Use of in situ Mössbauer 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 Mössbauer 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 Mössbauer 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.