Barium Hexaaluminates Research Articles

Redox oxidative cracking of n-hexane with Fe-substituted barium hexaaluminates as redox catalysts

Promoted hexaaluminate redox catalysts achieved excellent olefin yield while allowing autothermal redox oxidative cracking of naphtha with low COx formation.

Coking Study of Nickel Catalysts Using Model Compounds

The tendency of coke formation was investigated using nickel catalysts supported on calcium and barium hexaaluminates, compared with a commercial catalyst of natural gas steam reforming. It was developed a methodology in a microactivity unit using cyclohexane as model compound and hydrogen as gas carrier, at low temperature (300–500 °C). After the coking tests, the catalysts were characterized by elemental analysis (CHN) and thermogravimetric analysis using air and steam. 6NiO-BaAl presented the lowest coke removal rate with air. After that, the methodology was modified for ethanol and acetic acid, important model compounds used in studies of biofuels, steam reforming and bio-oil pyrolysis. All model compounds lead to carbon formation with the same chemical nature, as indicated by the temperature of the oxidation peak. So, the methodology can be used as a tool for selection of catalysts. Additionally, cyclohexane and acetic acid are ideal model compounds, because of the lowest and highest coke removal rates with air.

Catalytic activity of Mn-substituted barium hexaaluminates for methane combustion

The catalysts of hexaaluminate (BaMn x Al12−x O19−δ , x = 1.0, 2.0, 3.0, 4.0) to be used in methane combustion have been successfully synthesized by co-precipitation method and supercritical drying. The crystalline structure and surface area of catalyst were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis of BET method. BET analysis revealed that the preparing and drying method proposed here provides stable materials with higher surface area of 51.4 m2/g in comparison to materials prepared using conventional ambient drying method for BaMn x Al12−x O19−δ calcined at 1200° under oxygen. XRD analysis indicated that formation of a pure single phase BaMn x Al12−x O19−δ occurred up to x = 3 in the case of Mn-substituted barium hexaaluminates. Incorporation of Mn in excess leads to BaAl2O4 phase formation. As far as the valence state of Manganese ions was concerned, the introduced Mn ions were either divalent or trivalent. The first Mn ions were introduced in the matrix essentially as Mn2+ and only for BaMn3Al9O19−δ does manganese exist exclusively as Mn3+; the higher the Mn concentration, the higher the proportion of Mn3+. Catalytic activity for methane combustion has been measured for Mn-substituted barium hexaaluminates, light-off temperature was observed in the 512−624°C range. The highest activity was obtained for catalysts containing 3 Mn ions per unit cell, which reveals that the BaMn x Al12−x O19−δ catalyst was a promising methane combustion catalyst with high activity and good thermal stability. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

Synthesis and catalytic properties of barium hexaaluminates incorporated with chromium and lanthanum

Barium hexaaluminates incorporated with chromium and lanthanum (Ba1- xLaxCrAl11O19- a) were synthesized from aqueous metal nitrates and sulfates through the precipitation, and then followed by crystallization at 1300oC for 2 h. They had the ability to maintain heat resistance and high conversion of CH4in methane combustion at elevated temperature.

Observation of Eu3+→Eu2+ in barium hexa-aluminates with β′ or β-alumina structures prepared in air

Abstract In 1.30BaO · 6Al2O3:Eu and 0.82BaO · 6Al2O3:Eu with β′- or β-alumina structure respectively, prepared in air, the doped Eu3+ ions were partially reduced to Eu2+ ions. X-ray powder diffraction patterns and photoluminescent spectra were employed to characterize the compound structures and to detect the existence of Eu2+ ions, respectively. The reduction mechanism of Eu3+ → Eu2+ was proposed to be due to the existence of negative defects (e.g. O Na ‴ ) in these compounds. This is the first observation of the reduction of Eu3+ → Eu2+ in non-stoichiometric compounds prepared in an oxidizing atmosphere. And the luminescent properties of Eu2+ in the compounds of xBaO · 6Al2O3:Eu2+ (x = 0.82, 1.00, 1.30) showed similar behavior due to their similarity in crystal structures.

Synthesis of high-surface area Mn-doped barium hexa-aluminates by a combination of hydrothermal precipitation–calcination route

High-surface area (25 m2 g−1) Mn-doped barium hexa-aluminates with compositions BaMnAl11O19 and BaMn2Al10O19 were prepared by a combination of hydrothermal precipitation–calcination route. The precursors of the above two compounds were obtained by hydrolyzing a mixture of aqueous solutions of Ba, Al, and Mn at 180 °C for 1 h in the presence of urea. The analysis of the hydrothermally prepared samples showed the presence of carbonate and/or hydroxide phases of Ba and Mn and boehmite. Barium mono-aluminate (BaAl2O4), Ba–βII-Al2O3, γ-Al2O3, and a small fraction of MnO2 were observed after calcination of these hydrothermally prepared precursors at 1200 °C. A mixture of Ba–βI-Al2O3 and Ba–βII-Al2O3 phases were found after calcination at 1400 °C for 2 h with no other oxide phases of Ba, Al or Mn.

Synthesis of high-surface area Mn-doped barium hexa-aluminates by a combination of hydrothermal precipitation–calcination route

High-surface area (25 m 2 g −1) Mn-doped barium hexa-aluminates with compositions BaMnAl 11O 19 and BaMn 2Al 10O 19 were prepared by a combination of hydrothermal precipitation–calcination route. The precursors of the above two compounds were obtained by hydrolyzing a mixture of aqueous solutions of Ba, Al, and Mn at 180 °C for 1 h in the presence of urea. The analysis of the hydrothermally prepared samples showed the presence of carbonate and/or hydroxide phases of Ba and Mn and boehmite. Barium mono-aluminate (BaAl 2O 4), Ba–β II-Al 2O 3, γ-Al 2O 3, and a small fraction of MnO 2 were observed after calcination of these hydrothermally prepared precursors at 1200 °C. A mixture of Ba–β I-Al 2O 3 and Ba–β II-Al 2O 3 phases were found after calcination at 1400 °C for 2 h with no other oxide phases of Ba, Al or Mn.

Kinetics of Methane Catalytic Combustion on Mn Substituted Barium Hexa-Aluminates Catalysts

Kinetics of Methane Catalytic Combustion on Mn Substituted Barium Hexa-Aluminates Catalysts

Catalytic combustion of methane on substituted barium hexaaluminates

A sol–gel method using metallic barium, aluminum alkoxides and metal nitrates has been used to synthesize barium hexaaluminate partially substituted by either manganese, iron or both metal ions. The β-alumina structure was obtained by calcination under oxygen at 1200°C. X-ray analysis revealed that formation of a pure single phase BaM x Al 12− x O 19 occurred up to x=4 for Fe, x=3 for Mn and for Fe 1Mn 1 in the case of mixed substituted hexaaluminates. Incorporation of Mn in excess leads to another phase formation (manganese oxide or spinel). As far as the valence state of transition metal ions is concerned, the introduced Fe ions were always trivalent, whereas the Mn ones were either divalent or trivalent. In the latter case, the first Mn ions were introduced in the matrix essentially as Mn 2+ and only for BaMn 3Al 9O 19 does manganese exist exclusively as Mn 3+, the higher the Mn concentration, the higher the proportion of Mn 3+. All solids were aged at 1200°C under water and oxygen and showed a good thermal resistance. Activity for methane combustion has been measured for fresh and aged solids, light-off temperatures were observed in the 560–640°C range. However, the highest activity was obtained for catalysts containing either 3 Mn, 2 Fe or 1 Fe+1 Mn ions per unit cell. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.

Catalytic combustion of methane over copper- and manganese-substituted barium hexaaluminates

Copper- and manganese-substituted barium hexaaluminates were prepared by sol–gel method from metal alkoxides. The preparation conditions strongly influence the textural properties of the solids obtained. Manganese and copper occupy different crystallographic sites in the hexaaluminate structure: Mn 3+ ions are located in octahedral sites, while Cu 2+ enters only tetrahedral positions. The Cu sites are intrinsically more active than Mn sites for methane combustion, but the Cu-based catalysts are penalized by lower surface areas and by the lower limit of copper incorporation in the hexaaluminate matrix: manganese substitution for aluminium is possible up to ≈3 Mn per unit cell, while copper substitution is limited to about 1.3 Cu per unit cell. The catalytic activity of the Mn-substituted barium hexaaluminates increases with Mn substitution, the optimum composition being obtained when about 3 Mn ions per unit cell are incorporated, as regards the activity and the resistance to ageing at 1200°C in the presence of steam. This is related not only to the Mn content, but also to the higher Mn 3+/Mn 2+ ratio when the amount of manganese introduced increases, as shown by TPR and Auger parameter measurements. The catalytic activity can be correlated with the fraction of reducible manganese species.

Synthesis and characterization of aerogel-derived cation-substituted barium hexaaluminates

New methods have been demonstrated for the synthesis of high surface area cation-substituted hexaaluminates. These materials hold promise for use as catalysts in high temperature combustors. The hexaaluminates were prepared by calcining high surface area aerogels at temperatures up to 1600°C. In comparing unsubstituted and cation-substituted hexaaluminates, we found that phase transformations for the cation-substituted materials were more direct. Small amounts of BaCO 3 and BaAl 2O 4 were observed during transformation of the unsubstituted materials, while the cation-substituted materials transformed directly from an amorphous phase to crystalline hexaaluminate. The presence of substitution cations also caused the transformation to occur at lower temperatures. Manganese appears to be a better substitution cation than cobalt since BaMnAl 11O 19− α exhibited higher surface areas and methane combustion reaction rates than BaCoAl 11O 19− α . A doubly substituted material, BaMn 0.5Co 0.5Al 11O 19− α , was more active than its singly substituted counterparts. This result suggests a synergy between Mn and Co in the barium hexaaluminates.

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.

Synthesis, structure and catalytic properties of Fe-substituted barium hexaaluminates

A sol–gel method using Ba and Al isopropylates and iron nitrate has been used to synthesise barium hexaaluminate partially substituted with iron. After calcination under oxygen at 1200°C the β-alumina structure was obtained. Formation of the mixed BaFexAl12-xO19 phase occurred for x=1–4. XRD measurements showed a good crystallinity of the structure and expansion of unit cell parameters due to the presence of larger Fe3+ ions substituting Al3+ ones in octahedral sites only. Mossbauer spectroscopy revealed that Fe3+ ions are present in four different octahedral sites slightly distorted. Catalytic activity in methane combustion showed that an optimum was obtained for solid containing 2 Fe ions per unit cell: the increase of the amount of introduced iron was counterbalanced by the decrease of specific surface area. Intrinsic activities have been calculated for the four solids in both the fresh and aged states. It is observed that increasing iron content increases relative activities in the same ratio as the populations of iron located in two sites as deduced from Mossbauer spectroscopy. It is then tentatively assumed that activity is attributed to octahedrally coordinated Fe3+ ions in some specific sites.

Synthesis and properties of Eu 2+ activated blue phosphors

Blue phosphors Eu 2+ activated BaMgAl 10O 17, BaMg 2Al 16O 27, xBaO·6Al 2O 3 where x = 0.64−1.8 and LaMgAl 11O 19 have been prepared by the combustion of corresponding metal nitrates (oxidizer) and carbohydrazide (CH)/diformyl hydrazine (DFH)/urea (fuel) redox mixture at 400 500° C within 5 min. The phosphors were characterized by exposure to UV light, powder XRD, fluorescence and electron paramagnetic resonance spectroscopy. The phosphors showed a characteristic emission band in the region 435–462 nm when they were excited at 254 nm. With an increase in barium content in xBaO·6Al 2O 3 ( x = 0.64−1.8) the emission band showed a red shift. Addition of Mn 2+ in Eu 2+ doped barium hexa aluminates and Eu 2+ doped LaMgAl 11O 19 resulted in strong green emission at 515 nm. The fine particle nature of combustion derived phosphors has been studied by powder density (55–82% of theoretical value), particle size (5.7–9.5 μm) and BET surface area (5–22 m 2 g −1) measurements.

Single-crystal X-ray structure analysis of Mn-substituted barium hexaaluminates as-grown and after reduction

An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Ba0.780Mn0.254Al10.706O17.153) with the β-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 °C. Both as-grown and reduced crystals have a hexagonal structure (P63/mmc, Z= 2) and their unit-cell dimensions are a= 0.5591(1), c= 2.2659(2) nm and a= 0.5587(2), c= 2.2656(3) nm respectively. The structure refinement from the X-ray reflection data collected by using the ω-2θ scan technique was carried out by the least-squares method to give a final R value of 0.03. The resultant structure was identical with that of β-alumina (Ba0.75Al11.0O17.25) reported by Iyi et al.1 Di- and tri-valent manganese partially and preferentially replaced Al(2) sites in a tetrahedral environment in the spinel block. UV–VIS spectra showed that the evacuation treatment brought about the reduction of some of the Mn3+ to Mn2+. The charge compensation for the reduction was found to be effected by O2– defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.

Metastable phase relations and europium(II) luminescence of barium hexa-aluminates prepared by gel to crystallite conversion

Wet chemical reaction of hydrated alumina gels, Al 2O 3 · yH 2O(80 < y < 120) with Ba(OH) 2 gives rise to crystallites of the precursor, Ba 1+ x Al 12O 19+ x− z 2 (OH) z · nH 2O where −0.2 < x < 0.32, 15 < z < 20 and 5 < n < 9. Thermal decomposition of the precursor above 1150 °C produces monophasic hexa-aluminate, Ba 1+xAl 12O 19+x. Metastability of this nonstoichiometric phase is evident from the splitting of (00ℓ) reflections, on heat treatment above 1600 °C, indicating the subsolidus decomposition to 0.82BaO 6Al 2O 3 and 1.32BaO 6Al 2O 3. High resolution electron microscopy reveals extensive distribution of the blocking defects and structural intergrowths. The metastable Ba-hexaaluminates doped with Eu(II) exhibit predominantly the 436nm emission band for all the ‘x’ values. No deterioration in emission intensity is noticed on long term preservation of the phosphors. Differences prevailing in literature on the phase relations in Ba-hexa-aluminates can be attributed to preparative route dependent metastability.

Reactive coating on alumina substrates. Calcium and barium hexa aluminates

Modifying the surface of ceramics by coating with a substance different from that of the bulk is currently an important technological issue in structural as well as in functional applications (microelectric, environment, sensors, thermal and chemical barriers, catalysis, etc). About 80% of all advanced ceramics are based on alumina. In the present work the authors introduce reactive coating as a feasible route to design an appropriate surface layer in dense alumina compacts, i.e. CaO[center dot]6Al[sub 2]O[sub 3] (CA[sub 6]) and BaO[center dot]6Al[sub 2]O[sub 3] (BA[sub 6]). Doloma (CaO[center dot]MgO), calcium carbonate (CaCO[sub 3]) and barium carbonate (BaCO[sub 3]) reactive coatings on Al[sub 2]O[sub 3] substrates have been studied; the results have been discussed on the base of the high-temperature reactions predicted by the corresponding phase equilibrium diagrams.

ChemInform Abstract: Catalytic Properties of BaMAl11O19‐α (M: Cr, Mn, Fe, Co, and Ni) for High‐Temperature Catalytic Combustion.

AbstractThe cation‐substituted barium hexaaluminates of title exhibit high sintering resistance and retain a surface area of more than 10 m2/g even after annealing at 1300 °C. The Mn‐substituted system BaMnxAl12‐xO19‐α shows the highest catalytic activity for methane combustion.

Catalytic properties of BaMAl 11O 19-α (M = Cr, Mn, Fe, Co, and Ni) for high-temperature catalytic combustion

Series of cation-substituted barium hexaaluminates, BaMAl 11O 19-α (M = Cr, Mn, Fe, Co, and Ni), were investigated as catalysts for high-temperature combustion. Cation-substituted barium hexaaluminates showed high sintering resistance and retained a large surface area above 10m 2/g even after heating at 1300 °C. Transmission electron microscopic observation revealed that anisotropic crystal growth is the reason for the high heat resistance of the hexaaluminate. Manganese-substituted barium hexaaluminate, BaMnAl 11O 19−α, was most active for CH 4 combustion. From temperature-programmed desorption of oxygen and thermogravimetric analysis of oxidation states of substituted cations, it was concluded that the catalytic activity was accelerated by oxygen sorption accompanied by reduction-oxidation of M in the hexaaluminate lattice. Accordingly, the catalytic activity of BaMAl 11O 1-α is approximately regulated by the difference between the heats of formation of sesquioxides of M from monoxide, i.e., ( ΔH o f (MO 1.5) ΔH o f (MO), which is minimum at M = Mn. The catalytic activity of the BaMn x Al 12− x O 19−α system was measured as a function of Mn content. However, the large area and high catalytic activity were obtained only when the amount of Mn was small enough to not destroy the single phase of hexaaluminate. Retention of large surface area is the most prominent feature for high-temperature catalytic combustion.

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.