Takayuki Tsubota

Influence of metal ions on the structure of poorly crystallized iron oxide rusts

Abstract The poorly crystallized steel rusts have been prepared by hydrolysis of aqueous solutions dissolving Fe(III) and different metal ions such as Ti(IV), Cr(III), Ni(II) and Cu(II) at varied atomic ratios in metal/(Fe+metal). The precipitates formed by the hydrolysis were frozen with liquid N 2 to inhibit crystallization and particle growth. The rusts produced were poorly crystalline and nanosized particles with sizes of 4–10 nm and the rust formed without adding metal ions was assigned to ferrihydrite. The crystallite size decreased from 1.1 to 0.7 nm with increasing the added metal ions. It was shown by extended X-ray fine structure and Mossbauer spectroscopy that the short-range structure of poorly crystalline rusts was disturbed by adding Ti(IV).

Influences of Metal Chlorides and Sulfates on the Formation of Beta-FeOOH Particles by Aerial Oxidation of FeCl2 Solutions

To investigate the influences of metal ions on β-FeOOH formation, the β-FeOOH particles were prepared by aerial oxidation of FeCl 2 solutions in the presence of chlorides and sulfates of Ti(IV), Fe(III), Cu(II), and Ni(II), and CrCl 3 at different metal/Fe atomic ratios of 0-0.1 and characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, inductively coupled plasma, thermogravimetry and differential thermal analysis, and N 2 adsorption. The produced particles contained a large quantity of Ti(IV) and Cr(III) but little Cu(II) and Ni(II). All the added metal chlorides and sulfates made the formed particles less crystalline. The sulfate and chloride of Ti(IV) inhibited the formation of β-FeOOH at a high Ti/Fe by a marked depression of solution pH by hydrolysis of Ti(IV). The metal sulfate ions effectively reduced the crystallinty of β-FeOOH to give highly microporous particles. The strong inhibitory effect of SO 2- 4 on the crystallization of β-FeOOH can be ascribed to the adsorption of SO 2- 4 on the primary particles. This is also evident from the high content of SO 2- 4 in the formed particles.

Influence of hydrolyzed and nonhydrolyzed Ti, Cr, and Al ions on the formation of β-FeOOH particles

Beta-FeOOH particles were synthesized in the presence of Ti(IV), Al(III), and Cr(III) at metal/Fe atomic ratios of 0-0.1 by the following two methods: hydrolysis of aqueous FeCl3 solutions added to the hydrolysis products of these metal ions (subsequent hydrolysis, SH) and hydrolysis of aqueous FeCl3 solutions dissolving these metal ions (combined hydrolysis, CH). On increasing Al/Fe the particle size of the products with AlCl3 by SH method steeply rose at a low Al/Fe and then fell. The similar increase of particle size was seen in SH method with Ti(SO4)2 though the addition of TiCl4 decreased the particle size. In CH method, Ti(IV) markedly impeded the beta-FeOOH formation but Al(III) and Cr(III) showed no influence. The particles prepared by CH and SH methods contained a large amount of Ti(IV) but a few Al(III) and Cr(III). The large spindle-shaped and rod-shaped particles produced by SH method with AlCl3 and Ti(SO4)2 were highly microporous and poorly crystallized, indicating that the particles consist of fine primary particles and the aggregation of fine particles would be promoted by SO4(2-). The different influences of the metal ions on the beta-FeOOH formation were explained by their hydrolysis characteristics.

Influence of Metal Ions on the Transformation of γ-FeOOH into α-FeOOH

The transformation of γ-FeOOH into α-FeOOH in FeSO 4 solutions at 50°C was investigated in two different ways by dissolving Ti(IV), Cr(III), Cu(II), Ni(II), and Mn(II) in the solutions at atomic ratios of metal/Fe of 0-0.1 and adding these metal ions to the γ-FeOOH particles. The transition into α-FeOOH was examined by Fourier transform infrared spectroscopy and X-ray diffraction. Ti(IV), Cr(III), and Cu(II) dissolved in the solutions markedly interfered with the transition into α-FeOOH, whereas Ni(II) and Mn(II) showed no influence. The inhibitory effect of the former three metal ions is ascribed to the protection of γ-FeOOH particles against dissolution in FeSO 4 solution by surface coating with the hydrolysis species of metal ions, because transmission electron microscopy and inductively coupled plasma-Auger electron spectroscopy demonstrated that the conversion proceeds by dissolution and recrystallization. The metal ions added in γ-FeOOH particles promoted the conversion into α-FeOOH, which was interpreted by the enhancement of dissolution of γ-FeOOH particles due to the decrease in particle size by the addition of metal ions.

Structures of nanosized Fe–Ti mixed oxide particles produced by freezing method

Fe–Ti mixed oxide nanoparticles with different atomic ratios in Ti/(Fe + Ti) from 0 to 1.0 have been prepared by hydrolysis of aqueous solutions containing Fe(III) and Ti(IV) ions and a freezing method with liquid N2, which inhibits crystallization and particle growth. The particles produced were poorly crystalline according to XRD and the product, in the absence of Ti(IV), was assigned to 2-line ferrihydrite. The products had a high specific surface area of 369–460 m2 g−1, corresponding to a mean particle size of 3–4 nm. These nanosized particles were composed of crystallites of about 1 nm size. It was found by Mossbauer spectroscopy that the particles produced were superparamagnetic and their blocking temperature and inner electric field decreased with increasing Ti(IV) content.