Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Synthesis and low temperature fluorination of the alkaline-earth palladates Ba2−xSrxPdO3 (x = 0–2)

Since the discovery of superconductivity in Sr₂CuO₂F_2+δ there has been an increased interest in ternary oxide-fluorides. Sr₂CuO₂F_2+δ is prepared via low temperature (T = 220 °C) reaction routes. Low temperature fluorination induces an interesting structural rearrangement in the parent compound Sr₂CuO₃, which is a one-dimensional material containing linear chains of vertex sharing CuO₄ squares along the crystallographic b axis. Upon fluorination, one oxide is substituted by two fluorides and Cu²⁺ becomes octahedrally coordinated by four oxides and two fluorides. The fluorinated compound Sr₂CuO₂F_2+δ displays the T-type structure (La₂CuO₄). Insertion of excess fluorine, δ, also takes place and this fluorine occupies interstitial sites in the T structure. Although the starting material Ca₂CuO₃ is isostructural to Sr₂CuO₃, Ca₂CuO₂F_2+δ displays the T′ (Nd₂CuO₄) structure due to the smaller radius of Ca²⁺ compared to that of Sr²⁺.

The alkaline-earth palladates with the general formula A₂PdO₃ (A = Ba, Sr) are isostructural with the A₂CuO₃(A = Ca, Sr) materials. We prepared the Ba_2−xSr_xPdO₃ (x = 0–2) series and performed low temperature fluorination, which led to the synthesis of the series Ba_2−xSr_xPdO₂F_2+δ (0 ≤ x ≤ 1.5). All the compounds in the Ba_2−xSr_xPdO₂F_2+δ series show T′ structure (Ca₂CuO₂F_2+δ). Similarities and differences with Sr₂CuO₂F_2+δ and Ca₂CuO₂F_2+δ will be discussed.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.     

Influence of composition and thermal treatment on the properties of Cu2+xTa4O12+δ

Polycrystalline samples of Cu_2+xTa₄O_12+δ were prepared by solid-state reactions. Copper tantalate shows a remarkable compositional flexibility with respect to both the copper and oxygen stoichiometry. Single phase compounds could be synthesised for 0.125 ≤ x ≤ 0.5. Slowly cooled samples are green and possess a pseudo-tetragonal unit cell, which changes to a pseudo-cubic symmetry for x ≥ 0.45. Rapidly cooled aliquots are brown and have a (pseudo-) cubic structure. For both the slow-cooled and quenched samples a linear increase of the oxygen content with x was observed, the values of δ for the latter being significantly smaller. Magnetic measurements reveal a ferrimagnetic transition at 12.5 K, the strength of which is strongly reduced both by increasing the copper content and by quenching.     

Oxygen permeation through a Ce0.8Sm0.2O2−δ–La0.8Sr0.2CrO3−δ dual-phase composite membrane

A composite of oxygen ion conducting oxide Ce_0.8Sm_0.2O_2−δ (60 vol.%) and electron conducting oxide La_0.8Sr_0.2CrO_3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than 1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of 0.6 mm.     

The vapour pressure of di-arsenic pentoxide (As2O5)

The arsenic oxide pressure of As₂O₅ has been studied using mass spectrometry and a transportation method. Mass spectrometry revealed the presence of the species As₄O⁺₆, As₄O⁺₇, and As₄O⁺₈ in the vapour. The existence of volatile species up to As₄O₁₀(g) as a result of the reaction As₄O₁₀(g) As₄O_(10−y) (g) +1/2yO₂(g) has been assumed.

The oxygen pressure of this equilibrium builds up very slowly. The equilibrium pressure can be expressed by log(p_O2/atm) (880−952 K) = −(13940±930)/T + (14.53 ± 1.01)

A stationary arsenic oxide pressure has been measured using the transportation method. Since the oxygen pressure in the transportation gas did not influence the arsenic oxide pressure, it is assumed that only the As₄O₁₀(g) pressure has been measured. The results can be expressed by the linear function log(p_As4O10/atm) (865−1009 K) = −(15741 ± 410)/T + (13.87 ± 0.42).     

Preparation of epitaxial La0.6Ca0.4Mn1−xFexO3 (x = 0, 0.2) thin films: Variation of the oxygen content

Perovskite thin films with a nominal composition of La_0.6Ca_0.4Mn_1−xFe_xO₃ (x = 0, 0.2) were deposited by pulsed reactive crossed beam laser ablation. The film properties, such as electrical conductivity and magnetoresistance are studied as a function of the oxygen content and substrate type. The oxygen content of the thin films was determined by Rutherford Backscattering and controlled by varying the background gas pressure, pressure of the gas pulse and by using alternatively O₂ and N₂O as the gas pulse.

LaAlO₃ and SrTiO₃ were used as substrates at deposition temperature of 650 °C. The grown films were analyzed by X-ray diffraction in order to optimize the growth conditions, i.e. to obtain epitaxial thin films. Thin films doped with 20% Fe were grown under the same experimental conditions as the undoped LCMO films and the effect of the doping on the structural and transport properties of the thin films has been investigated.

The temperature of the metal–insulator transition was measured as a function of the oxygen content and substrate type.     

Structure analysis and the standard molar enthalpy of formation of a coordination polymer [Co(TO)2Cl2]n (TO = 1,2,4-triazole-5-one)

A coordination polymer was synthesized by the reaction of CoCl₂ with 1,2,4-triazole-5-one (TO) and charaterized by means of IR and TG–DTG. Single-crystal structure analysis showed that the complex crystallized in the monoclinic space group C2/c: a = 23.105(9) Å, b = 3.5683(2) Å, c = 13.589(6) Å,  = 90°, β = 124.038(4)°, γ = 90°, V = 928.4(7) Å³, Z = 4. The standard molar enthalpy of formation of the complex was determined to be (−1034.28 ± 0.95) kJ mol⁻¹.     

Molecular motion and phase transition in perovskite-type (CH3)nNH4−nSnCl3 (n=1–4) studied by proton NMR spin–lattice relaxation

Powder X-ray diffraction, ¹¹⁹Sn NMR spectra, and ¹H NMR spin–lattice relaxation times, T₁, were measured for (CH₃)_nNH_4−nSnCl₃ (n=1–4). From the Rietveld analysis, it is shown that all four compounds crystallize into deformed perovskite-type structures at room temperature. The temperature dependence of ¹H T₁ was analyzed in terms of the CH₃ reorientation and other motions of the whole cation. Except for the phase transition in CH₃NH₃SnCl₃, which is from monoclinic to rhombohedral at 331 K, ¹H T₁ was continuously changed at other phase transitions in this compound as well as in the n=2–4 compounds, suggesting that the transitions are not caused by the change of the motional state of the cation but by an instability of the [SnCl₃]_nⁿ⁻ perovskite lattice.     

Survey of the quasi-ternary system La0.8Sr0.2MnO3–La0.8Sr0.2CoO3–La0.8Sr0.2FeO3

An overview on the variation of the thermal expansion, the electrical conductivity as well as non-stoichiometry of the oxide content as a function of composition within the quasi-ternary system La_0.8Sr_0.2MnO_3−δ–La_0.8Sr_0.2CoO_3−δ–La_0.8Sr_0.2FeO_3−δ in air is presented. The various powders were synthesized under identical conditions. The DC electrical conductivity values of the compositions at 800 °C in air vary in a wide range from 15 to 1338 S cm⁻¹. The magnitude of electrical conductivity of the perovskites is mainly determined by the percentage of cobalt in the compositions. A similar behaviour was observed for the measured thermal expansion coefficients between room temperature and 1000 °C in air, increasing from 10.9 to 19.4 × 10⁻⁶ K⁻¹ as a function of cobalt content. Changes in the oxygen stoichiometry of the materials were characterized by temperature-programmed oxidation measurements.     

Refined heat capacity of LaPO4 in the temperature range 0–1600 K

In the present work temperature dependence of heat capacity of cesium tantalum tungsten oxide has been measured first in the range from 7 to 350 K and then between 330 and 630 K, respectively, by precision adiabatic vacuum and dynamic calorimetry. The experimental data were used to calculate standard thermodynamic functions, namely the heat capacity C_p° (T), enthalpy H°(T) − H°(0), entropy S°(T) − S°(0) and Gibbs function G°(T) − H°(0), for the range from T → 0 to 630 K. The structure of CsTaWO₆ is refined by the Rietveld method: space group F d3m, Z = 8, a = 10.3793(2) Å, V = 1118.14(4) Å³. The high-temperature X-ray diffraction was used for the determination of temperature of phase transition and coefficient of thermal expansion.     

Mechanical stability and transport properties of the Sn-promoted SrCo0.8Fe0.2O3−δ ceramic membrane

In this article, the phase compositions, thermal, mechanical and transport properties of both the SrCo_0.8Fe_0.2O_3−δ (SCF) and the SrCo_0.8Fe_0.1Sn_0.1O_3−δ (SSCF) ceramic membranes were investigated systematically. As compared with the SCF membrane, the SSCF one had a more promoted thermal shock resistance, which related to its small thermal expansion coefficient between them and an enhanced composite structure for it. For the SCF membrane, a permeation rate of 1.9 × 10⁻⁶ mol cm⁻² s⁻¹ was obtained at 1000 °C and under the oxygen partial pressure gradient of P_O2 (h)/P_O2 (l) = 0.209 atm/0.012 atm; however, the permeation rate was 2.5 × 10⁻⁶ mol cm⁻² s⁻¹ for the SSCF one in the same measuring condition. In addition, both peak values of total electrical conductivity (σ_e) for SSCF sample appeared with increasing temperature. The second peak value of σ_e for SSCF one was regarded as the contribution from its minor phase, which appeared with the mixed conducting behavior resulting from partly Co-dissolving into its lattice.     

Kinetics study of the gas-phase reactions of C2F5OC(O)H and n-C3F7OC(O)H with OH radicals at 253–328 K

The rate constants, k₁ and k₂ for the reactions of C₂F₅OC(O)H and n-C₃F₇OC(O)H with OH radicals were measured using an FT-IR technique at 253–328 K. k₁ and k₂ were determined as (9.24 ± 1.33) × 10⁻¹³ exp[−(1230 ± 40)/T] and (1.41 ± 0.26) × 10⁻¹² exp[−(1260 ± 50)/T] cm³ molecule⁻¹ s⁻¹. The random errors reported are ±2 σ, and potential systematic errors of 10% could add to the k₁ and k₂. The atmospheric lifetimes of C₂F₅OC(O)H and n-C₃F₇OC(O)H with respect to reaction with OH radicals were estimated at 3.6 and 2.6 years, respectively.     

Conductivity, calorimetry and phase diagram of the NaHSO4–KHSO4 system

Physico-chemical properties of the binary system NaHSO₄–KHSO₄ were studied by calorimetry and conductivity. The enthalpy of mixing has been measured at 505 K in the full composition range and the phase diagram calculated. The phase diagram has also been constructed from phase transition temperatures obtained by conductivity for 10 different compositions and by differential thermal analysis. The phase diagram is of the simple eutectic type, where the eutectic is found to have the composition X(KHSO₄) = 0.44 (melting point ≈ 406 K). The conductivities in the liquid region have been fitted to polynomials of the form κ(X) = A(X) + B(X)(T − T_m) + C(X)(T − T_m)², where T_m is the intermediate temperature of the measured temperature range and X, the mole fraction of KHSO₄. The possible role of this binary system as a catalyst solvent is also discussed.     

Supported Pd-perovskite catalyst for CNG engines'' exhaust gas treatment

Perovskite-type oxides of the series La_1−xA_xMn_1−yB_yO₃ (A = Sr; B = Fe or Co) were prepared by solution combustion synthesis and characterized by X-ray diffraction, specific surface analysis, transmission electron microscopy and field emission scanning electron microscopy techniques. Their activity towards the combustion of methane was evaluated in a temperature programmed combustion microreactor. The LaMn_0.9Fe_0.1O₃ catalyst was found to provide the best performance. The half-conversion temperature of methane over the LaMn_0.9Fe_0.1O₃ catalyst was 398 °C with a W/F = 0.12 g s/cm³ and a methane feed concentration of 0.4 vol% under oxygen excess. Via temperature programmed oxygen desorption (TPD) analysis as well as catalytic combustion runs, the prevalent activity of the LaMn_0.9Fe_0.1O₃ catalyst could be explained by its higher and increased capability to desorb suprafacial, weakly chemisorbed oxygen species. Further catalyst development allowed to maximise the catalytic activity of this compound by promoting it with CeO₂ (1:1 molar ratio) and with 1 wt% Pd. This promoted catalyst was lined on cordierite monoliths in a γ-Al₂O₃-supported form (catalyst weight percentage: 15 wt%) and then tested in a lab-scale test rig under realistic conditions for compressed natural gas-vehicles' exhaust gas treatment. Half methane conversion was achieved at 340 °C (gas high space velocity = 10 000 h⁻¹), nearly the same but with a fourfold lower amount of the expensive noble metal than that used in commercial 4wt%Pd–γ-Al₂O₃ catalysts.     

New p–ρ–T measurements up to 70 MPa for the system CO2 + propane between 298 and 343 K at near critical compositions

The vibrating tube densimeter method along with the Forced Path Mechanical Calibration model, is used to measure the high pressure isothermal p–ρ behavior of the CO₂+propane system along 17 isotherms between 293 and 343 K, at pressures up to 70 MPa. The compositions cover the range of mole fractions from x_CO2=0.45 to 1.0. The uncertainty in temperatures is ±0.015 K. The uncertainties in pressures are ±0.0013 MPa from 0.1 to 15.0 MPa and ±0.010 MPa from 5.0 to 70.0 MPa. The precision of the density measurements is ±0.014 kg m⁻³. The minimum global uncertainty is ±0.204 kg m⁻³, based on the calibration of the densimeter with pure water. A generalized Helmholtz energy model for mixtures is used to check the consistency of the new data with respect to previous p–ρ–T studies of this mixture. The average absolute deviation of our data with respect to the model is 0.64% which is fully consistent with the assessed accuracy.     

Thermogravimetric study on perovskite-like oxygen permeation ceramic membranes

The oxygen permeation properties of mixed-conducting ceramics SrFeCo_0.5O_3−δ (SFCO), Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), La_0.2Sr_0.8Co_0.8Fe_0.2O_3−δ (LSCFO) and Ba_0.95Ca_0.05Co_0.8Fe_0.2O_3−δ (BCCFO) were studied by thermogravimetric method in the temperature range 600–900 °C. The results show that the oxygen adsorption rate constants k_a of all material are larger than oxygen desorption rate constants k_d and both k_a and k_d are not strongly dependent on temperature in the studied temperature range. The oxygen vacancy contents δ(N₂) and δ(O₂) in nitrogen and oxygen and their difference Δδ = δ(N₂) − δ(O₂) play an important role in determining the temperature behavior of oxygen permeation flux J_O2.     

Synthesis and thermodynamic properties of a metal-organic framework: [LaCu6(μ-OH)3(Gly)6im6](ClO4)6

A metal-organic complex, which has the potential property of absorbing gases, [LaCu₆(μ-OH)₃(Gly)₆im₆](ClO₄)₆ was synthesized through the self-assembly of La³⁺, Cu²⁺, glycine (Gly) and imidazole (Im) in aqueous solution and characterized by IR, element analysis and powder XRD. The molar heat capacity, C_p,m, was measured from T = 80 to 390 K with an automated adiabatic calorimeter. The thermodynamic functions [H_T − H_298.15] and [S_T − S_298.15] were derived from the heat capacity data with temperature interval of 5 K. The thermal stability of the complex was investigated by differential scanning calorimetry (DSC).     

Thermodynamic properties of the system NH4NO3 + (NH4)2SO4 + H2O at 298.15 K

In this investigation, the mixed aqueous electrolyte system of nitrate and sulfate with common ammonium cation has been studied with the hygrometric method at the temperature 298.15 K. The water activities of the system [yNH₄NO₃ + (1 − y)(NH₄)₂SO₄](aq) are measured at total molalities from 0.4 mol kg⁻¹ up to saturation for different ionic-strength fractions y of NH₄NO₃ with y = 0.2, 0.5 and 0.8. The obtained data allow the deduction of osmotic coefficients. The experimental results are compared with the predictions of the Zdanovskii–Stokes–Robinson (ZSR), Leitzke and Stoughton (LSII), Kusik and Meissner (KM), and Pitzer models. From these measurements, new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture. The obtained results are used to calculate the excess Gibbs energy at total molalities for different ionic-strength fractions y.     

Hydrothermal synthesis and intercalation behavior of a layered titanium phosphate Ti2(H2PO4)(HPO4)(PO4)2 · 0.5C6N2H16, with an extended γ-phase intercalated into organic amine

With a hydrothermal technique, a layered titanium phosphate with the formula Ti₂(H₂PO₄)(HPO₄)(PO₄)₂ · 0.5C₆N₂H₁₆ (denoted TP-J2) has been prepared by treating the Ti/H₃PO₄/H₂O/1-methylpiperazine system directly. The as-synthesized products were characterized by powder X-ray diffraction, CP-MAS solid-state ³¹P NMR spectroscopy, thermogravimetric and differential thermal analyses (TG-DTA). The structure was solved by single-crystal X-ray diffraction analysis and it presents an extended γ-phase intercalated with organic amine. Crystal data: triclinic, , a = 8.106 (2) Å, b = 8.197 (2) Å, c = 11.658 (2) Å.  = 78.32 (3)°, β = 80.85 (3)°, γ = 77.90 (3)°, Z = 2. Additionally, the intercalation behavior of TP-J2 with n-alkyl monoamine (n = 2, 3, 4, 6, 8, 10 and 12) was investigated. Owing to the strong brønsted base, N,N′-dimethylpiperazine, resides in the interlayer, it presented unusual features of TP-J2 in contrast with that of γ-Tip.