Low-dimensional metallic nanostructures and their electrochemical relevance: Energetics and phenomenological approach

The energetic stability of a set of 36 adsorbate/substrate(1 1 1) couples involving Au, Ag, Cu, Ni, Pt, and Pd is analyzed for perfect metal surfaces and metal surfaces with different types of defects. Monte Carlo annealing at low temperatures is performed to obtain the minimum energy configurations. The simulation results are used to check some theories employed to calculate the binding energy of atoms on surfaces from simple energetic considerations. Besides, a new phenomenological formulation is developed, which can be employed to predict the existence of underpotential deposition for several types of 0-dimensional structures.     

Effect of montmorillonite treatment with supercritical CO2 on the morphology and properties of polypropylene nanocomposites

The effects of montmorillonite (MMT) treatment with supercritical carbon dioxide (SC-CO₂) on clay morphology and the properties of polypropylene nanocomposites were investigated by wide-angle X-ray diffraction, transmission electron microscopy, scanning electron microscopy, dynamic mechanical analysis, and differential scanning calorimetry. The use of poly(propylene glycol) (PPG) was evaluated. The results showed that the MMT morphology (structure formation, dispersion, and orientation) was affected by treatment with SC-CO₂, and the use of PPG, or the use of CO₂ in the liquid state. Consequently, different reinforcement measurements were obtained. The relationship between structure and properties was reported.     

On the invariance of the set of stable matchings with respect to substitutable preference profiles

For the many-to-one matching model we give a procedure to partition the set of substitutable preference profiles into equivalence classes with the property that all profiles in the same class have the same set of stable matchings. This partition allows to reduce the amount of information required by centralized stable mechanisms.     

The effects of ultrasound on organoclay dispersion in the PP matrix

PP/MMT nanocomposites were prepared by solution intercalation using sonication and quiescent conditions, and the effects on the morphological, thermal and mechanical properties were evaluated by WAXD, TEM, DMA, TGA and DSC analyses. The present study aims to clarify the effects of ultrasound use on the organoclay surface with different amounts of organic modifiers and on the exfoliation processes. The sonication process decreased around of 200 nm the aspect ratio of C15A organoclay. Besides, the effectiveness of the ultrasound process was only achieved with the C15A system because there is a small energetic barrier between their layers (clay with larger d 001). The sonication process increased the exfoliation and distribution of the C15A platelets in the PP matrix, increasing by 5% its reinforcement capacity. However, for I44P system, the use of ultrasound did not show any significant effect on the morphology and consequently on the final properties of the PP matrix. The T(c) temperature and the thermal stability of the PP nanocomposites were increased, independent of the clay type or of the ultrasound use.     

Molecular modification of coumarin dyes for more efficient dye sensitized solar cells

In this work, new coumarin based dyes for dye sensitized solar cells (DSSC) have been designed by introducing several substituent groups in different positions of the NKX-2311 structure. Two types of substitutions have been considered: the introduction of three electron-donating groups (-OH, -NH(2), and -OCH(3)) and two different substituents with steric effect: -CH(2)-CH(2)-CH(2)- and -CH(2)-HC=CH-. The electronic absorption spectra (position and width of the first band and absorption threshold) and the position of the LUMO level related to the conduction band have been used as theoretical criteria to evaluate the efficiency of the new dyes. The introduction of a -NH(2) group produces a redshift of the absorption maximum position and the absorption threshold, which could improve the cell efficiency. In contrast, the introduction of -CH(2)-CH(2)-CH(2)- does not modify significantly the electronic structure of NKX-2311, but it might prevent aggregation. Finally, -CH(2)-HC=CH- produces important changes both in the electronic spectrum and in the electronic structure of the dye, and it would be expected as an improvement of cell efficiency for these dyes.     

On group strategy-proof mechanisms for a many-to-one matching model

For the many-to-one matching model in which firms have substitutable and quota q–separable preferences over subsets of workers we show that the workers-optimal stable mechanism is group strategy-proof for the workers. Therefore, in centralized markets like entry-level professional labor markets if the proposed matching is the workers-optimal stable matching then, no group of workers can never benefit by reporting untruthfully their preference relations. We exhibit an example showing that this property fails if the preferences of firms are substitutable but not quota q–separable.Revised August 2004We are grateful to Flip Klijn, Howard Petith, William Thomson, a referee and an associate editor of this journal for helpful comments. The work of R. Martínez, A. Neme, and J. Oviedo is partially supported by the Universidad Nacional de San Luis through Grant 319502, by the Consejo Nacional de Investigaciones Científicas y Técnicas CONICET, through Grant PICT-02114, and by the Agencia Nacional de Promoción Científica y Técnica, through Grant 03-10814. The work of J. Massó is partially supported by the Spanish Ministry of Science and Technology, through Grant BEC2002–2130, and by the Generalitat de Catalunya, through Grant 2001SGR-00162 and the Barcelona Economics Program (CREA). All authors acknowledge financial support from the Grant PCI España-Iberoamérica 2003 (Programa de Cooperación Interuniversitaria de la Agencia Española de Cooperación Internacional-AECI).     

Reactivity of cyclohexene using ordered mesoporous materials based on calcined CoFeAl hydrotalcite-supported CoMo catalysts

Co²⁺, Fe³⁺ and Al³⁺ tertiary hydrotalcite-type solids were synthesized, calcined and impregnated with Mo (15 wt% MoO₃) and Co (3Mo:Co) in order to get different catalytic precursors: CoMo/CoFeAl. As-synthesized hydrotalcites and their catalytic precursors were characterized by different physicochemical techniques, such as: X-ray fluorescence, N₂ adsorption–desorption isotherms (BET specific surface area, BJH pore volume and diameter), Fourier transform infrared spectroscopy, X-ray diffraction and temperature-programmed desorption of carbon dioxide. These CoMo/CoFeAl catalytic precursors were previously pre-sulfided with CS₂ and tested in the cyclohexene hydrogenation reaction. Results indicated that these sulfide catalysts showed very low activity in comparison with the conventional CoMo/Al₂O₃ catalyst used as reference, possibly to their high basicity. These catalysts could have the advantage of retaining the octane number of gasoline, by low hydrogenation of olefins.     

Characterization of the main sources of chitin in cuba

The chemical composition and thermal behaviour of crab, shrimp and lobster shells were studied. The lobster cephalothorax and its main parts also constitute important sources of both polymers. Their chitin and protein content are 20 and 40 %, respectively (dry base) while in these cases the proteins are less associated to the matrix than in the carapace. The chitin level in the chitinous concentrates isolated from different sources is over 80 % in all cases but the polymer characteristics change in dependence on the raw material.

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Zusammenfassung Es wurde die chemische Zusammensetzung und das thermische Verhalten von Krabben-, Garnelen- und Hummerpanzern untersucht. Der Hummer cephalothorax und auch seine Hauptteile bilden wichtige Quellen für beide Polymere. Der Chitin- und Proteingehalt beträgt 20 bzw. 40% (bezogen auf Trockensubstanz), während in diesem Falle die Proteine weniger an der Matrix assoziiert sind als im Panzer. Der Chitingehalt in aus verschiedenen Quellen gewonnenen Chitinauszügen liegt in allen Fällen über 80%, die Eigenschaften der Polymere schwanken jedoch in Abhängigkeit vom Ausgangsmaterial.

     

A family of optimal weighted conjugate-gradient-type methods for strictly convex quadratic minimization

Numerical Algorithms - We introduce a family of weighted conjugate-gradient-type methods, for strictly convex quadratic functions, whose parameters are determined by a minimization model based on a...