ISO 15901-2 pdf download – Pore size distribution and porosityof solid materials by mercury porosimetry and gas adsorption —Part 2: Analysis of nanopores by gas adsorption.
In the case of a Type IV(a) isatherm. 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 adsurbcnts 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 niesopores that are closed at the tapered end.
Type V isotherms are characterized by a convexity to the relative pressure axis. Unlike Type Ill 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 ofType 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 IIJPAC classification of hysteresis loopst11 shown In Flure 3.
Type Hi 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.iS). some controlled pore glasses and ordered, mesoparous 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 Hi hysteresis Is often a clear sign that hysteresis Is entirely caused by delayed condensation, i.e. a metastable adsorption branch.
Hysteresis loops of Type 112 are given by more complex pore structures In which network effects are importantiWl. 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•dlmensional mati’rials with sllt•shapcd pores. 112(a) loops are for instance given by many silica gels, some porous gbsses (e.g. vycor) as well as some ordered mesoporous materials e.g. SRA-16 and KIT-5 sihcas). The Type H 2(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 mesacellular 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/p0. Loops of this type are given for instance by non-rigid aggregates of plate- like particles (e.g. certaIn days). but also lithe pore network consists of macropores which arc 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, hut the adsorption branch shows a more pronounced uptake at low p/p0 being associated with the filling of micropores. 114 100ps are often found with aggregated crystals olzeolltes. some mesoporous zeolites, and mlcro-mcsoporous carbons.
Care shall also be taken to properly select the equilibration conditions Too short of an equilibration time may lead to under-equilibrated data and isotherms shifted to too high relative pressures. Under- equilibration is often an issue in the very low relative pressure region o(the isotherm, since equilibration in narrow micropores tends to be very slow, For highest accuracy the saturation pressure p0 should be recorded for every data point (by means of a dedicated saturation pressure transducer). I.e. this Is most important for providing acceptable accuracy in the measurement ol p/p0 at high pressures, which is particularly important for evaluation of the size distribution of larger mesopores.
ISO 15901-2 pdf download – Pore size distribution and porosityof solid materials by mercury porosimetry and gas adsorption —Part 2: Analysis of nanopores by gas adsorption
