Hydrogen can either be adsorbed, or absorbed by materials. Absorption (H) is a chemical process that occurs in the bulk whereas adsorption (H or H2) is a surface process, and sorption is a general term that covers both mechanisms. The measurement of hydrogen sorption is of interest for both fundamental and applied reasons.
Macroscopic measurements help in determining the following aspects:
- H2 uptake as a function of P and T
- H2 sorption kinetics
- Cyclic stability
- Usable or reversible H2 capacities
Three techniques are most commonly used to measure the sorption of hydrogen by materials:
- The manometric technique (also known as Sieverts’ method) uses an isothermal system of fixed volume; P is measured to determine sorption
- The gravimetric technique makes use of a microbalance; weight change is measured to determine sorption
- Temperature-programmed desorption is where hydrogen is desorbed as a function of temperature (T)
Volume V1 is filled to a pressure Pi, whilst the sample cell is maintained at vacuum (Figure 1). The connecting valve is then opened to dose H2 to the sample, and the final pressure, Pf, is measured. The number of moles sorbed is then given by:
where V2 is the dead volume of the sample cell, and Zi,T and Zf,J are the H2 compressibility factors at Pi and Pf respectively.
In order to establish an entire isotherm, this dosing and measurement process is repeated for each data point.
Figure 4. Volume V1 is filled to a pressure Pi, while the sample cell is held at vacuum.
Two important aspects of manometric measurement technique are:
- Sample degassing and activation - this is the process of preparing a sample for hydrogen sorption
- Dosing and equilibration - this is the process of loading a sample with hydrogen
Sample Degassing and Activation
Samples have to be prepared for hydrogen sorption, and different types of processes are used for porous materials and hydrides. The process always begins with degassing, where there is deliberate removal of environmental contaminants, for example pre-adsorbed water.
Activation of Hydrides
Hydride activation involves hydrogenation or dehydrogenation of a metal host. This is often accompanied by decrepitation, where the brittle host breaks down into fine powder. Some metals, such as Pd, remain intact. However, hydrogenation must still be performed. Hydrogenation/dehydrogenation is performed under conditions appropriate for the particular materials and repeated cycling is often required.
Activation of Porous Materials
To activate the porous materials, the sample is exposed to a vacuum and elevated temperature, resulting in desorption of chemisorbed and physisorbed species. MOFs (metal organic frameworks) for example require the removal of solvents from the synthesis process. In some cases, the process is more involved. For instance, zeolites have to be degassed in stages because of hydrothermal instability, and microporous organic polymers or MOFs can be particularly fragile.
Practically, in a manometric instrument, the sample is exposed just to vacuum. Consequently, the sample and the system start the measurement process 'clean'. This process is similar to the baking process in UHV systems.
Dosing and Equilibration
For every isotherm point the calibrated volume, V1, is pressurized. The separating valve is opened; however, equilibrium is not achieved immediately. Here, thermal effects are important, and the sorption process must be allowed to reach completion. Hence, both T and P should be monitored carefully as a function of time, t.
The manometric technique is one of the three main methods used to measure hydrogen uptake by materials. It involves the measurement of pressure and temperature in a system of fixed volume. In this article we have covered the principles of the technique and some important aspects of measurement methodology.
This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.
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