Low Surface Area Analysis by Krypton Adsorption at 77 K
Measurement of very low surface area samples by manometric (volumetric) adsorption experiments using traditional nitrogen (77 K) or argon (87 K) is limited by the detection limits of even the best instrumentation. The recommended alternative is krypton adsorption at liquid nitrogen temperature (77 K), which improves the detection limit significantly and allows one to determine absolute surface areas down to 0.05 m² or less.
Using highly accurate volumetric adsorption equipment it is possible to measure absolute surface area as low as approximately 0.5-1 m² with nitrogen (77 K) as the adsorptive. At such low surface areas, the number of nitrogen molecules unadsorbed in the void volume of the cell is large, and can even exceed the number of molecules adsorbed on the surface, contributing to greater measurement uncertainty. Increasing the amount of sample increases the absolute surface area available; however, this is not always practical, due to cell size limitations or sample availability.
In order to measure even lower surface areas, the number of molecules contained within the void volume of the sample cell must be reduced. This can be achieved by using adsorptives with lower vapor pressures, such as krypton. At 77 K, krypton is about 38.5 K below its triple point temperature (Tr = 115.35 K) and it sublimates (i.e., P0, solid) at a pressure of about 1.6 torr. However, it has become customary to adopt the saturation pressure of supercooled liquid krypton for the application of the BET equation, which assumes that, despite the fact that the sorption measurement is performed far below the bulk triple point temperature, the adsorbed krypton layer is liquid-like. When taking the saturation pressure of the supercooled liquid krypton to be 2.63 torr, the number of molecules in the free space of the sample cell is significantly reduced to approximately 1/300 that of nitrogen (77 K) – illustrated in Figure 1. Combined with the fact that a pressure transducer with a much higher sensitivity is necessarily employed (a 10 torr transducer having 100 times the sensitivity of a 1000 torr transducer and a 1 torr transducer having 1000 times the sensitivity), krypton (77 K) adsorption is much more accurate for small adsorbed amounts and can be applied to assess absolute surface areas down to 0.05 m² or below.
Any problems with applying krypton (77 K) adsorption are associated with the fact that the nature and the thermodynamic state (solid or liquid?) of the adsorbed layer(s) are not well defined and therefore the reference state from which to calculate P/P0. Connected with this is some uncertainty with regard to the wetting behavior of the adsorbed krypton phase so far below the bulk triple point temperature – in the BET calculation, a complete wetting of the adsorbate phase is assumed. In the case of nitrogen (77 K) adsorption, a complete wetting behavior is observed for almost all materials, yet this situation may be different below the triple point temperature[1-3]. Another uncertainty is the effective cross-sectional area of krypton, which is very much dependent on the adsorbent surface and is therefore not well established. The cross-sectional area calculated from the density of the supercooled liquid krypton is 15.2 Å² , but larger cross-sectional areas of up to 23.6 Å² [1, 4] are often used.
Despite the uncertainties, krypton (77 K) adsorption remains the recommended adsorptive for low surface area materials and its use is also recommended for this application by IUPAC, ISO, ASTM, and USP.
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- ISO 9277, Determination of the Specific Surface Area of Solids by Gas Adsorption Using the BET Method, International Organization of Standardization (ISO).
- ASTM D4780-12(2017)e1, Standard Test Method for Determination of Low Surface Area of Catalysts and Catalyst Carriers by Multipoint Krypton Adsorption, ASTM International, West Conshohocken, PA, 2017, www. astm.org
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