25Mev/u~40Ar+~27Al、~58Ni、~115In在小角区产物的研究

重离子核反应机制的研究一直是人们关注的课题。用25MeV/u 40Ar的束流进行多种反应机制共存及其对碎片的贡献的研究，有助于我们更清楚地了解反应机制的演化和对放射性束流产额的预言。 实验中用25MeV/u40Ar的束流轰击115In、58Ni、27Al，这时对前方向6°、8.5°的碎片测量，发现它们的平行动量分布宽度满足Goldhaber关系式，碎裂部分提取的约化平行动量分布宽度σ0=54MeV/c，这比弹核碎裂部分的宽度要窄，说明转移反应比弹核碎裂受到相空间的严格限制更严格。转移反应主要是对弹核附近的碎片有贡献，当类弹碎片质量与弹核质量相差较大时，主要是弹核碎裂的贡献。对于弹核碎裂产物的最可几能量的提取，在实验能谱中拟合出碎片的最可几能量，对于接近弹核的碎片，我们利用外推法来确定碎裂部分的最可几能量，同时用实验提取的σ0值，拟合出碎裂能谱，从而将弹核碎裂和转移反应的贡献区分开来。 通过碎片同位素的测量，我们认碎片的同位素分布与靶核有一定的依赖关系，当靶核的N/Z值增加时，产物的N/Z也增加。弹核40Ar的N/Z为1.22，靶核的N/Z均大于1，所以产物N/Z的变化意味着系统由非平衡向平衡的演化，且在这一能区，重离子反应既有低能下一体耗散的特点，又有高能下核子-核子碰撞的特点，多种反应机制共存并竞争。

Structure and acidity of Mo/H-MCM-22 catalysts studied by NMR spectroscopy

A comprehensive study on physical and chemical properties of Mo/MCM-22 bifunctional catalysts has been made by using combined analytic and spectroscopic techniques, such as adsorption, elemental analysis, and Xe-129 and P-31 NMR of adsorbed trialkylphosphine oxide probe molecules. Samples prepared by the impregnation method with Mo loadings ranging from 2-10 wt.% have been examined and the results are compared with that obtained from samples prepared by mechanical mixing using MoO3 or Mo2C as agents. Sample calcination treatment is essential in achieving a well-dispersed metal species in Mo/MCM-22. It was found that, upon initial incorporation, the Mo species tend to inactivate both Bronsted and Lewis sites locate predominantly in the supercages rather than the 10-membered ring channels of MCM-22. However, as the Mo loading exceeds 6 wt.%, the excessive Mo species tend to migrate toward extracrystalline surfaces of the catalyst. A consistent decrease in concentrations of acid sites with increasing Mo loading < 6 wt.% was found, especially for those with higher acid strengths. Upon loading of Mo > 6 wt.%, further decreases in both Bronsted and Lewis acidities were observed. These results provide crucial supports for interpreting the peculiar behaviors previously observed during the conversion of methane to benzene over Mo/MCM-22 catalyst under non-oxidative conditions, in which an optimal performance was achieved with a Mo loading of 6 wt.%. The effects of Mo incorporation on porosity and acidity features of the catalyst are discussed. (C) 2004 Published by Elsevier B.V.