Hexaaluminate Structure Research Articles

Boosted carbon resistance of ceria-hexaaluminate by in-situ formed CeFexAl1−xO3 as oxygen pool for chemical looping dry reforming of methane

High potential to carbon deposition over metallic Fe0 greatly limits the improvement of methane-to-syngas selectivity via chemical looping technology. Herein, we found that the in-situ formed CeFexAl1−xO3 over ceria-hexaaluminate as “oxygen pool” could greatly improve carbon resistance even with presence of Fe0. Formation of CeFexAl1−xO3 only proceeds in ceria-hexaaluminate composite while not occurs in CeO2-Fe2O3-Al2O3, which was probably originated from the strong interaction between CeO2 and the adjacent Fe and Al ions in BaFe3Al9O19 hexaaluminate structure. CeFexAl1−xO3 with outstanding oxygen ion conduction capacity was in close contact with metallic Fe0 exsoluted from Fe-hexaaluminate in a unique CeFexAl1−xO3/Fe0/hexaaluminate sandwich-like structure, which provided a convenient pathway for CeFexAl1−xO3 as “oxygen pool” to supply sufficient oxygen for in-time oxidation of carbon over adjacent Fe0. Consequently, the recycled ceria-hexaaluminate displayed both outstanding carbon resistance and high CH4 conversion (∼90%) with enhanced syngas yield that 105% and 72% higher than CeO2-Fe2O3-Al2O3 oxide and Fe-hexaaluminate, respectively.

Anisotropic Rh3+ Diffusion in Layered Hexaaluminate Mitigates Thermal Deactivation of Supported Rhodium Catalysts

When exposed to a high-temperature oxidizing environment, Rh catalysts supported on Al2O3-based oxides lose their three-way catalytic activity as a result of unfavorable interface interactions that allow Rh3+ to diffuse into the support structure and occupy Al3+ sites. This study showed that the incorporated Rh3+ ions were not easily reduced to active Rh metal species and caused substantial thermal deactivation. The deactivation was most obvious for γ-Al2O3 and MgAl2O4 with a spinel-type structure but much less for hexaaluminate (LaMgAl11O19) with a layered structure consisting of alternative stacking of a spinel block and a La–O monolayer. After annealing at 900 and 1000 °C for 100 h in the air, Rh-deposited single crystals were studied by dynamic secondary ion mass spectrometry to analyze the Rh depth profile. The Rh depth profile in LaMgAl11O19 showed that the diffusion along the c axis (∥c) was significantly suppressed compared to that normal to the c axis (⊥c). The diffusion of Rh3+ was faster and nearly isotropic in a MgAl2O4 single crystal, which was used as a model crystal for γ-Al2O3. These results show that the layered structure of hexaaluminate influences the Rh3+ diffusion, i.e., the La–O interlayer between closely packed spinel blocks running at every approximately 1.1 nm interval is likely the diffusion barrier. This barrier effectively blocks further penetration of the incorporated Rh3+ ions from the surface and preserves their smooth reduction to active metallic Rh nanoparticles.

Efficient recovery of hydrogen and sulfur resources over non-sulfide based LaFexAl12-xO19 hexaaluminate catalysts by H2S catalytic decomposition

Simultaneous recovery of hydrogen and sulfur resources from acidic gas hydrogen sulfide (H2S), whose content may reach 20–100 % after hydrodesulfurization in oil refineries, is increasingly attractive. Nowadays, the widespread utilization of H2S decomposition technology is mainly restricted by the low hydrogen and sulfur yield, i.e. 20 % at 800 °C. In this work, for the first time a novel non-sulfide based LaFexAl12-xO19 hexaaluminate catalysts was used for high (50 %) hydrogen yield at 800 °C. At the same time, the active phases and reaction mechanism were confirmed by various characterization techniques. The XRD, Raman, XPS, Mössbauer and XAFS results verified that the Fe species in the hexaaluminate structure, especially octahedral-coordinated Fe3+, served as the main active phases. The reaction routes/mechanism for H2S decomposition were also verified by density functional theory (DFT) method. H2S was firstly adsorbed on the active sites and then completed the decomposition by direct dehydrogenation mechanism. Briefly, current study has minimized the challenges for H2S decomposition at industrial scale. It is anticipated that our findings will offer potential application of H2S decomposing technology.

Metal modified hexaaluminates for syngas generation and CO2 utilization via chemical looping

Chemical looping CH4CO2 reforming (CLDR) is an emerging technology for the generation of Fischer-Tropsch ready syngas and CO2 utilization, which is strongly dependent upon the improvement in the design of efficient oxygen carriers (OCs). In this present work, different metal additives (Si, Zr and Ce ions) were introduced into Fe-based hexaaluminates and used OCs for CLDR. The microstructure and reactivity of BaFe2.8M0.2Al9O19 (M = Fe, Si, Zr, and Ce) OCs were found greatly influenced by the metal additives and CH4/CO2 redox treatment. Pure Fe and Ce doped OCs showed the co-existence of both β-Al2O3 and MP hexaaluminate phases, while the introduction of Si and Zr in the hexaaluminate structure led to the phase transformation from β-Al2O3 into MP. During the CH4/CO2 redox process, large amounts of Fe species in both BaFe2.8Si0.2Al9O19 and BaFe2.8Zr0.2Al9O19 OCs were gradually stabilized in sintering FeAl2O4 with low oxygen-storage capacity, which resulted in low CH4 reactivity and weak cyclic stability. However, Ce-doped BaFe2.8Ce0.2Al9O19 OC showed good reactivity and stability during the 10 redox cycles with CH4 conversion of 93%, H2/CO ratio of ∼2, high syngas yield of 2.2 mmol/g, and high CO2 activation ability of 0.95 mmol/g, which was associated with the preservation of hexaaluminate main phase, the formation of CeFexAl1-xO3 and the abundant oxygen vacancies.

The Structure of Hexaaluminate and Application in High-Temperature Reaction

The Structure of Hexaaluminate and Application in High-Temperature Reaction

Fe-substituted Ba-hexaaluminate with enhanced oxygen mobility for CO2 capture by chemical looping combustion of methane

Abstract While Fe-based oxygen carriers (OC) are regarded to be promising for chemical looping combustion (CLC), the decrease of CO2 selectivity during deep reduction process and the severe agglomeration of Fe2O3 often occur after multiple redox cycles due to the low oxygen mobility. Herein, Fe-substituted Ba-hexaaluminates (BaFexAl12–xO19, denoted as BFxA-H, x = 1 and 2) prepared by a modified two-step method exhibited not only higher amount of converted oxygen (Ot) and CH4 conversion (77% and 81% vs. 17% and 75%) than those prepared by the traditional co-precipitation method (BFxA-C, x = 1 and 2) but also high CO2 selectivity above 92% during the nearly whole reduction from Fe3+ to Fe2+. Furthermore, the BFxA-H exhibited the excellent recyclability during 50 cycles. The better performance was ascribed to the markedly enhanced oxygen mobility which resulted from dominant occupancy of Fe cations in Al(5) sites (Fe5: 71% and 70% vs. 49% and 41%) in mirror planes of hexaaluminate leading to larger amount of lattice oxygen coordinated with Fe5 (O Fe5) (0.45 and 0.85 mmol/g vs. 0.31 and 0.50 mmol/g). The improvement of oxygen mobility also favored the preservation of chemical state of Fe cations in hexaaluminate structure in the re-oxidation step, resulting in the excellent recyclability of BFxA-H.

Microstructure and reactivity evolution of La[sbnd]Fe[sbnd]Al oxygen carrier for syngas production via chemical looping CH4[sbnd]CO2 reforming

The relationship between chemical looping CH4CO2 reforming performance and the microstructure of oxygen carrier (OC) is very important for the rational design of OC. In this paper, we studied the structural evolution of LaFeAl (LFA-t, t = 900–1200 °C) OCs as thermal treatment and ten periodic CH4/CO2 redox cycles, and correlated to their reactivity and stability for syngas production. Different calcination temperature brought about great discrepancy in phase composition of LFA OCs: LaFeO3, Fe2O3, and α-Al2O3 at 900 °C, LaFexAl1-xO3 and La-hexaaluminate at 1000 °C, and monophasic La-hexaaluminate at 1100–1200 °C. During the CH4/CO2 redox process, the repeated phase separation occurred over LFA-900 and LFA-1000 accompanied by the appearance of metallic Fe and FeAl2O4, which resulted in serious CH4 pyrolysis. La-hexaaluminate showed good phase stability during CH4/CO2 redox process via the charge compensation mechanism. The large hexaaluminate crystalline of LFA-1200 inhibited the oxygen transport from the bulk to surface, which led to carbon deposition. LFA-1100 hexaaluminate OC with moderate crystal size exhibited excellent reactivity and stability for producing syngas with desirable H2/CO ratio (∼2) during ten CH4/CO2 redox cycles thanks to high oxygen mobility and the reservation of hexaaluminate structure during redox process.

Fe‐substituted Ba‐hexaaluminates oxygen carrier for carbon dioxide capture by chemical looping combustion of methane

Fe‐substituted Ba‐hexaaluninates (BFA‐x (x = 1–3), x indicates Fe content) oxygen carrier (OC) were found to exhibit excellent sintering‐resistance under cyclic redox atmosphere at 800°C thanks to the reservations of the structure during the CH4 reduction step, thus preventing the agglomeration of particles during the subsequent reoxidation step. Lattice oxygen highly active for the total combustion of CH4 was observed in the hexaaluminate structure and its chemical state was influenced by Fe content. The highest amount of active O coordinated with Fe3+ in the mirror plane (O‐Fe3+(M)) for the total combustion was reacted (0.77 mmol/g) for BaFe3Al9O19 hexaaluminate OC. As a result, it exhibited the best reactivity with the CH4 conversion of 83% and CO2 selectivity of 100%. Moreover, superior regeneration and recyclability was also obtained, which originated from the fully recovery of O‐Fe3+(M) in the hexaaluminate structure. © 2015 American Institute of Chemical Engineers AIChE J, 62: 792–801, 2016

六铝酸盐催化剂的研究进展

本文从六铝酸盐复合氧化物的晶体结构及作为催化剂的应用方面入手，综述了相应的研究成果，并对六铝酸盐复合氧化物未来的应用进行了展望。 This paper reviews the structure of hexaaluminate and its application as the catalysis. On the other hand, we also briefly summarize the relative research of hexaaluminate compound and forecast the prospect of the future ap- plication of it.

Identification of the Crystallographic Sites of Ir in BaIr0.2FeAl10.8O19 Hexaaluminate

Noble metal ions in hexaaluminate structure are generally regarded as active centers for a variety of reactions, but their chemical states are still not clear for lack of effective characterization techniques. In this paper, in comparison with BaFeAl11O19, the crystallographic site of Ir in BaIr0.2FeAl10.8O19 hexaaluminate with Ba-beta(1)-Al2O3 structure was identified by Rietveld refinement and Fe-57 Mossbauer spectroscopy. In the BaIr0.2FeAl10.8O19 hexaaluminate structure, Fe occupied both the symmetric tetrahedral Al(2) sites in the spinel block and the distorted tetrahedral interstitial Al(5) sites in the mirror plane. In contrast to Fe, the framework Ir ions only occupied the distorted tetrahedral interstitial Al(5) sites in the loosely packed mirror plane, which originated from Ir ions in oxidic entities dispersed on the Ba-modified gamma-Al2O3 in the precursor. Ir ions in the Al(5) sites were highly active for N2O decomposition.

Stabilization mechanism and crystallographic sites of Ru in Fe-promoted barium hexaaluminate under high-temperature condition for N2O decomposition

The stabilization of volatile ruthenium from catalysts at high temperatures is an important issue in both academia and industry. In this paper, Ru-substituted barium hexaalumimates (BaRu0.2FeAl10.8O19) with βI-Al2O3 structure were prepared using the carbonates route and investigated for high-concentration of N2O decomposition. It was for the first time found that the evaporation of ruthenium species under high-temperature condition (1100–1200°C) could be effectively suppressed by the addition of Fe in the hexaaluminate precursor. Fe promoted the formation of intermediate stable BaRuO3 phase, which greatly alleviated the evaporation of Ru species during calcination, and thus allowed more Ru species enter into the final sintering-resistant hexaaluminate lattice after high-temperature treatment. Ru ions in the hexaaluminate structure only occupied the distorted tetrahedral interstitial Al(5) sites in the loosely packed mirror plane, which originated from Ru species in oxidic entities dispersed on the Ba-modified γ-Al2O3 and the intermediate BaRuO3 in the precursors. Ru ions in the Al(5) sites were key factors responsible for the high activity of N2O decomposition.

Evolution of Fe Crystallographic Sites from Barium Hexaaluminate to Hexaferrite

The substituted metal ions in the hexaaluminate structure are regarded as active centers, but their very limited substituted content prevents the obtainment of higher activity. Fe-substituted hexaaluminates exceptionally overcome such a limit. In this paper, we have studied the Fe crystallographic sites in BaFexAl12–xO19 (x = 1–12) and proposed the unique mechanism of stabilization of Fe ions in hexaaluminate structure at a high substitution level by employment of Rietveld refinement and 57Fe Mössbauer spectroscopy. When x = 1–4, the site occupancy of Fe3+ in both βI-Al2O3 and newly formed magnetoplumbite phases was kept constant, and the increased Fe3+ ions were accommodated by the increased amount of magnetoplumbite phase. When x = 5–12, the site occupancy of Fe3+ in the magnetoplumbite phase continuously increased, and thus the increased Fe3+ ions were accommodated by the magnetoplumbite structure. BaFexAl12–xO19 catalysts exhibited different N2O decomposition properties under conventional furnace heating and microwave heating modes.

AMnAl1111O19(A=La, Sr, Ba) 및 CeO2/LaAMnAl11O19를 이용한 메탄의 촉매 연소

Mn substituted La, Sr or Ba-hexaaluminate were prepared by co-precipitate method and calcined at for 5 h. Catalysts were characterized by X-ray diffraction and physisorption and scanning electron microscope (SEM). Compared to and , in which La located at mirror plane showed better crystallinity and high surface area, 13 . revealed well developed plate-like structure which is characteristic structure of hexaaluminate. The catalytic activity of methane combustion increased in the following order: > > and was dependent on surface area of catalysts. 60 wt% calcined at showed enhanced methane activity and methane was oxidized completely at low temperature (). It was confirmed that addition of ceria seems to be effective for the low and middle temperature combustion of methane. But, after calcination at high temperature of , it lost the promoting effect of ceria due to increase of ceria particle size and it had a limit to applying to the high temperature catalytic combustion.

Catalytic combustion of methane in simulated PSA offgas over Mn-substituted La–Sr-hexaaluminate (La xSr 1− xMnAl 11O 19)

Mn-substituted La, Sr-hexaaluminate was studied as a catalyst for the catalytic combustion of pressure swing adsorption (PSA) offgas from a hydrogen station based on steam reforming of petroleum gas. The catalytic activity and thermal stability were tested under simulated PSA offgas condition and the catalysts were characterized by X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), CO 2-temperature programmed desorption (TPD) and N 2 physisorption for BET. All the La x Sr 1− x MnAl 11O 19 catalysts prepared by co-precipitation method contained hexaaluminate structure as the major phase after 5 h calcination at 1200 °C. Partial substitution of La for Sr in SrMnAl 11O 19 increased the specific surface area and consequently the catalytic activity for oxidation of methane. The CO 2 in the simulated PSA offgas was found to suppress the catalytic activity in methane oxidation if the catalyst had appreciable surface basicity. The most active catalyst, La 0.6Sr 0.4MnAl 11O 19, retained its lattice structure and chemical composition even after 40-h thermal aging at 1000 °C under PSA offgas composition.

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.

High catalytic activity and stability of Pd doped hexaaluminate catalysts for the CH4 catalytic combustion

In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto θ-Al2O3. These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al2O3 less than 0.6 (xLa0.2Sr0.3Ba0.5MnAl11O19+(1−x)·Al2O3, with x=0.25, 0.60), samples with high specific surface area and θ-Al2O3 structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al2O3 catalyst. The highest activity was obtained for the samples presenting a θ-Al2O3 structure (with x=0.60) and a high surface area. Impregnation of 1wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700°C) were obtained on the stabilized θ-Al2O3 solids (x=0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700°C), due to a progressive reduction of the PdOx active phase into the less active Pd° phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported θ-Al2O3 samples. This clearly showed the beneficial effect of the support for the stabilization of the PdOx active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn3+/Mn2+ redox properties (in the case of the hexaaluminate and stabilized θ-Al2O3 samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface.

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.

Phase transformation and catalytic activity of hexaaluminates upon high temperature pretreatment

Both hexaaluminates powder have been synthesized with the co-precipitation method. High temperature pretreatment catalysts are characterized by X-ray diffraction (XRD) and BET method. The catalytic activity in methane combustion also is reported. The hexaaluminate structure is formed and retains the surface area of 20–30 m 2/g upon calcinations at 1200 °C for both SLMA (Sr 0.8La 0.2MnAl 11O 19) and SBLMA (Sr 0.3Ba 0.5La 0.2MnAl 11O 19). The surface area of SBLMA is larger than that of SLMA upon temperature higher than 1200 °C illustrating the superior thermal stability of the barium hexaaluminate. Partial replacement of Sr 2+ ion with Ba 2+ ion improves the thermal stability of the Ba-containing system. Also, the high surface area for SBLMA can be understood from the microstructural change of pore size. The SBLMA possesses larger pore than the SLMA to resist sintering under high temperature aged and shows a good activity in methane combustion.

In Situ Reacted Rare-Earth Hexaaluminate Interphases

A novel in situ reaction between a ceria-doped zirconia interphase coating on Saphikon fibers and an outer alumina coating has resulted in the formation of oriented hexaaluminate platelets which can act as a low fracture energy interface barrier for crack deflection in oxide-oxide ceramic-matrix composites (CMCs). The reaction proceeds only in reducing environments where the reduction of the cerium and zirconium ions to their 3+ valent state causes a destabilization phenomenon consistent with previously reported findings. The diffusion of the cerium from the zirconia into solid solution with the alumina can stabilize the layered hexaaluminate structure. Preferred orientational growth of the hexaaluminate parallel to the coating interface was observed which is the required orientation for enhanced debonding at the fiber/matrix interface in long-fiber-reinforced CMCs.

Characterization and catalytic combustion of methane over hexaaluminates

Both Sr 0.8 La 0.2 MnAl 11 O 19 (SLMA) and Sr 0.3 Ba 0.5 La 0.2 MnAl 11 O 19 (SBLMA) hexaaluminates are prepared by the co-precipitation method. Their thermal characterizations are examined by X-ray diffraction (XRD) and BET method. The activity of methane catalytic combustion also is reported. A temperature threshold of 1200 °C has been found for the formation of the hexaaluminate structure. The surface areas retain in the range of 20–30 m 2 g −1 upon calcination at 1200 °C for both SLMA and SBLMA. The surface area of SBLMA is larger than that of SLMA upon calcinations above 1200 °C illustrating the superior thermal stability of the barium hexaaluminate. Partial replacement of Sr 2+ ion with Ba 2+ ion improves the thermal stability of the hexaaluminate. The high surface area for SBLMA can be understood from the microstructural change of pore size. The SBLMA possesses larger pore than the SLMA to resist sintering under high temperature calcination. Upon calcinations at 1300 °C for 20 h, the SBLMA retains a surface area of 20 m 2 g −1 and shows a good activity in methane combustion.