CORRELATION OF PREDICTED AND MEASURED IRON OXIDATION STATES IN MIXED IRON OXIDES

CORRELATION OF PREDICTED AND MEASURED IRON OXIDATION STATES IN MIXED IRON OXIDES

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H. D. Rosenfeld, DuPont Central Research & Development, h-david.rosenfeld@usa.dupont.com W. L. Holstein, DuPont Central Research & Development

High surface area iron catalysts for catalytic reactions such as ammonia synthesis are fabricated by controlled reduction of iron oxide precursors prepared by solidifying an Fe-O melt formed by heating to >1600°C magnetite or hematite, to which iron is often added. They contain a mixture of crystalline magnetite (Fe₃O₄-) and poorly crystalline or amorphous wüstite (Fe_1-xO with x approx. eq. 0.05). A key factor in preparing high surface area irons is reportedly the composition of the iron oxide precursor [1]. Pure magnetite precursors yield iron catalysts with appreciably lower surface area and activity than mixtures of magnetite and wüstite. The present study describes work in progress on measuring variations in average Fe oxidation state of iron oxide catalyst precursors.

The magnetite and wüstite lattices are both derived from a cubic crystal lattice of oxygen anions. In Fe₃O₄, one-third of the iron atoms are on interstitial octahedral sites as Fe³⁺ and two-thirds are on tetrahedral sites, where they exchange their valence between the +2 and +3 oxidation states. In the idealized stoichiometric "FeO" structure, Fe²⁺ ions sit on octahedral sites, while in the actual Fe_1-xO structure, some of these octahedral sites are vacant and charge compensation results from Fe³⁺ on tetrahedral sites. For Fe₂O₃, oxygen ions form a hexagonal close packed structure and Fe³⁺ cations are located on octahedral sites. Fe cations on tetrahedral sites contribute to a pre-edge feature in the iron K edge spectra associated with the 1s -> 3d. A shift in edge position of 4.61 eV from Fe_1-xO to Fe₂O₃ results from a decrease in average electric charge at the Fe cations of about 0.10.

The present studies were performed on pure iron oxides: hematite [formal average Fe oxidation state of +3.00], magnetite [+2.67], and wüstite [+2.11]. Magnetite-wüstite and magnetite-hematite mixtures were prepared to give materials with average Fe oxidation states of 2.24, 2.42, 2.54, 2.80, and 2.89. The measured XAS spectra were normalized by fitting the pre-edge and post-edge regions with first and third order polynomials respectively. The average Fe oxidation states of the mixtures were calculated from the normalized, background corrected spectra by curve fitting with linear combinations of spectra from the pure components (open circles) and by interpolating to find the energy at which the edge amplitude reached 0.25 (closed circles). The agreement between measured and expected iron oxidation states was within ±0.04 and ±0.02 for the two methods, respectively. The extension of the technique to mixed valence iron oxide catalyst precursors is straightforward.

[1] R. Schlögl, in Catalytic Ammonia Synthesis - Fundamentals and Practice (J. R. Jennings, ed.), Plenum Press, New York, 1991, pp. 19-108