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排序方式: 共有251条查询结果,搜索用时 171 毫秒
91.
G.P. Costa A.H.A. Gonçalves L.A.V. Viana J.C.S. Soares F.B. Passos F.M.T. Mendes A.B. Gaspar 《Materials Today Chemistry》2021
Ruthenium catalysts supported on zinc-promoted amorphous-niobium mixed oxides were prepared, characterized, and studied in the additive-free partial hydrogenation of benzene reaction. The amorphous matrix of Nb2O5 was responsible for a highly active Ru/Nb2O5 catalyst, although less selective than those containing zinc. The ZnO-containing supports were prepared by wet impregnation technique, followed by incipient wetness of ruthenium chloride salt. The catalysts were characterized by textural analysis, X-ray fluorescence, X-ray diffraction, H2 chemisorption, temperature-programmed reduction (TPR), Scanning electron microscopy, H2 temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS) of the calcined-reduced samples. Chlorine retention was observed on zinc-containing samples. An unexpected ZnNb2O6 oxide phase, ascribed to a selectivity increase with less activity loss, was obtained for the supports at lower temperatures than those related on the literature. A very complex electronic environment of Ru- and Zn-containing species interactions was observed by TPR. The presence of surface-reduced (Ru0) and partially reduced (Ruδ+) Ru species observed by XPS justified well, respectively, the activity and selectivity achieved with every catalyst. The addition of water as a solvent resulted in very constant yield to cyclohexene, as expected, despite activity diminution due to low solubility of the reactants. 相似文献
92.
R. Hoppeler J. Viana Gomes D. Boiron 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,41(1):157-162
We have studied the atomic density of a cloud confined
in an isotropic harmonic trap at the vicinity of the Bose-Einstein
transition temperature. We show that, for a non-interacting gas
and near this temperature, the ground-state density has the same
order of magnitude as the excited states density at the centre of
the trap. This holds in a range of temperatures where the
ground-state population is negligible compared to the total atom
number. We compare the exact calculations, available in a harmonic
trap, to semi-classical approximations. We show that these latter
should include the ground-state contribution to be accurate. 相似文献
93.
Improved Potentiometric and Optic Sensitivity of Polyaniline Film to Dissolved Oxygen by Incorporating Iron‐Porphyrin
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Polyaniline (PAn) films can be used in an electrochemical and optical combination system to amplify the optical signal for dissolved oxygen (DO) detection. To further improve the sensitivity of PAn film to DO, an oxygen‐sensitive molecule, Fe(III) meso‐tetera (4‐sulfonatophenyl) porphyrin (FeTSPP) is incorporated into PAn during the film generation. Results show that, after incorporating FeTSPP into the PAn matrix, both optical and potentiometric responses to DO are improved. The optimal optical signal is obtained under the application of ?2.5 μA to the modified electrode. Under this applied current, both optical and potentiometric signals show linear relations with the DO concentration within the range of 0.00–4.63 mg L?1, and the sensitivities for optical and potentiometric signals are 4.18 grayscale units mg?1 L and 13.39 mV mg?1 L, respectively. 相似文献
94.
Leroy CM Wang HF Fargues A Cardinal T Jubera V Treguer-Delapierre M Boissière C Grosso D Sanchez C Viana B Pellé F 《Physical chemistry chemical physics : PCCP》2011,13(25):11878-11884
Periodic mesoporous Eu(3+) doped titania materials were obtained through the EISA (Evaporation Induced Self Assembly) process. Eu(3+) ions, entrapped within the semi-crystalline walls of the highly porous framework, appear to be advantageous during the probing of surface photochemical reactions. Its emission intensity is very sensitive to the presence of physisorbed molecules, in gas or liquid phase, that reside within the pores. In particular, strong fluctuations in intensity of the (5)D(0)→(7)F(2) transition were observed under UV light exposure on the time scale of tens of seconds. The emission modulation dynamics show a strong correlation with the crystallinity of the titania matrix. Correlation of the emission with the photocatalytic activity of the semiconductor for photodegradation of an organic molecule is observed. A model is proposed to describe the involved mechanisms. 相似文献
95.
Afonso ML Silva RA Matos M Viana AS Montemor MF Almeida M 《Langmuir : the ACS journal of surfaces and colloids》2012,28(10):4883-4888
The first stages of the electrocrystallization of (Per)(2)[Au(mnt)(2)] salt from dichloromethane on gold, platinum, and highly orientated pyrolytic graphite (HOPG) were investigated by cyclic voltammetry, atomic force microscopy, and X-ray photoelectron spectroscopy in order to understand the determinant factors for nucleation and crystal growth. The crystal growth occurs from adsorbed films of dithiolate on gold or platinum and of perylene on HOPG, after homogeneous nucleation, and it is controlled by the low diffusion of the species toward the growing surface. 相似文献
96.
Paulo Viana 《代数通讯》2013,41(11):4291-4302
For an algebraic nonsingular nonhyperelliptic complete curve of genus three defined over a field of characteristic two we show that the number of canonical Weierstrass points is one of the following: 24, 21, 20, 18, 17, 16, 15, 12, 11, 6 or 5. In most cases, we give also an explicit equation for the curve with the given number of Weierstrass points. 相似文献
97.
The spin-1 Blume–Capel model on a square lattice is studied by using an effective-field theory (EFT) with correlation. We propose an expression for the free energy within the EFT. The phase diagram is constructed in the temperature (T) and single-ion anisotropy amplitude (D) plane. The first-order transition line is obtained by Maxwell construction (comparison between free energies). Our results predict first-order transitions at low temperatures and large anisotropy strengths, which correspond in the phase diagram to the existence of a tricritical point (TCP). We compare our results with mean-field approximation (MFA), that show a qualitative correct behavior for the phase diagram. 相似文献
98.
I.L. CaldasR.L. Viana J.D. Szezech Jr.J.S.E. Portela J. FonsecaM. Roberto C.G.L. MartinsE.J. da Silva 《Communications in Nonlinear Science & Numerical Simulation》2012,17(5):2021-2030
We review symplectic nontwist maps that we have introduced to describe Lagrangian transport properties in magnetically confined plasmas in tokamaks. These nontwist maps are suitable to describe the formation and destruction of transport barriers in the shearless region (i.e., near the curve where the twist condition does not hold). The maps can be used to investigate two kinds of problems in plasmas with non-monotonic field profiles: the first is the chaotic magnetic field line transport in plasmas with external resonant perturbations. The second problem is the chaotic particle drift motion caused by electrostatic drift waves. The presented analytical maps, derived from plasma models with equilibrium field profiles and control parameters that are commonly measured in plasma discharges, can be used to investigate long-term transport properties. 相似文献
99.
We investigate the effect of white-noise perturbations on chaotic trajectories in open billiards. We focus on the temporal decay of the survival probability for generic mixed-phase-space billiards. The survival probability has a total of five different decay regimes that prevail for different intermediate times. We combine new calculations and recent results on noise perturbed Hamiltonian systems to characterize the origin of these regimes and to compute how the parameters scale with noise intensity and billiard openness. Numerical simulations in the annular billiard support and illustrate our results. 相似文献
100.
We investigated the transition to spatio-temporal chaos in spatially extended nonlinear dynamical systems possessing an invariant subspace with a low-dimensional attractor. When the latter is chaotic and the subspace is transversely stable we have a spatially homogeneous state only. The onset of spatio-temporal chaos, i.e. the excitation of spatially inhomogeneous modes, occur through the loss of transversal stability of some unstable periodic orbit embedded in the chaotic attractor lying in the invariant subspace. This is a bubbling transition, since there is a switching between spatially homogeneous and nonhomogeneous states with statistical properties of on-off intermittency. Hence the onset of spatio-temporal chaos depends critically both on the existence of a chaotic attractor in the invariant subspace and its being transversely stable or unstable. 相似文献