If the surface area increases after loading your catalyst to the support, with respect to your bare support, it means you're introducing pores or increasing surface roughness to the composite, not plugging pores. At this point it is important to know what your support and catalyst are, and the preparation method. As you mentioned somewhere, whatever this catalyst is, it has micropores. Your support is a mesoporous material. If the porosity of your catalyst is well developed, your catalyst should have higher surface area than your support to begin with. So, it would be expected to observe such increase in specific surface area from the support to the support-catalyst composite.

Now, because I would expect metal catalysts to be nonporous rather than microporous (assuming your catalyst is a metal), I have a very important question to you. When you normalize the amount of gas adsorbed per gram of material, do you use the weight of composite, or the weight of the support alone? The correct should be the total weight (support and metal catalyst). But if you used only the weight of support when normalizing the adsorption isotherm of your composite, then you end up with an artificially higher amount of gas uptake, and consequently higher calculated BET surface area.

Dear Pasquale Fulvio, I want to make more clear my question.. I had Used active metal MoP over AC. The surface area of catalyst, MoP/AC is high than the support, AC. The pore size distribution of AC showed micropores and macro... but after impregnation with MoP the pore size distribution of MoP/AC show some mesopores and very minute micropores with high surface area. I need your help to find a suitable reason of this high surface area.

Regarding your question, by normalizing the BET of MoP/AC we had used the weight of composite.

It really boils down to your synthesis method. Maybe your MoP crystals grow and form some interparticle mesopores, and aren't blocking much of the micropores, since crystals are larger than your micropores. EM should confirm that to you.

Hi Muhammad,

The behaviour you observed is really unusual, as I have always obtained the opposite (see some papers of metal-doping in my RG profile, and refs. therein). Whatever you put on your AC support generally leads to a decreased BET surface area, because what you add blocks some micropores, i.e. those for which the contribution to the surface area is the highest. If you observe the opposite, something different happened. Maybe you metal particles were so small and so numerous that their surface area is higher than that of the support, for which the surface area is not so high (if mesoporous, but how much ?). Or maybe your metal catalysed the oxidation of your AC, thus creating micropores and thereby increasing the surface area.

Alain

Dear Alain Celzard,

Yes I am agreed with you. I also found such behavior first time. after your and Dr paquale explanations, i draw the following explanations. Please add further improvement to it.

The commercial AC (BET=700 m2/g) contain micropores (0.5-2nm) (1 cm3/g) and little Macropores (50-200 nm) while the sample which i have doped (10% over Ac) is from 6-25nm... When I checked the pore size distribution of final sample, there is no macropores and the quantity of micropores was high (3.5 cm3/g) with a little amount of mesopores (2-5nm). This means that the macropores are convert to micropores, developed porosity (mention by Pasquale) while the high number of Micropores assist to increase surface area (750 m2/g) of the final material (as you mentioned).

Difference of surface area between 700 and 750 m2/g is not high and, in many cases, might correspond to the experimental uncertainty. Did you repeat your measurements ?However, given the much higher weight of metal compared to carbon, an absolute increase of surface area should indeed have occurred. What remains strange is the discussion about macropores, that you should not be able to estimate by adsorption, as they are too wide. By the way, a pore can be broadened by oxidation, but I don't see haw it could be narrowed, except by pore filling. Maybe your particles filled the biggest pores, leaving smaller pores between them, hence the overall surface increase ?

I agree that TEM observation would be useful to check the size and the global arrangement of your nanoparticles.

Alain

Dear Muhammad,

If I have understood correctly your surface area increased from 700 to 750 m2/g. This increase might not be significant. I mean, that the inherent error of the measurement (lower than 5%) and in weighing samples or the heterogeneity of your sample could explain partially this difference.

Have you repeated the measurements? You should repeat 2-3 times N2 sorption in different samples (I mean different weights) before extracting any conclusion.

Vanessa

The difference is very small for BET surface area. The reason is that it is very hard to accurately determine the monolayer region for BET calculations (very easy to over estimate it too), especially in high surface area materials.

"This means that the macropores are convert to micropores, developed porosity (mention by Pasquale)" - I didn't write that. I wrote that the catalyst you have could be forming larger particles than your micropores, and the voids between the latter catalyst particles were giving you some mesopores. And again, it really boils down to what material it is, amount of catalyst, and to your synthesis methodology.

Dear Muhammad

I have seen this effect in my research as well. the same as ref. (Amin MH, Tardio J, Bhargava SK. An investigation on the role of promoters in promoted gamma-alumina-supported nickel catalysts for dry reforming of methane.  Chemeca 2013. Brisbane, Australia2013). I am seeking to find good argue for this effect.

Dear Muhammad,

I also have the same effect, I do with ZSM-5, when I promote Zn on ZSM-5. The surface area of Zn/ZSM-5 is greater than that of ZSM-5. Until now, I have no idea to explain this result. If you have ideas for this, please help me?

Generally, BET surface area is inversely proportional to the pore diameter. This is because of the fact that particles with smaller pores diameter display larger specific surface area. However, porosity is another important parameters defining the alteration in BET surface area. In some cases, particles with small pore diameters shows small surface area due to small number of pores per gram.

Dear Mohammad,

I completely concur with Alain's assessment. The behavior observed in catalyst preparation is unconventional and has not been encountered by me before. However, I have read about similar phenomena in a few journal articles. It is likely that this anomaly may have resulted due to the promotion of AC oxidation by metal particles.

The BET surface area depends on slope of your adsorption isotherms within the relative pressure range you chose for fitting. The higher the slope, the higher the surface area is going to be. This may indicate increased strength in the interaction between N2 and your material's surfaces. Also, as you are filling micropores, you may be creating additional mesopores outside of your particles. These effects lead to another problem, which is properly estimating the volume for the gas monolayer (whatever monolayer means for micropore filling). You should check the range used for fitting, and make sure you are avoiding the multilayer region. I would have to look at your isotherms to verify that additional pores are being formed. But a quick PSD calculation, use NLDFT, should help you verify that.