Exploring the Redox Properties of Ce1-xUxO2±Î´ (x ≤ 0.5) Oxides for Energy Applications.

The Ce-U-O system, forming a solid solution in the fluorite structure, has gained much attention due to its unique properties. Mixed fluorite oxide powders of Ce1-xUxO2±Î´ compositions were found to be particularly active for H2 production through thermochemical water splitting. In the present work, we explore the reduction-oxidation properties of the mixed oxides with x = 0.1, 0.25, and 0.5. We report a particularly high oxygen storage capacity (OSC) for x ≥ 0.25 and show that the oxygen extracted from these mixed oxides is of a different origin than that extracted from CeO2. While in ceria, oxygen is extracted from the tetrahedral sites, leading to the formation of oxygen vacancies, the extracted oxygen in Ce1-xUxO2±Î´ (x ≥ 0.25) is essentially excess oxygen in the fluorite lattice (which spontaneously penetrates the oxide under ambient or oxidative conditions). This property, which is clearly related to the change in the valency of the U cations, is apparently responsible for the higher OSC and the lower activation energy for oxygen extraction from the mixed oxides compared to ceria. The mixed oxide powders are shown to be structurally stable, retaining their fluorite structure following reduction under Ar-5%H2 or oxidation in air until 1000 °C. The presented results provide new insights into the Ce-U-O system which may be exploited for future technical applications, as a catalyst for thermochemical water splitting, or as a solid electrolyte in solid oxide fuel cells.

Unidirectional rotation of micromotors on water powered by pH-controlled disassembly of chiral molecular crystals.

Biological and synthetic molecular motors, fueled by various physical and chemical means, can perform asymmetric linear and rotary motions that are inherently related to their asymmetric shapes. Here, we describe silver-organic micro-complexes of random shapes that exhibit macroscopic unidirectional rotation on water surface through the asymmetric release of cinchonine or cinchonidine chiral molecules from their crystallites asymmetrically adsorbed on the complex surfaces. Computational modeling indicates that the motor rotation is driven by a pH-controlled asymmetric jet-like Coulombic ejection of chiral molecules upon their protonation in water. The motor is capable of towing very large cargo, and its rotation can be accelerated by adding reducing agents to the water.