Reducible cerium oxide defect structure and catalytic function: the role of electron localization

Abstract

Cerium oxide (CeO2, ceria) surfaces are important for many applications, particularly catalysis. The importance relies to a large extent on its facile reducibility and the associated ability to release lattice oxygen. Yet, whether vacancies are at surface or subsurface sites, and whether they agglomerate or repel each other, is still under discussion. To this end, we apply density-functional theory (DFT) with the DFT+U approach to study the reduced CeO2111) surface. By combining DFT+U simulations [1] with STM imaging and spectroscopy [2], we predict and confirm that oxygen vacancies are likely to be bound to Ce4+ ions rather than to Ce3+ as priorly accepted. Moreover, we combine DFT+U with statistical thermodynamics and show that under a wide range of reducing conditions a (2×2) ordered subsurface vacancy structure is stable [3], providing support for recent experimental results. Defect-induced lattice relaxations are used to explain the findings. Furthermore, how the ability of ceria to stabilize reduced states −by accommodating electrons in very localized f-states, positively affects the catalytic activity of ceria-based systems, is briefly discussed using the examples of VOx/ceria [4] and Nix/ceria catalysts [5]. The former are particularly active for oxidation reactions such as the oxidation of methanol to formaldehyde CH3OH + ½ O2→CH2O+ H2O and the latter for the water-gas shift reaction CO+H2O→ CO2 + H2.