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排序方式: 共有135条查询结果,搜索用时 15 毫秒
71.
Yanping Sun Tracey A.Robson Trevor C.BrownSchool of Biological Biomedical & Molecular Sciences University of New England Armidale NSW Australia Manuscript received July revised August 《天然气化学杂志》2002,(Z1)
The oxidative dehydrogenation (ODH) of isobutane over Cr2O3/La2(CO3)3 has been investigated in a low-pressure Knudsen cell reactor, under conditions where the kinetics of the primary reaction steps can be accurately determined. By heating the catalyst at a constant rate from 150-300℃, temperature fluctuations due to non-equilibrium adsorption are minimized. The evolved gas profiles show that ODH to isobutene and water is a primary reaction pathway, while carbon dioxide, which forms from the catalyst during reaction, is the only other product. This CO2 evolution may enhance the activity of the catalyst. Isobutene formation proceeds with the participation of lattice oxygen from the Cr2O3/La2(CO3)3 catalyst. The intrinsic Arrhenius rate constant for the ODH of isobutane isk(s-1) = 1011.5±2.2exp{-((55±5) -ΔHads kJmol-1)/RT}The small pre-exponential factor is expected for a concerted mechanism and for such a catalyst with a small surface area and limited porosity. 相似文献
72.
pH-dependent x-ray absorption spectra of aqueous boron oxides 总被引:1,自引:0,他引:1
Duffin AM Schwartz CP England AH Uejio JS Prendergast D Saykally RJ 《The Journal of chemical physics》2011,134(15):154503
Near edge x-ray absorption fine structure (NEXAFS) spectra at the boron K-edge were measured for aqueous boric acid, borate, and polyborate ions, using liquid microjet technology, and compared with simulated spectra calculated from first principles density functional theory in the excited electron and core hole (XCH) approximation. Thermal motion in both hydrated and isolated molecules was incorporated into the calculations by sampling trajectories from quantum mechanics∕molecular mechanics simulations at the experimental temperature. The boron oxide molecules exhibit little spectral change upon hydration, relative to mineral samples. Simulations reveal that water is arranged nearly isotropically around boric acid and sodium borate, but the calculations also indicate that the boron K-edge NEXAFS spectra are insensitive to hydrogen bonding, molecular environment, or salt interactions. 相似文献
73.
D. J. Wilson R. J. Bradford J. H. Davenport M. England 《Mathematics in Computer Science》2014,8(2):263-288
Cylindrical algebraic decompositions (CADs) are a key tool in real algebraic geometry, used primarily for eliminating quantifiers over the reals and studying semi-algebraic sets. In this paper we introduce cylindrical algebraic sub-decompositions (sub-CADs), which are subsets of CADs containing all the information needed to specify a solution for a given problem. We define two new types of sub-CAD: variety sub-CADs which are those cells in a CAD lying on a designated variety; and layered sub-CADs which have only those cells of dimension higher than a specified value. We present algorithms to produce these and describe how the two approaches may be combined with each other and the recent theory of truth-table invariant CAD. We give a complexity analysis showing that these techniques can offer substantial theoretical savings, which is supported by experimentation using an implementation in Maple. 相似文献
74.
Joseph P. Tassone Emma V. England Preston M. MacQueen Michael J. Ferguson Mark Stradiotto 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(8):2507-2511
The base metal‐catalyzed C?N cross‐coupling of bulky α,α,α‐trisubstituted primary alkylamines with (hetero)aryl electrophiles represents a challenging and under‐developed class of transformations that is of significant potential utility, including in the synthesis of lipophilic active pharmaceutical ingredients. Herein, we report that a new, air‐stable Ni(II) pre‐catalyst incorporating the optimized ancillary ligand PhPAd‐DalPhos enables such transformations of (hetero)aryl chloride, bromide, and tosylate electrophiles to be carried out for the first time with substrate scope rivalling that achieved using state‐of‐the‐art Pd catalysts, including room temperature cross‐couplings of (hetero)aryl chlorides that are unprecedented for any catalyst (Pd, Ni, or other). 相似文献
75.
We report the first successful attempt to generate ultrashort (1-10 ps) relativistic electron bunches characterized by a ramped longitudinal current profile that rises linearly from head to tail and then falls sharply to zero. Bunches with this type of longitudinal shape may be applied to plasma-based accelerator schemes as an optimized drive beam, and to free-electron lasers as a means of reducing asymmetry in microbunching due to slippage. The scheme used to generate the ramped bunches employs an anisochronous dogleg beam line with nonlinear correction elements to compress a beam having an initial positive time-energy chirp. The beam current profile is measured using a deflecting mode cavity, and a pseudoreconstruction of the beam's longitudinal phase space distribution is obtained by using this diagnostic with a residual horizontal dispersion after the dogleg. 相似文献
76.
77.
[reaction: see text]. N-benzoylated quinone imine ketals undergo smooth cycloadditions in a [4 + 2] sense to yield the expected cycloadducts. The crude cycloadducts, when subjected to a short series of simple transformations, produce synthetically useful quantities of 5-methoxyindoles in excellent overall yields. 相似文献
78.
M.E. Cage D.L. Clough A.J. Cole J.B.A. England G.J. Pyle P.M. Rolph L.H. Watson D.H. Worledge 《Nuclear Physics A》1972
Differential cross sections have been measured for the elastic scattering of 33 MeV 3He particles from targets of 58,60,62,64Ni,63,65Cu and 64,66,68Zn over the angular range 5° to 175°. The data have been analysed with the optical model, with particular emphasis on the investigation of the ambiguities in the real potential, the form of the imaginary potential and the use of a spin-orbit term. 相似文献
79.
R. Ost N.E. Sanderson S. Mordechai J.B.A. England B.R. Fulton J.M. Nelson G.C. Morrison 《Nuclear Physics A》1976,265(1):142-152
Energy spectra and angular distributions of light particles (Z = 3–8) produced in the reactions 13C+58, 60, 64Ni have been measured at 105 MeV. Coincidence events between light outgoing particles Li, Be, B and α, p have been observed for the first time in the 13C+58Ni and 13C+64Ni reactions. The results show that at least part of the ‘deep-inelastic’ events are due to many-particle reactions. 相似文献
80.
The problems of charge stripe formation, spin-charge separation, and stability of the antiphase domain wall (ADW) associated with a stripe are addressed using an analytical approach to the t- J(z) model. We show that a metallic stripe together with its ADW is the ground state of the problem in the low doping regime. The stripe is described as a system of spinons and magnetically confined holons strongly coupled to the two dimensional spin environment with holon-spin-polaron elementary excitations filling a one-dimensional band. 相似文献