Hi,

Generally you should degas your samples for at least 3 h. IUPAC recommends 16 h time for degassing. You can also find exact time and temperature in Pharmacopoeia or books of excipients. You should degas your samples at 350 degrees if it does not cause any change in the sample. Also for deciding degas temperature, you can perform TGA and then keep degassing temperature below decomposition temperature. 

I hope my answer will clear your doubts.

Degassing time can definitely affect your data! In addition to the above limits, degas time needed will depend vastly on your sample composition. For crystalline, non-hygroscopic, non-porous powders (such as mica), it's rather rapid; for a bulky, porous piece of rubber, forget it! (you'd be better off with mercury intrusion as a porosity measurement there, and compute the surface area). Suggest: perform some ranging experiments on expendable samples, and look for the kinetics of surface clearance; then check occasionally that your samples' porosity doesn't greatly change from that in your "validation" runs (samples can't desorb rapidly through minute pores!). Hope this gives you a plan of attack. Remember also that you need a certain amount of uptake to provide precision, as well as the prep conditions to assure accuracy.

Thanks for your Reply. My sample is mesoporous silica and TEM also confirmed that. But BET surface area was less than what is normally reported in literature. I did only 2 hr degassing. Could it may be one reason for that???

Can't tell, but you should repetitively re-degas and retest the *same* sample. If your values keep climbing, you haven't pulled all the moisture off yet. When the BET surface area test values level off, then you know that your sample was finished degasing. My guess is that 2 hrs at 500C is more than adequate (silica gel in drying packs is usually regenerated at 200C, if I recall).

Together with the degassing time also the temperature could be a critical factor. When I measured mesoporous silica I used about 200°C overnight. But unfortunately there are only suggested values, if you have sufficient sample, you can try to repeat the measurement with different degassing conditions, I think that this is the only way.

Thanks Stanton de Riel and Marcella Bini. 

I did ~2 hrs degassing at about 150 oC.  Could it be sufficient?  

for mesoporous silica you should be aware of dehydration processes at high temperature. When you degas at temperature around 500°C, there will be a change in the surface and structure of the mesoporous sample. For mesoporous metal oxide their should be 3h at 500°C sufficient enough for degassing. 

It is actually optimization procedure and also depends on composition as well as method of preparation (synthesis temperature, calcination temperature, drying temperature perticularly for EISA sample). For porous materials, I think 4h is enough while 16 h is recomendated by IUPAC.

Degassing temperature should be selected below the decomposition temperature of materials to prevent unwanted error.

Best of luck.....

my material is organic so which time and temperature i will take for degassing for bet Characterization.

Dipak,

For organic materials the above discussion is largely inapplicable. Organics are inherently thermally unstable, so you might first decompose a bit of it by TGA, to find out where (and ideally, how) it decomposes, then degas at somewhat cooler temperature than the onset of the first decomposition step. Now, if you intend to decompose it in order to make a mesoporous catalyst (for example), then you would need to BET it both before and after that process!

respected Stanton de Riel sir, thank you for your valuable time and suggestions.

Depends on the material under investigation. You want to make sure you remove all residual moisture from pores to have an accurate BET surface area. So going with higher temperature with longer degassing time is safer as long as your material is not damaged/change structure under those conditions.

An empirical relation given by Orr and dallevalle suggests the safe temperature limit for ordinary degassing of solids at pressure lower than 5*10-6 mm Hg can be given as,

Θ = 14.4*104t-1.77 [so, if t=300C, Θ is appx. 5.9hrs]

Where θ is in hours and t is in °C (applicable between 100°C and 400°C). However, this is just a generalized formula.

[Ref. Particle Size Measurement: Volume 2: Surface Area and Pore Size Determination. Section 2.18.3, Page 79]