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

The thermodynamic equilibria between metallic iron, iron oxides, iron carbides and a hydrocarbon/hydrogen mixture were calculated at 600degreesC. 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 600degreesC 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 (Fe3O4) 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 (C) 2002 John Wiley Sons, Ltd.