中子管射频离子源放电特性仿真研究

国外射频离子源中子管的中子产额已经达到10^14n/s,明显优于潘宁源中子管。为了深入了解用于中子管射频离子源的放电特性,从射频感应耦合等离子体的放电原理入手,建立用于氢气放电模拟研究的理论模型。设计中子管射频离子源几何结构,运用麦克斯韦方程组在理论上推导了等离子体放电过程中影响粒子密度的因素。结合Comsol软件中的二维轴对称的电感耦合等离子体模型,采用单一变量法,通过仿真实验得到了线圈匝间距、线圈匝数、线圈直径、线圈功率和放电气压等参数对H^+密度分布和大小的影响。总结出H+在不同参数下的变化规律,得到了一些有价值的结论,为优化中子管射频离子源的实验参数和结构设计提供重要依据。

DFT Investigation of O_2 Adsorption on Si(001)-(2×2×1):H

A novel model was developed to theoretically evaluate the 02 adsorption on H-terminated Si（001）-（2×2×1） surface. The periodic boundary condition, the ultrasoft pseudopotentials technique based on density functional theory （DFT） with generalized gradient approxi,natior, （GGA） functional were applied in our ab initio calculations. By analyzing bonding energy oil site, the favourable adsorption site was determined. The calculations also predicted that the adsorption products should be Si=O and H2O. This theoretical study snpported the reaction mechanism provided by Kovalev et al, The results were also a base for further investigation of some more complex systems such as the oxida.tion on porous silicon surface.

Mechanistic Study of Palladium-Catalyzed Oxidative C-H/C-H Coupling of Polyfluoroarenes with Simple Arenes

Pd-catalyzed oxidative C-H/C-H coupling reaction is an emerging type of C-H acti- vation reaction, which attracts great interests because both reaction partners do not re- quire pre-functionalization. In the present study, we employed DFT methods to investigate the mechanism of Pd（OAc）2-catalyzed oxidative C-H/C-H coupling of pentafluoroben-zene with benzene. Four possible pathways were examined in the C-H activation part： path A benzene-pentafluorobenzene mechanism （C-H activation of benzene occurs before the C-H activation of pentafluorobenzene）, path B pentafluorobenzene-benzene mechanism （C-H activation of benzene occurs after the C-H activation of pentafluorobenzene）, path C benzene-pentafluorophenylsilver mechanism （C-H activation of benzene and subsequent transmetalation with pentafiuorophenyl silver complex）, path D pentafiuorophenylsilver- benzene mechanism （transmetalation with pentafluorophenyl silver complex and subsequent C-H activation of benzene）. Based on the calculations, the sequences of two C-H activation steps are found to be different in the oxidative couplings of same substrates （i. e. pentafiu- orobenzene and benzene） in different catalytic systems, where the additive Ag salts played a determinant role. In the absence of Ag salts, the energetically favored pathway is path B （i.e. the C-H activation of pentafluorobenzene takes place before the C-H cleavage of benzene）. In contrast, with the aid of Ag salts, the coordination of pentafluorophenylsilver to Pd center could occur easily with a subsequent C-H activation of benzene in the second step, and the second step significantly raises the whole reaction barrier. Alternatively, in the presence of Ag salts, the kinetically preferred mechanism is path C （i. e. the C-H activation of benzene takes place in the first step followed by transmetalation with pentafluorophenyl- silver complex）, which is similar to path A. The calculations are consistent with the H/D exchange experiment and kinetic isotope effects. Thus the present study not only offers a deeper understanding of oxidative C-H/C-H coupling reaction, but also provides helpful insights to further development of more efficient and selective oxidative C-H/C-H coupling reactions.

Elucidating the dominant reaction mechanism of methanol-to-olefins conversion in H-SAPO-18: A first-principles study

The reaction mechanism of zeolite- or zeotype-catalyzed methanol-to-olefins（MTO） conversion is still a subject of debate. Employing periodic density functional theory calculations, the olefin-based cycle was studied using tetramethylethene（TME） as a representative olefinic hydrocarbon pool in H-SAPO-18 zeotype. The overall free energy barrier at 673 K was calculated and found to be less than 150 kJ/mol in the TME-based cycle, much lower than those in the aromatic-based cycle（〉 200 kJ/mol）, indicating that olefins themselves are the dominant active hydrocarbon pool species in H-SAPO-18. The similarity of the intermediates involved between the aromatic-based cycle and the olefin-based cycle was also highlighted, revealing that both cycles were pattern-consistent. The selectivity related to the distribution of cracking precursors, such as higher olefins or carbenium ions, as a result of the olefin-based cycle for the MTO conversion. The enthalpy barrier of the crack-ing step scaled linearly with the number of carbon atoms of cracking precursors to produce ethene or propene with ethene being much less favored than propene for cracking of C7 and higher pre-cursors. This work highlighted the importance of the olefin-based cycle in H-SAPO-18 for the MTO conversion and established the similarity between the olefin-based and aromatic-based cycles.

Hydrogen-Assisted C-C Coupling on Reaction of CuC3H-Cluster Anion with CO

A fundamental study on C-C coupling,that is the crucial step in the Fischer-Tropsch synthesis(FTS)process to obtain multi-carbon products,is of great importance to tailor catalysts and then guide a more promising pathway.It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process,while the related mechanism is elusive.Herein,the reactions of the CuC3H-and CuC3-cluster anions with CO have been studied by using mass spectrometry and theoretical calculations.The experimental results showed that the coupling of CO with the C3H-moiety of CuC3H can generate the exclusive ion product COC3H-.The reactivity and selectivity of this reaction of CuC3H-with CO are greatly higher than that of the reaction of CuC3-with CO,and this H-assisted C-C coupling process was rationalized by theoretical calculations.

Catalytic roles of the acid sites in different pore channels of H‐ZSM‐5 zeolite for methanol‐to‐olefins conversion

H‐ZSM‐5 zeolite is a typical catalyst for methanol‐to‐olefins(MTO)conversion.Although the performance of zeolite catalysts for MTO conversion is related to the actual location of acid sites in the zeolite framework,the catalytic roles of the acid sites in different pore channels of the H‐ZSM‐5 zeolite are not well understood.In this study,the MTO reaction network,involving the aromatic cycle,alkene cycle,and aromatization process,and also the diffusion behavior of methanol feedstock and olefin and aromatic products at different acid sites in the straight channel,sinusoidal channel,and intersection cavity of H‐ZSM‐5 zeolite was comparatively investigated using density functional theory calculations and molecular dynamic simulations.The results indicated that the aromatic cycle and aromatization process occurred preferentially at the acid sites in the intersection cavities with a much lower energy barrier than that at the acid sites in the straight and sinusoidal channels.In contrast,the formation of polymethylbenzenes was significantly suppressed at the acid sites in the sinusoidal and straight channels,whereas the alkene cycle can occur at all three types of acid sites with similar energy barriers and probabilities.Consequently,the catalytic performance of H‐ZSM‐5 zeolite for MTO conversion,including activity and product selectivity,can be regulated properly through the purposive alteration of the acid site distribution,viz.,the location of Al in the zeolite framework.This study helps to elucidate the relation between the catalytic performance of different acid sites in the H‐ZSM‐5 zeolite framework for MTO conversion,which should greatly benefit the design of efficient catalyst for methanol conversion.