Aerogel preparation

Fig. 4. Comparison of physical properties of silica xerogels and aerogels. Note the similar properties of the aerogels prepared with and without supercritical...

Fricke J, Emmerling A (1991) Aerogels-Preparation, Properties, Applications. 77 37-88... 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

Malinowska, B., Walendziewski, J., Robert, D., Weber, J.V., Stolarski, M. 2003a. Titania aerogels preparation and photocatalytic tests. Int J Photoenergy 5 147-152. 

Mg. 1. Comparison of physical properties of silica xerogels and aerogels. Note the similar properties of the aerogels prepared with and without supercritical drying. Reproduced from C. J Brinkei and co-workeis, Mat. Res. Soe S mp. Proc. 271. 567 (1992). Courtesy of the Materials Research Society... 

Fricke, J., Emmerling, A. Aerogels-Preparation, Properties, Applications. Vol. 77, pp. 37-88. Frenking, G., Cremer, D. The Chemistry of the Noble Gas Elements Helium, Neon, and Argon - Experimental Facts and Theoretical Predictions. Vol. 73, pp. 17-96. 

For the glass wool support, the glass wool was added to the autoclave prior to aerogel preparation. The resulting catalysts were used as a catalyst for the conversion of the methanol-ethanol mixture the results are given in Table 6. Owing to the broadness of the peak corresponding to C3 alcohols and aldehydes, a clear distinction could not be made as to the identity or identities of the C3 species in these products that is, the major component could be propanal. 

Until a few years back, the crystal shape of solid materials was of academic curiosity only and shape was not considered to have an effect on the chemical properties and reactivities of a material. However, recent studies clearly indicate that the shape of nanocrystals does indeed affect the chemistry. For example, it has been shown that 4 nm nanocrystalline MgO particles adsorb six molecules of S02 per nm2 at room temperature and 20 Torr pressure.30 However microcrystalline MgO adsorbs only 2 molecules of S02 per nm2 under similar conditions. Similarly, the nanocrystalline aerogel prepared, AP-MgO material adsorbs four times as much C02 as the microcrystals. There are not only differences in the amounts of gaseous molecules adsorbed on these surfaces, but also the mode of surface binding can also be different. S02 binds more predominantly as a monodentate species on the AP-MgO crystal but favors a bidentate geometry on conventionally prepared, CP-MgO microcrystals. Clearly, these results indicate that the shape and size of the crystals affect the adsorptive properties of the MgO surfaces. The high reactivities of the... 

Fig. 9.3. Models of MgO crystal (a) polyhedralshaped, aerogel prepared AP-MgO nan crystals (b) hexagonal-shaped, conventionally prepared CP-MgO microcrystals and (c) cube-shaped commercial polycrystalline MgO. (Reprinted with permission from Klabunde, K.J. et al Chem. Eur. J 2001, 7, 2505-2510. Fig. 2, by permission ofWiley-VCH, Publishers.)...

Fig. 9.4. High-resolution transmission electron microscope image of aerogel prepared AP-MgO. (Reprinted with permission from Richards, R. etal., J. Am. Chem. Soc. 2000, 122, 4921-4925, Fig. 2, copyright (2000) American Chemical Society.)...

The aerogel-prepared metal oxide nanoparticles constitute a new class of porous inorganic materials because of their unique morphological features such as crystal shape, pore structure, high pore volume, and surface areas. Also, it is possible to load catalytic metals such as Fe or Cu at very high dispersions on these oxide supports and hence the nanocrystalline oxide materials can also function as unusual catalyst supports. Furthermore, these oxides can be tailored for desired Lewis base/Lewis acid strengths by incorporation of thin layers of other oxide materials or by preparation of mixed metal oxides. 

A full 23 factorial design was used to study the effect of drying conditions (temperature, pressure, autoclave volume) on the density of dry aerogel prepared by flushing the aerogel with hydrogen in order to reduce the catalyst. 

Figure 16.20 Claisen-Schmidt condensation of benzaldehyde with acetophenone using different crystallites of magnesium oxide at 110°C. (NAP-MgO aerogel prepared MgO,...

Liang, C. Sha, G. Guo, S. Resorcinol-formalde- 32. hyde aerogels prepared by supercritical acetone drying. J. Non-Cryst. Sol. 2002, 271, 167-170. 

M. Schneider, D. G. Duff, T. Mallat, M. Wildberger, A. Baiker, High-surface-area platinum-titania aerogels-preparation, structural-properties, and hydrogenation activity, J. Catal. 147(2) (1994) 500. 

Fig. 3. Comparison of thermal stability for xerogels and aerogels prepared with a molar ratio Zr/Si = 0.05.

In other Pt-doped monolithic carbon aerogels, prepared by adding the Pt pre-cnrsor to the initial R/F mixture [41], the Pt particle size determined by H2 chemisorption was mnch higher than that determined by high-resolution transmission electron microscopy (HRTEM). This indicates that some Pt particles were encapsnlated by the carbon matrix and were consequently inaccessible to H2. This can be the main problem of this preparation method when the metal-doped carbon gel is to be used as catalyst, because part of the metal will not be accessible to the reactant molecules. 

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