Thermomagnetic Investigations of Promoted and Unpromoted Iron Oxide and Iron Catalysts

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Abstract
The Curie temperature (θ) and intensity of magnetization of certain unreduced and reduced iron oxide catalysts have been determined. For the singly promoted unreduced systems of Fe3O4 containing small amounts of one of the following, Na2O, K2O, Cs2O, BaO, B2O3, Al2O3, or SiO2, θ was found to be practically the same as for Fe3O4, but large reductions in the saturation intensity of magnetization (Is) were observed. The doubly promoted unreduced systems studied contained additions to Fe3O4 of Al2O3 and one of the following (in small amounts): Na2O, K2O, BaO, or SiO2. θ for these materials varied only slightly and was either equal to or less than that for Fe3O4. Large reductions in Is were found. Magnetic measurements on the reduced materials gave values of θ the same as for pure iron. The extent to which the promoters go into solid solution, both for the unreduced and reduced materials, is determined. Additional unreduced unpromoted iron oxides were investigated that had variable ratios (γ) of Fe++/Fe+++. Starting at γ=0.352, Is was found to increase with increasing γ to a maximum at γ=0.50, agreeing with the known value for magnetite, and then to decrease to a small value for large γ. The value found for Is at γ=0.352 is in agreement with Néel's theory of ``ferrimagnetism'' as applied to cubic Fe2O3 and Fe3O4. θ was found to be constant at about 583°C for γ from 0.352 to 1.276.