分子束张弛谱——一种研究材料表面特性的近代方法

分子束张弛谱是诸如分子束、质谱、表面分析等技术相结合的产物,它是一种研究材料表面特性的有效手段。它的主要突破在于能从分子(原子)量度来研究诸如表面吸附、脱附、催化、腐蚀、能量适应等宏观气体——表面过程。本文论述了分子束张弛谱的基本原理,实验装置与技术。并举例说明这种方法的某些应用。     

分子束固体表面散射

分子束表面散射技术是研究气体-表面相互作用动力学的强有力的工具。它的主要突破是在于从分子(原子)量度来揭示诸如催化、腐蚀、吸附、解吸和能量适应等宏观的气体-表面过程。同时,它也是表面分析的手段之一。本文概述了气体-表面相互作用的基本原理,实验装置与技术,评论了这种技术作为表面分析(包括表面结构、德拜温度、声子谱、吸附质及气体-表面相互作用势)及作为表面研究(包括表面吸附与解吸动力学以及表面反应的机制)的原理与实验结果。最后简单地讨论了这种技术的优缺点及今后发展。     

NK—1型分子束—表面散射装置的研制

一、引言分子束可研究气体分子与表面单次碰撞作用,通过表面散射产物的驰予谱、角度及速度分布的检测,可洞察表面发生的微观动力学过程,能从分子(原子)层次来了解诸如吸附、脱附、催化、腐蚀和能量适应等气体——表面作用过程,是研究气体——表面     

热脱附谱的分析方法

一、引言热脱附谱法是研究固体吸附与脱附现象的手段之一。通过分析热脱附谱,可以得到脱附气体的脱附级数、脱附活化能及脱附速率常数,了解气体在固体表面的吸附特性。迄今为止,见到的有关热脱附谱的理论分析是在假定脱附谱为脱附活化能单值和多     

气体脱附机理的物理解释

采用一组新的方程来分析抽气过程。这个方程基于气体分子在不均匀的金属表面上所取位置的吸附与脱附的表面,而不是扩散过程。分析脱附位置、脱附率和气体脱附的来源.认为气体脱附现象必须理解为非平衡和非稳定过程。     

表面脱附(二)

§2.1热脱附 吸附分子在热能量作用下释离表面的热脱附(即通常的表面脱附)与表面吸附是同一个问题的两个方面,它们在固一气界面上是同时存在的。在一定的条件下,当吸附速率超过脱附速率时就呈现气体在表面上的吸附现象;反之,就呈现脱附现象。如果吸附速率与脱附速率相等,就达到了吸附平衡,这时单位时间落在表面上并被吸附的分子数与自表面上重新释出的分子数相等,即在真空系统的排气过程中系统内压强不断降低,上 相应减小,破坏了表面上的吸附平衡。脱附的气体使表现的系统抽气速率降低,并最终构成了限制极限压强的一个重要因素。 在吸附过程…     

激光-分子束-表面散射装置的研制

介绍了自行设计和加工的激光-分子束-表面散射装置，并对超高真空转动密封结构作了详细描述。差分泵浦的超音速分子束对准样品中心，射入超高真空主室。样品架安放在主室中央，四极质谱检测器可绕样品转动，用来测量表面散射分子的平动能及角分布。三个石英窗口作为激光窗口，可用LIF或MPI方法来测量表面散射分子的内能态分布，也可用于研究表面光化学。最后给出了分子束发散角及室温时CH2I2在Ag（110）表面用308um激光光解碎片碘的TOF谱的测量结果。     

全玻璃分子束装置及束强测量

高强度、超声速喷射式分子束源是分子束表面散射仪的一个重要组成部分。它与普通泻流源相比,具有强度大、速度分布与角分布窄等优点。文中叙述了我们实验室设计和制作的一种全玻璃结构的分子束装置,并在此装置上对不同气体的分子束强度进行了测试。测试结果表明,它与气体动力学理论符合得很好,设计符合要求。     

β-环糊精改性的多壁碳纳米管对NO的吸附性能研究

为开发NO气敏传感器,采用β-环糊精对模板法制备的多壁碳纳米管进行了表面修饰改性.利用扫描电镜、红外光谱、X-射线衍射、热分析-质谱等表征手段研究了改性的MWCNTs的结构性能,并对NO的吸附-脱附作用机理进行了讨论.结果表明:环糊精的小口端易与多壁碳纳米管的管壁相互作用并结合在一起,使环糊精包覆在多壁碳纳米管的表面.多壁碳纳米管外或内层的环糊精可以通过氢键与环糊精以层与层的方式进行连接,达到对MWCNTs的多层改性修饰作用.XRD研究证明经环糊精改性的多壁碳纳米管其结构并没有发生变化.热分析质谱研究发现,β-环糊精改性的MWCNTs对NO的吸附-脱附能力均大于未改性的MWCNTs,其中MWCNTs吸附NO主要是表面物理吸附和管体空腔内的化学吸附;而经β-环糊精修饰的MWCNTs除本身能够吸附NO外,其表面包覆的β-环糊精空腔也能够吸附NO分子,并与NO形成非共价键吸附.β-环糊精与MWCNTs质量比为10∶1时,其吸附NO的量最大,是未改性MWCNTs的1.69倍.     

分子束—表面散射装置的真空系统

一、引言分子束-表面散射装置是研究气体分子与表面相互作用的重要手段,也是表面分析的方法之一。该装置由两大部分组成。一是在进气压强为133～13300帕的条件下,为保证得到高强度超声速的分子束而采用差分抽气系统组成的分子束源部分;二是在进行反应散射、弹性散射及非弹性散射实验时所需的超高真空系统。该系统是由无油超高真空机组及升华钛泵超高真空分析室组成。本文阐述了全套装置系统的匹配问题。     

Gas-surface scattering effect on vacuum gas flows through rectangular channels

Flow of a rarefied gas through a channel of rectangular cross section is investigated numerically and experimentally. Emphasis is given on the study of the molecular scattering law influence at the wall boundary surfaces. The Cercignani-Lampis boundary conditions are considered in detail for the current problem, along with the linearized BGK kinetic model and the discrete velocity method. Numerical conductance values are tabulated for certain aspect ratios and accommodation coefficients in the whole range of the Knudsen number and compared with corresponding experimental results for certain gas-surface combinations. Compared to the classical Maxwell diffuse boundary conditions, the Cercignani-Lampis gas-surface interaction allows a better agreement with the measurements.     

Rates and equilibria of adsorption and desorption of propane and propylene on 13X molecular sieves determined experimentally using a differential sorption bed system

An adsorption/desorption system (apparatus) involving a differential sorption bed was constructed, and experimental procedures were developed to measure quickly and with reasonable accuracy adsorption and desorption rate and equilibrium data. The experimental adsorption and desorption rate and equilibrium data of propane and propylene in particles of 13X molecular sieves were measured with this experimental system for different adsorbate concentrations and temperatures and are presented in this work. The results indicate that the adsorption rates of propylene are higher than those of propane. There are differences, most often at earlier times, between the adsorption and desorption rates. The magnitude of the differences depends in a complex way on the concentration of the adsorbates and the temperature of the system. The potential use of the rate and equilibrium data of this work with respect to adsorption/desorption model building, parameter estimation, model discrimination, and adsorption/desorption process design is also discussed.     

Influence of ion radiation damage on surface reactivity

The role of ion irradiation on the reactivity of surfaces is reviewed with special attention to those experiments that make use of ultrahigh vacuum surface techniques. Ion irradiation effects on adsorption, incorporation and catalysis are discussed. The difficulties involved in directly probing surface defects are documented and a discussion of several techniques offering good prospects in this defect characterization is presented. Comparisons are made with related areas such as stepped sufaces and dislocation effects on reactivity but a review of the latter subjects is not attempted. Some examples of theoretical efforts in relation to the role of surface defects in gas-surface interactions are also included.     

Electrothermal properties of porous ceramic fiber media containing carbon materials

Electrically regenerable porous ceramic fiber media containing nanoporous carbon from 2.5% to 19.2% have been prepared for adsorption/regeneration system. An experimental apparatus was built for in situ measurement of the sample weight during adsorption and electrothermal desorption of gaseous adsorbates. Adsorption and electrothermal desorption behavior of gaseous adsorbates on carbon contained porous ceramic fiber media was explained by physical and electrothermal properties of these materials measured in this work. In situ thermal desorption and adsorption experiments show that a considerable amount of water vapor is adsorbed on the carbon contained media exposed to ambient air.     

The dynamic water vapour sorption behaviour of natural fibres and kinetic analysis using the parallel exponential kinetics model

Hygroscopic behaviour is an inherent characteristic of natural fibres which can influence their applications as textile fabrics and composite reinforcements. In this study, the water vapour sorption kinetic properties of cotton, filter paper, flax, hemp, jute, and sisal fibres were determined using a dynamic vapour sorption apparatus and the results were analyzed by use of a parallel exponential kinetics (PEK) model. With all of the fibres tested, the magnitude of the sorption hysteresis observed varied, but it was always greatest at the higher end of the hygroscopic range. Flax and sisal fibres displayed the lowest and highest total hysteresis, respectively. The PEK model, which is comprised of fast and slow sorption components, exhibited hysteresis in terms of mass for both processes between the adsorption and desorption isotherm. The hysteresis derived from the slow sorption process was less than from the fast process for all tested fibres. The fast processes for cotton and filter paper dominated the isotherm process; however, the hemp and sisal fibres displayed a dominant slow process in the isotherm run. The characteristic time for the fast sorption process did not vary between adsorption and desorption, except at the top end of the hygroscopic range. The characteristic time for the slow process was invariably larger for the desorption process. The physical interpretation of the PEK model is discussed.     

基于硅胶干燥剂的多孔基材性能测试

多孔性基材作为干燥剂的载体,对除湿转轮的性能有重要影响。本文选择了木浆滤纸(WPFP)、陶瓷纤维纸(CFP)和玻璃纤维纸(GFP)3种多孔基材进行研究。采用Hot Disk、扫描电镜、压汞仪及ASAP2460对3种材料的基本物性进行表征,将基材多次浸泡JN-30硅溶胶,并在恒温恒湿箱中高温高湿工况下进行吸附实验,对比3种基材本身及上胶后的吸附及脱附能力。结果表明:上胶次数对3种材料的影响相似,基材上胶后整体的饱和吸附量增大、达到饱和状态的时间变长,但上胶次数过多会导致每次上胶增加量减少,上胶次数不能超过5次。木浆滤纸相比于陶瓷纤维纸和玻璃纤维纸具有更好的亲水性及更高的上胶率,且亲水性相比于孔隙率对上胶率的影响更大,单位面积基材上胶后整体的吸附量最大、达到饱和状态的时间较长,符合LDF模型的模拟结果,且脱附能力较好,适用于除湿转轮的制作。     

Binary adsorption and desorption rates of propylene-propane mixtures on 13 X molecular sieves

The experimental adsorption and desorption rate data for pure propylene, pure propane, and propylene-propane mixtures on 13 X molecular sieves are presented for different temperatures and compositions; these rate data were efficiently determined using the differential sorption bed system described by Liapis et al. (Liapis et al. (1992) Rates and equilibria of adsorption and desorption of propane and propylene on 13 X molecular sieves determined experimentally using a differential sorption bed system. Sep. Technol. 2, 141–154). Propylene is the most preferentially adsorbed component, and the desorption rates of propylene and propane are slower than the adsorption rates. Mathematical models were constructed and solved, and it was found that the two-component model could provide a quantitative measure of the interactions between propylene and propane, and could describe the behavior of binary adsorption kinetics including the overshoot of the equilibrium loading by the less preferentially adsorbed propane. The theoretical results indicate that the desorption rates are not described by the same intracrystalline diffusion coefficients as the adsorption rates.     

Ce-Doped Smart Adsorbents with Photoresponsive Molecular Switches for Selective Adsorption and Efficient Desorption

Achieving efficient adsorption and desorption processes by controllably tuning the properties of adsorbents at different technical stages is extremely attractive. However, it is difficult for traditional adsorbents to reach the target because of their fixed active sites. Herein, we report on the fabrication of a smart adsorbent, which was achieved by introducing photoresponsive azobenzene derivatives with cis/trans isomers to Ce-doped mesoporous silica. These photoresponsive groups serve as “molecular switches” by sheltering and exposing active sites, leading to efficient adsorption and desorption. Ce is also doped to provide additional active sites in order to enhance the adsorption performance. The results show that the cis isomers effectively shelter the active sites, leading to the selective adsorption of methylene blue (MB) over brilliant blue (BB), while the trans isomers completely expose the active sites, resulting in the convenient release of the adsorbates. Both selective adsorption and efficient desorption can be realized controllably by these smart adsorbents through photostimulation. Moreover, the performance of the obtained materials is well maintained after five cycles.     

Different methods in TiO2 photodegradation mechanism studies: gaseous and TiO2-adsorbed phases

The development of photocatalysis processes offers a significant number of perspectives especially in gaseous phase depollution. It is proved that the photo-oxidizing properties of photocatalyst (TiO(2)) activated by UV plays an important role in the degradation of volatile organic compounds (VOC). Heterogeneous photocatalysis is based on the absorption of UV radiations by TiO(2). This phenomenon leads to the degradation and the oxidation of the compounds, according to a mechanism that associates the pollutant's adsorption on the photocatalyst and radical degradation reactions. The main objective of the study is the understanding of the TiO(2)-photocatalysis phenomenon including gaseous and adsorbed phase mechanisms. Results obtained with three different apparatus are compared; gaseous phases are analysed and mechanisms at the gaseous phase/photocatalyst interface are identified. This study leads to improve understanding of various mechanisms during pollutant photodegradation: adsorption of pollutants on TiO(2) first takes place, then desorption and/or photodegradation, and finally, desorption of degradation products on TiO(2). The association of analytical methods and different processes makes the determination of all parameters that affect the photocatalytic process possible. Mastering these parameters is fundamental for the design and construction of industrial size reactors that aim to purify the atmosphere.