It is unexpected indeed. I think that there is just an error in the way the data were treated for calculating such surface areas.

Alain

Are you sure that your raw data for the calculation of the BET surface area are reliable? You should check the recorded isotherm in terms of hyteresis and the size of the linear behaviour if the isotherm.

Please find enclosed the data, i think the problem is from the way that the softwar calculate the external surface area. So i need to do it manually ...

t plot is reporting -0.47 cc/g of micropore volume (cell AL27).

You should use alpha plot for external surface area.

Also, your equation : (BET surface area = Langmuir surface+ External surface) is not correct, each thing is different and you cannot compare as that in an algebraic equation.

Did you see the adsorption isotherm? You should ALWAYS start by analyzing qualitatively that, before getting into all the numbers. Well, the isotherm does not make any sense for an activated carbon. Attached are 3 images with my analysis, but you need to fix the isotherm first.

Notice that the numbers don't make any sense.

It isn't possible. The external surface area is equal to the mesopore surface area. Bet surface area corresponds to sum of the micro- and mesopore surface area. Therefore, it becomes always smaller than BET surface area.

Thanks for you all,

what type of this isotherm?

its I or I + IV ..?

What I wanted to say is: that is not a real isotherm, but something wrong with the equipment...

That's why I recommended to have a look at this isotherm and realize that it doesnt make any sense (totally linear? no way!)

Your isotherm looks like the calibration curve of an empty tube. Was the amount of material high enough for being able to measure something accurately ?

Alain

-Question: how do you know the external surface area is 2000 m2/g ? 

==> i know it via the Data obtained from the measurement, find enclosed the result obtained .

- Did you do the BET 1200 m2/g by yourself?

==> Yes i do it and i found the same result give by the measurement in Data enclosed by Software.

Finally, the problem of result of external surface area is T-plot ...  

Dear Zbair, your experiment is wrong. The cold and warm free volume is impossible: 16.00 and 45.00. Exact. Read the application note of Micromeritics

http://www.micromeritics.com/Repository/Files/apnote104.pdf

And use large time of 10 s to analyze a microporous carbon

Dear Zbair, Mr Parra is right. All the adsorbed volume are misscalculated, since you have not measered Warm and Cold free space.

To perform this kind of analysis you have two choices(In case you can't determine the sample density use B):

A) Measure sample density by He picnometry and determine warm and cold free space using appnote104 (as Mr Parra said). Then you have to use the option "calculate" in Analysis conditions\Free Space.

B) Measure warm and cold free space afterwards:

1-First you analize your sample in a similar way as you did for this sample (000-153.SMP), selecing the "Enter" option in in Analysis conditions\Free Space, to avoid that He enter in the pores of the sample.

2-Then, once the analysis has finished, you keep the sample tube connected to the analysis port and you run a second analysis only to determine the free space (see atached file) where you choose the option "measure" for the free space determination, no "low-dose pressure incremental mode", only one point in the pressure table (i.e. P/Po = 0.1).

when this second analysis is finished you copy the warm and cold free space values to the SMP file obtained in step 1.

Dear Dr, Zbair, I recommend the option A in the answer of David because He gas is necessary but, at the same time is a poisonous gas for the N2 adsorption isotherms in activated carbons. You can calibrate your holder tube empty and introduce the warm and cold free space in the previous isotherm. The best is use the He density of the sample, but if it is impossible, for one activated carbon you can use 2.0 g/cm3. And other recommendation, it is possible use a minimum of 0.1 g of sample.

Apologies for being late to the discussion. David and Jose are correct, the REAL free-spaces must be measured and re-entered into the software. The perfect integers "16" and "45" are arbitrary values. There's no knowing where they came from but they are clearly incorrect.  

The isotherm would be expected to be a type I with a good horizontal plateau. That very straight line positive slope is indicative of a void volume (sum of free spaces) error... too small a VV that is. Too large a void volume drives a type I isotherm downward after the knee. Taking a quick visual estimation of the isotherm, the knee is ~100cc/g uptake (at very low P the void volume error is of course much less) and the top of the isotherm is ~1000cc/g (but should be ~100). Therefore (very) approximately 900cc/g error, which for 0.0420g (per excel provided above) means ~38cc (too small) error. How this error is shared between cold and warm free spaces I have no idea.

Hopefully this discussion will alert other gas sorption analysts to understand what void volume/free space is all about, and such values must at some point be determined experimentally - be it at start of analysis, after analysis, or via blank cell "calibration" (which requires a correction for sample volume).

Dear Mohamed,

You can change the dead volumes entered after measuring the isotherm.

In the same Micromeritics ASAP 2020 equipment, I obtain 48.69 and 16.43 for warm and cold free space respectively. Try plugging those numbers in your sample information, then save and make the report again. Those numbers I obtain using the filler rod, if you are not using, your volumes will be larger.

You might see something close to the real isotherm after trying that. Then you can measure the free space and the isotherm again. Attached are the relevant pages from the ASAP 2020 user's manual.

It is not possible. Because, the BET surface area is equal to the sum of of micropores and external (meso pores) surface areas. If your measurements are correct, also Although this is rare, may occur in the mainly presence of large dimensional mesopore in the structure.

Sorry to do not answer this question, beacause I have the same doubt, but specifically, if the values shown in report from "Micromeritics Instrument Corporation" regarding to "BJH Desorption cumulative surface area of pores " is equivalent for this "external surface area"? If do not represents it parameter, where can I find this information?

with regards

At least in the t-plot report you can find the µ-pore volumes and surfaces plus the external surface areas.