There are a few issues to consider here: (i) sample preparation (ii) acquiring data points in a P/Po range suitable for surface area determination of a zeolite, (iii) appropriate calculation model, and (iv) other experimental factors.

(i) Sample must be properly prepared to empty the pores of adsorbed moisture. Zeolites typically require 400-450C as degassing temperature, but that temperature should be approached on a ramp (say 10deg/min) or stepwise (every 20 deg or so) to prevent rapid formation of steam from damaging the micropore structure.

(ii) zeolite micropores adsorb nitrogen at liquid nitrogen temperature at very low P/Po values. The monolayer capacity in reality corresponds to micropore filling... so one needs to measure adsorption data from say 0.005 to 0.05 P/Po. The "traditional" range of 0.05 to 0.35, or more commonly 0.1 to 0.3, is totally inadequate for zeolites.

(iii) Because the monolayer capacity in reality corresponds to micropore filling, fundamentally an accurate micropore area cannot be determined except by DFT calculations (must the correct "kernel". i.e. that which corresponds to the right chemistry, pore geometry, adsorbate and temperature). However, the use of the BET equation is recognized if it is applied in the best way possible: relatively low pressure data (see ii) and the best linear range is chosen for the BET plot/calculation. That approach is often referred to as the "Rouquerol method" and is described in the attached document (section 5.2.2).

BJH is never appropriate for microporous materials.

(iv) Other factors include proper free space (void volume) determination - beware "re-using" the values from a previous analysis if they are not from exactly the same sample cell/filler rod combination (and don't forget about accounting for sample volume differences if you do re-use free space values). If you have the instrument measure free-space post analysis, you must ensure that process is done properly (with helium, not nitrogen) and the sample tube is fully warmed - otherwise the free space will be over-estimated and your results will be lower than anticipated.

So, I encourage you to read the IUPAC report. Beyond these responses and that document, if you continue to have problems, I would recommend to upload your data file and/or isotherm data for further consideration by this community.

Good luck! 

From the N2 adsorption/desorption curve values, we can substitute them in to BET adsorption equation we can get surface area.  It is nothing but the multplication value of no. of nitrogen molecules adsorped  with crosssectional area of nitogen.

If the SSA is low then you have large particles and little mesoporosity,

Dear Collegue,

Im asking which the best method in the softwar is good to analyze the surface area for microporous materials (Activated Carbons)?

as you know generaly,  BET-BJH is just for mesoporous materials.

The software has nothing to do with it - the N2 molecule will absorb where it can - on surfaces and in pores.  It's tiny - around 14 square Angstroms in size.  Get a copy (if you already haven't) of Adsorption, Surface Area and Porosity S J Gregg & Ken Sing and read it.

You are correct that BET should only be used for non-porous samples.

For microporous materials, the best is using the subtracting pore effect method (SPE) developed by Kaneko et al.. There are many references, like:

DOI: 10.1016/S0001-8686(98)00050-5

It is similar in essence to the alphaS plot. You will need a reference material. For carbons you can find reference isotherms in the literature. SPE will give you a more realistic estimation of the real surface area.

BET and BJH are models to calculate different parameters. BET calculates the volume necessary to cover with one monolayer the surface of solid. BJH calculates de porosity distribution in the range of mesopores (approximate into 2 and 35 nm). In the software of 2020 you can select the automatic model to calculate de specific surface area from the rules of IUPAC. In my opinion are good, not the best, but is good method for un non expert research.

If you work with activated carbons, and the isotherms are type Ia o Ib (IUPAC classification) forgot the method BJH and applied the model NLDFT of your software. However, is better the model SAIEUS, free for N2 at 77 K in activated carbons. This model is 2D-NLDFT, and considers the corrugated form of activated carbon. This method calculated the distribution of microporosity.

 There are a few issues to consider here: (i) sample preparation (ii) acquiring data points in a P/Po range suitable for surface area determination of a zeolite, (iii) appropriate calculation model, and (iv) other experimental factors.

(i) Sample must be properly prepared to empty the pores of adsorbed moisture. Zeolites typically require 400-450C as degassing temperature, but that temperature should be approached on a ramp (say 10deg/min) or stepwise (every 20 deg or so) to prevent rapid formation of steam from damaging the micropore structure.

(ii) zeolite micropores adsorb nitrogen at liquid nitrogen temperature at very low P/Po values. The monolayer capacity in reality corresponds to micropore filling... so one needs to measure adsorption data from say 0.005 to 0.05 P/Po. The "traditional" range of 0.05 to 0.35, or more commonly 0.1 to 0.3, is totally inadequate for zeolites.

(iii) Because the monolayer capacity in reality corresponds to micropore filling, fundamentally an accurate micropore area cannot be determined except by DFT calculations (must the correct "kernel". i.e. that which corresponds to the right chemistry, pore geometry, adsorbate and temperature). However, the use of the BET equation is recognized if it is applied in the best way possible: relatively low pressure data (see ii) and the best linear range is chosen for the BET plot/calculation. That approach is often referred to as the "Rouquerol method" and is described in the attached document (section 5.2.2).

BJH is never appropriate for microporous materials.

(iv) Other factors include proper free space (void volume) determination - beware "re-using" the values from a previous analysis if they are not from exactly the same sample cell/filler rod combination (and don't forget about accounting for sample volume differences if you do re-use free space values). If you have the instrument measure free-space post analysis, you must ensure that process is done properly (with helium, not nitrogen) and the sample tube is fully warmed - otherwise the free space will be over-estimated and your results will be lower than anticipated.

So, I encourage you to read the IUPAC report. Beyond these responses and that document, if you continue to have problems, I would recommend to upload your data file and/or isotherm data for further consideration by this community.

Good luck! 

Here is some advice.

In our super fine powder activated carbon case, the QS-DFT (maybe the newest method applied in Quantachrome instruments) is recommended for the pore size distribution measurement.

As Mr. Thomas said above, the pretreatment is quite important. Because there is no way to measure the true total surface or pore volume 100% actually, the pretreatment should be check by several different conditions to get the stable and good data. When you increase under vacuuming condition, the protection procedures should be take to ensure the little damage to your samples.

Thanks for your help and advice

Mr Thomas has already given the appropriate and detailed response for your question. Just a few points to add further for practical consideration.

1. Surface area of advanced microporous materials such as MOF's, ZIF'S COF's etc. varies much depending upon the sample preparation conditions such as temperature, temperature ramp rate, activation duration and vacuum level.

2. For microporous materials numerous N2 isotherm points should be taken in low pressure range (up to P/P0= 0.05).

3. For any material check out the BET transform plot. The surface area calculations are meaningless if it has a negative intercept or correlation factor not close to unity. (points show considerable deviations from the straight line)

4. A guiding article to find the valid P/P0 range range for the BET calculation of microporous solids is attached herewith.