Specific surface area, changes

The properties of lot 34 and the preparation of the Ossar samples have been described by Dry and Beebe (4). Lot 33-43 has been used in an investigation, the results of which are to be published later from this laboratory, dealing with pore size distributions as well as the effect of outgassing temperature on weight loss and specific surface area changes. For reasons discussed elsewhere (4) we have adopted a procedure for dehydration which consists of degassing the samples in vacuo for at least 15 hours at 450°. 

Masteau, J. C., and Thomas, G. (1999), Modelling to understand porosity and specific surface area changes during tabletting, Powder Technol., 101(3), 240-248. 

Table 5.7 Specific surface-area change of the Si02 gel after calcination at atmospheric pressure...

X-ray data revealed no considerable effect of grinding on the phase composition of AC [19]. The total pore volume of composite catalysts varies only slightly, while their micropore and mesopore volumes and the specific surface area changes non-monotonically (Tables 1, 2) which will be discussed later. For the finest fraction of AC, specific surface area decreases markedly, while the micropore volume for fiactions less than 0.25 nun is negligible (Table 3). Probably, the number of cracks per particle decreases for fine fiactions of AC, so grinding leads only to further disappearance of cracks due to percussion riveting. However, it has no effect on the specific activity of AC (related per gram) (Fig.9)... 

Even in sintering a voluminous aerogel where shrinkage was 83% by volume, Teichner and Nicolaon foujid that the average pore diameter, dp, which is the ratio of specific pore volume to specific surface area, changed only by a modest amount (316). 

For adsorption on Spheron 6 from benzene-cyclohexane solutions, the plot of N N2/noAN2 versus N2 (cyclohexane being component 2) has a slope of 2.3 and an intercept of 0.4. (a) Calculate K. (b) Taking the area per molecule to be 40 A, calculate the specific surface area of the spheron 6. (c) Plot the isotherm of composition change. Note Assume that is in millimoles per gram. 

What is observed is that there are significant changes in specific surface, but that they are relatively modest and cannot account for large changes in reaction rates in shocked powders. The observed behavior can be characterized into typical behaviors as summarized in Fig. 7.1. If comminution is the dominant behavior, the specific surface area will be observed to increase. Such a behavior is called Type a. If consolidation is the dominant behavior, specific surface area will be observed to decrease. Such a behavior is called Type b. In the most typical case, the specific surface increases at low pres-... 

The prepared MAC adsorbents were tested for benzene, toluene, 0-, m-, p-xylene, methanol, ethanol, iso-propanol, and MEK. The modified content of all MACs was 5wt% with respect to AC. The specific surface areas and amounts of VOC adsorbed of MACs prepared in this study are shown in Table 1. The amounts of VOC adsorbed on 5wt%-MAC with acids and alkali show a similar tendency. However, the amount of VOC adsorbed on 5wt%-PA/AC was relatively large in spite of the decrease of specific surface area excepting in case of o-xylene, m-xylene, and MEK. This suggests that the adsorption of relatively large molecules such as 0-xylene, m-xylene, and MEK was suppressed, while that of small molecules was enhanced. It can be therefore speculated that the phosphoric acid narrowed the micropores but changed the chemical nature of surface to adsorb the organic materials strongly. 

Worz et al. give a numerical example to illustrate the much better heat transfer in micro reactors [110-112]. Their treatment referred to the increase in surface area per unit volume, i.e. the specific surface area, which was accompanied by miniaturization. The specific surface area drops by a factor of 30 on changing from a 11 laboratory reactor to a 30 m stirred vessel (Table 1.7). In contrast, this quantity increases by a factor of 3000 if a 30 pm micro channel is used instead. The change in specific surface area is 100 times higher compared with the first example, which refers to a typical change of scale from laboratory to production. 

We have recently tested the Tx model described above by obtaining T, measurements in powder samples with known S/V. Samples used were constructed from fumed silica (CAB-O-SIL M-5 and TS-500, Cabot Corp.), and were either hydrophilic (M-5) or treated by the manufacturer to be hydrophobic (TS-500). Powder of each type was pressed into a polycarbonate cylinder, with a degree of compression controlling the pore space volume of each sample. These materials have a very high specific surface area (200 m2 g 1 for M-5, 212 m2 g-1 for TS-500), which is not expected to change significantly even at the maximum compaction pressure used. 

Pellet density Void volume gram Specific surface area Enthalpy change for reaction Temperature Total pressure... 

FIGURE 2.1 Change of electrical conductivity with respect to Ni content in the Ni-YSZ cermets at 1,000°C. Two types of YSZ were used to prepare the cermets one was Toyo Soda powder with a specific surface area of 23 m2/g and an agglomerate size of -0.3 pm, and the other was Zircar powder with a specific surface area of 47 m2/g and an agglomerate size of -0.1 pm the NiO used has a specific surface area of 3.5 m2/g. (From Dees, D.W. et al., J. Electrochem. Soc., 134 2141-2146, 1987. Reproduced by permission of ECS-The Electrochemical Society.)... 

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