I want to know the relation between BJH surface area and BET surface area. because surface area from two parameters are different. And for mesoporous samples which surface area is to be considered? and what about pore radius and pore volume. Also if pore size distribution and pore radius is same.
hi
See the given paper , in which you can answer of your question
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20130000071.pdf
Hi!
BET surface area is more widely accepted (though bias exists). BET surface area is derived from low p/p0 points in the isotherm.
BJH method is based on Kelvin equation to calculate the pore size distribution (mainly for meso pores which is more accurate). The data is derived from p/p0 above around 0.3.
Herein, BET result and BJH result are using different set of data points. It looks that by integrate (BJH) pore size distribution (dA/dsize as unit), for a mesoporous material, the integration will be the total surface area. However, the result is not widely accepted due to its narrow suitability. Because BJH is only suitable for pores in meso-range. And also because everyone is using BET surface area, which becomes a protocal...
In summary, once the isotherm is tested (whole range from p/p0 ~ 0.05 to p/p0 ~ 0.995, both adsorption and desorption braches), the low pressure points give the BET surface area information, and the high pressure points give the BJH pore size distribution information.
For mesoporous as well microporous material surface area at P/Po less than 0.35 only
and for mesoporous materials BJH for pore radius and pore volume, usually surface area calculated using adsorption data
These are the answers to your three Q's
1) What is the relation between the BET surface area and the cumulative surface area computed from the BJH pore distribution profile ?
They need not be the same because
a) the BJH method with N2 as adsorbate does not correctly capture the data in the micro pore region because of enhanced adsorption due to the quadrupole moment of N2. e.g. in carbon molecular sieves or zeolites.
b) The BJH method cannot also distinguish between inkbottle or interconnected pores that could lead to errors in computation.
On the other hand for some samples with pores in the mesopore region there could be a good match.
2) For mesoporous samples which surface area is to be considered ?
The surface area is best determined by the BET method for all samples.. This method applies the BET equation is valid in the p/p0 range 0.05 to 0 .30 to determine the volume of a monolayer inspite of the fact that multilayers will form for measurements at p/p0 upto 0.3. In the case of microporous materials it is preferable to use Ar at liquid argon temperature for better accuracy
3) What about pore radius and pore volume. Also if pore size distribution and pore radius is same
The BJH Pore Size distribution is best determined for pores upto 300 A dia. Beyond this range the Mercurury porosimeter is best suited to complement the pore distribution in the macropore range.
For a good review paper please refer to
Pore size determination in modied micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis
Microporous and Mesoporous Materials 60 (2003) 1–17
*
Corresponding author. Tel.: +31-0-15-278-4371; fax: +31-0-15-278-4452.
E-mail address: [email protected] (J.C. Groen).
1387-1811/03/$ - see front matter . 2003 Elsevier Science Inc. All rights reserved.
doi:10.1016/S1387-1811(03)00339-1
Johan C. Groena, *,Louk A.A. Peera , Javier P erez-Ram iirezb
Applied Catalyst Characterization, DelftChemTech, Delft University of Technology,
Julianalaan 136, 2628BL Delft, The Netherlands
Norsk Hydro, Agri Research Centre, Nitric Acid Technology, P.O. Box 2560, N-3907, Porsgrunn, Norway
Received 11 December 2002; received in revised form 28 March 2003; accepted 28 March 2003
Dear Pratibha,
the BET model assumes monolayer formation followed by multilayer formation . What the method does is to calculate the volume of gase adsorbed within the monolayer. From this we get the numer of molecules adsorbed and with thr cross-section of the adsorptive we can calculate the surface covered by this numer of molecules. Hence, the BET surface area is the "probe acessible" surface area. It is (except for microporous materials) a physically meaningful value and thsu the BET-methode is the IUPAC recommended method for the determination of the specific surface area.
The BJH model assumes the formation of an adsorbate layer based on a statistical thickness function (e.g. deBoer or Hasely). At higher pressure the gas will condens in the pore as discribed by the Kelvin equation. Knowing the Kelvin radius and the thickness of the adsorbed layer will allow to calculate the pore size (with often times large errors) and based on this the pore volume. The pore volume can then bu used to calculate the pore size as
A = 2 V/ r
where A is the surface area, V is the pore volume and r is the radius of the pore.
This shows that the surface area calcualted in this way is a geometric surface area base don many assumption: Cylindircal pores, a theoretical statistical thickness of the adsorbate and capillary condensation based on the Kelvin equation. Therefore, the values obtained in this way have no physical meaning.
I hopre thi shelps,
Best,
Markus