Reduction of Alumina-Supported Iron Catalysts

Both Schlogl and Somorjai report on the interaction between oxidized iron and the promoters, mainly alumina and potassium oxide. In particular, the interaction with the structural promoter, alumina, can alter significantly the crystallography of the metallic iron surfaces formed during reduction. Since supported iron catalysts can contain much more alumina than normal for the industrial ammonia synthesis catalyst, any interaction between the iron oxide being reduced and the alumina support is much more easily observed. Some results, indicating unambiguously the effect of the interaction of iron with the alumina, will therefore be reviewed. 

The results of Wielers and co-workers have shown that Fe(II) is stabilized by some interaction with alumina. The stabilization may be due to the formation of FeAl204 or, more probably, to an interaction between ferrous oxide and an alumina or iron(II) aluminate surface. Kock and co-workers demonstrated the stabilization of Fe(II) by alumina using magnetic measurements. The authors studied the reduction of goethite (FeOOH), hematite (Fe203), a physical mix of FeOOH and alumina, and FeOOH deposited on alumina with carbon monoxide. They raised the temperature of their samples held in a flow of 5% carbon monoxide in helium, with a heating rate of 4.8 K min up to a temperature of 770 K and 

In the first instance, the results of Kock and co-workers have demonstrated the stability of iron(II) oxide (FeO) supported on alumina. Moreover, the formation of FeO on alumina apparently prevents further reaction to give iron carbide, even with carbon monoxide, which in the absence of alumina leads readily to carbide 

It can be concluded from the above experimental results that the interaction of hydrated iron oxides with supports such as alumina greatly affects the course of the reduction process. An intermediate wustite (FeO) species is stabilized by the support. It is difficult to assess the extent to which stabilization depends on the formation of ferrous compounds, such as Fe(II)Al204 and [Mg, Fe(II)] oxide. However, the results obtained in the presence of relatively small amounts of water vapor suggest that wustite (FeO) species are strongly stabilized without the simultaneous formation of an appreciable amount of Fe(II)Al204. 

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Since the properties of iron catalysts depend strongly on the detailed reduction procedure, the chapter will begin with a discussion of the reduction of alumina-supported iron catalysts. The interaction of alumina with iron(II) formed during the reduction will be evident. 

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