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18: Iron oxide model catalysts: preparation & characterization
Abstract:
Potassium promoted iron oxide model catalyst films were prepared by deposition of potassium onto
epitaxial Fe3O4(111) films at 200 K, followed by annealing in the range 200 to 970 K. Their formation
and composition was investigated by X-ray photoelectron spectroscopy (XPS) in combi-nation with
thermal desorption spectroscopy (TDS) and thermodynamic considerations. Already at 300 K a
solid-state reaction occurs and the iron oxide is partly reduced. Around 700 K a KFeO2 phase was
identified which transformed at higher temperatures into KxFe22O34(0.67<x<4). This transformation
started from the bulk of the film so that initially a potassium rich KFeO2 layer was formed on top of
KxFe22O34. The forma-tion of a single-crystalline KxFe22O34 (x=0.67) layer, which is terminated by a
submonolayer of potassium, is assumed to occur at 970 K. For a certain potassium content, this
surface develops a well ordered phase with a (2x2) superstructure. The potassium containing phases
are not stable in water atmosphere: In 10-8 mbar H2O, potassium hydroxide forms which decomposes
and desorbs beyond 400-500K resulting in a potassium-depleted near-surface laye
