Normally, when I calculate TOF values, I calculate TONs first, TON = n_desired product/n_catalyst. And then TOF value = TON/reaction time
TOF can be obtained by dividing the amount in moles of product produced by the product of the reaction time; the amount of active sites on the surface and
the mass of catalyst used. To obtain the number of active sites you need to perform NH3 or CO2-TPD analysis. NH3-TPD for solid acid catalyst, while CO2-TPD for solid base catalyst. I hope this answer your question.
You can get the details from my article titled "Synthesis and application of hierarchical mesoporous HZSM-5 for biodiesel production from shea butter" on Researchgate.
Turnover number (TON) = number of moles of reactant consumed/(mole of catalyst). Turnover frequency (TOF) = TON/time of reaction.
I agree with Mr. Alaba, Ms. Dang and Mr. Mandal. Following additional information may help in developing better understanding.
TON denotes how many maximum molecules of substrate can be converted into product per catalytic site of a given catalyst under defined conditions.
TOF is a measures of specific activity of a catalytic center of a given catalyst by the number of molecular reactions or catalytic cycles occurring at the centre per unit time. In other word, it is a number of revolutions of the catalytic cycle per unit
time
TOF = (Number of molecules reacted)/ (Number of sites) x (time)
In order to measure active sites of the acid or base catalyst, chemisorption studies by TPD analysis can be used.
You may refer these interesting papers:
Turnover Frequencies in Metal Catalysis:
Meanings, Functionalities and Relationships, DOI:10.1039/9781847553294-00320
Turnover Rates in Heterogeneous Catalysis, 10.1038/ncomms9618
I do hope that this will help you
If you use supported catalyst, then you can divide the number of product molecules over the number or catalyst molecules. this will be approximate value of TON
I think we can use surface area too if you cant determine the number of active sites. but you should note that TOF is related to surface area and not related to nomber of active sites.
TOF= No. of products molecules / (surface area x time)
with my best regards
For TOF , I agree Imteyaz Alamwith. To obtain the number of actives sites, the chemisorption of H2 is also used for supported catalyst in TPD analysis.
Please have a look at link below
https://www.researchgate.net/post/How_to_calculate_the_turnover_number_of_a_catalyst
Please find the following link of pdf for better understanding and how to calculate the TOF and TON, and how those two are related, If we know TOF , then how to calculate TON
https://application.wiley-vch.de/books/sample/3527331654_c01.pdf
In heterogeneous catalysis, TOF turnover frequency is a term to compare the efficiency of active sites between different catalyst. TOF is defined as the average numbers of chemical reactions happened on active sites per hour. To get the TOF, you need the conversion of products, which can be determined by GC, and then you need to measure the active sites of you catalyst. For metal catalysts, the number of active sites are always measured by H2 adsorption (although CO may be used sometime but not recommended because one CO molecule may occupy one or two active sites, and the uncertainty make the determination rather rough).
hi dear Alexandra Demeter
please calculate the dispersion of active metal in your catalyst
then follow under equation
TOF= (REACTION RATE *MW ACTIVE METAL)/(DISPERSION)
According to Zeinhom, is it really appropriate to use Surface area in calculating TOF, many reviewers kick against this. Please, what is your advice? Any reference to backup the use of SA?
Hello Alexandra,
TON = no. of moles of product/no. of moles of an active catalyst
TOF = TON/time of reaction
Generally,
TOF = Volumetric rate of reaction / (no. of moles of active centers/volume)
which can be rewrite in terms of catalyst weight,
TOF = Rate of reaction / concentration of active sites = -rA/Ct
where Rate of reaction (-rA) = mol/g cat * s
and Concentration of active centers or metal (Ct)= mol/g cat
We can write Ct in terms of % dispersion and % metal loading,
Ct = moles of active metal / g cat = Ns * 6.022 * 10^23 / g cat = (%Dispersion * Nt * 6.022 * 10^23) / (100 * g cat) = (%Dispersion * Metal loading in moles)/(100 * g cat)
Ct = D*Mt/g cat = D * mt / Mw * g cat = D * M / Mw = %D * %M / Mw * 10000
where, D - Dispersion fraction
Mt - Metal loading in moles
mt - Metal loading in g
M - Metal loading fraction
Mw - Molecular weight of acitve metal
Hope you get it!
why TOF is reported in units of (1/time) rather than (number of moles of reactant consumed/(mole of catalyst-time)
The formula is quite simple, however, how to obtain the number of the active sites is tricky and challenging.
Please read this article,Article “Turning Over” Definitions in Catalytic Cycles
Maybe it is difficult to quantitatively calculate the amount of active sites
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 Catalysis
Chem. 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 Bokhoven
https://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.
Thank you all for your valuable responses, Raimundo Crisostomo Rabelo Neto Pravin Bolne Mrinmay Mandal and other respondents
Dear Alexandra Demeter ,
I have attached a useful book. In the first chapter you can find your answer as clear as possible.
As said here before and according to the IUPAC, TOF is defined in heterogeneous catalysis as the number of reacting molecules per active site and unit time and is expressed in inverse of time units. The TON results from multiplication of the turnover frequency (TOF) and the lifetime of the catalyst (time).
For photocatalysis, the IUPAC ( Pure Appl. Chem. 83 (2011) 931–1014. https://doi.org/ 0.1351/PAC-REC-09-09-36 ) also provides definitions:
TOF as the number of photoinduced transformations (product formed or reactant consumed), per catalytic site and per time period.
TON is the number of times, n, that the overall reaction (the photo-chemical transformation) goes through a photocatalytic cycle.
So, the main issue (also mentioned in the discussion) is the "catalytic site", which is essentially unknown in photocatalysis. However, the literature presents some useful works. Maybe the best attempt considers measuring the (rate of) formation of radical species (photocatalytic mechanisms are of radical nature, most frequently OHs) and connecting it with surface species (through combination techniques such as probe-molecule EPR together with in-situ IR, etc.). Also, single site-single molecule approaches (mostly using flo probes) provides an answer. In the context of photocatalytic kinetic studies, we summarize some of these works in (Article Braiding kinetics and spectroscopy in photo-catalysis: the s...
). In any case, it is an open question.
As most colleagues agreed, TOF or TON definitions are clear and simple. The challenge however is how to accurately measure the number of active sites (i.e. dispersion), particularly in the cases where the active sites are not simple metallic sites or these concern bimetallic catalysts formulation (something which is currently very common in advanced catalytic systems).
Note also that particle size measurements, and thus estimation of the dispersion, is substantially depended from the measuring method used (e.g., HRTEM, XRD, H2-, CO-, or N2O-chemisorption).
However it is worth noticing that regardless of the difficulties, discrepancies and the existing disputation regarding the most valid method of measuring the population of active sites on heterogeneous catalysts, the only valid and scientifically acceptable method of comparing the intrinsic activity between different catalysts for a certain reaction is this one which is based on TOFs comparison and not via light-off diagrams; the latter is a relatively erroneous approach that is often made in the open literature...
Stay safe and healthy!
https://s3-eu-west-1.amazonaws.com/itempdf74155353254prod/12375047/Revisiting_Catalytic_Cycles__A_Broader_View_Through_the_Energy_Span_Model_v1.pdf
Responding directly: according to the definition of both physical quantities. But, the number of active sites is subject to a relatively large error.
How can the TON and TOF be calculated if the catalyst is taken in the form of weight % (i.e. mol wt of the catalyst is not known)?
Please, I am confused. can I use
TON= mole of product/mole of reactant
and, TOF= TON/time??????
Please, help...
Temitayo Falola
TON = Number of moles of reactant consumed OR number of moles of product produced / number of moles of catalyst
TOF = TON / reaction time
But my values from the calculations are low, even though I'm sure my material is a good catalyst.
Is there any other alternative to it?
Briefly, the turnover frequency (TOF) is a measure of how active a catalytic site is, while the turnover number (TON) is a measure of the stability of such an active site.
The turnover frequency (TOF) is defined as “Commonly called the turnover number, N, and defined, as in enzyme catalysis, as molecules reacting per active site in unit time” (See IUPAC’s definition). Judging from this description, it seems that the TOF and the TON are the same, but in the context of catalysis they are not. The common pitfalls are:
The TOF is calculated as an average value, dividing all molecules converted in a given period by the number of sites. This approach conditions the resulting TOF as it skews its value to a lower number.
The TON is calculated as the total number of transformations per site in a given time, even though the catalyst might still be active at the end of this period. This calculation is highly sensitive to the reaction time considered.
Thank you all, Researchers! But, how can I know the number of the active site, and how can I use that to determine the TOF and TON? like a formula!!!
I look forward to hearing from you.
Thank you.
Hi Temitayo Falola, this reply may be late, but the number of metal active sites can be determined by a catalyst characterization technique known as pulse chemisorption. In H2 pulse chemisorption, Hydrogen gas is required to titrate on the catalyst surface at controlled (programmed) conditions, and the amount of H2 uptake will be generated, as well as the dispersion and active metal surface area.
This equation can be of help:
TOF (min-1) = [Flowrate (cm3/min) * Conversion (%) * Selectivity (%) / Catalyst weight (g) * H2 uptake (cm3/g)]
(c7cy02440a.pdf)
WoW! Thank you. But how do i set up the experiment? Which instrument will I use?
Thank you.
Hello, Good day! You can get the required (TON/TOF) help from the following article.
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