The effect of active oxygen species in nano-ZnCr2O4 spinel oxides for methane catalytic combustion

The nano-ZnCr₂O₄ spinel oxides was synthesized by a ethylene glycol mediated solvothermal method. Catalytic combustion of methane test showed that an excellent activity over nano-ZnCr₂O₄ with T_10% = 300 °C and T_90% = 400 °C. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption-desorption measurements (BET) indicated that a uniform nano-ZnCr₂O₄ spinel oxides particles with the high surface area (96.2 m²g⁻¹) was successfully synthesized. Oxygen temperature programmed desorption (O₂-TPD) profile revealed there were two obvious desorption of oxygen species from nano-ZnCr₂O₄ in the range of 300–400 °C and 500–700 °C. It was clear that the desorption temperature range of the first oxygen species coincided with the methane catalytic combustion temperature. X-ray photoelectron spectroscopy (XPS) analysis exhibited that Cr⁶⁺ was present in the lattice of ZnCr₂O₄ apart from Cr³⁺. High valence cations of chromium in crystal lattice probable caused the presence of interstitial oxygen species in the structure to maintain the electroneutrality. Additionally, Raman spectra proved that there is the interstitial oxygen species in the crystal lattice of ZnCr₂O₄. Therefore, the excellent catalytic activity for methane combustion was contributed to the flexible interstitial oxygen in the ZnCr₂O₄.     

The influence of substituting metals (Ti, V, Cr, Mn, Co and Ni) on the thermal stability of magnetite

In this paper, a systematic study on the influence of substituting metals on the thermal stability of magnetite was carried out. Six series of substituted magnetite (Fe_3?x M _x O₄, M = Ti, V, Cr, Mn, Co and Ni) and Ti–V co-doped magnetite were prepared by a precipitation-oxidation method, followed by the characterization of X-ray diffraction (XRD), X-ray absorption near-edge structure (XANES) spectroscopy and thermogravimetry and differential scanning calorimetry (TG-DSC) analyses. XRD patterns confirmed the formation of samples with spinel structure and XANES probed the valence and site occupancy of the substituting ions. From the TG-DSC analysis results, the substitution of Ti⁴⁺, Mn²⁺, Co²⁺ and Ni²⁺ stabilizes the magnetite structure, while V³⁺ and Cr³⁺ do not show such an effect. For the thermal stability of maghemite, V³⁺ has a negative effect while the other studied ions show a positive effect. In Ti–V co-doped magnetites, the influence of Ti⁴⁺ and V³⁺ on the thermal stability of magnetite is similar to the case of their single-metal-substituted magnetites. The mechanism about the thermal stability change of magnetite by metal substitution was also discussed. The obtained results will be of high importance for the industrial applications of magnetite.     

New germanates RCrGeO5 (R=Nd-Er, Y): Synthesis, structure, and properties

The new complex germanates RCrGeO₅ (R=Nd-Er, Y) have been synthesized and investigated by means of X-ray powder diffraction, electron microscopy, magnetic susceptibility and specific heat measurements. All the compounds are isostructural and crystallize in the orthorhombic symmetry, space group Pbam, and Z=4. The crystal structure of RCrGeO₅, as refined using X-ray powder diffraction data, includes infinite chains built by edge-sharing Cr⁺³O₆ octahedra with two alternating Cr−Cr distances. The chains are combined into a three-dimensional framework by Ge₂O₈ groups consisting of two edge-linked square pyramids oriented in opposite directions. The resulting framework contains pentagonal channels where rare-earth elements are located. Thus, RCrGeO₅ germanates present new examples of RMn₂O₅-type compounds and show ordering of Cr⁺³ and Ge⁺⁴ cations. Electron diffraction as well as high-resolution electron microscopy confirm the structure solution. Magnetic susceptibility data for R=Nd, Sm, and Eu are qualitatively consistent with the presence of isolated 3d (antiferromagnetically coupled Cr⁺³ cations) and 4f (R⁺³) spin subsystems in the RCrGeO₅ compounds. NdCrGeO₅ undergoes long-range magnetic ordering at 2.6 K, while SmCrGeO₅ and EuCrGeO₅ do not show any phase transitions down to 2 K.     

Studies regarding the formation from metal nitrates and diol of NiM 2 III O4 spinels, inside a silica matrix

The present study deals with preparation and characterization of spinel mixed oxide systems NiM ₂ ^III O₄, where M^III?=?Fe^III, Cr^III. In order to obtain 50% NiFe₂O₄/50% SiO₂ and 50% NiCr₂O₄/50% SiO₂ nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO₂ matrix, of some particular precursors, coordination compounds of the involved M^II and M^III cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140?°C of the gels (tetraethylorthosilicate?Cmetal nitrates?C1,3-propanediol?Cwater). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350?°C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe₂O₄ starting with 600?°C, and NiCr₂O₄ starting with 400?°C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300?°C, the spinelic phase ??-Fe₂O₃ is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr₂O_3+x ) is formed. In the range 380?C400?°C, Cr₂O_3+x turns into Cr₂O₃ which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.     

Surface phenomena in conducting materials based on lanthanum chromite

The composition of the surface of a solid solution based on lanthanum chromite, La_0.975Ca_0.025·Cr_0.9Al_0.1O₃, and prepared by solid-phase synthesis followed by vacuum sintering and annealing in air was studied by X-ray photoelectron spectroscopy. The spectral line of chromium differs from the lanthanum line in the X-ray photoelectron spectrum in that the left wing of the chromium line is more gently sloping. The gentle slope of the line wing indicates that the given atom occurs in different charge states. For chromium, it appears possible to resolve the Cr³⁺ and Cr⁶⁺ bands in the X-ray photoelectron spectra, which allows estimation of the Cr³⁺ and Cr⁶⁺ concentrations.     

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 behaviour of cobalt oxides doped with ZrO2 and ThO2 and the possible cubic phase stabilization of zirconia

The effects of doping cobalt oxides with different amounts of ZrO₂ and ThO₂ (1.5–9 mol%) on the thermal stability of Co₃O₄ and the re-oxidation of CoO by O₂ to Co₃O₄ 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 Th⁴⁺ and Zr⁴⁺ (up to 6 mol%) exerted no appreciable effect on the thermal stability of Co₃O₄. Increasing the amount of the dopant ions to 9% resulted in no further change in the thermal stability of Co₃O₄ in the case of Th⁴⁺, and an increase of 16% in case of Zr⁴⁺-doping. However, ThO₂-doping of cobalt oxide was accompanied by an enhancement in the reactivity of CoO towards re-oxidation by O₂ to Co₃O₄ to an extent proportional to the amount of dopant oxide.The X-ray investigation of ZrO₂-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.     

High-temperature thermal behaviour of Cr-Doped LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> spinels synthesized by the sucrose-aided combustion method

Chromium doped spinels LiCr_YMn_2−YO₄ (0.2≤Y≤0.8) has been synthesized by the sucrose-aided combustion procedure. The thermal behaviour, phase homogeneity and structural characteristics of the samples were studied by thermal analysis, coupled mass spectrometry, and room-and high-temperature X-ray diffraction methods. It was found that the ‘as prepared’ samples contained residual organic impurities undetectable for X-ray diffraction, that burn out completely at 400°C. Samples treated between 400 and 750°C are single phase spinels, whose crystallites size increase from 10 to 50 nm on increasing the temperature. Cr-doping enhances the thermal stability of the spinels, which augments on increasing the Cr content Y. The enhanced thermal stability of the spinels has been accounted for based on the high excess stabilization energy of Cr³⁺ in octahedral ligand field.     

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr⁶⁺ ions (solution of potassium dichromate, K₂Cr₂O₇) in a water cathode was studied during a DC discharge in air. The concentration range of Cr⁶⁺ was (5.7–19) ×10^?5 mol/l and discharge current range was 20–80 mA. Cr⁶⁺ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr⁶⁺ kinetics. The main processes of Cr⁶⁺ reduction and Cr³⁺ oxidation were revealed. HO ₂ ^· radicals and hydrogen peroxide were shown to be responsible for Cr⁶⁺ reduction whereas ^·OH radicals and O₂ molecules provide the reverse process of Cr³⁺ oxidation to Cr⁶⁺. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.     

The sol–gel chromium-modified V<Subscript>6</Subscript>O<Subscript>13</Subscript> as a cathodic material for lithium batteries

A new V₆O₁₃-based material has been synthesized via the sol–gel route. This sol–gel mixed oxide has been obtained from an appropriate heat treatment of the chromium-exchanged V₂O₅ xerogel performed under reducing atmosphere. This new compound, with the chemical formula Cr_0.36V₆O_13.50, exhibits a monoclinic structure (C2/m) with the following unit cell parameters, a=11.89 Å, b=3.68 Å, c=10.14 Å, β=101.18°. The electrochemical characterization of this compound has been performed using galvanostatic discharge–charge experiments in the potential range 4–1.5 V and completed by ac impedance spectroscopy measurements. It exhibits a specific capacity of about 370 mAh g^?1, which makes the compound Cr_0.36V₆O_13.50 the best one in the V₆O₁₃-based system: 85% of the initial capacity (315 mAh g^?1) after the 35th cycle is still available at C/25 without any polarization. From impedance spectroscopy, a high kinetics of Li transport (D _Li=1.8×10^?9 cm² s^?1) is found at mid-discharge.     

Local structure distortion and spin Hamiltonian parameters for Cr4+ ions in Cr4+:α-Al2O3 crystals

The relations between the spin Hamiltonian (SH) parameters and crystal structure of Cr⁴⁺:α-Al₂O₃ crystals have been established. On the basis of this, the SH parameters including zero-field splitting parameter D and Zeeman g-factors (g_|| and g_⊥) for Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals, taking into account the spin–spin (SS), spin-other-orbit (SOO) and orbit–orbit (OO) magnetic interactions in addition to the spin–orbit (SO) magnetic interaction, are theoretically investigated using complete diagonalization method (CDM). The theoretical results are in excellent agreement with the experimental ones when the upper three O^2? ions rotate 0.94° toward [1 1 1] axis and the lower three O^2? ions rotate 0.92° toward it. Hence, the local structure distortion effect plays an important role in explaining the spectroscopic properties of Cr⁴⁺ ions in Cr⁴⁺:α-Al₂O₃ crystals. This study shows that for Cr⁴⁺:α-Al₂O₃ the contributions arising from SS, SOO, and OO interactions to the zero-field splitting (ZFS) parameter D are appreciable, whereas those to g_|| and g_⊥ are quite small.     

Synthesis,Crystal Structures,and Properties of (4,4'-Hbipy)2[Sn2(C2O4)3] and (4,4'-Hmbipy)[SnX2] (X = C2O42–, m = 2; X = Cl,m = 0; bipy = bipyridine)

Three new tin coordination compounds (4,4'-Hbipy)₂[Sn₂(C₂O₄)₃] ( 1 ), (4,4'-H₂bipy)[Sn(C₂O₄)₂] ( 2 ), and SnCl₂(4,4'-bipy) ( 3 ) were synthesized under hydro-(solvo-)thermal conditions and their crystal structures were determined by single-crystal X-ray diffraction. Compound 1 exhibits a ionic structure based on discrete [4,4'-Hbipy]⁺ cations and [Sn₂(C₂O₄)₃]^2– anions. These two units are linked via N–H ··· O hydrogen bonds to form a pseudo-one-dimensional zigzag hydrogen-bonded chain. In compound 2 , four-coordinate Sn atoms form monomeric tin dioxalato complexes, which are connected to the doubly protonated [4,4'-H₂bipy]²⁺ cations through N–H ··· O hydrogen bonded to give a one-dimensional zigzag hydrogen-bonded chain. Compound 3 forms a three-dimensional hydrogen-bonded network, in which ¹_∞[SnCl₂(4,4'-bipy)] linear chains are interconnected to each other by C–H ··· Cl hydrogen bonding. The solid-state UV/Vis/NIR diffuse reflectance spectroscopy shows that three compounds are broadband semiconductors. The thermogravimetric analysis evidences the thermal stability of the three compounds up to 175, 201, and 246 °C, respectively.     

时间分辨红外光谱研究Cr价态在La,Cr共掺杂SrTiO3光催化剂中的作用

使用时间分辨红外光谱研究了经H₂和O₂处理的La和Cr共掺杂SrTiO₃的光生电子动力学. X射线光电子能谱和Raman光谱结果表明, H₂处理后样品中的Cr均是+3价, 而O₂处理后Cr为+3和+6价. 使用355和532 nmm激光激发样品所得时间分辨红外光谱表明, 相比在Cr⁶⁺存在时, 光生电子衰减速率在Cr³⁺存在的情况下要慢, 这说明Cr³⁺更有利于抑制光生电子空穴的复合, 从而增加光催化产氢的活性.     

Synthesis,phase formation,and thermal expansion of sulfate phosphates with the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> structure

The NaFeZr(PO₄)₂SO₄ and Pb_2/3FeZr(PO₄)_7/3(SO₄)_2/3 sulfate phosphates with the NaZr₂(PO₄)₃ (NZP) structure were synthesized and studied using X-ray diffraction, electron microprobe analysis, IR spectroscopy, and simultaneous differential thermal and thermogravimetric analysis. The phase formation and thermal stability of the compounds were studied by powder X-ray diffraction and DTA–TG. The Pb_2/3FeZr(PO₄)_7/3(SO₄)_2/3 structure was refined by full-profile analysis. The structure framework is composed of randomly occupied (Fe,Zr)O₆ octahedra and (P,S)O₄ tetrahedra; the Pb²⁺ ions occupy extra-framework sites. The thermal expansion of Pb_2/3FeZr(PO₄)_7/3(SO₄)_2/3 in the temperature range from–120 to 200°C was studied by temperature X-ray diffraction. In terms of the average linear coefficient of thermal expansion (α_av = 1.7 × 10^–6°C^–1), this compound can be classified as having low expansion. The combination of different tetrahedral anions (a phosphorus and a smaller sulfur one) in the NZP resulted in a decrease in the framework size and cavities and enabled the preparation of low-expansion sulfate phosphate with a smaller extra-framework cation (cheap Pb) instead of larger cations (Cs, Ba, Sr) used most often in the monoanionic phosphates.     

Thermal characterization and compatibility studies of perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) oxide with Cr2O3 at high temperature

The chemical compatibility of perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) oxides with Cr₂O₃ has been examined between room temperature and 1,100 °C. Differential thermal analysis and thermogravimetric analysis were used to analyze the thermal behavior of BSCF–Cr₂O₃ binary mixtures in all composition ranges (0–100 mass% BSCF). The reaction products were identified by X-ray analysis after heating at 700–1,100 °C. As we expected, it was found that perovskite-type BSCF oxide had a poor chemical compatibility with the Cr₂O₃ oxide. In particular, the decomposition process of the BSCF–Cr₂O₃ binary mixture is quite complex and it starts at about 700–750 °C. The mixtures of BSCF and Cr₂O₃ oxides reacted forming mixed complex oxides based on (Ba/Sr)FeO₃, (Co/Fe)CrO₄, and (Ba/Sr)CrO₄ mixtures.     

Influence of annealing temperature on the electrochemical and surface properties of the 5-V spinel cathode material LiCr0.2Ni0.4Mn1.4O4 synthesized by a sol–gel technique

LiCr_0.2Ni_0.4Mn_1.4O₄ was synthesized by a sol–gel technique in which tartaric acid was used as oxide precursor. The synthesized powder was annealed at five different temperatures from 600 to 1,000 °C and tested as a 5-V cathode material in Li-ion batteries. The study shows that annealing at higher temperatures resulted in improved electrochemical performance, increased particle size, and a differentiated surface composition. Spinel powders synthesized at 900 °C had initial discharge capacities close to 130 mAh g^?1 at C and C/2 discharge rates. Powders synthesized at 1,000 °C showed capacity retention values higher than 85 % at C/2, C, and 2C rates at 25 °C after 50 cycles. Annealing at 600–800 °C resulted in formation of spinel particles smaller than 200 nm, while almost micron-sized particles were obtained at 900–1,000 °C. Chromium deficiency was detected at the surface of the active materials annealed at low temperatures. The XPS results indicate presence of Cr⁶⁺ impurity when the annealing temperature was not high enough. The study revealed that increased annealing temperature is beneficial for both improved electrochemical performance of LiCr_0.2Ni_0.4Mn_1.4O₄ and for avoiding formation of Cr⁶⁺ impurity on its surface.     

Reaction of chromium trifluoride with zirconium in NaF + ZrF4 melts

We report on the reaction of chromium trifluoride with zirconium in NaF:ZrF₄ = 50:50 (mol/mol) mixed melts. Chemical analysis, X-ray powder diffraction, differential thermal analysis (DTA), and IR spectroscopy show that at 400–600°C zirconium reduces chromium(III) fluoride to Cr²⁺ or Cr⁰ compounds and reduces zirconium tetrafluoride to ZrF_2–x , where 0 < x < 0.2; the particular products depend on the zirconium concentration in the batch.     

Preparation and crystallographic investigation of the spinel series: Mn1 + 2sCr2 − 3sSbsO4 (0.05 ⩽ , s ⩽ 0.30)

Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄, a new series of spinels, have been prepared and studied using X-ray powder data. For s going from 0.05 to 0.30, a gradually increases from 8.441(1) to 8.472(1) Å, and u slightly decreases, from 0.262 to 0.258. Interatomic distances are given. The Mn_{1 + 2s}Cr_{2 ? 3s}Sb_sO₄ (0.05 < s < 0.30) series may be conceived as the result of partial substitution of Cr^{3 +} by 2/3Mn^{2 +} + 1/3Sb^{5 +} in the normal spinel, MnCr₂O ₄.     

The pillared layered framework of Ba3(In1−xMx)2(HXO4)6 (0≤x≤1; M=Fe,Cr; X=P,As): synthesis,crystal structure,thermal stability and Mössbauer spectroscopy

The hydrothermal synthesis, single crystal structure analysis, spectroscopic and thermal stability studies of the compounds Ba₃(In_1−xM_x)₂(HXO₄)₆ (0≤x≤1; M=Cr, Fe; X=P, As) are reported. The 3D framework of these new phosphates can be described as a pillared layered framework. The metal cations (In³⁺, Fe³⁺, and Cr³⁺) occupy two crystallographically independent octahedral sites, M(1) and M(2). The layers are formed of M(2)O₆ octahedra and (HPO₄) tetrahedra sharing corners, with M(1)O₆ octahedra serving as pillars between adjacent layers. Single crystal study of Ba₃(In_0.5Fe_0.5)(HPO₄)₆ shows that indium and iron segregate between the two metal sites with Fe occupying primarily the site M(1) and In located primarily in M(2) site. Interactions between the building units within the layers occur through hydrogen bonding. Barium cations are located between the pillars, in 8-membered ring tunnels and are coordinated by 12 oxides. The phases loose three water molecules through condensation of six HXO₄ groups to form Ba₂M₂(X₂O₇)₃ at temperatures between 480 and 600 °C. Mössbauer spectroscopy shows the presence of high-spin Fe³⁺ in octahedral coordination.     

Preparation, structural and magnetic characterization of DyCrMnO5

The title compound has been first synthesized by a citrate technique followed by thermal treatments under moderate oxygen pressure conditions, and characterized by X-ray and neutron powder diffraction (NPD) and magnetization measurements. The crystal structure of DyCrMnO₅ has been refined from NPD data in the space group Pbam; a=7.2617(6) Å, b=8.5161(6) Å, and c=5.7126(5) Å at 295 K. This oxide is isostructural with RMn₂O₅ oxides (R=rare earths) and it contains infinite chains of (Cr, Mn)⁴⁺O₆ octahedra-sharing edges, linked together by (Mn, Cr)³⁺O₅ pyramids and DyO₈ units. The high degree of antisite disordering exhibited by DyCrMnO₅ is noteworthy. The octahedral positions are occupied by roughly 50% of Mn and Cr cations, and the pyramidal groups contain two thirds of Mn and one third of Cr cations. We assume that Mn and Cr cations at the octahedral positions exhibit a tetravalent oxidation state, whereas the metals at the pyramidal positions are trivalent, in order to preserve the electroneutrality of this oxide. The susceptibility vs temperature curve of DyCrMnO₅ does not suggest the establishment of a long-range magnetic structure even at low temperatures; the NPD technique does not provide any signal of magnetic ordering, since the reflections do not show any magnetic contribution.