Electronic and Structural Properties of Thin Iron Oxide Films on CeO₂

Modification of CeO₂ (ceria) with 3d transition metals, particularly iron, has been proven to significantly enhance its catalytic efficiency in oxidation or combustion reactions. Although this phenomenon is widely reported, the nature of the iron-ceria interaction responsible for this improvement remains debated. To address this issue, we prepared well-defined model FeO_<i>x</i>/CeO₂(111) catalytic systems and studied their structure and interfacial electronic properties using photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, coupled with density functional theory (DFT) calculations. Our results show that under ultrahigh vacuum conditions, Fe deposition leads to the formation of small FeO_<i>x</i> clusters on the ceria surface. Subsequent annealing results in the growth of large amorphous FeO_<i>x</i> particles and a 2D FeO_<i>x</i> layer. Annealing in an oxygen-rich atmosphere further oxidizes iron up to the Fe³⁺ state and improves the crystallinity of both the 2D layer and the 3D particles. Our DFT calculations indicate that the 2D FeO_<i>x</i> layer interacts strongly with the ceria surface, exhibiting structural corrugations and transferred electrons between Fe²⁺/Fe³⁺ and Ce⁴⁺/Ce³⁺ redox pairs. The novel 2D FeO_<i>x</i>/CeO₂(111) phase may explain the enhancement of the catalytic properties of CeO₂ by iron. Moreover, the corrugated 2D FeO_<i>x</i> layer can serve as a template for the ordered nucleation of other catalytically active metals, in which the redox properties of the 2D FeO_<i>x</i>/CeO₂(111) system are exploited to modulate the charge of the supported metals.