85 resultados para regime shift


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The water-gas shift (WGS) reaction was carried out in the presence of Pd and Pt substituted nanocrystalline ceria catalysts synthesized by solution combustion technique. The catalysts were characterized by powder XRD and XPS. The noble metals were found to be present in ionic form substituted for the cerium atoms. The catalysts showed highactivity for the WGS reaction with high conversions below 250 degrees C. The products of reaction were only carbon dioxide and hydrogen, and no hydrocarbons were observed even in trace quantities. The reactions were carried out with different amounts of noble metal ion substitution and 2% Pt substituted ceria was found to be the best catalyst. The various possible mechanisms for the reaction were proposed and tested for their consistency with experimental data. The dual site mechanism best described the kinetics of the reaction and the corresponding rate parameters were obtained.

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Noble metal substituted ionic catalysts were synthesized by solution combustion technique. The compounds were characterized by X-ray diffraction, FT-Raman spectroscopy, and X-ray photoelectron spectroscopy. Zirconia supported compounds crystallized in tetragonal phase. The solid solutions of ceria with zirconia crystallized in fluorite structure. The noble metals were substituted in ionic form.The water-gas shift reaction was carried out over the catalysts.Negligible conversions were observed with unsubstituted compounds. The substitution of a noble metal ion was found to enhance the reaction rate. Equilibrium conversion was obtained below 250 degrees C in the presence of Pt ion substituted compounds. The formation of Bronsted acid-Bronsted base pairs was proposed to explain the activity of zirconia catalysts. The effect of oxide ion vacancies on the reactions over substituted ceria-zirconia solid solutions was established. (c)2010 Elsevier B.V. All rights reserved.

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Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La(1-x)A(x)MnO(3), A=Ca, Sr, Ba, x similar or equal to 0.1-0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping (x), average A-site cation radius (< r(A)>), cationic disorder (sigma(2)) and strain (epsilon(zz)). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different < r(A)> reveals the importance of sigma(2) as a metric to qualify any analysis based on (r(A)). (c) 2006 Elsevier Ltd. All rights reserved.

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The purpose of this study is to experimentally investigate the interaction of inelastic deformation and microstructural changes of two Zr-based bulk metallic glasses (BMGs): Zr41.25Ti13.75Cu12.5Ni10Be22.5 (commercially designated as Vitreloy 1 or Vit1) and Zr46.75Ti8.25Cu7.5Ni10Be27.5 (Vitreloy 4, Vit4). High-temperature uniaxial compression tests were performed on the two Zr alloys at various strain rates, followed by structural characterization using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Two distinct modes of mechanically induced atomic disordering in the two alloys were observed, with Vit1 featuring clear phase separation and crystallization after deformation as observed with TEM, while Vit4 showing only structural relaxation with no crystallization. The influence of the structural changes on the mechanical behaviors of the two materials was further investigated by jump-in-strain-rate tests, and flow softening was observed in Vit4. A free volume theory was applied to explain the deformation behaviors, and the activation volumes were calculated for both alloys.

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Experiments were conducted in water and wind tunnels on spheres in the Reynolds number range 6 x 10(3) to 6.5 x 10(5) to study the effect of natural ventilation on the boundary layer separation and near-wake Vortex shedding characteristics. In the subcritical range of Re (<2 x 10(5)), ventilation caused a marginal downstream shift in the location of laminar boundary layer separation; there was only a small change in the vortex shedding frequency. In the supercritical range (Re > 4 x 10(5)), ventilation caused a downstream shift in the mean locations of boundary layer separation and reattachment; these lines showed significant axisymmetry in the presence of venting. No distinct vortex shedding frequency was found. Instead, a dramatic reduction occurred in the wake unsteadiness at all frequencies. The reduction of wake unsteadiness is consistent with the reduction in total drag already reported. Based on the present results and those reported earlier, the effects of natural ventilation on the flow past a sphere can be categorized in two broad regimes, viz., weak and strong interaction regimes. In the weak interaction regime (subcritical Re), the broad features of the basic sphere are largely unaltered despite the large addition of mass in the near wake. Strong interaction is promoted by the closer proximity of the inner and outer shear layers at supercritical Re. This results in a modified and steady near-wake flow, characterized by reduced unsteadiness and small drag.

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The chemical-shift of the X-ray K-absorption edge of Co was studied in a large number of compounds, complexes (spinels) and minerals of Co in its different oxidation states having widely different crystal structures and containing different types of bonding and various types of ligands, and were reported collectively, for the first time, in a single paper. A quadratic relationship was established on the basis of least-squares regression analysis to hold between the chemical-shift and the effective charge on the absorbing atom, but the dominance of the linear term was shown. This relation was utilized in evaluating the charge on the Co-ion in a number of minerals. The effect on chemical-shift of oxidation states of the absorbing atom, of the bond length, crystal structure and higher shell atoms of the molecule, and of electronegativity, atomic number and ionic radius of the ligand was discussed.

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The 1,2-shift observed during oxidation of organic substrates can arise by involvement of cation radicals.

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We demonstrate the activity of Ce0.78Sn0.2Pt0.02O2-delta, a new catalyst, towards water-gas shift (WGS) reaction. Over 99.5% CO conversion to H-2 is observed at 300 +/- 25 degrees C. Based on different characterization techniques we found that the present catalyst is resistant to deactivation due to carbonate formation and sintering of Pt on the surface when subjected to longer duration of reaction conditions. The catalyst does not require any pre-treatment or activation between start-up/shut-down reaction operations. Formation of side products such as methane, methanol, formaldehyde, coke etc. was not observed under the WGS reaction conditions indicating the high selectivity of the catalyst for H-2. Temperature programmed reduction of the catalyst in hydrogen (H-2-TPR) shows reversible reduction of Ce4+ to Ce3+, Sn4+ to Sn2+ and Pt4+ to Pt-0 oxidation state with oxygen storage capacity (OSC) of 3500 mu mol g(-1) at 80 degrees C. Such high value of OSC indicates the presence of highly activated lattice oxygen. CO oxidation in presence of stoichiometric O-2 shows 100% conversion to CO2 at room temperature. The catalyst also exhibits 100% selectivity for CO2 at room temperature towards preferential oxidation (PROX) of residual CO in presence of excess hydrogen in the feed. (C) 2010 Elsevier B.V. All rights reserved.

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The water gas shift reaction was carried out over noble metal ion substituted nanocrystalline oxide catalysts with different supports. Spectroscopic studies of the catalysts before and after the reaction showed different surface phenomena occurring over the catalysts. Reaction mechanisms were proposed based upon the surface processes and intermediates formed. The dual site mechanism utilizing the oxide ion vacancies for water dissociation and metal ions for CO adsorption was proposed to describe the kinetics of the reaction over the reducible oxides like CeO2. A mechanism based on the interaction of adsorbed CO and the hydroxyl group was proposed for the reaction over ZrO2. A hybrid mechanism based on oxide ion vacancies and surface hydroxyl groups was proposed for the reaction over TiO2. The deactivation of the catalysts was also found to be support dependent. Kinetic models for both activation and deactivation were proposed. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2662-2676, 2010

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Heteronuclear multiple-quantum coherence relaxation rate are calculated for the individual transitions of the S spin in an AIS nuclear spin system assuming that the heteronucleus (S spin) has relaxation contributions from both intramolecular dipole-dipole and chemical shift anisotropy relaxation. The individual multiplet components of the heteronuclear zero- and double-quantum coherences are shown to have different transverse relaxation rates. The cross-correlation between the two relaxation mechanisms is shown to be the dominant cause of the calculated differential line broadening. Experimental data are presented using as an example a uniformly 15N labelled sample of human epidermal growth factor.

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Nanocrystalline Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta of similar to 4 nm sizes were synthesized by a sonochemical method using diethyletriamine (DETA) as a complexing agent. Compounds were characterized by powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and transmission electron microscopy (TEM). Ce1-xFexO2-delta (0 <= x <= 0.45) and Ce0.65Fe0.33Pd0.02O2-delta crystallize in fluorite structure where Fe is in +3, Ce is in +4 and Pd is in +2 oxidation state. Due to substitution of smaller Fe3+ ion in CeO2, lattice oxygen is activated and 33% Fe substituted CeO2 i.e. Ce0.67Fe0.33O1.835 reversibly releases 0.31O] up to 600 degrees C which is higher or comparable to the oxygen storage capacity of CeO2-ZrO2 based solid solutions (Catal. Today 2002, 74, 225-234). Due to interaction of redox potentials of Pd2+/0(0.89 V) and Fe3+/2+ (0.77 V) with Ce4+/3+ (1.61 V), Pd ion accelerates the electron transfer from Fe2+ to Ce4+ in Ce0.65Fe0.33Pd0.02O1.815, making it a high oxygen storage material as well as a highly active catalyst for CO oxidation and water gas shift reaction. The activation energy for CO oxidation with Ce0.65Fe0.33Pd0.02O1.815 is found to be as low as 38 kJ mol(-1). Ce0.67Fe0.33O1.835 and Ce0.65Fe0.33Pd0.02O1.815 have also shown high activity for the water gas shift reaction. CO conversion to CO2 is 100% H-2 specific with these catalysts and conversion rate was found to be as high 27.2 mu moles g(-1) s(-1) and the activation energy was found to be 46.4 kJ mol(-1) for Ce0.65Fe0.33Pd0.02O1.815.

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We compute the entropy and transport properties of water in the hydration layer of dipalmitoylphosphatidylcholine bilayer by using a recently developed theoretical scheme two-phase thermodynamic model, termed as 2PT method; S.-T. Lin et al., J. Chem. Phys. 119, 11792 (2003)] based on the translational and rotational velocity autocorrelation functions and their power spectra. The weights of translational and rotational power spectra shift from higher to lower frequency as one goes from the bilayer interface to the bulk. Water molecules near the bilayer head groups have substantially lower entropy (48.36 J/mol/K) than water molecules in the intermediate region (51.36 J/mol/K), which have again lower entropy than the molecules (60.52 J/mol/K) in bulk. Thus, the entropic contribution to the free energy change (T Delta S) of transferring an interface water molecule to the bulk is 3.65 kJ/mol and of transferring intermediate water to the bulk is 2.75 kJ/mol at 300 K, which is to be compared with 6.03 kJ/mol for melting of ice at 273 K. The translational diffusion of water in the vicinity of the head groups is found to be in a subdiffusive regime and the rotational diffusion constant increases going away from the interface. This behavior is supported by the slower reorientational relaxation of the dipole vector and OH bond vector of interfacial water. The ratio of reorientational relaxation time for Legendre polynomials of order 1 and 2 is approximately 2 for interface, intermediate, and bulk water, indicating the presence of jump dynamics in these water molecules. (C) 2010 American Institute of Physics. doi:10.1063/1.3494115]

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A method is presented for the design of compact pulse burst signals, with amplitude and frequency stepping between individual pulses, for optimum rejection of radar clutter distributed arbitrarily in range. The method is illustrated by an example. It is shown that amplitude stepping plays a useful role only when the reciprocal of the individual pulse width is not insignificant compared to the bandwidth permitted to the signal. As an important and useful subclass of the amplitude-and-frequency-stepped signals, constant amplitude FSK bursts are studied and the extent of loss of clutter performance due to amplitude flattening is evaluated.

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We report the results of a comprehensive study on dc magnetization, ac susceptibility, and the magnetotransport properties of the La1-xSrxCoO3(0 <= x <= 0.5) system. At higher Sr doping (x >= 0.18), the system exhibits Brillouin-like field cooled magnetization (M-FC). However, for x < 0.18, the system exhibits a kink in the M-FC, a peak at the intermediate field in the thermoremnant magnetization and a non-saturating tendency in the M-H plot that all point towards the characteristic of spin glass behavior. More interestingly, dc magnetization studies for x < 0.18 do not suggest the existence of ferromagnetic correlation that can give rise to an irreversible line in the spin glass regime. The ac susceptibility study for x > 0.2 exhibits apparently no frequency dependent peak shift around the ferromagnetic transition region. However, a feeble signature of glassiness is verified by studying the frequency dependent shoulder position in chi `' (T) and the memory effect below the Curie temperature. But, for x < 0.18, the ac susceptibility study exhibits a considerable frequency dependent peak shift, time dependent memory effect, and the characteristic spin relaxation time scale tau(0) similar to 10(-13) s. The reciprocal susceptibility versus temperature plot adheres to Curie-Weiss behavior and does not provide any signature of preformed ferromagnetic clusters well above the Curie temperature. The magnetotransport study reveals a cross over from metallic to semiconducting-like behavior for x <= 0.18. On the semiconducting side, the system exhibits a large value of magnetoresistance (upto 75%) towards low temperature and it is strongly connected to the spin dependent part of the random potential distribution in the spin glass phase. Based on the above observations, we have reconstructed a new magnetic phase diagram and characterized each phase with associated properties.