TON and TOF will give you information about the efficiency and durability of a homogeneous catalyst. In a homogeneous catalyst, generally one active site will be there unless it is a poly-metallic system with each metal center performing the same catalytic activity simultaneously.

In heterogeneous catalysis, especially, at higher temperature, number of active sites are important and are obtained from sorption, i.e. the rate of the reaction with respect to number of active sites per volume.

TOF are very small in case of heterogeneous catalysis, often not more than 100, whereas in homogeneous catalysis, depending on the durability (recycling) and robustness of a catalyst, TOF can be in millions per second.

with best wishes

Balakrishna

Active sites and TOF are both important for designing a catalyst, but that is just the beginning. You also need density of sites (per unit volume or per unit mass). Once you know these values, you can calculated the maximum rate for the reactor. Then layered on top of that is mass (and heat) transfer to determine how the application (commercial) will work. This has to do with the formation of the catalyst (sphere, cylinder, solution).

So, yes, measure the active sites and TOF, but also keep the big picture in mind.

Hi there!

I am in agree with William G. Borghard. Both actives sites and TOF are so important for the design of a catalyst. (But, also all will depend on the specific application that you will use for)

As, for general characterization, active sites could be determined by different chemisorption techniques. And also, the nature and type of these active sites. These will be used for determining the value of TOF (Please, see: Article “Turning Over” Definitions in Catalytic Cycles

For general comparison, TOF will be necessary to establish the ACTIVITY of different catalysts in a specific reaction. If no, to define only the type of the active sites only will be an additional tool to complete the characterization of a catalyst.

So.... dearly, determine both active sites and TOF!!!

With kind regards,

Julián Sánchez.

There is no single measure that determines the suitability of a catalyst in a specific chemical process. In industry, the most important feature is productivity. However, one should not forget about selectivity and activity, but also about lifetime. TOF is also important, but mainly for enzymatic reactions.

Many thanks for your feedback. Mirosław Grzesik even though many studies in academy are based on industrial needs, somehow TOF values are encouraged to be given for heteroegenous catalysts in many papers, either for really to compare with other works or just reviewers keep thinking that that they are important. Howeve, the problem is, it is not easy to determine. In one of my works, a referee wanted me to calculate them but without finding active of number sites it is not easy to estimate TOF. Julián E. Sánchez-Velandia Thank you for your reply. Actually the paper you mentioned inspired me to start this discussion as well. By using the energetic span model described in the paper, I was able to calculate TOF values from my experimental data. Then, I realized without using chemisorption methods, one can also calculate the number of active sites by using the estimated TOF values from that model.Thanks to this reverse engineering method, I was able to find TOF value and then the number of active sites but then I started to ask which one is more important. If we can find TOF value without the number of active sites then do we really need them. But in terms of catalyst design, the number of active sites should be more important. Because when we try to scale up, the main part of the material we should focus on should be the number of active sites. William G. Borghard Thank you very much for mentioning the big picture. I remember how I was surprised when I saw an industrial catalyst which was nothing to do with our powders that we synthesized and characterized in laboratory. Sure the purpose is to obtain an industrial catalyst and design a catalytic reactor actually. I wonder that you mentioned about density of active sites but isn't it the same thing that we find for a certain amount of catalyst during chemisorption experiments? Or is there something that I miss to estimate it? In any case, if we really want to design the catalyst on molecular level, shouldn't it be the active sites that will be exposed to reactants to be understood well. TOF value should be a choice of saying that the catalyst is active. Balakrishna Maravanji Thank you for your feedback too. I can expect the difference of a TOF value for homogenous catalyst and heterogeneous catalyst due to transport properties. But I especially wonder why it is mentioned that TOF value of a heterogeneous catalyst can not be over 100 s-1. I read it in a paper too but it was not discussed there as well. It was mentioned that TOF values over 100 are not correct for heterogeneous catalysts and I don't understand where this rule of thumb comes from. Especially with my new semi-empirical method, I estimated my TOF values between 100-4000 s-1 and because I can't find any strong theroretical discussions about why TOF values should be below 100 s-1, I don't find this generalization scientifically correct. But if there is something I am missing, I will be happy to learn it. Thank you verey much again.

Dear Ebru Erunal you are asking an interesting question. Here's what we do when calculating TOFs for heterogeneous catalysts:

1. We start from its definition:

TOF = rate of reaction/concentration of active sites [=] [(mol reactant/s)/(mol active sites)] = [1/s]. Therefore, as it name implies, TOF is a frequency whose units are Hertz.

2. We consider that the rate of reaction that must be used to calculate TOF must be the intrinsic rate of reaction of the catalytic sites. Therefore, the following conditions must be fulfilled: (i) no transport limitations (mass and temperature gradients) must be present, (ii) as the reaction rate is a function of the advancement of the reaction, the intrinsic rate of the active sites must be as far from full conversion or equilibrium conditions as possible. For this, the rate should be extrapolated to zero conversion and free of the inhibition effects that products may have on the catalytic process. In a fixed-bed reactor, this means that the reactor should be operated in a differential mode; i.e. the conversion should be as close to 0 as experimentally feasible. Also, tests feeding products must also be done. In a batch reactor, this means that the initial rate of reaction is the most suitable for the calculation. For calculating the latter, samples must be taken as close to zero time as possible.

3. Measuring the concentration of active sites may be the most difficult part since one must answer the question: what is the active site for the reaction that you are investigating. Typically, you have the following possibilities: (i) for catalysts based on metallic nanoparticles (supported or not) where the latter are responsible for activating the most stable reactant (e.g. in methane oxidation, methane is hardest to activate than the oxidation agent -O2 or H2O, etc-), the number of moles exposed at the surface of the catalyst is well accepted as a metric for estimating the number of active sites. As you may know, this can be measured by chemisorption of some adequate probe; e.g. H2, CO, etc. (ii) for catalysts based on (supported or unsupported) oxides, sulfides or nitrides, the active site is also normally associated to an exposed metallic center or to a given structural site. Therefore, certain assumptions are always before defining a metric for the concentration of active sites. As an example, MoS2 catalysts have sites located in different parts of its geometrical structure and the specific role of these different sites depends on several factors associated with the type of molecule that is reacting, the reaction conditions, the possible interconversion of sites, among others. (iii) for bimetallic catalysts, e.g. Pd-Pt, one would need to define the specific role of each metal in the reaction. It may be the case that both metals are active in the reaction or that only one of them is. Therefore, as in the case of (ii) certain assumptions about the nature of the active sites -and reaction mechanism- must be made. (iv) For bifunctional catalysts; i.e. those where the support also contributes active sites, one should take into account the same as in (i).

Well, the points above are our rough guide for calculating TOFs. We have got help from literature such as the few ones cited below:

1. Foggler's Elements of Chemical Reaction Engineering

2. Chorkendorff and Niemantsverdriet's Concepts of Modern Catalysis and Kinetics

3. Froment and De Wilde's Chemical Reactor Analysis and Design

4. Harris et al. Consequences of product inhibition in the quantification of kinetic parameters, Journal of Catalysis 389 (2020) 468–475

Ebru Erunal , you are right about the density of sites being measured by techniques like chemisorption. My addition is for scaleup i.e. when you make the catalyst for an industrial application, the density of sites will change. For example, if the active catalytic phase is mixed with an inert phase (binder) for strength before forming (sphere, cylinder), the density of active sites will drop. And then there is the density of these catalyst particles in the reactor i.e. packing density. It is important to keep track of all of these issues when you estimate the activity in the ultimate application. So, you could have an active catalytic phase that has a lower TOF, but if it can be formed with less binder, it may be better in the application.

William G. Borghard many thanks for your reply. So far I have only chance to work with unsupported metal oxide catalysts at big scale and we haven't taken into account the decrease of active sites due to the binder. Víctor Baldovino thank you for the detailed procedure. When I tried to exploit the rate of my reaction which was in batch reactor, I considered each reaction rate for the same time interval. Because I conducted the same reaction at various temperatures, I thought it would be the best to consider the same time interval for each reaction. By this way only one parameter would be changed but actually I may not taken into account of the kinetic energy of molecules due to temeprature change. As far as I see from your explanation, it might be better to consider the initial time interval. At least I can calculate and see the deviation from my approach.

Hi Ebru Erunal

if your solid is a metal oxide, it does not make sense to determine the TOF, since this already universal concept is defined by the number of metal sites determined by the chemisorption of probe molecules. If the solid is a zeolite you could determine the TOF by determining total acid sites.

Trino Romero thank you for your notification. Currently I work on metal catalysts but I will keep in mind when I work on metal oxide catalysts.

Ebru Erunal, TOF and TON are mainly used as a result of small-scale research. They are also one of the favorite measures used by enzymologists. However, I don't think anyone asks about TON and TOF for solid industrial catalysts. Productivity is what matters most there. Moreover, the estimation of the number of active sites on the surface of the solid catalyst is difficult and unreliable. It is easier to estimate the catalyst surface area.

Dear All,

I would like to thank you again for your contributions in this discussion which helped me to finalize my recent paper about estimation of number of active sites through kinetic analysis. Last week I got a notification from the editor that it was accepted and soon it will be published. I can share it with you personally if you are interested in.

Best regards,

dear Ebru I am interested in your article.

Dear Ebru Erunal,

Very interesting discussion and also useful information has come through responses. When we are planning a catalyst design, to begin with, we should not worry about the TOF or TONs. We should focus on objectives set for making a novel catalyst. It can be cross coupling, hydrogenation, oxidation, polymerization or any typical organic transformation. Many times, or I should say, most of the time, theoretically predicted TOFs are of very little help while investigating catalytic efficacy, as many parameters are compromised in theory, whereas the same can not be done while doing experiments. Unless we are satisfied with optimized conditions, we will not proceed and naturally that takes care of everything.

Best regards

Balakrishna

Dear Balakrishna Maravanji many thanks again for your responses.

Dear Trino Romero I will send the published version as soon as I get it from the journal.

Dear,

Good question.

TOF is defined as the average numbers of chemical reactions happened on active sites per hour.TOF = rate of reaction/concentration of active sites.

Hello, Good day! You can get the required (TON/TOF) help from the following articles.