The longer analysis time for microporous materials with analysis starting from very low relative pressure(10-7)  is not unusual. It happens when the pore size is very small and gas molecule is not able to fill the pore and also gas molecule does not get enough time for equilibration. During this process, required relative pressure is not achieved to measure that particular point. The instrument keep trying again and again to measure that particular point. That,s how it is taking longer time. Some of our samples also take this much time. So there should not be a reason to worry.

Tank you, but i can classify  the obtained isotherm ? I or II or I+IV

Hi Zbair,

The isotherm seems type-I, since it has steep rise below the pressure region 0.01 and for isotherm to be type-IV it should have two steep rise that is missing in your isotherm. But i would suggest you to upload complete isotherm having both adsorption and desorption point, that would give better insight about the nature of isotherm and people will be in better position to give suggestion about your query.         

best

raeesh

Sometimes these measurements can take long, but 3-6 days seems excessively long to me. How was this measured? Did you use a cryocooler or an LN2 bath? I haven't seen an isotherm as linear as this one, you'd need a quite unique pore size distribution for that. Seems to me something went wrong. If you upload the desorption data and the raw data, maybe we can tell you what.

Pieter 

Please find enclosed the data of Adsorption and desorption and all results 

From your file I don't immediately see anything unusual, besides the isotherm itself.

Your sample seems to be highly microporous. After micropore filling there is a continuous linear increase of adsorbed amount which looks suspicious to me. There is also no condensation, i.e. sharp increase of Q, near p/p0=1, though you measured up to 0.995. Even if the sample has no meso and macroporosity, I'd expect to see at least some increase close to 1. Though adsorption increases linearly in the mesopore range, there is no hysteresis at higher pressures, which is also suspicious. Seems to me that the linear increase could be some instrumental issue, though I wouldn't know what exactly. The absence of hysteresis could be due to the lack of mesopores, but then the isotherm would be horizontal above p/p0 of about 0.2. A leak would probably give a time-dependency of Q, which wouldn't give such a linear isotherm.

I'd measure a standard first to be sure you don't have instrumental issues. I'd also recommend to measure p0 for each point if your measurements are this long. I don't know how an ASAP deals with p0, but it seems to me that the p0 values are extrapolated between those 3 p0 values it measured. Those are quite variable, probably actual p0 varies significantly with time in your lab. You should also think about the number of points you need for your analysis. Skipping some points in the low relative pressure range would shorten measurement time (the points above 0.1 only take a couple of min to measure for this sample). Also changing the dosing mechanism could speed up the measurement, but I don't know how and if this can be done for an ASAP.

Sorry I can't say more,

Pieter

Dear Mohamed,

You are again repeating the same question with an experimental isotherm which is wrong (you are using the plot that I made). You can't obtain good data if the isotherm is wrong.

There was some error in your experimental measurement procedure.

This is the original question (you are getting same answers: isotherm is wrong):

https://www.researchgate.net/post/How_can_I_interpret_the_external_surface_area_of_an_activated_carbon_which_is_greater_than_the_BET_surface/1

Could you properly measure the isotherm?

 Thanks for pointing that out, Fernando.

I'm happy to see that we came to the same conclusion.

Mohammed, it is always good practice to double-check your data before bothering other people with it: Check your instrument with a standard, and remeasure your sample. 

Pieter

I work with a Triflex device from Micromeritics, I supposed that the ASAP 2020 has a similar functioning.

I can indicate the analysis conditions I usually used for activated carbons:

- Backfill and match transducer with nitrogen

- Free space is measured after analysis with helium

- p0 is measured with p0 tube for each isotherm point

- Calculate the analysis temperature from p0 or Psat

I think there are not enough points on your isotherm, especially in the low pressures range. Up to p/p0 = 0,001, add a point every 10 cm3 STP (dose amount injected by the device) with an equilibration interval of 45 s. After that, you can use an equilibration time of 10 s and add more points.

For the analysis i repeat it several time Mr.Fernando Vallejos-Burgos and r.Pieter Bertier ·and i found the same result. find enclosed an article where they found the same  isotherm like me.

Best regards

Thank you Mr. Kévin Mozet  i will try it .

Dear Mohamed,

You obtain this kind of isotherms before measuring the cold and warm volumes. In my opinion, you did not measure them as they are exactly 45.000 and 16.000 cm3.  After measurement and introduction of these volumes your isotherm should be a perfect type I.

With best regards,

Vanessa

Both cold and warm volumes seem to be fixed by the operator to speed up the analysis. I think they are OK and should not incur in big error, Mohamed is using ASAP 2020 and I obtain similar values after measuring cold and warm volumes (e.g. 49.33 and 16.55 cm3).

Measurement of cold and warm volumes is essential to determine accurately textural properties. If solid is highly microporous, they should be measured after nitrogen adsorption in order to avoid helium entrapment.

But for sure, they have to be measured!

After seeing the article Mohamed shared before, I can see that there are more people obtaining same kind of isotherm (micropores + linear increasing uptake).

I have to recognize that I expected Mohamed isotherm was wrong, but it seems this is not the case, since it can happen. (I never expected so linear!).

Authors of the article say: "All these isotherms are clearly of Type I [32,33] that are typical of microporous materials. " However, the linear increasing uptake does not belong to microporous materials, since instead saturation should be expected.

This is a perfect Freundlich isotherm? The freundlich equation is not supposed to accurately describe the higher relative pressure range, like it does here, because as mentioned above, it does not account for saturation...

I still think this is an measurement artefact, if the linear increase would be real, it would imply there is substantial mesoporosity, and that would result in a hysteresis at higher relative pressure, due to capillary condensation etc. This isotherm has no hysteresis at all. There would be no hysteresis if the sample has no meso or macro porosity, but then the linear part would be horizontal.

The cold/hot volumes would explain that change of slope. These volumes are not unique for an instrument, they depend on the measurement cell (tube) that is used. Seems to me that for this measurement the volumes were fixed for a specific cell, but another cell was used for the actual measurement.

Pieter