Hydrogen reduction kinetics modeling of a precipitated iron Fischer–Tropsch catalyst

Abstract

Mechanisms and kinetics for the reduction of a precipitated iron-based Fischer–Tropsch catalyst in H 2 have been investigated using in situ Mössbauer effect spectroscopy (MES) and thermogravimetric (TG) method in the temperature range of 250–350 °C. In situ MES results indicate that the reduction of paramagnetic (PM) α-Fe 2O 3 (70%) and superparamagnetic (spm) Fe 3+ (30%) in the fresh catalyst proceed via different steps. PM α-Fe 2O 3 is firstly reduced to magnetite and then to metallic iron, while the reduction of spm Fe 3+ proceeds in three consecutive steps: it is first reduced to magnetite with a significantly rapid rate, then to non-stoichiometric wüstite, and finally to metallic iron. The reduction of PM α-Fe 2O 3 to Fe 3O 4 can be described by a two-dimensional Avrami–Erofe’ ev phase change model (formation and growth of nuclei). However, the corresponding overall reduction, which includes the reduction of PM α-Fe 2O 3 and spm Fe 3+ to Fe 3O 4, can be described by a three-dimensional phase-boundary-controlled reaction model based on the overall extraction ratio of oxygen. The difference between the two models selected for the PM α-Fe 2O 3 reduction and the corresponding overall reduction is attributed to the rapid reduction of spm Fe 3+ to Fe 3O 4. For the reduction of PM magnetite to α-Fe and its corresponding overall reduction (including the reduction of PM and spm Fe 3O 4 to α-Fe), it is found that both of them follow the Avrami–Erofe’ ev phase change model (two-dimensional or three-dimensional). The value of apparent activation energy for the overall reduction has been calculated and compared with the literature data.