Dual-Site Adsorption of Submolecular Fragments

This concept is illustrated best by considering the elementary steps involved when diatomic gas A2 dissociates and adsorbs atomically on two adjacent active sites. Two possibilities exist  

In (14-42a), diatomic A2 dissociates in the gas phase and then each atomic fragment adsorbs on a single site. There is no requirement that both sites must be adjacent to each other. In (14-42Z ), one end of diatomic A2 adsorbs on a single site. Then, dissociation occurs and the other atomic fragment adsorbs on an adjacent site. Both possibilities can be represented as follows if atomic A adsorbs on a single site  

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

The methanol synthesis from carbon monoxide and hydrogen represents an example where diatomic reactants produce a polyatomic product that contains only one bond that was present originally in the reactants. One molecule of C=0 chemisorbs on the catalytic surface via the carbon atom and two molecules of H2 must dissociate before forming three C-H bonds and one 0-H bond. 

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. 

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