Dual-site adsorption

A palladium-hydrogen-mordenite catalyst with a 10.8/1 silica/alumina mole ratio was evaluated for the hydroisomerization of cyclohexane. The rate of reaction followed a first-order, reversible reaction between cyclohexane and methylcyclopentane. The energy of activation for this reaction between 400° and 500°F was 35.5 it 2.4 kcal/mole. Cyclohexane isomerization rates decreased with increasing hydrogen and cyclohexane-plus-methylcyclopentane partial pressure. These effects are compatible with a dual-site adsorption model. The change of the model constants with temperature was qualitatively in agreement with the expected physical behavior for the constants. 

Pignet and Schmidt have fitted a Langmuir-Hinshelwood model for non-competitive dual-site adsorption and with dissociative oxygen adsorption ... 

In order to confirm their proposed mechanism (surface reaction between chemisorbed benzene and chemisorbed hydrogen), poisoning experiments were employed using thiophene as a poison. This showed that the deactivation rate decreases with increasing hydrogen and benzene partial pressures, and hence they concluded from their data that hydrogen and benzene are both chemisorbed on the catalyst surface and their proposed dual-site adsorption kinetic model is suitable to describe the hydrogenation of benzene on nickel. 

Zmcevic and Rusic (1988) used commercial nickel on a silica-alumina catalyst (code Rl-10, manufactured by BASF) containing 21% Ni. They proposed a mechanism based on the dual site adsorption model followed by a bimolecular surface reaction which is the rate-controlling step for the formation of cyclohexane from benzene. The rate equation obtained has the form ... 

Examples of dual-site adsorption of submolecular fragments are mentioned below, where each fragment adsorbs on a single site. 

Methane CH4 could fragment and undergo dual site adsorption as a methyl radical CH3 and a hydrogen radical H. Ammonia NH3 could fragment as NH2 and -H and adsorb on two active sites. 

Heterogeneons Catalytic Rate Laws Controlled by Dual-Site Adsorption of a Reactant... 

Components B, C, D, and A2B experience single-site adsorption, whereas diatomic A2 undergoes dual-site adsorption. Triple-site chemical reaction on the catalytic surface equilibrates on the time scale of the adsorption of reactant A2. The modified Langmuir-Hinshelwood mechanism is described by the following sequence of six elementary steps ... 

The Hougen-Watson model that is consistent with the six-step mechanism given by (14-138) is constructed by focusing on dual-site adsorption of A2. Hence,... 

This rate-limiting step is third order in the forward direction (i.e., adsorption) and second order in the reverse direction (i.e., desorption). Since two active sites are required for dual-site adsorption, one predicts that the kinetic model is proportional to the square of the vacant-site fraction. Substitution for a from (14-144) leads to... 

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