Its true that surface area and pore volume decreases with active species loading but I dont have any clue about pore diameter variation.
The suface area of active species also influence the pore volume. This could be proved with sem results before and after loading. Or in contrast, the 3D of pore and 3D of the active species in consideration will refelect the concept that the total area of catalyst and loaded species will increase.
We had measured BET surface are and pore volume by N2 adsorption method. It is found that surface area and pore volume decreases with increase in active species loading. While average pore diameter follows the reverse trend. Wish to know why is it so?
This is just an idea: maybe with your loading you're "selectively closing" the smaller pores. In this way, while you lost pore volume and surface area, your average pore diameter increases, because the only pore remaining for phisisorption are the larger ones.
Sorry for my bad english - hope I've been clear
We have loaded active species from 10 to 50 wt% on support. I agree "with selectively closing of smaller pores" concept for increase in pore diameter (avg.). But how to quantify it?
Just be careful with the interpretation here. The BET-surface area is given as m2 per gram of sample (and similarly the pore volume). Let's do a thought experiment and take silica (SBET = 250 m2/g; Vpore = 2 cm3/g) and Co3O4 (average crystallite size 10 nm) and make 10 wt.-% Co3O4/SiO2 by mixing these materials. The surface area of Co3O4 is 82 m2/g (SBET = 6000/rhoCo3O4(g/cm3)/dCo3O4(nm)). In order to obtain a 'loading' of 10 wt.-% of Co3O4 we need to mix 0.9 g of silica (corresponding to 225 m2) and 0.1 g of Co3O4 (corresponding to 8 m2). The BET-surface area of this 'new' material is thus 233 m2/g. THis change is caused by the normalization of the area to per unit mass of sample. A similar phenomenon may occur when looking at the pore volume.
Its also can possible that, after loading you might calcined the catalysts. This calcination also drop your BETSA, pore vol. and increase the pore diameter.
It's not always true that "surface area and pore volume decreases with active species loading". If your active component is like V2O5, there will be increase in BETSA
I think that I know the answer. Please have a look at my attached file. it's probably a good fit for your question.
at first, you have to make sure that your active species are well deposited on the support. if not, you can have the pore system of your active species separated from the support. You can verify with the pore size distribution results. In my example, I have Mn as active phase supported on alumina. At 30% Mn loading, it show that Mn can exist as bulk material separated from Alumina, and give pores around 400-500A°. Then, it made a higher value of average pore size.
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