The distribution of reaction rates and activation energies on catalytic surfaces: Exchange reaction between gaseous benzene and benzene adsorbed on platinum

Abstract

A mathematical derivation for the calculation of the distribution function of rate constants and activation energies for an isotopic exchange reaction between chemical species adsorbed on a solid surface and in the surrounding gas phase is presented. Starting from experimental kinetic isotherms, giving the rate of exchange as a function of time at various temperatures, it is shown how it is possible to derive the distribution functions of the rate constants and activation energies. The technique is applied to the exchange reaction between benzene molecules adsorbed on Pt black and molecules present in the gas phase. Experimental results on the rate of this exchange, obtained by means of C-14 labeled benzene, have been collected at 40 [deg], 60 [deg], 80 [deg], and 100 [deg] C, at constant surface coverage. The range of activation energies for the exchange reaction was found to be from about 10 to 35 kcal/mole, while the pre-exponential factor varied from about 107 to 1026 min-1. Additional observations on the chemisorption of benzene on Pt black are presented. The effects of various pretreatments of the Pt surface and of aging of the chemisorbed benzene on the exchange rate are discussed.