Nucleation of the Fe3C in reaction of methane with nanocrystalline iron

The carburization process of nanocrystalline iron in a flow of CH4/H2 mixture under atmospheric pressure at 580 °C in a differential reactor–thermobalance was studied. The course of reaction was followed by thermogravimetry, and the phase composition of the samples carburized to different degrees was determined by x-ray diffraction (XRD) and Mössbauer spectroscopy techniques. The XRD method was also used for calculating the mean crystallite size of unconverted iron after reaction at different time intervals. An unexpected relation between the average size of iron crystallites and the degree of conversion was found. The nucleation mechanism of the nanocrystalline iron carbide in the kinetic area of the reaction, limited by the dissociative adsorption of methane, has been suggested. According to this mechanism, iron crystallites are carburized successively, from the smallest to the largest.