In situ dilatometric and impedance spectroscopic study of core–shell like structures: insights into the exceptional catalytic activity of nanocrystalline Cu-doped CeO2

Abstract

Cu-doped CeO₂ (8 mol% Cu) nanoparticles were investigated by combined in situ dilatometry and impedance spectroscopy as a means of exploring the impact of temperature and pO₂ driven dopant redistribution and phase separation on both thermo-mechanical and electrical properties of the CuO_x–CeO₂ system. Dilatometry was used to track the thermal expansion of the CeO₂ nanoparticles as well as the reduction of segregated copper oxide into metallic copper when exposed to reducing conditions. The electrical properties of the nanoparticle array, extracted from impedance spectroscopy studies, point to proton conduction at low temperatures, with a transition to n-type electronic conductivity of CeO₂ at higher temperatures. After segregation of percolating interfacial layers, induced by exposure to reducing atmosphere, the electrical properties become dominated by p-type Cu₂O at intermediate pO₂ and metallic copper at low pO₂. The temperature and pO₂ dependent electrical properties of both the Cu₂O shell and the underlying ceria core were examined in light of defect chemical models. Based on these models, the standard formation enthalpy of copper vacancies and holes in Cu₂O and the standard formation enthalpy of oxygen vacancies and electrons in CeO₂ were found to be equal to Δ_ox,Cu2OH⁰ = (2.4 ± 0.4) eV and Δ_red,CeO2H⁰ = (1.5 ± 0.3) eV, respectively. These findings are discussed in relation to the exceptional catalytic activity of copper–ceria for various oxidation–reduction reactions, focusing on the roles of both nano-dimensions and the influence of Cu on the redox properties of CeO₂.