Adsorptives for Physisorption Experiments: Selection and Their Physical Properties

Summary of Quantachrome Technical Note #52

Characterization of porous materials is usually performed using gases at subcritical temperatures, such as nitrogen at 77 K, argon at 87 K and CO2 at 273 K. Below the critical temperature a clearly defined adsorptive phase (adsorbate) is present as a liquid like film is formed on the walls of the pores. This allows one to extract information about surface area, pore size and porosity from a proper analysis of physisorption isotherms. At temperatures above the critical point, other adsorption applications are the focus.

An important parameter for subcritical adsorption experiments is saturation pressure which depends on temperature. This is because the thickness of an adsorbed (liquid-like) film, as well as the pressure where pore filling and pore condensation occur in a pore of given width, is related to the difference in chemical potential of the adsorbed phase and the chemical potential of the bulk liquid at the same temperature at which the adsorption experiment is performed.

Experimental Considerations

Choice of adsorptive

The choice of adsorptive is dependent on both the information you wish to obtain about the adsorbent and the limitations of the instrument.

Compatibility

An adsorptive must be compatible with the instrument being used, for example the O-rings ratings must be appropriate.

Temperature

At temperatures which are below the critical point of the gas, it may be possible to determine surface area and pore size distributions of the adsorbent.

Experimental parameters

In order to start an adsorption experiment, the following three parameters must be known:

Temperature
Po option
Non-ideality factor

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 make it necessary to team nitrogen up with other gases to obtain full characterization of the adsorbent.

For Micropore Analysis

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. Argon at ~87 K fills micropores of dimensions 0.5-1 nm at higher relative pressures leading to faster equilibration and, subsequently, a reduction in analysis time.

Carbon dioxide
In the case of porous carbons carbon dioxide at ~273 K can be employed to measure pore sizes, particularly in the ultramicropore regime.

It is important to note that carbon dioxide is not suitable for exploring the pore size distribution of
zeolites, metal-organic frameworks (MOFs), or certain oxides because of its strong quadrupole
moment.

For Micropore Volume Determination

Water
Water is a very small molecule and can therefore penetrate very small pores which are not accessible for nitrogen and argon at cryogenic temperatures.

For Low Surface Area Determination

Krypton
At ~77 K, krypton has a lower vapor pressure than nitrogen meaning that a fewer number of
molecules are contained within the void volume of the sample leading to less uncertainty in the surface area measurement for low surface area samples.

For Thin Film Analysis

Krypton
Krypton can also be utilized at ~87 K for the pore size analysis of thin films.

Gases for Storage Applications

Hydrogen
Hydrogen is supercritical at room temperature; therefore, significant amounts of hydrogen can only be stored at elevated pressures.

Carbon Dioxide
Isosteric heats of adsorption can be calculated for carbon dioxide using the adsorption isotherms measured at different temperatures.

Methane
Generally cannot be used to characterize a material, but only to investigate its potential for methane storage.

More information on Adsorptives for Physisorption Experiments

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