Role of chemisorption in simple catalytic reactions

Abstract

Hydrogen and oxygen sorption and their interaction have been studied on evaporated films of Pt, Pd, Rh, Ni, Fe, Mo, Cu and Mn. Hydrogenation of cyclopropane has been followed on films of Ni, and sorption and interaction of hydrogen and nitrogen have been measured on films of Fe. Oxygen adsorbed on nickel does not react at 78°K with hydrogen adsorbed subsequently, but restricts the hydrogen adsorption roughly proportional to the degree of the surface coverage by oxygen. At room temperature there is extensive interaction of the preadsorbed oxygen with subsequently admitted hydrogen only if part of the surface had been left uncovered by oxygen, otherwise atomization of H₂ in the gas phase in necessary. Platinum behaves in the same way even at 78°K. With molybdenum there is no interaction between the preadsorbed oxygen and admitted hydrogen at 273°K even if hydrogen is present in the adsorbed state, atomization is again necessary.

Products of the interaction at 273°K differ with various metals in a characteristic way. With Ni and Mo the reaction is stopped after all oxygen had already reacted and the reaction cannot be repeated on the same surface at this temperature. With Pt, Pd and Rh, however, the sequence of oxygen and hydrogen sorption can be repeated without limitation. Cu behaves differently. There is no hydrogen adsorption and no interaction with oxygen even on surfaces not fully precovered by oxygen. We conclude that hydrogen enters the reaction with oxygen always in the adsorbed state. There seems no connection between the efficiency of the metal in this reaction and the presence or absence of hydrogen causing a “positive” effect in the change of the film conductivity. Two conditions are required if a metal is to be active in the O₂+ H₂ reaction: (i) all reactions must be adsorbed, (ii) their heat of adsorption has to be low.

In hydrogenation of cyclopropane on nickel a fast reaction takes place only on a very small portion of the surface, while on the remaining part the dissociated and dehydrogenated species are removed slowly. Thus, various adsorbed particles play a different role in the catalytic reaction. Further results show that adsorption is a necessary prerequisite for a catalytic reaction, but itself does not represent a sufficient condition. In all systems studied, adsorption of both components is largely competitive.