Gas adsorption-desorption

This relation can be simplified for nitrogen adsorption-desorption to with r j expressed in nm  

The pores in membranes of this type are formed by packing plate-shaped crystals in a parallel fashion. Slit-shaped pores where the slit width and plate thickness are about the same [8] are obtained in this way. 

The pore size distribution of some ceramic alumina membranes (AI2O3) treated at various temperatures are given in figure IV -17. These distribution curves were calculated from the corresponding adsorption-desorption isotherms and demonstrate that the alumina membranes all possess a narrow pore size distribution with individual pores being slitshaped. 

In summary, it may be cmcluded that the gas adsorptum-desorptUm method is simple if a suitable apparatus is available. The main problem is to relate the pore geometry to a model which allows the pore size and pore size distribution to be determined from the isotherms. Dead-end pores which do not contribute towards transport are measured by this technique. Ceramic membranes often give better results because their structure is generally more uniform and the membranes less susceptible to capillary forces. 

Schematic drawing of the extent of undercooling in relation to the pore diameter. Lsliquid (water) S=solid (ice) r=pore radius (tj T2 r3). 

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Following the convention in gas adsorption-desorption isotherms, the mercury isotherm, illustrated in Fig. 12.5, is plotted as volume versus relative pressure so that the radius increases from left to right. Curve I in Fig. 12.5 represents the condensation isotherm from the extrusion curve and curve II is the evaporation isotherm from the intrusion data. Since no adsorption takes place on the pore walls prior to the filling of pores in mercury porosimetry as occurs in gas adsorption, the usual knee of the isotherm is absent. However, condensation-evaporation isotherms from mercury porosimetry are strikingly similar to adsorption-desorption... 

Fig. 2. Gas adsorption/desorption isotherms obtained for pure silica gel (PS), silanized and activated silica (SPS), and immobilized derivatives in silica gels (ADS, silica with lipase physically adsorbed CB1, silica with lipase covalently bonded CB2, silica with lipase covalently bonded in presence of PEG EN1, silica with entrapped lipase EN2, silica with lipase entrapped in presence of PEG). (O),Adsorption ( ), desorption.

From other techniques (e.g. gas adsorption/desorption), it is known that the spherical pores in these mesoporous silicas are connected by some small channels. A similar property has been found in SBA-15 in which the straight cylindrical pores are connected by randomly located small channels. TEM cannot show these... 

The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/Po. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. 

The surface areas of dust samples as determined by optical and electron microscope have also been compared [167]. Pore size distributions of thin films of AI2O3, as measured by TEM, have also been compared with those determined by gas adsorption/desorption [168]. It has also been suggested that electron microscope gives a truer estimate of surface area than gas adsorption techniques [169]. Further information can be obtained in a recent review of specimen preparation for TEM [170]. 

These parameters can be determined from gas adsorption-desorption measurements, usually nitrogen, and from mercury penetration measurements [78,79]. 

Figure 3.2.9 Nitrogen gas adsorption-desorption isotherm (left) and BJH (adsorption branch) pore-size distribution (right) ofX2°°...

Gas adsorption desorption Kelvin (B.E.T. B.J.H.) Cylindrical or slits 2-50 nm Pore size distribution (including dead-end pores). Pore shape information. Specific surface area. Porosity Dry samples. Main problem relationship between the pore geometry eind a model which allows the pore sizes and pore size distribution to be determined from the isotherms. Network effect. 

Pore size distributions are determined from the hysteresis loop in gas adsorption/desorption isotherms and from calorimetric measurements by the shift in the melting (or freezing) peak for a phase transition of water inside the pores. The determination of the fractional rejection properties is done by permeation experiments of a macromolecular solute with a broad molecular weight distribution (MWD). The MWD of permeate and feed are compared and translated into a fractional rejection curve. The comparison of results obtained from these three independent methods for some characteristic membranes gives an indication of the strength and weakness of each of the methods studied. 

The gas adsorption/desorption. The determination of pore size and pore size distribution from gas adsorption/desorption isotherms is known from other types of adsorbents a hysteresis loop occurs between the adsorption and desorption curves when a full isotherm is measured. 

PPO membranes, calculated from gas adsorption/desorption Isotherms. 

The textural characterization of the supports and catalysts pore size distribution, pore volume, and surface areas were determined by use of mercury intrusion porosimetry using a Micromeritics Poresizer 9320 and nitrogen gas adsorption/desorption isotherms carried out on a Micromeretics ASAP 2000 respectively. For the porosimetiy analysis a contact angle of 140° and surface tension of480mNm for mercury were assumed. 

NMR imaging of gas adsorption/desorption in nanoporous solids, such as Y-AI2O3 and ZnO powders and partially sintered ceramics of these materials, as well as Vycor porous glass was analysed using Brunauer-Emmett-Teller (BET) theory. Visualization of gaseous xenon and methane in the void spaces of aerogels offered unique information and insights into the pore structure and molecular diffusivities of occluded sorbates. ... 

ABSTRACT Based on low-temperature nitrogen adsorption principle, the pore structure of coal particles is tested and adsorption isotherms of coal particles with different size are obtained by Quantachrome Autosorb-iQ automatic specific surface area and pore size distribution analyzer. Then, microstructure characteristic parameters such as specific surface area, pore volume and average pore size of coal particles are calculated. Besides, fractal dimension of the internal surface of coal particles is calculated with FHH fractal theory. The relationship between fractal dimension and pore structure parameters together with the adsorption capacity of coal particles is analyzed. Studies show that fractal dimension can characterize the variation of characteristic parameters such as specific surface area and total pore volume of coal particles. In addition, with the increase of fractal dimension, the surface heterogeneity of pore structure is strengthened and so is adsorption capacity. The findings can provide a certain theoretical foundation for mechanism study on coal gas adsorption, desorption and seepage. 

The studies on the microscopic pore structure characteristics of coal particles can provide certain theoretical references for mechanism studies such as gas adsorption, desorption and seepage in coal body. 

Figure 8.5 (a) Reversible nitrogen gas adsorption-desorption isotherm for PAF-2 ... 

Figure I - 20 gives the cumulative pore volume and the pore size distribution for a PPO poly(phenylene oxide) ultrafiltration membrane determined by thermoporometry [12]. Figure I - 21 gives the pore size distribution of a ceramic membrane determined by two methods gas adsorption-desorption and thermoporometry [13]. Both curves (and hence both methods) are in good agreement with each other. Similar results were found by Cuperus for Y-aJumina membranes [14].

There are a number of other techniques besides cut-off measurements for characterising ultrafiltration membranes. However, typical methods for microfiltration membranes, such as mercury intrusion or scanning electron microscopy cannot be used for the characterisation of ultrafiltration membranes. For this reason, other techniques have been developed such as thermoporometiy, liquid displacement and permporoinetry as have been discussed in chapter IV. Other more general techniques which are applicable are gas adsorption-desorption, permeability measurements and modified cut-off measurements. v, ... 

From calorimetric data and equation 6.13, the pore size and its distribution can be obtained. The pore size distribution of a ceramic membrane determined by DSC is in good agreement with data obtained by the gas adsorption-desorption method. 

Pore volume is related to porosity (see chapter Pore Volume (Porosity) in Porous Silicon ) and is generally described in terms of open volume (ml) per unit weight (g) of material, while surface area is defined by the exposed internal surface (m ) per unit weight of material these parameters can be measured using gas adsorption-desorption analysis (Gregg and Sing 1982). As well as surface area and pore volume, information on pore size and shape can also be surmised. 

The gas adsorption-desorption technique relates to the adsorption of nitrogen (or, less commonly, carbon dioxide, argon, xenon, and krypton), at cryogenic temperatures, via adsorption and capillary condensation from the gas phase, with subsequent desorption occurring after complete pore filling. An adsorption-desorption isotherm is constructed based upon the relationship between the pressure of the adsorbate gas and the volume of gas adsorbed/desorbed. Computational analysis of the isotherms based on the BET (Brunauer-Emmett-Teller) (Brunauer et al. 1938) and/or BJH (Barrett-Joyner-Halenda) (Barrett et al. 1951) methods, underpinned by the classical Kelvin equation, facilitates the calculation of surface area, pore volume, average pore size, and pore size distribution. 

A variety of instruments designed specifically for gas adsorption-desorption analysis are commercially available. Common to all is the sample preparation and measurement methodology... 

Fig. 1 (a) lUPAC classification of hysteresis loops from gas adsorption-desorption (b) schematic illustration of pore morphology effects that give rise to type H2 hysteresis curves (From Thommes 2010)... 

As a characterization tool for porous silicon materials and devices, gas adsorption-desorption analysis has proved to be extremely useful, particularly so for process development. 

Table 1 Parameters obtained from gas adsorption-desorption analysis after low- and high-current-density anodization and after static thermal oxidation in air porosity calculated gravimetrically and from total pore volume author s data) ...

Radzi AASM, Yusop SFM, Rusop M, Abdullah S (2012) Struetural and nitrogen gas adsorption-desorption studies of Bragg grating waveguide fabrieated on porous silicon nanostructure. lOP... 

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