Aerogels and xerogels

Pekala R.W., Alviso C.T. Carbon aerogels and xerogels. Mater Res Soc Symp Proc 1992 270-3. 

E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski, A. Baiker, D. Gunther and M. Schneider, Nondestructive Sol-Gel Immobilization of Metal (salen) Catalysts in Silica Aerogels and Xerogels, Chem. Mater., 2001, 13, 1296. 

Microporosity is a feature observed in many different materials (e g., activated carbons, aerogels, and xerogels). However, with regard to heterogeneous catalysis, zeolites are practically the only microporous catalysts used at present. The following chapter thus only addresses zeolites and their use in catalysis. 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

FIGURE 2.28 Formation of the aerogel and xerogel during the drying process. 

Figure 2.47. Comparison of the three-dimensional shape of an aerogel and xerogel formed from a gel. Reproduced with permission from Chem. Rev. 2004, 104, 3893. Copyright 2004 American Chemical Society.

The effect of the Zr/Si molar ratio is studied both for aerogels and xerogels, and a comparison allows to determine the effect of solvent evacuation procedure on the textural properties of silica supported sulfated zirconia samples. 

IR spectra of aerogels and xerogels prepared with the different Zr/Si molar ratios are shown in Fig. 4. In the spectrum of pure silica (spectrum a), five features are observed Asymetric Si-O-Si network stretches at 1089 and 1228 cm , the corresponding symmetric stretch at 801 cm , and Si-O-Si or Si-OH at 980 cm [5,10,12]. The band at 459 cm corresponds to the bending mode of silica. 

Comparison between aerogel and xerogel in n-hexane isomerization reaction at 493K. 

The comparison between aerogels and xerogels suggests the importance of the textural properties of this class of solids on their catalytic behaviour. In fact, the interesting... 

O.lZr-X-873. The difference between aerogels and xerogels is important Although the selectivities are similar, the n-hexane activity of the aerogel is higher owing to the presence of mesopores. 

To date, metal-doped carbon aerogels and xerogels have been used as catalysts in different reaction types, which can be grouped into environmental and fuel cell applications, C=C double-bond hydrogenation, skeletal isomerization, hydrodechlorination, and other reactions, including synthesis of methyl... 

Carbon aerogels and xerogels have been used as supports for Pt and Pt-based electrocatalysts for proton-exchange membrane fuel cells (PEMFCs), also known as polymer-electrolyte fuel cells [56,58,83-90], These fuel cells are convenient and environmentally acceptable power sources for portable and stationary devices and electric vehicle applications [91], These PEMFC systems can use H2 or methanol as fuel. This last type of fuel cell is sometimes called a DMFC (direct methanol fuel cell). 

Liu C, Komarneni S. Nitrogen and water sorption properties of ethyl substituted silica aerogels and xerogels. Mat Res Soc Symp Proc. 1995 371 217-22. 

Figure 8.7 Top Effect of density on the optical bandgap for aerogels prepared from naked CdSe nanopaiticles and CdSe-ZnS core-shell nanoparticles. Bottom CdSe-ZnS core-shell aerogel and xerogel monohths under normal (left) and UV (right) illumination. The native aerogel and xerogel are orange under normal hght and fluorescent green under UV stimulation.

Esoteric methods of forming supported metals or their analogues include the use of aerogels and xerogels/ ° photolithography and electron-beam lithography/ and a solvated metal-atom dispersion method/ ... 

Both aerogels and xerogels exhibit high surface areas and porosities. Typically die sol-gel materials are networks made up of nanometer-sized particles that are linked together in a network that contains pores of slighdy larger dimensions (mesoporous 2-20 nm diameter). The homogeneous distribution and small diameter of the pores provide a convenient location for a... 

Fig. 4 SEM images of resorcinol-formaldehyde cryogel, aerogel, and xerogel obtained by (a) freeze-drying, (b) critical extraction with liquid carbon dioxide, and (c) heating in an inert atmosphere, respectively. (From [185] with permission from Elsevier)...

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