a controlled rate of heating and cooling, X-ray diffraction, and IR spectrometry.The results obtained by DTA revealed that the addition of both Th 4+ and Zr 4+ (up to 6 mol%) exerted no appreciable effect on the thermal stability of Co 3O 4. Increasing the amount of the dopant ions to 9% resulted in no further change in the thermal stability of Co 3O 4 in the case of Th 4+, and an increase of 16% in case of Zr 4+-doping. However, ThO 2-doping of cobalt oxide was accompanied by an enhancement in the reactivity of CoO towards re-oxidation by O 2 to Co 3O 4 to an extent proportional to the amount of dopant oxide.The X-ray investigation of ZrO 2-doped cobalt oxides calcined in air at 1000°C revealed the presence of highly crystalline and stable zirconia in the cubic form. Such a stable phase could not be obtained at temperatures below 2370°C in the absence of stabilizing agents.X-ray and IR investigations of different solids showed the presence of free thoria and zirconia together with new thorium—cobalt and zirconium—cobalt compounds. However, the slow cooling of Zr-treated cobalt oxides from 1000°C to room temperature led to the decomposition of the newly formed compound. The d-spacings and absorption bands of the newly formed compounds were determined. 相似文献
5.
The nanocomposites based on cobalt oxide and nitrogen-doped carbon nanofibers (N-CNFs) with cobalt oxide contents of 10–90 wt% were examined as catalysts in the CO oxidation and supercapacity electrodes. Depending on Со3О4 content, such nanocomposites have different morphologies of cobalt oxide nanoparticles, distributions over the bulk, and ratios of Со3+/Co2+ cations. The 90%Со3О4-N-CNFs nanocomposite showed the best activity because of the increased concentration of defects in N-CNFs. The capacitance of electrodes containing 10%Со3О4-N-CNFs was 95 F/g, which is 1.7 times higher than electrodes made from N-CNFs. 相似文献
6.
The nature of the alkaline electrolyte (based on KOH, NaOH, LiOH), in which Co 3O 4 spinel type phases are synthesized by electrooxidation of CoO, is shown to play a key role on the composition, the structure and the electronic conductivity of the materials. In the materials, prepared in pure LiOH electrolyte or in mixed ternary electrolyte (KOH, NaOH, LiOH), Co 4+ ions are present in the octahedral framework, which entails electronic delocalization in the cobalt T2g band and a high conductivity. The structure of the sample, synthesized in KOH, is on the opposite closer to that of ideal Co 3O 4, with only Co 3+ in the octahedral sublattice, which leads to a semi-conducting behavior. Whatever the initial material, a thermal treatment induces an increase of the Co 4+/Co 3+ ratio in the octahedral network, resulting in a significant increase of the electronic conductivity. 相似文献
7.
The heteroleptic neutral tri‐ tert‐butoxysilanethiolate of cobalt(II) incorporating ammonia as additional ligand ( 1 ) has been prepared by the reaction of a cobalt(II) ammine complex with tri‐ tert‐butoxysilanethiol in water. Complex 1 , dissolved in hexane, undergoes oxidation in an ammonia saturated atmosphere to the ionic cobalt(III) compound 2 . Molecular and crystal structures of 1 and 2 have been determined by single crystal X‐ray structural analysis. 1 forms a dimeric molecule [Co{μ‐SSi(OBu t) 3}{SSi(OBu t) 3}(NH 3)] 2 with a folded central Co 2S 2 ring and distorted tetrahedral ligand arrangement at both Co II atoms (CoNS 3 core). The product 2 is composed of the octahedral Co III complex cation [Co{SSi(OBu t) 3} 2(NH 3) 4] + and the tri‐ tert‐butoxysilanethiolate anion. Within the crystal two pairs of ions interact by hydrogen bonds forming well separated entities. 1 and 2 are the first structurally characterized cobalt thiolates where metal is also bonded to ammonia and 2 is the first cobalt(III) silanethiolate. 相似文献
8.
The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe 0.5Co 0.5O 3 prepared using electrochemical oxidation. SrFe 0.5Co 0.5O 3 exhibits a second order paramagnetic-ferromagnetic transition close to room temperature ( TC=330 K). The maximal magnetic entropy change Δ SMMax , the maximal adiabatic temperature change Δ Tad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T. 相似文献
9.
以十六烷基三甲基溴化胺(CTAB)为模板剂,通过调变CTAB浓度水热合成了氧化钴前驱体,焙烧制得棒状形貌的Co 3O 4,在其表面浸渍K 2CO 3溶液制得K改性的Co 3O 4催化剂,用于N 2O分解。用X射线衍射(XRD)、N 2物理吸附(BET)、扫描电镜(SEM)、X射线光电子能谱(XPS)、H 2程序升温还原(H 2-TPR)和O 2程序升温脱附(O 2-TPD)等技术对催化剂进行了表征,考察了CTAB/钴及尿素/钴物质的量比等制备参数对Co 3O 4催化分解N 2O活性的影响。结果表明,CTAB浓度为0.05 mol/L、CTAB/钴离子物质的量比为1、尿素/钴离子物质的量比为4时,所制备的Co 3O 4催化剂具有较高的N 2O分解活性,而K改性可以进一步提升其催化性能。K改性的Co 3O 4在有氧有水气氛中400℃下进行N 2O分解反应,50 h后N 2O转化率仍保持在91%以上。 相似文献
10.
The axial single-crystal magnetic circular dichroism spectra of Cs 3ZnCl 5/Co 2+ and Cs 3ZnBr 5/Co 2+ have been measured over the 4000–7000 cm ?1 region of the 4A 2 → 4T 1 (F) transition at ambient and liquid-helium temperature. The B/D and C/D terms obtained give transition moment ratios, 〈t 2∥ m∥t 2〉/〈e∥ m∥t 2〉, in accord with the value required (?3 /2 ) by a dynamic ligand-polarisation model for d-electron transition probabilities in tetrahedral metal complexes. 相似文献
11.
In acetate buffer media (pH 4.5–5.4) thiosulfate ion (S 2O 32?) reduces the bridged superoxo complex, [(NH 3) 4Co III(μ‐NH 2,μ‐O 2)Co III(NH 3) 4] 4+ ( 1 ) to its corresponding μ‐peroxo product, [(NH 3) 4Co III(μ‐NH 2,μ‐O 2)Co III(NH 3) 4] 3+ ( 2 ) and along a parallel reaction path, simultaneously S 2O 32? reacts with 1 to produce the substituted μ‐thiosulfato‐μ‐superoxo complex, [(NH 3) 4Co III(μ‐S 2O 3,μ‐O 2)Co III(NH 3) 4] 3+ ( 3 ). The formation of μ‐thiosulfato‐μ‐superoxo complex ( 3 ) appears as a precipitate which on being subjected to FTIR shows absorption peaks that support the presence of Co(III)‐bound S‐coordinated S 2O 32? group. In reaction media, 3 readily dissolves to further react with S 2O 32? to produce μ‐thiosulfato‐μ‐peroxo product, [(NH 3) 4Co III(μ‐S 2O 3,μ‐O 2)Co III(NH 3) 4] 2+ ( 4 ). The observed rate ( k0) increases with an increase in [T Thio] ([T Thio] is the analytical concentration of S 2O 32?) and temperature ( T), but it decreases with an increase in [H +] and the ionic strength ( I). Analysis of the log At versus time data ( A is the absorbance of 1 at time t) reveals that overall the reaction follows a biphasic consecutive reaction path with rate constants k1 and k2 and the change of absorbance is equal to { a1 exp(– k1t) + a2 exp(– k2t)}, where k1 > k2. 相似文献
12.
Iron and its binary oxides are meticulously exploited for environmental remediations. However, only limited studies have been carried out on the degradation of industrial organics by advanced oxidation process. In this study, iron oxide, cobalt oxide, and iron–cobalt binary oxides were synthesized by a modified hydrothermal method as heterogeneous Fenton-like catalysts for the removal of methylene blue (MB) from wastewaters. The oxide nanostructures were characterized by different analytical techniques. Studying the effects of various parameters such as catalyst dose, MB concentration, and H 2O 2 concentration, the reaction conditions were optimized to enhance the removal of MB dye. The results revealed that α-Fe 2O 3–Co 3O 4 shows much higher activity than both Co 3O 4 and α-Fe 2O 3 for the degradation of MB at room temperature and beyond. The binary α-Fe 2O 3–Co 3O 4 shows degradation efficiency of 96.4% at 65 °C within 60 min. Furthermore, the binary α-Fe 2O 3–Co 3O 4 catalyst retains its activity for up to four successive cycles. A probable mechanism is also proposed, involving the generation of ‧OH radical as well as Fe 2+/Fe 3+ or Co 2+/Co 3+ redox couple of the binary α-Fe 2O 3–Co 3O 4 catalyst. 相似文献
13.
Only rarely have polyoxometalates been found to form core–shell nanoclusters. Here, we succeeded in isolating a series of rare giant and all-inorganic core–shell cobalt polyoxoniobates (Co−PONbs) with diverse shapes, nuclearities and original topologies, including 50-nuclearity {Co 12Nb 38O 132}, 54-nuclearity {Co 20Nb 34O 128}, 62-nuclearity {Co 26Nb 36O 140} and 87-nuclearity {Co 33Nb 54O 188}. They are the largest Co−PONbs and also the polyoxometalates containing the greatest number of Co ions and the largest cobalt clusters known thus far. These molecular Co−PONbs have intriguing and atomically precise core–shell architectures comprising unique cobalt oxide cores and niobate oxide shells. In particular, the encapsulated cobalt oxide cores with different nuclearities have identical compositions, structures and mixed-valence Co 3+/Co 2+ states as the different sized Co−O moieties of the bulk cubic-spinel Co 3O 4, suggesting that they can serve as various molecular models of the cubic-spinel Co 3O 4. The successful construction of the series of the Co−PONbs reveals a feasible and versatile synthetic method for making rare core–shell heterometallic PONbs. Further, these new-type core–shell bimetal species are promising cluster molecular catalysts for visible-light-driven CO 2 reduction. 相似文献
14.
The partial substitution of Co by Rh in the [Pb 0⋅7Co 0.4Sr 1.9O 3] RS[CoO 2] 1.8 family has been investigated. By transmission electron microscopy and X-ray powder diffraction, it is shown that the substitution of Rh for Co takes place at the two cobalt sites of the structure but for the low enough Rh contents, this substitution is made preferentially at the level of the CdI 2-like layer. Thus, a generic formula [Pb 0.7(Co 0.4−zRh z)Sr 1.9O 3] RS[Co 1−yRh yO 2] b1/b2 (0? y?0.5 and 0? z?0.3) can be proposed for this new family of misfit phase. As observed for the pure misfit cobaltite, the thermoelectric power is also very large, close to +140 μV/K at room temperature. The Rh cation can adopt a mixed valency Rh 3+/Rh 4+ (4 d6/4 d5) with low spin states t2g6/ t2g5 equivalent to the ones of low spin Co 3+/Co 4+ (3 d6/3 d5). The large thermopower observed in the Rh substituted compounds is therefore a direct proof that the coexistence of low spin states t2g6/ t2g5 contributes to the thermoelectric power enhancement in these oxides. 相似文献
15.
The specific conductivity logσ
T
and thermal electromotive force coefficient Q
T
of perovskite-like lanthanum cobaltite were measured over the temperature range 1173 K ≤ T ≤ 1323 K depending on the partial pressure of oxygen (10−8 atm ≤ p
O
2 ≤ 1 atm). The p
O
2-T-δ phase diagram of solid LaCoO3-δ studied earlier was used to recalculate the electric property data into the log σ
T
-δ and Q
T
-δ dependences. The isoconcentration temperature dependences of specific conductivity (logσ)δ-1/T and Seebeck coefficient Q
δ-1/T for LaCoO3-δ were analyzed. The conclusion was drawn that charge transfer was activation in character and had a low activation energy. This was evidence that small-radius polarons (Co
•Co
and Co
′Co
) were responsible for charge transfer in perovskite-like lanthanum cobaltite. The calculated equilibrium constants for the formation of electronic defects and functional dependences of defect concentration [Co
•Co
]
T
= f
1(δ) and [Co
′Co
]
T
= f
2(δ) were used to analyze polaron transfer parameters in LaCoO3-δ. 相似文献
16.
By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co 3O 4, MgCo 2O 4, Co 2FeO 4, Co 2AlO 4 and CoFe 2O 4. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H 2O 2 decomposition. Studying the effect of dominant surface termination, isotropic Co 3O 4 and CoFe 2O 4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co 3+ plays the major role for high activity. In H 2O 2 decomposition, Co 2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction. 相似文献
17.
Poly(vinylamme) complexes with cobalt chloride hexahydrate exhibit the largest glass transition temperature enhancements that have been measured in this laboratory. T g increases from 56°C in the undiluted polymer to 193°C when the molar concentration of cobalt is only 3%, based on the moles of polymeric repeat units. This translates to a 45°C enhancement in T g per mol% cobalt. Higher glass transition temperatures have been measured for other polymeric complexes with d-block salts, but this study reports the largest ΔT g=T g(complex)-T g(undiluted polymer) when transition-metal catalyzed chemical crosslinking reactions do not occur. A plausible explanation for this effect is based on the fact that cobalt chloride hexahydrate maintains pseudo-octahedral symmetry in the undiluted crystalline state and in an amorphous glassy complex with the polymer. The lone pair on nitrogen in the amino sidegroup is a strong base and, most likely, displaces all four waters of hydration that coordinate directly to the metal center. In fact, estimates of the ligand field splitting and the ligand field stabilization energy for octahedral cobalt complexes that contain methyl amine (CH 3NH 2) model ligands suggest that five or six amino sidegroups from several different chains might coordinate to each metal center, drastically reducing the mobility of the polymer. Both chloride anions remain near each divalent Co 2+ center, but either one or two Cl − might not reside in the first-shell coordination sphere of the transition metal. This bonding picture suggests that octahedral cobalt acts as a multi-functional bridge between five or six amino sidegroups. Empirical ligand field stabilization calculations support the concept that octahedral Co 2+ is a multi-functional coordination crosslinking agent, and the inorganic literature provides experimental verification that similar small-molecule complexes (i.e., [Co(NH 3) 6] 2+) exist. It is proposed that the onset of T g occurs when sufficient thermal energy is supplied to remove either x or (x-1) amino sidegroup ligands from the coordination sphere of cobalt, where x is either 5 or 6. This is an endothermic process based on bond energies, as well as calculations which focus on the relative energies of the metal d-electrons. 相似文献
18.
Two new transition metal dicyanamide complexes [Co 2(tppz)(dca) 4]·CH 3CN ( 1 ) [tppz=tetra(2‐pyridyl)pyrazine, dca=dicyanamide] and [Co(tptz)(dca)(H 2O)](dca) ( 2 ) [tptz=2,4,6‐tri(2‐pyridyl)‐1,3,5‐triazine] were synthesized and characterized by single crystal X‐ray diffraction analysis. In 1 each cobalt(II) atom is coordinated to three dca anions and one tppz molecule to form a distorted octahedral geometry, the neigbour two cobalt(II) atoms are bridged by one tppz ligand to form a dimer, then the cobalt(II) atoms in each dimer are joined together to form a ladder chain structure. In 2 the coordination geometry around the central metal is also distorted octahedral, each cobalt(II) atom is coordinated by two dca anions, one tptz molecule and one water ligand to form a cationic part, and the cationic part is linked with the free dca anions via the electrostatic attraction to give an infinite chain structure. Magnetic susceptibility measurement in the range of 2–300 K indicates that there are antiferromagnetic couplings between adjacent metal ions in 1 ( T>29 K, (=?9.78 K, C=4.92 cm 3·K·mol ?1) and ferromagnetic couplings in 2 ( T>150 K, (=7.97 K, C=2.59 cm 3·K·mol ?1) respectively. 相似文献
19.
Photolytic decomposition of the complex [Co(NH 3) 5(H 2O)](ClO 4) 3 under the action of a laser with a wavelength of 355 nm, which is resonant in energy to the energy of the ( 1 A 1g → 1 T 2g ) d–d transition, was studied. Decomposition of the complex is accompanied by a release of ammonia with its subsequent oxidation to nitrogen oxides and by partial cobalt reduction with the formation of the mixed cobalt(II, III) oxide Co 3O 4. 相似文献
20.
During precipitation and calcination at 200°C nanocrystalline Co 3O 4 was obtained with average size crystallites of 13 nm and a well developed specific surface area of 44 m 2 g ?1. A small addition of a structural promoter, e.g. Al 2O 3, increases the specific surface area of the cobalt oxide (54 m 2 g ?1) and decreases the average size of crystallites (7 nm). Al 2O 3 inhibits the reduction process of Co 3O 4 by hydrogen. Reduction of cobalt oxide with aluminium oxide addition runs by equilibrium state at all the respective temperatures. The apparent activation energy of the recrystallization process of the nanocrystalline cobalt promoted by the aluminium oxide is 85 kJ mol ?1. Aluminium oxide improves the thermostability of both cobalt oxide and the cobalt obtained as a result of oxide phase reduction. 相似文献
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