Hello everyone,
I have 2 BET diagrams for precipitated silica, and tests were held under the N2 atmosphere. There isn't any sign of isotherm in none of them. I don't know how I can explain it.
I would appreciate your help.
I attach these diagrams for you to see.
properties:
sample 1: specific surface area > 600m2/g, porosity = 6nm
sample 2 specific surface area > 200m2/g, porosity = 16nm
By studying the Linearity, BET Slope, Crossover Point, Hysteresis of BET plot, which is a plot of the quantity (P/P₀)/(V(gas)/V(solid)) against P/P₀. Here, V(gas) is the volume of gas adsorbed, and V(solid) is the volume of the solid adsorbent.
The BET is only applied in the linear part of the graph, between approximately 0.1 and 0.3 p/p0. You have very good plots with well evident linear parts and you can obtain certainly good values of surface area and pore distribution
Up to me really good isotherms. In the desorption part is something strange, why is p/p0 higher during desorption?
1. At first, two graphs you represents, should be adsorption-desorption isotherms - dependence of volume adsorbed Va againt repative pressure p/po (not BET-diagrams!!) but they are not right. The first is absolute nonsens, red points representing desorption part of isotherm can not start before ending of adsorption isotherms, it means at relative pressure p/po=01.999 - 1. The second graph is better, but on desorption isotherm only three points are presented, it should be measeres down to relative pressure about 0.40to calculate pore size distribution.
2. BET-isotherm is calculated by linearization of data of adsorption isotherm in range of relative pressure of p/po = 0.05 - 0.30, from obtained slope and intercept values is calculated specific surface area.
3. What kind of equipment was used for adsorption data measuring? Generally, the adsorption isotherms are mesured by gaseous nitrogen at the temperature of liquid nitrogen, and data are automatically treated by BET-isotherm. Try to ask to measure isotherm once again.
Except for the desorption part the isotherms are OK. But they have to be transformed into BET calculations: selecting the region of p/p0 0.05 to 0.32 if You follow the standard BET assumptions. If the sample is highly microporous a smaller p/p0 range must be used. If this is made properly the C-value and y-intercept should both be positive.
Yes, Henrik Romar, I agree with you, that adsorption parts of isotherms are relatively good (only in the first picture up to p/po = about 0.95), and, if the material contains micropores (it is reflected also in negative value of C-constant in standard BET-equation), it is necessary to use t-plot method (or less used alpha-plot method) to separate volume of micropores and surface of mesopores.
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