The Effect of Water on the Stability of Iron Oxide and Iron Carbide Nanoparticles in Hydrogen and Syngas Followed by in Situ X-ray Absorption Spectroscopy

Abstract

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 irreversi...