N2-sorption isotherms must close at P/P°=0.42. If they do not it indicates that the measurements are not of good quality. It is usually due to the fact that the amount of sample in the cell was too small(one usually advises to put the equivalent of 5-20 m2 in the cell). Another (related) problem would be a poor evaluation of the dead volume of the sample cell (and of its evolution during measurement)

Thanks Juliette.

Once more, does it relate with the setting of degassing process from the measurement? Could you give me any suggestions about these artifact.

Because my sample is the hydrochar from biomass actually, which contain a lot of oil content.

degassing of the sample during measurement could also lead to this kind of shape. However, if, indeed, this shape is due to oil in your sample, this means that you contaminated the whole manifold with oil. Another possibility is a leak on the cell (or on the nut that connect the cell to the manifold). What amount of sample did you use for your measurement ?

My sample mass was 0.1448 g with the degassing temp. at 200oC. I attach the measurement settings below.

what is the value of the BET surface area you measured for this sample: if your surface area is below 30 m2.g-1 (which is likely considering your N2 uptake), the amount of sample was too low

That's right, I got very low BET surface area of 6.4 m2.g-1 with the pore volume of 0.037 cm3.g-1. What should I do next? Could you please give me any suggestion for it?

You should attempt a second measurement with about 1g of sample. If you don't have enough sample, you could consider using Kr instead of N2 as adsorbant. You could also attempt to decrease the overall duration of your experiment (you don't need so many experimental points to evaluate surface area and porosity, they will simply make your measurement longer and increase error on the evaluation of the dead volume (dead volume will vary during measurement and may not be perfectly corrected). You should also pay attention to the insulation jacket (use standard cell and jacket)

Also, another consideration would be check the equilibrium time. Maybe it is not enouhg

Open hysteresis is usually an indication of poor equilibrium as it has been pointed out on several answers to your query. Some other times it could be due to experimental problems such as leaks in the system. However, some times this can be due to the sample itself. If the sample has too small pores, nitrogen will have diffusion limitations in those pores leading to poor equilibrium in automatic systems. it is true that this limitation could be overcome by larger equilibrium times but as a matter of fact I find that certain types of sample never reach equilibrium in reasonable times. This is the case of biomass samples activated by certain hydrothermal methods.

I would like to inform you If the BET If smaal that not meaning the catalyst is bad as a Nano carbon.