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

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 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 was identified which transformed at higher temperatures into K_xFe₂₂O₃₄(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 K_xFe₂₂O₃₄. The formation of a single-crystalline K_xFe₂₂O₃₄ (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 × 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.