I have seen in many journal papers, researchers used adsorption branch for analysis, so far from my colleagues I know that adsorption branch shouldn't be used since its not saturated while analysis.
Why there is vast pore size difference between these branches, which is the correct / low error value?
Thanks in advance.
In the range of the BET equation validity (generally 0.1 - 0.3 relative pressure) there should be no difference between adsorption and desorption branches, this hysteresis is due to mesopores filling and emptying processes and occurs at higher relative pressures.
Pore size is not to be determined from BET analysis. BET is in a strict sense just used for calculation of surface area in a non-porous surface (from BET calculation you will obtain the amount of adsorptive that will cover a monolayer), although it is widely accepted to be used as a reference for porous samples, even if the non-porous filling mechanism is not the most physically correct.
another answer i heard from an engineer today. 'in certain samples, the secondary or pseudo pores are formed due to clumping / aggregation of the particles in the materials, in such cases, the desorption branch falls down steeply (as in case of cage like porous materials). In such cases, maximum N2 is desorbed at those points giving it as pore diameter which isnt true. hence the adsorption branch is chosen. not sure how far its true, but researchers used adsorption branches in such cases of isotherms (H2 hystereses).eg: SBA-16
If the isotherm is type IV, i.e., the adsorption branch reaches saturation, it can be used with the right implementation of the BJH method for calculating pore sizes. This is particularly useful when the hysteresis loop is non-H1, i.e., the are bottleneck effects that make the use of the desorption branch unreliable. However, the use of DFT models instead of BJH to calculate pore sizes is strongly advisable, no matter if the adsorption branch reaches saturation or not, and the implementation of such models intrinsically imply the use of data from the adsorption branch.
I think the current BET adsorption isotherm is well suit to mesoporous and higher microporous compounds. In other cases it shows it shows deviation. Type II for mesoporous and Type IV is for microporous. In any case at higher pressure it will not reach saturation.
There is some confusion. The question refers to pore size distribution, not to surface area. For PSD the desorption branch is frequently used, only as it gives more elegant plots. For serious work the adsorption branch should be used.
Dear N. Venkatathri,
I think you are confusing people. Isotherms reach saturation if there is no more pore space available, e.g. a microporous sample.
Also, I hope you realize the following regarding adsorption isotherms:
Type I is for microporous
Type II for non-porous
Type IV for mesoporous
if you are using the results that come from the software, you need to know exactly how it's calculating. microporous could cause differences between adsorption and desorption branches at low pressures, i suggest you to look for the methods used for pore size calculations (e.g. BJH) and read about their considerations and the principles they are based on. You need to remember that every case must be treated separately and analized as well, everything depends on the material, previous the analysis you need no have at least some expectations.
Hi Rajesh,
Attached review discusses the problems related to pore size calculations.
The authors of the review strongly recommend the use of adsorption branch of the isotherm to calculate Pore size distribution due to tensile strength effect, phase change of adsorbate and monolayer formation in micro and mesopore materials.
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