Relative surface area goes higher with more porous structure. More porous structure involves more micropores, and not necessarilly macropores.

Take for instance, a particle having only one large macropore with no smaller pores, the relative surface are will be too small despite the fact that the size of the pore is too large.

Conversely, if the particle has many small pores, the surface area will go high even if the total pore size volume is relatively small.

 Much of the relative surface area is due to the presence of high number of smaller pores.

As Hikmat said, it is because you can put a lot of area in a small volume of micropores.

Yes, the result is quite normal, the difference between micropore and mesopore is pore size, not pore volume.

BR,

Thank you all for the answers. They were very helpful.

Do you have any journal paper/book recommendation which supports this idea? 

So, how should I classified my sample microporous (because of high microporos area, Type I isotherm) or mesoporous (high mesoporous volume)?

Any general surface science/solid state book explains variations of surface area.

To characterize the surface: the mesopores and their sizes can be characterized directly by micrography and by mercury porosimetry under pressure  (which is not advisable).

Smaller pores can and relative surface area can be characterized by adsorption (N2 and/or acetic acid). Procedure is available in many sources.

Melike, please notice that reports of automatic instruments are sometimes misleading. If your isotherm is really Type 1 there are no mesopores.

Sorry Melike

I forget the second answer to your question

Mesopores 2-50 nm in diamater

Micropores less than 2 nm pores

Hikmat, what I meant to ask is how do you classify a material with high microporous area and high mesoporos volume? Is it microporous, mesoporous or both?

Thanks,

Please send me the isotherm and I will give you the answer. [email protected] 

Although there are six isotherm shapes those are ideal samples. In real life samples can have a mix of pore types. So if your isotherm is rectangular and looks like the classic Langmuir shape it is Type I. The micropore volume and the External Surface Area are calculated from a t-plot analysis. However, if your sample has wide micropores be careful where you choose to  take the linear region in this calculation. Also with your Adsorption apparatus the mesopore volume is calculated at a point that you choose. Sometimes people use the penultimate point on the adsorption branch but with isotherms that go up nearly vertically at the end a slight difference in relative pressure leads to a great difference in pore volume. Personally I take the point at a relative pressure of 0.96 on the desorption branch as being equivalent to the filling of all pores of less than 50 nm (micro 0-2 and meso 2-50) then subtract the micropore volume calculated from the t-plot to give me the volume of the mesopores.

One last point is that the filling of the micropores is a Volume Filling Effect and so micropore area is a misnomer. You have the total surface area from the BET equation in relative pressures from 0.02-0.12 for microporous materials. The External Area i.e. the area not involved with the filling of micropores from the slope of the t-plot and the micropore volume from the intersection of that line with the y axis.

A rather long explanation that I hope settles your doubts.

For activated carbons, pore volumes ranging between 0.5 and 1.5 m3/g  are associated with high surface areas ranging from 850 m2/g  to 1400 m2/g  or even more. However this cannot be said there pore size which may be as low as 21 (Å)  and as high as 36 (Å)  

H3PO4 is usually preferred as a chemical activation agent because it bubbles through the carbon matrix and gives a well-developed microporosity leading to a high surface area after opening up the cellulose structure

Humphrey

The BET equation usually gives a good representation of the frequently appearing Type II and IV isotherms within the range of relative pressures 0.05-0.3, and this range is generally used in practice for measurement of the surface area. At higher relative pressures, the BET equation is usually inaccurate because of capillary condensation effect, while at P/Po values below about 0.05, the amount of adsorbed gas is too small to be measured with sufficient accuracy. A poorer description is obtained for the type I, III and V isotherms, but in practice, they are often analyzed by the BET method. In order to calculate the surface area, it is necessary to know the mean cross-sectional area Am occupied by one molecule of adsorbate gas.

Humphrey

Humphrey, the BET equation can not be applied to Type III and Type V isotherms because the c value is too low. For Type I isotherms the BET equation leads to a straight line with a positive intercept and usually a high c value due to the strong interaction between the adsorbate and the adsorbate. In these cases the linear part of the BET equation is located between relative pressures of 0.02 and 0.12 or 0.15. The narrower range being an indication of narrow micropores, the slightly wider range indicating wider micropore although the shape of the Type I isotherm would also indicate this difference.

Yates

Thank you