with no doubt, the three-phases test is the most conclusive (more than hot filtration tests or other leaching tests). Concerning your 2nd point, the answer is no. The TOF may increase with the amount of catalyst up to a certain point, and the it starts to decrease. So you will only have linearity in a certain range of substrate to catalyst ratio. And this has nothing to do with leaching or any contribution from homogeneous species

Dynamic light scattering! I prefer it

Your question is not very clear. In heterogeneous catalysis, a reaction normally occurs over a solid surface and the reaction rate relates to surface area of the catalyst and not its amount. To explore this relationship, the surface area of a solid catalyst is normally measured by N2 adsorption (BET) method.

Hai Godinda

Nice question ,

Usually this is the trend followed as you explained in ur question

but we cant say its purely heterogeneous all the time

some of the active sites of catalyst may leach in the reaction media but may not be active in those medium as homogeneous catalyst. This is the one glimpse to say its heterogeneous !!!

It is usually called as leaching test involving hot filtration!

As you said the leached can be confirmed by the ICP and AAs analysis!!

The change in the Weight of catalyst before and after reaction is one can think but still its not accurate to measure the weight of catalyst after reaction

i don't think so any other methods are there on this.

regards

vijay

- evaluate the activity and activation energy with and without catalyst.

- in the case you have a loss of the catalytic specie from the support due to leaching you should observe a decrease of apparent TON or TOF after some cycles of utilization of the heterogenous catalyst. AAS, TEM Chemisorption can then confirm the change in the state of the catalyst

@Antonio comite Decrease of apparent TON or TOF may be due to the blockage of active sites or change of active sites not apparently leaching off active sites right !!

@Mohammad Najafpour can i know about how it works

Other than above mentioned, you can plot the conversion or yield for many of trials with the same batch of catalyst, or Evan you can check XRD, may be slightly helpful. Depends up on your metal other characterizations also applicable.

Many Thanks to all for their remarks

If the catalysis is heterogenoeous the most important parameter is the surface. if the surface of the catalyst is blocked no reaction will occur. You can do the following:

1. performe your reaction;

2. filtrate,

3. add a poisoned catalyst (the poison depends on your material and reaction I suppose)

4. run the reaction again. If it goes like in the first step, the catalysis is homogeneous; if it does not go is heterogeneous; if it goes but the selectivity/TON/TOF is intermediate, then you might have a combination of the two processes (adsorption/desorption of ions - dissolution/recrystallization of the catalyst and so on).

Dear Monica

Thanks a lot for your suggestion. I'll definitely follow the procedure.

The generalization is always dangerous. Sometimes, the surface area could is not the only responsable by catalyst activity. Depends on type of reaction (redox, acid catalyzed).

In addition, is necessary check blank-reactions under all reaction conditions (support effect, temperature effect, etc. This one with the others suggestion above (check masss, recycle, quantify leaching, ICP, hot filtration) can be enough.

Determining the true nature of a catalyst is complex, and most methods (including the ones discussed above) have limitations, often there is an exception to the rule. I suggest great care be taken in applying any of the above methods - as you have not reported your reaction/conditions etc, some may not be suitable.

I would recommend the following reviews which address the issues of homo vs heterogeneity (and everything in between), as well as discuss all of the current methods for assessing a given reaction:

Robert Crabtree, Chem Rev 2011, page 1536

Finke J of Mol. Catal A: Chemical, 2003, 198, page 317

In general, no one method is conclusive, though through the use many of the tests - including those above - should provide very strong support one way or the other. The three phase test is often viewed as the 'ultimate answer' to this question, but beware that it also has inherent limitations and can be applied incorrectly resulting in incorrect conclusions.

Happy reading.

A "leaching" experiment might be helpful; after one reaction, filter the solid catalyst from the reaction mixture, apply new reagents and try the reaction again (as though no fresh catalyst were added). Compare the reaction parameters (in particular, rate constant) to an uncatalysed reaction. It should point you in the right direction at least...

My particpation on these discussion was minimum, however, it was extremely profitable. A special thanks to Prof. Behera and mainly Karl Collins! Excelent the suggested references. Im so gratefull by you have cited it, and adivise all to reading. Best Regards

you can try "hot filtration test" as described by Lempers, H. E. B.; Sheldon, R. A. J. Catal. 1998, 175, 62-69. Moreover if you have metal/metal ions as the active centre, you can also try for AAS study of the reaction filtrate(separating from catalyst) after proper treatment. This will help you in finding any active species soluble in reaction filtrate.

Many thanks to all for their valuable suggestions. There are so many tests for the Heterogeneity. Your suggestion will definitely help to test the heterogeneity. But what could we conclude while plotting amount of catalyst (lower to optimum) vs shorter time period( very less that the optimum reaction period)? Could anyone suggest?

As catalyst amount increase , the active sites of catalyst obviously increases and as a result it reaches the desired products in a shorter time !!! as more the catalyst more is the collision between the catalyst and the reactants and equllibrium to the product is reached in shorter time !!

Another thing is some time the increase in the catalyst amount may also decrease your activity due to rapid deactivation by the product formed :

Hello ,

@monica distaso

what does it mean by

add a poisoned catalyst what exactly it mean could you explain me in brief

I dont think soo any poisoned catalyst can run the reaction :)

regards

vijay

To ensure that the reaction is truly heterogeneous, seperate the catalyst and put in measured quantity of the reactants in the filtrate/centrifugate and run the reaction for the appropriate time. if the reaction shows negligible progress, then it indeed is heterogeneous.

@ Marakatti

so, the point is exactly that. If the reaction does not work, then it is heterogeneous (we have intentionally de-activated the surface). It can happen that the reaction works, because of the metals leached from the first cycle. Then it is homogeneous. Intermediate behaviours are well described above.

Review "Distinguishing between the homogeneous and heterogeneous mechanisms..."

Article Distinguishing Between the Homogeneous and Heterogeneous Mec...

Thank you for you valuable Reply @monica

but i don't agree with your answer :(

I don't think so it can be used to differentiate the homogeneous and heterogeneous

Because leaching of active sites may take place when catalyst is fresh

When it become deactivate or poisons there are no active sites at all to carry the reaction even if it goes by heterogeneous or Homogeneous then their will be no activity at all .

Therefore hot filter for the fresh catalyst that to within the conversion rate of 50 % is good method to find the heterogeneity :)

If we will plot amount of the catalyst vs shorter time period (less than the actual time period of the reaction) in relation to the conversion and we will get a linear relation

Two possibilities:

1. catalyst amount increasing-----active site increasing---- conversion increasing

or

2. catalyst amount increasing-----active site increasing----leaching increasing----- conversion increasing

So no conclusion can be made regarding heterogeneity of the reaction from the above mentioned plot.

Dear Apurba

Thank you very much for this nice reply

@apurba

catalyst amount increasing-----active site increasing----leaching increasing----- conversion increasing

This is only when leaching of active sites are there in your catalyst unless its heterogeneous only

And it is not mandatory all the time that leached species should be active as catalyst in homogeneous or heterogeneous :)

@Vijaykumar i am agreeing with you………

There are so many possibilities regarding the matter I only try to say that the plot (as mentioned by Gobinda Behera sir) could not able to draw conclusion about heterogeneity of the reaction. However it may support the other analysis result.

Dear Gobinda,

I don't think that analysis and heat filtration alone are reliable techniques to distinguish between homogeneous and heterogeneous reactions : almost all the catalysts undertake some leaching, but it is not because you find some complex in the liquid phase that it will responsible for the (totality of the) reaction. This may be completed by a series of tests where you dissolve increasing amounts of the active principle (for example a noble metal salt) in the liquid solution of reactants, you measure the activity, then you compare the obtained results with those of your batch containing the solid catalyst as well.

Some researchers suggest also to put the conformed heterogeneous catalyst inside a basket and to introduce and remove it from the reaction bath several times, at given intervals, to test the steady state of the reaction. This is valid, of course, after the initial induction time and in the absence of poisoning.

As you can see, there is no universal rule : you should adapt to the specific case you are testing.

Dear Marco

Thank you very much for your suggestion. Hot filtration test is a general technique used by all. Besides this we can analyze through AAS and ICP-OES.

Yes, the hot filtration (or split) test is becoming a diffuse fashion. It can give some indications, but in many cases it cannot give a final answer.

It is interesting to read, for example, the report of Richardson and Jones in J. Catal. 251 (2007) 80–93.

Many thanks Marco for this article and also for your contribution to this discussion.

In transition metal catalysis, you should perform a CS2 poisoning test. In this test, if you have a heterogeneous catalyst, a sub-stoichiometric amount of CS2 (considering the metal load) led to a great decrease in conversion. On the other hand, homogeneous metal solution will linearly decrease its conversion with the amount of CS2 added.

Dear Ricardo

Many thanks. Could you say me the detail procedure for this test( CS2 poisoning test)?

Regards:

Gobinda

we have used this method in: http://dx.doi.org/10.1016/j.jcat.2008.01.015 . Check this out! Best Regards..

Homogeneous processes are faster than heterogeneous ones. If you have an import homogeneous contribution the reaction rate will be less sensitive to the temperature.

Dear Gobinda,

There is a recent review on this topic.

http://pubs.acs.org/doi/pdf/10.1021/cr2002905

Regards

Jacques

@jacques Le paih

its very nice article

thank you for that

Sir, the mercury drop test is one method to determine heterogeneity.

@Ninad

can you explain in brief what is the mercury drop test

The best one is the proportionality between the rate constant and the weight of a catalyst under the condition of small conversions. It is necessary to be sure that no diffusion effects influence the reaction rate and that the reaction proceeds under stationary conditions..Small catalyst weights are desirable in all experiments.

In some reactions, the chains originate at the surface but the reaction continues in the volume (heterogeneous-homogeneous reactions). In these cases, when you poison the surface, we also poison the formation of active particles. You can think that the reaction proceeds at the surface, however, it is really heterogeneous-homogeneous. The experiments with different weights of the catalyst are better . The occurrence of the homogeneous reaction is easily determiined because the kiinetics of homogeneous reactions and of heterogeneous reactions are different. In addition, this procedure is simpler and you will not polute your apparatus by sulfur.

Many thanks Prof. Ostrovskii for your contribution in this discussion.

Many thanks to Jacques for that informative review article on heterogeneity (Resolving Heterogeneity Problems and Impurity Artifacts in Operationally Homogeneous Transition Metal Catalysts). Its really a very nice article which could give a lot of information on heterogeneity.

There is one more techniques: a reaction is heterogeneous, if the productivity of any weight of a catalyst does not depend on the form and volume of the working volume of the reactor. Therewith, the conditions named by me in the preceding answer should be performed.

Dear Prof. Ostrovskii

Thank you very much for your suggestion. I'll definitely try this procedure.

Regards:

Gobinda

I do not know how much it works with heterogeneous catalysis but I remember that the heterogeneity of adsorption centers can be tested by measuring the absorption heat vs. coverage, i.e. differential adsorption heat test (microcalorimetry). I presume that the same thing can be done by inverse gas chromtography using various concentrations of the adsorbed gas. Maybe tge presence of chemical reaction blurs the picture, but perhaps if you do the adsorption tests at temperatures where the reaction itself is slow, you may get information on the pure adsorption behavior, which still reflects the heterogeneity of the surface - nad adsoprtion is the first step of reaction anyway.

Dear Prof. Banghegyi

Many thanks for your suggestion in this discussion. I got many procedures for the heterogeneity test through this discussion. It will be better if there will be any simple technique for this test. Anyway many thanks to all who have participated and suggested in this discussion.

Regards:

gobinda

I don't know if I can add much more to what Professor's Banhegyi & Ostrovskii have mentioned previously. First my assumption is you doing batch reactions with a homogeneous fluid mixture (either gas or liquid) and an solid catalyst of known surface area and defect morphology. Professor Ostrovskii catalyst loading suggestion is a good starting place. Additionally, running a series of constant loading where you've change the relative surface area and morphology would complement that work. Assaying the fluid for soluble metal species (ICP or AA) would tell you if you have the potential for multi-phase reactions as warned earlier. One other thing, if there is an induction period to form the reactive species, that may also be an indication that you may have a may homogeneous component contributing to your kinetics.Ultimately your kinetic expressions from the loading studies should help you eliminate reaction pathways.

Dear Prof. King

Many thanks for your suggestion in this discussion.

Dear Dr. Karnahi

Thanks a lot for this informative article.

Have you tested if the reaction ocurred without catalyst?

@Joao Poco

Yeah, Me have tested the blank reaction.

I think if we do a reaction in a particular reaction medium, first we have to check the medium alone for presence of any basic materials and secondly we should stirr the catalyst in the medium alone and the filtrate should be tested for any soluble basic species, and thirdly we should do a blank reaction without the catalyst.

One method is to see if the reaction if poisoned by Hg (homogeneous will not be poisoned).

If you extract the catalyst in the middle of the reaction and after that dont see any progress, it means your catalyst is heterogeneous 

Article Preparation and characterization of Ni/mZSM-5 zeolite with a...

It is sufficient to use two reactors of different volumes with the same quantity of a catalyst  under the same reaction conditions. If the reaction rate is higher in the reactor of  larger volume, this means that the reaction is homogeneous or heterogenno-homogeneous. If the reaction rates with different reactors are the same, this means that the reaction is heterogeneous.  

Hi, Hot filterationtest 

Hot filtration test will work

I liked the answer from Omid Mazaheri. If your catalyst is an acid an the product is acid. You could only detect the homogeneous catalysis after some time.