Before any sorption experiment a material and the apparatus must be degassed to a certain extent. In the case of adsorbents this process is crucial in preparing the sample’s surface for adsorption. In general, it is necessary to begin an adsorption measurement with the surface in a state appropriate for the application for which the material is being considered. For hydrogen adsorption a ‘clean’ surface is required, although there may be exceptions, depending on the definition of a ‘clean’ surface. Generally, however, any environmental adsorbates that could react with hydrogen or be desorbed in the temperature range of interest must be removed. This issue is covered by the IUPAC guidelines, and two approximate thresholds for degassing pressures are identified. Firstly, around 10 mPa (10^-4 mbar) is suggested as a satisfactory residual pressure for degassing a sample for the purpose of a surface area or porosity determination measurement; this is in the high vacuum range. Secondly, it suggests that an ultra high vacuum (UHV) pressure of less than 1 μPa (10^-8 mbar) may lead to changes in surface composition, the formation of surface defects or irreversible changes in texture. In a later section, it also refers to a basic UHV pressure of 100 μPa (10^-6 mbar), which is closer than 1μPa to an achievable level in sorption apparatus constructed from UHV components. However, the important point is that even the highest of these pressures requires a pump suitable for UHV systems, which typically means a turbomolecular pump. The BET Method standard suggests 1 Pa as “usually sufficient” for degassing, but also mentions the requirement of a vacuum “better than 10^-2 Pa” for the zero point of a gravimetrically-determined isotherm. The standard also includes instructions for monitoring the degassing process.

During the degassing procedure it is the pressure above the sample that is important and this often cannot be measured directly. The geometry of the sorption apparatus will have a significant effect on the achievable vacuum over the sample and the rate at which it will be achieved, due to the conductance of the tubing. A system constructed of UHV components, large bore tubing and having a relatively direct path from the vacuum pump to the sample will achieve a different level of vacuum in the sample cell than a system constructed of narrow tubing with a more complex route (including, for example, several valves) from the pump to the sample cell. As an example, from recent hydride work, Vajo et al replaced their sample cell with an ionization

gauge to determine the quality of the vacuum achievable on their sample and found that a base pressure at their pump of < 1.3 × 10^-6 Pa resulted in a pressure of 1.3 × 10^-4 Pa near their sample after pumping overnight. In this case the sample cell was separated by approximately 1 m of 0.953 cm outside diameter tubing, several valves and a 2 μm filter gasket. With a different length of tubing, different geometry, different filtration, and so on, this value would be different and the length of time taken to achieve an equivalent vacuum would be different. In addition, there are further material-related considerations, with the degassing temperature being another essential factor in the degassing process. This will depend on both the adsorbate species to be removed and the thermal stability of the material. In the case of carbon materials, for example, the results of degassing the sample at different temperatures can be significantly different and can have implications for the material’s subsequent readsorption of environmental adsorbates upon exposure to air. The sample’s subsequent storage conditions will then have an effect on the material’s properties.

An important related issue is the accurate determination of the dry sample mass. In gravimetry the sample mass can be monitored during the degassing procedure. Once the mass has stabilized under conditions suitable for the particular material, the measured value can be used as the dry sample mass31. In the volumetric technique this is not possible and an alternative method of determining the degassed sample mass must be used. The significance of this as a source of error is dependent on the type of material; for example, if an adsorbent is hydrophilic (for example, a zeolite) there may be a larger wt.% uptake of water to be removed than for a hydrophobic adsorbent (for example, a carbon). Another example is the removal of solvent from the pores of a MOF, which could also contribute to a larger reduction in sample mass during the degassing process.

For hydrides that can be exposed to air, degassing is not as crucial as for porous adsorbents but it is still an important part of the activation procedure. If an unactivated sample is not degassed sufficiently the environmental adsorbates are likely to inhibit the initial surface activation process.