The effect of water on iron-based nanoparticles under hydrogen
and syngas was investigated by in situ X-ray absorption spectroscopy.
The iron oxide (γ-Fe2O3) nanoparticles,
dispersed as a monolayer on flat silica surfaces, were readily converted
into metallic iron in dry hydrogen at 350 °C and into iron carbide
in dry syngas (H2/CO 2/1 vol/vol) at 325 °C. However,
in the presence of water, the reduction did not proceed beyond magnetite
(Fe3O4) up to 350 °C. Wustite (Fe(II)O
or FeO(1–x)) was formed at 450
°C in wet syngas and 550 °C in wet hydrogen. Once formed,
the iron carbide nanoparticles proved remarkably stable against oxidation
in wet syngas at 350 °C. However, we observed the formation of
a surface iron(II) oxide phase that increases with increasing H2O/CO ratio. This implies that the active surface of iron-based
Fischer–Tropsch catalysts is covered by considerable amounts
of adsorbed oxygen during the Fischer–Tropsch reaction. Reducing
the temperature by only 20 K results in complete and irreversible
oxidation to magnetite. We propose that the surface iron(II) oxide
plays an important role during Fischer–Tropsch synthesis by
regulating the relative rates of CO hydrogenation versus water gas
shift and by stabilizing the iron carbide catalyst against irreversible
deactivation by oxidation to magnetite.
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