| Title | Surface and porosity of nanocrystalline boehmite xerogels. | | Author(s) | Alphonse P, Courty M | | Institution | CIRIMAT, UMR-CNRS 5085, Université Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France. alphonse@chimie.ups-tlse.fr | | Source | J Colloid Interface Sci 2005 Oct 1; 290(1):208-19. | | MeSH | Aluminum Hydroxide
Aluminum Oxide
Crystallization
Gels
Hydrolysis
Microscopy, Electron, Transmission
Nanostructures
Nitric Acid
Particle Size
Porosity
Research Support, Non-U.S. Gov't
Sensitivity and Specificity
Surface Properties
Temperature
X-Ray Diffraction
| | Abstract | 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 degrees 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 degrees C for nonpeptized gels and 400 degrees 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 degrees 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 degrees C, micropores are still present. | | Language | eng | | Pub Type(s) | Journal Article
| | PubMed ID | 15936767 |
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