Oxygen Storage and Mobility on Model Three-Way Catalysts

Oxygen storage capacity (OSC) of TW catalysts has a strong impact on their performances under oscillatory conditions. This paper reviews the different methods developed for OSC measurements, the effects of catalyst composition on OSC as well as the impact of OSC on TW catalysis, concentrating on the results obtained in our laboratory. Use of OSC measurements for catalysts control (on-board diagnostic) is also examined. The role of superoxide species as intermediates in the oxygen storage process will be emphasized. Finally, we shall review the recent developments on CeZr-based oxides.     

Magnetization reversal in Pt/Co(0.5 nm)/Pt nano-platelets patterned by focused ion beam lithography

Arrays of ultrathin Pt/Co(0.5 nm)/Pt nano-platelets with lateral sizes ranging from 30 nm to 1 μm have been patterned by focused ion beam (FIB) lithography under a weak Ga(+) ion fluence. From polar magneto-optical Kerr microscopy it is demonstrated that nano-platelets are ferromagnetic with perpendicular anisotropy down to a size of 50 nm. The irradiation process creates a magnetically soft ring at the nano-platelet periphery in which domain nucleation is initiated at a low field. The magnetization reversal in nano-platelets can be interpreted using a confined droplet model. All of the results prove that ultimate FIB patterning is suitable for preparing discrete magnetic recording media or small magnetic memory elements and nano-devices.     

From Zr-Rich to Ce-Rich: Thermal Stability of OSC Materials on the Whole Range of Composition

Car manufacturers use close coupled three way catalysts to reduce cold start emissions. These close coupled catalysts must show long term durability and thermal stability at temperatures higher than 1000 °C. Thus, during the past 10 years, a significant amount of attention has been paid to the design of CeO₂-ZrO₂ OSC materials with improved surface area, phase and OSC stability. Solid solutions of Ce/Zr mixed oxides are now commonly available on industrial scale. Recent studies have demonstrated that the introduction of rare earth oxides promoters such as La, Pr, Nd, Y, Sm [ENGELHARD WO patent o2 30546; OMG-DMC² EP Patent 1 181 970; DAIHATSU EP Patent 1 174 174; DELPHI US Patent 6 387 338; ENGELHARD WO Patent 02 22242] or aluminium oxide [TOYOTA EP Patent 1 172 139; Kanzawa et al. SAE Technical Paper 2003-01-0811] in the Ce/Zr matrix is a very good way to improve the thermal stability and the redox properties of these materials. Positive interactions between PGM and these 'promoters' are also claimed. Nevertheless, not only the composition but the optimization of the manufacturing processes is a key factor to obtain both high thermal stability (expressed as surface area and XRD phase purity) and OSC. To meet the increasingly stringent emission limits, Rhodia has developed a new generation of CeZr based mixed oxides. These materials show phase stability and thermal stability at temperatures higher than 1100 °C in a wide composition range: from Zr-rich to Ce-rich mixed oxides. These characteristics have been obtained with a new manufacturing process which leads to highly porous materials. The controlled morphology at microscopic and macroscopic scale prevents particle sintering under severe ageing conditions. Such materials are preferred PGM supports for cost effective catalysts with low PGM loading     

Pressure-Induced Frequency Lineshifts in the nu2 Band of Ammonia: An Experimental Test of the Rydberg-Ritz Principle

The hemolytic lectin, CEL-III, is a Ca2+-dependent, galactose/N-acetylgalactosamine-specific lectin purified from the marine invertebrate, Cucumaria echinata (Holothuroidea). After binding to specific carbohydrates on the erythrocyte surface, CEL-III forms ion-permeable pores by oligomerizing in the membrane, which leads to colloid osmotic rupture of the cells. When incubated with liposomes composed of total lipids from the human erythrocyte membrane, CEL-III efficiently induced the leakage of carboxyfluorescein (CF) trapped in the vesicles, suggesting the presence of its receptor in the membrane lipids. The rate of CF-leakage increased with increasing temperature, although the hemolytic activity of CEL-III had been found to be much higher at lower temperatures (around 10 degrees C). Identification of the receptor for CEL-III was performed by examining the ability of individual lipids from human erythrocytes to induce CF-leakage from DOPC-liposomes. As a result, the most effective receptor was found to be lactosyl ceramide (LacCer), while globoside (Gb4Cer) also showed slight induction of CF-leakage. On the other hand, a binding assay involving CEL-III-horseradish peroxidase conjugate indicated that CEL-III exhibits similar affinity for LacCer and Gb4Cer, suggesting that the structure or length of the carbohydrate portion of sphingoglycolipids is also relevant as to their ability to induce CF-leakage in addition to their affinity. Electron micrographs of CEL-III-treated liposomes revealed that CEL-III induced considerable morphological changes in the vesicles, while a clearly distinguishable oligomeric structure of the protein was not observed.     

From Zr-rich to Ce-rich: thermal stability of OSC materials on the whole range of composition

Topics in Catalysis - Car manufacturers use close coupled three way catalysts to reduce cold start emissions. These close coupled catalysts must show long term durability and thermal stability at...     

Rare earths based oxides as alternative materials to Ba in NOx-trap catalysts

The reduction of NOx from Diesel Engines is a major issue of automotive catalysis. In this paper, we investigate basic rare earths oxides (such as La₂O₃, Nd₂O₃ or Pr₆O₁₁) used as NOx-trap sites in replacement of Ba. These species are intimately mixed with (CeZr) solid solutions and the thus obtained mixed oxides are considered as Pt carrier. Investigations on powder model Pt-(CeZr)/RE₂O₃ (or Pr₆O₁₁) catalysts show promising results. These model catalysts present high NOx storage capacity, especially at low temperature, and improved resistance to sintering at 700 °C under lean hydrothermal conditions. Moreover, they show total desulfation at 600 °C i.e. 150 °C lower than current Pt-(BaO/Al₂O₃) technologies and are thus considered as more durable technologies.     

Progress in the studies of the advanced plutonium fuel assembly

A new assembly concept, designated APA (for dvanced lutonium fuel ssembly), should make it possible to multi-recycle plutonium in pressurized water reactors. The basic idea is founded on the manufacture of a large plutonium thin annular fuel rod with an inert support, cooled on both faces. The absence of plutonium generation, combined with moderate fuel temperature should make it possible to achieve substantial burn-up fractions in these rods. The assembly is compatible with the internals of a Pressurized Water Reactor (PWR), and provides for permanent reversibility. Neutronic studies showed a compliance with actual safety/control criteria. A multi-recycling scenario was simulated for 84 years' operation with a 65 GW electrical power installed capacity, comprising forty-five 1450 MW electrical power PWRs, 32 of which are loaded with UO₂ and 13 with APAs. It showed that the plutonium inventory is controlled. Thermal-hydraulic studies showed one can find an annular rod geometry allowing one to respect both margins to Critical Heat Flux (CHF) during normal and accidental operations and void fraction limitations.