On the preparation and composition of potassium promoted iron oxide model catalyst films

Potassium promoted iron oxide model catalyst films were prepared by deposition of potassium onto epitaxial Fe3O4 (111) Elms at 200 K, followed by annealing in the range 200 to 970 K. Their formation and composition were investigated by X-ray photoelectron spectroscopy (XPS) in combination with thermal desorption spectroscopy (TDS) and thermodynamic considerations. Already at 300 K a solid-state reaction occurred and the iron oxide was partly reduced. Around 700 K a KFeO2 phase identified which transformed at higher temperatures into KxFe22O34(0.67 < x < 4). This transformation started the bulk of the film so that initially a potassium-rich KFeO2 layer was formed on top of KxFe22O34. The formation 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 (2 x 2) superstructure. The potassium containing phases are not stable in water atmosphere: In 10(-8) mbar H2O, potassium hydroxide forms, then decomposes and desorbs beyond 400-500 K resulting in a potassium-depleted near-surface layer.