There are the micropores in your sample. As in case of analyzing the micropores, t plot and NLDFT method are used and according to your sample results, there is micropore volume of 0,028 cm3/g which you can mention in your research work. From SBET value from the BET analyzer, only you can analyze the surface area, not the micopore or mesopore.

Check your plot, if at low p/po you have a "knee" this is presence of micropores.

Applying the t-plot method you can separate and measure the microporous volume and the external specific surface ares

Your sample gives rise to a type I isotherm with wide micropores. Use relative pressure values between 0.02 to 0.12 for the BET analysis. This Surface Area is only a Finger Print for your sample due to the microporous nature. This is the reason why the measurement is denominated SBET (where the S means specific): The filling of micropores at low relative pressure is a Volume effect and not a surface area effect.

Best regards.

Thank you, M. Yates for your response. I have tried the recommended relative pressure range (in my isotherm 0.01 - 0.15) and the surface area was 676 m2/g, and the linearity was worse then in the above evaluation (0.999425).

It is possible to say, that when I measure only part of the whole isothem (in the range of BET, i.e. 0.05-0.30 p/p0) and find low and positive C constant, and good linearity, micropores can still be present in the sample? It is necessary to measure the whole isotherm, to be sure, that micropores are not present?

If possible t is always worth measuring the whole adsorption/desorption isotherm since then you can also get information about the mesoporosity of the sample and know more about the sample. I have analsed a series of titanias made by the sol-gel method whose surface areas and isotherm shapes ran from 100 m2/g with a classical rectangular Type I isotherm up to a few hundred m2/g with isotherms that looked like yours, so microporous but with much wider micropores. I suggest that you use Rouquerol´s method to determine the highest relative pressure to use for the linear portion of your BET plot. If the maximum is less than bout 0.15 p/p° that indicates micropores. Next, with the t-plot analysis take care where you take the linear portion. For non-microporous materials the linear portion starts at a thickness of 0.354 (the thickness of a monolayer of nitrogen) and extends up to when there is an upswing in the measured points. This could be at a thickness of about 1. If your sample has narrow mesopores the upswing starts sooner and so the linear region can be quite short. For microporous materials the linear region may start at about 0.8 and extend to 2.5. The important point is that after the linear region the remaining points lie above the line. So if you have a material that you think is not microporous and you choose a t-plot with points from 0.354 to 0.6 and get good linearity that passes through the origin this only means that there are no micropores if the subsequent points lie above your regression line. If they fall below then change your range of calculation to larger values of t since your sample is microporous.

Sorry for the long answer but I hope that this helps to clear up your doubts.

Thank you for the time, you have spent by this answer. I really appreciate all the infomation contained in it, especially the information about t-plot method usage!

By the way, the Rouquerol´s method (Micropore BET assitent by Quantachrome) recommends the range of 0.009 - 0.25 p/p0 for my sample for BET determination.

Thanks a lot once more.