Ethene Oxidation on Pd(111): Kinetic Hysteresis Induced by Carbon Dissolution

The catalytic oxidation of ethene was studied on Pd(111) in the 10⁻⁷–10⁻⁶ mbar pressure range by a molecular beam TPR hysteresis experiment between 400 K and 1,000 K. Two important effects were identified: the reaction-blocking effect of a dense chemisorbed adlayer of oxygen and the promotional effect of dissolved carbon segregating back to the surface and efficiently reducing the adsorbed oxygen. A strong dependence of the catalytic activity on the oxygen partial pressure is explained by the inhibiting effect of oxygen adsorption; high oxygen pressures in fact extinguish the reaction. The presence of oxygen-free metal surface area, where ethene can dissociate, is necessary for high activity. During heating the highest activity is observed at T ∼ 620 K, where a combination of oxygen clean-off by carbon segregating back to the surface is combined with a high ethene adsorption rate, thus forming additional reaction sites and additional reaction products. During heating this carbon-induced clean-off of O(ad) is very efficient because the dissolved C atoms rather accumulate in the surface-near region and largely segregate back to the surface at T > 600 K. In contrast, during cooling from higher temperatures a high surface-near carbon bulk concentration does not build up because the bulk mobility of C atoms is also high and the faster diffusion of C into deeper layers counteracts carbon enrichment in the surface-near metal bulk. This effect favours a higher oxygen surface coverage and a stronger deactivation during cooling. If the carbon loading of the surface-near region was increased by decomposition of clean ethene prior to the reaction experiment, the promotional effect during the heating cycle was strongly enhanced, but the cooling cycle showed no memory of the C presaturation. Generally, the observed hysteresis effects stem from an interplay of combined oxygen site blocking and carbon diffusion effects. Konrad Hayek—deceased.     

First-Principles Phase Stability Calculations of Pseudobinary Alloys of (Al,Zn)3Ti with L 12, D 022, and D 023 Structures

The thermodynamic and mechanical stability of intermetallic phases in the Al₃Ti-Zn₃Ti pseudobinary alloy system is investigated from first-principles total energy calculations through electronic density-functional theory within the generalized gradient approximation. Both supercell calculations and sublattice-cluster-expansion methods are used to demonstrate that the addition of Zn to the Al sublattice of Al₃Ti stabilizes the cubic L1₂ structure relative to the tetragonal D0₂₂ and D0₂₃ structures. This trend can be understood in terms of a simple rigid-band picture in which the addition of Zn modifies the effective number of valence electrons that populate bonding and anti-bonding states. The calculated zero-temperature elastic constants show that the binary end members are mechanically stable in all three ordered phases. These results point to a promising way to cost effectively achieve the stabilization of L1₂ precipitates in order to favor the formation of a microstructure associated with desirable mechanical properties. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by The Alloy Phase Committee of the joint EMPMD/SMD of The Minerals, Metals, and Materials Society (TMS), held in San Antonio, TX, March 12-16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany.     

Responses of beaver (Castor canadensis Kuhl) to predator chemicals

Free-ranging beaver (Castor canadensis) in two different beaver populations in New York State were exposed to predator chemicals to test feeding inhibition. Solvent extracts of feces were applied to stem sections of aspen, the preferred food tree of beavers, permitting smelling and tasting the samples. Predator odors were from wolf (Canis lupus), coyote (Canis latrans), dog (Canis familiaris), black bear (Ursus americanus), river otter (Lutra canadensis), lynx (Lynx canadensis), and African lion (Panthera leo). The experiment was repeated. The predator odors reduced feeding compared to untreated or solvent-treated controls. One population consumed 17.0% of the samples with predator odor and 27.0% of the controls in summer, and 48.4% and 60.0%, respectively, in autumn. The other population accepted 3.15% of the predator odor samples and 11.05% of the controls in summer. Coyote, lynx, and river otter odors had the strongest effects. Diesel oil and bitter-tasting neem extract had weaker effects. Predator odors are promising as feeding repellents for beaver.     

A multichannel time-domain scanning fluorescence mammograph: performance assessment and first in vivo results

We present a scanning time-domain fluorescence mammograph capable to image the distribution of a fluorescent contrast agent within a female breast, slightly compressed between two parallel glass plates, with high sensitivity. Fluorescence of the contrast agent is excited using a near infrared picosecond diode laser module. Four additional picosecond diode lasers with emission wavelengths between 660 and 1066 nm allow to measure the intrinsic optical properties of the breast tissue. By synchronously moving a source fiber and seven detection fiber bundles across the breast, distributions of times of flight of photons are recorded simultaneously for selected source-detector combinations in transmission and reflection geometry either at the fluorescence wavelength or at the selected laser wavelengths. To evaluate the performance of the mammograph, we used breastlike rectangular phantoms comprising fluorescent and absorbing objects using the fluorescent dye Omocyanine as contrast agent excited at 735 nm. We compare two-dimensional imaging of the phantom based on transmission and reflection data. Furthermore, we developed an improved tomosynthesis algorithm which permits three-dimensional reconstruction of fluorescence and absorption properties of lesions with good spatial resolution. For illustration, we present fluorescence mammograms of one patient recorded 30 min after administration of the contrast agent indocyanine green showing the carcinoma at high contrast originating from fluorescence of the extravasated dye, excited at 780 nm.     

Correlation between Surface Texture and Chemical Composition in Undoped, Hard, and Soft Piezoelectric PZT Ceramics

The electrical and mechanical properties of piezoelectric lead zirconate titanate (PZT) ceramics are strongly influenced by domain-wall motion, which can be tailored via the substitution of ions within the perovskite structure. Different domain mobilities are achieved by lead and/or oxygen vacancies, according to the valency and ionic radius of the dopants. To quantify the surface near domain mobility, hard (silver-doped), soft (lanthanum-doped), and undoped PZT ceramics have been prepared. An applied mechanical stress after sintering causes texturing near the surface, because of the ferroelastic behavior of the 90° domains. The texture is quantified via X-ray diffractometry (XRD) analysis of the tetragonal F _T(002)- and F _T(200)-peak intensities, using least-squares refinement with Gaussian profile functions. The samples are subsequently annealed to remove the surface texture and again characterized via XRD measurements. However, annealed samples still reveal a preferred domain orientation that can be removed only by a second annealing of the pulverized samples. A comparison of the tetragonal additive systems clearly reveals the greatest domain mobility for lanthanum-doped PZT ceramics, whereas the silver-doped and undoped samples have similar ferroelastic behavior. Furthermore, the surface texture of all the compositions is dependent on the applied mechanical stress and cannot be removed completely by heat treatment.