Eley-Rideal type adsorption

The kinetics of the ethylene oxidation are rather complicated as they depend not only on ethylene and oxygen pressure but also on the concentration of the reaction products. These influence the rate by adsorption competition with the reactants. Moreover, different forms of adsorbed oxygen may occur on the catalyst surface. Consequently, the rate equations proposed in the literature consist of either Langmuir—Hinshelwood and Eley—Rideal types or power rate models with non-integer coefficients. Power rate models are less appropriate as their coefficients inevitably depend on the reaction conditions. 

Presentation of a Reaction Mechanism. In the Na0 -CO reactions, after the reaction had achieved a steady state, the reaction gas mixture was switched into either a pure helium stream or a 02-He stream. The responses of C02 and CO were then followed. The CO(dec. , 0)-CO response obtained instantaneously responded zero with no delay, indicating that there was no reversibly adsorbed CO. Furthermore, the C0(dec.,0)-C02 response obtained was not affected by the presence of 02 in the stream, suggesting the nonexistence of irreversible adsorption of CO which could react with oxygen. Thus, a model of the direct reaction of gaseous CO with adsorbed oxygen, an Eley-Rideal type mechanism, may be proposed. 

Within the frameworks of the lattice gas model it is reasonable to classify the elementary processes by the number of sites m, which a given process occurs on, i.e., one- and two-site cases. In the first case the changing parameter is the occupancy state of one site. The processes such as these include isomerization associated with changes in the internal degrees of freedom of the adspecies (ZA- ZB, i.e., transition of the adspecies from state A to state B), adsorption-desorption of the atoms and nondissociating molecules (A + Z- ZA), reaction according to the collision mechanism (A + ZB ->ZD + C, Eley-Rideal s-type mechanism). It should be remembered that ZA, Z and A denote adspecies A, empty lattice site and species A in the gaseous phase, respectively. 

From adsorption measurements during reaction one can also examine whether the reaction rate is correlated with the amounts of adsorbed reactants or with the pressure of the reactants, i.e., whether the mechanism is of Langmuir-Hinshelwood type or of the Eley-Rideal variety. 

Closure. After reading this chapter, the reader should be able to discuss the steps in a heterogeneous reaction (adsorption, surface reaction, and desorption) and describe what is meant by a rate-limiting step. The differences between molecular adsorption and dissociated adsorption should be explained by the reader, as should the different types of surface reactions (single site, dual site, and Eley-Rideal). Given heterogeneous reaction rate data, the reader should be able to analyze the data and to develop a rale law for Langmuir-Hinshelwood kinetics. The reader should be able to discrimi-... 

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