Langmuir方程Γ∞在被吸附的重金属量测定中的应用

为研究吸附剂吸附的原理和影响因素,采用数学建模和模拟方法对Langmuir方程的常量Γ∞进行了定量分析,发现吸附剂总表面积是影响吸附能力的主要因素,吸附剂吸附能力随总表面积增大而增大,随粒径减小而急剧增大,悬浮物颗粒的几何形态影响甚小。天然状态下,单位水体悬浮物含量的不同将导致总表面积的不同,进而影响被吸附重金属的量。总之,改变悬浮物颗粒表面都会影响试验精度。     

微狭缝中甲烷吸附特性的分子动力学模拟

为了研究甲烷在纳米尺度狭缝中的吸附特性,采用分子动力学模拟的方法研究了温度、压力以及狭缝表面的水滴对甲烷吸附情况的影响。研究表明:降低温度和增大压力均能使吸附相的密度增大、吸附层的层数增加、吸附区扩大;升高压力将使吸附作用减弱,温度较低时,升高温度将使吸附作用增强,温度较高时,升高温度将使吸附强度减弱;狭缝表面存在水滴时,狭缝对甲烷的吸附作用被明显削弱。     

Alkali,alkaline earth and transition metal doped B6H6 complexes for hydrogen storage

Alkali, alkaline earth and transition metal doped B₆H₆ complexes are considered for the hydrogen storage. Density functional theory (DFT) and second order Møller–Plesset methods with 6–311++G** basis set have been used for the study. B₆H₆Li, B₆H₆Be, B₆H₆Sc, B₆H₆Li₂, B₆H₆Be₂, B₆H₆Sc₂ complexes can interact with maximum three, two, four, six, four and eight H₂ molecules respectively with respective H₂ uptake capacity of 7.2, 4.8, 6.5, 12.5, 8.3 and 9.1 wt%. This uptake capacity is well above the target set by the U.S. Department of Energy by 2020 except for the B₆H₆Be complex. Thermo chemistry calculations are carried out to estimate the Gibbs free energy corrected H₂ adsorption energy which reveals whether adsorption of hydrogen on these complexes is favourable or not at different temperature. It is observed that H₂ adsorption on all the six complexes are unfavourable at ambient conditions where as it is favourable below 150, 135, 75, and 50 K on B₆H₆Sc, B₆H₆Be, B₆H₆Li and B₆H₆Li₂ complexes respectively. Various interaction energies in H₂ adsorbed complexes are obtained using Many-body analysis approach. The H₂ desorption temperature for the B₆H₆Li, B₆H₆Be, B₆H₆Sc, B₆H₆Li₂, B₆H₆Be₂ and B₆H₆Sc₂ complexes is found to be 25, 165, 265, 10, 265 and 373 K respectively.     

Electronic structure calculations and molecular dynamics simulations of hydrogen adsorption on Beryllium doped complexes

Interaction of molecular hydrogen with Be and Be₂ decorated acetylene is studied using first principles calculations and molecular dynamics simulations. C₂H₂Be and C₂H₂Be₂ complex can interact with maximum of two and four H₂ molecules respectively thereby showing respective H₂ uptake capacity of 10.3 and 15.5 wt % and is well above the target set by U.S. department of energy. Temperature dependent Gibbs free energy corrected adsorption energy shows that H₂ adsorption on C₂H₂Be is energetically favorable below 250 and 200 K and 1 atm. pressure at MP2/6-311++G** and CCSD(T)/6-311++G** level whereas it is energetically favorable on C₂H₂Be₂ at entire temperature and pressure range considered here from 50 K to 400 K and 50 atm to 400 atm. The desorption temperature obtained for C₂H₂Be(2H₂) and C₂H₂Be₂(4H₂) complexes at MP2/6-311++G** level using van't Hoff equation is 281 and 638 K respectively. Molecular dynamics simulations performed at different temperature show that all the adsorbed H₂ molecules remain adsorbed during the simulations on a complex at temperature T only if Gibbs free energy corrected H₂ adsorption energy at that temperature T is positive. H₂ adsorption on Be decorated ethylene is also studied and the results are compared with Be decorated acetylene.     

A molecular dynamics simulation study of PVT properties for H2O/H2/CO2 mixtures in near-critical and supercritical regions of water

Supercritical water gasification technology is an efficient and clean way to use the coal. This technology can convert carbon and hydrogen elements of coal into the mixtures of H₂O/H₂/CO₂ that can be used for electricity generation. The acquisition of PVT properties of H₂O/H₂/CO₂ mixtures is one of the most critical issues in realizing the design and operation of thermal power generation system using this technology. However, no experimental, theoretical and simulation studies exist regarding the PVT properties of H₂O/H₂/CO₂ in the near-critical and supercritical regions of water. In this paper, the molecular dynamics simulations of the PVT properties of H₂O/CO₂ mixtures are carried out and the theoretical calculations are conducted based on the equation of state, and the results are compared with the experimental values. Moreover, the PVT properties for H₂O/H₂ mixtures and H₂O/H₂/CO₂ mixtures in the near-critical and supercritical regions of water are predicted using molecular dynamics simulation and compared with the calculation results of the equation of state. The results of this paper are of great significance to the development of supercritical water gasification of coal, and could offer the reference for the application of H₂O/H₂/CO₂ mixtures in practical production.     

Multi-scale design simulation of a novel intermediate-temperature micro solid oxide fuel cell stack system

This paper presents a multi-scale simulation technique for designing a novel intermediate-temperature planar-type micro solid oxide fuel cell (mSOFC) stack system. This multi-scale technique integrates the fundamentals of molecular dynamics (MD) and computational fluid dynamics (CFD). MD simulations are carried out to determine the optimal composition of a potential electrolyte that is capable of operation in the intermediate-temperature region without sacrifice in performance. A commercial CFD package plus a self-written computational electrochemistry code are employed to design the fuel and air flow systems in a planar five-cell stack, including the preheating manifold. Different samarium-doped ceria (SDC) electrolyte compositions and operating temperatures from 673 K to 1023 K are investigated to identify the maximum ionic conductivity. The electrochemical performance simulation using an available 5-cell yittria-stablized-zirconia (YSZ) mSOFC stack shows good agreement with our experimental results. The same stack design is used to predict a novel SDC-mSOFC performance. Feasibiulity studies of this intermediate-temperature stack are presented using this multi-scale technique.     

The effects of hydrogen and vacancy on the tensile deformation behavior of Σ3 symmetric tilt grain boundaries in pure fe

The interplay of hydrogen, vacancy and grain boundary plays an important role in hydrogen induced premature fracture in the metallic materials. In this work, two models with the representative high angle grain boundaries with the low and high grain boundary energy have been built according to the coincidence site lattice theory. The effects of hydrogen and hydrogen-vacancy combination on the deformation behaviors of the two models were investigated by means of molecular dynamics simulation with the straining direction vertical to the grain boundary. It is found that in both cases hydrogen tends to segregate and maintain a high local stress state around the grain boundary, and promote the premature fracture compared with the hydrogen-free model. Vacancy enhanced the effects of hydrogen in the model with grain boundary of the lower energy. However, vacancy promoted the dislocation evolution behavior in the model with grain boundary of the higher energy. The simulation results were further explained by considering the effects of hydrogen on the generalized stacking fault energy and the work of separation of the grain boundary.     

Interferents for hydrogen storage on a graphene sheet decorated with nickel: A DFT study

In the present work, the decoration of a graphene sheet with nickel is considered as a hydrogen storage material by means of density functional theory calculations. A number of factors relevant for the handling and operation of this material were analyzed. This includes the interaction with potentially interfering chemicals, hydride formation and the hydrogen storage capacity. The present results show that unless the access of oxygen to the surface is restricted, its strong bond to the decorated systems will preclude the practical use for hydrogen storage. In the best case, the energy required to replace an adsorbed oxygen molecule by hydrogen is of the order of 1.7 eV, something that indicates the severe problem that the presence of oxygen represents for this type of systems.     

Molecular dynamics simulation on the effect of water uptake on hydrogen bond network for OH− conduction in imidazolium-g-PPO membrane

OH⁻ conduction involved in the hydrophilic channel of anion exchange membrane strongly depends on the water uptake. To investigate the effect of water uptake on the hydrogen bond network for OH⁻ conduction, a series of molecular dynamics simulations based on all-atom force field were performed on the hydrated imidazolium-g-PPO membranes with different water uptakes. The systems were well verified by comparing the membrane density and OH⁻ conductivity with previous experiments. By means of local structural properties and pair-potential energy, reasonable hydrogen bond criteria were determined to describe the hydrogen bond network confined in the membrane. Increasing water uptake enhances the hydration structures of water and OH⁻, and facilitates the reorganization of the hydrogen bond network. Water and OH⁻ are nearly saturated with water when the water uptake reaches λ = 10, where well-connected hydrogen bond network is produced. Further increasing water uptake has much less contribution to improving the hydrogen bond network, but inevitably swells the membrane channel. This work provides a molecular-level insight into the effect of water uptake on the hydrogen bonding structures and dynamics of OH⁻ and water confined in the imidazolium-g-PPO membrane.     

Nd2−xGdxZr2O7 electrolytes: Thermal expansion and effect of temperature and dopant concentration on ionic conductivity of oxygen

The effect of temperature and complex dopant composition on oxygen ion conductivity in solid oxide electrolyte fuel cells was investigated by atomistic molecular dynamics simulation. A new electrolyte (Nd_2−xGd_xZr₂O₇) was selected to study oxygen ion conductivity using three Gd compositions (x = 0.8, 1.0, and 1.2) in a wide range of temperature (T = 1273 K–1873 K). MSD results of cations showed these groups of electrolyte are stable at high operating temperature. The first composition (x = 0.8) had the highest ionic conductivity that was in good agreement with the experimental data. A simple effective model that works with configuration energy of the oxygen crossing plate was applied to explain the observed conductivity trend. The model illustrated the point as well. Increasing Gd concentration decreases existence probability of easy crossing plate. Radial distribution function analysis also confirmed results. Thermal expansion of the electrolyte has a major effect on the selecting of the electrolyte materials; thus, this important factor was also studied. Results showed the first composition had the greatest thermal expansion.     

Atomistic simulations of hydrogen embrittlement

It is well known that hydrogen weakens strengths of metals, and this phenomenon is called hydrogen embrittlement. Despite the extensive investigation concerning hydrogen related fractures, the mechanism has not been enough clarified yet. In this study, we applied the molecular dynamics method to the mode I crack growth in α-Fe single crystals with and without hydrogen, and analyzed the hydrogen effects from atomistic viewpoints. We estimated the hydrogen trap energy in the vicinity of an edge dislocation in order to clarify the distribution of hydrogen atoms, using the molecular statics method. We also evaluated the energy barrier for dislocation motion under a low hydrogen concentration. Based on these results, we propose a mechanism for hydrogen embrittlement of α-Fe under monotonic loading.     

The effect of temperature and topological defects on H2 adsorption on carbon nanotubes

Physisorption of molecular hydrogen on pristine single-walled carbon nanotube and three types of topologically defected nanotubes (Stone-Wales, vacancy and interstitial defects) at different temperatures 77, 300 and 600 K has been investigated via molecular dynamics simulation. The interatomic interactions (covalent bonds) between the carbon atoms within the nanotube wall were modeled by the well-known bond order Tersoff potential. The applied intermolecular forces are modeled using the modified form of the well-known Lennard-Jones potential based on the nanotube curvature. The adsorption/desorption cycle was followed by increasing the operating temperature under the pressure of 1 bar. The simulation results of exposing 6.5%wt of H₂ on defected and pristine (3,3) nanotubes reveal that the highest and lowest adsorption energies and storage capacities are obtained from the nanotubes with interstitial and vacancy defects, respectively.     

H2 and CO production through coking wastewater in supercritical water condition: ReaxFF reactive molecular dynamics simulation

The degradation mechanism of benzo[a]pyrene (BaP), a representative component of coking wastewater, and the pathway for the production of H₂ and CO in supercritical water have been investigated via ReaxFF reactive molecular dynamics simulations. The BaP molecules in the SCWG, SCWPO and SCWO systems show different degradation pathways. The maximum H₂ yield is obtained at the oxygen ratio of 0.2. There are three routes for the generation of H₂ molecules and production from H radical-rich water is the main route. CO molecules are formed by the CC bond breakage and CO bond breakage in the reforming fragments. There is a time delay between the fuel gas generation reaction and the side reactions due to the change of the instantaneous concentrations of H₂ and CO, providing a possible pathway to increase the amount of the produced fuel gases by designing a suitable reactor and recovering the gas fuel in time. Finally, kinetic behaviors of coking wastewater have been analyzed.