Surface potential studies in chemisorption and catalysis

• Main Author: Delchar, Trevor Anthony
• Published: Imperial College London 1963
• Subjects: 541
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602181

A static capacitor method has been developed for study of surface potential changes during adsorption on metal films. The metal film forms the outer cylinder of a cylindrical capacitor across which is maintained a constant field. Perturbation of this field by an adsorbed gas layer results in rapid automatic compensation by a servomechanism. The compensating voltage is equal to the contact potential change to within 0.2%, and is simultaneously displayed on a pen recorder. The preparation and use of a new reference surface is described. The rapid response of the measuring system has been utilised to examine transient surface potential changes occuring at the instant of interaction of a gas with a metal surface. These transient changes are identified with two phenomena a) Formation of a weakly held 2nd layer which diffuses across the adatom layer and becomes chemisorbed, this behaviour is found for oxygen and hydrogen on nickel at 77°K and 90°K and for nitrogen on tungsten at 77°K. It is not found for CO on nickel and copper, or for oxygen on copper. Activation ener- -gies for diffusion have been derived and are of the order of Kcal. b) Addition of oxygen to nickel or copper films at temperatures greater than 90°K is accompanied by incorporation in the metal lattice. The kinetics of incorporation have been examined. It is suggested that the surface field is the controlling factor in incorporation. Some preliminary experiments on nickel and copper oxide are described as well as some on mixed gas studies. Surface potential changes on oxides are found to be small and positive. Surface potential changes for nitrogen on tungsten show that the recorded S.P. depends on the temperature of measurement and the temperature of addition. An attempt is made to explain this behaviour in terms of the alpha, beta and b states.