There has been much debate about the best way to analyze micropore size distribution using a gas sorption apparatus. Historically, these studies have been performed using nitrogen at 77 K, but recent studies have shown that argon adsorption measured at 87 K 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 great deal of useful information about solid characteristics such as surface area and pore size. Surface area is calculated from the amount of monolayer, often using the BET method, and pore size is calculated from pore fill pressures.
Nitrogen (chemical element symbol N) is a generally inert diatomic gas that is normally colorless, odorless, and tasteless. At atmospheric pressure, nitrogen is liquid between 63K and 77K and remains colorless and odourless. It makes up 78% of the volume of the Earth’s atmosphere and was discovered in 1772 by Daniel Rutherford, originally named noxious air.
The reason for using nitrogen adsorption is that both gas and cryogen are cheap and abundant, however the disadvantages are:
* A very high vacuum is required on the sample (particularly in the case of ultramicropores <0.7nm)
* leading to long analysis times
* difficulties in determining the break-even point
* associated with adsorption forces between gas and surface difficulties
* leading to preferential adsorption on more active surface sites or even the possibility of pore blockage
However, the 87K argon analysis has the real advantages of:
* ultra-micropore filling at much higher relative pressures
* leading to much faster balance times and overall parsing times (parsing can be up to 50% faster)
* faster balance time means the balance point can be determined much more reliably, minimizing the risk of errors caused by insufficient balance
* Argon also has a much weaker surface interaction, reducing selective adsorption problems on specific surface functional groups
Argon (symbol for the chemical element Ar) is also colorless and odourless, and more importantly, it is very inert being one of the noble gases. It makes up just under 1% of the Earth’s atmosphere by volume, making it the third most common gas. At atmospheric pressure, argon is a 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 evaluating the volume of micropores (pores with internal width less than 2 nm) and the specific surface area of ​​microporous material by gas adsorption at low temperature (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. it is difficult, because the filling of pores of dimension 0.5 -1nm occurs at relative pressures from 10-7 to 10-5 where the rate of diffusion and adsorption equilibrium is very slow… Therefore, it is advantageous to analyze microporous materials by using argon as adsorbent at the temperature of liquid argon (87.3 K).
Other methods of pore size analysis include capillary flow porometry (also known as the liquid ejection technique) and mercury porosimetry (using the physical principle that a non-reactive, non-wetting liquid will not penetrate pores until enough pressure is applied to force it out). entry).
