Tech Articles
A sample’s history, including periods of storage, can have a significant effect on its sorption properties and so it is important that this is known and recorded, particularly if the results from two samples are to be compared. This was identified as one of the key problems in (volumetric) kinetic measurements on intermetallic hydrides by Wang and Suda [1]. In this case, the sample’s history includes periods of exposure to air, which are shown to greatly affect the initial hydrogenation reaction, and the number of hydrogenation/dehydrogenation cycles. Robens et al [2] noted, with regard to high surface area candidate reference materials, that the surface could be substantially affected during storage and sample preparation. Therefore, when testing reproducibility, results from microporous materials that have been stored differently should only be compared, for the purpose of assessing measurement accuracy, if the sample degassing procedure can be guaranteed to return the sample’s surface to its original state. The sensitivity of a microporous carbon to its storage conditions was shown by Avraham et al [3]. The importance of a complete knowledge of a sample’s thermal history, in the context of intermetallics, was discussed in detail by Buckley et al [4]. They concluded that the sensitivity of the physical properties of LaNi5-H with regard to the sample’s thermal history is such that the detailed thermal history of the sample should be quoted when data are published.
Pressure measurement is obviously important in both the volumetric and gravimetric techniques. In the volumetric method it is used to calculate the sorbed quantity and therefore must be determined to sufficient accuracy; however, the accurate determination of the pressure is also crucial for gravimetric techniques. In temperature-programmed techniques its importance depends on how the desorbed hydrogen is being measured. JIS H 7201-1991 [5], referring to the volumetric method, states that the pressure should be measured using an instrument “with precision of three or more significant digits at the maximum service pressure”. The IUPAC guidelines [6] suggest that for nitrogen BET the pressure should be measured to within ± 10 Pa, which applies to both volumetric and gravimetric methods, although in the sub-ambient pressure range. As with temperature measurement, this requirement is omitted from the subsequent standard. The choice of pressure measuring device for volumetric measurements must be so that the uncertainty in the pressure measurement is not significant compared to the expected
drop in pressure upon hydrogen absorption, or vice versa, for the chosen sample size. If measurements are to be made over a wide range of pressures then this may require different devices covering different pressure ranges, in addition to the vacuum gauge or gauges used to monitor the pump system.
[1] Wang X. –L., Suda S., "Consistent determination of the intrinsic kinetic properties between hydrogen and hydriding alloys" Journal of Alloys and Compounds 231(1-2)
[2] Robens E., Krebs K. -F., Meyer K., Unger K. K., Dabrowski A., "Progress in the standardization of particle and surface characterisation" Colloids and Surfaces A: Physicochemical and Engineering Aspects 208
[3] Avraham I., Danon A., Koresh J. E., "Study of carbon molecular sieve fibres by atmospheric TPD-MS of H2O, CO and CO2" Journal of the Chemical Society, Faraday Transactions 94(13)
[4] Buckley C. E., Gray E. Mac A., Kisi E. H., "Stability of the hydrogen absorption and desorption plateaux in LaNi5-H. Part 4: thermal history effects" Journal of Alloys and Compounds 231
[5] Japanese Industrial Standards Committee, Method of determining the PCT relations of hydrogen-absorbing alloys JIS H 7201
[6] Sing K. S. W., Everett D. H., Haul R. A. W., Moscou L., Pierotti R. A., Rouquérol J., Siemieniewska T., "Reporting physisorption data for gas/solid systems - with special reference to the determination of surface area and porosity" Pure and Applied Chemistry 57(4)
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