Oxidation of nitric oxide (NO) for subsequent efficient reduction in selective catalytic reduction or lean NO(x) trap devices continues to be a challenge in diesel engines because of the low efficiency and high cost of the currently used platinum (Pt)-based catalysts. We show that mixed-phase oxide materials based on Mn-mullite (Sm, Gd)Mn(2)O(5) are an efficient substitute for the current commercial Pt-based catalysts. Under laboratory-simulated diesel exhaust conditions, this mixed-phase oxide material was superior to Pt in terms of cost, thermal durability, and catalytic activity for NO oxidation. This oxide material is active at temperatures as low as 120°C with conversion maxima of ~45% higher than that achieved with Pt. Density functional theory and diffuse reflectance infrared Fourier transform spectroscopy provide insights into the NO-to-NO(2) reaction mechanism on catalytically active Mn-Mn sites via the intermediate nitrate species.
Steady-state IR measurements for adsorption of only CO and under WGS reaction indicate that formates are present on the surface of partially reduced ceria, in contrast to a recent study, and that they are strongly limited at high CO conversions. At low temperatures and conversions, the formates are close to the equilibrium adsorption/desorption coverages obtained from CO adsorption alone. The formates are close to saturation at low temperatures. These IR results favor the bidentate formate mechanism in explaining WGS. However, more kinetic studies are required and over a wider range of temperatures. While low-temperature kinetic studies have found a zero-order dependency for CO and related this to saturation of a noble metal surface, this study indicates that one cannot rule out the possibility of the formate mechanism on this basis, as CO is also close to saturation as an adsorbed formate at the low temperatures used in previous studies.
Nanoceria is used as a catalyst in diesel fuel, as an abrasive in printed circuit manufacture, and is being pursued as an antioxidant therapeutic. Our objective is to extend previous findings showing that there were no reductions of cerium in organs of the mononuclear phagocyte (reticuloendothelial) system up to 30 days after a single nanoscale ceria administration. An ~5% aqueous dispersion of citrate-stabilized 30 nm ceria, synthesized and characterized in-house, or vehicle, was iv infused into rats terminated 1, 7, 30, or 90 days later. Cageside observations were obtained daily, body weight weekly. Daily urinary and fecal cerium outputs were quantified for 2 weeks. Nine organs were weighed and samples collected from 14 tissues/organs/systems, blood and cerebrospinal fluid for cerium determination. Histology and oxidative stress were assessed. Less than 1% of the nanoceria was excreted in the first 2 weeks, 98% in feces. Body weight gain was initially impaired. Spleen weight was significantly increased in some ceria-treated groups, associated with abnormalities. Ceria was primarily retained in the spleen, liver, and bone marrow. There was little decrease of ceria in any tissue over the 90 days. Granulomas were observed in the liver. Time-dependent oxidative stress changes were seen in the liver and spleen. Nanoscale ceria was persistently retained by organs of the mononuclear phagocyte system, associated with adverse changes. The results support concern about the long-term fate and adverse effects of inert nanoscale metal oxides that distribute throughout the body, are persistently retained, and produce adverse changes.
This team of investigators revealed that nanoceria, which is being studied as an anti-oxidant, has very limited uptake by the brain regardless of the range of sizes studied, suggesting major challenges in the application of this novel approach in the central nervous system.
The objective was to characterize the biodistribution of nanoscale ceria from blood and its effects on oxidative stress endpoints. A commercial 5% crystalline ceria dispersion in water (average particle size Â3194 nm) was infused intravenously into rats (0, 50, 250 and 750 mg/kg), which were terminated 1 or 20 h later. Biodistribution in rat tissues was assessed by microscopy and ICP-AES/MS. Oxidative stress effects were assessed by protein-bound 4-hydroxy 2-transnonenal (HNE), protein-bound 3-nitrotyrosine (3-NT), and protein carbonyls. Evans blue (EB)-albumin and Na fluorescein (Na 2 F) were given intravenously as blood-brain barrier (BBB) integrity markers. The initial ceria t ½ in blood was Â7 min. Brain EB and Na 2 F increased some at 20 h. Microscopy revealed peripheral organ ceria agglomerations but little in the brain. Spleen Ce concentration was !liver !blood !brain. Reticuloendothelial tissues cleared ceria. HNE was significantly increased in the hippocampus at 20 h. Protein carbonyl and 3-NT changes were small. The nanoparticle characterizations before and after biodistribution, linked with the physiological responses, provide a foundation for evaluating the effects of engineered nanomaterial physico-chemical properties on peripheral organ distribution, brain entry and resultant toxicity.
Particles and fibres affect human health as a function of their properties such as chemical composition, size and shape but also depending on complex interactions in an organism that occur at various levels between particle uptake and target organ responses.
While particulate pollution is one of the leading contributors to the global burden of disease, particles are also increasingly used for medical purposes. Over the past decades we have gained considerable experience in how particle properties and particle-bio interactions are linked to human health. This insight is useful for improved risk management in the case of unwanted health effects but also for developing novel medical therapies. The concepts that help us better understand particles’ and fibres’ risks include the fate of particles in the body; exposure, dosimetry and dose-metrics and the 5 Bs: bioavailability, biopersistence, bioprocessing, biomodification and bioclearance of (nano)particles. This includes the role of the biomolecule corona, immunity and systemic responses, non-specific effects in the lungs and other body parts, particle effects and the developing body, and the link from the natural environment to human health. The importance of these different concepts for the human health risk depends not only on the properties of the particles and fibres, but is also strongly influenced by production, use and disposal scenarios.
Lessons learned from the past can prove helpful for the future of the field, notably for understanding novel particles and fibres and for defining appropriate risk management and governance approaches.
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