Thermal transformation of synthetic euhedral and fibrillar crystals of boehmite into aluminas

Santos, Helena de Souza; Kiyohara, Pedro Kunihiko; Santos, Pérsio de Souza

doi:10.1016/0025-5408(96)00068-2

Cited by 21 publications

(10 citation statements)

References 21 publications

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“…It was shown that, strangely, the phase present was d-alumina (PDF 00-046-1131, Tetragonal, P-4m2; Repelin and Husson 1990;De Souza Santos et al 1996, instead of the expected c-alumina mentioned on the label. According to De Souza Santos et al (1996Santos et al ( , 2000, d-Al 2 O 3 is produced from c-Al 2 O 3 , being the transformation pseudomorphic either from hexagonal laths of boehmite or from fibrillar pseudoboehmite as precursors of c-Al 2 O 3 .…”

Section: Resultsmentioning

confidence: 99%

Gold on oxide-doped alumina supports as catalysts for CO oxidation

2011

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The effect of doping a commercial alumina support with metal oxides of Ce, Co, Cu, Fe, La, Mg, Mn, Ni and Zn was investigated. Doped d-Al 2 O 3 samples were obtained by simple physical mixture (PM) of the alumina with the desired commercial oxide and by traditional impregnation of alumina with precursor salts of the same metals followed by calcination (IC). The metal load (7% wt.) was the same in both cases. Gold (1% wt.) was loaded using a liquid phase reductive deposition method. The obtained materials were characterized by adsorption of N 2 at -196°C, temperature programmed reduction, X-ray diffraction, energy-dispersive X-ray spectrometry and transmission electron microscopy. Both samples prepared by PM and IC showed a mixture of the d-alumina phase with the respective metal oxide, but the BET surface areas of the IC samples were, in general, higher than those of the PM materials. The particle size of the oxide phases were larger for the PM samples than for the IC materials. Nevertheless, catalytic experiments for CO oxidation showed that PM samples were much more active than IC. That could be explained by the size of gold nanoparticles, well known to be related with catalytic activity, that was lower in samples prepared by PM (7-16 nm) than by IC (11-17 nm). Gold was found to be in the metallic state. The most active samples were aluminas containing Zn and Fe prepared by PM that had the smallest gold nanoparticles sizes (7-13 and 8-12 nm, respectively) and had room temperature activities for CO conversion of 0.62 and 1.34 mol CO h -1 g Au -1 , respectively, which are larger than those found in the literature for doped c-alumina samples.

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Section: Resultsmentioning

confidence: 99%

Gold on oxide-doped alumina supports as catalysts for CO oxidation

2011

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“…They were previously used for reference in X-ray studies to characterize the aluminas formed in the thermal transformation of synthetic crystals of aluminum hexachloride, hydroxiacetate, fibrillar pseudoboehmite and euhedral boehmite 19 .…”

Section: Samplesmentioning

confidence: 99%

“…As reference, sintered particles of α-Al203 formed at 1200°C from pseudoboehmite 19 and at 1500°C from microcrystals of synthetic gibbsite, are also presented.…”

Section: Samplesmentioning

confidence: 99%

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Standard transition aluminas. Electron microscopy studies

2000

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The aim of this paper is to present the results of characterization of the particle shapes of six standard transition aluminas samples using transmission and scanning electron microscopies; selected area electron diffraction, in parallel with X-ray powder diffraction were used for confirmation of the different transition aluminas types. The transition aluminas -chi; kappa; gamma; theta; delta; and eta were supplied by ALCOA Central Laboratory. The chi-; kappa-;gamma-and delta-Al203 microcrystals are pseudomorphs from their respective precursors gibbsite and boehmite. However, theta-Al203 microcrystals are not pseudomorphs after the standard delta-Al203 sample. Also, eta-Al203 are not pseudomorphs after bayerite somatoids.

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“…The thermal transformation of the freshly (non aged) precipitated boehmite, named "gelatinous boehmite" or "alumina gel", was studied by Lippens (1961) Souza Santos andSouza Santos (1992); Souza Santos et al ( , 1996a; Souza Santos and Toledo (1994); Campos and Souza Santos (1996) and Souza characterizing the aluminas formed by the thermal dehydroxilation of colloidal crystals of fibrillar pseudoboemite and of hydrothermal lamellar boehmite crystals by electron microscopy, X-ray and electron diffractions and differential thermal analysis. Antunes et al (1996); Antunes and Souza Santos (1998) and Antunes (1998) described the aluminas formed by the thermal dehydroxilation of nordstrandite and bayerite.…”

Section: Introductionmentioning

confidence: 99%

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Kiyohara

Santos

Coelho

et al. 2000

An. Acad. Bras. Ciênc.

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Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (AL-COA 123) and aqueous solution of acetic acid at 96 • C±1 • C. The white powder of Al(OH)(CH 3 COO) 2 is constituted by agglomerates of crystalline plates, having size about 10µm. The crystals were fired from 200 • C to 1550 • C, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300 • C); delta (750 • C); alpha (1050 • C). The aluminas maintain the original morphology of the Al(OH)Ac 2 crystal agglomerates, up to 1050 • C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550 • C. High surface area aluminas are formed in the temperature range of 700 • C to 1100 • C; the maximum value of 198m 2 /g is obtained at 900 • C, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac 2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al 2 O 3 which acts as precursor for the sequence gamma-to alpha-Al 2 O 3 .

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Thermal transformation of synthetic euhedral and fibrillar crystals of boehmite into aluminas

Cited by 21 publications

References 21 publications

Gold on oxide-doped alumina supports as catalysts for CO oxidation

Gold on oxide-doped alumina supports as catalysts for CO oxidation

Standard transition aluminas. Electron microscopy studies

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

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