Transient initial kinetic regimes of Fischer-Tropsch synthesis

Transient kinetic regimes of Fischer-Tropsch (FT) synthesis with a potassiu m-promoted iron catalyst have been observed and characterized by time-resol ved conversion and selectivity studies using H-2/CO and a H-2/CO2 synthesis gases. Up to six episodes relating to catalyst transformations/reconstruct ions could be distinguished. Amazingly, with the H-2/CO2 synthesis gas a Fi scher-Tropsch product of nearly the same composition as that with the H-2/C O synthesis gas was finally obtained at the steady state. However, the tran sient episodes lasted for a long time. Selectivity has been related to steps of elemental reactions by use of a ki netic model. As intrinsic FT feature, the principle of selective inhibition is established fron the detailed selectivity results. Spatial constraints at the FT sites appear also to control selectivity The iron catalyst exhibi ts fundamental differences in how generating FT sites, as compared with cob alt catalysts. FT sites on iron are stable, whereas FT sites on cobalt are of dynamic nature. (C) 1999 Elsevier Science B.V. All rights reserved.