The iron/iron oxide interface morphologies formed during the reduction of dense wustite and magnetite samples have been examined using scanning electron microscopy. Under conditions where porous iron products are obtained, a range of structures, which depend critically on the gas composition, reaction temperature, and bulk oxide composition, may be formed at the inter-face during reduction. Three principal mechanisms of porous iron growth have been identified: (1) a continuous coupled reaction, involving cooperative pore and iron growth, (2) a continuous dendritic growth mechanism, in which pores advance ahead of the iron formation, and (3) a discontinuous mechanism, involving the successive formation and breakdown of dense iron lay-ers on the oxide surface. The reaction mechanisms are explained in terms of the relative mag-nitudes of the various chemical reactions and mass transport processes which occur during the decomposition of the solids.

Mechanisms of porous iron growth on wustite and magnetite during gaseous reduction

The true fundamental trends of the different phase transformation steps (Fe 2 O 3 –Fe 3 O 4 –FeO–Fe) during iron oxide reduction process are necessary requested. Both of the macroscopic and microscopic changes during these steps have a direct bearing on the overall reduction kinetics. Magnetite single crystals were isothermally reduced in pure hydrogen at 900–1100 °C. The oxygen weight loss during reduction process was recorded as a function of time. The magnetite samples and its reduction products were characterized by X-ray diffraction analysis, reflected light microscope, scanning electron microscope and vibrating sample magnetometer to reveal the effect of hydrogen reduction at different temperatures on reduction behavior, composition, microstructure and magnetic properties of the produced samples. The reduction rate increases with reaction temperature. The reduction at 900 and 950 °C was stopped at 83 and 89% extent, respectively. Depending on the kinetics obtained data from reduction process, the reduction rate was found to be controlled by the solid-state diffusion mechanism. The magnetic characterizations of the reduced magnetite at 900–1100 °C were measured whereas the magnetization value increased from 114.1 to 132.1 emu/g with increasing temperature. Magnetite single crystal was partially reduced at 20, 40, 60, and 80%, its magnetic properties was found to be dependent on the morphology, core–shell like structure is formed at 40% reduction extent.

Reduction kinetics, magnetic behavior and morphological changes during reduction of magnetite single crystal

We have investigated the electronic structure of nanosized iron oxide by scanning tunneling microscopy and spectroscopy as well as by photoelectron spectroscopy. The nanoparticles were produced by the thermal treatment of ferritin molecules containing a self-assembled core of iron oxide. Depending on the thermal treatment we were able to prepare different phases of iron oxide nanoparticles resembling $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, and a phase which apparently contains both $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$. The changes to the electronic structure of these materials were studied under reducing conditions. We show that the surface band gap of the electronic excitation spectrum can differ from that of bulk material and is dominated by surface effects.

Electronic structure of nanoscale iron oxide particles measured by scanning tunneling and photoelectron spectroscopies

The impact of gas-phase pretreatment of supported iron-oxide catalyst utilized in aligned carbon nanotube (CNT) growth is studied to understand the correlation between the catalyst oxidation state and the growth characteristics of the aligned CNT forests. By varying the pretreatment conditions from a reducing to an oxidizing environment, notable changes are observed in both the collective CNT array growth behavior and the individual CNT characteristics. Although the greatest catalytic activity was observed following a full reduction to the zerovalent (metallic) Fe catalyst, growth is also observed from a catalyst composed of both Fe2O3 and Fe3O4 particles. XPS core-level analysis, following pretreatment of the catalyst, emphasizes the critical nature of the combined catalyst–underlayer interaction to achieve optimal catalyst activity during growth and hence the most efficient catalyst reduction process. Additionally, CNT diameters during growth were strongly affected by the pretreatment process. Overall, ...

Role of Catalyst Oxidation State in the Growth of Vertically Aligned Carbon Nanotubes

Dense wustite was reduced at temperatures from 430 to 1100 °C in CO-A, CO-CO2, H2-A, and H2-H2O mixtures. Most of the experiments were conducted on a hot stage microscope, providing a clear record of the sequence of events. The morphology of phases was further studied on the optical and the scanning electron microscopes. Two types of nuclei were identified: (1) dense nuclei, ranging from regular whiskers to simple protrusions, around which flat bases develop to form a protective film, and (2) porous nuclei with lenticular shapes, which remain level with the sample surface as they grow (both radially and into the wustite) to form a porous layer. Mild gases and moderate temperatures favor dense iron, whereas strong gases and extreme temperatures (either low or high) favor porous iron. The domains of the two types are represented in morphology maps. They overlap in transition zones where mixed nuclei are found; in this case the porous nuclei appear later than the dense ones but they grow much faster. The morphology maps support the view that sponge iron in iron ore reduction does not originate from porous iron nuclei but develops as a maze of dense iron growths. Experiments with cold-worked and annealed samples indicate that dislocations are essential to the growth of whiskers.

The nucleation of iron on dense wustite: A morphological study

An “environmental cell” located in a high voltage transmission electron microscope has been used to study the reduction of single crystal iron oxides by hydrogen and hydrogen-argon mixtures. The cell enables a direct observation of the solid during reaction, thus permitting the nucleation and growth of solid reaction products to be observed. Hematite was reduced at temperatures in the range 387 to 610°C with gas pressures up to 5.3 kP. Reduction with pure hydrogen was considerably faster than when argon was present. Lath magnetite which rapidly transforms to porous magnetite and thence (more slowly) to porous iron was observed. The reduction of magnetite and of wustite single crystals was observed in the temperature range 300 to 514°C using both hydrogen and hydrogen-argon mixtures at gas pressures up to 6.6 kP. Incubation periods were found for magnetite reduction; during these periods faceted pits formed in the oxide. Iron formed in the early stages was epitaxial with the host magnetite; at later stages the epitaxy was lost and fissures frequently formed in the metal. The morphology of the iron differed between the gas mixtures. Disproportionation accompanied the reduction of wustite, producing intermediate polycrystalline magnetite despite reducing conditions. The disproportionation appeared to be promoted by the reduction reaction. For both oxides, reduction in the hydrogen-argon mixture was slower than in pure hydrogen.

David Rieck

Papers

Investigation of iron oxide reduction by TEM