Micropore Analysis Using Argon As Adsorptive

There has been much debate as to the best way to analyse micropore size distribution using gas sorption apparatus. Historically these studies have been performed using Nitrogen at 77K, but recent studies have shown that Argon adsorption measured at 87K has many real advantages in micropore analysis.

The tendency of all solid surfaces to attract surrounding gas molecules gives rise to a process called gas sorption. Monitoring the gas sorption process provides a wealth of useful information about the characteristics of solids such as surface area and pore size. Surface area is calculated from the monolayer amount, often using the BET method, and pore size is calculated from pore filling pressures.

Nitrogen (chemical element symbol N) is a generally inert diatomic gas which is normally colourless, odourless and tasteless. At atmospheric pressure nitrogen is liquid between 63K and 77K and remains colourless and odourless. It constitutes 78% of Earth’s atmosphere by volume and was discovered in 1772 by Daniel Rutherford, originally termed noxious air.

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

However, 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

Argon (chemical element symbol Ar) is also colourless and odourless, and more importantly is very inert being one of the noble gases. It constitutes just under 1% of the Earth’s atmosphere by volume making it the third most common gas. At atmospheric pressure argon is liquid between 84K and 88K. It was discovered in 1894 by Lord Rayleigh and Sir William Ramsay after isolating and examining the residue obtained by removing nitrogen, oxygen, carbon dioxide and water from clean air.

Part 3 of ISO 15901:2007 (Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption) describes methods for the evaluation of the volume of micropores (pores of internal width less than 2 nm) and the specific surface area of microporous material by low-temperature adsorption of gases (i.e. when neither chemisorption nor absorption takes place).

This ISO standard states that 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).

Other methods of pore size analysis include capillary flow porometry (also know as the liquid expulsion technique) and mercury porosimetry (using the physical principle that a non-reactive, non-wetting liquid will not penetrate fine pores until sufficient pressure is applied to force its entry).

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