I have been performing surface area analysis of various carbon materials using a Gemini VII Micromeretics Surface Area analyzer. Typically, I get a full BET and DFT analysis, and the surface areas for these numbers are usually in alignment (a BET surface area of 7 m2/g might be 7.7 m2/g for DFT, but fairly close). These analyses were typically performed on graphite or carbon black materials.
More recently, I have performed this analysis on some carbon nanotube (CNT) material. The CNT material has extremely large discrepancies between the BET and the DFT surface areas. For example, a BET surface area of 7 m2/g was 35 m2/g on DFT, and a BET that was 15 m2/g was 144 m2/g from DFT.
One explanation is that the microporosity is contributing substantially to the DFT surface area. However, I have no experience dealing with porous carbon materials and do not know if this result is at all typical. Any help or interpretations of this would be valuable. I have attached images of BET and DFT analysis for a graphite sample and a CNT sample.
Hello,
the more complex your material is (like your CNT) the more it is necessary to deal with the applicability of the different theories for the characterization of your material. So it is impossible to say a priori here what is "right" - BET or DFT.
It is necessary to remember that the equation of calculation of the specific surface area is independent from any theory. One variable in the equation is the monolayer capacity (amount of adsorbate needed to cover the surface with a complete monolayer of molecules, where each molecule of the adsorbate covers a certain area of the surface). Just for this the BET theory is applied. In the original publication of Brunauer, Emmett and Teller the term "specific surface area" even does not appear.
So it is necessary to "growth" a monolayer on your material, as a rule this works nicely if you have a type II or IV standard isotherm. Microporosity does not allow to growth such a monolayer, by this reason Dubinin developed the theory of volume filling of micropores. Materials containing only microporosity show type I isotherms.
So analysis of your isotherm itself gives already a lot of information. A good orientation how good your monolayer is forming you can get also from the value of C in the BET equation (printed in the protocols), which is describing the intensity of adsorbate-surface interaction. It should be in a certain range.
I have some conference publications written more than 20 years ago when I was responsible for the equipment (also micromeritics of that generation). They were written with the aim to explain colleagues these backgrounds for handling the results. I can share them on the private base here.
Best regards
Michael
I would expect DFT results to be more correct in the case you assume a cylindrical model of the pores.
BET should be erroneous in most cases, since the phenomenon of pore filling is not well represented in the BET equation. Can you try to fit the BET in the very low pressure range? It sometimes gives acceptable results for porous samples.
Hi Danny,
From my understanding, if the results are in disagreement, then Fernando is correct. You generally trust the DFT result. BET is based on some wide-ranging assumptions that may not hold true in your case.
However, it's interesting the case you've got. The numbers are suggesting that your carbons are not particularly porous. I believed porous carbons were generally in the range of hundreds of m2/g? For very small pores (as you can see with porous carbons and as you might particularly be seeing with the nanotubes), BET is not a particularly good measure of surface area as it relies upon a pressure range of 0.1 < P/P0 < 0.3. At this pressure range, it is my understanding that very small pores, (< 1 - 2 nm) in size are usually completely filled and as such they do not contribute to the growing multilayer that BET relies on to calculate surface areas. I would recommend reading a paper on the area however, which might help to clarify things.
If you hunt for sections of this paper referring to BET analysis I think you will find some guidance.
Article Phase Separation in Confined Systems
Danny and Jacob,
> BET is not a particularly good measure of surface area as it relies upon a pressure range of 0.1 < P/P0 < 0.3
For microporous materials, the latest IUPAC guidelines recommend using alternative criteria to select the appropriate relative pressure range for BET analysis:
http://sol.rutgers.edu/~aneimark/PDFs/IUPAC_Report_PAC_2015.pdf
rather than fixed values. This tends to be much lower than 0.1 < P/P0 < 0.3 for purely microporous materials (see ref [9] in the IUPAC report).
However, I think that using too high a range should overestimate the BET area; yet the value of 7 m2/g seems too low for a CNT sample (as Jacob suggests) unless, perhaps, it contains very few actual CNTs.
I think this suggests there could be some other problem (e.g. with the measurements) so that it is not just a question of the analysis method (BET vs DFT). An example, although I know very little about the samples or measurement protocol, would be problems with insufficient sample degassing or not allowing enough time for equilibration at each isotherm point...
Best regards,
Darren
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