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

Mechanisms and kinetics for the reduction of a precipitated iron-based Fischer–Tropsch catalyst in H2 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-Fe2O3 (70%) and superparamagnetic (spm) Fe3+ (30%) in the fresh catalyst proceed via different steps. PM α-Fe2O3 is firstly reduced to magnetite and then to metallic iron, while the reduction of spm Fe3+ 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-Fe2O3 to Fe3O4 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-Fe2O3 and spm Fe3+ to Fe3O4, 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-Fe2O3 reduction and the corresponding overall reduction is attributed to the rapid reduction of spm Fe3+ to Fe3O4. For the reduction of PM magnetite to α-Fe and its corresponding overall reduction (including the reduction of PM and spm Fe3O4 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.

The reduction of iron FT catalyst in the temperature range of 250–350 °C was separated into two processes for the mathematical modeling. The overall reduction of α-Fe2O3 (PM and spm phases) to Fe3O4 can be described by a three-dimensional phase-boundary-controlled reaction model, while the overall reduction of Fe3O4 to α-Fe follows the formation and growth of nuclei model (two-dimensional or three-dimensional).Figure optionsDownload as PowerPoint slide