Hi Alireza

First BET surface area is not an absolute value. In fact studies show that the BET method gives values with over 30% error. Still it is the most contemporary standard method. Everybody reports BET so it gives you handle to compare data. 

Now here's the explanation

1- BET is measured using adsorption isotherm. In the BET model it is assumed the only single sites with homogeneous energy level are available.  

2- Metal dopant is too small it does not occupy much of the a room so it might not be the reason for change in surface area "if it is not clogging the pores".

3- Metal dopant adds inhomogeneity to the surface, so using BET model which is for single site adsorption "might" give you some deviation.

4- Finally and the most important, it depends how to add dopants though what chemistry and what solvents etc. Because metal dopants can clog the pores and make some pores inaccessible to the probe molecule (N2). This can significantly alter the measured surface area and pore volume.

Ali

Hi

The variation of BET surface in your case could be related to the size of metal-dopant. At low concentration of dopant, the partical size of metal could be smaller than in case at high concentration of dopant. Then, the BET area could be varied in several ways with different metal loadings

BR, 

Hi Alireza

First BET surface area is not an absolute value. In fact studies show that the BET method gives values with over 30% error. Still it is the most contemporary standard method. Everybody reports BET so it gives you handle to compare data. 

Now here's the explanation

1- BET is measured using adsorption isotherm. In the BET model it is assumed the only single sites with homogeneous energy level are available.  

2- Metal dopant is too small it does not occupy much of the a room so it might not be the reason for change in surface area "if it is not clogging the pores".

3- Metal dopant adds inhomogeneity to the surface, so using BET model which is for single site adsorption "might" give you some deviation.

4- Finally and the most important, it depends how to add dopants though what chemistry and what solvents etc. Because metal dopants can clog the pores and make some pores inaccessible to the probe molecule (N2). This can significantly alter the measured surface area and pore volume.

Ali

with increase the metal dopants  BET surface area decreases because pore blockage occur. Also total pore volume decreases because probe gas will not adsorb inside the pores

You could have pore blocking or the formation of a second low surface phase (there is a limit in dopant concentration). The BET surface is the result of it

If more dopants are added, assuming dopants have different atomic sizes than mother crystal,

then the crystal will involve more imperfection.

Such imperfections cause more surface roughness and disorder (Line defects, dislocations, may cause surface texture.

This may increase the specific surface area.