Lanthanum Hexaaluminates Research Articles

Lattice Oxygen Lability of La-Hexaaluminates Substituted by Different Metals and Their Effects on the Thermal Stability of Supported Nano-Ir Particles.

Oxide supports with high lattice oxygen lability can stabilize the supported nanoparticles at high temperatures. The lattice oxygen lability of lanthanum hexaaluminates (LHAs) substituted with other metals (such as Mg and Fe) as well as their effects on the thermal stability of supported Ir particles were investigated via CO chemisorption, hydrogen temperature-programmed reduction (H2-TPR), oxygen temperature-programmed desorption (O2-TPD), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), and scanning electron microscopy/transmission electron microscopy (SEM/TEM) techniques. The H2-TPR results showed that the lattice oxygen lability of lanthanum iron hexaaluminate (LFA) was much higher than that of lanthanum magnesium hexaaluminate (LMA). This variation could be attributed to the difference in the reducibility of Fe/Mg atoms and their substitution sites in the crystallographic lattice. Under the reductive condition, the H2-TPR presented that the amount of reducible lattice oxygen of LFA supported by metallic Ir decreased significantly, implying the existence of the migration of lattice oxygen and formation of oxygen vacancies, as revealed by O2-TPD and XPS results. After thermal aging at 1200 °C, the amount of residual Ir in LFA was about 4 times that of LMA, as shown in the ICP results. The mean size and dispersion of Ir particles in LFA were better than those in LMA, as revealed by the SEM/TEM results, showing the superior thermal stability of the Ir particles in LFA support. Hence, this study concludes that the lattice oxygen lability plays an important role in improving the thermal stability of the Ir@LHAs at high temperatures. Based on characterization results, a model was proposed to explain the interaction between Ir and LHAs and its effect on the thermal stability of the Ir particles.

Heat and moisture resistance of divalent metals substituted La-Hexaaluminates prepared by reverse microemulsion

Lanthanum hexaaluminates (LHAs) substituted with divalent metals (Mg, Zn) were successfully prepared by the reverse microemulsion (RM) method and high-temperature treatment. Phase composition, microstructure, porosity characteristics, and their thermal evolution were investigated using X-ray diffractometry, scanning electron microscopy, N2 isothermal sorption and thermogravimetry-differential scanning calorimetry, respectively. The thermal stability of substituted hexaaluminates was also evaluated in both dry and moisture atmosphere at 1200-1600 °C. Experimental results showed that the RM method promoted the formation of pure and well crystallized magnetoplumbite hexaaluminates with high specific surface areas at temperatures 1100-1200 °C. For both obtained hexaaluminates, their phase composition and nanometer-sized crystallites were retained at temperature as high as 1600 °C in dry air. However, LaMgAl11O19 presented severe deterioration in flowing water vapor at 1300 °C including about 50% weight loss and the remarkable crystalline structure change, whereas the substitution of Zn enabled the LaZnAl11O19 to possess superior moisture resistance. According to the partial charge model of cations in aqueous media, a “hydrolysis-corrosion” mechanism was proposed to describe the degradation of hexaaluminates in high-temperature water vapor. It was revealed that the high-temperature moisture resistance of substituted LHAs was inversely proportional to the hydrolysis tendency of substituted metal cations.

Effect of seeding on the formation of lanthanum hexaaluminates synthesized through advanced sol gel process

Advanced sol gel processing was used to synthesize high purity lanthanum hexaaluminate powders. The effect of seeding on the formation of lanthanum hexaaluminate was also studied by seeding the gel. Dry gel was calcined at various temperatures starting from 1100°C to 1600°C for 2h to study the phase evolution. The combine effects of advanced sol gel processing and the presence of seeds promoted the formation of lanthanum hexaaluminate phase at lower temperature than the conventional routes. Lanthanum hexaaluminate phase was detected at 1201°C and 1300°C in seeded and un-seeded gels, respectively. The presence of seed decreases the temperature of formation of lanthanum hexaaluminate by 99°C. Single phase lanthanum hexaaluminate was formed at 1600°C in seeded gel whereas trace of lanthanum monoaluminate phase still present in un-seeded gel even at 1600°C.

Preparation and investigation of rare earth magnesium hexaaluminate solid solutions

Conditions of preparation, by the method of solid state reactions, of rare-earth hexaaluminates RE1−x−yMxM’yMgAl11O19 (RE = La, Sm; M, Mt’ = Gd, Yb, Lu, Y, Sc; x, y = 0, 0.15, 0.3), were investigated. For a number of compositions, high-degree single-phase products were obtained applying multi-step heat treatments in Ar/H2 atmosphere at 1650–1690°C. Intense (107) and (114) diffraction lines typical for the hexaaluminate phase have been observed in X-ray diffraction patterns. Studies of microstructure and of elemental composition showed that magnesium deficiency on the sample surface may reach some 20%, while composition in the bulk is more homogeneous and close to stoichiometric. Our estimations of structural homogeneity and thermal conductivity show that lanthanum hexaaluminates La1−x−yMxM’yMgAl11O19 with pair additives Gd-Yb, Gd-Y, Y-Yb, Y-Lu, Y-Sc (x = y = 0.15) and samarium hexaaluminates Sm1−x−yMxM’yMgAl11O19 with pair additives Gd-Yb, Y-Yb (x = y = 0.15), as well as Sm0.7Yb0.3MgAl11O19, may present interest as thermal barrier coatings.

Effect of the Precursor Solution Concentration and Modification Ions on the Catalytic Property of La-Hexaaluminate Catalysts

Lanthanum hexaaluminates LaAl12O19-δ modified by transition-metal ion (Mn, Co and Ni ) were prepared through co-precipitation of metal nitrates within the water phase of a Triton X-100/n-hexanol/cyclohexane reverse microemulsion. The effects of different modification ion and the precursor solution concentration on the catalytic property of catalysts were investigated. The structure and properties of these samples were characterized by XRD, BET, SEM and TG-DTA. The stabilities of catalysts were assessed by measuring the activities for combustion of methane. Upon calcination at 1200°C, when the precursor solution concentration is 0.75M, the sample LaMnNiAl10O19-δ retained a specific surface area of 40.34m2/g and showed a excellent activity for methane combustion, which temperature at 10% , 90% and 99% conversion were 420 °C, 690 °C and 760 °C, respectively.

Catalytic combustion of methane over iron- and manganese-substituted lanthanum hexaaluminates

A series of hexaaluminates, LaMnFexAl11−xO19−δ samples (x = 1, 2, 4, 6, 8) as new catalysts were prepared by carbonate precipitation and calcined at high temperature. Fe and Mn ions were used as active components to replace part of aluminum ions in the hexaaluminate lattices. The structures and properties of these samples were characterized by XRD, BET, and XPS. The series of hexaaluminates exhibited significant catalytic activity and stability at high temperature. The LaMnFe2Al9O19−δ retains a larger surface area and shows a good activity in methane combustion.

Catalytic properties of a (A=Ba, Ca, Sr and Y) modified lanthanum hexaaluminates for catalytic combustion of methane

A series of hexaaluminates, La0.8A0.2MnAl11O19−δ samples (A = Ba, Ca, Sr and Y) as new catalysts were prepared by carbonate precipitation and calcined at high temperature. The structure and properties of these samples were characterized by XRD, BET and XPS techniques. Upon calcination at 1200°C, the hexaaluminate structure was formed and it retained the specific surface area of 17∼20 m2g−1. The La0.8Ca0.2MnAl11O19−δ catalyst has a surface area of 19.3 m2g−1 and shows a good activity in CH4 combustion.

Cationic disorder effects in complex oxide laser materials and phosphors

A comparative analysis of the high-resolution spectra of the Nd 3+ and Pr 3+ in strontium lanthanum hexa-aluminates, charge compensated with magnesium Sr 1− x (Nd, Pr) y La x– y Mg x Al 12− x O 19, on a large composition range is presented. The spectra are mainly dependent on the composition parameter x and show two families of centers for x < 0.5 and three for x ⩾ 0.8, with distinct spectral characteristics. Based on spectral data, composition dependence and crystal structure, tentative structural models for the these centers are proposed. It is inferred that this complex picture results from the interplay between the effects determined by the anionic coordination of the centers and the perturbative effects caused by the multiple occupation of specific sites by cations of different valence.

Effect of water in the precursor solution on the catalytic property of LaMnAl11O19 high temperature combustion catalyst

Manganese-substituted lanthanum hexaaluminates were prepared by the sol-gel method. The effect of water content on the catalyst preparation was examined. Large water content caused a rapid decrease in the surface area. When the R (R is the mole ratio of H2O/ Al(OC3H7)3) is equal to 1.5, the catalyst retains a specific surface area of 13.5 m2 g−1 and shows higher activity in CH4 combustion.

Catalytic properties of Fe and Mn modified lanthanum hexaaluminates for catalytic combustion of methane

Abstract A series of Fe and Mn ions modified lanthanum hexaaluminates, LaFeMn x Al 11−x O 19−δ ( x = 0, 0.5, 1, 2, 3, 4, 5) as new catalysts for combustion of methane, was prepared by sol–gel method and calcined at high temperature. Fe and Mn ions were used as active components to replace part of aluminum ions in the hexaaluminates lattices. The structure and properties of these samples were characterized by XRD, BET, TG-DTA and XPS. The series of hexaaluminates exhibited significant catalytic activity and stability at high temperature. The LaFeMnAl 10 O 19− δ catalyst possesses a larger specific surface area and shows a higher activity than the others in methane combustion.

X-ray induced defects and thermoluminescent properties of lanthanum hexaaluminates with magnetoplumbite-like structure

This work presents an electron spin resonance and optical study of the defects produced by X-ray irradiation of the lanthanum aluminate La(Mg 1− y Mn y ) x Al 11O 18+ x ( x, y ≤ 1). The thermal bleaching of these defects is responsible for an intense green thermoluminescence due to the 4 T 1- 6 A 1 transition of Mn 2+ ions of the lattice. The mechanism of this thermoluminescence and its variation with manganese concentration are discussed.