Heat of adsorption and density distribution in slit pores with defective walls: GCMC simulation studies and comparison with experimental data

The adsorption behavior (capacity, density distribution and packing density) and the isosteric heat versus loading in a slit pore whose walls contain defective graphene layers are investigated in this paper. The defective wall is characterized by the extent and size of the defect. Simulation results obtained with the Grand Canonical Monte Carlo method reveal complex patterns of isosteric heat, and this complex behavior is a result of the interplay between three factors: (i) the surface heterogeneity (solid-fluid interaction, sites with varying degree of affinity), (ii) fluid-fluid interaction and (iii) the overlapping of potentials exerted by the two defective walls. We illustrate this with argon adsorption in pores of various sizes, and results obtained from the simulation agree qualitatively with the experimental data at 77 K on Saran microporous S600H and micro-mesoporous S84 charcoals of Beebe et al. [R.A. Beebe, B. Millard, J. Cynarski, J. Am. Chem. Soc. 75 (1953) 839]. The S600H was found to contain pores predominantly in the neighborhood of 7 Å with 30% of defect and a defective size of 2.84 Å. This is consistent with the argument made by Beebe et al. that this sample is a microporous solid and most pores can accommodate only one layer. The other sample, S84, has larger pores than S600H, and it is found that it has a wider pore size distribution and the pore width is centered at about 12 Å.     

Configurational entropy of adsorbed rigid rods: Theory and Monte Carlo simulations

The configurational entropy of straight rigid rods of length k (k-mers) adsorbed on square, honeycomb, and triangular lattices is studied by combining theory and Monte Carlo (MC) simulations in grand canonical and canonical ensembles. Three theoretical models to treat k-mer adsorption on two-dimensional lattices have been discussed: (i) the Flory-Huggins approximation and its modification to address linear adsorbates; (ii) the well-known Guggenheim-DiMarzio approximation; and (iii) a simple semi-empirical model obtained by combining exact one-dimensional calculations, its extension to higher dimensions and Guggenheim-DiMarzio approach. On the other hand, grand canonical and canonical MC calculations of the configurational entropy were obtained by using a thermodynamic integration technique. In the second case, the method relies upon the definition of an artificial Hamiltonian associated with the system of interest for which the entropy of a reference state can be exactly known. Thermodynamic integration is then applied to calculate the entropy in a given state of the system of interest. Comparisons between MC simulations and theoretical results were used to test the accuracy and reliability of the models studied.     

Thermodynamic properties of a hard-core Lennard-Jones fluid from computer simulation and the coupling parameter series expansion

ABSTRACT

The thermodynamic properties of a fluid with an interaction potential consisting in a hard-sphere core plus a Lennard-Jones tail have been obtained by Monte Carlo (MC) NVT simulation as a function of the density along several isotherms. In addition, the liquid–vapour coexistence has been determined by means of histogram-reweighting MC. These data have been used to analyse the performance of perturbation theory. To this end, the first three perturbation terms of the inverse temperature expansion of the Helmholtz free energy have been obtained by means of MC NVT simulations to test the convergence of the perturbation series and to compare with the predictions of the coupling parameter series expansion. Then, the predictions of the latter theory for the thermodynamic properties have been compared with the simulations, revealing the overall excellent performance of this perturbation theory for this model fluid, except in the vicinity of the critical point.     

Phase behavior and fluid-solid surface tension of argon in slit pores and carbon nanotubes

Phase behaviors of argon in several types of cylindrical and slit pores are examined by grand-canonical Monte Carlo simulations. Condensation processes in single- and multi-walled carbon nanotubes along with those in hard-wall tubes are compared. Effects of the pore size on pressure-tensor components, the fluid-wall surface tension, and the adsorption are also compared for the different fluid-pore interactions. The chemical potential at which the fluid begins to condense in the single-walled nanotube is greater than that in the multi-walled nanotube by an amount nearly equal to the difference in the potential-well depth of the fluid-pore interaction, and the adsorption isotherms overlap each other almost completely for narrow pores and partially for wider pores. Similar analyses are performed for slit pores of two different hydrocarbon models.     

Adsorption of argon and nitrogen in cylindrical pores of MCM-41 materials: application of density functional theory

Adsorption of argon and nitrogen at their respective boiling points in cylindrical pores of MCM-41 type silica-like adsorbents is studied by means of a non-local density functional theory (NLDFT), which is modified to deal with amorphous solids. By matching the theoretical results of the pore filling pressure versus pore diameter against the experimental data, we arrive at a conclusion that the adsorption branch (rather than desorption) corresponds to the true thermodynamic equilibrium. If this is accepted, we derive the optimal values for the solid-fluid molecular parameters for the system amorphous silica-Ar and amorphous silica-N₂, and at the same time we could derive reliably the specific surface area of non-porous and mesoporous silica-like adsorbents, without a recourse to the BET method. This method is then logically extended to describe the local adsorption isotherms of argon and nitrogen in silica-like pores, which are then used as the bases (kernel) to determine the pore size distribution. We test this with a number of adsorption isotherms on the MCM-41 samples, and the results are quite realistic and in excellent agreement with the XRD results, justifying the approach adopted in this paper.     

Molecular simulation study of triangle-well fluids confined in slit pores

Grand canonical Monte Carlo simulations are used to study the behaviour of triangle-well (TW) fluids with variable well widths confined inside slit pores. The effect of individual factors influencing the properties of confined fluids such as fluid–fluid interactions, pore size and pore wall–fluid interactions are obtained using simulations as it is difficult to experimentally determine the same. An interesting observation of this study is that inside the narrow pore of slit height h* = 5 at the high-pressure condition of P* = 0.8, for the TW fluid with long-range attraction or for the fluid at a low temperature for even a short-range attraction, the density profiles show layering such that there is a sticking tendency of the particles at centre, while there is a depletion of particles near the wall (as the layers at the centre have higher density peak heights than near the walls).     

密度泛函理论研究碱金属原子在完美氧化镁(001)表面的吸附

基于平面波赝势法的密度泛函理论系统地研究了孤立碱金属原子(锂、钠、钾、铷、铯)在完美氧化镁(001)表面的吸附. 锂在氧位表面上的吸附能是0.72 eV,大约是其它碱金属的3倍. 锂和表面氧之间较强的相互作用主要是来源于共价键的作用,这可由态密度和差分电荷密度的分析所证实. 讨论碱金属在MgO(001)表面吸附的成键机理.     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Adsorption of V on a hematite (0 0 0 1) surface and its oxidation: Submonolayer coverage

The adsorption of submonolayer V on an idealized model hematite (0 0 0 1) surface and subsequent oxidation under atomic O adsorption are studied by density functional theory. The preferred adsorption sites, adsorption energy and configuration changes due to V and O adsorption are investigated. It is found that in most cases V forms threefold bonds with surface O atoms, inducing a large geometry change at the hematite surface and near surface region and a bond stretch between surface Fe and O. The adsorption energy is mainly decided by interplay between adsorbed metal-surface oxygen bonding and adsorbed metal - subsurface metal interaction. The relative energy of subsequent O adsorption and geometry depends on the reformed V/hematite structure. Electronic properties such as projected densities of states and chemical state change upon V adsorption are studied through both periodic slab and embedded cluster localized orbital calculations; both strong vanadium-oxygen and vanadium-iron interactions are found. While V generally donates electrons to a hematite surface, causing nearby Fe to be partially reduced, the Fe and V oxidization state depends very much on the coverage and detailed adsorption configuration. When the V/hematite system is exposed to atomic O, V is further oxidized and surface/near surface Fe is re-oxidized. Our theoretical results are compared with X-ray surface standing wave and X-ray photoelectron spectroscopic measurements. The influence of d-electron correlation on the predicted structures is briefly discussed, making use of the DFT + U scheme.     

Thermodynamic properties of the Lennard-Jones FCC solid: perturbation theory parameterisation and Monte Carlo simulation

This work is concerned with a valid representation of the solid-phase equation of state (EOS), the validity of which is evaluated by comparing to Monte Carlo (MC) simulation results. The proposed EOS has been developed by employing an optimal division of the Lannard-Jones (LJ) potential and an effective temperature- and density-dependent diameter into the framework of the simplified perturbation theory. Then, with the aim of extending to the chain systems, the conventional chain contribution (i.e. TPT1) is added to the proposed model (i.e. the atomic LJ system). Finally, the solid-state EOS based on Helmholtz free energy will be introduced for low temperature and high density conditions. To verify the accuracy of the proposed model, its performance is compared with the results of MC simulation. The comparison between the obtained results from the proposed model and the MC simulations shows that the EOS can satisfactorily predict the properties of the solid LJ system, both for the atomic system and for the chains.     

Computer simulations of critical phenomena and phase behaviour of fluids

Computer simulation techniques such as Monte Carlo (MC) and Molecular Dynamics (MD) methods yield numerically exact information (apart from statistical errors) on model systems of classical statistical mechanics. However, a systematic limitation is the restriction to a finite (and often rather small) particle number N (or box linear dimension L, respectively). This limitation is particularly restrictive near critical points (due to the divergence of the correlation length of the order parameter) and for the study of phase equilibria (possibly involving interfaces, droplets, etc.). Starting out with simple lattice gas (Ising) models, finite size scaling analyses have been developed to overcome this limitation. These techniques work for both simple Lennard-Jones fluids and their mixtures, including generalizations to approximate models for quadrupolar fluids such as carbon dioxide, benzene etc. and various mixtures, whose phase behaviour can be predicted. A combination of MC and MD allows the study of dynamic critical phenomena, and specialised techniques (umbrella sampling plus thermodynamic integration) yield the surface free energy of droplets as function of droplet size. Thus, computer simulation has become a versatile and widely applicable tool for the study of fluids.     

Adsorption of NO and N2O on Fe-BEA and H-BEA zeolites

FT-IR (Fourier-transform infrared) spectroscopy and density function theory (DFT) methods have been applied to the investigation of the interaction of NO and N₂O with Fe³⁺ species in a beta zeolite (BEA). The geometries for H-BEA and Fe-BEA represented as 10T cluster, and NO and N₂O adsorption on them in η¹-O and η¹-N modes have been completely optimized. The results show that NOx could be adsorbed on Fe³⁺ species and Brønsted acid sites in two modes, but NOx is mainly bonded by N to H or Fe atom and the iron site is preferred. NOx adsorbed on Fe³⁺ species is more stable than on Brønsted acid sites. Adsorption energies for N₂O and NO follow the order of NO > N₂O, predicating that the affinity of NO molecule on BEA zeolite is much stronger than N₂O molecule on BEA zeolite.     

Adsorption in one-dimensional channels arranged in a triangular structure: Theory and Monte Carlo simulations

Adsorption thermodynamics of interacting particles adsorbed on one-dimensional channels arranged in a triangular cross-sectional structure is studied through Bragg-Williams approximation (BWA), Monte Carlo (MC) simulations and the recently reported Effective Substates approximation (ESA) [J.L. Riccardo, G. Zgrablich, W. A. Steele, Appl. Surf. Sci. 196 (2002) 138]. Two kinds of lateral interaction energies have been considered: (1) w_L, interaction energy between nearest-neighbor particles adsorbed along a single channel and (2) w_T, interaction energy between particles adsorbed across nearest-neighbor channels. We focus on the case of repulsive transversal interactions (w_T>0), for which a rich variety of ordered phases are observed in the adlayer, depending on the value of the parameters k_BT/w_T (being k_B the Boltzmann constant) and w_L/w_T. Comparisons between analytical data and MC simulations are performed in order to test the validity of the theoretical models. Appreciable differences can be seen for the different approximations, ESA being the most accurate for all cases.     

过渡金属Pd掺杂AsP结构及其吸附甲醛和一氧化碳的性能研究

基于第一性原理方法,研究了单层本征磷砷AsP和过渡金属钯（Pd）掺杂磷砷AsP的结构,并对比研究了本征和掺杂后的AsP吸附甲醛（HCHO）和一氧化碳（CO）气体分子的稳定性、能带结构、态密度以及电荷差分密度。研究结果表明：经Pd掺杂后AsP由半导体转变为导体;本征AsP吸附一氧化碳最稳定的位置为P-As键顶上,吸附甲醛最稳定的位置为P原子顶上;本征吸附时气体分子与基底之间的距离在3 Å左右,气体分子与基底之间未形成化学键。过渡金属Pd原子掺杂AsP后形成两种结构,分别为Pd原子替换超胞结构中的As原子或P原子。两种掺杂结构分别吸附一氧化碳或甲醛气体分子时,除了Pd原子替换AsP中的As原子形成的结构吸附甲醛的吸附能未明显增加外,其余掺杂结构吸附一氧化碳或甲醛的吸附能和电荷转移较本征吸附时均显著增强,吸附CO分子时,C原子与Pd原子之间形成了化学键。特别是,Pd原子替换AsP中的P原子形成的结构对一氧化碳和甲醛气体分子的吸附性能明显强于Pd原子替换AsP中的As原子所形成的结构。     

Adsorption behaviour of SO2 and SOF2 gas on Rh-doped BNNT: a DFT study

Abstract

As the insulating medium, SF₆ is widely used in gas insulation equipment. Partial discharge and local overheating can cause the decomposition of SF₆, resulting in a decrease in insulation strength of the equipment. The detection of SF₆ decomposition gas can be used for on-line insulation detection of gas insulation equipment in electric power industry. In order to develop a new sensor gas sensing material for gas detecting. In this work, based on the first-principles density functional calculation (DFT) method of DMol³, the adsorption of SF₆ decomposition gas on (5,0) Z-type Rh-BNNT in different ways was explored. The adsorption energy, adsorption distance, charge transfer as well as density of states were discussed. The results show that the adsorption strength between SO₂ molecule with Rh-BNNT is larger than with SOF₂ molecule, combined with desorption time, theoretically predicts Rh -BNNT have the potential to be a material for SO₂ gas sensors.     

Modifying a graphene layer by a thymine or a uracil nucleobase: DFT studies

The hybridizations of a graphene layer by a thymine and a uracil nucleobase have been investigated by performing density functional theory (DFT) calculations. The isolated and hybrid structures have been firstly stabilized to reach the minimum energy and the electronic properties have been subsequently evaluated for the optimized structures. The structural and atomic scale parameters indicated that the tip of graphene is important in determining the properties of new hybrids. Moreover, different effects of thymine and uracil nucleobases have been identified in the hybrid structures. Quadrupole coupling constants have been evaluated to characterize the atomic scale properties, in which the most notable effects of hybridizations have been observed for the atoms close to the linking regions whereas negligible effects have been seen for other atoms.     

Dense skyrmion crystal stabilized through interfacial exchange coupling: Role of in-plane anisotropy

A Monte Carlo simulated-annealing algorithm was used to study the magnetic state in an in-plane helimagnet layer on triangular lattice that exchange couples to an underlayer with strong out-of-plane anisotropy. In the single helimagnet layer with in-plane anisotropy (K), the formation of labyrinthlike domains with local spin spirals, instead of parallel stripes, is favored, and these domains rapidly transform into dense skyrmion crystals with increasing interfacial exchange coupling (J′), equivalent to a virtual magnetic field, and finally evolve to an out-of-plane uniform state at large enough J′. Moreover, with increasing K, the skyrmion crystal state can vary from regular 6-nearest-neighboring circular skyrmion arrangement to irregular squeezed skyrmions with less than 6 nearest neighbors when the in-plane anisotropy energy is higher than the interfacial exchange energy as the skyrmion number is maximized. Finally, we demonstrated that the antiferromagnetic underlayer cannot induce skyrmions while the chirality inversion can be achieved on top of an out-of-plane magnetization underlayer with 180◦ domain walls, supporting the experimental findings in FeGe thin film. This compelling advantage offers a fertile playground for exploring emergent phenomena that arise from interfacing magnetic skyrmions with additional functionalities.     

Interpretation of experimental N K NEXAFS of azide, 1,2,3-triazole and terpyridyl groups by DFT spectrum simulations

Experimental N K-edge NEXAFS data of surface immobilized azide, 1,2,3-triazole and terpyridyl groups are interpreted with the help of DFT spectrum simulations. Assignments of π* resonances in experimental N K-edge NEXAFS spectra to nitrogen atoms within these functional groups have been made. The azide was immobilized on gold as the head group of a thiol SAM, 1,2,3-triazole was formed on this SAM by click reaction and terpyridyl groups were introduced as substituents of the acetylene used for the click reaction. For azide-terminated molecules, DFT spectrum simulations are found to be useful to find measurement conditions delivering experimental N K-edge NEXAFS data with negligible X-ray damage. The 1,2,3-triazole group is found to be rather stable under X-ray irradiation.