I have seen the same type but with samples having low BET areas. Ad You cannot affect the measurements there must be some problems with the pretreatment/degassing of the samples. Try to double the degassing time or increase the temperature during degassing.

Are you trying to make a chemical activation of the samples as indicated by the addition of Ni and Zn. What was the temperature used for activations? Did  You  was the product from the activation in order to remove all activation metals? From the results is seems that 1) You did not get any increase in pores 2) You created pores but they are clogged or 3) You created a lot of micropores that are really hard to evacuate before the BET measurement. What instrument was used and what was the degassing protocol used

I'm trying to synthesised a catalyst by using biochar as my catalyst-support, followed by impregnating with metals on it.

300oC is the temperature I used to calcine my catalyst (I suspect it was too low for calcination, but I have adapted this calcination temperature from a high impact publication).

Micrometric ASAP 2020 is the instrument I used to determine BET. I'm not sure about the degassing protocol.

In fact, my samples did gave me low BET surface area (less than 1m2/g). By looking at the isotherms, it seems like my sample violating some nature law (I supposed), higher pressure gives lesser volume adsorbed which it should be the other way round (higher pressure, higher volume adsorbed).

From my XRD analysis, it was clearly shows that my sample was in amorphous structure. Does this property having any effects to my isotherms? 

To degass carbons we use some 140 C for 2-3 hours

Thank you for the precious advice.

Try to treat the Ni/Zn treated char at 500-600C in an inert atmosphere (nitrogen), wash the carbon after therma tretment with several portions of distilled water. Dry and measure BET after proper degassing. 

Once again, thank you so much for the precious suggestions =) 

The shape of this isoterm can be caused by Free Space determination error or dewar overfilling.

Have you cheked the values of the Warm and Cold Free Space?

The values of the warm and cold free space were 26.8749cm3 and 76.5088cm3 respectively. 

May I know why the free space would have effects on the isotherms? Thanks. =)

If your instrument is a Micromeritics ASAP or Tristar the Cold/Warm ratio should be arond 3. In your case is 2,85 that could indicate He contamination (Leaks or trapped air in the He line, use of flexible plastic instead metal for He lines...)

Did you fill the dewar with liquid nitrogen do you use the tool to avoid overfilling? (This could also cause wrong FS determination, which affect more to the result a P/Po>0,4 )

All the volumes adsorbed at any P/Po are calculated using FS. Basically when your instruments doses a certain amount of adorbate to the manifold, it calculates the number of moles in the manifold (n gas= P1*Vmanifold/R*T). Then it opens the valve (V9 for ASAP) so part of this gas enters the sample tube. Some of the gas molecules will adsorb onto the sample surface, but the rest will remain in the free space of the sample tube. So if you determine P2 in the manifold once the equilibrium has been reached you can determine how many moles of gas remained in the manifold. Therefore you can determine how many moles of gas have been dosed to the sample tube (n dosed=n1-n2). To calculate the number of moles adsorbed you have to substract the moles in the free space to the moles dosed (n ads = n dosed - n FS).

Can the previous comment really explain the negative values obtained at some points?

 Up to me a BET measurement needs as a minimum about 20 m2to give proper results. If you have less than 1 m2 there should be at least 20 grams of sample in the tube and it does not fit in. If degassing is properly done the conclusion is that you have too small surface area in the sample. If the instrument gives good results before and after this sample the problems are in the sample only.

To Dr. David Otero, thank you for your detailed technical explanation. Unfortunately, I'm not granted to run the analysis on my own. Perhaps I can bring your suggestion to the operator that run analysis in my institution. Thank you so much.

To Dr. Henrik Romar, I guess the major problem would be came from my sample. Anyway, I will make a final conclusion and search for any other possible reason for that small BET surface area, IF I still got low BET surface area (