Hexaaluminate Phase Research Articles

Effect of Preparation Methods on Activation of Catalysts BaNi 0.2Mn 0.8Al 11O 19-δ for Dimethyl Ether Combustion

Catalytic combustion of dimethyl ether (DME) over hexaaluminate catalyst BaNi 0.2Mn 0.8Al 11O 19-δ has been investigated. The catalysts were prepared with the sol-gel method and reverse microemulsion method respectively and characterized by thermogravimetry-differential thermal analysis, X-ray diffraction and transimission electron microscope. It was found that the formation of Mn, Ni modified hexaaluminate was a relatively slow process via two solid state reactions and spinel structure was a transition phase. At the same calcined temperature and time, the catalyst prepared with the reverse microemulsion method could form the hexaaluminate phase more easily than that prepared with the sol-gel method. The catalyst BaNi 0.2Mn 0.8Al 11O 19-δ prepared with the reverse micro-emulsion method appeared a plate-like morphology, while it appeared a needle-like morphology when using the sol-gel method. The catalytic activities of catalysts BaNi 0.2Mn 0.8Al 11O 19-δ prepared with two different methods for DME combustion were tested. It showed that catalyst prepared with the reverse microemulsion method had better catalytic activity, i.e. T 10% of DME had decreased by 45°C, about 90% conversion of diemthyl ether at 380°C.

Carbon-templated hexaaluminates with enhanced surface area and catalytic performance

Carbon templating was used to synthesize high-surface area LaFeAl 11O 19 hexaaluminates leading to improved catalytic performance. The oxide precursor–template composites were prepared by impregnation, complexation with citric acid, and coprecipitation routes varying the amounts of acetylene black incorporated. Calcination in static air at 1473 K led to single-phased hexaaluminates. The samples were characterized at different stages of the synthetic protocol by ICP–OES, in situ and ex situ XRD, TEM, N 2 adsorption, He pycnometry, TGA, H 2-TPR, and XPS. The templating effect of carbon originates fine oxide particles. Consequently, the specific surface area of the non-templated hexaaluminates (1–11 m 2 g −1 depending on the preparation method) increased up to a factor of 25 upon incorporation of carbon in the synthesis. The stabilization of small particles occurs despite the temperature difference between the elimination of carbon (973 K) and the attainment of the hexaaluminate phase above 1373 K. The morphology of the oxide obtained upon carbon removal determines the textural properties of the hexaaluminate particles. Our results demonstrate that the confined space synthesis is not exclusively responsible for small oxide particles. The stabilization of small crystallites on the carbon surface in the composite and breakage of the oxide layer caused by template combustion are key aspects for attaining finely dispersed particles when the carbon is macroporous. The enhanced catalytic activity and time-on-stream stability of the templated hexaaluminates was demonstrated in the direct N 2O decomposition using model and simulated feed mixtures. In this application, a quasi-linear relation between the reaction rate and the specific surface area of the samples was obtained. Characterization by H 2-TRP and XPS indicated that the nature of iron in LaFeAl 11O 19 was not altered by carbon incorporation in the synthesis.

In situ studies during thermal activation of dawsonite-type compounds to oxide catalysts

The thermal decomposition of metal-substituted ammonium aluminium carbonate hydroxide (AACH) materials with dawsonite-like structure has been studied by using in situ XRD, FT-IR, TGA, and TPD-MS in the temperature range 298–1573 K. The in situ approach enables accurate monitoring of the decomposition process and the nature and stability of the resulting oxide phases. This information is essential to optimize the thermal activation of these versatile materials for subsequent catalytic applications. Pure AACH and the corresponding substituted systems with La and/or transition metals (Fe or Mn) were synthesized by co-precipitation using the carbonate route. In situ XRD indicated the destruction of the AACH phase at 473 K, independent of the composition of the material, leading to an amorphous alumina phase. This transition temperature is in excellent agreement with FT-IR, TGA, and TPD-MS data. The later techniques substantiate the one-step removal of H2O, NH3, and CO2 within a narrow temperature range. In pure AACH, amorphous alumina is transformed into α-Al2O3 at 1373 K, while La-containing samples display the hexaaluminate structure above 1423 K. Incorporation of transition metals into the Al and La–Al systems promotes the formation of α-Al2O3 and hexaaluminate phases at significantly lower temperatures.

Study on hexaaluminate MnLaAl 11O 19-δ catalyst for catalytic combustive reaction of dimethyl ether as a new fuel

Study on catalytic combustive reaction of dimethyl ether as a new fuel was presented. Hexaaluminate catalysts were used to reduce ignition temperature so that dimethyl ether completely combusted at low temperature. Hexaaluminate catalysts MnLaAl 11O 19-δ were prepared by reverse microemulsion method. Crystalline phase and structure of the catalyst were analysed by means of TG-DTA, XRD and BET. The results show that the hexaalunminate is of magnetoplumbite structure when La is taken as mirror plane cation. Hexaalunminate phase is formed slowly via 1050 °C calcined for 4 h and it can keep hexaaluminate phase and high surface area of 48 m 2·g −1 even calcined at 1200 °C for 2 h. Catalytic activity of MnLaAl 11O 19-δ was tested in combustion reaction of dimethyl ether. It shows that hexaaluminate is of high activity with T 10% at 170 °C and almost 100% conversion at 370 °C.

High-temperature catalysts with a synergetic effect of Pd and manganese oxides

A synergetic effect in the catalytic activity has been found after palladium introduction in Mn–Al–O systems. The magnitude of the synergetic effect depends on the types of the oxidic manganese species: oxide Mn 3O 4, spinel (Mn, Mg)[Mn, Al] 2O 4 or hexaaluminate (Mn, Mg)LaAl 11O 19. The synergetic effect of Pd and manganese-containing compounds is observed only if palladium is introduced to the low-temperature precursor of the manganese alumina spinel or manganese hexaaluminate. The synergetic effect is not observed when high-temperature samples with formed spinel or hexaaluminate phases are modified with Pd.

Hydrothermal synthesis of LA-MN-Hexaaluminates for the catalytic combustion of methane

Hydrothermal synthesis by using urea hydrolysis at 1.0-3.0 MPa and 120-130 ‡C was employed to prepare Mn-substituted hexaaluminate catalysts for methane combustion. The results from DTA-MS demonstrated that CO3- and Off anions co-exist in the hydrothermal reaction. XRD reveals that the components of carbonates and hydroxides in the hydrothermal reaction are more favorable than those in the (NH4)2CO3 co-precipitation for the formation of the Mn-substituted hexaaluminate phase. After calcination at 1,200 ‡C for 2 h, LaMnAl11O19 is the major phase of the catalyst prepared by the hydrothermal synthesis method while LaAlO3 is the major one of the catalysts prepared by NH4OH and (NH4)2CO3 co-precipitation. The catalyst prepared by hydrothermal synthesis has higher activity than that prepared by NH4OH and (NH4)2CO3 co-precipitation. The major reason is that more Mn2+ ions have incorporated into the hexaaluminate lattice. The effect of drying methods on the formation of hexaaluminate phase was also discussed.

Ageing of palladium, platinum and manganese-based combustion catalysts for biogas applications

During recent years, catalytic combustion of low heating value gases has received increased attention. The purpose of the present work was to study the effect of ageing for 30 days at 1000°C in air saturated with 12% steam on Pd- and Pt-impregnated as well as Mn-substituted lanthanum hexaaluminate materials. Both hexaaluminate powders and 400 cpsi cordierite monoliths, washcoated with hexaaluminate powder, were aged. Powders were characterised by BET and XRD, whereas the catalytic activity of the washcoated monoliths was evaluated in a bench-scale rig for conversion of synthetic gasified biomass. The surface areas decreased significantly during the first day of ageing, whereas further ageing had only a minor influence. The pure lanthanum–alumina sample was a mixture of the hexaaluminate LaAl 11O 18 phase and the less preferable perovskite LaAlO 3 phase, which increased after ageing. The Mn-substituted lanthanum–alumina mainly showed pure hexaaluminate phase both before and after ageing. The catalytic activity tests showed that Pd-impregnated lanthanum hexaaluminate was the most active catalyst for combustion of carbon monoxide and hydrogen, retaining low light-off temperatures also after 30 days of ageing. However, the ignition temperature for 50% conversion ( T 50) of methane was approximately 300°C higher than for the fresh sample. Pt-impregnated samples were less active than the Pd ones. The Pt-loading decreased after ageing, whereas the Pd-loading remained fairly constant. However, the amount of Pd oxide decreased after ageing. Further, the Mn-substituted samples were less active than the precious metal ones. Here, the activity for the combustion of carbon monoxide was substantially affected by ageing. The formation of nitrogen oxides from ammonia was lower over the aged samples than over fresh ones; the Mn-substituted sample aged 30 days showed the lowest yield, only 30% of ammonia was converted to nitrogen oxides.

The effect of temperature on a ceria–alumina–baria–cordierite monolith combination under oxidising and reducing conditions

XRD and surface area determinations have been carried out on samples of a commercial ceria–baria–alumina on cordierite monolith heated in air or H 2/He at temperatures from 500 to 1200°C. Below 700°C the fall in surface area is slightly greater in the reducing atmosphere but stability is better above 1000°C. Sintering in air proceeds with continuous growth of CeO 2 particles and the development of α-Al 2O 3 above 1000°C. Traces of a previously unreported phase are evident after 24 h above 1100°C. CeO 2 is barely detectable after treatment in H 2/He and less α-Al 2O 3 is formed. Instead the predominant crystalline phase (apart from cordierite) is CeAlO 3 at temperatures to 1100°C while the new phase forms at 1200°C in considerably greater amounts than under oxidising conditions. Sintering for different periods of time at 1200°C shows that the new phase arises with consumption of α-Al 2O 3 and CeAlO 3. It is fluorescent in ultraviolet light and can be separated as a heavy fraction by gravimetric settling. The XRD pattern closely resembles that of a known cerium terbium magnesium hexaaluminate phosphor and electron microprobe analysis suggests an approximate composition of Ce(Ba 1− x Mg x )Al 11O 19 ( x ∼ 0.87). The presence of magnesium shows that its formation involves mingling of cordierite and washcoat components in a process driven by the conversion of Ce 4+ to Ce 3+ in a reducing atmosphere. The new hexaaluminate phase was observable by XRD in four out of 19 three-way converters recovered from used vehicles.

The Aluminum-Rich Part of the System BaO–Al2O3–MgO: I: Phase Relationships

The subsolidus phase relations in the Al-rich part of the system BaO–Al2O3–MgO at 1800°C in air have been determined. The hexaaluminate phases in this part of the system all exhibitβ-alumina structure types. In the binary system BaO–Al2O3, the phases Ba1.21Al11O17.71and Ba0.75Al11O17.25were confirmed. In the ternary system, two solid solution series, BAM-I and BAM-II, exist. The solid solution range of BAM-I between the defect-freeβ-alumina end member BaMgAl10O17and the Mg-free end member Ba0.75Al11O17.25can be described by the formula Ba1−x/4Mg1−xAl10+xO17+x/4(0≤x≤1). The solid solution series BAM-II contains higher Mg concentrations. It can be expressed by the formula Ba2Mg6−3yAl28+2y□yO50(O≤y≤0.3). The Mg-rich end member of BAM-II is plotted exactly at the join between the defect-free end member of BAM-I and the stoichiometric Mg–Al–spinel. Thus it is concluded that the structure of BAM-II is a mixed-layer model containing both structure units.

Catalytic combustion of methane on copper-substituted barium hexaaluminates

Barium hexaaluminates partially substituted by copper have been synthesized by a sol–gel method. Formation of the parent BaAl12O19 and the mixed BaCuAl11O18.5 catalysts occurred during calcination at 1200°C. XRD showed a good crystallinity of the structure and expansion of the unit cell parameter due to the presence of larger mathrmCu2+ ions in place of Al 3+ in tetrahedral sites. The presence of copper strongly enhances the catalytic activity in methane combustion, which should be due to the Cu2+ species evidenced by FTIR of adsorbed CO and by TPR. The introduction of a second copper ion in the structure was not possible, however, as revealed by the formation of copper (II) aluminate in addition to the Cu-substituted hexaaluminate phase.

Phase Relations in the BaO–Al2O3–AIN System: Materials with the β-Alumina Structure

It is generally known that aluminates with the β-alumina-type structure exhibit very useful optical and electrical properties. In this paper the synthesis of a new compound, namely barium aluminium-oxynitride with the β-alumina structure (space groupP63/mmc) is reported. To obtain additional information, the BaO–Al2O3–AlN phase diagram was investigated at 1400, 1500, 1650, and 1700°C. The ideal composition of this new material is BaAl11O16N. The unit cell dimensions of the material with the ideal composition (after firing at 1700°C) area=5.6019 (2) Å andc=22.6579 (8) Å, soc/a=4.0447 (2). We determined a solid solution between the already known barium β-alumina (or barium hexaaluminate phase I) with the composition BaAl13.8O21.7and the new barium aluminium-oxynitride. The extent of this solid solution is dependent on the firing temperature. For this solid solution thecaxis is decreasing, while theaaxis is increasing with increasing nitrogen content.

Changes of Crystalline Phase and Catalytic Properties by Cation Substitution in Mirror Plane of Hexaaluminate Compounds

Mn-substituted hexaaluminate compounds have been investigated as catalysts for high temperature combustion above 1000°C. In this study, changes of crystalline phase and catalytic properties of Sr1−xLnxMnAl11O19−α(Ln= Pr, Nd, Sm, Gd) were investigated. Two kinds of hexaaluminate phases existed as equilibrium phases in the unsubstituted sample, theLnions were preferentially soluble in one of these phases. Accompanied with the dissolution, unequilibrium LnAlO3phase was sometimes formed for highxsamples. The amount of this phase decreases with an increase in temperature and/or ionic radius ofLn. The surface area of the samples generally decreased with increasing extent ofLnsubstitution. Catalytic activity for methane combustion at low conversion level was improved by the substitution.

Influence of the preparation procedure and of the barium content on the physicochemical and catalytic properties of barium-modified platinum/alumina catalysts

Addition of barium (from 2 to 16 wt.-%) to alumina was performed by impregnation of the support with barium nitrate and by a sol-gel procedure using alkoxide precursors. After calcination at 1073 K and for high barium loading, the formation of barium monoaluminate was shown by X-ray measurements. The barium hexaaluminate phase was observed after calcination at 1473 K. The solids prepared by the sol-gel method showed a very high specific area which was not preserved for a calcination temperature exceeding 1473 K. Platinum was introduced onto the barium-modified alumina using hexachloroplatinic acid. The properties of the metallic phase were strongly dependent on the nature of the solvent used for H 2PtCl 6 (water or ethanol). In the case of an aqueous solution, the BaAl 2O 4 phase disappeared during the impregnation procedure because of an acidic attack and the metallic area accessible to gases was low. On the other hand, when H 2PtCl 6 dissolved in ethanol was used, the barium monoaluminate phase was still present and the chemisorptive properties of platinum were less affected by comparison with impregnation with aqueous solutions. A Fourier transform IR study of carbon monoxide adsorption showed that the surface properties of the platinum particles were strongly modified by the presence of barium additive; the changes in surface properties were less marked when platinum was introduced by ethanolic solutions. It was postulated that the dissolution of barium aluminate in aqueous medium led to the formation of barium containing compounds onto the metal particles or close to the platinum crystallites. The influence of the medium used for the impregnation by the platinum precursor was also shown in cyclohexane dehydrogenation: the catalytic activity was less affected by barium addition when platinum was introduced by using ethanolic solutions.

F + center as a paramagnetic probe in the EPR investigation of barium hexaaluminate phases I and II and barium-lanthanum hexaaluminate ceramics

An X-ray-induced F + center is used as a paramagnetic probe to study sintered powders of barium hexaaluminates phases I and II (referred to as Ba( Φ I) and Ba( Φ II)) and mixed barium-lanthanum hexaaluminate (referred to as BLA). This F + center was previously shown to exist only in β-alumina-type mirror planes containing interstitial oxygen ions as charge-compensating defects, so that its study yields information about the local structure of this type of mirror plane. By comparison of the EPR spectra of Ba( Φ I) and Ba( Φ II) it is shown that the latter contains only one type of F + center with a reduced hf interaction. This indicates that there is only one type of mirror plane containing interstitial oxygen in Ba( Φ II) and that the separation between spinel blocks is larger in Ba( Φ II) than in Ba( Φ I). This result agrees with models proposed by other authors for barium-lead hexaaluminate and barium hexagallate. Investigation of the F + center in mixed barium-lanthanum hexaaluminate ceramics clearly supports the hypothesis of segregation of Ba 2+ ions in β-alumina-type mirror planes. It is also shown that the structure of these planes is very similar to that of Ba( Φ II) defect planes.

Superstructure of barium lead hexaaluminate phase II (BaPb β(II)-alumina) revealed by high-resolution electron microscopy

Superstructure of barium lead hexaaluminate phase II (BaPb β(II)-alumina) revealed by high-resolution electron microscopy

The crystal structure of barium lead hexaaluminate phase II

A refinement was performed on the average crystal structure of barium lead hexaaluminate phase II ((Ba 0.8Pb 0.2) 2.34Al 21.0O 33.84) using single crystal X-ray diffraction data, giving a final R value of 0.030 with space group symmetry P 6m2 . The structure is essentially of a β-alumina type, but contains a lot of defects and interstitials. Inside the spinel block were found Ba(Pb) ions at 12-coordinated polyhedral sites, formed by complex defects, including triple Reidinger defects, in the same unit cell. Barium lead hexaaluminate phase II was found to consist of two kinds of unit cells with formulae “(BaPb) 3.0Al 20.0O 35.0” and “Ba 2.0Al 22.0O 34.0” in a 1 : 2 ratio; these three cells combine to form an a√3 × a√3 superstructure.

The crystal structure of barium hexaaluminate phase I (barium β-alumina)

The crystal structure of barium hexaaluminate phase I (Ba 0.79Al 10.9O 17.14) was determined by single crystal X-ray reflection data. The refinements were carried out by the least-square method to give a final R-value of 0.023. The structure was revealed to be essentially of a β-alumina type and the Ba ion was detected only at the 6 h site near the Beevers-Ross site (2 d site). The charge compensation for nonstoichiometry was found to be principally effected by the interstitial oxygen due to Frenkel defects of Al ions. From the structural point of view, phase I was referred to as “barium β-alumina.”