Which value should be considered as the actual surface area of thenanomaterials?
Dear Prashanth,
1)BET method use the Brunauer-Emmett-Teller equation to determine the surface area of solid materials:
1/(W.(P0/P)-1)) = 1/(Wm.C) + (C-1)(P/P0)/(Wm.C) [1]
C is a constant related to the energy adsorption indicate the adsorbent/adsorbate interactions. P/Po is relative pressure.
From the equation we can calculate the surface area St.
In case of multipoint BET, we use the slope s and the intercept i from the BET plot (usually requires at least 3 points) to calculate the Wm.
Singlepoint BET is just a simplified case of multipoint BET. For routine measurements, if the value of C is sufficiently large to ignore the value of i, the equation [1] will become Wm = W(1/P/P0).
As a result, the calculation will be faster. Singlepoint BET is also used for determine the appropriate range of P/P0.
2)For your second question, multipoint BET is the better choice. But in most case, the value of C is large enough to warrant the relative error between the two methods less than 2%.
For more details, you can consult this reference:
http://www.google.co.jp/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&sqi=2&ved=0CF0QFjAH&url=http%3A%2F%2Fnanonet.rice.edu%2Fresearch%2FBET%2FBET%2520Seminar%2520Part%25201%2520morning.ppt&ei=g8ToUsqkFK6eiAehsYGYBw&usg=AFQjCNFt7qLvJq1kBkhuKvy82D9_3rPrxg&sig2=uhJgiJckvk3BAFGwU-XxgA
Regards
Dear Prashanth,
1)BET method use the Brunauer-Emmett-Teller equation to determine the surface area of solid materials:
1/(W.(P0/P)-1)) = 1/(Wm.C) + (C-1)(P/P0)/(Wm.C) [1]
C is a constant related to the energy adsorption indicate the adsorbent/adsorbate interactions. P/Po is relative pressure.
From the equation we can calculate the surface area St.
In case of multipoint BET, we use the slope s and the intercept i from the BET plot (usually requires at least 3 points) to calculate the Wm.
Singlepoint BET is just a simplified case of multipoint BET. For routine measurements, if the value of C is sufficiently large to ignore the value of i, the equation [1] will become Wm = W(1/P/P0).
As a result, the calculation will be faster. Singlepoint BET is also used for determine the appropriate range of P/P0.
2)For your second question, multipoint BET is the better choice. But in most case, the value of C is large enough to warrant the relative error between the two methods less than 2%.
For more details, you can consult this reference:
http://www.google.co.jp/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&sqi=2&ved=0CF0QFjAH&url=http%3A%2F%2Fnanonet.rice.edu%2Fresearch%2FBET%2FBET%2520Seminar%2520Part%25201%2520morning.ppt&ei=g8ToUsqkFK6eiAehsYGYBw&usg=AFQjCNFt7qLvJq1kBkhuKvy82D9_3rPrxg&sig2=uhJgiJckvk3BAFGwU-XxgA
Regards
I'd like to add just a couple of things to the more than exhastive Son Nguyen's reply.
1) You have to select a dataset giving a positive value of the constant C
2) Also, if your material are nanoporous (pores having d
So, do we need to determine the porosity before going for BET analysis ?
No, you don't. I just mean that if the BET will show a linear relationship in the ISO range (0.01-0.1) rather than the conventional one if your material is micro or nanoporous,
single point surface area is usually used to determine surface area for quick testing but multipoint test is more reliable than single point , anyhow there is no big difference between both ones.
May I know that any difference occur in surface area between single point and multi point. For a compounds with micro or nano pores, does the difference in SA will be huge or marginal?
there is no huge difference between both ones , but usually multipoint SA is higher than single point which not exceeds 4% difference. so the single point value is very close to the multi point value, but typically a bit lower in value. This derives directly from the assumptions that were used to derive the single point equation, in a production environment that difference proves to be completely negligible.
but I also need to Know why I should determine surface area by adopting the BET multipoint method in the pressure range 0.01-0.1 for nanoporous materials? I 'll try to test a nano material with both ranges to see the differences.
I would like also to add some question here: what is the difference between t-Plot external surface area and BET surface area? I have seen that the 1st one is 1.6 times larger than the second one. Is the first one the external surface area and the second the total surface area? A second question is: in case that you have used the normal relative pressure range, in materials that have some nanoporosity (but the majority is mesoporosity), will it be any error? for instance that the micropore volume is negative or the calculated external surface area is larger than the total surface area?
if huge difference occur between surface area calculated by multipoint method and DFT method. then which one is reliable?
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