Investigation of Surface Phenomena by Radioactive Tracers

This thesis describes investigations of a catalyst system which have been made by means of a new radioactive-tracer technique whereby ethylene-C14 was observed directly on the surface of a nickel film during hydrogenation. A thin-wall G.-M. counter of special design, capable of detecting the radiation from carbon-14, was mounted within a catalyst vessel in such a way that the surface of an evaporated nickel film could be monitored directly. In this way the adsorption and desorption of carbon-14 labelled ethylene at the surface could be followed continuously. Hydrogen and non-labelled ethylene were admitted to the catalyst on which ethylene-Cl4 had been adsorbed, and the change in the count rate due to the adsorbed ethylene-C14 was followed continuously as hydrogenation proceeded. It was found that only a fraction of the preadsorbed ethylene was removed from the catalyst surface during hydrogenation; the remainder was firmly held during prolonged hydrogenation of the non-labelled ethylene. Experiment also showed that there was negligible exchange between adsorbed and gas-phase ethylene. A number of films were prepared by evaporation in an inert atmosphere; the results of experiments on these films have provided new information on the role of film orientation and the effect of particular crystal planes on catalysis. A series of experiments was carried out which involved the adsorption on the nickel film of both labelled and non-labelled ethylene. By investigating the effect of different admission sequences on the fraction of the adsorbed ethylene-C14 removed during hydrogenation it has proved possible to arrive at conclusions as to the factors which influence the magnitude of the active ethylene fraction. The results of this research have implications for the correlation between chemisorption and catalysis