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Introduction to Heterogeneous Catalysis in Organic Transformation
Published in Varun Rawat, Anirban Das, Chandra Mohan Srivastava, Heterogeneous Catalysis in Organic Transformations, 2022
Garima Sachdeva, Gyandshwar Kumar Rao, Varun Rawat, Ved Prakash Verma, Kaur Navjeet
A second important reaction mechanism was proposed by Eley and Rideal [20]. The Eley–Rideal mechanism explains a reaction between a chemisorbed reactant and a non-chemisorbed reactant. An Eley–Rideal reaction is distinguished by the fact that one of the reactants is not chemisorbed locally and so not in equilibrium with the surface. According to this mechanism, only one of the reactants is adsorbed on the surface in this process, and it combines immediately with an incoming molecule from the gas phase, bypassing the need for an adsorption site. Since, the gas-phase temperature does not have to be the same as that of the surface, the reaction can be classified as non-thermal (Figure 1.5(II).
Theoretical and experimental investigation of the influence of intraphase diffusion on the oxidation of toluene over manganese-based oxide
Published in Environmental Technology, 2022
Z. Gomzi, M. Duplančić, V. Tomašić
Knowledge of intrinsic reaction kinetics is essential for reactor performance analysis and reactor model development. Several kinetic models have been used in the literature to describe the catalytic oxidation of volatile organic compounds and toluene as their representatives. In general, it is possible to use a simple empirical power-low kinetics and a more complex mechanistic approach based on the proposed reaction mechanism. The simple power-low rate equation describes the oxidation rate as a function of a rate constant and the concentrations (or pressures) of the reactants. Unlike other industrially significant catalytic oxidation reactions, catalytic oxidation of volatile organic compounds (VOCs) generally occurs in excess oxygen (or air). Therefore, the concentration or partial pressure of oxygen remains unchanged, and the oxidation rate depends only on the concentration/pressure of the reactant (toluene in our case). On the other hand, more complex mechanistic kinetic models can be proposed to describe the reaction rate in more detail, including the Langmuir–Hinshelwood (L-H) and the Eley-Rideal (E-R) mechanisms, and the Mars-van Krevelen expression (MvK). Different types of reaction mechanisms can be proposed and appropriate kinetic models can be derived for each of them, taking into account the nature of the interaction between the reactants and the active sites on the catalytic surface, the rate-limiting step, the number of species in the rate-determining step, etc. [3, 15]. The Langmuir–Hinshelwood mechanism is often used in the literature, where the reaction occurs between the adsorbed oxygen species and the adsorbed reactants. The controlling step is the surface reaction between two adsorbed molecules at analogous active sites, and the final form of the mechanistic kinetic model usually depends on the nature of the adsorption of the reactant molecules on the catalyst surface (associative or dissociative adsorption of toluene and oxygen at active sites). It is also possible to use the Eley-Rideal mechanism, in which the reaction proceeds between adsorbed reactant molecules and oxygen molecules in the gas phase. The controlling step is usually the reaction between an adsorbed molecule and a gas-phase phase molecule. Some researchers use the Mars-van Krevelen expression based on the mobility of surface lattice oxygen and the formation of oxygen vacancies on the catalyst surface [16, 17]. According to Vannice [18] the MvK expression is the form of rate expression that has no physical relevance and must be considered only as a mathematical fitting function. It should be emphasized that validation of mechanistic models requires independent determination of adsorption constants, which is always difficult. Therefore, in this work reaction of toluene oxidation was approximated by the first-order reaction.