Langmuir-Rideal mechanisms

The Hougen-Watson kinetic model that is consistent with the Langmuir-Rideal mechanism can be obtained from the rate law in equations (14-63) and (14-64) via the following modification of each generic term ... 

Consider the three-step Langmuir-Rideal mechanism of heterogeneous catalysis ... 

Figure 6.19 shows the partial-pressure-dependent reaction rates for a unimolecular reaction (to be derived as an exercise), for bimolecular reaction (derivation given above), and for a Langmuir-Rideal mechanism (explained below). 

In the Langmuir-Rideal mechanism (also shown in Figure 6.19), a molecule from the gas phase reacts with an adsorbed species although both reactant molecules are adsorbed and competitively occupy surface sites. In this model there is no surface reaction between adsorbed species. 

Mechanisms in which a gas molecule reacts with an adsorbed molecule of the other kind are known as Langmuir-Rideal mechanisms. If, for example, the interaction is between gaseous A and adsorbed B the rate is proportional to [A]6>b and is therefore... 

For the case of two substances that react with each other at a solid surface we consider two possible mechanisms. If only one of the reactants is adsorbed the mechanism is called the Langmuir-Rideal mechanism ... 

If both of the reacting molecules are adsorbed and if at least one of them can move around on the surface, a reaction between two adsorbed molecules can occur. This mechanism is called the Langmuir-Hinshelwood mechanism. It is said to occur more commonly than the Langmuir-Rideal mechanism. It can be represented by the mechanism... 

Similar equations were written by Eley [204] for the exchange of N2 with N2 catalyzed by Fe or W, and mechanisms such as Eq. XVIII-33 have come to be known as Eley-Rideal mechanisms. Mechanisms such as that of Eq. XVIII-32 are now most commonly called Langmuir-Hinshelwood mechanisms (see... 

Derive the probable rate law for the reaction CO + j02 = CO2 as catalyzed by a metal surface assuming (a) an Eley-Rideal mechanism and (b) a Langmuir-Hinshelwood mechanism. 

Figure A3.9.1. Schematic illustrations of (a) the Langmuir-Hinshelwood and (b) Eley-Rideal mechanisms in gas-surface dynamics.

Rettner C T 1994 Reaction of an H-atom beam with Cl/Au(111)—dynamics of concurrent Eley-Rideal and Langmuir-Hinshelwood mechanisms J. Chem. Phys. 101 1529... 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

Mechanistic kinetic expressions are often used to represent the rate data obtained in laboratory studies, and to explain quantitatively the effects observed in the field. Several types of mechanisms have been proposed. These differ primarily in complexity, and on whether the mechanism assumes that one compound that is adsorbed on the catalyst surface reacts with the other compound in the gas phase, eg, the Eley-Rideal mechanism (23) or that both compounds are adsorbed on the catalyst surface before they react, eg, the Langmuir-Hinshelwood mechanism (25). 

Whether a catalytic reaction proceeds via a Langmuir-Hinshel vood or Eley-Rideal mechanism has significant implications for the kinetic description, as in the latter case one of the reactants does not require free sites to react. However, Eley-Rideal mechanisms are extremely rare, and we will assume Langmuir-Hinshelwood behavior throughout the remainder of this book. 

What is the essential difference between the Langmuir-Hinshelwood and Eley-Rideal mechanisms Which of the two is the more likely mechanism ... 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

Fig. 1. Variation of rate with reactant pressure for a bimolecular surface reaction proceeding by a Rideal—-Eley or a Langmuir—Hinshelwood mechanism.

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