用实验和模拟计算确定SiO2空心微球的孔径分布

用巨正则系综蒙特卡罗（GCMC）模拟方法结合统计积分方程（SIE）计算了SiO2空心微球球壳上的孔径分布（PSD）.HRTEM、XRD及氮气吸附等实验测试表明,SiO2空心微球的球壳上有无序的介孔孔道.在模拟中,基于实验数据,将SiO2空心微球模型化为具有一定孔径分布的园柱孔,流体模型化为Lennard-Jones(LJ)球,流体分子和孔壁间的相互作用采用Wang等人[10]最近提出的完全解析的势函数描述.模拟结果显示,用孔径分布拟合的吸附数据和实验吸附等温线吻合良好,说明PSD能够十分有效地表示SiO2空心微球的微孔结构.     

基于密度泛函理论研究二元排斥Yukawa流体的表面结构性质

基于自由能密度泛函理论(DFT)考察了二元排斥Yukawa (HCRY)流体在不同外场下的密度分布. 基于微扰理论, 体系的Helmholtz自由能泛函采用硬球排斥部分和长程色散部分贡献之和, 其中Kierlik和Rosinberg的加权密度近似(WDA)被用来计算硬球排斥部分, 而色散部分采用平均场理论(MFT)进行描述. 为了验证DFT计算结果的合理性, 研究中采用巨正则Monte Carlo(GCMC)模拟计算了在不同主体相密度、硬核直径和位能参数比的条件下二元HCRY混合流体的密度分布. 结果表明, 该DFT计算结果与GCMC模拟值吻合良好.     

氮气在MCM－41分子筛中的吸附：实验和分子模拟

用美国Micromeritics公司生产的ASAP2010物理吸附仪测定了低温（77 K） N_2在MCM-41分子筛中的吸附等温线，获得了表征MCM-41特征的BET比表面、BJH孔 容和平均孔径。同时用巨正则Monte Carlo（GCMC）模拟方法考究了N_2在MCM-41中 的吸附，得到了N_2在MCM-41中的模拟吸附等温线，分析了流体在MCM-41分子筛中 的微观结构。GCMC模拟中MCM-41介孔材料模型化为圆柱孔，N_2模型化为Lennard- Jones（LJ）球。N_2和MCM-41介孔墙壁间的相互作用采用Tjatjopoulos-Feke- Mann（TFM）势能模型进行表征。通过使模拟和实验结果有一个好的吻合，确定了 一组有效的MCM-41分子筛的势能参数（σ_(ww) = 0.265 nm，∈_(ww)/k = 190 K ）。这为以后其他吸附质在MCM-41中吸附的预测奠定了基础、提供了依据。     

AB2型聚合物流体的表面结构性质

在密度泛函理论(DFT)框架下, 应用改进的基本度量理论(MFMT)表达硬球作用对自由能泛函的贡献, 根据统计力学理论结合加权密度近似(WDA)表达聚合作用对自由能泛函的贡献, 建立了描述AB2型聚合物流体的化学势, 得到了聚合物流体在硬球颗粒表面的密度分布表达式, 计算了聚合物流体在硬球颗粒表面附近的密度分布, 并探讨了体积分数、聚合程度和硬球颗粒尺度对体系密度分布的影响. 此外, 通过体系密度分布, 进一步分析了体积分数、聚合程度和硬球颗粒尺度与剩余吸附的关系.     

超临界甲烷在氯化锆层柱纳米材料中吸附的分子模拟

用巨正则MonteCarlo(GCMC)方法模拟了甲烷在氯化锆层柱材料中的吸附。模拟中,氯化锆层柱材料模型化为柱子均匀分布在层板间的层柱孔,非极性分子甲烷采用Lennard-Jones分子模型,层板墙采用Steele的10-4-3模型,流体分子与柱子的相互作用采用点-点(sitetosite)的方法计算。在高度理想化模型的基础上,引入交互作用参数kfw,建立了有效势能模型。通过实验数据确定交互作用参数kfw,从而使该模型能有效地表征流体与层板墙的相互作用。根据77K温度下氮气的实验吸附数据,确定了流体和层板墙间的交互相作用参数。然后用这个有效的参数kfw=0.65模拟了三个超临界温度下氯化锆层柱材料中甲烷的吸附情形,得到了它位的吸附等温线,局部密度分布以有流体分子在层柱微孔中的瞬时构象,并分析了温度对材料吸附性能的影响。结果表明GCMC方法是预测材料吸附性能的一种强有力的工具。     

超临界甲烷在氯化锆层柱纳米材料中吸附的分子模拟

用巨正则MonteCarlo(GCMC)方法模拟了甲烷在氯化锆层柱材料中的吸附。模拟中,氯化锆层柱材料模型化为柱子均匀分布在层板间的层柱孔,非极性分子甲烷采用Lennard-Jones分子模型,层板墙采用Steele的10-4-3模型,流体分子与柱子的相互作用采用点-点(sitetosite)的方法计算。在高度理想化模型的基础上,引入交互作用参数kfw,建立了有效势能模型。通过实验数据确定交互作用参数kfw,从而使该模型能有效地表征流体与层板墙的相互作用。根据77K温度下氮气的实验吸附数据,确定了流体和层板墙间的交互相作用参数。然后用这个有效的参数kfw=0.65模拟了三个超临界温度下氯化锆层柱材料中甲烷的吸附情形,得到了它位的吸附等温线,局部密度分布以有流体分子在层柱微孔中的瞬时构象,并分析了温度对材料吸附性能的影响。结果表明GCMC方法是预测材料吸附性能的一种强有力的工具。     

高阶FMSA展开研究缔合流体界面张力

结合一阶平均球近似(First-order mean-spherical approximation, FMSA)与重整化群(Renormalization group, RG)变换计算了流体全局性相行为. 应用FMSA理论解析得到的径向分布函数(Radial distribution function, RDF)和直接相关函数(Direct correction function, DCF)建立密度泛函方法, 并在其展开项中考虑了高阶微扰项作用, 即考虑了主体流体密度不一致性, 避免原有方法在计算密度分布时存在难以收敛、误差大等问题. 将高阶展开扩展应用到缔合流体, 计算表明, 和分子模拟数据相比, 界面密度分布和界面张力较之原有的密度泛函方法均有了明显改善.     

平行板间超支化聚合物流体的密度分布和溶剂化力

在密度泛函理论(DFT)框架下,应用改进的基本度量理论(MFMT)和统计力学理论结合加权密度近似(WDA)分别表示硬球作用和聚合作用对自由能泛函的贡献,建立了描述ABg型超支化聚合物流体的化学势,得到了聚合物流体在平行板间的密度分布表达式,计算了聚合物流体在两平行板间的密度分布,并探讨了体积分数、反应程度和单体中B类官能团数对体系密度分布的影响.此外,通过体系密度分布,进一步分析了反应程度、板间宽度与溶剂化力的关系.     

氢气在碳纳米管内吸附的密度泛函理论研究

本文应用密度泛函理论研究纯流体氢气在单壁碳纳米管内吸附过程,采用硬球状态方程改进的基本测量理论表征硬球的斥力作用,离子吸引项的贡献则用微扰理论描述.在温度为300k,氢气本体对比密度范围为0.2～0.7条件下,计算了三种不同尺寸的碳纳米管氢气吸附的密度分布,其密度泛函计算结果与计算机分子模拟数据完全一致.     

Yukawa流体的相平衡和界面张力研究

应用二阶微扰理论, Duh-Mier-Y-Teran状态方程和在平均球近似(mean spherical approximation, MSA)的基础上获得的直接相关函数, 建立了适用于均匀流体和非均匀流体的状态方程. 结合此状态方程, 重整化群理论(renormalization group theory, RG)和密度泛函理论(density functional theory, DFT), 分别研究了Yukawa流体的相平衡和界面张力. 结果与分子模拟数据吻合良好.     

Molecular simulation of binary mixture adsorption in buckytubes and MCM-41

MCM-41 and buckytubes are novel porous materials with controllable pore sizes and narrow pore size distributions. Buckytubes are carbon tubes with internal diameters in the range 1–5 urn. The structure of each tube is thought to be similar to one or more graphite sheets rolled up in a helical manner. MCM-41 is one member of a new family of highly uniform mesoporous silicate materials produced by Mobil, whose pore size can be accurately controlled in the range 1.5–10 nm. We present grand canonical Monte Carlo (GCMC) simulations of single fluid and binary mixture adsorption in a model buckytube, and nonlocal density functional theory (DFT) calculations of trace pollutant separation in a range of buckytubes and MCM-41 pores. Three adsorbed fluids are considered; methane, nitrogen and propane. The GCMC studies show that the more strongly adsorbed pure fluid is adsorbed preferentially from an equimolar binary mixture. Ideal adsorbed solution theory (IAST) is shown to give good qualitative agreement with GCMC when predicting binary mixture separations. The DFT results demonstrate the very large increases in trace pollutant separation that can be achieved by tuning the pore size, structure, temperature and pressure of the MCM-41 and buckytube adsorbent systems to their optimal values.     

GCMC investigation into adamantane-based aromatic frameworks with diamond-like structure as high-capacity hydrogen storage materials

A new class of 3D adamantane-based aromatic framework (AAF) with diamond-like structure was computationally designed with the aid of density functional theory (DFT) calculation and molecular mechanics (MM) methods. The hydrogen storage capacities of these AAFs were studied by the method of grand canonical Monte Carlo (GCMC) simulations. The calculated pore sizes of three AAFs reveal that AAF-1 and AAF-2 belong to microporous materials, while AAF-3 is a member of mesoporous materials. The GCMC results reveal that at 77 K and 100 bar, AAF-3 exhibits the highest gravimetric hydrogen uptake of 29.50 wt%, while AAF-1 shows the highest volumetric hydrogen uptake of 63.04 g L(-1). In particular, the gravimetric hydrogen uptake of AAF-3 reaches the Department of Energy's target of 6 wt% at room temperature. The extraordinary performances of these new AAFs in hydrogen storage have made them enter the list of top hydrogen storage materials up to now.     

Adsorption Study of Methane on Activated Meso-carbon Microbeads by Density Functional Theory

A combined method of density functional theory (DFT) and statistics integral equation (SIE) for the determination of the pore size distribution (PSD) is developed based on the experimental adsorption data of nitrogen on activated mesocarbon microbead (AMCMB) at 77K. The pores of AMCMB are described as slit-shaped with PSD.Based on the PSD, methane adsorption and phase behavior are studied by the DFT method. Both nitrogen and methane molecules are modeled as Lennard-Jones spherical molecules, and the well-known Steele‘s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. In order to test the combined method and the PSD model, the Intelligent Gravimetric Analyzer (IGA-003) was used to measure the adsorption of methane on the AMCMB. The DFT results are in good agreement with the experimental data. Based on these facts,we predict the adsorption amount of methane, which can reach 32.3ω at 299K and 4 MPa. The results indicate that the AMCMBs are a good candidate for adsorptive storage of methane and natural gas. In addition, the capillary condensation and hysteresis phenomenon of methane are also observed at 74.05K.     

Structure and adsorption of a hard-core multi-Yukawa fluid confined in a slitlike pore: grand canonical Monte Carlo simulation and density functional study

Because of the increasing interest in studying the phenomenon exhibited by charge-stabilized colloidal suspensions in confining geometry, we present a density functional theory (DFT) for a hard-core multi-Yukawa fluid. The excess Helmholtz free-energy functional is constructed by using the modified fundamental measure theory and Rosenfeld's perturbative method, in which the bulk direct correlation function is obtained from the first-order mean spherical approximation. To validate the established theory, grand canonical ensemble Monte Carlo (GCMC) simulations are carried out to determine the density profiles and surface excesses of multi-Yukawa fluid in a slitlike pore. Comparisons of the theoretical results with the GCMC data suggest that the present DFT gives very accurate density profiles and surface excesses of multi-Yukawa fluid in the slitlike pore as well as the radial distribution functions of the bulk fluid. Both the DFT and the GCMC simulations predict the depletion of the multi-Yukawa fluid near a nonattractive wall, while the mean-field theory fails to describe this depletion in some cases. Because the simple form of the direct correlation function is used, the present DFT is computationally as efficient as the mean-field theory, but reproduces the simulation data much better than the mean-field theory.     

Monte Carlo Simulation and Experimental Studies of CO2, CH4 and Their Mixture Capture in Porous Carbons

Adsorption of carbon dioxide and methane in porous activated carbon and carbon nanotube was studied experimentally and by Grand Canonical Monte Carlo (GCMC) simulation. A gravimetric analyzer was used to obtain the experimental data, while in the simulation we used graphitic slit pores of various pore size to model activated carbon and a bundle of graphitic cylinders arranged hexagonally to model carbon nanotube. Carbon dioxide was modeled as a 3-center-Lennard-Jones (LJ) molecule with three fixed partial charges, while methane was modeled as a single LJ molecule. We have shown that the behavior of adsorption for both activated carbon and carbon nanotube is sensitive to pore width and the crossing of isotherms is observed because of the molecular packing, which favors commensurate packing for some pore sizes. Using the adsorption data of pure methane or carbon dioxide on activated carbon, we derived its pore size distribution (PSD), which was found to be in good agreement with the PSD obtained from the analysis of nitrogen adsorption data at 77 K. This derived PSD was used to describe isotherms at other temperatures as well as isotherms of mixture of carbon dioxide and methane in activated carbon and carbon nanotube at 273 and 300 K. Good agreement between the computed and experimental isotherm data was observed, thus justifying the use of a simple adsorption model.     

A density functional theory for Lennard-Jones fluids in cylindrical pores and its applications to adsorption of nitrogen on MCM-41 materials

A density functional theory (DFT) constructed from the modified fundamental-measure theory and the modified Benedict-Webb-Rubin equation of state is presented. The Helmholtz free energy functional due to attractive interaction is expressed as a functional of attractive weighted-density in which the weight function is a mean-field-like type. An obvious advantage of the present theory is that it reproduces accurate bulk properties such as chemical potential, bulk pressure, vapor-liquid interfacial tension, and so forth when compared with molecular simulations and experiments with the same set of molecular parameters. Capabilities of the present DFT are demonstrated by its applicability to adsorption of argon and nitrogen on, respectively, a model cylindrical pore and mesoporous MCM-41 materials. Comparison of the theoretical results of argon in the model cylindrical pore with those from the newly published molecular simulations indicates that the present DFT predicts accurate average densities in the pore, slightly overestimates the pore pressure, and correctly describes the effect of the fluid-pore wall interaction on average densities and pressures in the pore. Application to adsorption of nitrogen on MCM-41 at 77.4 K shows that the present DFT predicts density profiles and adsorption isotherms in good agreement with those from molecular simulations and experiments. In contrast, the hysteresis loop of adsorption calculated from the mean-field theory shifts toward the low pressure region because a low bulk saturated pressure is produced from the mean-field equation of state. The present DFT offers a good way to describe the adsorption isotherms of porous materials as a function of temperature and pressure.     

Improvement of the Derjaguin-Broekhoff-de Boer theory for the capillary condensation/evaporation of nitrogen in spherical cavities and its application for the pore size analysis of silicas with ordered cagelike mesopores

In a previous work, we proposed an improvement of the Derjaguin-Broekhoff-de Boer (DBdB) theory for capillary condensation/evaporation in open-ended cylindrical mesopores. In this paper, we report a further extension of this approach to the capillary condensation/evaporation of nitrogen in siliceous spherical cavities. The main idea of this improvement is to employ the Gibbs-Tolman-Koenig-Buff equation to predict the variation of the surface tension in spherical mesopores. In addition, the statistical film thickness (the so-called t-curve), which is evaluated accurately on the basis of adsorption isotherms measured for MCM-41 materials, is used instead of the originally proposed t-curve to take into account the excess chemical potential due to the surface forces. It is shown that the aforementioned modifications of the original DBdB theory that was refined by Ravikovitch and Neimark have significant implications for the pore size analysis of cagelike mesoporous silicas. To verify the proposed improvement of the DBdB pore size analysis (IDBdB), two series of FDU-1 samples, which are well-defined cagelike mesoporous materials (composed of siliceous spherical cavities interconnected by short necks), were used for the evaluation of the pore size distributions (PSDs). The correlation between the spinodal condensation point in the spherical pores predicted by the nonlocal density functional theory (NDFT) developed by Ravikovitch and Neimark and that predicted by the IDBdB theory is very good in the whole range of mesopores. This feature is mirrored to the realistic PSD characterized by the bimodal structure of pores computed from the IDBdB theory. As in the case of open-ended cylindrical pores, the improvement of the classical DBdB theory preserves its simplicity and simultaneously ensures a significant improvement of the pore size analysis, which is confirmed by the independent estimation of the average pore size by the NDFT and the powder X-ray diffraction method.     

Effect of Structure and Pore Size of Mesoporous Molecular Sieve Materials on the Growth of Carbon Nanotubes

Microporous and mesoporous molecular sieve materials with different channel structures and pore sizes were applied as supports for Fe-loaded catalysts to catalytically synthesize carbon nanotubes. The deposited carbon materials were characterized by the TEM technique. It was shown that the structures and pore sizes of supports greatly influence the forms and quality of the deposited carbon materials. The larger the pore size of the support used, the larger the diameter and pore size of the carbon nanotubes formed. It seems that the growth of carbon nanotubes can be orientated by the one-dimensional mesoporous structure of hexagonal mesoporous molecular sieve materials.     

Porous structure of natural and modified clinoptilolites

The evaluation of the pore-size distribution (PSD) of natural and modified mesoporous zeolites, i.e., clinoptilolites is presented. We demonstrate the SEM results showing that the pores of fracture-type from 25-50 nm to 100 nm in size between clinoptilolite grains, as well as pores between crystal aggregates up to 500 nm in size are present in the studied material. The detailed distribution of pore sizes and tortuosity factor of the above-mentioned materials are determined from the adsorption-desorption isotherms of nitrogen measured volumetrically at 77 K. To obtain the reliable pore size distribution (PSD) of the above-mentioned materials both adsorption and desorption branches of the experimental hysteresis loop are described simultaneously by recently developed corrugated pore structure model (CPSM) of Androutsopoulos and Salmas. Evaluated pore size distributions are characterized by well-defined smooth peaks placed in the region of the mesoporosity. Moreover, the mean pore diameter calculated from the classical static measurement of nitrogen adsorption at 77 K correspond very well to the pore diameters from SEM, showing the applicability of the CPSM for characterization of the porosity of natural zeolites. We conclude that classical static adsorption measurements combined with the proper modeling of the capillary condensation/evaporation phenomena are a powerful method which can be applied for pore structure characterization of natural and modified clinoptilolites.