Kinetics of the iron carbide formation in the reaction of methane with nanocrystalline iron catalyst

The kinetics of the formation of iron carbide was studied on a doubly and triply promoted nanocrystalline iron catalyst under the flow of methane/nitrogen or methane/hydrogen gas mixture. The catalyst samples were characterised using XRD, AES-ICP, low-temperature nitrogen adsorption and oxygen chemisorption. The carburisation process was studied using thermogravimetry, through the measurements of the mass increase. The studies were carried out in a thermobalance (differential reactor) under atmospheric pressure. Contrary to other reported studies of the methane dissociatior, these were carried out at low temperature (500-580degreesC). The use of the nanocrystalline iron allowed neglecting the effect of diffusion on the process rate. The limiting step of the carburisation process is the dissociative adsorption of methane on the iron surface. The overall rate of :he process is the difference between the rate of the iron carbide formation and that of the decarburisation. The rate of the process depends on the chemical composition of the catalyst. The difference in activity between doubly and triply promoted iron catalyst was explained taking into account the role of the surface oxygen. The apparent activation energy does not depend on the chemical composition of a catalyst and it is equal E-a = 158 0 mol(-1) for the iron carbide formation and Ea = 132 U mol(-1) for the decarburisation process. The new model of an active site for the carburisation process is proposed. The site is composed of four surface iron atoms-one of them is uncovered and three of them are covered by oxygen atoms. (C) 2004 Elsevier B.V. All rights reserved.