Characterization of iron oxide in Fe₂O₃ SiO₂ catalyst

Silica-supported iron oxide catalyst was prepared by hydrolysis of a mixed solution of ethyl silicate and iron(III) nitrate dissolved in ethylene glycol. A gel obtained by the hydrolysis was dried and calcined at various temperatures. The iron oxide particles thus prepared were maghemite (γ-Fe₂O₃), deduced by EXAFS (extended X-ray absorption fine structure) measurements and the sample color. The maghemite particles were classified into two groups: one is small but detectable by transmission electron microscopy (TEM) and the other is too small to be detected by TEM. The former was well controlled to a consistent size, which depended on the calcination temperature, and exhibited ferrimagnetic properties. The latter showed superparamagnetic properties and was estimated to be smaller than 10 Å in particle size by magnetic and Mössbauer measurements at low temperature. For comparison, the catalyst was also prepared by a conventional impregnation method using silica powder and an aqueous solution of iron(III) nitrate. Iron oxide thus prepared was hematite (α-Fe₂O₃) and the particle sizes were broadly distributed.