From Surface to Bulk and Back: Dynamic Redistribution of Iron in CeO2(111) Model Catalysts under Reducing and Oxidizing Conditions

Abstract

Transition-metal modification of cerium dioxide (CeO2) is widely employed to enhance catalytic performance through the creation of active sites, promotion of oxygen-vacancy formation, and improved oxygen mobility. All of these effects can be strongly influenced by the dynamic redistribution of dopant species under reaction conditions, a phenomenon that remains poorly understood. Here, we investigate the redox-dependent redistributing of Fe in a well-defined FeOx/CeO2(111) model systems using scanning tunneling microscopy, near-ambient pressure X-ray photoelectron spectroscopy, low-energy electron diffraction, and ion sputtering depth profiling by introducing ultra-high vacuum conditions and reducing (H2) or oxidizing (O2) environments at temperatures up to 850 K. Under reducing conditions, the FeOx overlayer becomes unstable above 600–750 K and undergoes extensive disintegration accompanied by Fe diffusion into the CeO2 bulk. Depth profiling reveals a non-uniform Fe distribution with preferential accumulation in deeper regions of the ceria film, eventually approaching the CeO2(111)/ Pt(111) interface. In contrast, oxidizing conditions stabilize Fe at the CeO2(111) surface over the entire investigated temperature range accompanied by substantial restructuring of FeOx species on the surface. Fe incorporation is shown to be reversible, as oxygen annealing of samples with Fe-depleted surfaces restores Fe to the surface. These results demonstrate that FeOx/CeO2(111) systems exhibit fundamentally different structural and chemical behavior under reducing and oxidizing environments, enabling a dynamic redistribution of Fe. The observed activation temperatures for Fe redistribution closely coincide with those reported for Fe–CeO2 catalyst activation in hydrogen temperature-programmed reduction experiments, suggesting that redox-controlled Fe redistribution represent a key structural process governing the performance of Fe-containing ceria catalysts.