中能重离子反应中靶碎片的质量分布研究

本文利用核化学方法研究了中能重离子反应机制。在135MeV/u~(12)C + Fe反应中鉴别了39个放射性核素，并得到了这些靶余核的生成截面，通过电荷分布假设导出了该反应中靶余核的质量分布。结果表明，中能重离子与非裂变靶核相互作用中靶余核质量分布的形状相似，随着入射能的增加，质量分布曲线变得更加平缓。同时，在低质量区（A < 30）反应产物的产额明显增加，且产物有偏向丰中子一侧的趋势。利用熔合碎裂统计模型和级联两体统计模型对实验结果进行了拟合，两种理论模型都能较好地符合实验数据的总体趋势，但考虑了多重碎裂衰变模式的熔合碎裂统计模型的计算结果再现了A < 30质量区产额的回升现象，而级联两体模型更好地符合了蒸发余核的质量分布，预示着在很高入射能的情况下，多重碎裂成为靶余核形成的主要反应机制之一。在20MeV/u~(40)Ar引起~(238)U的裂变反应中测到了101个核素的生成截面，通过高斯电荷分布曲线拟合得到了各同量民位素的独立产额以及各质量链的质量产额。从电荷分布曲线中可以看出，裂变碎片来源于两种不同的反应机制，即丰中子产物来自高激发核的裂变过程，而缺中子产物来自低能裂变过程。质量分布曲线在裂变质量区（80 < A < 140）有一很宽的对称裂变峰而在近靶区的重靶余核也能以很高的截面通过周边碰撞产生

Template-assisted syntheses of two novel porous zirconium methylphosphonates

Two porous zirconium methylphosphonates (designated as ZMPmi and ZMPme respectively) were synthesized by using dibutyl methylphosphonate (DBMP) as a template. Two efficient post-synthetic treatments were developed to remove the incorporated template without destroying the hybrid structures. The materials were characterized by SEM, EPMA, TG, DTA, FTIR, and NMR. Specific surface area and porosity were evaluated by BET, alpha(s)-plots and DFT methods based on N-2 adsorption-desorption isotherms. The specific surface areas of ZMPmi and ZMPme are determined to be 279 and 403 m(2) g(-1) and the maxima of pore size distributions are at 0.7 and 1.3 nm respectively. (c) 2005 Elsevier Inc. All rights reserved.

Performance comparison of low-temperature direct alcohol fuel cells with different anode catalysts

Low-temperature polymer electrolyte membrane fuel cells directly fed by methanol and ethanol were investigated employing carbon supported Pt, PtSn and PtRu as anode catalysts, respectively. Employing Pt/C as anode catalyst, both direct methanol fuel cell (DMFC) and direct ethanol fuel cell (DEFC) showed poor performances even in presence of high Pt loading on anode. It was found that the addition of Ru or Sn to the Pt dramatically enhances the electro-oxidation of both methanol and ethanol. It was also found that the single cell adopting PtRu/C as anode shows better DMFC performance, while PtSn/C catalyst shows better DEFC performance. The single fuel cell using PtSn/C as anode catalyst at 90degreesC shows similar power densities whenever fueled by methanol or ethanol. The cyclic voltammetry (CV) and single fuel cell tests indicated that PtRu is more suitable for DMFC while PtSn is more suitable for DEFC. (C) 2003 Elsevier B.V. All rights reserved.

Light alkenes preparation by the gas phase oxidative cracking or catalytic oxidative cracking of high hydrocarbons

The preparation of light alkenes by the gas phase oxidative cracking (GOC) or catalytic oxidative cracking (COC) of model high hydrocarbons ( hexane, cyclohexane, isooctane and decane in the GOC process and hexane in the COC process) was investigated in this paper. The selection for the feed in the GOC process was flexible. Excellent conversion of hydrocarbons ( over 85%) and high yield of light alkenes ( about 50%) were obtained in the GOC of various hydrocarbons including cyclohexane at 750 degreesC. In the GOC process, the utilization ratio of the carbon resources was high; CO dominated the produced COX (the selectivity to CO2 was always below 1%); and the total selectivity to light alkenes and CO was near or over 70%. In the COC of hexane over three typical catalysts (HZSM-5, 10% La2O3/HZSM-5 and 0.25% Li/MgO), the selectivity to COX was hard to decrease and the conversion of hexane and yield of light alkenes could not compete with those in the GOC process. With the addition of H-2 in the feed, the selectivity to COX was reduced below 5% over 0.1% Pt/HZSM-5 or 0.1% Pt/MgAl2O4 catalyst. The latter catalyst was superior to the former catalyst due to its perfect performance at high temperature, and with the latter, excellent selectivity to light alkenes ( 70%) and the conversion of hexane (92%) were achieved at 850 degreesC ( a yield of light alkenes of 64%, correspondingly).

Coking kinetics on the catalyst during alkylation of fcc off-gas with benzene to ethylbenzene

The production of ethylbenzene from the alkylation of dilute ethylene in fee off-gases with benzene has been commercialized in China over a newly developed catalyst composed of ZSM-5/ZSM-11 co-crystallized zeolite. The duration of an operation cycle of the commercial catalyst could be as long as 180 days. The conversion of ethylene could attain higher than 95%, while the amount of coke deposited on the catalyst was only about 10 wt.%. Thermogravimetry (TG) was used to study the coking behavior of the catalyst during the alkylation of fee off-gas with benzene to ethylbenzene. Based on effects of reaction time, reaction temperature, reactants and products on coking during the alkylation process, it is found that the coking rate during the alkylation procedure follows the order: ethylbenzene > ethylene > propylene > benzene for single component, and benzene-ethylene > benzene-propylene for bi-components under the same reaction condition. Furthermore, the coking kinetic equations for benzene-ethylene, benzene-propylene and ethylbenzene were established. (C) 2003 Elsevier B.V. All rights reserved.