La-doped Alumina Research Articles

La-Doped Alumina, Lanthanum Aluminate, Lanthanum Hexaaluminate, and Related Compounds: A Review Covering Synthesis, Structure, and Practical Importance

Advanced materials based on lanthanum–aluminum oxides are useful for many practical applications, not the least in catalysis, energy conversion and energy storage systems designed for a sustainable world economy. The three main La–Al–O (LAO) combinations of commercial interest are La-doped aluminas (LDA), lanthanum monoaluminate (LMA, a perovskite), and lanthanum hexaaluminate (LHA). Usually, the literature treats doped aluminas, perovskites, and hexaaluminates separately. The current review tries to identify common trends and features with respect to structure, synthesis, and applications of LAOs, which should lead to a more holistic understanding of their chemistry. This, in turn, should trigger innovative ideas with respect to new or improved applications. Apart from a variety of uses at low temperatures, the main advantages of LDA, LMA, LHA, and related compounds are their high-temperature stability and the extended applicability that comes with it.

Size effects on the formation of LaAlO3 phase in La-doped γ-Al2O3 after hydrothermal treatment

A series of La-doped alumina ceramics were prepared by a strong electrostatic adsorption technique in comparison with a conventional incipient wetness impregnation method. The commercial La-doped alumina support was used as a reference sample. The samples were calcined in a temperature range of 800–1200 °C. The characterization of the ceramics was performed using low-temperature nitrogen adsorption, transmission electron microscopy, X-ray diffraction analysis, and photoluminescence. A strong electrostatic interaction between alumina and lanthanum was found to affect the stabilization of alumina and the resistance of La2O3 towards agglomeration during the wet procedures. The size effects related to the formation of LaAlO3 of ∼10 nm in size from the amorphous oxide phase take place on the surface of alumina doped with lanthanum. In cases of the commercial La-modified alumina sample and the sample prepared by wetness impregnation with lanthanum nitrate, the hydrothermal treatment under mild conditions has two effects. Firstly, the La2O3 particles weakly bonded with the support grow in size. Secondly, the high-temperature calcination of the samples leads to the formation of the LaAlO3 phase. Oppositely, the samples prepared by the adsorption of La(EDTA)- complexes showed no formation of the LaAlO3 phase neither before nor after the hydrothermal treatment procedure. The key factor accelerating the dissolution of the La2O3 particles was found to be the regime of the suspension stirring.

A comparative study on the thermal stability, textural, and structural properties of mesostructured γ-Al2O3 granules in the presence of La, Sn, and B additives

Mesoporous γ-alumina is widely used as catalyst support in various catalytic reactions of industrial interest. However, due to the instability of γ-alumina at elevated temperatures, many efforts have been reported to inhibit the α-alumina phase transition through doping with suitable metalloids, as well as transition, post-transition, or rare-earth elements. In the present study, undoped and La-, Sn-, and B-doped alumina granules were synthesized via sol-gel/oil drop method with the aim to clarify the role of the additives and their content on the porous structure as well as on the chemical, structural, and microstructural behavior of γ-alumina. XRD and DTA/TG results demonstrated that thermal stability of transition aluminas increases more than 100 °C by 3 wt% lanthanum and tin doping; however, boron doping seems to have only negligible effect on the thermal stability. On the other hand, based on nitrogen adsorption-desorption analysis, tin and boron-doped aluminas showed a higher surface area at 750 °C (between 214.74 m2/g to 245.97 m2/g) but higher loss in the surface area after calcination at 1200 °C (between 25.45 m2/g to 8.57 m2/g). On the contrary, the 3 wt% La-doped alumina sample, with a relatively high surface area at 750 °C (227.17 m2/g), exhibited the highest surface area after calcination at 1200 °C (53.07 m2/g). 27Al MAS NMR and HRTEM studies indicated that the presence of 3 wt% La in alumina structure leads to thermal and mesoporous structure stability up to 1200 °C by inhibiting oxygen lattice restructuring. These results provide a comparative perspective of La, B, and Sn additives' behavior in γ-alumina.

Luminescence of alumina ceramic doped with lanthanum under medium- and high-dose irradiation

The synthesis of La-doped alumina ceramics results in a formation of new pulsed cathodoluminescence bands and thermoluminescence peaks which associated with phases of lanthanum aluminate (LaAlO3) and lanthanum aluminium oxide (LaAl11O18). Annealing at high temperatures (up to 1600 °C) with maximum lanthanum concentration allows to create ceramics with high intensity of 510 pulsed cathodoluminescence band and 370 K thermoluminescence peak. The high intensity of new peaks thermoluminescence demonstrates the material sensitivity to ionizing radiation and the possibility of its use as ionizing radiation detectors in the case of expanding the sublinear dose recording range.

The Impact of La Doping on Dry Reforming Ni-Based Catalysts Loaded on FSP-Alumina

The activity of nickel-based catalysts makes them promising for the dry reforming of methane (DRM), but their implementation is limited by instability which stems from undesirable selectivity to side reactions that promote carbon formation. Cobalt can be added in the form of a bimetallic Ni–Co catalyst to improve stability although this can be at the expense of activity. Here we report the impact of lanthanum doping of an alumina support on the performance of neat Ni and bimetallic Ni–Co dry reforming catalysts with the intent of improving both stability/selectivity and activity. Direct investigation of the DRM side reactions demonstrated that introducing La to the alumina support decreased the selectivity of Ni towards the Boudouard and methane cracking reactions. This afforded Ni an enhanced resistance to catalyst coking, thereby improving catalyst stability. Monometallic Ni supported by La-doped alumina was found to be the most active catalyst for the DRM with the La addition also improving catalyst stability via suppression of the Boudouard and methane cracking reactions. The benefits of La-doping on catalyst performance were less pronounced when Co was present alongside the Ni as the La superseded the positive contribution of the Co. Characterisation of the materials illustrated La inclusion into the support was in the form of single atoms highly dispersed across the Al2O3 surface, which improved support basicity and strengthened the interactions between the Ni/Co metal and the support.

Ammonia as an efficient COX-free hydrogen carrier: Fundamentals and feasibility analyses for fuel cell applications

A COX-free 1 kW-class hydrogen power pack fueled by liquid ammonia is presented. For applications in a practical-scale hydrogen production system in conjunction with a polymer electrolyte membrane fuel cell, Ru catalysts supported on La-doped alumina (Ru/La(x)-Al2O3) were pelletized by varying the lanthanum doping content (x mol%) to control catalytic activities. An optimized Ru(1.06 wt%)/La(20)-Al2O3 pellet catalyst presents a >99.7% conversion efficiency at 500 °C under a gas hourly space velocity of 5000 mL gcat−1 h−1. Various materials were screened to remove residual ammonia from the product stream, and the X zeolite was chosen as a highly capable adsorbent. Based on the synthesized catalyst and screened adsorbent, a power pack consisting of a dehydrogenation reactor, an adsorbent tower, and a 1 kW-class polymer electrolyte membrane fuel cell was designed and manufactured. The as-integrated system can convert 9 L min−1 of ammonia into 13.4 L min−1 of hydrogen, powering a 1 kW-class fuel-cell continuously for >2 h without any performance degradation. To achieve autothermal and COX-free operations, heat required for ammonia dehydrogenation was provided by unutilized hydrogen from the fuel cell, drastically increasing the overall efficiency of the system to >49% while removing the external heat source, isobutane. Finally, a drone tethered to the system was operated, demonstrating the feasibility of an elongated flight time of >4 h, much longer than 14 min with Li-polymer battery loaded on the drone. The system is expected to meet the United States Department of Energy’s 2020 gravimetric and volumetric hydrogen storage targets of 4.5 wt% and 30 gH2 L−1 at system weights of 43 kg and 50 kg, respectively.

Peculiarity of Rh bulk diffusion in La-doped alumina and its impact on CO oxidation over Rh/Al2O3

The catalytic behavior and stability of rhodium supported on pure and La-doped alumina were studied towards the CO oxidation reaction. Diffusion of Rh3+ ions into the bulk of support was shown to cause rapid deactivation in both cases. Alumina doping with La leads to modification of the alumina donor sites, concentrating of Rh near such sites, and acceleration of Rh3+ diffusion process. Diffused Rh ions were shown by means of photoluminescence to localize in the nearest coordination environment of the La ions in La-enriched alumina starting from a calcination temperature of 800°C.

Kinetics of grain growth in La-doped ultrapure Al2O3

Kinetics of grain growth was measured in dense polycrystalline ultrapure alumina doped with 100 ppm of lanthanum. The evolution of grain size followed cubic kinetics, which can be attributed to the influence of solutes. Based on the increase of Gibbsian excess of segregating dopants with increase in the average grain size, it can be approximated that boundary velocity scales as inverse-square of grain size, leading to cubic grain growth kinetics. The activation energy of grain growth was found to be 418 ± 22 kJ mol−1 for La-doped alumina; comparable to that of undoped ultrapure alumina, and much lower than the activation energy of creep in La-doped alumina. The grain boundary mobility was found to be almost an order of magnitude lower than that of undoped ultrapure alumina, which can be attributed to the drag of oversized dopants segregated to the grain boundary core. These findings provide a basic understanding of the effects of segregating dopants on grain growth, and can be exploited in the production of coarsening resistant nanograined materials.

Effect of amount of doping agent on sintering, microstructure and optical properties of Zr- and La-doped alumina sintered by SPS

SPS-produced α-alumina samples are prepared from powders doped with different amounts of Zr4+ and La3+ cations. Zr4+ cations segregate at grain boundaries. m-ZrO2 particles are formed at 570 but not at 280 cat ppm. A β-alumina LaAl11O18 structure is found at 310 cat ppm when the lanthanum grain boundary solubility limit is exceeded (∼200 cat ppm). 100 cat ppm La is sufficient to block the diffusion path across grain boundaries and inhibit grain growth. Both doping cations disturb the grain boundary diffusion whatever their amount. They delay the densification at higher temperatures while limiting grain growth. The real in-line transmittance (RIT) of α-alumina is improved due to the reduced grain size. Nevertheless, increasing the cation amount leads to an increase in porosity or even the formation of secondary phase particles, both detrimental for optical properties. Finally, optimised amounts of cation of 200 and 150 cat ppm are found for La- and Zr-doped alumina, respectively.

ChemInform Abstract: Preparation and Characterization of High-Temperature Thermally Stable Alumina Composite Membrane.

A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C.

Formation of featured nano-structure in thermal stable La-doped alumina composite catalyst

Nanometer-order structural changes of lanthanum-modified γ-alumina at elevated temperature were examined for application toward heat-stable catalytic support. The composite powder was prepared by aqueous process using lanthanum (III) nitrate and alumina. The La added aluminas were heated in the temperature range of 600–1200 °C, and then characterized by surface area measurement, X-ray powder diffraction, scanning electron microscope (SEM), and transmission electron microscope (TEM). The solid-state reaction between lanthanum oxide and alumina lead to the nano-structure where the nanometer-size particles of LaAlO 3 are dispersed in the secondary particles of alumina. The tailored microstructure provides a new nano-structured catalyst of La-alumina, which is stable catalytic support at high temperatures.

Effect of La addition on thermal microstructural evolution of macroporous alumina monolith prepared from ionic precursors

Porous alumina monoliths with high thermal stability have been prepared using sol-gel processing from ionic precursors. Samples were synthesized from AlCl3·6H2O and LaCl3·7H2O dissolved in a H2O/etanol solution containing poly(ethylene oxide) (PEO) via propylene oxide (PO) involved sol-gel reaction accompanied by phase separation. The heat-treated gels consist of controllable size of macropores and La-doped alumina skeletons. Doped lanthanum ions are homogeneously embedded in the gel matrix, which retards the phase transition from γ-alumina to α-alumina during the sintering. The 3 mol% La-doped aluminate exhibits the BET surface area of 55.3 m2/g, after the heat treatment at 1100°C for 24 h. Drying condition are also related to the thermal stability of obtained gels. Aluminate aerogels show higher thermal stability than corresponding xerogels due to relatively large mesopore size and high porosity. As a result, the 3 mol% lanthanum doped aluminate aerogel shows BET surface area as high as 82.5 m2/g even after heat treatment at 1100°C for 24 h.

Structural transformation of PdPt nanoparticles probed with X-ray absorption near edge structure

Structural transformation of bimetallic PdPt nanoparticles has been investigated with X-ray absorption near edge structure at high temperatures. PdPt bimetallic nanoparticles have been prepared on a La-doped alumina by a sequential loading of H 2PtCl 6 onto the Pd loaded catalyst. The results of extended X-ray absorption fine structure at Pd K and Pt L III edges showed that Pt was surface-enriched or anchored on the Pd metal core with an increase of the Pt content. The structure of the obtained bimetallic PtPd nanoparticles seemed to be retained upon heating up to 1273 K under ambient condition. The measurement of catalytic activity of these PdPt bimetallic nanoparticles over methane combustion showed that the difference in activity with increasing and decreasing reaction temperatures was disappeared probably due to the synergistic effect of the formation of the PdPt bimetallic nanoparticles.

Synthesis, characterization and catalytic properties of Pt/CeO 2–Al 2O 3 and Pt/La 2O 3–Al 2O 3 sol–gel derived catalysts

Platinum supported on La 2O 3–Al 2O 3 and CeO 2–Al 2O 3 were prepared by the sol–gel method. High specific surface areas between 572–418 m 2/g were obtained. FTIR-pyridine absorption band proved the formation of Lewis sites with comparable intensity in the supports. The stabilization of the γ-Al 2O 3 in the La 2O 3–Al 2O 3 support was observed by X-ray diffraction when the sample was calcined at 1100°C, while segregation of CeO 2 was observed at such temperature in the CeO 2–Al 2O 3 preparation. The formation of Al V and Al VI coordinated aluminum was observed by MAS-NMR 27 Al spectra in La doped-alumina. In the Ce doped-sample only Al VI coordination was identified. Propene and acetone were the products in the iso-propanol decomposition over CeO 2–Al 2O 3, meanwhile, for the La 2O 3–Al 2O 3 support only propene was obtained. The platinum (0.5 wt.%) particle size distribution as determined by electron microscopy showed very small particles <1.0 nm. On the other hand, in the n-heptane dehydrocyclization the activity and selectivity to toluene of the Pt/La 2O 3–Al 2O 3 catalyst was found bigger than those of Pt/CeO 2–Al 2O 3 catalyst. It is proposed that doped-alumina prepared by the sol–gel method has structural defects which induce modification in the metal activity.

Foreword

The synthesis of La-doped alumina ceramics results in a formation of new pulsed cathodoluminescence bands and thermoluminescence peaks which associated with phases of lanthanum aluminate (LaAlO3) and lanthanum aluminium oxide (LaAl11O18). Annealing at high temperatures (up to 1600 °C) with maximum lanthanum concentration allows to create ceramics with high intensity of 510 pulsed cathodoluminescence band and 370 K thermoluminescence peak. The high intensity of new peaks thermoluminescence demonstrates the material sensitivity to ionizing radiation and the possibility of its use as ionizing radiation detectors in the case of expanding the sublinear dose recording range.