POWDER TECH NOTE
51
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 equipment, 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
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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 adsorbate 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 crosssectional area of krypton 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 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) [6].
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 configuration is available on several models of gas
sorption analyzers from Quantachrome instruments, including the Autosorb iQ, Quadrasorb SI
and Autosorb 6B. Typical analysis conditions for an
Autosorb are given in Table 1.
Table 1. Autosorb analysis parameters.
Parameter
Value
P/Po range
0.05 – 0.30
Number of points
3 -7, evenly spaced
Tolerance
0
Equilibration
4 (minutes)
Po
2.63 (torr)
Bath temperature
77.4 (K)
Equilibration
4 (minutes)
Summary
Typical krypton BET data (for an approximately
1g oxide sample on a Quadrasorb) are shown in
Figures 1 and 2. The calculated total BET
surface area of 0.20 m2 was based on an
assumed cross-sectional area of 0.205 nm2
(20.5 Å2).
Krypton adsorption at ~77 K is considered to be
the standard method for routine surface area
measurements of materials with low surface area.
Within this context it has also been described in
standard methods for surface area determination
published by ASTM [5] and ISO [6].
Figure 1. BET plot for krypton adsorption at 77.4K.
Figure 2. The corresponding BET tabular report.
References
[1] S. Lowell, J. Shields, M.A. Thomas, and M. Thommes “Characterization of Porous Solids and Powders: Surface
Area, Porosity, and Density” Springer (2004).
[2] J.G. Dash “Films on Solid Surfaces” Academic Press, New York (1975).
[3] H. Dominguez, M.P. Allen and R. Evans, Mol. Phys. (1998) 96, 209.
[4] A.L. McClellan and H.F. Harnsberger, J. Colloid Interface Sci. (1967) 23, 577.
[5] D4780 “Standard Test Method for Determination of Low Surface Area of Catalysts by Multipoint Krypton
Adsorption” ASTM International (2007).
[6] ISO FDIS 9277 “Determination of the specific surface area of solids by gas adsorption using the BET method”
International Organization for Standardization (ISO).
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