Kinetics of free-surface decomposition of magnesium and barium sulfates analyzed thermogravimetrically by the third-law method

The application of the third-law method to the determination of the E parameters in cases of free-surface decomposition of MgSO₄ and BaSO₄ in vacuum and investigation of the effect of oxygen in reactor atmosphere on the rate of MgSO₄ decomposition allowed to support the dissociative evaporation mechanism of decomposition of these sulfates with releasing of atomic oxygen as one of the primary products. The experimental values of the E parameter for MgSO₄ and BaSO₄ are equal to 335.7±1.7 and 411±4 kJ mol⁻¹. The serious difference in decomposition temperatures in vacuum (about 1000 and 1400 K) and the E parameters for these two sulfates is connected with the difference in transfer of condensation energy of oxides to the reactants. In cases of MgSO₄ and BaSO₄, the values of τ parameter are 0.42 and 0. Dependence of the τ parameter on the ratio of the equivalent pressure to the saturation pressure of corresponding metal oxide has been revealed from a comparison of τ parameters reported in the literature for eight different reactants. This correlation is of prime consideration for understanding of the mechanism of consumption of the condensation energy by the reactant.     

ⅡB族元素氧化物与γ-Al2O3相互作用的XRD研究

The interaction between γ-Al_2O_3 and the oxides of Zinc, Cadium and Mercury have been studied by x-ray diffraetion(XRD). Samples of ZnO/γ-Al_2O_3, CdO/γ-Al_2O_3 and HgO/γ-Al_2O_3 were prepared by impregnating γ-Al_2O_3 with the corresponding nitrate solutions, then drying and ealeinating. It has been shown that CdO and HgO can be dispersed on γ-Al_2O_3 surface to form submonolayers. Their dispersion capaeieies derived from Fig.2 and Fig.3 are 0.48g CdO/g γ-Al_2O_3 and 0.16g HgO/g γ-Al_2O_3, or 0.20g CdO/100m~2 and 0.067g HgO/100m~2 surface of γ-Al_2O_3 (specific surface of γ-Al_2O_3 is 240m~2/g). On the other hand, ZnO/γ-Al_2O_3 system can not be in a monolayer dispersion state, because ZnO can react with γ-Al_2O_3 to form Zinc spinel at rather low temperature (200 ℃). The difference between these three systems may be attributed to their different ion radius.     

Reaction of bis(trimethylsilyl) sulfate with peroxides and oxides

Conclusions Bis(trimethylsilyl) sulfate when reached with Na₂O₂, BaO₂, PbO₂, V₂O₅, MnO₂, gives hexamethyldisiloxane and oxygen (50–80% yield), and the same product when reacted with the oxides of the Group Ib and IIb Metals (CuO, ZnO, CdO, HgO).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2395–2397, October, 1978.     

Quantitative interpretation of the evaporation coefficients for the decomposition or sublimation of some substances in vacuo

An attempt is made at a quantitative interpretation of the low evaporation coefficients (10⁻⁷–10⁻¹) observed under thermal decomposition of 12 different compounds (AgNO₃, BaSO₄, BaCO₃, BN, Be₃N₂, Mg₃N₂, AIN, GaN, ZnO, CdS, CdSe and NH₄Cl) in vacuo under the conditions of free-surface evaporation after Langmuir. It is suggested that the main reason for the low evaporation coefficients lies in the difference between the true scheme of thermal decomposition of a given compound and in assuming its direct decomposition to the products corresponding to thermodynamic equilibrium. These differences consist, firstly, in a primary gasification of all decomposition products, including low-volatile components (metals and metal oxides) and, secondly, in a partial or total evolution of nitrogen, oxygen, sulfur, selenium and hydrogen chloride in the form of free atoms or H + Cl mixture rather than of molecules. The interpretation of the evaporation coefficients for elemental red phosphorus, arsenic and antimony is based on the assumption that the sublimation of these substances occurs, accordingly, through a primary formation of P₂ molecules, or of As₂ + As₄ and Sb₂ + Sb₄ mixtures, rather than by direct formation of the P₄, As₄, or Sb₄ molecules corresponding to thermodynamic equilibrium, as is generally accepted. The proposed mechanisms of decomposition or sublimation are validated by a comparison of the experimental and calculated activation energies.     

A simple method for preparation of Zinc,Cadmium, Mercuric and Cobalt Acetylacetonates

By mixing ZnO, CdO HgO, CoO and Co₃O₄, respectively, with acetylacetone at room temperature, in the presence of the proper solvents, the corresponding metal acetylacetonates were obtained directly in good yields. The molecular structure was confirmed on the basis of UV spectroscopy, melting points, metal analysis and X-ray powder photography.     

A theoretical study of the dehydration and the dehydrogenation processes of alcohols on metal oxides using MOPAC

Using a semi-empirical molecular orbital method, PM3, and 2-propanol as an example, the dehydration and the dehydrogenation processes of alcohol on oxide catalysts were studied. The catalysts addressed here were four kinds of oxides (Al₂O₃, SiO₂, ZnO, CdO) whose reaction selectivities had been experimentally determined. The usual models consisting of a surface metal ion, several oxide ions and an isopropoxy group were used in calculations. For the dehydration, heats of formation of the models were calculated at each point of the process where the distance between a β-hydrogen of the group and a basic site (i.e. oxygen of the group or a surface oxide ion) or a metal ion was gradually shortened, or where the length of the C–O bond of the group was gradually increased. A reasonable dehydration mechanism was estimated by comparing activation energies calculated from the transitions of the heats of formation. The most probable dehydrogenation mechanism was also estimated in a similar way by gradually making an -hydrogen close to a surface oxide ion, the metal ion or a surface proton. It was concluded that the dehydration proceeds by scission of the C–O bond of the group after its oxygen was attacked by some electrophile on the surface and that the dehydrogenation proceeds by a mechanism in which an -hydrogen of the group was extracted by the metal ion.

Based on the dehydration mechanism thus deduced, alkoxy groups generated by adsorption of the primary, secondary and tertiary alcohols on SiO₂ were calculated in order to estimate the activation energies of their decompositions. In result, the order of the energies was found to be in good agreement with that of the decomposition rates experimentally determined by Kitahara. This agreement gives support to the validity of the mechanism deduced for the dehydration of alcohol.     

Mechanism of carbothermal reduction of iron, cobalt, nickel and copper oxides

A new scheme of thermal dissociation which is based on the dissociative evaporation of the reactant with simultaneous condensation of the low-volatile product has been invoked to interpret the kinetics of reduction of FeO, CoO, NiO and Cu₂O by carbon. A critical analysis of literature data and their comparison with theoretical calculations has shown that the main kinetic characteristics of carbothermal reduction, including the initial decomposition temperature and activation energy are in full agreement with the proposed mechanism of decomposition. Condensation of the low-volatile product (metal vapour) in the reaction zone and partial transport of condensation energy to the oxide account for the features which are typical of solid state reactions and manifest themselves in the appearance of periods of induction and acceleration in the course of the process. Carbon fulfils the role of buffer in this process. This is supported by an appearance of metals in the condensed phase and a higher than equilibrium partial pressure of oxygen in high-vacuum experiments with Knudsen cells.     

General, spontaneous ion replacement reaction for the synthesis of micro- and nanostructured metal oxides

A novel spontaneous ion replacement route based on the solubility difference as the driving force to synthesize a number of metal oxides has been established. We present a comprehensive study on the ion replacement reaction for chemical synthesis of micro- and nanostructured Mn2O3, ZnO, CuO, CdO, Al2O3, and CaO samples. This novel approach described herein is derived from the solubility difference between two carbonate salts, in which a metal cation can be driven from one liquid phase into another solid phase in the solution system. The resulting metal carbonate salts are initially formed and subsequently calcined to form highly crystallined metal oxides. The variation of pH values, reaction temperature, and reagent shapes can vary the solubility of these two carbonate salts, which thus changes the final morphology of metal oxides. The present work makes a progress to simply and mildly synthesize metal oxides with various morphologies, due to the fact that materials with a desired morphology are a key engineering step toward their shape-dependent chemical and physical properties.     

Valence-bond calculations on ZNO and HGO using integrals computed through the semiempirical AM1 method

Valence-bond calculations have been carried out on ZnO and HgO using a basis set of Slatertype atomic orbitals and the one- and two-electron integrals as computed in the semiempirical AM 1 molecular orbital method. The zero differential overlap approximation has been used to calculate integrals between atomic orbital Slater determinants using the rules for matrix elements between determinants formed by orthogonal orbitals. Diabatic and adiabatic curves have been analyzed for the two systems, and results compared with molecular orbital AM 1 results. © 1992 John Wiley & Sons, Inc.     

Thermal investigations of cefadroxil complexes with transition metals

The cefadroxil (Cef) complexes with transition divalent metals of the formula MCef·nH₂O (where n=2 for M=Cu²⁺, Ni²⁺, Zn²⁺ and n=3 for Co²⁺) and CdCef_1.5·4H₂O were prepared and characterized by elemental and infrared spectra. The thermal analysis of the investigated complexes in air atmosphere was carried out by means of simultaneous TG-DSC technique. During heating in air they lose bound water molecules and then decompose to oxides: Co₃O₄, NiO, CuO, ZnO and CdO. The CdCef_1.5·4H₂O complex forms probably an intermediate product Cd₂OSO₄. The combined TG-FTIR technique was employed to study of decomposition pathway of the investigated complexes. The first mass loss step is the water loss of the complexes. Next, decomposition of cefadroxil ligand occurs with evolution of CO₂ and NH₃. At slightly higher temperature COS is observed according to decomposition of cephem ring. Additionally, as decomposition gaseous products: HCN, HNCO (HOCN), H₂CNH, CO, SO₂, hydrocarbons and carbonyl compounds were observed. The formation of metal sulfates is postulated as solid intermediate product of decomposition in the argon atmosphere. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ce1-xTbxO2-δ复合氧化物氧缺位的拉曼光谱研究

采用溶胶-凝胶法制备了Ce1-xTbxO2-δ复合氧化物, 利用不同Raman激发波长(514和785 nm), 结合X射线衍射(XRD)、氢气-程序升温还原(H2-TPR)和氧气-程序升温脱附(O2-TPD)表征, 考察了Ce1-xTbxO2-δ复合氧化物在O2, He和H2气氛下氧缺位的原位变化情况和CeO2的F2g特征Raman峰位的偏移. 实验结果表明, 随着Tb掺杂量的提高, 由于晶胞收缩使得CeO2的F2g特征Raman振动峰发生蓝移. 514 nm Raman激发波长反映了催化剂的表面信息, 而785 nm激发波长反映了整体信息. 正是由于表面和整体变化的不一致, 造成原位Raman实验过程中氧缺位浓度变化趋势的不同. 在He和H2气氛下, 由于温度升高时伴随着Ce1-xTbxO2-δ中O2气的脱出, 使复合氧化物的微观结构发生改变, 以致Ce0.9Tb0.1O2-δ中的氧缺位浓度(A587/A465)在785 nm激发波长下出现先升高后下降的现象.     

Thermal decomposition of Cd(CH3COO)2·2H2O studied by a coupled TG-DTA-MS method

The thermal decomposition of cadmium acetate dihydrate in helium and in air atmosphere has been investigated by means of a coupled TG-DTA-MS method combined with X-ray diffraction analysis. Dehydration of Cd(CH₃COO)₂·2H₂O is a two-stage process with Cd(CH₃COO)₂·H₂O as intermediate. The way of Cd(CH₃COO)₂ decomposition strongly depends on the surrounding gas atmosphere and the rate of heating. CdO, acetone and CO₂ are the primary products of decomposition in air. In helium decomposition goes by two parallel and consecutive reactions in which intermediates, Cd and CdCO₃, are formed. Metallic cadmium oxidizes and cadmium carbonate decomposes giving CdO. Some of the metallic cadmium, depending on the heating rate and the concentration of oxygen, evaporates. Acetone is partially oxidized in secondary reactions with oxygen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study of adsorption and decomposition of H2O on Ge(100)

The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H2O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H2O molecules are bound to the surface via Ge-O dative bonds between the O atom of H2O and the electrophilic down atom of the Ge dimer. In the dissociative adsorption geometry, the H2O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H2O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H2O molecule from the molecular adsorption to the dissociative adsorption geometry.     

MgSO4-K2SO4-H2O三元体系在75 ℃沸腾蒸发非平衡态的成盐特征

采用105 ℃恒温热源, 75 ℃恒温沸腾, 蒸发强度为240～260 g/(h·L)等控制条件, 对MgSO4-K2SO4-H2O体系进行蒸发结晶实验, 研究了该体系在沸腾蒸发条件下的成盐规律, 根据实验现象, 定义并确定了实验条件下的初级成盐区, 扩展成盐区和条件成盐区. 实验结果表明: (1)该体系K2SO4, Leonite, langbenite和MgSO4·H2O等盐能够形成初级晶核的初级成盐区与溶解平衡相区存在明显偏离, 其中K2SO4成盐区域缩小, 而Leonite和MgSO4·H2O结晶区域均有不同程度的增大; (2) 在蒸发结晶过程中, 成盐晶种的存在使该盐的成盐区比初级成盐区有所扩展; (3) 由于成盐区的扩展, 使成盐区域出现不同程度的交错. 在交错区域成盐的种类取决于晶种的种类, 即存在条件成盐区, 这是非平衡态成盐相关系区别于平衡相图和介稳相图的特征区域.     

Thermal decomposition of iron(VI) oxides, K2FeO4 and BaFeO4, in an inert atmosphere

The thermal decomposition of solid samples of iron(VI) oxides, K₂FeO₄·0.088 H₂O (1) and BaFeO₄·0.25H₂O (2) in inert atmosphere has been examined using simultaneous thermogravimetry and differential thermal analysis (TG/DTA), in combination with in situ analysis of the evolved gases by online coupled mass spectrometer (EGA-MS). The final decomposition products were characterized by ⁵⁷Fe Mössbauer spectroscopy. Water molecules were released first, followed by a distinct decomposition step with endothermic DTA peak of 1 and 2 at 273 and 248 °C, respectively, corresponding to the evolution of molecular oxygen as confirmed by EGA-MS. The released amounts of O₂ were determined as 0.42 and 0.52 mol pro formula of 1 and 2, respectively. The decomposition product of K₂FeO₄ at 250 °C was determined as Fe(III) species in the form of KFeO₂. Formation of an amorphous mixture of superoxide, peroxide, and oxide of potassium may be other products of the thermal conversion of iron(VI) oxide 1 to account for less than expected released oxygen. The thermogravimetric and Mössbauer data suggest that barium iron perovskite with the intermediate valence state of iron (between III and IV) was the product of thermal decomposition of iron(VI) oxide 2.     

有氧气氛中碱金属改性CuAl复合氧化物催化分解N2O的活性

合成了Cu/Al原子比分别为2.0、3.1、4.1的CuAl类水滑石样品,焙烧得到CuAl复合氧化物。在Cu/Al原子比为3.1的CuAl氧化物表面浸渍碱金属盐溶液,制备改性CuAl复合氧化物,用AES、XRD、FT-IR、BET、H₂-TPR、XPS等技术对催化剂进行了结构表征,考察了CuAl复合氧化物组成、碱金属助剂类型和K的前驱物对改性催化剂在有氧气氛中催化分解N₂O活性的影响。结果表明,Na、K、Cs改性CuAl复合氧化物均提高了催化剂活性,但K助剂的增强效应最显著;钾的不同前驱物改性CuAl复合氧化物的催化活性有显著差异,加入碳酸钾、草酸钾提高了催化剂的活性,而加入醋酸钾、硝酸钾反而降低了催化剂活性。优化出的K改性CuAl复合氧化物催化剂在含氧含水气氛的N₂O分解反应中表现出了较高的活性。     

Mechanism of low temperature decomposition of NiO single crystals

The decomposition of NiO single crystal was investigated under dissociative conditions in the temperature interval between 330 and 850 °C in the absence of reducing gas species. An unusually fast and constant decomposition rate was measured at the lowest temperatures, coupled with an unusual largely porous microstructure of the metallic product layer. This anomalous high reaction rate was interpreted on the basis of a decomposition mechanism implying the dissociative vaporization of the oxide followed by the condensation of the metal. The proposed mechanism is supported by the microstructure of the product and of the reacting interface. The complex dependence of reaction rate from temperature was shown to be related to a collapse of the porous product to form a compact metal layer at higher temperatures due to sintering.     

碱金属助剂类型及前驱物对改性NiAl复合氧化物催化分解N2O活性的影响

制备了不同Ni/Al原子比的NiAl类水滑石样品,焙烧获得NiAl复合氧化物,用于N₂O分解反应,研究了NiAl复合氧化物组成对催化活性的影响。在活性较高的NiAl复合氧化物表面浸渍碱金属碳酸盐溶液,制备改性NiAl复合氧化物,考察了碱金属类型（Na、K、Cs）和钾前驱物（K₂CO₃、K₂C₂O₄、CH₃COOK、KNO₃）对改性催化剂活性的影响。用XRD、ICP-AES、FT-IR、BET、H₂-TPR、XPS技术表征了催化剂的组成结构。结果表明,Ni/Al原子比为2.7的NiAl复合氧化物催化活性较高;Na、K、Cs碳酸盐改性NiAl复合氧化物均提高了催化剂活性,其中K的助剂效应最强。钾前驱物对K改性NiAl复合氧化物的催化活性有显著影响,其中碳酸钾、醋酸钾、草酸钾的加入明显提高了改性催化剂的催化活性,而加入硝酸钾反而降低了催化剂活性。     

On the convergence of isolated neutral oxygen vacancy and divacancy properties in metal oxides using supercell models

The properties of isolated neutral oxygen vacancies and divacancies of metal oxides of increasing complexity (MgO, CaO, alpha-Al2O3, and ZnO) have been studied by means of density-functional theory within a supercell periodic approach. Vacancy formation energies, vacancy-vacancy interactions, and geometry rearrangements around these point defects have been investigated in detail. The characterization of the electronic structure of these point defects has been established by analysis of the density of states and of the topology of the electron density and of electron localization function. It is found that the chemical character of the oxide determines the properties of the oxygen vacancies. For the covalent ZnO oxide, a more complex scheme arises in which the relaxation around the oxygen vacancy is much larger leading to the formation of Zn4-like almost metallic particles in the crystal. The relationship of these structures with the crystal shear planes is discussed. The present study shows that supercells containing approximately 200-300 atoms provide converged values for the geometric and electronic structure of oxygen vacancies of these metal oxides in the point defect low concentration limit.