Influence of chlorine on the decomposition of ethylene over iron and cobalt particles

The interaction of cobalt and iron powders with ethylene and ethylene/hydrogen mixtures containing trace concentrations of chlorine has been studied using a combination of flow reactor and transmission electron microscopy techniques. Detailed analysis of both the gaseous products and the amount of solid carbon (a filamentous form) deposited on the metal surfaces has permitted us to gain an insight into some of the factors surrounding the promotional effect of low concentrations of chlorine on the catalytic action of both cobalt and iron. The optimum carbon deposition activity was achieved when either of these metals was treated at 400 degrees C in an ethylene/hydrogen environment containing 75-100 ppm chlorine. If the halogen was removed from the reactant, then the high activity for carbon filament growth could not be sustained. Reintroduction of chlorine after a suitable period of time resulted in restoration of the carbon deposition activity to its original level, demonstrating the reversible nature of the ''activation-deactivation'' processes. The results of this study are rationalized according to the notion that the presence of adsorbed chlorine species is responsible for causing reconstruction of the metal surface; however, the possibility that the halogen is capable of inducing a perturbation in the electronic properties of the particles is also considered. It is possible that the charge transfer between adsorbed chlorine species and the metal surface atoms leads to a strengthening of the metal-ethylene bond and a concomitant weakening of the C-C bond in the olefin, making the latter more susceptible to decomposition and enhancing the formation of the solid carbon product.