RATE OF HETEROGENEOUS CATALYTIC REACTIONS INVOLVING IONIC INTERMEDIATES

A solution to the problem of the computation of the rate law of a catalytic reaction involving ionic intermediates and taking into account the ionic nature of the surface species is proposed in this work. It is shown that such a rate law is computable within the Langmuir-Hinshelwood-Hougen-Watson (LHHW) theory when an electric charge conservation equation is added to the usual set of equations. Using the approximation that the number of sites equal the number of negative charges of the surface species and neglecting the contribution of the bulk, a rate law can be computed exactly. This is illustrated by considering four examples of catalytic reactions involving ionic intermediates, namely the isomerization of unsaturated hydrocarbons by Bronsted acids, the hydrogenation of unsaturated hydrocarbons involving the heterolytic dissociation of hydrogen, the generation of methyl radicals in methane oxidative coupling, and the partial oxidation of hydrocarbons. With no approximations the rate law obtained is complex and includes, in an intricate manner, several initial concentrations of surface and bulk species characterizing the catalyst.