The evolution of Fe phases of a fused iron catalyst during reduction and Fischer–Tropsch synthesis

Abstract

The reducing temperature and gas hourly space velocity were varied to obtain different Fe phase compositions for a fused-iron catalyst. These model catalysts were subject to the Fischer–Tropsch synthesis (FTS) and phase evolution was monitored. The BET surface area decreased as the reduction temperature was increased from 350 °C to 650 °C, and the pore diameter changed by the opposite trend. XRD and Mössbauer spectroscopy were used to quantitatively determine the phase composition. They indicated that a high reduction temperature and high GHSV (gas hourly space velocity) favored the transformation from Fe₃O₄ to α-Fe, although a higher reduction temperature led to larger α-Fe crystallite sizes. Furthermore, α-Fe gradually transforms to an iron carbide (χ-Fe₅C₂) phase in a period of 12 h when exposed to syngas (CO + H₂). After a 500 h run in the FTS, the partial carbide regenerated into Fe₃O₄ again. Magnetite had negligible catalytic activity for the FTS, whereas iron carbide was active. Nonetheless, the activity was proportional to the surface area of iron carbide. Higher reduction temperatures guaranteed lower methane selectivity and a higher olefin to paraffin ratio for C_2–4 hydrocarbons, and possibly high reducibility and large pore sizes as well.