Selective Poisoning of Catalytic Activity

Abstract

A RECENT investigation¹ has shown that the exchange reaction between alkylbenzenes and deuterium may provide a useful method of investigating changes in the catalytic activity of evaporated metal films subjected to thermal sintering. One of the most suitable alkylbenzenes for this purpose is p-xylene because it contains only two different kinds of hydrogen atoms—the six in the methyl groups and the four on the benzene ring. Nickel films laid down on a glass substrate at 0° C will catalyse the exchange of the first six hydrogen atoms (reaction I) about five times faster than the exchange of the remaining four (reaction II) at 0° C. Pre-sintering such nickel films for 30 min at 200° C before use reduces their activity for reaction I by a factor of about 600 but has an even more drastic effect of about 10⁵ on the rate of reaction II. The earlier work did not establish whether these marked changes of catalytic activity on sintering were due solely to the influence of temperature on the films or whether contamination, and consequent poisoning of the films, had occurred by gas evolved from the reaction vessel. Because of this uncertainty, we have now carried out a series of experiments on the influence of carbon monoxide as a poison for reactions I and II.