Hi I'm carrying out N2 adsorption-desorption for porous sample, but the isotherms I got are open Type IV based on IUPAC rules. They shouldn't be open right?
I have read literature about similar sample, they are all mesoporous. Some answers I found in other similar questions say that there might be some other pores other than mesopore. Is this reliable?
All my porous materials have this phenomenon. I think you can combine your PSD to analyze it.
1) What amount of sample do you use for your measurements ? This could be an artifact coming from the measurement errors due to the use of a too small sample. The usual recommendation for N2 77K measurement is to use a sample with a surface of 20m2 (i.e. 100 mg if your sample as a surface of 200m2/g).
2) Also, if the diffusion of N2 into your sample is very slow, you will underestimate the adsorption curve and over estimate the desorption curve.
Therefore, using a bigger sample and/or increasing the equilibrium time could make this open loop disappears.
Regards
Sébastien Vaesen Thank you for your advice. I'll try one more time depending on your suggestion. Basically, I everytime use about 0.13g or 0.14g (after vacuum) to start desorption. I think your 2nd advice is correct because our equipment will automatically stop at 1000 minutes. Thank you in advance.
Yujing Liu , do you mean 1000 minutes per point ? This is pretty long actually... Did you check the size of your particles and the activation conditions ?
I think that you should stop your Adsorption Branch at a Relative Pressure of about 0.98 and not force the isotherm to reach saturation which will cause condensation of the nitrogen on the glass walls of the sample holder. This then leads to a wider gap between the adsorption and desorption loops during the first few desorption points due to the long equilibration times that would be neccesary. The most obvious reason for this isotherm not closing would be a slow leak.
Sébastien Vaesen Sorry to confuse you. Basically every point would be obtained within 30 minutes. The 1000 is the maximum time can be used for per sample.
Either the equilibration time for the measured points is not set sufficiently or the material undergoes structural changes during adsorption. Could you please provide some more details about the material?
In general, to my understanding this open hysteresis is due to the equipment error (leakage) or an inappropriate sample preparation.
However, I have noticed some researchers to propose that open hysteresis can be a result of the characteristic of the sample as well. I recall some type of activated carbon was given as an example. So for this reason, I am assuming there might be rare(?) cases in which this open hysteresis is possible.
Does someone have more detailed information about this? What kind of materials exactly give this kind of result? Are there any specific features about the sample which might result this kind of sorption behavior?
Please, share your view and perhaps relevant literature as well.
Just off the top of my head the two reasons other than equipment failour for low pressure hysteresis are: in the case of swelling clays the structure changes during adsorption, giving access to surfaces that were not available at the beginning of the isotherm. On outgassing the sample these surfaces are again hidden to the adsorbing gas until higher relative pressures causes them to be made available again. With activated carbons with very narrow micropores the adsorbing gas does not have enough kinetic energy to enter the pores, adsorbing in the pore mouths and requiring higher relative pressures and more gas molecules to push them further down the pores. Longer equilibration times will not solve this. For this reason many researchers use carbon dioxide at 0°C to determine the micropores of carbons since at this much higher temperature the gas molecules have a far greater kinetic energy. I hope that this explanation helps.
Thank you for your sharing your thought. I think your thoughts follow pretty much what I have in my mind what comes to carbon materials. Release of the N2 gas in the desorption may be restricted and curve does not meet the sorption curve due to the microporous nature. CO2 studies indeed seem to be a standard procedure when studying microporous carbon materials. However, if the sample is micro-mesoporous CO2 analysis alone is not satisfactory..
Of cause you are right. Using CO2 at 0°C is only good for the very low part of the isotherm since you are so far above the liquefaction/freezing point of the gas. The curve does not come down to meet the adsorption branch not because restriction of the gas during desorption but due to "Activated entry" during the adsorption that causes an underestimation of the adsorption capacity.
Okay after a year and a half, I think one of the possibilities is the porous structure of my sample is somewhat flexible, e.g. aerogels. As I found a literature online: 10.1016/j.apenergy.2016.11.043. I already measure my sample on two different BET machine, and results are same/similar. So I hope I can be helpful if someone in the future has same problem.
Swelling clays do this. On the Adsorption branch the pores fill and expand but on the desorption branch they do not go back to where they were. On outgassing the same sample a second time you should get a similar isotherm to what you have already found. Your isotherm is a Type IIb, that is similar to a Type II non-porous but with hysteresis due to the presence of mesopores that extend into the macropore range.
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