Low Surface Area Analysis by Krypton Adsorption at 77.4K
Abstract Measurement of very low surface area samples by manometric (volumetric) adsorption experiments using traditional nitrogen (at 77.4 K) or argon (at 87.3 K) is limited by the detection limits of even the best equipment. The recommended alternative is krypton adsorption, at liquid nitrogen temperature (i.e. 77.4 K), which improves the detection limit significantly and allows one to determine absolute surface areas down to 0.05 m2 or less.
Problems Associated with Low Surface Area Measurements Using highly accurate volumetric adsorption equip- ment, it is possible to measure absolute surface areas as low as approximately 0.5 – 1 m2 with nitrogen as the adsorptive. At such low surface areas, the number of nitrogen molecules unadsorbed in the void volume of the cell can be large compared to, and even exceed, the number of molecules adsorbed on the surface, contributing to a larger measurement uncertainty. Increasing the amount of sample can increase 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 a lower vapor pressure, such as krypton. At 77.4K krypton is about 38.5K below its triple point temperature (Tr = 115.35K), 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, i.e., one assumes that, despite the fact that the sorption measurement is performed that far below the bulk triple point temperature, the adsorbed krypton layer is liquid-like. The saturation pressure of the supercooled liquid krypton is 2.63 torr, therefore the number of molecules in the free space of the sample cell is significantly reduced to approximately 1/300th that of the nitrogen case. Hence, krypton adsorption at ~77K is made much more accurate, and can be applied to assess absolute surface areas down to 0.05 m2 or below. Any problems with applying krypton adsorption at 77.4K are of course associated with the fact that the nature and the thermodynamic state (solid or liquid?) of the adsorbed layer(s) are not well defined, and hence the reference state
to calculate P/P0. Connected with this is some uncertainty with regard to the wetting behavior of the adsorbed krypton phase that far below the bulk triple point temperature (i.e., in the BET approach a complete wetting of the ad- sorbate phase is assumed). In the case of nitrogen adsorption at its boiling temperature 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 that the effective cross- sectional area of krypton is very much depend- ent on the adsorbent surface and is therefore not well established. The cross-sectional area calculated from the density of the supercooled liquid krypton is 0.152 nm2 (15.2 Å2), but larger cross-sectional areas of up to 0.236 nm2 (23.6 Å2) [1, 4] are often used. One commonly adopted value is 0.202 nm2 (20.2 Å2) .
Krypton Adsorption Techniques Because of the low saturation pressure of krypton, the relative pressures in the classical BET range (i.e. 0.05 – 0.3) correspond to absolute pressures below 1 torr. In order to achieve and measure pressures in this range, a turbomolecular pump and 1 torr or 10 torr gauges are required. This config- uration is available on several models of gas sorption analyzers from Quantachrome instru- ments, including the Autosorb iQ, Quadrasorb SI and Autosorb 6B. Typical analysis conditions for an Autosorb are given in Table 1.