Reaction between two adsorbed molecules

For a surface bimolecular reaction to occur, the reactants or intermediates must have enough energy, usually through thermal activation, to surmount an activation barrier. 

Additionally, the reactants or intermediates must be in close proximity to react, while for molecular (as opposed to atomic) species, the relative orientation can also be critical. The frequency with which all of these conditions are met determines the reaction rate. 

For a surface reaction between two reactants A and B, there are two distinct mechanisms Langmuir-Hinshelwood and Rideal-Eley. In the former mechanism, the reaction occurs between A and B when both are adsorbed on the surface. In the latter, the reaction occurs between an adsorbed molecule and a molecule in the gas phase. These mechanisms may be distinguished on the basis of the corresponding kinetic equations. 

The rate of reaction of the Langmuir-Hinshelwood mechanism is proportional to the fractions of the molecules of A and B that are adsorbed. 

These fractions are given by eqs. (10.17) and (10.18), such that the rate is therefore 

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Example 18 is a reaction between two adsorbed molecules of C2H3D. Aldag, Lin, and Clark (13) postulated that the reactant molecules are mobile and that both reactants and activated complex are free to rotate. That is. Step 9 is the rate-determining step. For that step logL = 12, a reasonable value. 

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... 

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