纳米圆柱孔材料的限定空间内流体的吸附及相行为

2019-02-24 21:05:48

吸附 adsorption SWNTs pore DFT









中文题名纳米圆柱孔材料的限定空间内流体的吸附及相行为

 





副题名 





外文题名 Adsorption and phase behavior of fluids in confinement of nano-cylindrical materials 





论文作者张现仁   





导师汪文川教授   





学科专业化学工程   





研究领域\研究方向 





学位级别博士 





学位授予单位北京化工大学   





学位授予日期2001   





论文页码总数141页   





关键词碳纳米管  密度泛函  纳米材料   





馆藏号BSLW

/2001

/TB383

/61 





【中文摘要】

    被限制在几个分子直径的纳米孔中的流体分子,表现出一系列不同于主体流体的物理性质。如何更好的理解有限尺寸效应,表面作用以及维数的变化的影响具有重要的理论意义。此外由于吸附是日常生活及工业单元操作中较普遍存在的一种现象,了解吸附微观机理和限定空间内流体的相行为也具有重要的实际意义。本文应用计算机模拟和密度泛函理论研究了简单流体分子在圆柱状孔MCM-41和单壁碳纳米管束(SWNTs)中的吸附及相行为。
   本文提出了一个解析的势函数来描述圆柱状孔中的LJ分子与壁面的相互作用势。经验证这个势模型和虚拟原子模型相一致,并在孔壁厚度趋于零的极限条件下和圆柱面的势函数吻合较好。
   通过这个势函数模拟了氮气在MCM-41中的吸附等温线并与本校实验测定的氮气在MCM-41中吸附的实验数据相比较,拟合出适合表征MCM-41孔墙分子的势参数。利用这套势参数和巨正则系综Monte Carlo(GCMC)方法模拟了不同孔径和温度下甲烷在MCM-41中的吸附等温线和其在孔中的排列方式。由模拟可知,当在室温300K,压力为5.4MPa和7.1MPa时,吸附在3.525nm的MCM-41中的甲烷可达7.86mmol/g和9.75mmol/g。。
   本文比较了甲烷在MCM-41中吸附的密度泛函理论(DFT)计算和GCMC模拟结果。通过比较可以看出,DFT计算和GCMC模拟的吸附等温线吻合较好。本文采用DFT方法计算了甲烷在不同的孔径和不同温度下的吸附等温线。研究了孔径,温度,孔墙势阱深度和孔墙厚度对吸附等温线的影响。
   本文采用GCMC方法和DFT方法考察了温度和墙势作用对层状转变的影响。GCMC模拟发现当孔壁的粒子密度分别为ρ〓(ρ〓=2.0g/cm〓)和2ρ〓时,两种情况下0->1层状转变的临界点都在T〓=0.44和0.5之间。模拟表明当孔壁的粒子密度为ρ〓时层状转变处于毛细凝聚对应的回滞环中。而当孔壁的粒子密度为2ρ〓时则不处于毛细凝聚对应的回滞环中。此外在65K温度下GCMC模拟发现,这时的层状转变伴随着很窄的回滞环。总体来言,DFT和GCMC方法给出了大体相同的结论。不同的是,当孔壁内的粒子密度为2ρ〓和温度为74.05K时,GCMC方法给出的层状转变是连续的,而DFT的计算结果表明,这时的层状转变是一阶相变并伴有明显的回滞环。本文应用分子动力学(MD)方法考察了在可能的0->1层状转变前的状态中流体分子的构型的变化和扩散性质,以推断温度,孔墙作用和粒子密度对LJ粒子在圆柱状孔中的排列方式以及它们所处的状态。通过MD模拟还发现LJ粒子在圆柱状孔的表面上有明显的聚集效应,第一层未排满前粒子不是均匀的分布在圆柱状孔的内表面上,而是分布在圆柱状孔的表面上的流体粒子聚在一起,呈片状或簇状分布。
   本文采用GCMC方法模拟研究了甲烷在SWNTs的管内和管隙中的吸附和相行为。与狭缝孔相比,室温下甲烷在4.077nm的SWNTs中具有较高的吸附储存能力。在较低温度下,除毛细凝聚外,甲烷在孔径为4.077nm的正方晶格的碳管阵列的管隙中会出现类似固体的分子构型。在这个体系中甲烷的固化温度应在125K和135K之间,这个温度高于LJ甲烷分子在主体流体中的转变温度101K。本文也采用DFT方法研究了甲烷在单壁碳纳米管中丰富的相行为。
   本文采用GCMC方法模拟研究了乙烷在SWNTs的管内和管隙中的吸附。模拟发现碳管中碳原子的结构对乙烷分子在管内中的吸附几乎没有影响。模拟结果表明,对乙烷分子在管内和管隙中的吸附能力来讲,孔的体积和墙势作用是一对相互竞争的因素。而且无论是管内吸附还是管间吸附时,靠近壁面处的分子密度较高且乙烷分子倾向于顺着壁面排列。当碳管的孔径为2.719nm时,由吸附等温线可以判断,这时的管内吸附具有中孔吸附的特征,而管隙中的吸附具有微孔吸附的特征。在讨论管内乙烷分子的吸附时发现,在低温下一般乙烷分子在达到饱和吸附的分子数密度会随孔径的增加而增加。但孔径在1nm附近时,增加孔径会引起孔内吸附的分子数密度的突然减少。
   本文采用DFT方法研究了氢气在SWNTs的管内和管隙中的吸附。结果表明温度对吸附量和流体分子在孔中的排列均有较大影响。DFT计算可知,77K和6MPa时,氢气在2.719nm的SWNTs的总的吸附的重量百分比分别可达到13.2wt%。而在300K和6MPa时,氢气在2.719nm的SWNTs的总的吸附的重量百分比仅为1.5wt%。以前对氢在单壁碳纳米管束的模拟和理论计算仅仅局限于较小的微孔,而本文的计算表明较大的孔径有利于氢气在单壁碳纳米管束的管隙中的吸附。本文中还比较了氢气在管内和管隙的吸附,发现氢气在管隙中的吸附占吸附总量的一个较大比重,不可忽略。并且管径越大所占的比重越大。











【外文摘要】

 Abstract
   Molecules confined within narrow pores, with the size of a few molecular diameters, can exhibit different physical behaviors from the bulk fluid. To understand these behaviors resulting from the finite-size effect, surface force and varying dimensionality is of significant scientific interest. In addition, understanding these phenomena is necessary for many industrial operations. Computer simulation and the density functional theory (DFT) are used in this work to study the adsorption of simple molecules and their phase behaviors in cylindrical pores of MCM-41 and in single walled carbon nanotubes (SWNTs) arrays.
   An analytical potential model describing the interaction between the wall of a cylindrical pore and an LJ molecule confined in the pore has been proposed. The model gives good fit to the results from the pseudoatom model and the cylindrical surface model in the limit where the wall thickness reaches zero.
   GCMC simulations have been carried out to study the adsorption of nitrogen in MCM-41 at 77K. By comparing the simulated adsorption isotherm and the experimental data reported by He et al, a set of potential parameters for the surface of MCM-41 is obtained. With the set of potential parameters, the adsorption of methane in MCM-41 of different pore sizes at different temperatures has been simulated. And simulated results show that the amounts of methane adsorption in the MCM-41 pore of 3. 525nm can reach 7. 86mmol/g and 9. 75mmol/g at 300K and at 5. 4MPa and 7. 1MPa, respectively.
   A comparison of the adsorption isotherms from DFT calculations and from GCMC simulations indicates that the two methods agree well in general. The effects of temperature, pore diameter, wall thickness and energy parameter of fluid-solid interactions on adsorption isotherms have been studied by the DFT method in this work.
   Layering transitions within cylindrical pores at several temperatures and different fluid-solid interactions have been studied by both the GCMC and DFT methods. With GCMC method, the critical temperatures of 0->1 layering transitions with two fluid-solid potentials, corresponding to ρ〓 and 2 ρ〓 (ρ〓=2. 0g/cm〓) in the pore of 3. 525nm are all at the reduced temperature T〓=0.44~0.5. Layering transitions are found within the hysteresis loops of capillary condensations at low fluid-wall potentials. While the layering transitions of the pore with the stronger fluid-wall interaction get out the hysteresis loops of capillary condensations. Moreover, a small hysteresis loop of 0->1 layering transition is found in this case. However, the critical temperatures of 0-> layering transitions determined by the DFT method is higher than 0.5. The diffusion properties of the fluid and the effects of temperature, the fluid-wall potential and number density of fluid particles on the diffusivities nearby layering transitions are investigated by MD method. MD results show that fluid molecules are not evenly distributed on the surface of the wall but aggregate in some areas.
   GCMC simulations have been carried out to investigate the adsorption and the phase behavior of methane in SWNTs. Simulation results demonstrate that the total amount of adsorptive storage of methane in SWNT square array can reach 22mmol/g for the tube of 4. 077nm at 6MPa and 300K, more than that in a slit pore of the same size. Simulation results also suggest that in the interstice formed by outer surfaces of square arranged carbon tubes of 4. 077nm, the solid-like structure can be recognized at 125K or below this temperature. DFT calculations also have been performed to study the phase transitions of methane molecules within the tubes of SWNTs.
   Molecular simulations have been carried out to study ethane adsorption in SWNTs, in which ethane molecules are represented by two-site LJ particles. The structure of SWNTs is found to hardly make any difference regarding the amount of ethane adsorption in tubes of SWNTs. Ethane molecules near the wall both in the tubes of SWNTs and interstices intend to lay along the surface of SWNTs. When the pore diameter of SWNTs is 2. 719nm, the adsorption isotherm in the tubes of SWNTs exhibits the feature of mesopores while the adsorption in the interstice shows the feature of micropores. At low temperature, generally, the number density of ethane molecules adsorbed inside the tube of SWNTs would increase as the pore size increases. However, when the diameter of a tube is about 1nm, the number density of the fluid would suddenly decrease as the pore size increases.
   In this work, hydrogen adsorption both in the tubes and the interstices of SWNTs has been investigated by DFT method. Calculated results indicate that the temperature of adsorption affects both the storage amount and the arrangement of hydrogen molecules. For example, the simulated gravimetric storage of hydrogen in SWNTs of 2. 719nm can reach 13. 2wt% at 77K and 6Mpa. However, the gravimetric storage of hydrogen would only be 1. 5wt% at 300K and 6MPa. In contrast with the small pore studied in previous work, a serious of larger pores are considered here and found that the larger pore is in favor of the gravimetric storage. In comparison with the adsorption in the tubes of SWNTs, calculations suggest that the adsorption in interstices for large size SWNTs is not neglectable, especially for large pore and at low temperature and large pore.