Search results for "Dispersion"
showing 10 items of 1101 documents
Hydrogenation of acetylene-ethylene mixtures on Pd catalysts: study of the surface mechanism by computational approaches. Metal dispersion and cataly…
2000
The hydrogenation mechanism of acetylene–ethylene mixtures on Pd catalysts under different experimental conditions was studied by employing a time-dependent Monte Carlo approach set to use a fixed series of event probabilities. The dependence of the catalyst activity and selectivity on the sizes of the metal particles was simulated at microscopic level and the results, also refined by fitting procedures, suggested proper explanations for the apparent nonuniformity of the related experimental findings. The use of the steric hindrance parameter of the surface species and the available surface energy on the metallic catalyst sites was decisive for reproducing the experimental results.
Hydrogenation of 2,4-Dinitro-toluene on Pd/C Catalysts: Computational Study on the Influence of the Molecular Adsorption Modes and of Steric Hindranc…
2002
Abstract A new time-dependent Monte Carlo algorithm was developed to simulate isobar and isotherm three-phase batch hydrogenation of 2,4-dinitro-toluene on Pd/C catalysts. A new reaction mechanism was formulated, involving 9 and 27 toluene derivatives, in solution and adsorbed on the surface, respectively. In fact, three different ways of adsorption were considered for all surface derivatives. Microscopic mechanistic hypotheses were formulated analyzing the mimicked surface populations able to reproduce the experimental catalytic activity–selectivity patterns at different times, temperatures, reagent concentrations, and catalyst particle morphologies. The three different adsorption modes, g…
13C and17O NMR study of methoxy groups in chlorinated Di- and trimethoxybenzenes
1990
13C and 17O NMR data [chemical shifts and 1J(CH) values] for the methoxy groups in isomeric 1,2-, 1,3- and 1,4-dimethoxybenzenes, 1,2,3-trimethoxybenzenes and most of their chlorinated derivatives and some related brominated compounds were measured for CDCl3 solutions. The 17O NMR chemical shifts show up to 60 ppm dispersion. Comparison between the compounds with and without adjacent chlorine atoms (2,6-di- and 2,4,6-tri-substitution) also showed a clear methoxy carbon chemical shift change. The number and position of the chlorine atoms in the aromatic ring give small but observable effects on the 17O NMR chemical shifts of the methoxy group if it is coplanar with the aromatic plane. Simila…
Energy interactions in amyloid-like fibrils from NNQQNY.
2014
We use large-scale MP2 calculations to analyze the interactions appearing in amyloid fibers, which are difficult to determine experimentally. To this end, dimers and trimers of the hexapeptide NNQQNY from the yeast prion-like protein Sup35 were considered as model systems. We studied the energy interactions present in the three levels of organization in which the formation of amyloid fibrils is structured. The structural changes in the hydrogen bonds were studied too. It was found that the most energetic process is the formation of the β-sheet, which is equally due to both hydrogen bonds and van der Waals interactions. The aromatic rings help stabilize these aggregates through stacking of t…
The Instability of Ni{N(SiMe3 )2 }2 : A Fifty Year Old Transition Metal Silylamide Mystery.
2015
The characterization of the unstable Ni(II) bis(silylamide) Ni{N(SiMe3 )2 }2 (1), its THF complex Ni{N(SiMe3 )2 }2 (THF) (2), and the stable bis(pyridine) derivative trans-Ni{N(SiMe3 )2 }2 (py)2 (3), is described. Both 1 and 2 decompose at ca. 25 °C to a tetrameric Ni(I) species, [Ni{N(SiMe3 )2 }]4 (4), also obtainable from LiN(SiMe3 )2 and NiCl2 (DME). Experimental and computational data indicate that the instability of 1 is likely due to ease of reduction of Ni(II) to Ni(I) and the stabilization of 4 through dispersion forces.
Influence of Uniaxial Pressure on Dielectric Properties of the PLZT-x/65/35 (x = 11 and 13) Ceramics
2012
Uniaxial pressure (0–1000 bars) effects on the dielectric properties of the PLZT-x/65/35 ceramic samples (x = 11 and 13) samples was investigated. The obtained results reveal that the applied stress largely changes the dielectric properties of these ceramics. Applying uniaxial pressure leads to a reduction of the peak intensity of the electric permittivity and a change of the frequency dispersion. Besides with increasing the pressure the ϵ-peak shifts to a higher temperature. The results show that applying the uniaxial pressure induces similar effects as increasing the Ti−ion concentration in PZT system. These effects can be connected with the change in domain structure under the action of …
New model describing the total dispersion of dry powder agglomerates
2010
Micron and submicron particles tend to form agglomerates due to adhesive and cohesive forces. The disintegration of these dry powder agglomerates often represents a major task in powder technology. One model describing the mechanical stability of granulates is given by the planar fracture model of Rumpf. In the present work, a new model is introduced intended for applications where a complete disintegration of dry powder agglomerates is required (e.g. for pharmaceutical aerosol generation). Therefore, the breakup of every single connection inside an agglomerate is considered and the dispersion strength σdisp of model agglomerates is calculated and discussed. In order to balance the stabilit…
Efficient energy conversion for cubic third-harmonic generation that is phase matched in KTiOPO(4).
2000
We demonstrate, for the first time to our knowledge, that efficient third-harmonic generation can be achieved with a cubic contribution much larger than the quadratic processes. An energy-conversion efficiency of 2.4% is achieved for cubic third-harmonic generation that is phase matched along the x axis of a KTiOPO(4) crystal by use of a picosecond fundamental laser emitting at 1618 nm. The associated cascading processes are only 10% of the pure cubic interaction, which is very suitable for study of the specific quantum optical correlations. Calculations of the third-harmonic generation conversion efficiency with respect to group-velocity dispersion and to the longitudinal Gaussian beam pro…
Layered double hydroxide nanocomposites based on carbon nanoforms
2020
Abstract Carbon nanoform (CNF)/layered double hydroxide (LDH) nanocomposites have been widely studied in recent years thanks to the combination of properties of both LDH and carbon nanomaterials. They exhibit improved or unexpected properties as a consequence of their hierarchical structures. There are three main reported approaches to prepare the nanocomposites: reassembly of nanocarbons and LDHs, direct formation of LDHs on nanocarbon materials, and the direct synthesis of nanocarbons on the LDH phase. The resulting CNF/LDH nanocomposites exhibit improved conductivity, mechanical properties, and redox reactivity; moreover, the hybridization confers hierarchical porosities and better dispe…
Temperature dependence of the magnetic Casimir-Polder interaction
2009
We analyze the magnetic dipole contribution to atom-surface dispersion forces. Unlike its electrical counterpart, it involves small transition frequencies that are comparable to thermal energy scales. A significant temperature dependence is found near surfaces with a nonzero DC conductivity, leading to a strong suppression of the dispersion force at T > 0. We use thermal response theory for the surface material and discuss both normal metals and superconductors. The asymptotes of the free energy of interaction and of the entropy are calculated analytically over a large range of distances. Near a superconductor, the onset of dissipation at the phase transition strongly changes the interac…