1,057 research outputs found
Деградация человеческого потенциала как фактор латентной составляющей деятельности высшей школы Украины
Рассмотрены проблемы тенизации и коррупционности функционирования украин-ской высшей школы на фоне вектора развития показателей потенциала населения страны.Розглянуті проблеми тінізації і коррупційності функціонування української вищої школи на тлі вектору розвитку показників потенціалу населення країни
In Situ Transmission Electron Microscopy to Study the Location and Distribution Effect of Pt on the Reduction of Co3O4–SiO2
The addition of Pt generally promotes the reduction of Co3O4 in supported catalysts, which further improves their activity and selectivity. However, due to the limited spatial resolution, how Pt and its location and distribution affect the reduction of Co3O4 remains unclear. Using ex situ and in situ ambient pressure scanning transmission electron microscopy, combined with temperature-programmed reduction, the reduction of silica-supported Co3O4 without Pt and with different location and distribution of Pt is studied. Shrinkage of Co3O4 nanoparticles is directly observed during their reduction, and Pt greatly lowers the reduction temperature. For the first time, the initial reduction of Co3O4 with and without Pt is studied at the nanoscale. The initial reduction of Co3O4 changes from surface to interface between Co3O4 and SiO2. Small Pt nanoparticles located at the interface between Co3O4 and SiO2 promote the reduction of Co3O4 by the detachment of Co3O4/CoO from SiO2. After reduction, the Pt and part of the Co form an alloy with Pt well dispersed. This study for the first time unravels the effects of Pt location and distribution on the reduction of Co3O4 nanoparticles, and helps to design cobalt-based catalysts with efficient use of Pt as a reduction promoter
Carbon Nanofiber Growth Rates on NiCu Catalysts: Quantitative Coupling of Macroscopic and Nanoscale In Situ Studies
Since recently, gas-cell transmission electron microscopy allows for direct, nanoscale imaging of catalysts during reaction. However, often systems are too perturbed by the imaging conditions to be relevant for real-life catalyzed conversions. We followed carbon nanofiber growth from NiCu-catalyzed methane decomposition under working conditions (550 °C, 1 bar of 5% H2, 45% CH4, and 50% Ar), directly comparing the time-resolved overall carbon growth rates in a reactor (measured gravimetrically) and nanometer-scale carbon growth observations (by electron microscopy). Good quantitative agreement in time-dependent growth rates allowed for validation of the electron microscopy measurements and detailed insight into the contribution of individual catalyst nanoparticles in these inherently heterogeneous catalysts to the overall carbon growth. The smallest particles did not contribute significantly to carbon growth, while larger particles (8-16 nm) exhibited high carbon growth rates but deactivated quickly. Even larger particles grew carbon slowly without significant deactivation. This methodology paves the way to understanding macroscopic rates of catalyzed reactions based on nanoscale in situ observations
Synthesis and Characterization of Supported Mixed MoW Carbide Catalysts
For mixed MoW carbide catalysts, the relationship between synthesis conditions, evolution of (mixed) phases, extent of mixing, and catalytic performance of supported Mo/W carbides remains unclear. In this study, we prepared a series of carbon nanofiber-supported mixed Mo/W-carbide catalysts with varying Mo and W compositions using either temperature-programmed reduction (TPR) or carbothermal reduction (CR). Regardless of the synthesis method, all bimetallic catalysts (Mo:W bulk ratios of 1:3, 1:1, and 3:1) were mixed at the nanoscale, although the Mo/W ratio in individual nanoparticles varied from the expected bulk values. Moreover, the crystal structures of the produced phases and nanoparticle sizes differed depending on the synthesis method. When using the TPR method, a cubic carbide (MeC1-x) phase with 3-4 nm nanoparticles was obtained, while a hexagonal phase (Me2C) with 4-5 nm nanoparticles was found when using the CR method. The TPR-synthesized carbides exhibited higher activity for the hydrodeoxygenation of fatty acids, tentatively attributed to a combination of crystal structure and particle size
Magnetic Properties of a Quantum Ferrimagnet: NiCu(pba)(D_2O)_3 . 2D_2O
We report the results of magnetic measurements on a powder sample of
NiCu(pba)(D_2O)_3 \cdot 2D_2OS\chi\chi TT\chi TJ/k_B=121
K^{2+}^{2+}g_{Ni}g_{Cu}\chi T$ at low temperatures
is reproduced fairly well by the calculation for the same ferrimagnetic model.Comment: 7pages, 4 postscript figures, usues REVTEX. appear in J. Phys. Soc.
Jpn vol 67 No.7 (1998
Magnetic properties of the spin-1 chain compound NiClCHCHCHNH
We report experimental results of the static magnetization, ESR and NMR
spectroscopic measurements of the Ni-hybrid compound
NiClCHCHCHNH. In this material NiCl octahedra are
structurally arranged in chains along the crystallographic -axis. According
to the static susceptibility and ESR data Ni spins are isotropic
and are coupled antiferromagnetically (AFM) along the chain with the exchange
constant K. These are important prerequisites for the realization of
the so-called Haldane spin-1 chain with the spin-singlet ground state and a
quantum spin gap. However, experimental results evidence AFM order at K presumably due to small interchain couplings. Interestingly,
frequency-, magnetic field-, and temperature-dependent ESR measurements, as
well as the NMR data, reveal signatures which could presumably indicate an
inhomogeneous ground state of co-existent mesoscopically spatially separated
AFM ordered and spin-singlet state regions similar to the situation observed
before in some spin-diluted Haldane magnets
Electronic density of states derived from thermodynamic critical field curves for underdoped La-Sr-Cu-O
Thermodynamic critical field curves have been measured for
over the full range of carrier concentrations
where superconductivity occurs in order to determine changes in the normal
state density of states with carrier concentration. There is a substantial
window in the plane where the measurements are possible because the
samples are both thermodynamically reversible and the temperature is low enough
that vortex fluctuations are not important. In this window, the data fit
Hao-Clem rather well, so this model is used to determine and
for each temperature and carrier concentration. Using N(0) and the ratio of the
energy gap to transition temperature, , as fitting
parameters, the curves give over the
whole range of . Values of N(0) remain rather constant in the optimum-doped
and overdoped regime, but drops quickly toward zero in the underdoped regime.
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