Catalysis, Langmuir-Hinshelwood mechanism

This Langmuir-Hinshelwood mechanism is the one most commonly encountered in the heterogeneous catalysis of gas reactions and the appropriate rate expressions for various special cases are well known [9, 31, 42], In general, we may write... 

Keywords Photocatalysis and catalysis Photocatalytic activity Band structure and excitation Energy conversion Langmuir-Hinshelwood mechanism Electron-hole recombination Quantum efficiency Physical property-activity correlation Synergetic effect. 

According to the lUPAC, the Langmuir-Hinshelwood mechanism is defined as a mechanism for surface catalysis in which the reaction occurs between species that are adsorbed on the surface. This mechanism is expected to exhibit a second order kinetics with respect to the surface coverage of the two reactants. 

The simplest mechanism for interpreting critical phenomena in heterogeneous catalysis is the Langmuir adsorption mechanism, also referred to as the Langmuir-Hinshelwood mechanism. This mechanism includes three elementary steps (1) adsorption of one type of gas molecule on a catalyst active site (2) adsorption of a different type of gas molecule on another active site (3) reaction between these two adsorbed species. For the oxidation of carbon monoxide on platinum, this mechanism can be written as follows ... 

The quasi-equilibrium assumption is frequently used in the heterogeneous catalysis, since the surface reaction steps are often rate-Hmiting, while the adsorption steps are rapid. This is not necessarily true for large molecules. Here we consider the application of the quasi-equilibrium hypothesis on two kinds of reaction mechanisms, an Eley-Rideal mechanism and a Langmuir-Hinshelwood mechanism. The rate expressions obtained with this approach are referred to as Langmuir-Hinshelwood-Hougen-Watson (LHHW) equations in the literature, in honor of the pioneering researchers. 

The catalytic oxidation of CO on a Pt snrface has been extensively studied in view of its relevance to automotive gas pollution. Moreover, its relative simplicity motivated its extensive use as a model systan for heterogeneous catalysis. Molecular beam studies of the catal3dic oxidation of CO on the plane (111) on platinum indicate that the reaction follows a Langmuir-Hinshelwood mechanism [2],... 

All heterogeneous catalysis relies on the existence of reactions between adsorbed molecules or atoms (the Langmuir-Hinshelwood mechanism) or between adsorbed molecules or atoms and free molecules (the Rideal mecharrism). 

A reason for using microkinetics in heterogeneous catalysis is to have comprehensive kinetics and a transparent reaction mechanism that wonld be useful for re or design or catalyst development. Furthermore, in the long run, the exparimental effort to develop a microkinetics scheme can be less than that for a Langmuir-Hinshelwood (LH) or powa--law scheme because of the more fundamental nature of the reaction kinetics parameters. 

The electrooxidation of CO to C02 is, similar to its electroless counterpart in gas-phase catalysis, one of the most widely studied electrochemical reaction processes [131,152]. It is generally assumed that the electrooxidation of adsorbed CO proceeds primarily via a Langmuir-Hinshelwood type mechanism involving either adsorbed water molecules or, at higher electrode potentials, adsorbed surface hydroxides (see Figure 6.25) according to... 

The reaction mechanism is based on the Langmuir-Hinshelwood model for heterogeneous catalysis. This represents sorption and desorption of reactants and products as equilibria the ligand-exchange reaction is considered rate limiting. By... 

In the case of photogenerated catalysis, two different but equivalent models are worth considering the Langmuir-Hinshelwood photocatalytic process and the Eley-Rideal photocatalytic process. The former is described by Mechanism II, in which the reaction occurs at a photochemically active surface when light is absorbed by the catalyst and leads to the generation of surface electrons (e ) and holes (h" ). 

The turnover rate becomes proportional to the surface coverage 0, Equation (1.6) is the Langmuir - Hinshelwood expression and is very similar to the Michaelis - Menten expression used in enzyme catalysis. Consider the following mechanism describing... 

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