On the La–Cr–O Phase Change During Methane Catalytic Combustion Investigated with in situ HT-XRD

La₂CrO₆ (Cr6), LaCrO₃ (Cr3), LaCrO₃–La₂CrO₆ (Cr6–Cr3) and LaCrO₃–La₂O₃ (Cr3-L) catalysts were synthesised and investigated with in situ X-ray diffraction (ISXRD) during methane catalytic combustion in order to characterise the solid phases present under reactants and to determine the effect of chromium oxidation state on the catalytic activity. Methane conversion was evaluated over a temperature range of 300–800 °C, using oxygen-to-methane ratio of 4 and GHSV of 8,000 h^?1. The TPR provided information about the oxygen depletion temperatures characteristic of lattice oxygen mobility in the samples and ISXRD results evidenced a cooperative effect of Cr3 and Cr6 phases at low temperatures (<725 °C) and of Cr3 and LaCrO₄ (Cr4) phases at high temperatures (>750 °C). The relative phase composition determined the oxygen activation capability and hence the corresponding activity for the oxidation of methane. It was observed that while direct and back Cr6 ? Cr4 transition temperatures were unaffected by Cr6 content in the samples, the methane conversion was strongly modified. This suggests that Cr³⁺/Cr⁶⁺ and Cr³⁺/Cr⁵⁺ species involved substantial modification of the surface chemistry which affected the catalytic activity. These results provide the first direct evidence of the presence of Cr4 metastable phase during methane combustion over La–Cr–O catalysts.     

Interaction between metallic interconnect and constituent oxides of (La,Sr)MnO3 coating of solid oxide fuel cells

The chemical interaction between Fe–Cr alloy interconnect and constituent oxides of Sr-doped LaMnO₃ (LSM) coating, La₂O₃, SrO and Mn₂O₃, is investigated at 800–900 °C in air. The Cr deposition reaction between the Fe–Cr alloy metallic interconnect and oxides varies significantly with the nature of the oxides. The interaction between the Fe–Cr alloy and La₂O₃ and Mn₂O₃ oxide coatings primarily results in the formation of LaCrO₃ and (Cr, Mn)₃O₄ while in the case of SrO oxide coating, Cr₂O₃ is the main product. In the case of LSM coating, the formation of (Cr, Mn)₃O₄ and Cr₂O₃ is identified. The results indicate that the chemical interaction between the Fe–Cr alloy interconnect and LSM coating is most likely related to the surface oxide species such as SrO and MnO_x initially enriched or segregated on the surface of LSM particularly in the early stages of the reaction.     

On the effect of La–Cr–O– phase composition on diesel soot catalytic combustion

A series of samples of La–Cr–O– perovskites were designed as catalysts for diesel soot combustion. They were prepared by using co-precipitation method, at ambient temperature using ammonia followed by a hydrothermal treatment (T = 200 °C, P = 20 atm, t = 24 h). All the chromium-containing precursors were then calcined at high temperature to develop the oxide catalyst. Two phase composition 86%LaCrO₃–(14%) La₂CrO₆ or 94%LaCrO₃–6%La₂O₃ were formed depending on the atmosphere of calcination (oxygen or hydrogen respectively) used. Their respective BET surface areas were 1.1 and 6.5 m² g⁻¹. Highly crystalline, pure phase of LaCrO₃ and La₂CrO₆ powders were also prepared, with BET area of 4 and 3 m² g⁻¹, respectively. The crystalline structure and properties of all samples were characterised by X-ray diffraction (XRD), using Rietveld refinement, and temperature-programmed reduction (TPR) techniques. O₂-TPD measurements performed on all samples showed the presence of suprafacial, weakly chemisorbed oxygen only for LaCrO₃, which contributes actively to soot combustion. TPR study performed on all catalysts showed that while pure LaCrO₃ and La₂O₃ samples did not reduce, the biphasic catalysts showed the presence of oxygen depletion peaks characteristic of lattice oxygen mobility in the samples at ca. 665 °C. Catalytic combustion of diesel soot was studied over all catalysts. The results showed that pure LaCrO₃ exhibited significant catalytic activity which was sensitively affected by the modifier La₂CrO₆ or La₂O₃.     

Ca2+‐Doped LaCrO3: A Novel Energy‐Saving Material with High Infrared Emissivity

The present study describes the successful synthesis of a Ca²⁺‐doped LaCrO₃ ceramic with high infrared (IR) emissivity, which is important for high‐temperature applications for significant energy saving. It is demonstrated that 20 mol% Ca²⁺‐doped LaCrO₃, i.e., La_0.8Ca_0.2CrO₃, exhibited an IR emissivity as high as 0.95 in the spectral region of 3–5 μm, which was 33.8% higher than that of LaCrO₃. By using La_0.8Ca_0.2CrO₃ as IR radiation agent in surface coating of heating unit, the radiative heat transfer could be enhanced significantly. The mechanism of the high IR emissivity of La_0.8Ca_0.2CrO₃ was attributed to the following aspects: Ca²⁺ doping introduced an impurity energy level of Cr⁴⁺ into LaCrO₃ and increased the hole carrier concentration, enhancing both impurity absorption and hole carrier absorption in the IR region; moreover, the doping caused lattice distortion enhanced the lattice vibration absorption. This novel high IR emissivity ceramic shows a promising future in high‐temperature applications for the purpose of energy‐saving.     

Studies on the redox properties of chromite perovskite catalysts for soot combustion

This paper deals with the preparation (by combustion synthesis), the characterization (by XRD, AAS, BET, SEM, TEM, TPD/R, and XPS analyses), the catalytic activity testing (in a temperature-programmed combustion microreactor and in a DSC analyzer), and the assessment of the reaction mechanism of a series of nanostructured soot combustion catalysts based on La–Cr substoichiometric or alkali-metal-substituted perovskites (La_0.9CrO₃, La_0.8CrO₃, La_0.9Na_0.1CrO₃, La_0.9K_0.1CrO₃, La_0.9Rb_0.1CrO₃, La_0.8Cr_0.9Li_0.1O₃), whose performance is compared with that of the standard LaCrO₃. Some conclusions are drawn concerning the role of each single constituting element on the activity of the most promising catalyst, La_0.8Cr_0.9Li_0.1O₃, which is already active well below 400 °C. The role of weakly chemisorbed O⁻ surface species in particular is pointed out as crucial for the soot combustion process. This indicates the way for the development of new, more active catalysts, possibly capable of delivering amounts of these oxygen species even higher than those obtained (about 700 μmol / g) for the most active Li-substituted lanthanum chromite catalyst developed.     

A study based on the oxidation vaporization of LaCrO3

This is a study based on the assumption that the volatilization of Cr gives rise to the poor sinterability of LaCrO₃. We have studied the effect of buried materials (none, Al₂O₃, La₂O₃, Cr₂O₃, LaCrO₃) on the sinterability of LaCrO₃ by conventional sintering in air. It was observed that the sinterability of LaCrO₃ in air is largely enhanced by burying with La₂O₃ (with a linear shrinkage of 12.5% and a relative density of 86.90%). The energy dispersive spectroscopy (EDS) infers that there is a volatilization of Cr in sintering of LaCrO₃ in air and the volatilization of Cr does affect the sinterability of LaCrO₃.     

Electrochemical Determination of Gibbs Energy of Formation of LaCrO3 Using a Composition‐Graded Bielectrolyte

Presented are new measurements of the standard Gibbs free energy of formation of rhombohedral LaCrO₃ from component oxides La₂O₃ and Cr₂O₃ in the temperature range from 875 to 1175 K, using a bielectrolyte solid‐state cell incorporating single crystal CaF₂ and composition‐graded solid electrolyte (LaF₃)_y·(CaF₂)_1?y (y = 0–0.32). The results can be represented analytically as (±2270)/J·mol^?1 = ?72329 + 4.932 (T/K). The measurements were undertaken to resolve serious discrepancies in the data reported in the literature. A critical analysis of previous electrochemical measurements indicates several deficiencies that have been rectified in this study. The enthalpy of formation obtained in this study is consistent with calorimetric data. The standard enthalpy of formation of orthorhombic LaCrO₃ from elements at 298.15 K computed from the results of this study is /kJ·mol^?1 = ?1536.2 (±7). The standard entropy of orthorhombic LaCrO₃ at 298.15 K is estimated as 99.0 (±4.5) J·(mol·K)^?1.     

Chromium in lead metasilicate glass: Solubility,valence, and local environment via multiple spectroscopy

In this article the doping of chromium in a lead metasilicate glass is explored using Raman, UV–Vis, and Cr K-edge XANES spectroscopy. Our results indicate that the Cr ions are predominantly present as CrO₄^2? complexes, with a minor amounts of Cr(III). The solubility of Cr is limited to ～1 mol % Cr₂O₃. This boundary was inferred from the intensity changes to the CrO₄^2? stretching vibrations and confirmed by the inability to produce a homogeneous glass containing 2 mol % Cr₂O₃. The establishment of the solubility limit of Cr in lead glasses is important to design more effective glass compositions depending on the desired application. Finally, this low solubility indicates that Cr₂O₃ could be used as a potential nucleating agent for lead-silica based glass-ceramics.     

Stability of calcium substituted lanthanum chromites used as SOFC anodes for methane oxidation

La_0·7Ca_0·32CrO₃, La_0·75Ca_0·15Sr_0·1Cr_0·95Mg_0·05O₃, La_0·85Ca_0·15CrO₃ and La_0·85Ca_0·15Cr_0·9Mg_0·1O₃ compounds were studied in reducing atmospheres containing hydrogen, methane and CO/CO₂. They were found to exhibit liquid phase sintering except for La_0·85Ca_0·15CrO₃. Mg enhanced the densification of stoichiometric LaCrO₃. Current was found to affect the stability of the heavily doped samples, whereas the reducing atmospheres did not have a detectable impact. Calcium and or strontium and chromium enriched secondary phases were detected on current treated La_0·7Ca_0·32CrO₃ and La_0·75Ca_0·15Sr_0·1Cr_0·95Mg_0·05O₃ samples. La_0·85Ca_0·15Cr_0·9Mg_0·1O₃ did not degrade under current, at least for 100 h. A current enhanced demixing could be responsable for the segregation. The solubility limit of calcium in LaCrO₃ is thought to be low at 800°C (around 15%).     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.     

Mechanochemical synthesis of LaCrO3 by grinding constituent oxides

Hydrous amorphous chromium oxide (Cr₂O₃·nH₂O) and crystalline one (Cr₂O₃) powders prepared by heating Cr(OH)₃·nH₂O at different temperatures were separately ground with La₂O₃ powder using a planetary ball mill at room temperature to investigate mechanochemical synthesis of LaCrO₃. Hydrous amorphous chromium oxide prepared reacts with La₂O₃ to form LaCrO₃ and the reaction proceeds with an increase in grinding time. No reaction takes place in the case of crystalline Cr₂O₃. The ground sample consists of agglomerates and large particles with size up to around 100 μm and shows low specific surface area of about 5 m²/g. The mechanochemical method can be also used to synthesize other chromium rare earth complex oxides by grinding the mixture of hydrous chromium oxide and rare earth (Pr, Nd and Sm) oxides.     

Thermal evolution of Al2O3–CaO–Cr2O3 castables in different atmospheres

Al₂O₃–CaO–Cr₂O₃ castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al₂O₃–CaO–Cr₂O₃ castables were prepared and heated at 300–1500 °C in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO₄ and Ca₄Al₆CrO₁₆ formed in air at 300–900 °C and 900–1300 °C respectively, while C₁₂A₇ and CA₂ were generated rather than forming Cr(VI) compounds in coke bed at 700–1300 °C. However, at 1500 °C, nearly all the chromium existed in the form of (Al_1-xCr_x)₂O₃ solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0–1697.8 mg/kg of Cr(VI) at 500–1300 °C but only 17.2 mg/kg of Cr(VI) at 1500 °C, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr₂O₃ and Al₂O₃ was suppressed and a trace of Cr₂O₃ would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.     

Phase diagram of the Al2O3–ZrO2–La2O3 system

The phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Three new ternary and two new binary eutectics were found. The minimum melting temperature is 1665 °C and it corresponds to the ternary eutectic LaAlO₃ + T-ZrO₂ +  La₂O₃·11Al₂O₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. The latter fact is the theoretical basis for creating new composite ceramics with favorable properties in the Al₂O₃–ZrO₂–La₂O₃ system.     

Preparation of calcium doped LaCrO3 fine powders by hydrothermal method and its sintering

The synthesis of fine powders of LaCrO₃ and its solid solutions doped with calcium under hydrothermal conditions and the sintering of these powders were investigated. Precursor alkaline coprecipitated lanthanum chromite gels with three different compositions: LaCrO₃, La_0.9Ca_0.1CrO₃ and La_0.8Ca_0.2CrO₃, were processed under hydrothermal conditions at low temperatures (350–425 °C), for a reaction time between 30 and 120 min. Powders of a single phase with orthorhombic structure of LaCrO₃, La_0.9Ca_0.1CrO₃ and La_0.8Ca_0.2CrO₃ were obtained at a temperature as low as 350, 400 and 425 °C, respectively, for a short reaction interval of 1 h. SEM and TEM micrographs showed that particles with an irregular morphology and an average particle size of 300 nm, were mainly obtained under hydrothermal conditions. The powders were pressed by cold isostatic pressing at 200 MPa, and then sintered in air at a temperature range of 1200–1500 °C for various intervals (1 to 5 h). A maximum apparent density of 97.7% was achieved on specimens with high calcium content, La_0.8Ca_0.2CrO₃, at 1400 °C for 5 h. The average grain size measured on the sintered specimens was 6 μm.     

Investigating the solid phase synthesis of lanthanum chromite

Conclusions We studied the kinetics of solid phase synthesis of lanthanum chromite and solid solutions based on it in the system La₂O₃-Cr₂O₃-CaO-O₂.A relationship was obtained between the degree of conversion in the system and the synthesis temperature, the exposure time, and the CaO concentration. The synthesis of pure LaCrO₃ from oxides is quantitatively complete at 1695 K and with an exposure time of 2 h. The presence in the original mixture of CaCO₃ leads to the formation, in addition to LaCrO₃, of the compound -Ca(CrO₂)₂, and then with increase in the synthesis time to 10 h, at 1695 K, to the formation of solid solutions of the type La_1–xCa_xCrO₃ (x0.25). We determined the constants for the crystal lattices for the synthesized solid solutions.The technological parameters determined for the solid-phase synthesis can be used in work on the preparation of resistor elements made of lanthanum chromite.Translated from Ogneupory, No. 3, pp. 9–12, March, 1990.     

Complex Effect of Sm3+/W6+ Codoping on α‐β Phase Transformation and Phonon Scattering of Oxygen‐Deficient La2Mo2O9

Lanthanum molybdate, La₂Mo₂O₉, has been attracted considerable attention owing to its high concentration of intrinsic oxygen vacancies, which could be reflected by enhanced phonon scattering and low thermal conductivity. A new series of La₂Mo₂O₉‐based oxides of the general formula La_2?xSm_xMo_2?xW_xO₉, where x ≤ 0.2, were synthesized by citric acid sol–gel process. The variation in thermal conductivity with Sm³⁺and W⁶⁺ fractions was analyzed based on structure information provided by X‐ray diffraction and Raman spectroscopy. The fully dense La_2?xSm_xMo_2?xW_xO₉ ceramics showed a minimum thermal conductivity value [κ = 0.84 W·(m·K)^?1,T = 1073 K] at the composition of La_1.8Sm_0.2Mo_1.8W_0.2O₉, which stems from the multiple enhanced phonon scatterings due to mass and strain fluctuations at the La³⁺ and Mo⁶⁺ sites as well as the high concentration of intrinsic oxygen vacancies embedded in the crystal lattice. The thermal conductivities present an abrupt decrease at the structural transition, which is due to the phase transformation from a low‐temperature ordered form (monoclinic α‐La₂Mo₂O₉) to a high‐temperature disordered form (cubic β‐La₂Mo₂O₉₎.     

Fabrication and characterization of La2O3–Fe2O3–Bi2O3 nanopowders: Effects of La2O3 addition on structure,optical, and radiation-absorption properties

In this study, we fabricated and characterized six new nanopowders representing variations of La₂O₃–Fe₂O₃–Bi₂O₃, i.e., 100Bi₂O₃, 30Fe₂O₃–70Bi₂O₃, 3La₂O₃–27Fe₂O₃–70Bi₂O₃, 7La₂O₃–23Fe₂O₃–70Bi₂O₃, 10La₂O₃–20Fe₂O₃–70Bi₂O₃, and 20La₂O₃–10Fe₂O₃–70Bi₂O₃ (represented by 100B, 30F70B, 3L27F70B, 10L20F70B, and 20L10F70B, respectively). These nanopowders were prepared by the microwave-assisted hydrothermal synthesis method. Saponin extract from soapnuts was used as the nanoparticle capping agent. The structural, optical, and gamma radiation characteristics were measured, calculated, and analysed, respectively. The chemical structures of the nanocomposites influenced their optical and radiation shielding characteristics. The optical bandgaps of the 100B, 30F70B, 3L27F70B, 7L23F70B, 10L20F70B, and 20L10F70B nanopowders were 3.16, 3.13, 3.43, 3.45, 3.46, and 3.58 eV, respectively. The ranges of the mass attenuation coefficients of the nanopowders were computed, using XCOM, to be 0.0412–5.1624, 0.0401–4.5406, 0.0401–4.5285, 0.0401–4.5129, 0.0401–0.5015, and 0.0400–4.4156 cm²/g, respectively, and the ranges of mass energy absorption coefficients were found to be 0.0232–1.7525, 0.0228–1.5484, 0.0228–1.5598, 0.0288–1.5746, 0.0228–1.5853, and 0.0227–1.6192 cm²/g, respectively, for photon energies in the range of 0.1–10 MeV. The order of the dose rate trend was as follows: 30F70B < L27F70B < 7L23F70B < 10L20F70B < 20L10F70B. Analysis of the photon interaction parameters showed that the synthesized nanopowders could function well as fillers in radiation-shielding matrices.     

Structure and high-temperature tribological properties of Pb-Cr-O composite films prepared by reactive magnetron sputtering

The formation of ternary composite oxides in a high-temperature environment has laid the foundation for the design of high-temperature wear-resistant self-lubricating film. A series of Pb-Cr-O films with different Cr contents were prepared by incorporating different ratios of Pb-Cr into a target in the reactive magnetron sputtering system. The results showed that the hardness of the Pb-Cr-O films is greatly improved compared to the pure Pb-O film. In addition, the Pb₂₉Cr₄O₆₇ film with the highest Cr content forms an amorphous structure due to the accumulation of Cr⁶⁺ at the grain boundary, which improves the H/E and H³/E² of the film. At 600 °C, in contrast with the single PbO lubricating phase formed by pure Pb₃₃O₆₇ film, the Pb₂₉Cr₄O₆₇ film forms a composite lubricating phase of Pb₅CrO₈ and PbO. This leads to a decreased wear rate as low as 7.2 × 10^?6 mm³N^?1m^?1 while maintaining low coefficient of friction comparable to pure Pb₃₃O₆₇ film. At higher temperature of 700 °C, Cr element in Inconel 718 matrix diffuses into the Pb-based oxide film and forms Pb₅CrO₈ phase similar to Pb₂₉Cr₄O₆₇ film, which improves the wear resistance of the Pb₃₃O₆₇ film while maintaining low friction coefficient of 0.15.     

Fabrication and Upconversion Luminescent Properties of Er3+‐Doped and Er3+/Yb3+ Codoped La2O2CN2 Nanofibers

La₂O₂CN₂:Er³⁺and La₂O₂CN₂:Er³⁺/Yb³⁺ upconversion (UC) luminescence nanofibers were successfully fabricated via cyanamidation of the respective relevant La₂O₃:Er³⁺ and La₂O₃:Er³⁺/Yb³⁺ nanofibers which were obtained by calcining the electrospun composite nanofibers. The morphologies, structures, and properties of the nanofibers are investigated. The mean diameters of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are 179.46 ± 12.58 nm and 198.85 ± 17.07 nm, respectively. It is found that intense green and weak red emissions around 524, 542, and 658 nm corresponding to the ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_l5/2 energy levels transitions of Er³⁺ ions are observed for La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers under the excitation of a 980‐nm diode laser. Moreover, the emitting colors of La₂O₂CN₂:Er³⁺ and La₂O₂CN₂:Er³⁺/Yb³⁺ nanofibers are all located in the green region. The upconversion luminescent mechanism and formation mechanism of the nanofibers are also proposed.