Procedure to determine the number of active sites in a Heterogeneous Catalyst?
Regarding heterogeneous catalysis: Calculating no. of active sites is not easy, because surface sites are not the same, and normally speaking not all surface sites are active.
Moreover, relative number of surface sites depends on relative surface area, which varies with particle site.
However, one acceptable measure for TN would be:
- number of reacted moles (or produced moles)/gross number of moles catalyst
- no of reacted moles (or produced moles)/no of specific element moles in the catalyst
Therefore, TN value will increase with relative surface area, which should be specified in your writing up.
Wishes
For heterogeneous catalytic system, it is quite difficult to calculate TON accurately. TON calculation is actually based on various surface properties of catalyst and those are called as catalyst active sites (surface area, acid amount, base amount, acid or base strength, metal loading etc.). This means in heterogeneous system, there might be more than one factor which can be considered as catalyst active sites. So in my opinion, we should understand actual catalyst active site which is playing a major role in a particular reaction and then only we can calculate TON.
Yes! Both are well written...Now (for TOF) you have to measure the active sites (relatively) by using TPD (Adsorption/Desorption) then calculate how many moles of reactant was converted per active site in given time.
Note: Catalyst Researchers always think to "How to increase the no. of active site?" But mostly unknowingly they improve the TOF of the active site.
Turnover number and turnover frequency are two different values. See the thread here for further information.
https://www.researchgate.net/post/How_to_calculate_the_turnover_number_of_a_catalyst
TOF and TON are mainly used for homogeneous catalyst. For heterogeneous systems are more industrially relevant parameters like catalyst activity (mol product / hour and kg catalyst) and catalyst stability preferred.
TON and TOF are sometimes used for comparison with homogenous systems. In this case a measurment of active sites is needed and generally performed by absorption/desorption techniques like H2/CO/NH3 TPD or similar. The choice should be done case by case, depending on catalyst and wanted comparison.
Also single cristal measurments are performed on model system. This Physiscal Chemistry niche is the bridge between well defined homogeneous systems and more statistical heterogeneous ones and TOn and TOF can have a sense there.
ToF is also used in bench-scale, fundamental studies of heterogeneous catalysis, not just homogeneous or single crystal systems. The ToF is particularly important as first-order benchmark to compare one catalyst system to another after all reactor-dependent mass transport parameters are removed from rate measurements.
Thank you all for your suggestions........If i am not wrong one should determine the active sites before calculating TON or TOF.
For ToF, you certainly need to know the specific number density of active sites on your catalyst in one way or another, for example as number per unit catalyst area or number per unit catalyst mass.
TOF is TN per unit time. TOF = TN/(Time) this is not the issue., keeping in mind TNF is more describing (about catalyst efficiency) than TN
Now, the issue is how can TN (or TNF) be calculated from measured active sites? The measured active sites (even for same material) is relative term depending on condition of the solid catalysts (such as relative surface area, surface texture, cracks, .......).
TOF works for homogeneous and heterogeneous, and is a good criterion for catalyst efficiency. It has been used in photocatalytic research as well.
The safest way to report then will be: TN = moles reacted/moles catalyst a showing gross TN (or TNF) value and defining the term inside the report.
In case of metal oxide or chalcogenide catalysts, we may also use: TN = moles reacted /moles metal
In such a case, we get different values for different catalyst systems, as follows:
- With higher Relative Surface Area --> we get higher TN (because it is relative)
- with more relative active sites --> we get higher TN (since it is relative)
- With less poisoned sites --> we get higher TN (since it is relative)
I have done this way of reporting many times (in homogeneous, heterogeneous and photocatalytic publications), and reviewers never rejected that.
Despite accuracy, it is straight forward, non-demanding and accurate way.
Wishes
Actualy I have a supported catalyst.......I mean to say that....some metal is supported over clay........and I have used it in a reaction as a catalyst.......so will you please suggest me how I can calculate the TON or TOF!
What have you measured in your reaction? What have you measured for your catalyst?
Suppose you have specific rate as moles reacted per time per total mass of catalyst (clay + metal) - r, mass of metal loaded per total mass of catalyst (the wt fraction loading) - w_m, and the molar mass of your metal M_m. ToF would roughly be ...
ToF = r * M_m / w_m
... in units of molecules reacted per unit time per atom of metal (as you can confirm). This applies strictly for a uniform monolayer of the metal on the clay. Otherwise, it has to be adjusted by the dispersion of your metal.
Always it is very difficult to count the number the active sites in heterogeneous catalyst system because it is tell exactly which is the active site during reaction. An example is that many scientists believe that defect sites are more active in case of metal cluster catalysis.
Hello Jeffrey Weimer
Why should you involve the clay support in TON calculation???
TNO should equal = No of reacted moles/No of moles catalyst itself
The catalyst which is supported on the clay, be it metal metal oxide or metal chalcogenide. This applies to any system without molecular monolayers or supported particles, as I showed earlier.
The support has nothing to do with, unless if you yourself wish to involve for technical reasons. But for efficiency and activity study purposes, no need to involve it.
Just take the easy direct route unless you need to do otherwise.
HIkmat
@H Hilal: ToF depends on active sites. ToN may or may not. When the clay is inert, ToF cannot include it.
I can suggest to read CATTECH 7,196,2003. I can send you reprint if you send your email address to [email protected]
TON and TOF are per mole of catalyst, not per mole of active site. In both homogeneous and heterogeneous catalysis, some of the metal is inactive, but you still have to pay to buy it, and it still counts in the calculation. Don't use the number of active sites, that is both subjective and of little value. In homogeneous catalysis, scientists don't say "well, only 10% of my metal is active, so I'll only count those moles of catalyst when I calculate TON". That would artificially inflate the TON and TOF values. The same standard should apply to heterogeneous catalysis.
Certainly, to be precise, the entire units must be given. I find in most cases they are not. The ToN or ToF groups below could each be entirely different numerical values calculated from the same catalyst for the same simple reaction aX -> bY.
ToN - mols X consumed per mol of total catalyst
ToN - mols X consumed per mol of active catalyst
ToN - mols Y produced per mol of total catalyst
ToN - mols Y produced per mol of active catalyst
ToF - molecs X consumed per total atom/site on catalyst per time
ToF - molecs X consumed per active atom/site oncatalyst per time
ToF - molecs Y produced per total atom/site on catalyst per time
ToF - molecs Y produced per active atom/site on catalyst per time
Stoichiometry a b relates values between X Y. Fractional amount of catalyst that is active relates values between total active.
In heterogeneous catalysis, some folks indeed do care about, go to great lengths to verify, and subsequently do report ToF values based on the fractional amount of catalyst that is active.
Other than what Philip Jessop said, which I agree with, I'd like to emphasize that there is a big gap about the utility of TON and TOF in academia and industry. While TON can be used for the estimation of the longevity of a catalyst system, TOF says nothing about the real kinetics of a reaction. It can only be used for a very rough comparison of a catalytic activity in the same (or a very similar reaction) under very similar reaction conditions only at low conversions. (TOF depends on the rate order !). It is funny to see TOF values given above 90 or even at 100 % conversion! I would not suggest that the rate order and rate constants should be determined for each and every time, but showing the reaction profile (conversion versus reaction time, 3,4 values) at given reaction conditions (not a lot of work) could give more practical information about the rate and usability of the reaction than a vague TOF.
It can be calculated by dividing No of moles of the product/No of moles of catalyst used. But I am not sure when you are using solid supported catalysts with active sites and also some acidic/basic support. In that case it may be differ.
Hi
Can you please find the attached image that shows the equation I used during the calculation of TOF
Have a very good day and best of luck
Turnover number (TON) is defined as number of substrate/aimed product per unit of active site during the whole experiments. The number of catalytic active sites is usually measured by chemisorption. If the actual content of active site cannot be calculated, the unit mass of catalyst is also adopted. TON is usually used for static experiment such as in autorclave. TOF is rather different to TON, which is commonly used for fixed bed reaction. Hope my answer to help you.
The volume of any chemisorbed compound has nothing to do with the number of sites active for any catalytic reaction. Stop with this erroneous assumption.
Canwe calculate TOF by wt of product wt kf products at 50 % conversion level / {(wt of catalyst in g)* surface area*time.
Turnover number (TON) = number of moles of reactant consumed/(mole of catalyst
The Turnover Frequencies (TOF) is calculated according to Equation: TOF= (Number of molecules reacted)/(Number of sites) x (time) The number of sites can be estimated from the relationships between size, area and dispersion of metallic particles using various techniques such as:
- Measurements by Gas Chemisorption;
- X‐Ray Diffraction Line Broadening Analysis;
- X-Ray Absorption Spectroscopy (XAS);
- Small‐Angle X‐Ray Scattering Analysis (SAXS);
- Transmission Electron Microscopy;
- Measurements by Magnetic Methods;
- Estimated by X-ray photoelectron spectroscopy (XPS);
Book: Handbook of Heterogeneous Catalysis / Prof. Dr. G. Ertl, Prof. Dr. H. Knözinger, and Prof. Dr. J. Weitkamp
Book: Characterization of Heterogeneous Catalysts (Chemical Industries) / Francis Delannay
Book: Characterization of Solid Materials and Heterogeneous Catalysts / Michel Che and Jacques C. Vedrine
Book: Principles and Practice of Heterogeneous Catalysis / John Meurig Thomas and W. John Thomas. - Turnover Rates in Heterogeneous CatalysisChem. Rev. 1995, 95, 661-666 661/ M. Boudart - Article “Turning Over” Definitions in Catalytic Cycles / ACS Catal. 2012, 2, 12, 2787–2794 / Sebastian Kozuch and Jan M. L. Martin - The dynamics of overlayer formation on catalyst nanoparticles and strong metal-support interaction / Nature Communications volume 11, Article number: 3220 (2020) / Arik Beck, Xing Huang, Luca Artiglia, Maxim Zabilskiy, Xing Wang, Przemyslaw Rzepka, Dennis Palagin, Marc-Georg Willinger & Jeroen A. van Bokhovenhttps://www.nature.com/articles/s41467-020-17070-2 - High-Surface-Area Catalyst Design: Synthesis, Characterization, and Reaction Studies of Platinum Nanoparticles in Mesoporous SBA-15 Silica. / J. Phys. Chem. B 2005, 109, 2192-2202 (DOI: 10.1021/jp048867x) / R. M. Rioux, H. Song, J. D. Hoefelmeyer, P. Yang, and G. A. Somorjai - Determination of Nanoparticle Size by Measuring the Metal−Metal Bond Length: The Case of Palladium Hydride / J. Phys. Chem. C 2015, 119, 854−861(DOI: 10.1021/jp510730a) / Jianqiang Wang, Qi Wang, Xinghua Jiang, Zhongneng Liu, Weimin Yang, and Anatoly I. Frenkel - Article Determination of the Size of Supported Pd Nanoparticles by X... / J. Phys. Chem. C 2010, 114, 39, 16677–16684 / R. Wojcieszak, M. J. Genet, P. Eloy, P. Ruiz, and E. M. Gaigneaux. - Article Subnanometer Substructures in Nanoassemblies Formed from Clu... / J. Phys. Chem. C 2018, 122, 37, 21686–21693 / Janis Timoshenko, Avik Halder, Bing Yang, Soenke Seifert, Michael J. Pellin, Stefan Vajda, and Anatoly I. Frenkel.
Jin Chen thank you for your explanation but could you provide me with relevant resource material that clearly report explanation. Thank you once again
Ahmed I. Osman could please provide detailed information of the material where the picture was obtained? Thank you.
Turnover number (TON) specifies the maximum use that can be made of a catalyst for a special reaction under defined reaction conditions.TON = Moles of desired product formed/ surface area of heterogeneous catalyst.
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