Here I have attached the isotherm of two samples? I have used BET multipoint for surface area (SA) calculation. For First sample SA is ~4.55 and for 2nd sample ~53.95 m2/g. Which isotherm shall I use for calculating pore size distribution? How I understand which type of pore it is?
Hi,
I can't see the attached files but it seems that at least the first material is mostly non-porous because of the very low specific surface area. Usually the PSD is calculated for mesoporous materials from the desorption branch (from BJH method) but there are no strict rules, the most important being to compare several materials on a similar basis (DFT and BJH methods do not lead to strictly similar values). You can have a look at the papers from Matthias Tommes (from Quantachrome) for general information, some papers are already available from google.
In my opinion, it is useless to indicate very accurate values (54 instead of 53.95 m2/g will be better), since the reproducibility of Sbet measurements is not that accurate.
Thank You Bruno Sir, for your Kind response. Here again I have attached the files..please give a look and if possible give response..
Nasima,
The correct spelling was "Thommes" not "Tommes", sorry for that; you can find a paper on google "Physical adsorption characterization of nanoporous materials" which deeply discusses the ways to characterize the PSD of materials and presents many types of isotherms.
It seems that both of your samples have some mesopority. Sample 2 has type IV isotherm with H2 hysteresis. For sample 1, it is more difficult because the adsorption is very weak and the experimental data may lack of accuracy (the adsorption and desorption branchs are not superimposed at low P/P0 values). You can either apply BJH (on desorption branch) or NLDFT methods to determine the pore size distribution.
Hope it helps
Hi Nasima,
Because Sample 1 has a much lower reported surface area (as compared with Sample 2), most likely you are not running a large enough sample mass for Sample 1. You should check with your instrument provider what is the recommended sample amount to be run on the instrument.
With type H2 hysteresis, the desorption branch has delayed desorption associated with networking effects. The desorption branch is therefore not representative of the complete pore size distribution. The only way to get a complete and accurate pore size distribution in a mesoporous material with Type H2 hysteresis is to use an adsorption branch DFT kernel. If the delayed desorption is due to pore blocking effects (e.g., as occurs in ink-bottle type pores), you can potentially use the desorption branch to obtain information about the neck sizes in the material (but not the pore body behind the neck). If the delayed desorption is instead associated with cavitation, you will have a pore size artifact which is correlated with the thermodynamic state of the adsorbed fluid and not with any particular pore size.
Hi the pore distribution is usually given by the instrument which uses the bjh methods if you start your measurement for a complete adsorption and desorption analysis.
Dear Francisco and Hammou ......thank you both of you for your kind response...It really helpful for me.
The question of whether the material is porous can be easily answered by plotting number of moles versus -ln-ln(P/Po). If one gets a straight line, there is no porosity. If this is the case then the slope of this plot yields the monolayer coverage as calculated by quantum mechanics.
If this plot has a negative curvature, then the standard plot method is probably at the moment the best method to approximate the porosity, or one could use the NLDFT along with the low pressure results of the plot mentioned above for the monlayer coverage. Do not use the BET for the standard plot method - it’s off by a factor of 2-3.
Is there any possibility for hysteresis in the first material (non porous) due to intra-particle porousity ?