I want to do BET analysis for samples such as agricultural biomass derived char (biochar) and also alkali (KOH) activated Bio chars. how long must be the degassing time for both bio char and activated biochar. And also, what must be the preferred degassing temperature?
it can be at 500 ºC for 8 hours, but the temperature depends on which the biochar was formed.
Test it experimentally, say 200 C at 2-5 µm Hg for 4, 8 and 16 hours. The weight should go to a constant value. Look up for yellowish stuff in the tubes, most likely tars!
Hi Uddipta Nath,
Dry samples in a oven before to remove access of adsorbed water (or other solvents). For the degas step you can use 110 oC for 2 hours. There is no need for prolongated hours. Activated carbons are not difficult to degas. During analysis, try to perform free space measurements after the nitrogen adsorption measurements because micropores can retain helium (slow desorption) which can affect analysis (s-shape curve at low P/Po).
Nice.. To. Hear, I am also facing same issues. Evacuation timeout error I am getting. As its taking a longer time for evacuation. Hope I can follow wat Alexandre have suggested. Biochars are really cumbersome to handle. Hope I can heat it in oven at.. 250 degrees to remove moisture and other solvents for 1 hour and than degas externally for 2 to 3 hrs at 350 to 400 degrees and than analyze. Hope I can follow this henrik?
The only way to ensure proper degassing procedures is to increase degassing time and/or temperature. The suggeston of using 100 C for 2 hours is up to me too short. We use 130 C for 3 hours BUT our carbons have been at 800 C and they are measured as soon as possible after that.
Thanks.. Professor Henrik, even I am doing the same, my biochars are prepared at temperature 800 degrees only and analysied as soon as possible. I guess temperature 250 for 1 hour in oven or muffle Furnace, will help as it can also remove the bounded moisture if any after Charing. Would you prefer external degassing at 350 degrees Celsius or vaccum degassing at 350 degrees for 2 to 3 hrs after heating it inMuffle furnace?. Clarify me this so dat I can try this and check if I can analyse the sample.
Vaccum degadssing is always better.
I suggest that You make a series using vacuum, 150 degrees and test 1, 2, 4 and 8 hours degassing. Measure the SSA, pore volume and pore size distribution for each sample. Also measure the weight of the samples before and after regarding.
Dear all, the degassing procedure - temperature and time - depends on two factors: thermal stability of sample, and on its porous structure. If the char was prepered at 800 °C, it is stable enought for degasing temperature e.g. 350 °C. From mesoporous and macroporous-nonporous solids the humidity could be removed at relatively lower temperatures durin shorter time. In the case of char it is possible the presence of micropores, and for removing the humidity from micropores it is suitable higher temperature than 150 °C - I use for all micropores-containing solids temperature 350 °C and degasing overnight. There is one more important factor during degasing - level of reached vacuum. I use oil rotary vacuum pump, and after overnight degassing I reach vacuum about 2-3 Pa, that near to possible maximum by oil vacuum pump, and such vacuum is necessary to remove all adsorbed species from micropores. So I recommend for char prepared at 800 °C degassing temperature about 350 °C and time sufficient to reach maximum vacuum possible.
Whenever 2 BET runners meet they present at least 3 procedures that are absolutely right!
Dear all thank you for your valuable suggestions. My samples are prepared at 300 and 350 degree Celsius and activated with cold alkali process. (i.e. KOH activation). As everyone has different views and judgements. I would like to know which degassing time and temperature will suit my samples? Thank you.
Henrik Romar Shivakumar R Pavol Hudec Alexandre A. S. Gonçalves Trino Romero
Hi.. Uddipta... I guess u can go for temperature 250 degree Celsius according Prof. Pavol Hudec, as ur samples are prepared 300 to 350. Degas it under vaccum for 8 hrs. Set evacuation time double than normal analysis if u have faced issues earlier. Main thing is helium trapping in mesopores and micropores,so free space analysis should be carried after nitrogen adsorption
Degassing of samples is of extreme importance in achieving reproducible results. I had some mesoporous alumina catalysts that were giving strange results after degassing overnight under vacuum at 140°C, which should have been sufficient for moisture removal. Since these materials had been prepared with a heat treatment step of 500°C, I chose to outgas at 300°C. While outgassing I noticed the dramatic loss in vacuum when heating from 140°C to 200°C due to the loss of moisture that was strongly adsorbed on the active centres of the catalyst. So a low temperature outgassing and a good final vacuum did not mean a well outgassed sample. After these results I always use an outgassing regime of 300°C overnight on samples that have been previously heat treated above this temperature.
For bio-char like materials, higher degas time is highly recommendable in order to achieve vacuum in sample, You may try out 150 C for 12 h condition to do degassing
Hii Uddipta,
You can degas the samples at 200 deg. Celcius for 8 hours in order to get good results. Degassing at 200 deg. Celcius for 2 h may give you less value as compared to 8 hr degassing.
All the best.
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