Role of rare-earth oxides and thoria as promoters in precipitated iron-based catalysts for Fischer-Tropsch synthesis

The catalytic results in Fischer-Tropsch synthesis indicate that the addition of La, Ce, Nd, Eu and Th oxides to a precipitated iron-based catalyst increases the syngas conversion to hydrocarbon products and shifts the selectivity to the gasoline and middle fractions with significant suppression of both methane and wax formation. Rare-earth (RE) oxides are more effective than thoria, of which Eu₂O₃ shows a unique promoting effect. The results of catalyst characterization obtained from the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR),carbon monoxide temperature-programmed desorption (TPD), carbon dioxide TPD and temperature-programmed surface reaction (TPSR) measurements demonstrate that the presence of these promoters greatly changes both the bulk structure and surface states of the iron catalyst, especially when promoted with europium. It is found that they increase the dispersion of the iron particles, inhibiting their growth and further reduction due to the formation of a mixed Fe-RE(Th) oxide interface, encourage the adsorption and dissociation of carbon monoxide and decrease the reactivity of the surface carbon species towards hydrogen due to the electronic donation properties brought about by their basicities. These results explain the observed variation in the catalytic performances. Eu₂O₃ exhibits a unique promoting effect on the catalytic activity and selectivity due to its superior structural and chemical effects.