Specific surface area methods

The representative particle size most appropriate to describe the agglomeration process is the surface equivalent diameter, (Table 2.5), because porosity is surface dependant. As described in Chapter 2, such diameter is the size of a spherical particle which, if the powder consisted of only these particles, would have the same specific surface area as the actual sample. When determining the specific surface area, methods must be chosen at only measure the outer particle surface excluding the accessible irmer surface due to open particle porosity. One of these recommended methods is permeability, also discuss in Chapter 2. 

Permeability. The resistance of a packed powder to the flow of a gas can be used under certain circumstances to measure particle size or specific surface area. Methods of this type are described by Arnell (111) and Deryagin, Fridlyand, and Krylova (112). Such procedures are probably now seldom used since the rapid flow method of nitrogen adsorption has become available. 

The specific surface area of a solid is one of the first things that must be determined if any detailed physical chemical interpretation of its behavior as an adsorbent is to be possible. Such a determination can be made through adsorption studies themselves, and this aspect is taken up in the next chapter there are a number of other methods, however, that are summarized in the following material. Space does not permit a full discussion, and, in particular, the methods that really amount to a particle or pore size determination, such as optical and electron microscopy, x-ray or neutron diffraction, and permeability studies are largely omitted. 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

The external surface area of the filler can be estimated from a psd by summing the area of all of the equivalent spheres. This method does not take into account the morphology of the surface. It usually yields low results which provide Htde information on the actual area of the filler that induences physical and chemical processes in compounded systems. In practice, surface area is usually determined (5) from the measured quantity of nitrogen gas that adsorbs in a monolayer at the particle surface according to the BET theory. From this monolayer capacity value the specific surface area can be determined (6), which is an area per unit mass, usually expressed in m /g. 

Another standard industry method for surface area is based on the adsorption of cetyltrimethylammonium bromide (CTAB) from aqueous solution. This is ASTM method D3765-85 (2). This method measures the specific surface area of carbon black exclusive of the internal area contained in micropores that are too small to admit the large CTAB molecules. Eor mbber-grade nonporous blacks the CTAB method gives excellent agreement with nitrogen surface areas. 

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. 

The difference between the specific surface areas of materials treated by the wet or dry method can also be explained by the rewelding mechanism. The surface area of wet milled powders continuously increases during milling, while extended dry milling causes a decrease in surface area. 

Brown et al. [494] developed a method for the production of hydrated niobium or tantalum pentoxide from fluoride-containing solutions. The essence of the method is that the fluorotantalic or oxyfluoroniobic acid solution is mixed in stages with aqueous ammonia at controlled pH, temperature, and precipitation time. The above conditions enable to produce tantalum or niobium hydroxides with a narrow particle size distribution. The precipitated hydroxides are calcinated at temperatures above 790°C, yielding tantalum oxide powder that is characterized by a pack density of approximately 3 g/cm3. Niobium oxide is obtained by thermal treatment of niobium hydroxide at temperatures above 650°C. The product obtained has a pack density of approximately 1.8 g/cm3. The specific surface area of tantalum oxide and niobium oxide is nominally about 3 or 2 m2/g, respectively. 

Niobium oxide obtained by plasma chemical decomposition is an ultra-fine powder with a specific surface area, as determined by the BET method, of about 20-30 m2/g. The estimated average particle size does not exceed 0.1 pm. 

Separation methods, multichromato-graphic la 56 Serine la 246,356 lb 132 Serotonin la 70,76,239,240,262,355, 380 lb 37-39,231,243,348 Serotonin metabolites lb 327 Serum lipids la 89 Serum proteins la 74 Sesquiterpene derivatives lb 239,446 Sesquiterpene esters lb 239 Sesquiterpene glucosides la 327 Sesquiterpene lactones lb 448 Sevin lb 387-389 Si 50 000, specific surface area la 91 Silica gel, caffeine-impregnated la 85 -, surface modified la 3 Silica gel 60, specific surface area la 91... 

The elimination of HCHO by gas adsorption using activated carbon having high specific surface area is one of the effective removal methods. However, in this method, periodical replacement of carbon adsorbents, regeneration, and combustion after saturation with HCHO are required. An ideal method is the catalytic oxidation of HCHO to CO2 at room temperature, using molecular oxygen in ambient atmosphere. 

The new method produces TiN powders with surface areas exceeding 200 m g that are otherwise only accessible using a forced flow reactor and a microwave plasma activator in which titanium metal is reacted with N2 in the gas phase [14]. TiN powders with considerably lower specific surface area (Sg<60m g ) were also synthesized using the nitridation of 10-15 nm-sized... 

The major gaseous components were analyzed by a gas chromatograph equipped with a TCD and a molecular sieve 13X column. The specific surface areas of carbon produced were measured by the BET method(ASAP 2010, Micromeritics). The morphology and particle size of the formed carbon were investigated by the scanning electron microscopy(S-4200, Hitachi... 

The AC used in this study was a granular type (30 35 mesh) prepared from coconut shell. The purified AC (PAC) was prepared by boiling the AC for 5 hr in a water bath. The acidic and alkaline solutions for preparation of MACs were made with HNO3 (NA), H2SO4 (SA), HCl (HA), H3PO4 (PA), CH3COOH (AA), KOH (PH), and NaOH (SH). The AC was modified into each solution according to the conventional wet process. The specific surface area of the adsorbents was measured by BET method (ASAP 2020, Micrometries, USA). 

The specific surface areas of the samples were measured by the single point BET method (p/pQ=0.3). 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

Supported iron catalysts are notoriously difficult to reduce [6-8] and thus a substantial fraction of the iron can be expected to remain inactive for the catalysis of hydrogenation. Particular attention has therefore been paid to the preparation of Fe/MgO catalysts by several different methods and examination of their effectiveness in producing metallic iron of adequate specific surface area after reduction in hydrogen. The activity and selectivity for primary amine formation have been determined for the hydrogenation of ethanenitrile (acetonitrile) and propanenitrile. 

The platinum concentrations in the platinized carbon blacks are reported to be between 10 and 40% (by mass), sometimes even higher. At low concentrations the specific surface area of the platinum on carbon is as high as lOOm /g, whereas unsupported disperse platinum has surface areas not higher than 10 to 15m /g. However, at low platinum concentrations, thicker catalyst layers must be applied, which makes reactant transport to reaction sites more difficult. The degree of dispersion and catalytic activity of the platinum depend not only on its concentration on the carrier but also on the chemical or electrochemical method used to deposit it. 

BET method. The most commonly used method for determining the specific surface area is the so-called BET method, which obtained its name from three Nobel prize winners Brunauer, Emmett and Teller (1938). It is a modification of the Langmuir theory, which, besides monolayer adsorption, also considers multilayer adsorption. The equation allows easy calculation of the surface area, commonly referred to as the BET surface area ( bet). From the isotherms also pore-radii and pore-volumes can be calculated (from classical equation for condensation in the pores). 

There are two widely used methods for the determination of the specific surface area of particles. One is based on permeability and the other on gas adsorption. 

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