Formation of Magnetite in Highly Alkaline Media in the Presence of Small Amounts of Ruthenium*

The effect of small amounts of ruthenium on the formation of magnetite in highly alkaline media was investigated using X-ray powder diffraction (XRD), Mössbauer and FT-IR spectroscopies, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). Acicular a-FeOOH particles precipitated in a highly alkaline medium with the addition of tetramethylammonium hydroxide (TMAH) were used as a reference material. Initial addition of small amounts of Ru(NO)(NO3)3 to that precipitation system had a drastic effect on the formation of iron oxide phases and their properties. The addition of Ru(NO)(NO3)3 favoured the formation of stoichiometric Fe3O4. With an increase of the initial Ru(NO)(NO3)3 concentration in the precipitation systems less time was needed for the formation of Fe3O4 as a single Fe-bearing phase in the precipitates. Ruthenium ions made solid solutions a-(Fe,Ru)OOH; however, there was no indication of the formation of solid solutions with a-Fe2O3 and Fe3O4. Mössbauer and FT-IR spectroscopies supported the conclusion on the formation of solid solutions a-(Fe,Ru)OOH. FE-SEM showed the formation of octahedral Fe3O4 particles of a mm range size. Ruthenium particles (≈ 20 nm in size) were deposited onto the surfaces of Fe3O4 particles. They were also present in the form of clusters containing octahedral Fe3O4 particles in the nanosize range (≈ 100 nm or less). The formation of Fe3O4 was interpreted as a combining effect of the thermal decomposition products of TMAH under autoclaving conditions and the catalytic action of ruthenium. In such a way strong reductive conditions in the investigated precipitation system were created.