Suppression of the reversible thermal behavior of the layered double hydroxide (LDH) of Mg with Al: stabilization of nanoparticulate oxides

The layered double hydroxide of Mg with Al decomposes below 600 degrees C with the loss of nearly 48% mass, resulting in the formation of an oxide residue having the rock salt structure and nanoparticulate morphology. However, this product reconstructs back into the parent LDH, owing to its compositional and morphological metastability. The oxide can be kinetically stabilized within an amorphous phosphate network built up through an ex situ reaction with a suitable phosphate source such as (NH4)H2PO4. This oxide transforms into a thermodynamically more stable phase with a spinel structure on soaking in an aqueous medium. The oxide residue has a nanoparticulate morphology as revealed by the Scherrer broadening of the Bragg reflections as well as by electron microscopy. This work shows that the hydroxide reconstruction reaction and spinel formation are competing reactions. Suppression of the former catalyzes spinel formation as the excess free energy of the metastable oxide residue is unlocked to promote the diffusion of Mg2+ ions from octahedral to tetrahedral sites, which is the essential precondition to the formation of a normal spinel. This reaction taking place as it does at ambient temperature and in solution helps in the retention of a nanostructured morphology for the spinel. Another way of stabilizing the oxide is by incorporating the thermally stable borate anion into the LDH. This paves the way for an in situ reaction between the cations of the host LDH and the borate guest. The in situ reaction directly leads to the formation of an oxide with a spinel structure.     

Topochemical synthesis of co-fe layered double hydroxides at varied fe/co ratios: unique intercalation of triiodide and its profound effect

Co-Fe layered double hydroxides at different Fe/Co ratios were synthesized from brucite-like Co(2+)(1-x)Fe(2+)(x)(OH)(2) (0 ≤ x ≤ 1/3) via oxidative intercalation reaction using an excess amount of iodine as the oxidizing agent. A new redoxable species: triiodide (I(3)(-)), promoted the formation of single-phase Co-Fe LDHs. The results point to a general principle that LDHs with a characteristic ratio of total trivalent and divalent cations (M(3+)/M(2+)) at 1/2 may be the most stable in the oxidative intercalation procedure. At low Fe content, e.g., starting from Co(2+)(1-x)Fe(2+)(x)(OH)(2) (x < 1/3), partial oxidation of Co(2+) to Co(3+) takes place to reach the M(3+)/M(2+) threshold of 1/2 in as-transformed Co(2+)(2/3)-(Co(3+)(1/3-x)-Fe(3+)(x)) LDHs. Also discovered was the cointercalation of triiodide and iodide into the interlayer gallery of as-transformed LDH phase, which profoundly impacted the relative intensity ratio of basal Bragg peaks as a consequence of the significant X-ray scattering power of triiodide. In combination with XRD simulation, the LDH structure model was constructed by considering both the host layer composition/charge and the arrangement of interlayer triiodide/iodide. The work provides a clear understanding of the thermodynamic and kinetic factors associated with the oxidative intercalation reaction and is helpful in elucidating the formation of LDH structure in general.     

New insights into the intercalation chemistry of Al(OH)3

This paper reports a number of recent developments in the intercalation chemistry of Al(OH)(3). From Rietveld refinement and solid-state NMR, it has been possible to develop a structural model for the recently reported [M(II)Al(4)(OH)(12)](NO(3))(2)·yH(2)O family of layered double hydroxides (LDHs). The M(2+) cations occupy half of the octahedral holes in the Al(OH)(3) layers, and it is thought that there is complete ordering of the metal ions while the interlayer nitrate anions are highly disordered. Filling the remainder of the octahedral holes in the layers proved impossible. While the intercalation of Li salts into Al(OH)(3) is facile, it was found that the intercalation of M(II) salts is much more capricious. Only with Co, Ni, Cu, and Zn nitrates and Zn sulfate were phase-pure LDHs produced. In other cases, there is either no reaction or a phase believed to be an LDH forms concomitantly with impurity phases. Reacting Al(OH)(3) with mixtures of M(II) salts can lead to the production of three-metal M(II)-M(II)'-Al LDHs, but it is necessary to control precisely the starting ratios of the two M(II) salts in the reaction gel because Al(OH)(3) displays selective intercalation of M nitrate (Li > Ni > Co ≈ Zn). The three-metal M(II)-M(II)'-Al LDHs exhibit facile ion exchange intercalation, which has been investigated in the first energy dispersive X-ray diffraction study of a chemical reaction system performed on Beamline I12 of the Diamond Light Source.     

Polytypism, disorder, and anion exchange properties of divalent ion (Zn, Co) containing bayerite-derived layered double hydroxides

Incorporation of Zn(2+) into bayerite results in the formation of a cation-ordered layered double hydroxide (LDH) of monoclinic symmetry in which about half the vacancies of Al(OH)(3) are occupied by Zn(2+) giving rise to positively charged layers. Charge compensation takes place by the incorporation of sulfate ions in the interlayer region. Structure refinement reveals that the adjacent layers in the crystal are related by a 2(1) axis (we call it 2M(1) polytype) with sulfate coordinating in the D(2d) symmetry in the interlayer region. Another polytype in which adjacent layers are related by a 2-fold axis (2M(2) polytype) can also be envisaged. Faulted crystals arising from intergrowths of the 2M(2) polytype within the 2M(1) structure were also obtained. These bayerite-based LDHs have a distinctly different interlayer chemistry when compared to the better known brucite-based LDHs, in that they have a strong affinity for tetrahedral ions such as SO(4)(2-), CrO(4)(2-), and MoO(4)(2-) and a poor affinity for CO(3)(2-) ions. These observations have implications for the use of LDHs in applications related to chromate sorption.     

Shifting Oxygen Charge Towards Octahedral Metal: A Way to Promote Water Oxidation on Cobalt Spinel Oxides

Cobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox‐active building block Co?O. The charge of oxygen anion tends to shift toward the octahedral‐occupied Co instead of tetrahedral‐occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox‐active metal center in CoO₆ octahedra.     

Mixed oxides obtained from Co and Mn containing layered double hydroxides: Preparation, characterization, and catalytic properties

Co-Mn-Al layered double hydroxides (LDHs) with various Co:Mn:Al molar ratios (4:2:0, 4:1.5:0.5, 4:1:1, 4:0.5:1.5, and 4:0:2) were prepared and characterized. Magnesium containing LDHs Co-Mg-Mn (2:2:2), Co-Mg-Mn-Al (2:2:1:1), and Co-Mg-Al (2:2:2) were also studied. Thermal decomposition of prepared LDHs and formation of related mixed oxides were studied using high-temperature X-ray powder diffraction and thermal analysis. The thermal decomposition of Mg-free LDHs starts by their partial dehydration accompanied by shrinkage of the lattice parameter c from ca. 0.76 to 0.66 nm. The dehydration temperature of the Co-Mn-Al LDHs decreases with increasing Mn content from 180 °C in Co-Al sample to 120 °C in sample with Co:Mn:Al molar ratio of 4:1.5:0.5. A subsequent step is a complete decomposition of the layered structure to nanocrystalline spinel, the complete dehydration, and finally decarbonation of the mixed oxide phase. Spinel-type oxides were the primary crystallization products. Mg-containing primary spinels had practically empty tetrahedral cationic sites. A dramatic increase of the spinel cell size upon heating and analysis by Raman spectroscopy revealed a segregation of Co-rich spinel in Co-Mn and Co-Mn-Al specimens. In calcination products obtained at 500 °C, the spinel mean coherence length was 5-10 nm, and the total content of the X-ray diffraction crystalline portion was 50-90%. These calcination products were tested as catalysts in the total oxidation of ethanol and decomposition of N₂O. The catalytic activity in ethanol combustion was enhanced by increasing (Co+Mn) content while an optimum content of reducible components was necessary for high activity in N₂O decomposition, where the highest conversions were found for calcined Co-Mn-Al sample with Co:Mn:Al molar ratio of 4:1:1.     

Periodic DFT study of the structural and electronic properties of bulk CoAl2O4 spinel

In this study, structural and electronic properties of CoAl2O4 spinel are investigated for the first time by means of quantum chemical computational tools. Coupling supercell periodic calculations under the density functional theory formalism with a nonempirical quasi-harmonic Debye model, we examine the influence of temperature on the relative stability of several cation distributions of Co2+ and Al3+ over tetrahedral and octahedral interstices of the oxygen sublattice. Our simulations are able to reproduce the experimentally observed trend: (i) the normal spinel is calculated to be the stable structure at static and low-temperature conditions, and (ii) as the temperature increases, the preference of structures with Al3+ at tetrahedral sites (and Co2+ at octahedral sites) is found to progress following an asymptotic conduct. The effects of the cation distributions on geometrical variations of electronic and magnetic properties of CoAl2O4 can be interpreted as dominated by the local behavior of Co2+ at octahedral sites.     

Dinuclear [Co(II)(NCMe)5Co(II)(NCS)4] possessing octahedral and tetrahedral Co(II) sites

The structural and magnetic properties of dinuclear [Co(II)(NCMe)(5)Co(II)(NCS)(4)]·MeCN have been investigated. The structure consists of an octahedral Co(II)(NCMe)(5) center connected to a tetrahedral Co(II)(NCS)(4) center bridged by a μ(1,3)-NCS(-) ligand. The bridging NCS(-) weakly couples the pair of S = (3)/(2) Co(II) spin sites, as evidenced by the magnetic data being best fit by the Curie-Weiss expression with θ = -15.5 K.     

Tyrrellite,Cu(Co0.68Ni0.32)2Se4, isostructural with spinel

Tyrrellite, a naturally occurring Co–Ni–Cu selenide, has been studied by single‐crystal X‐ray diffraction. It possesses the normal spinel‐type structure, with Cu occupying the tetrahedral site and (Co+Ni) the octahedral site. The average Cu—Se distance of 2.3688 (2) Å is close to that of 2.3703 (8) Å in CuCr₂Se₄, whereas the average (Co+Ni)—Se distance of 2.3840 (1) Å appears to be slightly shorter than most octahedral Co—Se or Ni—Se distances (∼2.40–2.50 Å) in other selenides. The refined structure provides a basis for a redefinition of the ideal chemical formula of tyrrellite, which should be Cu(Co,Ni)₂Se₄, rather than the previously suggested (Cu,Co,Ni)₃Se₄.     

Synthesis and exfoliation of Co2+-Fe3+ layered double hydroxides: an innovative topochemical approach

This paper describes a topochemical synthetic approach to Co2+-Fe3+ layered double hydroxides (LDHs). Micrometer-sized hexagonal platelets of brucite-like Co2/3Fe1/3(OH)2 were first prepared by a homogeneous precipitation of an aqueous solution of divalent cobalt and ferrous ions through hexamethylenetetramine (HMT) hydrolysis under a nitrogen gas atmosphere. A subsequent oxidative intercalation process, by the action of iodine (I2) in chloroform (CHCl3), transformed the precursory brucite-like Co2+-Fe2+ hydroxides into hydrotalcite-like Co2+-Fe3+ LDHs, in which the oxidization of Fe2+ into Fe3+ induced positive charges to the octahedral hydroxyl layers while anions (I-) were intercalated into the interlayer space. Co2+-Fe3+ LDHs inherited the high crystallinity and hexagonal platelet morphology from their brucite-like precursor due to the topotactic nature of the transformation, which was verified by abundant microscopic and spectroscopic characterizations. After a normal ion-exchange process, Co2+-Fe3+ LDHs accommodating perchlorate anions were exfoliated into unilamellar nanosheets in formamide by an ultrasonic treatment.     

利用程序升温还原方法研究钴,钌,钼加氢脱硫催化剂氧化态的还原过程

钴或镍在加氢脱硫(HDS)催化剂中的助剂作用文献已有很多的讨论。近年来,研究结果发现,少量的钌加入到Mo/Al_2O_3和Co-Mo/Al_2O_3催化剂上可以显著地提高其加氢脱硫活性,可是关于钌的助剂作用前人研究的较少。因此对比研究钴和钌的的助剂作用有助于认识各种不同类型的助剂在加氢脱硫催化剂中的功能。本文应用程序升温还原方法对钴和钌在加氢脱硫催化剂的前身态氧化物还原过程中的助剂作用进行了考察。     

Magnetic polyoxometalates: anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.     

Thermal, solution and reductive decomposition of Cu-Al layered double hydroxides into oxide products

Cu-Al layered double hydroxides (LDHs) with [Cu]/[Al] ratio 2 adopt a structure with monoclinic symmetry while that with the ratio 0.25 adopt a structure with orthorhombic symmetry. The poor thermodynamic stability of the Cu-Al LDHs is due in part to the low enthalpies of formation of Cu(OH)₂ and CuCO₃ and in part to the higher solubility of the LDH. Consequently, the Cu-Al LDH can be decomposed thermally (150 °C), hydrothermally (150 °C) and reductively (ascorbic acid, ambient temperature) to yield a variety of oxide products. Thermal decomposition at low (400 °C) temperature yields an X-ray amorphous residue, which reconstructs back to the LDH on soaking in water or standing in the ambient. Solution decomposition under hydrothermal conditions yields tenorite at 150 °C itself. Reductive decomposition yields a composite of Cu₂O and Al(OH)₃, which on alkali-leaching of the latter, leads to the formation of fine particles of Cu₂O (<1 μm).     

Phenolate and phenoxyl radical complexes of Co(II) and Co(III)

The new phenol-imidazole pro-ligands (R)LH react with Co(BF(4))(2).6H(2)O in the presence of Et(3)N to form the corresponding [Co(II)((R)L)(2)] compound (R = Ph (1), PhOMe (2), or Bz (3)). Also, (Bz)LH, reacts with Co(ii) in the presence of Et(3)N and H(2)O(2) to form [Co(III)((Bz)L)(3)](4). The structures of 1.2.5MeCN, 2.2DMF, 3.4MeOH, and 4.4DMF have been determined by X-ray crystallography. 1, 2, and 3 each involve Co(II) bound to two N,O-bidentate ligands with a distorted tetrahedral coordination sphere; 4 involves Co(III) bound to three N,O-bidentate ligands in a mer-N(3)O(3) distorted octahedral geometry. [Co(II)((R)L)(2)](R = Ph or PhOMe) undergo two, one-electron, oxidations. The products of the first oxidation, [1](+) and [2](+), have been synthesised by the chemical oxidation of 1 and 2, respectively; these cations, formulated as [Co(II)((R)L*)((R)L)(2)](+), comprise one phenoxyl radical and one phenolate ligand bound to Co(II) and are the first phenoxyl radical ligand complexes of tetra-coordinated Co(II). 4 undergoes two, one-electron, ligand-based oxidations, the first of which produces [4](+), [Co(III)((Bz)L*)((Bz)L)(2)](+). Unlike [1](+) and [2](+), product of the one-electron oxidation of [Co(II)((Bz)L)(2)], [3](+), is unstable and decomposes to produce [4](+). These studies have demonstrated that the chemical properties of [M(II)((R)L*)((R)L)(2)](+)(M = Co, Cu, Zn) are highly dependent on the nature of both the ligand and the metal centre.     

Solution decomposition of the layered double hydroxide of Co with Fe: phase segregation of normal and inverse spinels

The nitrate-intercalated layered double hydroxide of Co with Fe decomposes on hydrothermal treatment to yield an oxide residue at a temperature as low as 180 degrees C. The oxide product is phase segregated into a Co(3)O(4)-type normal spinel and a CoFe(2)O(4)-type inverse spinel. Phase segregation is facilitated as decomposition in a solution medium takes place by dissolution of the precursor hydroxide followed by reprecipitation of the oxide phases. In contrast, thermal decomposition takes place at 400 degrees C. This temperature is inadequate to induce diffusion in the solid state whereby phase segregation into the thermodynamically stable individual spinels is suppressed. The result is a single-phase metastable mixed spinel oxide. This is rather uncommon in that a hydrothermal treatment yields thermodynamically stable products where as thermal decomposition yields a metastable product.     

Tetrahedral Co(II) coordination in alpha-type cobalt hydroxide: Rietveld refinement and X-ray absorption spectroscopy

We report a Rietveld refinement analysis and X-ray absorption study on a green-color Cl(-)-intercalated alpha-type cobalt hydroxide phase. The refinement clearly demonstrated that one-fifth to one-sixth of the Co(II) at octahedral sites was replaced by pairs of tetrahedrally coordinated Co(II) on each side of the hydroxide plane, represented by a structural formula of [Co(octa)(0.828)Co(tetra)(0.348)(OH)2](0.348+)Cl(0.348).0.456H2O. X-ray absorption spectroscopy also indicated that the divalent cobalt were in local neighboring environments of both octahedral and tetrahedral coordination. Furthermore, UV-vis spectroscopic measurements elucidate the typical green/blue color of an alpha-type cobalt hydroxide.     

Order-disorder transition in the complex lithium spinel Li2CoTi3O8

Li₂CoTi₃O₈ has an ordered Li₂BB′₃O₈ spinel structure, space group P4₃32, at room temperature with 3:1 ordering of Ti and Li on the octahedral sites, and Li, Co disordered over the tetrahedral site. Rietveld refinement of variable temperature neutron powder diffraction data has shown an order-disorder phase transition in Li₂CoTi₃O₈ which commences at ∼500 °C with Li and Co mixing on the tetrahedral and 4-fold octahedral sites and is complete at a first order structural discontinuity at ∼915 °C. The fraction of Ti on the 12-fold octahedral site exhibits a small decrease with increasing temperature, which may suggest that the disordering involves all three cations. Above 930 °C, the structure, space group Fd3¯m, has Li, Co and Ti sharing a single-octahedral site and Li, Co sharing a tetrahedral site, although Co still exhibits a preference for tetrahedral coordination. A labelling scheme for ordered and partially ordered 3:1 spinels is devised which focuses on the occupancy of the Li,B cations.     

Monodisperse MFe2O4 (M = Fe, Co, Mn) nanoparticles

High-temperature solution phase reaction of iron(III) acetylacetonate, Fe(acac)(3), with 1,2-hexadecanediol in the presence of oleic acid and oleylamine leads to monodisperse magnetite (Fe(3)O(4)) nanoparticles. Similarly, reaction of Fe(acac)(3) and Co(acac)(2) or Mn(acac)(2) with the same diol results in monodisperse CoFe(2)O(4) or MnFe(2)O(4) nanoparticles. Particle diameter can be tuned from 3 to 20 nm by varying reaction conditions or by seed-mediated growth. The as-synthesized iron oxide nanoparticles have a cubic spinel structure as characterized by HRTEM, SAED, and XRD. Further, Fe(3)O(4) can be oxidized to Fe(2)O(3), as evidenced by XRD, NEXAFS spectroscopy, and SQUID magnetometry. The hydrophobic nanoparticles can be transformed into hydrophilic ones by adding bipolar surfactants, and aqueous nanoparticle dispersion is readily made. These iron oxide nanoparticles and their dispersions in various media have great potential in magnetic nanodevice and biomagnetic applications.     

CoMn2O4 spinel from a MOF: synthesis, structure and magnetic studies

A hydrothermal reaction of Mn(OAc)(2)·4H(2)O, Co(OAc)(2)·4H(2)O and 1,2,4 benzenetricarboxylic acid at 220 °C for 24 h gives rise to a mixed metal MOF compound, [CoMn(2){C(6)H(3)(COO)(3)}(2)], I. The structure is formed by the connectivity between octahedral CoO(6) and trigonal prism MnO(6) units connected through their vertices forming a Kagome layer, which are pillared by the trimellitate. Magnetic susceptibility studies on the MOF compound indicate a canted anti-ferromagnetic behavior, due to the large antisymmetric DM interaction between the M(2+) ions (M = Mn, Co). Thermal decomposition studies indicate that the MOF compound forms a tetragonal mixed-metal spinel phase, CoMn(2)O(4), with particle sizes in the nano regime at 400 °C. The particle size of the CoMn(2)O(4) can be controlled by varying the decomposition temperature of the parent MOF compound. Magnetic studies of the CoMn(2)O(4) compound suggests that the coercivity and the ferrimagnetic ordering temperatures are dependent on the particle size.     

Speciation of ternary cobalt(II) phenanthroline-silica surface complexes as a function of pH and ligand steric bulk

Co(II) solution species containing 1 equiv of phenanthroline (phen), 2-methyl-1,10-phenanthroline (MMP), or 2,9-dimethyl-1,10-phenanthroline (DMP) ligand formed inner-sphere surface complexes when grafted on silica. The speciation on the silica surface depended on both the pH of the grafting solution and the steric bulk of the ligand. [Co(DMP)](2+) formed tetrahedral surface adducts exclusively, with a 1:1 ligand-Co ratio. These surface adducts were first detectable at pH values above 5.1. [Co(MMP)](2+) and [Co(phen)](2+) formed exclusively octahedral adducts on the surface with a 1:1 ligand-Co ratio at pH values below 5. The [Co(MMP)](2+) complex formed a tetrahedral adduct initially at pH 6 and increasingly as the pH was raised. The [Co(phen)](2+) complex did not produce a comparable tetrahedral surface species under any conditions. Instead, mixtures of octahedral surface species with both 1:1 and 2:1 ligand-Co ratios began to form at pH values above 6. Taken together, the results indicated that the development of tetrahedral stereochemistry was strongly influenced by steric factors in the presence of a nitrogen-donating ligand. All three phenanthroline derivatives promoted surface binding of the Co(II) ion adducts, so that maximal binding occurred at lower pH values than for binding of [Co(H(2)O)(6)](2+), which formed exclusively tetrahedral adducts.