The cryoTune for easy handling: BET surfaces with argon 87 K according to ISO 9277 and IUPAC recommendation 2015

The specific surface of solids – e.g. adsorbents and catalyst pellets – is an important property that is routinely determined by nitrogen adsorption at 77 K and the BET model (Brunauer, Emmett and Teller). However, the suitability of nitrogen for surface characterization must be considered critically, as is also stated in the current IUPAC Technical Report 2015 and ISO 9277:2010.

The reason behind this is the quadrupole moment caused by the non-spherical symmetrical charge distribution of the electrons around the nuclei of the nitrogen molecule. The quadrupole moment causes nitrogen molecules to interact specifically with functional surface groups and ions. In comparison, the atomic adsorbing gases argon or krypton have no quadrupole moment and posess spherical geometry. These noble gases always occupy the same place on the surface – in contrast to the ellipsoidal nitrogen molecules.

Schematic comparison of the nitrogen molecule at 77 K with the two noble gases argon 87 K

Schematic comparison of the nitrogen molecule at 77 K with the two noble gases argon 87 K and krypton 120 K, each of which has only one position.

The article by the working group of Prof. Fröba (University of Hamburg) in Partikelwelt 17 deals with the comparative investigation of BET surfaces of different Metal-Organic Frameworks (MOFs) with argon at 87 K and nitrogen at 77 K (see table). It can be concluded that all BET surface values determined by nitrogen sorption are higher than those determined with argon. By comparing the difference in value of the differently determined values (table, right column) it can be noted that there is no constant, certain factor between the values depending on the adsorptive in the MOFs investigated. Therefore, to obtain scientifically accurate results, the measurement with the atomic adsorptive argon at 87 K is recommended.

BET surface Ar 87 K / m2g-1

BET surface N2 77 K / m2g-1

Comparison BET N2 77 K vs. BET Ar 87 K / %

HKUST-1

1533

1674

+ 9,2

UHM-30

1648

1806

+ 9,6

UHM-31

1376

1808

+ 31,4

UHM-36

1223

1385

+ 13,2

UHM-37

1178

1305

+ 10,8

UHM-38

1198

1264

+ 5,5

UHM-39

1019

1213

+ 19,0

The cryoTune with a nitrogen Dewar.

In practice, the measurement of argon isotherms is often associated with the problem of the availability of liquid argon for cooling. With the cryoTune, we offer an excellent alternative to the use of liquid argon (87 K). By using liquid nitrogen, the cryoTune can be used to set a measuring temperature of 87 K. The temperature of the liquid nitrogen can be adjusted by means of the cryoTune. Measuring temperatures in the range up to 135 K can be realized very energy-saving and completely noiseless with the cryoTune.

In the meantime, 3P Instruments has installed cryoTunes in several adsorption instruments of different manufacturers. We would be pleased to check whether this is also possible for your sorption device with the corresponding software version. Please send your inquiries to info@3P-instruments.com. In the following you can see some possibilities for using the cryoTune:

applications of cryoTune

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