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Bulk and surface phases of iron oxides in oxygen and water atmosphere at low pressure

MPS-Authors
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Ketteler,  Guido
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Weiss,  Werner
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Ranke,  Wolfgang
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Schlögl,  Robert
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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FHIAC_KWR01_Fe_O_phases.pdf
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Citation

Ketteler, G., Weiss, W., Ranke, W., & Schlögl, R. (2001). Bulk and surface phases of iron oxides in oxygen and water atmosphere at low pressure. Physical Chemistry Chemical Physics, 3(6), 1114-1122. doi:10.1039/B009288F.


Cite as: http://hdl.handle.net/11858/00-001M-0000-0011-18E6-D
Abstract
Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p£1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe-H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4-Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5x10-5 mbar oxygen at 920-1000K results in a new g-Fe2O3(111)-like intermediate sur-face phase during the oxidation to a-Fe2O3(0001). A model is suggested for the growth of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.