There have been many disputes regarding the best way to analyze the size distribution of micropores using a gas sorption device. Historically, these studies were 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 the surrounding gas molecules causes a process called gas sorption. Monitoring the sorption process of a gas gives a lot of useful information about the characteristics of solids, such as surface area and pore size. The surface area is calculated from a monolayer amount, often using the BET method, and the pore size is calculated from the pore filling pressure.
Nitrogen (symbol of the chemical element N) is usually an inert diatomic gas, which is usually colorless, odorless and tasteless. At atmospheric pressure, nitrogen is a liquid between 63K and 77K and remains colorless and odorless. It makes up 78% of the Earth’s atmosphere in terms of volume and was discovered in 1772 by Daniel Rutherford, originally called harmful air.
The diet for using nitrogen adsorption is that gas and cryogen are cheap and numerous, but the disadvantages are:
* very high vacuum is required for the sample (especially in the case of ultra-microfores <0.7 nm)
* leads to long analysis time
* difficulty in determining the equilibrium point
* difficulties associated with adsorption forces between the gas and the surface
* which leads to preferential adsorption to more active surface areas or even the possibility of blocking pores.
However, the analysis of argon at 87K has very real advantages:
* filling with ultramicropores at much higher relative pressures
* leads to a significantly faster equilibration time and overall analysis time (analysis can be 50% faster)
* faster balancing time means that the balancing point can be determined much more reliably, minimizing the risk of errors caused by balancing
* Argon also has a much weaker surface interaction, reducing the problems of selective adsorption on specific surface functional groups.
Argon (the symbol of the chemical element Ar) is also colorless and odorless and, more importantly, very inert, being one of the noble gases. It makes up just under 1% of the earth’s atmosphere in terms of volume, which makes it the third most abundant 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 studying the residue obtained by removing nitrogen, oxygen, carbon dioxide and water from clean air.
Part 3 of ISO 15901: 2007 (Distribution of pore sizes and porosity of solid materials using mercury porosimetry and gas adsorption) describes methods for estimating the volume of micropores (pores of internal width less than 2 nm) and the specific surface area of a Microporous material with low-temperature gas adsorption (t. E. When chemisorption or absorption occurs).
This ISO standard states that the size and volumetric analysis of microporous materials, such as zeolites, carbon molecular sieves, etc., is difficult because the filling of pores 0.5–1 nm in size occurs at a relative pressure of 10–7 to 10–5. 5, the rate of diffusion and adsorption is very low. using argon as a liquid argon adsorbing at a temperature (87.3 K).
Other methods for analyzing pore sizes include capillary flow measurement (also known as the liquid ejection method) and mercury porosimetry (using the physical principle that non-reactive non-wetting liquid will not penetrate the small pores until sufficient pressure is applied to enter it).
