Thermochemical Study of Gaseous Salts of Oxygen-containing Acids: VII. Alkaline-Earth Metal Niobates

The existence of gaseous alkaline-earth metal niobates was established by high-temperature mass spectrometry. The constants of equilibria involving the niobates and the corresponding oxides were calculated, and the standard enthalpies of formation and atomization of gaseous BeNbO₃, CaNbO₃, SrNbO₃, BaNbO₂, BaNbO₃ and BaNb₂O₅ were determined.     

New thermochemical reference system expressing the relative energy of organic compounds

To facilitate the comparison of the energies of nonisomeric organic compounds, we introduce a new thermochemical reference system. The reference substances, instead of elements, are series of compounds: unbranched alkanes and their selected derivatives (halogenides, ethers, sulfides, and tertiary amines). The relative enthalpies calculated from the heats of formation directly reflect the relative energy of both isomeric and nonisomeric compounds, and they can be used in the calculation of the heats of reaction. The relative enthalpies of the elements (in kJ/mol) are C, –22.846; H₂, 43.426; O₂, 251.66; N₂, –178.62; S, –41.62; F₂, 396.12; C1₂, 94.22; Br₂, 8.16; and I₂, –107.64.     

Mass spectrometric investigation of vaporization in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The thermodynamics of vaporization in the Bi₂O₃-Fe₂O₃ quasibinary system was studied by high-temperature mass-spectrometry. The partial pressures of the constituents of a saturated vapor over the system at 1100 K were determined. Based on the experimental data, the following parameters were calculated: the activities of the components of the Bi₂O₃-Fe₂O₃ system condensed phase, the standard enthalpies of some heterogeneous reactions, and standard enthalpies of formation and enthalpies of formation for crystalline BiFeO₃ and Bi₂Fe₄O₉ from individual oxides. An optimal temperature for the solid-phase synthesis of bismuth ferrites from simple oxides is recommended.     

Thermodynamics of CoAl2O4-CoGa2O4 solid solutions

CoAl₂O₄, CoGa₂O₄, and their solid solution Co(Ga_zAl_1−z)₂O₄ have been studied using high temperature oxide melt solution calorimetry in molten 2PbO·B₂O₃ at 973 K. There is an approximately linear correlation between lattice parameters, enthalpy of formation from oxides, and the Ga content. The experimental enthalpy of mixing is zero within experimental error. The cation distribution parameters are calculated using the O’Neill and Navrotsky thermodynamic model. The enthalpies of mixing calculated from these parameters are small and consistent with the calorimetric data. The entropies of mixing are calculated from site occupancies and compared to those for a random mixture of Ga and Al ions on octahedral site with all Co tetrahedral and for a completely random mixture of all cations on both sites. Despite a zero heat of mixing, the solid solution is not ideal in that activities do not obey Raoult's Law because of the more complex entropy of mixing.     

The standard enthalpies of formation of mixed antimony and bismuth oxides

The Bi₂O₃-BiSbO₄ and BiSbO₄-Sb₂O₄ sections of the phase diagram of the Bi-Sb-O system were studied by the Knudsen effusion method with mass spectrometric analysis of the gas phase. The standard enthalpies of formation of the Bi₃SbO₆(g), Bi₂Sb₂O₆(g), BiSb₃O₆(g), Bi₃SbO₇(s), and BiSbO₄(s) mixed antimony-bismuth oxides and the standard enthalpies of some reactions with the participation of these oxides were calculated.     

LiMO2 (M=Mn, Fe, and Co): Energetics, polymorphism and phase transformation

LiMO₂ materials (M=Mn, Fe, and Co) with different structures were synthesized and their enthalpies of formation from oxides (Li₂O and M₂O₃, M=Mn and Fe), or from oxides (Li₂O and CoO) plus oxygen at 25 °C were determined by high-temperature oxide melt solution calorimetry. The relative stability of the polymorphs of the compound LiMO₂ was established based on their enthalpies of formation. Phase transformations in LiFeO₂ were investigated by differential scanning calorimetry and high-temperature oxide melt solution calorimetry. The phase transition enthalpies at 25 °C for β→α, γ→β, and γ→α are 4.9±0.7, 4.3±0.8 and , respectively. Thus the γ phase (ordered cations) is the stable form of LiFeO₂ at room temperature, the α phase (disordered cations) is stable at high temperature and the β phase may have a stability field at intermediate temperatures.     

Oxidation behavior of rare earth oxide-coated Fe20Cr and Fe20Cr5Al alloys

This article presents the influence of surface additions of nanocrystalline rare earth (RE) oxides CeO₂, La₂O₃, and CeO₂ + La₂O₃ on the isothermal oxidation behavior of Fe20Cr and Fe20Cr5Al at 1000 °C. Thermogravimetric studies revealed parabolic kinetics in all cases and the scale thickness on specimen surfaces varied with the nature of RE oxide. The oxidation resistance of specimens coated with two RE oxides was significantly higher than those coated with either one of the two oxides. The marked increase in the oxidation resistance of the alloys coated with two RE oxides is due to optimization of RE ion radius and RE oxide grain size/shape.     

Vanadium oxide monolayer catalysts. I. Preparation,characterization, and thermal stability

Vanadium oxide catalysts of the monolayer type have been prepared by means of chemisorption of vanadate(V)-anions from aqueous solutions and by chemisorption of gaseous V₂O₃(OH)₄. Using Al₂O₃, Cr₂O₃, TiO₂, CeO₂ and ZrO₂, catalysts with an approximately complete monomolecular layer of vanadium(V) oxide on the carrier oxides can be prepared, if temperature is not too high. Divalent metal oxides like CdO and ZnO may already form threedimensional surface vanadates at moderate temperature. The thermal stability of a monolayer catalyst is related to the parameter z/a, i. e. the ratio of the carrier cation charge to the sum of ionic radii of carrier cation and oxide anion. Thus, monolayer catalysts will be thermally stable only under the condition that z/a is not too high (aggregated catalyst) nor too small (ternary compound formation).     

“嵌入模型”在γ-Al2O3负载单组分和双组分金属氧化物催化剂中的应用

探讨负载型金属氧化物催化剂的表面组分与载体之间的相互作用, 有助于理解相关催化剂的催化作用本质. 近年来, 我们对单组分CuO以及双组分CuO-Mn₂O₃, CuO-CoO等金属氧化物在γ-Al₂O₃载体表面的分散行为和存在状态, 及其物理化学性质和催化性能(CO+O₂和NO+CO模型反应)进行了研究. 结果表明, 这些金属氧化物在γ-Al₂O₃载体表面的分散行为和所得负载型催化剂样品的一些物理化学性质及其催化性能均可参照“嵌入模型”来解释. 在此基础上, 我们讨论了这些样品的“组成-结构-性质”间的关系, 并针对表面负载双组分金属氧化物样品提出了表面协同氧空位参与的NO+CO反应机理.     

Enthalpy of formation of BaCe<Subscript>0.9</Subscript>In<Subscript>0.1</Subscript>O<Subscript>3−δ</Subscript>(s)

Enthalpy of formation of the perovskite-related oxide BaCe_0.9In_0.1O_2.95 has been determined at 298.15 K by solution calorimetry. Solution enthalpies of barium cerate doped with indium and mixture of BaCl₂, CeCl₃, InCl₃ in ratio 1:0.9:0.1 have been measured in 1 M HCl with 0.1 M KI. The standard formation enthalpy of BaCe_0.9In_0.1O_2.95 has been calculated as −1611.7±2.6 kJ mol⁻¹. Room-temperature stability of this compound has been assessed in terms of parent binary oxides. The formation enthalpy of barium cerate doped by indium from the mixture of binary oxides is Δ_ox H ⁰ (298.15 K)=−36.2±3.4 kJ mol⁻¹.     

Pyrochlore and Perovskite Potassium Tantalate: Enthalpies of Formation and Phase Transformation

Alkali niobates and tantalates are currently important lead‐free functional oxides. The formation and decomposition energetics of potassium tantalum oxide compounds (K₂O?Ta₂O₅) were measured by high‐temperature oxide melt solution calorimetry. The enthalpies of formation from oxides of KTaO₃ perovskite and defect pyrochlores with K/Ta ratio of less than 1 stoichiometry—K_0.873Ta_2.226O₆, K_1.128Ta_2.175O₆, and K_1.291Ta_2.142O₆—were experimentally determined, and the values are (?203.63±2.92) kJ mol^?1 for KTaO₃ perovskite, and (?339.54±5.03) kJ mol^?1, (?369.71±4.84) kJ mol^?1, and (?364.78±4.24) kJ mol^?1, respectively, for non‐stoichiometric pyrochlores. That of stoichiometric defect K₂Ta₂O₆ pyrochlore, by extrapolation, is (?409.87±6.89) kJ mol^?1. Thus, the enthalpy of the stoichiometric pyrochlore and perovskite at K/Ta=1 stoichiometry are equal in energy within experimental error. By providing data on the thermodynamic stability of each phase, this work supplies knowledge on the phase‐formation process and phase stability within the K₂O?Ta₂O₅ system, thus assisting in the synthesis of materials with reproducible properties based on controlled processing. Additionally, the relation of stoichiometric and non‐stoichiometric pyrochlore with perovskite structure in potassium tantalum oxide system is discussed.     

Thermal instabilities of organic carbonates with discharged cathode materials in lithium-ion batteries

Thermal instability of lithiated cathode materials with organic carbonate were investigated using DSC. Lithium transition metal oxides of LiFePO₄, LiMn₂O₄, and LiCoO₂ were mixed with diethyl carbonate, dimethyl carbonate, ethylene carbonate, ethyl methyl carbonate, and propylene carbonate then dynamically screened to about 500 °C. Curves were acquired and analyzed to determine exothermic onset temperatures and reaction enthalpies. These data for assessing the thermal hazards of lithium-ion batteries under discharged conditions were compared to those data published in the literature.     

Calorimetric study of the adsorption of air pollutants on alumina-supported tin and gallium oxides

The differential enthalpies of adsorption of air pollutants such as SO₂, NO₂, NO on aluminasupported tin and gallium oxides were measured by calorimetry coupled with isothermal volumetry. Whatever the amount of tin or gallium on alumina, the enthalpies of adsorption of SO₂ at low coverage were of the same order of magnitude or up to 50 kJ/mol lower than those of the support. The amount of SO₂ adsorbed decreased with increasing SnO₂ loading and increased with increasing Ga₂O₃ loading on the alumina. The differential enthalpies of adsorption of NO₂ are close to those on the support whatever the amount of tin or gallium (around 120–130 kJ/mol). NO is only reversibly adsorbed on the samples. The calorimetric data of ammonia adsorption are given for comparison.     

Catalytic transfer hydrogenation of 2-butanone over oxide catalysts

Catalytic transfer hydrogenation of 2-butanone with 2-propanol was studied in gas phase over a series of oxides of different acid-base properties. Although the basic oxides (MgO, La₂O₃) gave high initial conversions, these oxides underwent deactivation during the reaction. This deactivation could be partially prevented by a previous treatment with chloroform of the oxide. The amphoteric oxides (TiO₂, ZrO₂, Al₂O₃) were also active in this reaction. Increasing the acidic character of the catalyst (Nb₂O₅, WO₃) led to a pronounced dehydration of 2-propanol. The results obtained over a series of rare earth oxides (La₂O₃, Sm₂O₃, Gd₂O₃, Dy₂O₃, Er₂O₃) revealed that beside the role of basic and acid sites a correlation seems to exist between the number of unpaired electrons of the metal ion and the catalytic activity, indicating the role of one electron donor sites.     

The thermodynamic properties of gaseous salts formed by some 3d metal oxides

The existence of 48 new gaseous salts formed by 3d metal oxides was proved experimentally, and their standard enthalpies of formation were calculated. Gaseous oxides CrO, CrO₂, VO, VO₂, FeO, CoO, and NiO were found to possess amphoteric properties and form salts of two types as both cation-and anion-forming oxides.     

A thermal and thermochemical study of natural hemimorphite

A thermal and thermochemical study of natural aqueous hydroxyl-containing diorthosilicate, hemimorphite Zn₄[Si₂O₇](OH)₂ · H₂O, was performed. The step character of its thermal decomposition was studied using FTIR spectroscopy. Melt solution calorimetry was used to determine the enthalpies of formation from oxides Δ_f H _O^OX (298.15 K) = −69.3 ± 9.9 kJ/mol and elements {ie1481-2} (298.15 K) = −3864.3 ± 10.2 kJ/mol.     

Correlating the Surface Basicity of Metal Oxides with Photocatalytic Hydroxylation of Boronic Acids to Alcohols

Photoredox catalysis provides opportunities in harnessing clean and green resources such as sunlight and O₂, while the acid and base surface sites of metal oxides are critical for industrial catalysis such as oil cracking. The contribution of metal oxide surfaces towards photocatalytic aerobic reactions was elucidated, as demonstrated through the hydroxylation of boronic acids to alcohols. The strength and proximity of the surface base sites appeared to be two key factors in driving the reaction; basic and amphoteric oxides such as MgO, TiO₂, ZnO, and Al₂O₃ enabled high alcohol yields, while acidic oxides such as SiO₂ and B₂O₃ gave only low yields. The reaction is tunable to different irradiation sources by merely selecting photosensitizers of compatible excitation wavelengths. Such surface complexation mechanisms between reactants and earth abundant materials can be effectively utilized to achieve a wider range of photoredox reactions.     

Effect of the oxidation state of manganese in the manganese oxides used in the synthesis of Mn-substituted cordierite on the properties of the product

Catalysts based on Mn-substituted cordierite 2MnO · 2Al₂O₃ · 5SiO₂ have been synthesized using different manganese oxides (MnO, Mn₂O₃, and MnO₂) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn₂O₃ or MnO₂ are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn³⁺ cations. The highest NO_x yield (about 76% at 800–850°C) is observed for the MnO₂-based catalyst, whose surface contains the largest amount of manganese oxides.     

Standard enthalpy of formation of platinum hydrous oxide

Standard enthalpies of formation of amorphous platinum hydrous oxide PtH_2.76O_3.89 (Adams' catalyst) and dehydrated oxide PtO_2.52 at T=298.15 K were determined to be -519.6±1.0 and -101.3 ±5.2 kJ mol^-1, respectively, by micro-combustion calorimetry. Standard enthalpy of formation of anhydrous PtO₂ was estimated to be -80 kJ mol^-1 based on the calorimetry. A meaningful linear relationship was found between the pseudo-atomization enthalpies of platinum oxides and the coordination number of oxygen surrounding platinum. This relationship indicates that the Pt-O bond dissociation energy is 246 kJ mol^-1 at T=298.15 K which is surprisingly independent of both the coordination number and the valence of platinum atom. This may provide an energetic reason why platinum hydrous oxide is non-stoichiometric. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal instabilities of organic carbonates with charged cathode materials in lithium-ion batteries

Thermal instabilities of de-lithiated materials with organic carbonates were verified by DSC. Transition metal oxides FePO₄, NiO₂, Mn₂O₄, and Co₃O₄ were mixed with DEC, DMC, EC, EMC, and PC then dynamically screened to about 500 °C. Curves were acquired and analyzed to determine exothermic onset temperatures and reaction enthalpies. These data for assessing the thermal hazards of conditions were compared to those data published in literature.