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Micropore Analysis Using Argon As Adsorptive
There has been much debate as to the best way to analyze micropore size distribution using gas sorption instrumentation. Historically these studies have been performed using nitrogen at 77K, but recent studies have shown that argon absorption measured at 87K has many real advantages in micropore analysis.
The tendency of all solid surfaces to attract surrounding gas molecules results in a process called gas adsorption. Observing the gas adsorption process provides a wealth of useful information about the properties of solids such as surface area and pore size. Surface area is calculated from monolayer amounts, often using the BET method, and pore size is calculated from pore filling pressures.
Nitrogen (chemical element symbol N) is an inert diatomic gas that is usually colorless, odorless, and tasteless. Nitrogen is a liquid between 63K and 77K at atmospheric pressure and remains colorless and odorless. It makes up 78% of the Earth’s atmosphere by volume and was discovered by Daniel Rutherford in 1772, originally called noxious air.
The rationale for using nitrogen absorption is that both gas and cryogen are cheap and plentiful, however there are disadvantages:
* A very high vacuum is required on the sample (especially in the case of ultramicropores <0.7nm).
* Leading to long analysis times
* Difficulties in determining the equilibrium point
* Difficulties related to adsorption forces between gases and surfaces
* Also leading to the possibility of preferential adsorption or pore blocking on more active surface sites
However, argon analysis at 87K has real advantages:
* Ultramicropore filling at high relative pressures
* Leading to faster balance times and overall analysis times (analysis can be up to 50% faster).
* Faster equilibration times mean that the equilibrium point can be determined more reliably reducing the risk of errors due to equilibration
* Argon also has a very weak surface interaction which reduces the problems of selective adsorption on specific surface functional groups.
Argon (chemical element symbol Ar) is also colorless and odorless, and most importantly, being one of the noble gases, it is very heavy. It makes up just less than 1% of Earth’s atmosphere, making it the third most common gas. Argon is a liquid between 84K and 88K at atmospheric pressure. It was discovered in 1894 by Lord Rayleigh and Sir William Ramsay after separating 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 solids by mercury porosimetry and gas adsorption) describes methods for assessing micropores (pores with an internal width of less than 2 nm) and the specific surface area of micropores. materials by low-temperature absorption of gases (ie, when the chemical is not adsorbed or adsorbed).
This ISO standard states that the pore size and volume analysis of microporous materials such as zeolites, carbon molecular sieves, etc. is difficult, because pore filling of 0.5 -1nm size occurs at relative pressures of 10-7 to 10-5 where the diffusion and adsorption equilibrium rates are very slow. …. Therefore, it is advantageous to analyze microporous materials using argon as an absorbent at liquid argon temperature (87.3 K).
Other methods of pore size analysis include capillary flow porometry (also known as liquid expulsion technique) and mercury porosimetry. access).
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#Micropore #Analysis #Argon #Adsorptive