Investigations on AgI catalyst surface structure by isotope exchange reactions

Abstract The authors have investigated the isotopic exchange reaction between solid AgI precipitates and CH 3 131 I vapour and the dependence of the exchange reaction upon the preparation—and at the same time upon the surface structure—of the solid catalyst. Precipitates have been prepared in a well defined way so that Ag + and I − in excess as well as “neutral” surfaces were obtained. From the investigation it appears that on positively charged surfaces the exchange goes further and more rapidly compared to the negatively charged surfaces. The mechanism of the exchange also varied with the surface structure. It has been found that the exchange is preceded by an adsorption equilibrium and takes place between the adsorbed intermediate and the solid surface. This is followed by desorption. In properly chosen experimental conditions (temperature, pressure) the aforementioned elementary steps can be separated and thoroughly studied. On the basis of a mathematical analysis of the kinetic curves it has been shown that the AgI crystals took part in the reaction as a uniformly heterogeneous surface. Among the elementary steps the exchange itself is a complex one. On surfaces with a Ag + excess it can be realized as an S N 1-type nucleophyllic substitution and the first step of this is a slow heterolysis, while on surfaces with an I − excess the exchange process takes place as a pseudo- S N 2-type reaction by an electron transfer.