2. Catalysts (See also references II/26 to II/30 at end of this section).
(a) Preparation.
Fuzed as well as precipitated catalysts have been studied, but the ease in handling and reproduction and mechanical strength of the fuzed type led to the almost exclusive use of the latter. It is substantially identical with the Leuna ammonia catalyst. The catalyst may be prepared as follows:
Pure electrolytic iron or iron from carbonyl (99.9% minimum purity) was mixed with alumina to give a final mixture of 305% Al2O3 based on Fe, 0.15 to 0.5% K2O are added and the iron is oxidized. If iron metal was the raw material, oxygen has to be used for the oxidation step. The exact final state of oxidation was not known, but the catalyst could be readily reproduced.
The melt is broken up into 1-2 mm. particles and reduced. The reduction requires hydrogen of high purity and a large excess of hydrogen is used. A temperature, however, of at least 420° had to be reached to obtain reduction.
Some X-ray studies had been made regarding the influence of the reduction temperature. It was found that at 400° C, Fe3O4 and Fe were present in the ratio of around 1:3. At 425° the ratio was nearer 1:4 while at 500° no more Fe3O4 could be found. By the same method it was further determined that the space velocity of the H2 over the iron influenced the Fe3O4: Fe ratio. Thus: At 300 V/H/V, after 6 days, the ratio was still 0.75, but at 300 V/H/V after 4 days it was 0.3.
In a patent application, dated July 1941, the use of reduction with H2 is disclosed, whereby the product of space velocity x time (days) must exceed 200. This is apparently the lower limit.