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91.
Summary Pt/SiO2 and Pt-Fe/SiO2 catalysts having a Pt loading ranging from 0.5 to 3.0 wt.% and a fixed amount of Fe in the bimetallic series, 1.0 wt.% have
been prepared by the impregnation procedure, followed by calcinations and reduction in H2 flow at 773 K. The samples were characterized by N2 adsorption at 77 K, H2 chemisorption at 298 K, TEM, TPR and XPS. The hydrogenation of citral at 363 K and 8.3 bar over a series of Pt/SiO2 and Pt-Fe/SiO2 catalysts was studied. Thus, the selectivity towards the unsaturated alcohol (geraniol + nerol) decreases at high loads of
monometallic Pt. An effect of polarization of the C=O bond due to the presence of Fe3+ species leads to catalysts active and highly selective to the hydrogenation of the carbonyl bond. Characterization results
showed that Pt is present as Pto and Fe mainly asFe3+. 相似文献
92.
Gina Pecchi Patricio Reyes M. Graciela Jiliberto T. López J. L. G. Fierro 《Journal of Sol-Gel Science and Technology》2006,37(3):169-174
Platinum-supported catalysts prepared by impregnation of mixed CeO2/Al2O3 and CeO2/ZrO2 oxides using the sol-gel method were characterized and used in the combustion of ethyl acetate. In each series, the effect
of CeO2 loading (3 and 5 wt%) was studied. Characterization data from the studied catalysts (specific area measurements, hydrogen
chemisorption, programmed temperature reduction (TPR), X-ray diffraction (XRD), photoelectron spectra (XPS) and transmission
electron microscopy (TEM)) revealed significant changes in porosity and metal dispersion in each series. The catalytic activity
of the solids, evaluated in the total combustion of ethyl acetate, exhibited a positive effect with the addition of Ce in
the zirconia series whereas no significant changes was observed in the alumina series. 相似文献
93.
Jose Patricio Díaz Varela Antoni Torrens Torrell 《Archive for Mathematical Logic》2006,45(8):1011-1020
?ukasiewicz implication algebras are {→,1}-subreducts of Wajsberg algebras (MV-algebras). They are the algebraic counterpart of Super-?ukasiewicz Implicational logics investigated in Komori, Nogoya Math J 72:127–133, 1978. The aim of this paper is to study the direct decomposability of free ?ukasiewicz implication algebras. We show that freely generated algebras are directly indecomposable. We also study the direct decomposability in free algebras of all its proper subvarieties and show that infinitely freely generated algebras are indecomposable, while finitely free generated algebras can be only decomposed into a direct product of two factors, one of which is the two-element implication algebra. 相似文献
94.
While every theorem has a proof in mathematics, in US geometry classrooms not every theorem is proved. How can one explain the practitioner’s perspective on which theorems deserve proof? Toward providing an account of the practical rationality with which practitioners handle the norm that every theorem has a proof we have designed a methodology that relies on representing classroom instruction using animations. We use those animations to trigger commentary from experienced practitioners. In this article we illustrate how we model instructional situations as systems of norms and how we create animated stories that represent a situation. We show how the study of those stories as prototypes of a basic model can help anticipate the response from practitioners as well as suggest issues to be considered in improving a model. 相似文献
95.
Stability of closed timelike curves in the Gödel universe 总被引:1,自引:0,他引:1
We study, in some detail, the linear stability of closed timelike curves in the Gödel universe. We show that these curves are stable. We present a simple extension (deformation) of the Gödel metric that contains a class of closed timelike curves similar to the ones associated to the original metric. This extension correspond to the addition of matter whose energy-momentum tensor is analyzed. We find the conditions to have matter that satisfies the usual energy conditions. We study the stability of closed timelike curves in the presence of usual matter as well as in the presence of exotic matter (matter that does satisfy the above mentioned conditions). We find that the closed timelike curves in the Gödel universe with or without the inclusion of regular or exotic matter are stable under linear perturbations. We also find a sort of structural stability. 相似文献
96.
Rodrigo?CastroEmail author Patricio L.?Felmer 《Communications in Mathematical Physics》2005,256(2):411-435
We consider the nonlinear Schrödinger equation with superlinear and subcritical nonlinearity. Assuming that the potential is radially symmetric we find radial sign-changing solutions of the equation that concentrate in a ball, as the parameter goes to zero. We study the asymptotic profile of these highly oscillatory solutions, completely characterizing their behavior by means of an envelope function.Partially supported by FONDECYT Lineas Complementarias Grant 8000010.Partially Supported by FONDAP Matemáticas Aplicadas and FONDECYT Lineas Complementarias Grant 8000010. 相似文献
97.
The title problem is solved assuming that the thickness varies symmetrically with respect to the x-axis. The edges defined by are elastically restrained against rotation while the remaining edges are clamped or simply supported.Approximate expressions for four of the lower natural frequencies of vibration (including the fundamental) are given.A forced vibrations situation is also dealt with. 相似文献
98.
Manuel del Pino Patricio L. Felmer Peter Sternberg 《Communications in Mathematical Physics》2000,210(2):413-446
We examine the asymptotic behavior of the eigenvalue w(h) and corresponding eigenfunction associated with the variational problem m(h) o infy ? H1(W;C ) \fracòW \abs(i?+hA)y2 dx dy òW\absy2 dx dy \mu(h)\equiv\inf_{\psi\in H^{1}(\Omega;{\bf C} )} \frac{\int_{\Omega } \abs{(i\nabla+h{\bf A})\psi}^{2}\,dx\,dy} {\int_{\Omega }\abs{\psi}^{2}\,dx\,dy} in the regime h>>1. Here A is any vector field with curl equal to 1. The problem arises within the Ginzburg-Landau model for superconductivity with the function w(h) yielding the relationship between the critical temperature vs. applied magnetic field strength in the transition from normal to superconducting state in a thin mesoscopic sample with cross-section W ì \R2\Omega\subset\R^{2}. We first carry out a rigorous analysis of the associated problem on a half-plane and then rigorously justify some of the formal arguments of [BS], obtaining an expansion for w while also proving that the first eigenfunction decays to zero somewhere along the sample boundary ?W\partial \Omega when z is not a disc. For interior decay, we demonstrate that the rate is exponential. 相似文献
99.
Patricio René Perrotta Nelio Roberto Vettorazzi Fernando Javier Arévalo Adrián Marcelo Granero Sofía Noemí Chulze María Alicia Zón Héctor Fernández 《Electroanalysis》2011,23(7):1585-1592
The quantification of ochratoxin A is studied at cysteamine self‐assembled monolayer modified gold electrodes in red wine samples by square wave voltammetry. Detection and quantification limits of 0.004 µg L?1 and 0.012 µg L?1, respectively, were determined. The recovery percentages were in the range from 146 % to 94.0 % at spiking levels ranging from 0.02 to 5 µg L?1. The variation coefficients for within‐laboratory repeatability varied from 31.4 to 11.5 % for spiked level from 0.02 to 2.0 µg L?1. The developed electrochemical method is efficient, reproducible, and ultrasensitive for the quantification of OTA in red wine samples. 相似文献
100.
Gaete Patricio Helayël-Neto José 《The European Physical Journal C - Particles and Fields》2014,74(11):1-8
Dark energy models with a slowly rolling cosmological scalar field provide a popular alternative to the standard, time-independent cosmological constant model. We study the simultaneous evolution of background expansion and growth in the scalar field model with the Ratra–Peebles self-interaction potential. We use recent measurements of the linear growth rate and the baryon acoustic oscillation peak positions to constrain the model parameter \(\alpha \) that describes the steepness of the scalar field potential. 相似文献