Study of bimetallic interactions and promoter effects of FeZn, FeMn and FeCr Fischer–Tropsch synthesis catalysts

The promotional effects of transition metals of Zn, Mn and Cr on the textural properties, reduction behavior, surface basicity, structural changes during reduction and reaction, and the catalytic performances of Fe-based Fischer–Tropsch synthesis (FTS) catalysts were investigated by N2 physisorption, X-ray diffraction (XRD), Mössbauer spectroscopy (MES), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), H2-differential thermogravimetric analysis (H2-DTG) and CO2 temperature-programmed desorption (CO2-TPD). The FTS reaction behaviors of the catalysts were measured at 1.5 MPa, 260 °C and syngas with H2/CO ratio of 2.0. The results show that there are two distinct forms of bimetallic interactions for the promoted catalysts, namely (1) ZnFe2O4 compound formed for Zn-promoted iron catalyst and (2) solid solutions observed for those promoted by either Cr or Mn promoters. The presence of ZnFe2O4 compound in the Zn-promoted catalyst leads to the phase separations between Zn and Fe oxides, and therefore very similar catalytic behavior to that of unpromoted catalyst. The stability of activity was improved due to the increased dispersion of active site through the formation of ZnFe2O4 compound. In contrast, for the Mn- and Cr-promoted catalysts, the solid solutions in FeMn and FeCr systems strongly inhibit the reduction of the catalysts, and enhance the stability of catalytic activity. The FTS tests show that Mn and Cr promoters enhance the olefin and C5+ hydrocarbon selectivity and restrain the methane selectivity due to their more strong basic sites. Besides the surface basicity the selectivities of Mn-promoted catalyst are also correlated with the enrichment of Mn on the catalyst surface.