I've got the following results from a lab, to which I've sent two very different powders.
I believe the isotherms are type III in both of them.
However, every reference that I see shows the hysteresis loop over the adsorption line, but the hysteresis in both materials is under the adsorption line, with the hysteresis in powder2 resembling the shape of the number 8.
Could these hysteresis loops be considered as H3 type? or does the analysis contain errors?
Also, what is the cause for such behavior? Is it related to heating during desorption? (because I know many equipments perform this to speed up the analysis)
This is a type II isotherm for a (practically) non-porous material. It is not type III because the adsorption at low pressures is non-negligible, this is not obvious because your first point is at bit high pressure (around P/P0=0.05).
Regarding the origin of the hysteresis, or if there should be more or not, I think it looks OK. In any case, ResearchGate is full of people having similar question. I recommend you to have a look (or using the here unpopular google):
https://www.researchgate.net/post/Why_does_mesoporous_material_have_hysteresis_loop_in_adsorption-desorption_N2_curve
https://www.researchgate.net/post/How_to_interpret_the_following_N2_adsorption_desorption_isotherms
https://www.researchgate.net/post/BET_hysteresis_loop_analysis2
https://www.researchgate.net/post/Why_is_the_desorption_curve_crossing_the_adsorption_isotherm
Indeed it seems to be type 2 isotherms for the two samples. Regarding the "crossing" of adsorption and desorption isotherms, it is likely due to the fact that the instrument did not collect enough data on the adsorption isotherm. For some reasons, the last point of the adsorption isotherm is at P/P°=0.8. With more data between P/P°=0.8 and P/P°=1, the adsorption isotherm would likely remains below the desorption isotherm. For the second curve, another small problem can be detected which is that the adsorption and desorption isotherm do not overlay for P/P°
From the way your graphs look, I would suggest you increase the degassing time or to some extent degassing temperature if you were the one running the samples especially for the second hysteresis loop.
https://www.researchgate.net/post/Why_does_mesoporous_material_have_hysteresis_loop_in_adsorption-desorption_N2_curve
https://www.researchgate.net/post/Why_is_the_desorption_curve_crossing_the_adsorption_isotherm
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