Physical Properties of Potential Adsorptive Gases for Physisorption Experiments

Porous materials are normally characterized using gases at subcritical temperatures, such as nitrogen at 77 K (T/Tc = 0.61), argon at 87 K (T/Tc = 0.58), CO2 at 273 K (T/Tc = 0.90) (a schematic phase diagram for a pure fluid is shown in Fig. 1).

Below the critical temperature, Tc, a clearly defined adsorptive phase (adsorbate) is present as a liquid like film is formed on the walls of the pores.

Schematic phase diagram of a fluid. The vapor pressure line, which defines the temperatures and pressures where gas and liquid are in coexistence, terminates at C, the critical point. Tr is the triple point.

Figure 1. Schematic phase diagram of a fluid. The vapor pressure line, which defines the temperatures and pressures where gas and liquid are in coexistence, terminates at C, the critical point. Tr is the triple point.

Essential experimental parameters necessary to start a physisorption analysis are discussed. After that there is an overview of gases used in characterization of adsorbents as well as a discussion of gases often studied for storage applications.

Experimental Considerations

Choice of Adsorptive

Choosing an adsorptive should be based on both the instrument limitations and the data you want to know about the adsorbent. The adsorptive choice will dictate the necessary pressure range for the experiment.

Compatibility

Before an adsorptive is chosen, its compatibility with the instrument must be assessed. O-rings are used in many Quantachrome instruments as gas and vacuum-tight seals.

Temperature

At temperatures less than the critical point of the gas, it may be possible to determine surface area and pore size distributions of the adsorbent. Below the critical temperature, it is convenient to measure the Po value continuously during the experiment using a dedicated Po cell.

At temperatures above the critical point, a saturation pressure, Po , is no longer defined. Pore size analysis and a classical surface area (BET) are also not possible. If the experiment is performed at supercritical temperatures, the user cannot use the measure Po option. Hence the user must enter a pseudo- Po value which is recommended as 760 torr in the software. With this entered value, the data obtained reflects the adsorbed amount as a function of absolute pressure (in atm).

Experimental parameters

For beginning an adsorption experiment, the following three parameters must be known:

  • Temperature
  • Po option (Table 1)
  • Non-ideality factor (Table 2)

Table 1. Instrument configuration and Po type

Quantachrome instruments Transducers [torr] Lowest P/Po range Turbopump Po options
Autosorb-iQ 1000 1 x 10-3 No Stationa, User Entered
Autosorb-iQ MP 1000, 10, 1 1 x 10-7 Yes Stationa, User Entered
Autosorb-6B 1000 1 x 10-3 No Stationa, User Entered, Open to ambient, Calculated
Autosorb-6B MP 1000, 10 1 x 10-5 Yes Stationa, User Entered, Open to ambient, Calculated
Nova 1000 1 x 10-3 No Measure, Entered, Daily, Calculate, Continuous
Quadrasorb SI 1000 1 x 10-3 No User Entered, Calculate, Measure every X points
Quadrasorb SI MP 1000, 10 4 x 10-5 Yes User Entered, Calculate, Measure every X points

a. Station = continuous measurement of Po with a dedicated Po cell, and pressure transducer

Table 2. Physical properties of adsorptives

Gas Temp [K] Non-ideality factor [torr-1] Poa[torr] Molecular Weight [g/mol] Critical Temperature [K]
Acetone 298 1.208 x 104 184.62 58.08 508.1
Acetonitrile 298 3.456 x 10-4 72.825 41.05 548
Acetylene 298 1.163 x 10-5 37946b 26.04 308.1
Ammonia 298 1.461 x 10-5 7490.2 17.03 405.7
Argon 77.35
87.45
11.4 x 105
3.94 x 105
205 (s) 230 (l)
771.94
39.948 151
Benzene 298 1.095 x 10-4 94.519 78.11 562
Butane 273
298
1.42 x 10-4
3.95 x 10-5
769.87
1816.3
58.12 425
n-Butanol 298 1.497 x 10-4 4.79736 74.12 563.1
Carbon Dioxide 195
273
298
2.57 x 105
9.078 x 10-6
6.842 x 10-6
760.98b
26037
48095
44.01 304.2
Carbon Monoxide 87.45 4.64 x 10-5 1404 28.01 133.9
Cyclohexane 298 9.341 x 10-5 96.978 84.16 553.5
Ethane 298 9.912 x 10-6 31329 30.07 305.3
Ethanol 298 2.716 x 10-4 44.604 46.07 513.9
Ethyl Acetate 298 1.327 x 10-4 74.38 88.105 530.6
Hexane 298 1.137 x 10-4 121.41 86.17 507.4
n-Hexanol 298 6.63 x 10-5 0.6697b 102.18 611
Hydrogen 77.35
87.45
97
107
2.16 x 10-6
1.18 × 10-6
5.35 × 10-7
9.85 × 10-8
N/A
N/A
N/A
N/A
2.016 33.3
Isopropanol 298 1.885 x 10-4 33.105 60.09 508.3
Krypton 77.35
87.45
3.00 x 10-5
2.00 × 10-5
1.6
2.63c
13
83.8 209
Methane 273
298
3.15 x 10-6
2.31 × 10-6
N/A
N/A
16.04 190.4
Methanol 293
298
303
313
1.004 x 10-4
9.441 × 10-5
8.875 × 10-5
7.835 × 10-5
96.958
126.38
163.14
264.55
32.04 512.6
Nitrogen 77.35
298
6.58 x 10-5
2.168 × 10-7
759.55
N/A
28.02 126.2
Nitrous Oxide 298 7.004 x 10-6 42251 44.02 309.7
Oxygen 77.35
87.45
298
6.799 x 10-5
4.65 × 10-5
8.627 × 10- 7
155.61
565.32
N/A
31.999 154.6
n-Pentanol 298 5.32 x 10-5 1.5464 88.15 588.2
n-Propanol 298 1.587 x 10-4 14.922 60.09 536.8
Sulfur Dioxide 298 1.19 x 10-5 2928.4 64.06 430.6
Sulfur Hexafluoride 298 1.503 x 10-5 17678 146.05 318.7
Tetrahydrofuran 298 3.240 x 10-5 129.64 72.11 540.1
Toluene 298 1.505 x 10-4 21.894 92.13 592.2
Water 293
298
303
313
6.852 x 10-5
6.246 × 10-5
5.706 × 10-5
4.793 × 10-5
17.384
23.565
31.582
54.95
18.02 647.1

Gases for Characterization: Alternatives to Nitrogen

Although nitrogen at ~77 K is the adsorptive traditionally used for characterization of porous materials, some of its limitations, especially to characterize microporous or very low surface area materials, make it necessary to team nitrogen up with other gases to obtain full characterization of the adsorbent.

Argon

The characterization of microporous materials with nitrogen at ~77 K is difficult because filling of pores with dimensions of 0.5-1 nm occurs at very low relative pressures (P/P0 =10-7 – 10-5). Pore filling pressure regions of argon at ~87 K in zeolites are correlated with the pore size/structure which allows one to resolve small differences in pore size (resolution of 0.1 nm). A complete micro- and mesopore size analysis is not possible because at ~77 K argon is ~6.5 K below the triple point temperature of bulk argon. Analysis is restricted to pore diameters less than 15 nm due to the fact that pore condensation cannot be observed above this pore size. Argon adsorption at ~77 K does not offer the same experimental benefits as compared to argon adsorption at ~87 K [3].

Carbon Dioxide

Carbon dioxide has a higher diffusion rate than nitrogen, which allows the carbon dioxide molecules to more easily access the ultramicropores than nitrogen at ~77 K, thus enabling quicker analysis. Pore size distributions can be obtained from isotherm using NLDFT or Grand Canonical Monte Carlo (GCMC) methods.

For Micropore Volume Determination

Water

Using water as an additive is an alternative for the determination of the total micropore volume is the use of water as adsorptive. Water is a quite small and can therefore penetrate very small pores which are not accessible for nitrogen and argon at cryogenic temperatures. Because of its strong dipole and the resulting specific interactions with surfaces, it is not a good choice for pore size analysis.

Krypton

Krypton may be used as an alternative for hydrogen for low surface area samples. Krypton has a lower vapor pressure than nitrogen meaning that a fewer number of molecules is contained within the void volume of the sample leading to less uncertainty in the surface area measurement for low surface area samples.

Gases for Storage Applications

Hydrogen

Hydrogen is supercritical at room temperature hence it is possible to store considerable amounts of pressure at high pressures. However, hydrogen adsorption experiments can still be performed at subatmospheric pressures in order to obtain information about the hydrogen storage potential of an adsorbent.

Cryogenic temperatures can also be studied to increase hydrogen uptake. Hydrogen isotherms measured at different temperatures can be used to determine the isosteric heat of adsorption for the adsorbent.

Carbon Dioxide

Carbon dioxide can be used not just for ultramicropore characterization but there is also a lot of interest in the storage of carbon dioxide. Storage capacities may be determined at different temperatures from the isotherm.

Methane

Using porous materials for storing methane is also of key interest. As for hydrogen, in these storage experiments, methane is supercritical and can therefore not be used to characterize a material, but only to investigate its potential for methane storage.

Conclusions

Gases other than nitrogen play a key role in characterizing porous materials and offer better alternatives for determining properties such as pore size, particularly for microporous materials. An important consideration in the physisorption of gases is the temperature at which the experiment is being performed.

If a temperature below the critical point of the gas is selected, information about the surface area, pore size distribution, and porosity of the adsorbent can be obtained. Advice has been given on setting up physisorption experiments, including discussion of some of the important experimental parameters relative to the adsorptive of choice.

This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.

For more information on this source, please visit Quantachrome Instruments.

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