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Surface and porosity of nanocrystalline boehmite xerogels

Alphonse, Pierre and Courty, Matthieu Surface and porosity of nanocrystalline boehmite xerogels. (2005) Journal of Colloid and Interface Science, 290 (1). 208-219. ISSN 0021-9797

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Official URL: http://dx.doi.org/10.1016/j.jcis.2005.04.013


Boehmite xerogels are prepared by hydrolysis of Al(OC4H9)3 followed by peptization with HNO3 (H+/Al = 0, 0.07, 0.2). XRD and TEM show that these gels are made of nanosized crystals (5–9 nm in width and 3 nm thick). According to the amount of acid, no significant differences are found in size and shape, but only in the spatial arrangement of the crystallites. Nitrogen adsorption–desorption isotherms of nonpeptized gels are of type IV, whereas isotherms of peptized gels are of type I. These isotherms are analyzed by the t-plot method. The majority of pore volume results from intercrystalline mesopores, but the peptized gels also contain intercrystalline micropores. The particle packing is very dense for the gel peptized with H+/Al = 0.2 (porosity = 0.26), but it is less dense in non-peptized gel (porosity = 0.44). Heating these gels under vacuum creates, from 250 °C onwards, an intracrystalline microporosity resulting from the conversion of boehmite into transition alumina. But heating also causes intercrystalline micropores collapsing. The specific surface area increases up to a limit temperature (300 °C for nonpeptized gels and 400 °C for peptized) beyond which sintering of the particles begins and the surface decreases. The PSD are calculated assuming a cylindrical pore geometry and using the corrected Kelvin equation proposed by Kruk et al. Peptized xerogels give a monomodal distribution with a maximum near 2 nm and no pores are larger than 6 nm. Nonpeptized gels have a bimodal distribution with a narrow peak near to 2 nm and a broad unsymmetrical peak with a maximum at 4 nm. Heating in air above 400 °C has a strong effect on the porosity. As the temperature increases, there is a broadening of the distribution and a marked decrease of small pores (below 3 nm). However, even after treatment at 800 °C, micropores are still present.

Item Type:Article
Additional Information:Thanks to Elsevier editor. The definitive version is available at http://www.sciencedirect.com. The original PDF of the article can be found at Journal of Colloid and Interface Science website : http://www.sciencedirect.com/science/article/pii/S0021979705003875
HAL Id:hal-03601087
Audience (journal):International peer-reviewed journal
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Institution:Université de Toulouse > Institut National Polytechnique de Toulouse - Toulouse INP (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UT3 (FRANCE)
French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
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Deposited On:24 Sep 2008 15:20

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