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 CeO 2 (8 mol% Cu) nanoparticles were investigated by combined in situ dilatometry and impedance spectroscopy as a means of exploring the impact of temperature and pO 2 driven dopant redistribution and phase separation on both thermo-mechanical and electrical properties of the CuO x -CeO 2 system. Dilatometry was used to track the thermal expansion of the CeO 2 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 CeO 2 at higher temperatures. After segregation of percolating interfacial layers, induced by exposure to reducing atmosphere, the electrical properties become dominated by p-type Cu 2 O at intermediate pO 2 and metallic copper at low pO 2 . The temperature and pO 2 dependent electrical properties of both the Cu 2 O 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 Cu 2 O and the standard formation enthalpy of oxygen vacancies and electrons in CeO 2 were found to be equal to D ox,Cu 2 O H 0 ¼ (2.4 AE 0.4) eV and D red,CeO 2 H 0 ¼ (1.5 AE 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 CeO 2 .

On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

“…It is well known that zero-dimensional or point defects play a particularly important role in the control and optimization of semiconductor physicochemical properties. − Despite the strong interest in α-Fe 2 O 3 as a PEC photoanode candidate, its simple binary character, and its abundance in the earth’s crust, there have been surprisingly limited amounts of reports on its underlying defect structure and corresponding bulk transport properties. − There are even fewer studies regarding the defect properties of nanosized α-Fe 2 O 3 , which show particular promise for energy conversion applications. There is clear evidence that such nanosized particles can exhibit substantially different defect chemistries than their bulk counterparts. − In this study, we are presenting our approach for achieving a detailed understanding of the defect regimes and electrical properties of α-Fe 2 O 3 nanostructures (doped and undoped) and comment on their significance in relation to their use as a photoanode with enhanced PEC performance.…”

Solubility Limit of Cu and Factors Governing the Reactivity of Cu–CeO2 Assessed from First-Principles Defect Chemistry and Thermodynamics

“…In fact, the Cu−CeO 2 system evolves chemically and topologically at reaction conditions. For example, Knauth et al 20 performed dilatometry and impedance spectroscopy experiments to find that a Cu 2 O "shell" may form at the surface of the Cu−CeO 2 nanoparticles after annealing at 650 °C. If the sample had been at equilibrium, the solubility of Cu would have increased at higher temperatures and stayed in the CeO 2 lattice, instead of forming a Cu 2 O shell.…”

CeO2 Nanorods and Nanocubes: Impact of Nanoparticle Shape on Dilatometry and Electrical Properties

“…The total sample length change, over the whole duration of an experiment (18 days), was only 0.8 μm, below 2% of the initial thickness (~50 μm, see below). This change is much lower than for any other studied nanoceria samples and a very distinctive feature of nanocube samples; it is likely related to the very high initial packing density attainable with cubes.…”

“…There is also limited work on nanosized CeO 2 powders . In previous studies on spherical nanodimensioned particles, their P O2 ‐dependent oxygen deficiency was related to preferential surface reduction or oxygen adsorption/desorption, but to our knowledge, the electrical properties of variously shaped nanoparticles have not been previously studied. Cerium dioxide is thus a model material for the investigation of shape dependence of the electrical properties of nanosized oxide particles.…”

“…CeO 2 has also numerous applications in heterogeneous catalysis, e.g., for the water gas shift (WGS) reaction, SO 2 reduction by CO, and CO or methane oxidation . Important variations in catalytic activity for the WGS reaction have been noticed not only by reducing the ceria particle size down to nanometers, or by doping e.g., with copper, but also when the shapes of the nanoobjects are modified. For example, ceria nanorods have a much higher catalytic activity than nanopolyhedra, whereas nanocubes exhibit nearly no catalytic activity …”