Contemporary Approach To Micropore Analysis

There has been much debate as to the best way to analyse micropore size distribution using gas sorption apparatus.

Historically gas sorption studies are performed using Nitrogen at 77K, but recent studies have shown that Argon adsorption measured at 87K has many real advantages in micropore analysis.

The rationale for using Nitrogen adsorption is that both the gas and cryogen are cheap and plentiful, however the disadvantages are:

very high vacuum required over the sample (particularly in the case of ultramicropores <0.7nm)
leading to long analysis times
difficulties in determining the point of equilibration
difficulties associated with adsorption forces between gas and surface
leading to preferential adsorption onto more active surface sites
or even the possibility of pore blockage

Argon analysis at 87K has the very real advantages of:

filling ultramicropores at much higher relative pressures
leading to much faster equilibration times and overall analysis times (analyses can be up to 50% faster)
faster equilibration time means that the point of equilibration can be determined much more reliably minimising the risk of errors caused by under equilibration
Argon also has a much weaker surface interaction reducing the problems of selective adsorption onto specific surface functional groups

ISO Standard 15901-2007

Pore Size Distribution and Porosity of Solid Materials by Mercury Porosimetry and Gas Adsorption - Part3, Section 5.1 states:

“The pore size and volume analysis of microporous materials such as zeolites, carbon molecular sieves etc. is difficult, because the filling of pores of dimension 0.5 -1nm occurs at relative pressures of 10-7 to 10-5 where the rate of diffusion and adsorption equilibration is very slow. ... Hence, it is of advantage to analyse microporous materials by using argon as adsorptive at liquid argon temperature (87.3 K)”

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Mike Tucker at our latest installation of three twin micropore Autosorb-IQ2 analysers.