Classifications of Isotherms According to Their Shape or Their Hysteresis Loop Sample Clauses

Classifications of Isotherms According to Their Shape or Their Hysteresis Loop. In 1940, Brunauer et al. [6] proposed to classify the physical adsorption isotherms in five different groups numbered from I to V. The classification relates the shape of the isotherms to both the mean pore size of the adsorbent and the intensity of adsorbate-adsorbent interactions [7]. The five types of adsorption isotherm are schematically represented on Figure 19.  Isotherms of type I are characteristic of microporous adsorbents  Isotherms of types II and III are observed for macroporous adsorbent. Type II isotherms frequently occur. On the contrary type III isotherms are rather unusual and correspond to very weak adsorbate-adsorbent interactions  Isotherms of types IV and V are obtained for mesoporous adsorbents. Type IV isotherms are widespread whereas type V isotherms (which correspond to very weak adsorbate-adsorbent interactions) are seldom encountered. In everyday practice, the shape of the isotherm usually gives a rather good idea of the mean size of the pores present in the solid. Sometimes however, an isotherm cannot be related to one definite group of this classification because different types of pores are present in the adsorbent. These "mixed" isotherms are also indicative of the general porosity of the solid. Another easy way to get information on the porous texture of the adsorbent is to look at the shape of the hysteresis loop. The presence of a hysteresis loop is always observed with isotherms of types IV and V and sometimes with isotherms of types II and III. A classification of the hysteresis loops has been proposed for the first time by de Boer [8] and four categories of hysteresis loops were adopted by the IUPAC [2] and were related to the general shape of the pores present in the solid, as illustrated in Figure 20.  Type H1 hysteresis had been associated with "cylinder shaped" pores of rather constant cross section or with narrow pore size distribution.  Type H2 hysteresis corresponds to a wide pore size distribution or to pores with narrow necks and wide bodies (often referred to as 'ink bottle' pores)  Type H3 hysteresis, which does not exhibit any limiting adsorption at high relative pressure, is observed with aggregates of plate-like particles giving rise to slit-shaped pores.  Type H4 hysteresis appears to be associated with narrow slit-like pores, but in this case the Type I isotherm character is indicative of microporosity. In practice, the hysteresis loop shape may appear rather different from the ideal curves presen...