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| Titel |
Gas adsorption and desorption effects on cylinders and their importance for long-term gas records |
| VerfasserIn |
M. C. Leuenberger, M. F. Schibig, P. Nyfeler |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1867-1381
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Measurement Techniques ; 8, no. 12 ; Nr. 8, no. 12 (2015-12-18), S.5289-5299 |
| Datensatznummer |
250116728
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| Publikation (Nr.) |
 copernicus.org/amt-8-5289-2015.pdf |
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| Zusammenfassung |
| It is well known that gases adsorb on many surfaces, in particular metal
surfaces. There are two main forms responsible for these effects (i) physisorption
and (ii) chemisorption. Physisorption is associated with lower
binding energies in the order of 1–10 kJ mol−1, compared to
chemisorption which ranges from 100 to 1000 kJ mol−1. Furthermore, chemisorption only forms
monolayers, contrasting physisorption that can form multilayer adsorption.
The reverse process is called desorption and follows similar mathematical
laws; however, it can be influenced by hysteresis effects. In the present
experiment, we investigated the adsorption/desorption phenomena on three
steel and three aluminium cylinders containing compressed air in our
laboratory and under controlled conditions in a climate chamber,
respectively. Our observations from completely decanting one steel and two aluminium
cylinders are in agreement with the pressure dependence of
physisorption for CO2, CH4, and H2O. The CO2 results for
both cylinder types are in excellent agreement with the pressure dependence
of a monolayer adsorption model. However, mole fraction changes due to
adsorption on aluminium (< 0.05 and 0 ppm for CO2 and
H2O) were significantly lower than on steel (< 0.41 ppm and
about < 2.5 ppm, respectively). The CO2 amount adsorbed
(5.8 × 1019 CO2 molecules) corresponds to about the fivefold monolayer
adsorption, indicating that the effective surface exposed for adsorption is
significantly larger than the geometric surface area. Adsorption/desorption
effects were minimal for CH4 and for CO but require further attention
since they were only studied on one aluminium cylinder with a very low mole
fraction. In the climate chamber, the cylinders were exposed to temperatures
between −10 and +50 °C to determine the corresponding
temperature coefficients of adsorption. Again, we found distinctly different
values for CO2, ranging from 0.0014 to 0.0184 ppm °C−1 for
steel cylinders and −0.0002 to −0.0003 ppm °C−1 for aluminium
cylinders. The reversed temperature dependence for aluminium cylinders points
to significantly lower desorption energies than for steel cylinders and due
to the small values, they might at least partly be influenced by temperature,
permeation from/to sealing materials, and gas-consumption-induced pressure
changes. Temperature coefficients for CH4, CO, and H2O adsorption
were, within their error bands, insignificant. These results do indicate the
need for careful selection and usage of gas cylinders for high-precision
calibration purposes such as requested in trace gas applications. |
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