Pore-specific surface area

Absorption of harmful organic compounds by activated carbons from gas and liquid media is of interest and importance for human and environmental protection purposes.1"21 The influence of the texture of carbon granules (size and volume of pores, specific surface area, granule size d, and carbon bed depth V), gas stream humidity and velocity, and amounts of pre-adsorbed water are investigated on adsorption of organics in different media.1 21... 

The typical observations from the BET N2 adsorption measurements are the following pore volume and pore size decrease after incorporation of metal compounds and nanoparticle formation within the pores (Table 1). These findings unambiguously confirm presence of the host species in the pores. Specific surface area obtained from BET measurements also normally decreases, but it is not necessarily the case. New surface of nanoparticles may add to the total surface available for N2 adsorption, so if particles are small (large surface area) and numerous, decrease in BET surface is not a necessary indication of the guest particle formation. 

Bulk Density Total Volume Volume Pore with Radii Sorption Volume of Pores Specific Surface Area... 

The pore specific surface area an the average radius values could be used to determined the fractal properties of TVEX resins following the mathematical models describing the disorder and polydispersity of materials. The fractal dimension Df, defines self-similarity of porous materials [8]. It was calculated based on differential function of pore volume distribution of radius [9] ... 

FIGURE 8.6 Dependence of pore specific surface area on pore radius for TVEX with following TBP/DVB content, % 1-50/10 2-50/25 3-50/45. 

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. 

Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

Soaking a siUca gel in dilute ammonium hydroxide solution at 50—85°C can result in significant coarsening of the gel texture (5). Aging and thermal treatments result in a one-way process, ie, loss of specific surface area and in increase in pore size. The pore size can also be enlarged by dissolution of some of the siUca. Treating a siUca gel with O.S-N KOH or dilute HF can enlarge the pores from 0.7 to 3.7 nm (3). 

Aluminium oxide is available in grades with neutral, acidic and basic reactions, which can also vary in the specific surface area and pore size. This makes the separations achieved vary and care must be taken to document precisely. 

The macroporous particles prepared by using only linear polystyrene as diluent yielded lower pore volume and specific surface area values. 

The pore volume and the specific surface area of the uniform macroporous particles increased and the average pore size decreased with the increasing divinylbenzene concentration within the monomer phase. 

In which the ratio m/n is close to 3. The silane was produced by free radical copolymerization of vinyltriethoxysilane with N-vinylpyrrolidone. Its number-average molecular weight evaluated by vapour-phase osmometry was 3500. Porous silica microballs with a mean pore diameter of 225 A, a specific surface area (Ssp) of 130 m2/g and a pore volume of 0.8 cm3/g were modified by the silane dissolved in dry toluene. After washings and drying, 0.55% by weight of nitrogen and 4.65% of carbon remained on the microballs. Chromatographic tests carried out with a series of proteins have proved the size-exclusion mechanism of their separation. 

Some authors have suggested the use of fluorene polymers for this kind of chromatography. Fluorinated polymers have attracted attention due to their unique adsorption properties. Polytetrafluoroethylene (PTFE) is antiadhesive, thus adsorption of hydrophobic as well as hydrophilic molecules is low. Such adsorbents possess extremely low adsorption activity and nonspecific sorption towards many compounds [109 111]. Fluorene polymers as sorbents were first suggested by Hjerten [112] in 1978 and were tested by desalting and concentration of tRN A [113]. Recently Williams et al. [114] presented a new fluorocarbon sorbent (Poly F Column, Du Pont, USA) for reversed-phase HPLC of peptides and proteins. The sorbent has 20 pm in diameter particles (pore size 30 nm, specific surface area 5 m2/g) and withstands pressure of eluent up to 135 bar. There is no limitation of pH range, however, low specific area and capacity (1.1 mg tRNA/g) and relatively low limits of working pressure do not allow the use of this sorbent for preparative chromatography. 

Depending on the metal foam configuration, its specific surface area varies from 500 for original foam to 10,000 m /m for compressed foam. Aluminum foam of 40 pores per inch (ppi) was studied. The structure of the porous material is presented in Fig. 2.77. 

The second case study. This involves all silica micro- and mesoporous SBA-15 materials. SBA-15 materials are prepared using triblock copolymers as structure-directing templates. Typically, calcined SBA-15 displays pore sizes between 50 and 90 A and specific surface areas of 600-700 m g with pore volumes of 0.8-1.2cm g h Application of the Fenton concept to mesoporous materials looks simpler since mass transfer would be much less limited. However, it is not straightforward because hydrolysis can take place in the aqueous phase. 

KoTHmex activated carbon fiber fabrics AW-1101 (BET specific surface area of 880m /g, average pore diameter of 2nm) was provided by Taiwan Carbon Technology Co. Ltd. The support is pretreated in a boiling aqueous solution of 6.5wt.% HNO3 for 1 h. Then the support is rinsed with distilled water, air-dried for 12h at room temperature and for 5h at 393 K. BET specific surface area of the ACF is 950m /g. 

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). 

Because of the preceding reasons, sorbents for partition chromatography should have rather small specific surface areas in combination with large specific pore volumes. 

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