I just want to know that if the catalyst support contains two or three metals, then how can I determine the number of active sites. Please suggest what is the characterization technique of the catalyst to evaluate that active sites.
Chemisorption techniques, it is not easy because a catalyst can have different types of active sites.
M. Baerns---Actually I am working with hydrotalcite catalyst. The catalyst contains three metal support. I want to calculate the active sites of each metal support. How can I able to evaluate that active sites?Suppose,if my catalysts support contains Fe(Iron), Si(silicon), Ni(Nickel).
I think you should think about "the number of active sites on the surface". This data is more important in heterogenous catalysis. Normally, you can know the number of your active sites by ICP, AAS or XRF. It's not a problem. But to determine metal dispersion, you need to know "the number of active sites on the surface". So, you have to work with chemisorption, for example H2 on Pt, CO on transition metal...
@Sụoy Bepari: you can use CO adsorption (FTIR)
BR,
Assuming the surface has same composition
assuming you use same type of molecules as adsorbates
you must accept the following assumptions:
1) That the surface has different types of adsorption sites
2) The number of active sites is relatively only very small to total sites
Determining number of real active sites exactly is a real challenge that needs to be addressed. Moreover, an active site may become inactive at some time.
In our heterogeneous catalysis studies, we normally calculate turn over number (no. of molecules reacted /total no of sites). The sites here are considered as number of involved atoms.
@P Butolia: EDX or XPS give relative amounts of metals, not absolute amounts. Serious assumptions have to be made to get to any value for wt% loading, and the numbers are subject to wide variations accordingly. DRIFTS is but one of a few different options to determine number of active sites.
I suggest that a selection of characterization techniques (XPS, EDS, SEM, TEM) should be used to define the chemistry and structure of your materials. As to the number of active sites, that depends on your reaction, as others have noted.
The identification and counting of active sites in heterogeneous catalysis became the "Holy Grail" of heterogeneous catalysis in 1925 abd the situation remains the same today :)
its all relative methods and not accurate s to find them a defined by Taylor in 1925 :)
Le Phuc Nguyen---please tell me how to find active sites of the catalyst from FTIR?
you need to do CO-adsorption and monitoring of adsorption by FTIR. Pls have a look at this article http://www.ijvs.com/volume6/edition1/review.pdf
Hope this helps
BR,
I agree with Hikmat S. Hilal and Vijaykumar Marakatti.
For which reaction you want to know the active sites of your catalyst?
> First you should understand the active sites for your reaction, not all the possible adsorption/absorption sites of CO or other molecules?
> TOF is one of wide accepted way to determine the catalyst efficiency
Komateedi N. Rao---I want to perform steam reforming reaction with this catalyst. So, please tell me how to calculate active sites of my catalyst? Please give me some specific idea about this matter.
I refer to some fundamental some the art sources on active sites/centers which you might want to consult:
Handbook of Heterogeneous Catalysis, ds.: Ertl, Knoezinger. Schueth, Weitkamp.
Vol. 1, p.1 to 2, 26, 1561, 1268-1645
There is also information on the methodology of how to determine the sites themselves, their strength, and experimental determination.
I do hope that this is helpful for you.
Manfred Baerns
You need to define the nature of your active site. Are you looking at acid sites or base sites? A quick standard is NH3 for acid sites, and CO2 for base sites. There are other adsorbates of choice of course.
I would start there first. As Soheil mentions, TPD is a great technique to use, though take your pick of inert atmosphere, or UHV. The surface kinetics for both processes differ from each other, so if you perform one, stick with that.
If you saturate a known amount of catalyst (presumably you also know the specific surface area), you perform a TPD and observe the desorption of gas through your detection system.
You also need to perform a calibration curve for the known gases. For NH3, I had used H-ZSM5 zeolite for calibration, adsorbing NH3 on it (since the active sites equal to the number of moles of aluminium).
Once you know how much gas was desorbed from the catalyst, you can calculate the number of moles of acid/base sites per area/mass.
Can anybody suggest any absolute standard material for NH3 TPD analysis? Any material?
Dear Sujoy:
Hi
currently it is well accepted that H2-chemisorption and TPR techniques provide essential information about the catalytic active sites . if you want to measure the active sites only on the surface of the catalyst you should do a special H2-chemisorption. in this technique you need to purge H2 over the sample to reduce all the active catalytic sites then a secondary gas (i.e. NO2) reacting with surface sites should be passed over it. after that the third reducing gas (H2) purged again. through the difference in H2 gas consumption in step 3 rather than step1, you would be able to measure the active catalytic sites on the surface.
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