PROMOTIONAL EFFECT OF CARBON-MONOXIDE ON THE DECOMPOSITION OF ETHYLENE OVER AN IRON CATALYST

We have used the growth of filamentous carbon in conjunction with the gas-phase product distribution as a probe of the reactivity of a powdered iron catalyst when exposed to a carbon-containing gas environment. During the course of a series of reactions designed to examine the differences in carbon deposition characteristics exhibited by CO and C2H4 over iron, an unexpected pattern of behavior was found. Exposure of the metal to a C2H4-H2 mixture at 600°C resulted in only minor decomposition of the olefin; however, upon addition of a small amount of CO to the system, there was a dramatic increase in the amount of filamentous carbon formed, of which the major fraction could be attributed to decomposition of C2H4. Maximum reactivity was achieved with a C2H4-CO-H2 (3:1:1) mixture, and it was apparent that CO was responsible for not only promoting the formation of solid carbon, but also inducing the conversion of C2H4 to C2H6. Removal of CO from the system resulted in a rapid decline in catalytic activity, however, upon reintroduction of CO activity was restored to its initial high level, indicating that the reversible nature of the activation-deactivation processes. This behavior is rationalized in terms of a reconstruction of the iron surface in the presence of coadsorbed CO which results in the possible formation of various faces with differing reactivity characteristics, and a relaxation when CO is removed from the reactant. © 1993 Academic Press, Inc.