ISO 15901-2:2022 pdf download – Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 2: Analysis of nanopores by gas adsorption.
In this case, the initial monolayer- multilayer adsorption on the mesopore walls, which takes the same path as the corresponding part of a Type II isotherm, is followed by pore condensation. Pore condensation is the phenomenon whereby a gas condenses to a liquid-like phase in a pore at a pressure p less than the saturation pressure p 0 of the bulk liquid. This leads to the appearance of a Type IV adsorption isotherm. A typical feature of Type IV isotherms is a final saturation plateau, of variable length (sometimes reduced to a mere inflexion point). In the case of a Type IV(a) isotherm, capillary condensation is accompanied by hysteresis. This occurs when the pore width exceeds a certain critical width, which is dependent on the adsorption system and temperature (e.g. for nitrogen and argon adsorption in cylindrical pores at 77 K and 87 K, respectively, hysteresis starts to occur for pores wider than ~ 4 nm). With adsorbents having mesopores of smaller width, completely reversible Type IV(b) isotherms are observed. In principle, Type IV(b) isotherms are also given by conical and cylindrical mesopores that are closed at the tapered end. Type V isotherms are characterized by a convexity to the relative pressure axis. Unlike Type III isotherms there occurs a point of inflection at higher relative pressures. Type V isotherms result from weak gas-solid interactions on microporous and mesoporous solids (e.g. water adsorption on micro-or mesoporous carbons). Type VI isotherms are notable for the step-like nature of the sorption process. The steps result from sequential multilayer adsorption or uniform non-porous surfaces. Amongst the best examples of Type VI isotherms are those obtained with argon or krypton at low temperature on graphitized carbon blacks. There are various phenomena which contribute to the occurrence of hysteresis, and this is also reflected in the IUPAC classification of hysteresis loops [1] shown in Figure 3. Type H1 hysteresis loops are observed for mesoporous materials with relatively narrow pore size distributions as for instance in ordered mesoporous silicas (e.g. MCM-41, MCM-48, SBA-15), some controlled pore glasses and ordered, mesoporous carbons, and materials with mesoporous cylindrical pores and for agglomerates of spheroidal particles of uniform size. Usually, network effects are minimal and occurrence of Type H1 hysteresis is often a clear sign that hysteresis is entirely caused by delayed condensation, i.e. a metastable adsorption branch. Hysteresis loops of Type H2 are given by more complex pore structures in which network effects are important [10] . The very steep desorption branch, which is a characteristic feature of H2(a) loops, can be attributed either to pore-blocking/percolation in a narrow range of pore necks or to cavitation- induced evaporation, as well as for some 2-dimensional materials with slit-shaped pores. H2(a) loops are for instance given by many silica gels, some porous glasses (e.g. vycor) as well as some ordered mesoporous materials (e.g. SBA-16 and KIT-5 silicas). The Type H2(b) loop is also associated with pore blocking, but the size distribution of neck widths is now much larger. Examples of this type of hysteresis loops have been observed with mesocellular silica foams and certain mesoporous ordered silicas after hydrothermal treatment. A distinctive feature of Type H3 is that the lower limit of the desorption branch is normally located at the cavitation-induced p/p 0 . Loops of this type are given for instance by non-rigid aggregates of plate- like particles (e.g. certain clays), but also if the pore network consists of macropores which are not completely filled with pore condensate. Capillary condensation between small particles can also lead to Type H3 hysteresis. Hysteresis of Type H4 is somewhat similar to Type H3, but the adsorption branch shows a more pronounced uptake at low p/p 0 being associated with the filling of micropores. H4 loops are often found with aggregated crystals of zeolites, some mesoporous zeolites, and micro-mesoporous carbons.
ISO 15901-2:2022 pdf download – Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption — Part 2: Analysis of nanopores by gas adsorption
