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951.
Microorganisms produce toxins against its competitors sometimes, and variable yields are useful to explain the observed oscillatory
behavior in the reactor. In this paper, a model with general quadric yields of competition in the bioreactor of two competitors
for a single nutrient where one of the competitors can produce toxin against its opponent, is proposed. We analyze the asymptotic
behavior of the model in terms of the relevant parameters. The conditions of the three dimensional Hopf bifurcation, and the
existence of limit cycles in the nutrient-organism phase plane are obtained. 相似文献
952.
Dispersion of copper(Ⅱ) phthalocyanine (CuPc), copper(Ⅱ) phthalocyaninesulfonate (CuPcS) and cobalt(Ⅱ)phthalocyaninetetrasulfonate (CoPcTS) on the surface of titanium dioxide was investigated by XRD, XPS, FT-IR and UV-Vis techniques. Results show that interaction between CuPc and TiO2 was very weak and CuPc was difficult to disperse on the surface of the support. While partly sulfurized CuPcS could be dispersed on the surface of support through sulfo-groups and its dispersion capacity was determined to be 0.085 g CuPcS/g TiO2. Completely sulfurlzed CoPcTS could also be dispersed on the surface of TiO2 as a monolayer and its dispersion capacity was 0.12 g CoPcTS/g TiO2. Interactions of the sulfo-groups as well as the electrons of CoPcTS with the surface of TiO2 could be evidenced by FT-IR characterization. Therefore, it was suggested that CoPcTS molecules be adsorbed on the surface of TiO2 in a flat-lying mode while CuPcS in a slanting one. UV-Vis spectra show that the dispersed CuPcS and CoPcTS molecules exist in both forms of monomers and dimers. 相似文献
953.
Kinetic control of the stereoselectivity of protonation of enolates and other strongly delocalized anionic species is involved in a large number of organic reactions. Protonation occurring from the less hindered side of the, e.g., enolic system affords the less stable of two diastereomers. However, one apparent discrepancy has been in the synthesis of prostaglandins. The present research deals with the source of this behavior. A curious effect of the substituent at the enolic alpha carbon was uncovered. In certain instances an alpha substituent is forced to twist into a conformation blocking the proton donor from its side, thus reversing the stereochemistry of protonation. In the course of this research, a number of five-ring enols of varying structure were investigated. Finally, the ketonization reaction course has been studied theoretically. 相似文献
954.
Sittichai Natesakhawat Xueqin Wang Lingzhi Zhang Umit S. Ozkan 《Journal of molecular catalysis. A, Chemical》2006,260(1-2):82-94
Chromium-free iron-based catalysts were prepared and studied in regard to their performance in the high-temperature water-gas shift reaction (HTS). The effects of various catalyst preparation variables (i.e., Fe/promoter ratio, pH of precipitation medium, calcination and reduction temperatures) and preparation methods were investigated. Aluminum is a potential chromium replacement in HTS catalysts. Further improvement in WGS activity of Fe–Al catalysts can be achieved by the addition of small amounts of copper or cobalt. Catalysts were characterized using BET surface area measurements, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). As a textural promoter, aluminum and chromium prevent the sintering of iron oxides and stabilize magnetite phase by retarding its further reduction to FeO and metallic Fe. The promotional effect of Cu is found to be strongly dependent on the preparation method. 相似文献
955.
We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using modified embedded atom method potentials and the Monte Carlo method. The nanoparticles are constructed with disordered fcc configurations at two fixed overall concentrations (50 at. % Pt and 75 at. % Pt). We use octahedral and cubo-octahedral nanoparticles terminated by {111} and {100} facets to examine the extent of the Pt segregation to the nanoparticle surfaces at T=600 K. The model particles contain between 586 and 4033 atoms (particle size ranging from 2.5 to 5 nm). Our results imply that a complete {100}-facet reconstruction could make the cubo-octahendral Pt-Ni nanoparticles most energetically favorable. We predict that at 600 K due to segregation the equilibrium cubo-octahedral Pt50Ni50 nanoparticles with fewer than 1289 atoms and Pt75Ni25 nanoparticles with fewer than 4033 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that, due to an order-disorder transition, the Pt50Ni50 cubo-octahedral nanoparticles containing more than 2406 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogenous core. 相似文献
956.
957.
Ernest Wenkert Edward W. Hagaman Nicole Kunesch Nai-yi Wang Bla Zsadon 《Helvetica chimica acta》1976,59(8):2711-2723
The 13C shifts of 16α- and 16β-substituted derivatives of quebrachamine, 14,15-dehydroquebrachamine, cleavamine, 15,20α-dihydrocleavamine and 15,20β-dihydrocleavamine are determined and correlated with possible conformations of these tetracycles. The method of analysis of the C(16) configuration of these compounds, which emanated from this study, is used for the determination of the configuration of the site of coupling of vindoline and cleavamine β-chloroindolenine. 相似文献
958.
活性酚醛树脂中羟甲基含量的测定 总被引:12,自引:0,他引:12
本文提出一种酚醛树脂中羟甲基含量的化学分析法,分别以^13CNMR和化学分析法测定酚醛树脂的羟甲基含量。将两种测试结果与已知的投料量进行比较,证明所建立的化学分析法可行。 相似文献
959.
强酸阳离子交换树脂催化性能的研究 总被引:1,自引:1,他引:1
通过叔丁醇脱水生成异丁烯的反应研究对比了国内外常见种类的凝胶型和大孔型强酸阳离子交换树脂的催化性能,并对树脂的性质,预处理方法,以及其用量和再生程度等因素对树脂催化活性的影响进行了研究。在此基础上,合成了具有较高催化活性的新型大孔强酸阳离子交换树脂。 相似文献
960.