Rate expressions for bimolecular surface reactions

Throughout this section, it will be assumed that the mass transfer of reactants and products and the adsorption of reactants or desorption of products are not rate-limiting. 

For a reaction proceeding via a Langmuir—Hinshelwood mechanism A(ads.) + B(ads.) - Products 

If the pressure of B is kept constant and PA varied, the rate will change 

The reaction becomes first order in each reactant and second order overall. The hydrogenation of ethylene over copper [27] under certain conditions obeys such a rate law. If reactant A is weakly adsorbed, then bAPA 1 + bBPB and the relevant rate equation is 

Examples of the applicability of eqns. (7) and (8) are, respectively, the hydrogenation of carbon dioxide over platinum [28] and the hydrogenation of ethylene over copper [29] and platinum [30] under certain conditions. There are, of course, an infinite variety of intermediate cases lying between the extreme conditions considered above. 

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