Pore structure adsorption effect

It is obvious that the samples exhibit observable differences in adsorption capacity as a consequence of their different pore structures. These effects can be nicely... 

As discussed in Chapter 8, the pre-adsorption of n-nonane can be used as a means of blocking narrow micropore entrances (see Section 8.2.3). Thus, in the case of an ultramicroporous adsorbent such as Carbosieve, the pre-adsorption of nonane leads to complete blockage of the pore structure. The effect of progressively removing the pre-adsorbed nonane from a supermicroporous carbon is shown in Figure 9.13. The adsorbent used in this work was a well-characterized carbon cloth with the following properties a(BET), 1330 m2g-1 a(ext), 25m2g 1 fp(mic), 0.44 cm3 g-1 wp, 0.6-2.0 nm (Carrott et al., 1989). 

Everett concludes that in systems where pore blocking can occur, pore size distribution curves derived from the desorption branch of the isotherm are likely to give a misleading picture of the pore structure in particular the size distribution will appear to be much narrower than it actually is. Thus the adsorption branch is to be preferred unless network effects are known to be absent. 

A discussion of the adsorption of water on oxides would be incomplete without some reference to the irreversible effects which are often encountered when samples of oxide, hydroxide or oxide-hydroxide are exposed to the vapour. These effects ( low-temperature ageing ), which manifest themselves in changes in surface area, in pore structure and sometimes in the lattice structure itself, are complex and difficult to reproduce exactly. ... 

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

The expected structural transformation of VPI-5 into the more stable AIPO4-8 structure upon dehydration no longer occurred, while the porosity disappeared almost completely. This was attributed to presence of stacks of FePc complexes, filling the pores. No such effects were found upon adsorption of pre-synthesized complex into the VPI-5 voids. Indeed, application of the washing procedures that remove FePc from the external surface of zeolite crystals1591 do not result in any extraction of FePc from VPI-5. 

A relatively simple pore structure of fairly uniform tubular pores would 1) expected to give a narrow Type HI hysteresis loop (see Figure 7.3) and in this cas the desorption branch is generally used for the analysis. On the other hand, if there i a broad distribution of interconnected pores it would seem safer to adopt the adsoif tion branch since the location of the desorption branch is largely controlled b network-percolation effects. If a Type H2 loop is very broad, neither branch canb used with complete confidence because of the possibility of a combination of effect (i.e. both delayed condensation and network-percolation). Furthermore, the condeii sate becomes unstable and pore emptying occurs when the steep desorption branch j located at a critical pjp° (i.e. at c. 0.42 for N2 adsorption at 77 K). 

The specific surface area of a ceramic powder can be measured by gas adsorption. Gas adsorption processes may be classified as physical or chemical, depending on the nature of atomic forces involved. Chemical adsorption (e.g., H2O and AI2O3) is caused by chemical reaction at the surface. Physical adsorption (e.g., N2 on AI2O3) is caused by molecular interaction forces and is important only at a temperature below the critical temperature of the gas. With physical adsorption the heat erf adsorption is on the same order of magnitude as that for liquefaction of the gas. Because the adsorption forces are weak and similar to liquefaction, the capillarity of the pore structure effects the adsorbed amount. The quantity of gas adsorbed in the monolayer allows the calculation of the specific surface area. The monolayer capacity (V ,) must be determined when a second layer is forming before the first layer is complete. Theories to describe the adsorption process are based on simplified models of gas adsorption and of the solid surface and pore structure. 

Even today, adsorbents which are employed in separation techniques do not fulfil the following criteria a high capacity combined with fast mass transfer kinetics. The solution to this problem is provided by nature itself. There, aU adsorption processes are performed in a very effective manner using the principle of hierarchy. It is the ultimate goal to mimic this property and to manufacture a man-made adsorbent featuring a high degree of hierarchically ordered pore structure. 

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