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.

Defect Chemistry of Oxides for Energy Applications.

Oxides are widely used for energy applications, as solid electrolytes in various solid oxide fuel cell devices or as catalysts (often associated with noble metal particles) for numerous reactions involving oxidation or reduction. Defects are the major factors governing the efficiency of a given oxide for the above applications. In this paper, the common defects in oxide systems and external factors influencing the defect concentration and distribution are presented, with special emphasis on ceria (CeO2 ) based materials. It is shown that the behavior of a variety of oxide systems with respect to properties relevant for energy applications (conductivity and catalytic activity) can be rationalized by general considerations about the type and concentration of defects in the specific system. A new method based on transmission electron microscopy (TEM), recently reported by the authors for mapping space charge defects and measuring space charge potentials, is shown to be of potential importance for understanding conductivity mechanisms in oxides. The influence of defects on gas-surface reactions is exemplified on the interaction of CO2 and H2 O with ceria, by correlating between the defect distribution in the material and its adsorption capacity or splitting efficiency.

The Effect of Lithium Doping on the Sintering and Grain Growth of SPS-Processed, Non-Stoichiometric Magnesium Aluminate Spinel.

The effects of lithium doping on the sintering and grain growth of non-stoichiometric nano-sized magnesium aluminate spinel were studied using a spark plasma sintering (SPS) apparatus. Li-doped nano-MgO·nAl2O3 spinel (n = 1.06 and 1.21) powders containing 0, 0.20, 0.50 or 1.00 at. % Li were synthesized by the solution combustion method and dense specimens were processed using a SPS apparatus at 1200 °C and under an applied pressure of 150 MPa. The SPS-processed samples showed mutual dependency on the lithium concentration and the alumina-to-magnesia ratio. For example, the density and hardness values of near-stoichiometry samples (n = 1.06) showed an incline up to 0.51 at. % Li, while in the alumina rich samples (n = 1.21), these values remained constant up to 0.53 at. % Li. Studying grain growth revealed that in the Li-MgO·nAl2O3 system, grain growth is limited by Zener pining. The activation energies of undoped, 0.2 and 0.53 at. % Li-MgO·1.21Al2O3 samples were 288 ± 40, 670 ± 45 and 543 ± 40 kJ·mol-1, respectively.

Clinical utility of stent cytology for the diagnosis of pancreaticobiliary neoplasms.

OBJECTIVE: ERCP is frequently the technique of choice for the diagnosis and palliative treatment of pancreaticobiliary neoplasms. The sensitivity of tissue sampling techniques has been disappointing. Two small series have reported a high diagnostic yield from cytological examination of cellular debris from removed biliary stents. The aim of this study was to determine the clinical utility of stent cytology for the diagnosis of biliary and pancreatic neoplasia. METHODS: Patients who had biliary or pancreatic stents removed at ERCP and evaluated cytologically were the subjects of this study. Stents were scraped and washed, and the cellular debris was examined cytologically and recorded as positive, atypical, or negative. Patient demographics, ductal stricture, and stent characteristics were recorded. RESULTS: Between January, 1994 and January, 2001 a total of 126 biliary stents from 101 patients (61 male, 40 female, mean age 62 yr) and 29 pancreatic stents from 19 patients (seven male, 12 female, mean age 58 yr) were examined. The sensitivity, specificity, positive predictive value, and negative predictive value of biliary and pancreatic stents for the diagnosis of neoplasia were: 11%, 100%, 100%, 53%, and 25%, 100%, 100%, 77.8%, respectively. The sensitivity improved slightly if all degrees of atypia were considered positive for neoplasia. The mean duration of stent placement was 93.6 days for biliary stents and 43.5 days for pancreatic stents. CONCLUSIONS: Biliary and pancreatic stent cytology has limited clinical utility in the diagnosis of pancreaticobiliary neoplasia, owing to poor sensitivity. In addition, the time lag between stent placement and removal for cytologic examination further limits its contribution to patient management.