Use of in situ M ssbauer Spectroscopy and X-Ray Diffraction Techniques for the Characterization of Activated Fe-Ce Catalysts Employed in Fischer-Tropsch Synthesis

Several Fe-Ce catalysts for FT synthesis were prepared following two different methods: coprecipitation from Fe and Ce nitrate solutions and a physical mixture of pure Fe and Ce oxide precursors. Previously to CO hydrogenation, catalysts were activated in syngas (50 ml/min, H2/CO = 2) at 553 K for 1 h at atmospheric pressure. The iron phases developed after activation pretreatment were identified by XRD and in situ M ssbauer spectroscopy with the objective to study the effect of the addition of cerium on the reduction behavior and catalytic properties of Fe systems. A good correlation between iron phases detected by both techniques was found. The results revealed that the cerium oxide in the samples prepared by coprecipitation produces two effects: (i) lower reduction rate leading to the metastable Fe1-xO species, and (ii), a decrease in the crystallite size of the iron species upon increasing Ce-contents, as inferred from an increase in superparamagnetic species detected by in situ M ssbauer spectroscopy. Since iron carbides are the “real” active phases in FT synthesis, the stabilization of Fe1-xO phase after activation is suggested to be responsible for the drop in catalytic activity in Fe-Ce catalysts prepared by coprecipitation during FT synthesis.