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

Abstract

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 de- veloped after activation pretreatment were identified by XRD and in situ Mossbauer 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 correla- tion 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 su- perparamagnetic species detected by in situ Mossbauer 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 activ- ity in Fe-Ce catalysts prepared by coprecipitation during FT synthesis.