Study of the oxide/carbide transition on iron surfaces during catalytic coke formation

Abstract

Abstract The thermodynamic equilibria between metallic iron, iron oxides, iron carbides and a hydrocarbon/hydrogen mixture were calculated at 600°C. On the basis of metastable Fe–C–O phase diagram, iron oxides can be converted directly into carbides in a reducing and carburizing atmosphere. Experimental results on the rate of oxide/carbide conversion are reported. Thermogravimetric measurements have been performed in an iC 4 H 10 –H 2 –Ar atmosphere at 600°C on preoxidized iron samples. The kinetics of the oxide layer transformation were studied by sequential exposure experiments. Scanning electron microscopy observations and x‐ray diffraction analysis have been carried out. The results lead to the conclusion that magnetite (Fe 3 O 4 ) is transformed into carbide particles, acting as a catalyst for graphitic filament growth. The initial stages of the oxide/carbide transition were studied by x‐ray photoelectron spectroscopy. The results confirm that no metallic iron was formed during the transformation. Copyright © 2002 John Wiley & Sons, Ltd.