This question is rather important for the researchers working in the field of catalysis. It is under discussion for about a century. What is your opinion on its solution?
I should add the following. The rate of catalytic reactions is rather often determined by the rate of diffusion; this can be the so-called external diffusion when the reaction rate is determined by the rate of diffusion from the gas medium to the external surface of a catalyst, and the so-called internal diffusion, when the reaction rate is determined by the rate of diffusion of source substances or products within the porous structures of catalysts. There are no doubts that the kinetics of catalytic processes say nothing on their catalytic mechanisms in all cases when the rate of reactions is determined by the rate of diffusion.
I would like to know the opinion of the members of the community whether the kinetic dependences are applicable for clarification of the reaction mechanisms in those cases when the observable reaction rates are determined just by the rates of catalytic reactions but not by the rates of diffusion.
It is really hard to say...In heterogeneous catalysis, you are not even sure how your catalyst is coordinate to the mesoporous surface..but still you can suggest.
I think it really depends. I do not think all the heterogeneous catalytic reaction could be guessed by kinetic study. But kinetic study could give a proof of the proposed mechanism.
I am not sure if I understand your question well? The mechanism of catalytic heterogeneous reaction is part of mysterious in science to find very high speed equipment to characterize formed intermediates in a catalytic reaction sequence. Once the influences of heat and mass transfer gradients for reaction and reactor system have a legible influence on rate, the rate is considered as surface reaction and no diffusion limited rate. I assume there is possibility to speculate (predict) some formed intermediates from catalytic rate equation. I published few works in early 90, and found useful information concerning design of the catalyst.
With respect to today’s sophisticated equipment, maybe more information can be gained with regards to formed intermediates (adsorbed on the surface of the catalyst).
1.Kinetic and mechanisms in the ammoxidation of toluene over a TiO2(B)-supported vanadium-oxide monolayer catalyst; Sanati M, Andersson A; INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH 30 (2): 312-320 FEB 1991
2.Kinetics and Mechanisms in the Ammoxidation of Toluene over a TiO2 (B)-supported Vanadium Oxide Monolayer Catalyst. 2. Combustion Reactions; Sanati M, Andersson A, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH 30 (2): 320-326 FEB 1991
3.Direct propane ammoxidation to acrylonitrile – kinetics and nature of active phase, Andersson A, Andersson SLT, Genti G, Grasseli RK, Sanati M, Trifiro F, Sinev M, Haber J, Suresh DD, Krylov OV, Borders E, Portela MF, Villa PL; STUDIES IN SURFACE SCIENCE AND CATALYSIS 75: 691-705 Part A, 1993
4.Drift study of the oxidation and the ammoxidation of toluene over a TiO2 (B)-supported vanadia catalyst; Sanati M, Andersson A; JOURNAL OF MOLECULAR CATALYSIS 81 (1): 51-62 APR 22 1993
The mechanism can be guessed by study on reaction kinetics. A simple rate calculation never give any clue to a mechanism. For a heterogeneous system, we have to know the diffusion limitations on the surface too. You may find interest in some of the following papers on heterogeneous catalysts
https://www.researchgate.net/publication/239940262_Kinetic_investigations_on_the_modified_chitosan_catalyzed_solvent-free_synthesis_of_jasminaldehyde?ev=prf_pub
https://www.researchgate.net/publication/239940255_Hydroformylation_of_propene_heterogeneously_catalyzed_by_HRh(CO)(PPh3)3_encapsulated_in_to_hexagonal_mesoporous_silicaParametric_variation_and_mass_transfer_study?ev=prf_pub
https://www.researchgate.net/publication/239940265_Investigations_on_the_kinetics_of_hydroformylation_of_1-hexene_using_HRh(CO)(PPh3)3_encapsulated_hexagonal_mesoporous_silica_as_a_heterogeneous_catalyst?ev=prf_pub
Article Kinetic investigations on the modified chitosan catalyzed so...
Article Hydroformylation of propene heterogeneously catalyzed by HRh...
Article Investigations on the kinetics of hydroformylation of 1-hexe...
You can develop the micro-kinetic model based on experimental results.
The answers to your questions are discussed here … Chemical Reactor Analysis and Design, 3rd Ed, G. F. Froment, K. B. Bischoff, and J. De Wilde (John Wiley & Sons, Inc, Hoboken NJ) 2011.
In a more general sense, reaction dynamics has three parts -- rates, kinetics, and mechanisms. Rates are how fast it goes, kinetics are the dependencies on concentration and temperature, and mechanisms are sequences of steps to document what the molecules are doing. Absent diffusion limitations, the rates are intrinsic (surface) rates. From intrinsic rate measurements at different concentrations and temperatures, we obtain a kinetic expression. We also obtain a kinetic expression from proposed mechanisms. We can use the former (rate measurements) to validate the latter (proposed mechanisms). Better still, we use rate measurements to create kinetic expressions that then invalidate proposed mechanisms whose kinetic expressions do not fit with observations. This is a more rigorous test. We can also use kinetic expressions to "guess" at reaction mechanisms. This process is however just a guess, not science.
Dear All,
Unfortunately, it is an illusion and a chronic wrong belief that it is possible to guess the mechanism of any heterogeneous catalytic reaction on the basis of studies of its kinetics. I devoted many years and several review papers to this problem, and I will list most detailed of them. But, at first, I will present the opinions of several experienced catalytists of different epochs.
‘‘Chemical kinetics was directed by Arrhenius toward an ambitious goal. For 50 years now it has attempted to understand the basic nature of chemical change, to follow the meandering of every atom and the leaps of every electron and even to comprehend the reasons for these motions. . . The present reviewer is a scarred and disillusioned veteran of such attempts. He can see no real advance after half a century. Little parties of intrepid adventurers have pushed far into the wilderness, and established outposts beneath many various banners. These banners have proclaimed that reaction rates are related to heat changes, or to free energy changes. . . But always in the end a luxuriant growth of experiments has overcome the hardiest theorists and left of their venture only a little Chichen-Itza in an endless jungle. . . Superhuman intelligence could perhaps deduce truth from the available imperfect experiments, and practicing kineticists might draw correct conclusions from perfect data. Since neither combination will exist in the foreseeable future, it would seem that practical catalysis must continue to limp along with little help from limping theory.’’(Kassel L.S., Review on the book ‘‘Catalysis from the standpoint of chemical kinetics’’ by Schwab G-M. In: Twelfth report of the committee on catalysis, Nat. Res Counsil (USA); 1940, 332–334.).
N. Hanney, expert in the fields of catalysis and physics of solids wrote that few of the problems of chemistry were subjected to so prolonged and intensive investigations but with no significant successes as catalysis (Hanney N.B. Solid state chemistry. NJ: Prentice-Hall Englewood Cliffs; 1967.)
Much later, Weller wrote: “The author’s interest in kinetics and chemisorption has been developing for some 50 years. One hopes that prejudices have softened over the years, but one knows that they have not disappeared. . . With the proliferation of physical and mathematical techniques for obtaining and interpreting kinetic data, it is fruitful to reexamine, occasionally, the premises on which elaborate theoretical structures may be built.’’(Weller S.W. Kinetics of heterogeneous catalyzed reactions. Catal Rev – Sci Eng, 34 (1992) 227–80.).
In reality, apparently, all heterogeneous catalytic reactions proceed by the same mechanism, which is determined by the oscillation theory of catalysis (OTHC) (V.E. Ostrovskii, Sci. Isr.-Techn. Adv., 14 (2012) № 4, 57-79; Fuel, 110 (2013) 124-132), according to which all reactions are, factually, one-step. This concept is proved by about 20 catalyst/reaction systems.
In the context of the OTHC, the catalyst of any stationary chemical reaction proceeding under some external conditions in any gaseous or liquid mixture containing source substances and a desired product is a solid capable of forming at its surface a chemically-adsorbed phase (chemadphase, ChPh) or a set of ChPhs that are under equilibrium with these source substances and product; the rate of any reaction is determined by its rate-determining step (RDS).
The OTHC leads to the following important statements, which should be taken into consideration in the procedure of deduction of kinetic equations for stationary heterogeneous catalytic processes:
(1) The catalytic properties of a solid in a gas medium are determined by the capability of this solid to form a ChPh (or a system of ChPhs) that is in equilibrium with the source substances and desired product;
(2) In each cross-section of a catalyst, a catalyst/reacting-medium equilibrium exists because its “self-healing” is much faster than its disruption by catalytic reactions;
(3) The reaction rate of any catalytic reaction is determined by a single RDS, the equation of which can include not only chemisorbed particles but also gas particles (case 1) or chemisorbed particles only (case 2), and no other reaction step influences the observable reaction rate;
(4) The reaction scheme necessary and sufficient for deduction of the kinetic equation for any catalytic reaction should include the RDS equation and a set of equations of subtotal equilibriums, which, being summed together and with the RDS equation, should give the equation of this reaction;
(5) The set of equations of particular equilibriums should include (n – 1) independent equations (n is the number of the ChPhs at the catalyst surface).
The art of studying consists in the ability of a researcher to determine the RDS and the chemical nature of the ChPhs. The grounds, approaches, examples, and the proofs of impossibility to reveal the real reaction mechanisms from kinetic studies of catalytic processes are considered in the reviews above-referred and in: V.E. Ostrovskii, Sci. Isr.-Technol. Adv., 13 (2011) №4, 1-39; Ind. Eng. Chem. Res., 43 (2004) 3113-3126; Thermochim. Acta, 489 (2009) 5-21; J. Therm. Anal. Calorim., 108 (2012) 559-581; J. Therm. Anal. Calorim., 95 (2009), 609-622, and in earlier papers published in International and Russian journals. All papers are available at my ResearchGate page.
- Badges
- Science topic
- Similar topics
- Methodology
- Research Methodology
- Research Papers