Characterization of iron oxides embedded in silica gel obtained by two different methods

Abstract

A new way of obtaining iron oxides embedded in amorphous silica gel matrices using Fe²⁺, oxygen and tetraethoxysilane (TEOS) as precursors is described. The product of this new method was compared to the compound produced when hydrochemically synthesized iron ferrite (magnetite) was directly embedded in silicon oxide (SiO₂). The two xerogels were characterized and found to differ substantially. The compound obtained using the first procedure is amorphous iron oxide-hydroxide embedded in silica gel and lacks magnetic properties. In contrast, the silicon oxide particles in the second compound become partly coated by the magnetite, such that its magnetic properties are similar to those of magnetite proper.

Introduction

The synthesis of silica aerogels, xerogels and their derivatives is currently generating much interest due to the possible applications of these products in several areas such as the pharmaceutical and chemical industry and agriculture. The optical, magnetic, electronic and chemical (surface) properties of these compounds make them ideal for specific heterogeneous catalysis processes, especially when doped with Fe₂O₃, Al₂O₃ and V₂O₅ [1], [2], [3].

The first descriptions of xerogels formed by iron oxides embedded, in silica gel, and more specifically of the Fe₂O₃–SiO₂ system, date back to the 1980s. Yoshio et al. [4] obtained a set of compounds they described as amorphous magnetic materials, whose characterization was later completed by other authors [5], [6]. This was followed by an attempt to obtain useful compounds for catalysis or materials with specific properties for other applications (e.g., coatings, etc.) by reducing the iron oxide in these amorphous materials to metallic iron [7], [8]. When the embedded compound is hematite, the composites obtained may be used as pigments, catalysts and anticorrosion agents [9]. To obtain solids with magnetic properties, methods have been developed for the synthesis of silica gel doped with maghemite and magnetite (Fe₃O₄). These involve adding tetraethylorthosilicate (TEOS) to the maghemite [10], [11] or magnetite [12], [13], [14] previously synthesized in aqueous medium. Today, new magnetic materials are still being developed using this method, adding magnetite as the precursor and varying the reaction conditions.

In this paper, we explore a new way of embedding iron oxides in an amorphous xerogel matrix using Fe²⁺, oxygen and TEOS as precursors. Nevertheless, since the compound obtained (hereafter referred to as xerogel-1) lacks magnetic properties, we also evaluated an alternative synthesis route whereby iron ferrite (magnetite) obtained hydrochemically is directly used the as precursor along with TEOS in the gelation process. Using this method, we obtained a second compound (xerogel-2) with magnetic properties. Both xerogels were then characterized and their properties compared. The interest in these xerogels comes from the fact that sorption properties can be efficiently modified by the use of silica gel as a host matrix, generating composite materials useful in the design of materials for chemical sensors, ion-exchange coatings and chromatographic applications [15].