Electronic\nand Structural\nProperties of Thin Iron\nOxide Films on CeO<sub>2</sub>

Modification of CeO<sub>2</sub> (ceria) with 3d transition\nmetals,\nparticularly iron, has been proven to significantly enhance its catalytic\nefficiency in oxidation or combustion reactions. Although this phenomenon\nis widely reported, the nature of the iron–ceria interaction\nresponsible for this improvement remains debated. To address this\nissue, we prepared well-defined model FeO<sub><i>x</i></sub>/CeO<sub>2</sub>(111) catalytic systems and studied their structure\nand interfacial electronic properties using photoelectron spectroscopy,\nscanning tunneling microscopy, and low-energy electron diffraction,\ncoupled with density functional theory (DFT) calculations. Our results\nshow that under ultrahigh vacuum conditions, Fe deposition leads to\nthe formation of small FeO<sub><i>x</i></sub> clusters on\nthe ceria surface. Subsequent annealing results in the growth of large\namorphous FeO<sub><i>x</i></sub> particles and a 2D FeO<sub><i>x</i></sub> layer. Annealing in an oxygen-rich atmosphere\nfurther oxidizes iron up to the Fe<sup>3+</sup> state and improves\nthe crystallinity of both the 2D layer and the 3D particles. Our DFT\ncalculations indicate that the 2D FeO<sub><i>x</i></sub> layer interacts strongly with the ceria surface, exhibiting structural\ncorrugations and transferred electrons between Fe<sup>2+</sup>/Fe<sup>3+</sup> and Ce<sup>4+</sup>/Ce<sup>3+</sup> redox pairs. The novel\n2D FeO<sub><i>x</i></sub>/CeO<sub>2</sub>(111) phase may\nexplain the enhancement of the catalytic properties of CeO<sub>2</sub> by iron. Moreover, the corrugated 2D FeO<sub><i>x</i></sub> layer can serve as a template for the ordered nucleation of other\ncatalytically active metals, in which the redox properties of the\n2D FeO<sub><i>x</i></sub>/CeO<sub>2</sub>(111) system are\nexploited to modulate the charge of the supported metals.