Study of the carburization of an iron catalyst during the Fischer-Tropsch synthesis: Influence on its catalytic activity

Abstract

The fast transformation of an iron/alumina catalyst into Fe 2+ x C during the H 2 + CO reaction was followed by in situ Mössbauer spectroscopy at 523 K and the behavior was compared with changes in catalytic activity for Fischer-Tropsch synthesis. After a few hours of synthesis, no metallic iron could be detected by either Mössbauer or IR spectroscopy, whereas the CO conversion was still half of that observed initially. The nature of the sites responsible for the remaining activity is discussed. The interpretation of the Mössbauer spectra has permitted the determination of the stoichiometry of the Fe 2+ x C carbide (0 ≤ x ≤ 0.4), and hence the following of the change of x during the reaction. Thus, the activity in CO hydrogenation could be related to the number of carbon vacancies in the iron carbide, i.e., to the extent of the metallic character of this carbide. This concept of the variation of hydrogenating properties of the carbide with the carbon content has been supported by similar catalytic results obtained in the room temperature hydrogenation of ethylene performed on carbides of different stoichiometry.