Sample 1: Carbon material (Lignocellulose was subjected to hydrothermal carbonization followed by calcination at 650 oC), Sample 2: Carbon material (sample 1 after KOH activation). I have referred a similar thread on N2 adsorption/desorption isotherms that was still open to discussion (https://www.researchgate.net/post/What_is_the_reason_for_the_open_loop_in_N2_adsorption-desorption_isotherm). Thank you in advance.
Please go through the following link: http://goo.gl/iNJa7E
You need to give a bit more information about your measurement, before we can say something relevant about it, the required information is in the link that was suggested above.
At first sight both your samples are strongly microporous, don't have a lot is meso and macro pores. Sample 2 has a far bigger micropore volume.
The hysteresis at low relative pressure for sample 1 can in theory not result form physisorption. N2 physisorption isotherms of materials with mesopores will have a hysteresis at p/p0 higher than about 0.4, but never much lower than that.
There are 3 obvious mechanisms/processes that result in low pressure hysteresis:
-chemisorption: some of the adsorbate is sorbed to high energy 'sites' and does not come off easily. If there is a lot of chemisorption, it will be visible from the mass of your sample after measurement. Another check is by thermal desorption: increase the T after desorption at low relative pressure, see if and at what T the chemisorbed sorbate is removed. Another test is to run another ad/desorption cycle without outgassing after your first measurement. The second adsorption branch ought to be lower in case a substantial amount of sites is filled by chemisorption
-Sample volume changes: swelling/shrinking of your sample due to interaction with N2 can result in low pressure hysteresis. Rouquerol addresses this issue in detail for carbons in the latest edition of his handbook on adsorption
-Measurement not at equilibrium: this is the most common cause of low pressure hysteresis. The test is simply to increase the equilibration time of the measurement and see whether it affects the hysteresis.
regards,
Pieter
Thanks to both Pieter Bertier and Venkata Chandra Sekhar Palla for suggestions/answers.
Pieter Bertier, I really couldn't find out the points you have mentioned as I got this analysis done outside. My questions are; under which category these isotherms belong to as per IUPAC? are they mixed type of I and IV, comprising micro and mesopores? How to account for opren loop? Following is the information I have
Instrument; NOVA-1000 ver 3.70, Adsorbate; Nitrogen, Adsorption and desorption equilibrium time; 60 sec, Adsorption and desorption tolerance; 0.1000 mm Hg
Bath temperature; 77.4 K, P0; 693.41 mm Hg,
Sample 1: Adsorption and desorption dwell time; 120 sec, weight; 0.0096 g, volume; 0.0593 cc
Sample 2: Adsorption and desorption dwell time; 1200 sec, weight; 0.0330 g, Sample volume; 0.3434 cc
I think they are both type I, but the shape is a bit distorted because of your measurement conditions (and the scale of your adsorbed amount axis for sample 2). Dwell time suggests you did an isotherm scan with fixed equilibration time, and that time was definitely not long enough for the first sample, hence the low pressure hysteresis. That open loop is a measurement artefact. The dwell time was 10 times longer for the second sample?
Dwell time scans are normally used for fast scans to get a first rough estimate, or for quality control, when the rate of adsorption is known beforehand.
In most cases it is better to measure with an equilibration condition, i.e. a criterium for the change of pressure with time. This way the equilibration time is flexible. At low relative pressures equilibration typically takes longer, a criterium allows for that, a constant dwell time doesn't. The tolerance value for QC devices is not for the equilibration condition, it is a tolerance for the relative pressure, i.e. how close each measured point will be to the pressure points you selected.
Pieter
Pieter is correct. Sample 1's low pressure hysteresis is indicative of "under-equilibrated" data, i..e not enough time being allowed for nitrogen to diffuse into/out of a narrow/constricted micropore network. The relatively low uptake of nitrogen could be due to truly there being little micropore volume developed, or the pores being too small to admit nitrogen. In cases like this it is enlightening to analyze the sample using CO2 (at 0degC) since CO2 molecule is somewhat smaller and the temperature of measurement is that much higher, greatly accelerating diffusion.
The KOH activation seems to have done a really nice job of opening up (exfoliating) the pores and developing significant microporosity accessible to nitrogen. The "permanent" hysteresis here looks to be nothing more than experimental uncertainty. Plot the data from 0 on the y-axis and you'll see what I mean. All things considered, it looks like very high quality data!
It would be interesting to re-run sample 1 with the longer "dwell" time as used on sample 2.
Both isotherms are predominantly type I, but with a measurable (reversible) slope above the knee which is fairly typical of carbons (they allow reversible multilayer adsorption which means some "type II" character also). It is zeolites that tend to give type I isotherms with extremely sharp knees and a near horizontal plateau. Sample 2's isotherm does have a hint of mesoporosity in it (which means a touch of type IV) as identified by the barely discernible change in the desorption slope between P/Po = 0.5 and 0.4.
One last suggestion. Rather than have the NOVA measure sample volume I recommend that the operator simply enters the density (2.0 g/cc for sample 1, 2.2g/cc for sample 2).
Thnk you Martin A Thomas for nice comments. I should discuss some of these points with operator.
By the way, any idea on availabilty of facility for analysis using CO2 in Bangalore or India? I did try at few Indian Institutes of Technology (IITs) here in vain.
@Pieter Bertier
Could you provide the latest edition of handbook on adsorption by Rouquerol.
regards
raeesh
Dear Raeesh,
You can order it here:
http://www.amazon.com/Adsorption-Powders-Porous-Solids-Second/dp/0080970354/
if you want a free copy or a signed version, you'll need to ask Rouquerol himself
Pieter
I have a question regarding negative slope for adsorption curve. It would be great if some of the experts ( Pieter Bertier Martin A Thomas) in this conversation could help me. The question is posted in the following link:
https://www.researchgate.net/post/Why_N2_adsorption_isotherm_has_negative_slope?view=5bbc293a979fdca1287e146d
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