scholarly journals Requiem for the Rate-Determining Step in Complex Heterogeneous Catalytic Reactions?

Reactions ◽  
2020 ◽  
Vol 1 (1) ◽  
pp. 37-46 ◽  
Author(s):  
Dmitry Yu. Murzin

The concept of the rate determining step, i.e., the step having the strongest influence on the reaction rate or even being the only one present in the rate equation, is often used in heterogeneous catalytic reactions. The utilization of this concept mainly stems from a need to reduce complexity in deriving explicit rate equations or searching for a better catalyst based on the theoretical insight. When the aim is to derive a rate equation with eventual kinetic modelling for single-route mechanisms with linear sequences, the analytical rate expressions can be obtained based on the theory of complex reactions. For such mechanisms, a single rate limiting step might not be present at all and the common practice of introducing such steps is due mainly to the convenience of using simpler expressions. For mechanisms with a combination of linear and nonlinear steps or those just comprising non-linear steps, the reaction rates are influenced by several steps depending on reaction conditions, thus a reduction in complexity to a single rate limiting step can lead to misinterpretations. More widespread utilization of a microkinetic approach when the reaction rate constants can be computed with reasonable accuracy based on the theoretical insight, and availability of software for kinetic modelling, when a system of differential equations for reactants and products will be solved together with differential equations for catalytic species and the algebraic conservation equation for the latter, will eventually make the concept of the rate limiting step obsolete.

2007 ◽  
Vol 14 (03) ◽  
pp. 419-424 ◽  
Author(s):  
VLADIMIR P. ZHDANOV

In the middle of the previous century, G. K. Boreskov, and J. Horiuti and S. Enomoto independently showed that for reversible reactions, running via a one-route mechanism with a rate-limiting step, there exist general relationships between the reaction rates in the forward and backward directions and also between the corresponding apparent activation energies and reaction heat. Their treatments are formally applicable to gas- and liquid-phase reactions and also to heterogeneous catalytic reactions (HCR) occurring in an ideal overlayer adsorbed on a uniform surface. In reality, HCR often run on heterogeneous surfaces, and the HCR kinetics are complicated by adsorbate–adsorbate lateral interactions. I explicitly demonstrate that in such situations the Boreskov–Horiuti–Enomoto rules are applicable as well.


2021 ◽  
Author(s):  
Sihang Liu ◽  
Nitish Govindarajan ◽  
Hector Prats ◽  
Karen Chan

Kolbe electrolysis has been proposed an efficient electrooxidation process to synthesize (un)symmetrical dimers from biomass-based carboxylic acids. However, the reaction mechanism of Kolbe electrolysis remains controversial. In this work, we develop a DFT- based microkinetic model to study the reaction mechanism of Kolbe electrolysis of acetic acid (CH3COOH) on both pristine and partially oxidized Pt anodes. We show that the shift in the rate-determining step of oxygen evolution reaction (OER) on Pt(111)@α-PtO2 surface from OH* formation to H2O adsorption gives rise to the large Tafel slopes, i.e., the inflection zones, observed at high anodic potentials in experiments on Pt anodes. The activity passivation as a result of the inflection zone is further exacerbated in the presence of Kolbe species (i.e., CH3COO* and CH3*). Our simulations find the CH3COO* decarboxylation and CH3* dimerization steps determine the activity of Kolbe reaction during inflection zone. In contrast to the Pt(111)@α-PtO2 surface, Pt(111) shows no activity towards Kolbe products as the CH3COO* decarboxylation step is limiting throughout the considered potential range. This work resolves major controversies in the mechanistic analyses of Kolbe electrolysis on Pt anodes: the origin of the inflection zone, and the identity of the rate limiting step.


2020 ◽  
Vol 5 (9) ◽  
pp. 1682-1693
Author(s):  
Kin Wai Cheah ◽  
Suzana Yusup ◽  
Martin J. Taylor ◽  
Bing Shen How ◽  
Amin Osatiashtiani ◽  
...  

Application of tetralin as a source of hydrogen for catalytic conversion of oleic acid to diesel-like hydrocarbons using a bimetallic Pd–Cu catalyst.


2013 ◽  
Vol 19 (2) ◽  
pp. 273-279 ◽  
Author(s):  
Gh. Bakeri ◽  
M. Rahimnejad

In this study, the kinetics of hydrazodicarbonamide (HDCA) synthesis reaction was investigated. Hydrazodicarbonamide is prepared by reaction of urea and hydrazine in acidic medium. Synthesis of HDCA from urea and hydrazine is a two steps reaction. In the first step, semicarbazide is synthesized from the reaction of one mole of urea and one mole of hydrazine and in the second step, semicarbazide reacts with urea to produce hydrazodicarbonamide. By controlling the temperature and pH in the reaction, hydrazine concentration and the amount of produced hydrazodicarbonamide were measured and using these data, reaction rate constants were calculated. Based on this study, it was found that the semicarbazide formation reaction from hydrazine is the rate limiting step. Rate of semicarbazide synthesis is -r1 = 0.1396 [NH2NH2]0.5810 and the rate of hydrazodicarbonamide synthesis is -r2 = 0.7715 [NH2NHCONH2]0.8430.


2007 ◽  
Vol 06 (02) ◽  
pp. 323-330 ◽  
Author(s):  
LAI-CAI LI ◽  
JUN-LING LIU ◽  
JING SHANG ◽  
XIN WANG ◽  
NING-BEW WONG

The reaction mechanism of the activation of ethane by nickel atom has been investigated by density functional theory (DFT). The geometries and vibration frequencies of reactants, intermediates, transition states and products have been calculated at the B3LYP/6-311 + +G(d, p) level. Two main pathways, C – C bond activation and C – H bond activation, are identified. In former channel, the rate-limiting step is found to be hydrogen-transferring step with a high barrier of 227 kJ · mol-1. In the C – H bond activation pathway, the second hydrogen-transferring step is the rate-determining step of the whole reaction. The barrier of the step is 71 kJ · mol-1. Our results show that the studied reaction would undergo along C – H bond activation pathway to form the products H 2 molecule and Ni ⋯ethene complex. The present theoretical work indicates that Ni atom is more active than Ni + cation in activating ethane.


1997 ◽  
Vol 62 (9) ◽  
pp. 1429-1445 ◽  
Author(s):  
Marek Janík ◽  
Vladimír Macháček ◽  
Oldřich Pytela

The cyclization kinetics of methyl S-(2,4,6-trinitrophenyl)mercaptoacetate to 2-methoxycarbonyl-5,7-dinitrobenzo[d]thiazol-3-oxide have been studied in acetate, methoxyacetate or N-methylmorpholine buffers. In the acetate and methoxyacetate buffers, the cyclization obeys the rate equation v = [SH](k'MeO[CH3O-] + k'B[B-] + k'B,MeO[B-][CH3O-]) and goes by two reaction paths differing in the order of their reaction steps, the splitting off of the proton from C-H group being the rate-limiting step in either path. In the N-methylmorpholine buffers, increasing concentration of the base results in gradual decrease of reaction order in the base and change in the rate-limiting step of cyclization. Methyl S-(2,4-dinitrophenyl)mercaptoacetate undergoes cyclization neither in the given buffers nor in methoxide solution.


1979 ◽  
Vol 44 (10) ◽  
pp. 3102-3110 ◽  
Author(s):  
Jaromír Kaválek ◽  
Karel Janák ◽  
Vojeslav Štěrba

5-Phenyl-1,3,4-thiadiazol-2-diazonium ion (I) is transformed into 2-amino-5-phenyl-1,3,4-thiadiazol (II) in diluted mineral acids. The reaction rate measured in solutions of diluted sulphuric acid reaches its maximum at concentrations of 2 to 2.5M-H2SO4. The reaction intermediate is 5-phenyl-1,3,4-thiadiazol-2-diazo hydroxide (III). The rate-limiting step in formation of III consists in base-catalyzed reaction of the diazonium ion I with water; it is 4.3 times slower in 0.1M-D2SO4 than in 0.1m-H2SO4. Ratio of the rate constants of the transformation of the diazo hydroxide III into the diazonium ion I and into the amine II increases rapidly with increasing sulphuric acid concentration.


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