MOESSBAUER SPECTROSCOPIC STUDIES OF IRON-57-SURFACE ENRICHED IRON-56 CORE FISCHER-TROPSCH SYNTHESIS CATALYSIS (IRON CATALYSTS)
Abstract
('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.
Degree
Ph.D.
Subject Area
Chemical engineering
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