| ('57)Fe(CO)(,5) was synthesized directly from ('57)Fe(,2)O(,3) by the method of Hagen. A pentane solution of ('57)Fe(CO)(,5) applied the Mossbauer-active ('57)Fe isotope to the surface of a metallic ('56)Fe core. Evaporation of the pentane at -42(DEGREES)C and subsequent air exposure of the sample provided an ('57)Fe-surface-enriched ('56)Fe powder allowing Mossbauer spectroscopy to selectively probe the surface of an unsupported iron Fisher-tropsch synthesis (FTS) catalyst. The consistency in the characterization of the coated sample by x-ray diffraction, x-ray photoelectron spectroscopy, secondary ion mass spectrometry and Mossbauer spectroscopy shows that the ('57)Fe is retained in the surface layer. The proposed structure of the as-coated sample has amorphous ('57)Fe(,2)O(,3) clusters or patches resting on or imbedded in a passivation layer of ('56)Fe, the surface of which is enriched in ('57)Fe. This oxide layer covers an intact ('56)Fe metallic core and is covered by a carbonaceous overlayer. A series of catalysts with the same metallic core but different surface compositions was made by varying the pretreatment of the as-coated sample. Catalysts with surfaces of iron oxide only, metallic iron only, an iron/iron oxide mixture and an iron/iron oxide/iron carbide mixture were tested in FTS using 3:1 H(,2):CO at 523K. Regardless of initial surface composition, the post-reaction surface consisted exclusively of (chi)-Fe(,5)C(,2). Room tempera- ture titration of the post-reaction with N(,2)O perturbed the iron carbide Mossbauer spectrum as a whole inducing a collapse in the carbide magnetic structure. This indicates that the majority of the ('57)Fe giving the post-reaction Mossbauer spectrum still resides in the surface region of the catalyst, allowing us to conclude that, at our reaction conditions, (chi)-Fe(,5)C(,2) is the catalytic phase for FTS.; A catalyst with a starting surface of 68% magnetite, 13% metallic iron and 19% (chi)-Fe(,5)C(,2) was the only one to show fully oxidized iron after post-reaction N(,2)O titration of the carbide surface. This special catalyst was the most active by a factor of at least 2 and has a carbon poor iron carbide site which is the one completely oxidized by N(,2)O. The higher kinetic rates for this catalyst and a bulk magnetite catalyst demonstrate a synergistic role between magnetite and iron carbide which promotes the maintenance of free active sites even after the magnetite has been removed by reduction and carburization. |
