A considerable amount of research is being conducted in the use of materials as a hydrogen storage medium. A wide range of microporous materials are being studied as potential storage media. The accurate determination of hydrogen sorption properties at different temperatures and pressures is critical when choosing a new storage material. However, requirements such as rigorous degassing when preparing samples for hydrogen adsorption and the need for pure hydrogen make the measurement of hydrogen uptake technically demanding.
Earlier studies on hydrogen uptake by zeolites at low temperatures involved measurements at a single temperature. However, this study demonstrates the determination of the hydrogen uptake of a commercial Na-X zeolite by using the gravimetric technique at various temperatures between that of liquid nitrogen and ambient temperatures.
Experimental Procedure
An IGA-001 was used to make the measurements in this experiment. The IGA-001 is a fully automated gravimetric analyzer used for determining a material’s gas sorption properties at pressures of up to 2 MPa.
The experiment involved degassing about 176.5 mg of Na-X zeolite sample from Sigma-Aldrich at 623 K for 18.5 hours. The base pressure at the turbomolecular pump inlet was maintained at 10 µPa prior to cooling to 87 K. A liquid nitrogen circulation cryofurnace was used to control the temperature. The average temperature deviation near the sample at each equilibrium uptake point was observed to be less than ±0.1 K. Measurements were made at the average temperatures indicated in Figure 1 using 99.9999% purity hydrogen. A buoyancy scan was carried out at room temperature before the hydrogen uptake measurement to determine the helium density of the sample needed for correcting for the buoyancy effect.
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Figure 1. Hydrogen adsorption and desorption isotherms for Na-X zeolite, measured at the temperatures indicated in the legend
Results
The isotherms for target temperatures of 87 K, 97 K, 107 K, 117 K, 137 K and 237 K are shown in the Figure 1, they exhibit reversibility between adsorption and desorption, showing that only physisorption occurred. It can also be observed that the maximum excess adsorbed quantity achieved at a temperature of 87 K and a pressure of 2.0 MPa was 2.02 wt.%. The uptake at the highest temperature of 237 K at 2 MPa was observed to be 0.245 wt.%. The Sips equation, a combination of the Langmuir and Freundlich equations, which expresses the excess adsorbed quantity, n, as a function of pressure, p, has been used to fit the data. The equation is as follows:
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Figure 2. Hydrogen adsorption isotherms for Na-X zeolite, showing least squares fits to the Sips (Langmuir-Freundlich) equation
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Figure 3. Hydrogen adsorption isobars for Na-X zeolite in the temperature range 87-237 K. The values are taken from the fits to the data shown in Figure 2 using the Sips (Langmuir- Freundlich) equation.
Conclusion
This experiment demonstrates the determination of hydrogen sorption isotherms for Na-X zeolite at different temperatures, ranging from 87 to 237 K, and at pressures of up to 2 MPa. The reversibility of the adsorption-desorption isotherms indicate the occurrence of physisorption, rather than chemisorption.
At 87 K and 2 MPa, the maximum uptake was found to be 2.02 wt.%. At the highest temperature, 237 K, and at 2 MPa, the maximum uptake was measured to be 0.245 wt.%.
This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.
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