2239.     MERKEL, H.  [Study of Fischer-Tropsch Iron Catalysts.  I.  Magnetic Investigation of Unused Iron Catalysts.  II.  Model Test With Copper Ferrite.]  Brennstoff-Chem., vol. 31, 1950, pp. 208-212; Chem. Abs., vol. 44, 1950, p. 8564.

        Fe catalysts with and without the addition of Cu were investigated thermomagnetically after precipitation and in different stages of drying at 105°.  Directly after the precipitation, the precipitate appeared strongly ferromagnetic, yet its magnetization curve was still very irregular.  After several hours of drying, Fe catalysts containing Cu gave uniform magnetization curves with a Curie point at 470°-475°.  The intensity of magnetization was just as great as after the formation.  If drying was continued for 29 hr. the Curie point, in spite of progressive oxidation of the constituents, remained virtually unchanged.  The compound taken as a basis for the Curie point 470°-475° was no longer stable in the temperature range of the synthesis.

        Heating to 220° for 3 ½ hr. resulted in complete decomposition whereby Fe₃O₄ in magnetically equivalent amounts appeared as the conversion product.  Heating to 800° resulted in a sharp decline in the magnetization.  By means of a model test it was seen that the compound taken as a basis for the Curie point 470°-475° had to do with a strongly ferromagnetic Cu ferrite whose magnetization curve still appeared after mild reduction conditions.  In contrast to the behavior of Cu-containing Fe catalysts, a prepared Cu ferrite showed a sharp increase in magnetization after a heat treatment at 800°.  This different behavior is due to the presence of oxidic iron in the catalyst after the ferrite formation, which does not appear magnetically.  A Fe preparation without the addition of Cu showed after 8 hr. of drying, strong Fe₃O₄ formation.  With progressive oxidation an increase in the Curie point was established.  After a 30-hr. drying the Fe had been oxidized quantitatively into the trivalent form with formation of ferromagnetic Fe₂O₃.  The Curie point of this oxide could not be accurately determined since the magnetization curve falls off almost linearly in the ferromagnetic conversion range.  It lies in the temperature range of 600°-610°.  Heating to 800° causes the loss of the magnetism of the Fe₂O₃, so that only the magnetism of the stable Fe₃O₄ remains.  Beside the known effect of Cu on the forming and synthesis processes it is shown that this metal also influences the oxidation of Fe during the drying.  By using Fe chloride for the initial solution, Cu-free materials easily form ferromagnetic γ-Fe₂O₃ in quantitative amounts.  In this condition the catalyst is difficult to reduce and cannot be used for the synthesis at normal pressure.  On the other hand, ferritic combined a-FE₂O₃ is formed with extensive addition of Cu.  Oxidation is thereby incomplete, 20-25% of the Fe remains in the bivalent form.

        ----------.  See abs. 2683, 2684, 2685.