Nonporous carbons, physisorption

Recent fundamental studies of physisorption by nonporous carbons have involved the application of molecular simulation [23, 29], density functional theory, and lattice gas models [30]. Important aspects studied have included the way in which the molecular packing in each layer is likely to be affected by a systematic alteration of the lattice parameters and interaction energies and the resulting changes in the isotherm shape [21, 30]. 

Figure 5.19 shows an idealized form of the adsorption isotherm for physisorption on a nonporous or macroporous solid. At low pressures the surface is only partially occupied by the gas, until at higher pressures (point B on the curve) the monolayer is filled and the isotherm reaches a plateau. This part of the isotherm, from zero pressures to the point B, is equivalent to the Langmuir isotherm. At higher pressures a second layer starts to form, followed by unrestricted multilayer formation, which is in fact equivalent to condensation, i.e. formation of a liquid layer. In the jargon of physisorption (approved by lUPAC) this is a Type II adsorption isotherm. If a system contains predominantly micropores, i.e. a zeolite or an ultrahigh surface area carbon (>1000 m g ), multilayer formation is limited by the size of the pores. 

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

Isotherm I is typical of adsorption in micropores, which occurs, for instance, in zeolites and in activated carbons. Isotherm II represents multilayer physisorption on a flat surface (valid for many nonporous substances). Type HI and V isotherms are characteristics of weak gas-solid interactions (water adsorption on gold and bromine adsorption on silica are good examples). The type IV isotherm is frequently found in the study of heterogeneous catalysts its shape is characteristics... 

Carbon blacks with specific surface areas of up to 100 m /g can be regarded as essentially nonporous [15] since they give reversible Type II isotherms in the lUPAC classification [10]. Early physisorption measurements on carbon blacks [1] were designed to test the validity of surfiice areas determined the BET method [6]. Carbon blacks were considered [5] to be especially suitable for this purpose because the discrete nature of their spheroidal particles allowed electron microscopy to be used for the evaluation of the particle size distribution. Certain well-characterized carbon blacks are still extremely useful as reference adsorbents [11, 16]. 

The isotherm (I) represents microporous materials, generally found in zeolites and activated carbons. The isotherm (II) is the multilayer physisorption on a flat surface (usually nonporous). The isotherms (III) and (V) are characteristic of gas-solid weak interactions, and the isotherm (IV) is the most frequent in heterogeneous catalysts, representing multilayer adsorption and capillary condensation in mesoporous materials. The isotherm (VI) shows the behavior of nonporous materials, energy uneven. 

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