Sol-gel chemistry of transition metal oxides

This paper describes a kinetic model for the decomposition of hydrogen peroxide by ferric ion in homogeneous aqueous solution (pH < 3). The reaction was investigated experimentally at 25.0 °C and I = 0.1 M (HClO4/NaClO4), in a completely mixed batch reactor and under a wide range of experimental conditions (1 ≤ pH ≤ 3; 0.2 mM ≤ [H2O2]0 ≤ 1 M; 50 μM ≤ [Fe(III)]0 ≤ 1 mM; 1 ≤ [H2O2]0/[Fe(III)]0 ≤ 5000). The results of this study demonstrated that the rate of decomposition of hydrogen peroxide by Fe(III) could be predicted very accurately by a kinetic model which takes into account the rapid formation and the slower decomposition of Fe(III)−hydroperoxy complexes (FeIII(HO2)2+ and FeIII(OH)(HO2)+). The rate constant for the unimolecular decomposition of the Fe(III)−hydroperoxy complexes was determined to be 2.7 × 10-3 s-1. The use of the kinetic model allows a better understanding of the effects of operational parameters (i.e., pH and [H2O2]0/[Fe(III)]0) on the complex kinetics of decomposition of H2O2 by Fe(III).

Catalytic Decomposition of Hydrogen Peroxide by Fe(III) in Homogeneous Aqueous Solution: Mechanism and Kinetic Modeling

Storage of ferrihydrite in aqueous suspensions at 24°C and pHs between 2.5 and 12 for as long as three years resulted in the formation of goethite and hematite. The proportions and crystallinity of these products varied widely with the pH. Maximum hematite was formed between pH 7 and 8, and maximum goethite at pH 4 and at pH 12. The crystallinity of both products, as indicated by X-ray powder diffraction line broadening and magnetic hyperfine field values and distribution widths, was poorer, the lower the proportion of the corresponding product in the mixture. The existence of two competitive formation processes is suggested: goethite is formed via solution, preferably from monovalent Fe(III) ions [Fe(OH)2+ and Fe(OH)4−], and hematite by internal rearrangement and dehydration within the ferrihydrite aggregates. This concept relates the proportions of goethite and hematite to the activity of the Fe(III) ion species in solution, and implies that conditions favorable for the formation of goethite are unfavorable for that of hematite and vice versa.

Effect of pH on the formation of goethite and hematite from ferrihydrite

Photolysis of Fe (III)-hydroxy complexes as sources of OH radicals in clouds, fog and rain

Iron oxide-based porous solids were prepared by a sol−gel process using Fe(III) salts in various solvents. It was observed that the addition of propylene oxide to Fe(III) solutions resulted in the formation of transparent red-brown monolithic gels. The resulting gels were converted to either xerogels by atmospheric drying or aerogels by supercritical extraction with CO2(l). Some of the dried materials were characterized by nitrogen adsorption and desorption analysis and transmission electron microscopy (TEM). The results of those analyses indicate that the materials have high surface areas (∼300−400 m2/g), pore sizes with mesoporic dimensions (2−23 nm), and a microstructure made up of 5−10 nm diameter clusters of iron(III) oxide. The dependence of both gel formation and its rate was studied by varying the epoxide/Fe(III) ratio, the Fe(III) precursor salt, amount of water (H2O/Fe(III)) present, and the solvent employed. All of these variables were shown to affect the rate of gel formation and provide a con...

R.N. Sylva

Papers

Spectrophotometric investigation of iron(III) hydrolysis in light and heavy water at 25°C

Precipitation in hydrolysed iron(III) solutions