The Precursor Oxide Mixture

THE PRECURSOR OXIDE MIXTURE 2.3.1. Structure of the Catalyst Precursor 

The catalyst components are mixtures of oxides which have been fused in an electric arc furnace at temperatures of ca 2000 K. The resulting large blocks of black hard material are broken into lumps of usually ca 1 cm diameter. The visual homogeneity of these lumps is, in general, a good indicator for the quality of the final activated catalyst. Poor catalysts exhibit white spots of segregated promoter oxides and bubble holes caused by evaporation of impurities during the fusion process. Primary sources of iron can be iron ores, scrap metal, or iron oxides (oxyhydrates) from other industrial processes. Potassium is added as potassium carbonate, nitrate, or potassium hydroxide, aluminum as alumina and calcium as oxide or carbonate. 

In the following discussion the X-ray diffraction (XRD) patterns of a series of six industrial catalysts are compared. High-resolution scans were obtained with monochromated Co radiation on a transmission Guinier diffractometer. Phase analyses were carried out on an automated Phillips APD 10 powder diffractometer using postmonochromated Cu radiation. In Fig. 2.2, relevant sections of the diffraction patterns of catalysts from three industrial sources are displayed. The observed reflections can be assigned to magnetite and wustite as the main components of the catalysts. The catalysts differ markedly in their content of crystalline wustite. It should be stressed here that XRD is not a suitable method for quantitative determination of the wustite content, since it depends on the crystallinity of the phase analyzed. The nonstoichiometric nature of wustite and its close structural 

It should be pointed out here that areas of amorphous or glassy material can also be found in reduced catalysts (see later in Section 2.6). This material, which is also rich in promoter oxides, remains therefore unreduced in the activation process but is, in contrast to the large inclusions in the catalyst precursor, very finely divided between the iron crystallites. Using selected area electron diffraction, it can be identified as a mixture of a highly defective spinel phase with a truly amorphous (ring pattern) phase. The nature of this material, which seems to be a common ingredient in all types of iron catalyst, is difficult to assess because of the notorious analytical difficulties presented by amorphous minor phases. It may, however, play an important role for the operation of the catalyst in acting as an inert spacer material between active iron particles. This role will be further discussed in the final section of the chapter. 

In summary, we see that the catalyst precursor consists of the dominant phases, magnetite and wustite, within small amounts of various ternary iron oxides and binary promoter additives. The crystallinity and texture of the samples differ widely. It is concluded that the initial solid state for the reduction process catalyst of activation is inhomogeneous for a given catalyst. It is further concluded that it is difficult to derive characteristic differences for a set of catalysts from XRD alone and also that very accurate quantitative chemical analyses are of limited value only, since they will not necessarily be representative for the catalyst charge as a whole. 

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Fig.2 (a) The dependence of the activity for methanol production upon the calcination temperature of the precursor of the copper and ytterbium oxide mixture and (b) XRD pattern of the precursor oxide mixture calcined at various temperatures. 

Finally, it should be kept in mind that the application of the phase diagram for prediction of phase composition is only valid for equilibrium conditions. The precursor oxide mixture is, however, metastable and, furthermore, contains impurities (promoters) which may affect activation, so that the process is not always carried out under equilibrium conditions. This makes it possible for solid solutions of a-iron, wustite, and magnetite to exist as metastable forms of the catalyst. The phase boundary from pure iron to such a solid solution is so sharp, and the region for the existence of pure iron so small, that very precise methods of analysis of the final catalyst will be required to determine whether the active form of the catalyst is pure iron or any of the contributing solid solutions. 

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