Specific surface area measurement

Much of the difficulty in demonstrating the mechanism of breakaway in a particular case arises from the thinness of the reaction zone and its location at the metal-oxide interface. Workers must consider (a) whether the oxide is cracked or merely recrystallised (b) whether the oxide now results from direct molecular reaction, or whether a barrier layer remains (c) whether the inception of a side reaction (e.g. 2CO - COj + C)" caused failure or (d) whether a new transport process, chemical transport or volatilisation, has become possible. In developing these mechanisms both arguments and experimental technique require considerable sophistication. As a few examples one may cite the use of density and specific surface-area measurements as routine of porosimetry by a variety of methods of optical microscopy, electron microscopy and X-ray diffraction at reaction temperature of tracer, electric field and stress measurements. Excellent metallographic sectioning is taken for granted in this field of research. 

Specific Surface Area Measurements (M2/gm) Microscope Micromerograph Sorptometer Scd(1)... 

Alkaline earth oxides (AEO = MgO, CaO, and SrO) doped with 5 mol% Nd203 have been synthesised either by evaporation of nitrate solutions and decomposition, or by sol-gel method. The samples have been characterised by chemical analysis, specific surface area measurement, XRD, CO2-TPD, and FTIR spectroscopy. Their catalytic properties in propane oxidative dehydrogenation have been studied. According to detailed XRD analyses, solid solution formation took place, leading to structural defects which were agglomerated or dispersed, their relative amounts depending on the preparation procedure and on the alkaline-earth ion size match with Nd3+. Relationships between catalyst synthesis conditions, lattice defects, basicity of the solids and catalytic performance are discussed. 

The specific surface area measured through BET method at liquid nitrogen temperature for AC-ref, SC-100, SC-155 and S-155 are shown in Table 1. This table shows that the value obtained for AC-ref is higher than that obtained for the SC-155 sample, and the corresponding SC-155 value is higher than those obtained for SC-100 and S-155. The first comparison is related with the carbon level of the samples, but the second one (SC-100 and SC-155) is related with the expansion of the carbon microdomains because of the higher treatment temperature. The low value observed for S-155 denotes that the carbon network contributes with the major part of the specific surface area. 

The mean particle size of the starting felodipine was 60 pm, and reduced after micronization with the PGSS process to 42 pm. Specific surface areas measured using the BET method increased from 0.33 m2/g for the starting felodipine to 1.33 m2/g for micronized felodipine. 

The specific surface area measured Dy BET metnod, after a desorotion at 2 1... 

Later the kinetics of the process on a high-temperature catalyst of the type commercially employed at present was studied (126). The catalyst contained, prior to reduction, 93% Fe203 and 7% Cr203 specific surface area measured after kinetic experiments was near to 20 m2/g, bed density... 

In most applications, measurement of specific surface area is synonymous with the use of nitrogen and the BET isotherm. For most commercial surface area instruments, at least 1 m2 of surface area must be present in the sample cell for the measurement to be repeatable. This requires that a significant mass of material (> 1 g) must be present in the sample cell for low surface-area materials. When these conditions cannot be met due to material availability, an instrument that uses krypton as the adsorbate should be utilized. Krypton allows specific surface area measurement with as little as 0.1 m2 present in the sample. 

As stated previously, IGC can also be used to measure the specific surface area of a given powder. The adsorbate gas is not restricted to N2 and Kr rather, the same probes that are used to characterize the surface thermodynamic properties of the powder can be used as probe to measure its specific surface area. Table 13.2 contains a comparison of specific surface area measurements via traditional N2/Kr adsorption vs IGC. 

To conclude this section, it must be reiterated that following the discussion above, it is obvious that different causes exist for the spreading of the specific surface area measured in an adsorption experiment. Thus, it is usually estimated, by measuring repeatedly the tested samples, that the relative error in the BET surface area measurements of the adsorption parameters is normally around 20% [5], For samples with very large surface areas, the relative error could be even 30% [2],... 

Specific surface area measurements were performed with the unsupported bulk membrane material before and after SASRA, to obtain information about the stability of (doped) y-alumina under steam-reforming conditions. 

Table 2 shows results of specific surface area measurements of the same powders as mentioned in Figure 3 and Figure 4. 

The author wishes to thank Dr. R. Bredesen (SINTEF) for performing the SASRA treatment and the XRD and specific surface area measurements and for the fruitful discussions about hydrothermal stability of intermediate layers. 

The results of specific surface area measurements of different unsupported (doped) silica membrane materials is provided in Table 3. Unfortunately due to time limitations no measurements were performed on Mg/Al-doped material fired at lower temperatures. 

The author wishes to express his thanks to Dr. H. Weyten (VITO) for performing the high temperature measurements, Dr. R. Bredesen (SINTEF) for performing SASRA treatments, XRD-measurements and specific surface area measurements, Dr. J.A. Dalmon (IRC) for performing permeance and permselectivity measurements on the tubular membranes and Dr. A. Vredenburg (University of Utrecht) for performing the RBS measurements. These people are also kindly thanked for fruitful discussions on their respective measurments. 

For the application of microporous silica membranes in steam-containing environments it is of major importance that the silica membranes will be tested on hydrothermally stable supports. Silica membranes should be prepared on the basis of the results of the specific surface area measurements described in chapter 6. Unsupported silica membrane material of which the specific surface area does not change under SASRA conditions is most promising. An example is silica fired at 825°C (chapter 5). The need of doping the silica with foreign ions or atoms is currently uncertain. 

Similarly as for carbon materials, the maximum hydrogen uptake in different zeolites shows a good correlation with the BET specific surface area measured by nitrogen adsorption [73]. Beyond the specific surface area the interaction of hydrogen molecules with zeolites is strongly influenced by the type and the concentration of cations present in the framework channels. This is well reflected by, for example, the increase in the isosteric heat of adsorption in zeolites with increasing aluminum... 

S1M1N2 - specific surface area, measured by the N2 adsorption Wz - volume adsorbed at a relative pressure - 0.95, correspontb to the (microporet-mesopore) volume V ,i - micropore volume Vme - mesopore volume MBads - Methylene Blue adsorption Irads-b adsorption... 

Correlation between specific surface areas measured using a simple and robust technique such as gas permeametry and SSA determined by more sophisticated techniques such as PSD, MIP and BET have been carried out. A good agreement between Sbf (surface measured by gas permeametry, Blaine Fisher, Sbf) and measured Sbet was obtained. This is due to the fact that the DS studied does not exhibit intraparticular microporosity (both Krypton and air can access all the surface developed by the powder). Sbf compared with estimated Sng (estimated from MIP results) and Spsd (estimated from PSD results) show a good linearity, but Shb and Spsd values are overestimated. This arises due to the simplifying approximations for particles shape included in the theoretical models for PSD and PIM... 

Physical characterization of TBO includes particle size and size distribution measurement by laser diflraction (macroporosity) as well as specific surface area measurement (microporosity). Particle size measurement by FSSS (Fisher Sub-Sieve Sizer), as also sometimes used, is misleading, because of the porosity of the TBO particles. An empirical relationship between FSSS and particle size measured by laser scattering can, however, be detected if the microporosity of the samples is uniform (constant blueing conditions). 

The carbon nanofibers obtained had a specific surface area measured by N2 adsorption ranging from 100 to more than 250 m /g depending on the synthesis temperatures. Such a surface area was of the same order as those reported in the literature [5,14,15]. The presence of graphite edges which could act as adsorption sites for nitrogen was proposed to explain this high surface area. 

Specific surface area measurements corresponding to fresh Nio,5Coo.5Mo04 catalysts prepared at different pH and the loss of surface area after 9 h of reaction are presented in Table 1. It can be observed that increasing the pH from 6.0 to 8.5 the surface area progressively increases from 23 to around 103 m /g, indicating that the size of the catalyst particles decreases. After 9 h of reaction, all samples undergo a surface area loss. The samples prepared at pH 7.5 and 8.5 are the most affected by the reaction conditions. 

The aerodynamic diameter dj, is the diameter of spheres of unit density po, which reach the same velocity as nonspherical particles of density p in the air stream Cd Re) is calculated for calibration particles of diameter dp, and Cd(i e, cp) is calculated for particles with diameter dv and sphericity 9. Sphericity is defined as the ratio of the surface area of a sphere with equivalent volume to the actual surface area of the particle determined, for example, by means of specific surface area measurements (24). The aerodynamic shape factor X is defined as the ratio of the drag force on a particle to the drag force on the particle volume-equivalent sphere at the same velocity. For the Stokesian flow regime and spherical particles (9 = 1, X drag... 

Several traditional techniques have been used to determine the composition, structure and texture of the catalysts. These include X-ray fluorescence. X-ray diffraction, specific surface-area measurements, mercury porosimetry, and electron microscopy. The application of each technique is straightforward and will not be discussed here. For descriptions of these techniques see the respective sections in Part A of this volume. 

In order to get a deeper insight in the deactivation process, BET specific surface area measurements and XPS studies were undertaken. X-ray diffraction was utilized for the purpose of monitoring the possible changes in Pd particle size due to possible sintering. 

Specific surface area measurements and nitrogen isotherm adsorptions were determined on a Micromeritics ASAP 2405, with a pretreatment of 3 h at 150°C under secondary vacuum, to avoid thermal degradatation of grafted ligands. Castaing microprobe analyses were recorded on a JEOL JXA 8800 in order to confirm homogeneous distribution of ligands and active species. 

The BET specific surface area measurements were carried out on a Micromeritics ASAP 2000 analyser, using nitrogen or krypton at -196°C. Samples were previously outgassed under vacuum at 150°C. The pore size distribution and total pore volume were determined by using the BJH (desorption) method. 

The precursors were characterized by differential thermal analysis (DTA) and thermogravimetiy (TG) and the catalysts were characterized by chemical analysis, specific surface area measurements, X-ray difiraction, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). 

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