Investigations of the reduction behavior of iron-impregnated alumina gels (Fe/AlOOH) and the formation of Fe⁰–Al₂O₃ metal-ceramic composites

Abstract

Fe/AlOOH gels calcined and reduced at different temperatures have been investigated by a combined use of Mössbauer spectroscopy, x-ray diffraction, and electron microscopy in order to obtain information on the nature of the iron species formed as well as the various reduction processes. Calcination at or below 1070 K mainly gives reducible Fe³⁺ while calcination at higher temperatures gives substitutional Fe³⁺ in the form of Al_2−xFe_xO₃. The Fe³⁺ species in the calcined samples are, by and large, present in the form of small superparamagnetic particles. Crystallization of Al₂O₃ from the gels is catalyzed by Fe₂O₃ as well as FeAl₂O₄. Fe (20 wt. %)/AlOOH gels calcined at or below 870 K give FeAl₂O₄ when reduced in hydrogen at 1070 K or lower and a ferromagnetic Fe⁰–Al₂O₃ composite (with the metallic Fe particles >100 Å) when reduced at 1270 K. Samples calcined at 1220 K or higher give the Fe⁰–Al₂O₃ composite when reduced in the 870–1270 K range, but a substantial proportion of Fe³⁺ remains unreduced in the form of Al_2−xFe_xO₃, showing thereby the extraordinary stability of substitutional Fe³⁺ to reduction even at high temperatures. Besides the ferromagnetic Fe⁰–Al₂O₃ composite, high-temperature reduction of Al_2−xFe_xO₃ yields a small proportion of superparamagnetic Fe⁰–Al₂O₃ wherein small metallic particles (<100 Å) are embedded in the ceramic matrix. In order to preferentially obtain the Fe⁰–Al₂O₃ composite on reduction, Fe/AIOOH gels should be calcined at low temperatures (∼1100 K); high-temperature calcination results in Al_2−xFe_xO₃. Several modes of formation of FeAl₂O₄ are found possible during reduction of the gels, but a novel one is that involving the reaction, 2Fe³⁺ + Fe⁰ → 3Fe²⁺.