Low La Content Research Articles

Comparison of conventional and solid-state ion exchange procedures for the incorporation of lanthanum in H-beta zeolite

Abstract Lanthanum-exchanged beta zeolites have been prepared by a solid-state ion-exchange procedure which consists of calcining in static conditions a mechanical mixture of LaCl 3 .7H 2 O and H-beta zeolite. The exchanged samples have been characterized at various steps of their preparation by XRD, BET, FTIR, TEM, EDS and chemical analyses. The results have been compared to those obtained on samples exchanged in solution. In the case of conventional ion exchange in solution, the incorporation of La into the zeolitic pores is strongly limited due to the steric hindrance caused by the large hydration sphere of the La 3+ ions. As a consequence, strongly heterogeneous samples contaminated by La-rich crystalline particles are obtained even at rather low La contents or after repeating the exchange several times. In contrast, the yield for lanthanum incorporation reaches values as high as 100% when the solid-state exchange procedure is used. Moreover, the samples are highly homogeneous and no loss of crystallinity is observed in spite of the very simple conditions used (calcination in static). The exchange by La species of protons initially present in the H-beta parent sample is confirmed by FTIR spectra of OH groups, which also reveal that part of the incorporated La species interact with the silanol groups.

A comparative study of the ac-field dependence of ‘soft’ and relaxor PLZT ferroelectrics

Significant differences in the ac-field dependence of the dielectric constant, polarization hysteresis response, and the fatigue behavior have been found between low La-content “soft” and high La-content relaxor ferroelectric behaviors in lead zirconate titanate (PZT) ceramics. X-ray diffraction and transmission electron microscopy studies revealed a decrease in domain regularity with La concentration in “soft” ferroelectrics. This domain breakdown is believed to be responsible for the anomalous ac field effects. An ac field induced relaxor behavior observed in low La content “soft” PZT requires an extended concept of relaxor behavior.

AC Field Induced Polarization Disordering in The Ferroelectric Phase Of Modified Pzt Ceramics

ABSTRACTPronounced nonlinear enhancements in dielectric responses and decreases in phase transformation temperatures were induced under moderate ac fields and high field cycling for La modified lead zirconate titanate (PZT) ceramics. Polarization disordering could be inhibited by introducing acceptor dopants into La modified PZT compositions and by DC bias field. Analysis of the frequency dispersion revealed that the field driven transformation in lower La content PZT is a field assisted activation process, while the transformation in higher La content PZT relaxors is intrinsically a thermal activated process.

Role of charge compensation mechanism in La-modified Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics: Enhanced ordering and pyrochlore formation

Selected area electron-diffraction (SAED) investigations were performed on La-modified (1−x)Pb(Mg1/3Nb2/3)O3–x(PbTiO3) ceramics for compositions near the morphotropic phase boundary for various nonstoichiometric ordering conditions of Mg and Nb on the B-site cation positions. Studies were performed on specimens with La contents between 0 and 10 at. % and for x=0.35. Specimens were fabricated either by: (i) keeping the average Mg/Nb/Ti ratio unchanged and while creating B-site vacancies (La–PMN–PT), or (ii) changing the average Mg/Nb/Ti ratio (PLMN–PT). For La–PMN–PT, SAED studies revealed the presence of F spots for high La contents. The superlattice reflection intensity was found to increase with increasing La content. However, for PLMN–PT, distinct diffuse superlattice reflections were observed at low La contents, which then decreased significantly in intensity with increasing La substitution. Quantitative analysis using standardless energy-dispersive x-ray spectroscopy was also performed. These investigations revealed that enhanced pyrochlore formation accompanies enhanced B-site cation ordering with increasing La content for La–PMN–PT.

The effect of lanthanum promoter on the selectivity of Pd/zeolite-X in methanol synthesis

The promoting effect of various amounts of lanthanum on Pd/zeolite-X in enhancing selectivity in CO hydrogenation toward methanol/oxygenates has been investigated. The catalysts were characterized by temperature-programmed reduction, O 2-H 2 titration, and temperature-programmed desorption. The dispersion and H 2 chemisorption of Pd/zeolite-X catalysts were almost unchanged on La promotion. The virtually unchanged chemisorption capacity of the Pd/zeolite-X catalysts on La promotion is attributed to the cationic exchange method adopted to introduce La after Pd introduction into the zeolites. The La 2O 3 formed during calcination was partially reduced during reduction with hydrogen due to its contact with palladium. PdLaO x entities formed thereby trigger the activation of coordinated CO/H 2 and facilitate the hydrogenation of CO to methanol/oxygenates at low temperature. Thus, the Pd sites responsible for methanol synthesis are stabilized by La at low and high pressures. Dimethyl ether, dominant for low La content Pd/zeolite-X catalysts, may be ascribed to the acid sites developed, but other mechanisms cannot be ruled out. The effect of the zeolite matrix lies in the stabilization of PdLaO x entities, but otherwise it behaves in a similar fashion to other supports. The zeolite lattice, however, ensures the maintenance of small PdLaO x particles. The turnover frequency values obtained with the optimum La promotion on Pd/zeolite-X are consistent with the reported values observed on Pd/La 2O 3 catalysts. The zeolites are therefore potential alternative supports in the preparation of suitable bimetallic systems.

ChemInform Abstract: Multiphase Alloys La2‐xCaxMg17 for Hydrogen Storage.

AbstractThe multiphase alloys La2‐xCaxMg17 with a low La content (1.4 ≤ x ≤ 1.8) show good hydrogen absorption‐desorption characteristics and exhibit the highest hydrogen capacity at the lowest hydriding pressure (5 bar).

Study of phase transitions and properties of tetragonal (Pb,La)(Zr,Ti)O 3 ceramics—II : Diffuse phase transitions and thermodynamics

The diffuse phase transition in tetragonal PLZT materials has been investigated. Strongly broadened permitivity curves were observed for materials with a relative high La content. Curie-Weiss behaviour in the permittivity curves is observed only at temperatures far above the apparent Curie temperature. A latent heat effect is observed at the sharp FE t → PE c transition in PLZT x/30/70 materials with a low La content ( x<16 at° La). This heat effect decreases linearly with increasing La content. The diffuse tetragonal → cubic transition is attended with a very small heat-effect and a low value (≈0.04 cal/mol°C) for the entropy difference between high and low temperature phase could be determined. Consequently, polarisation fluctuations are possible in a certain temperature interval around the transition.

Study of phase transitions and properties of tetragonal (Pb,La)(Zr,Ti)O 3 ceramics—I : Phase diagram and β-phase

The phase diagram ( T− x plot) of thermally depoled tetragonal PLZT materials has been investigated by means of X-ray diffraction, Capacitance and DSC measurements. Materials with a low La content show a classical Fe t ⇇PE c transition. This transition has first or second order character, dependent on the Zr/Ti ratio. Materials with a medium and high La content show a diffuse transition from the cubic high temperature phase to a tetragonal, so called β phase. This β phase has no clear FE properties and a spontaneous transition to the FE t phase on lowering the temperature only takes place for materials with a medium La content. For materials with a high La content a spontaneous β t → FE t transition is not observed.

Measurements relating to the genesis of the Tsumeb pipe, South West Africa

The Tsumeb ore-body is one of a number of steep pipe-like structures that pierce the Otavi Series of the Damara System in South West Africa. These structures lie on a well-defined ENE belt of ring-complexes; included in the latter are carbonatites. Strontium isotope and La and Ce analyses were carried out on 13 carbonate rocks taken from within the pipe and country-rocks. The mean 87Sr/ 86Sr value of ≈0.712 combined with the low La and Ce contents of all the samples does not support a carbonatitic origin for the pipe. The problem of the age of the Tsumeb pipe remains unresolved. The most likely events that could be responsible for the development of the pipe are the post-kinematic Damara granites of Cambrian age or alternatively they could be associated with the magmatic activity responsible for the emplacement of the post-Triassic ring complexes.