Corundum-Type Catalysts

The Physical Chemistry of Materials Energy and Environmental Applications 

FIGURE 2.13 A Layer of O2 with 2/3 of the octahedral sites occupied by Fe3+. 

Iron oxide with a corundum-type structure, that is, aFe203, is a good catalyst for reactions such as the catalytic oxidation of S02 [53], Studies carried out on this reaction show that S02 adsorption is the rate-determining step it contributes electrons to the conduction band and the adsorption of 02 withdraws the conduction electrons from an oxygen vacancy [53], 

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As an example, the joint analysis of IR and Raman spectra provided evidence of the partial ordering of cations in a Fe-Cr corundum-type mixed sesquioxides, which are used industrially as high temperature water-gas shift catalysts, but are also active in olefin oxidative dehydrogenation. X-ray diffraction (XRD) patterns of these solids indicate the conmdum-type structure without any superstructure. This implies that iron and chromium ions are randomly distributed. IR and Raman spectra instead definitely show that cations are at least partially ordered in layers such as in the ilmenite-type superstructure. Similarly, XRD analysis shows a cubic (non-ferroelectric) structure of nanometric BaTi03, while vibrational spectroscopies reveal microscopic asymmetry of this structure. Similarly, IR spectroscopy allowed the determination of the state of vanadium in solid solution in Ti02 anatase catalysts, and the presence of Ti" + in the silicalite framework of TSl catalysts, " used for the selective oxidation of phenol and the ammoximation of cyclohexanone with hydrogen peroxide. 

It is difficult to understand the structures of chromia-alumina catalysts without being familiar with the various types of aluminas. Excellent reviews of this subject may be found in Lippens Ph.D. thesis (3) and the Alcoa Technical Paper No. 10 Alumina Properties (36). The dehydration sequence from aluminum hydroxide to corundum (36,37) is shown in Fig. 4, although a variation in experimental conditions pro-... 

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