Promotional Effect of Carbon Monoxide on the Decomposition of Ethylene over an Iron Catalyst

Abstract

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 C 2H 4 over iron, an unexpected pattern of behavior was found. Exposure of the metal to a C 2H 4-H 2 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 C 2H 4. Maximum reactivity was achieved with a C 2H 4-CO-H 2 (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 C 2H 4 to C 2H 6. 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.