Controlled Recrystallization of Hematite from Two Highly Different Phases of Ferric Trihydroxide
- PDF / 371,555 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 77 Downloads / 134 Views
Concordia University, Laboratory of Solid State Chemistry and M8ssbauer Spectroscopy, Laboratories for Inorganic Materials, Department of Chemistry and Biochemistry, 1455 De Maisonneuve Blvd. W., Montreal, Quebec, H3G 1M8, Canada ABSTRACT
Hematite Fe2 0 3 , is a semiconductor, and its electrical properties are highly sensitive to the preparation methods and purity. Its precursors can be hydrated ferric oxides; however, these are usually obtained from poorly defined ferric gels, which are obtained by hydrolysis of an aqueous solution of a ferric salt by a base. We have designed a novel synthetic route to ferric hydroxide, by reaction of a peroxocompound with an aqueous solution of a ferrous salt, which involves simultaneous oxidation of Fe(ll) to Fe(lIl), and hydrolysis, in the same reaction process. The two kinds of ferric hydroxide are highly different, however, both give hematite by dehydration/recrystallization, however, the way this occurs for each is different. INTRODUCTION
Hematite a-Fe 20 3 crystallizes in the A120 3 corundum type (trigonal space group R3c) [1]. It is slightly oxygen deficient at low temperatures, and therefore it is an ntype semiconductor when it is pure, but p-type conduction has also been reported [23]. Ithas been established to be an attractive candidate for electrode material in high energy-density batteries, both in the form of bulk or thin films [4]. Its Seebeck voltage, its resistance under ultra high pressure and the anisotropy of its electrical properties have also been studied [5-7]. Their semiconducting properties also make unsupported and supported ferric oxides commonly used catalysts for the production and transformation of hydrocarbons [8, 9]. The electrical and catalytic properties are highly dependent on the method of preparation as well as on the crystallite size and degree of sintering [3, 9]. Hematite can be prepared by heating hydrated ferric oxides. Hydrolysis of aqueous solutions of ferric salts precipitate a gel, which after freezing and thawing gives a very fine powder of iron(Ill) oxide hydrate (hereafter called "Fe(OH)3 gel" forshort, to remind its origin) [10]. These powders dehydrate to give Fe20 3 , with dehydration and recrystallization being separate processes. We have designed a novel method of preparation of ferric hydroxide, which is a new kind of Fe(OH) 3 (hereafter called "Fe(OH)3 powder", to differentiate it from the Fe(OH) 3 obtained from the gel) [11-13]. This new kind of Fe(OH) 3 is obtained by addition of a peroxo- aqueous solution to an aqueous solution of a ferrous salt. This results in the slow precipitation of a fine light brown powder of Fe(OH) 3, which bares 307 Mat. Res. Soc. Symp. Proc. Vol. 358 ©1995 Materials Research Society
little resemblance to the dark brown gel. In this work, we carried out a comparative study of the dehydration and recrystallization of both kinds of Fe(OH) 3 to hematite. EXPERIMENTAL
Materials Preparation Fe(OH) 3 gel was prepared according tothe method described by van der Giessen [10]. Concentrated ammonium hy
Data Loading...
