Adsorption and purification of biogas inside graphitic nanopores: molecular dynamics simulation approach.

Biogas is one of the most common sources of biomass energy. Due to the associated environmental pollution and costs, desulfurization, and purification are the most important challenges of biogas power generation. Using all-atom molecular dynamics (MD), we systematically simulated the isothermal adsorption behavior of biogas (comprising CH4, CO2, H2O, H2S, and H2) in graphite (Gr) slit nanopores. The impact of slit width, system temperature, and moisture content on the adsorption energy, adsorption ratio, and diffusion coefficient of biogas molecules was investigated. Simulation results revealed that due to strong interactions between graphite and H2S, graphite slits of width d = 48 ~ 80 Å displayed significant selective adsorption of H2S molecules. At temperatures between 300 and 500 K, Gr slits can effectively separate H2S in biogas. Moreover, as the moisture content of biogas (vol%) increases from 0 to 20%, the formation and interactions of hydrogen bonds between water molecules create H2O films accumulating on the Gr surface and taking up the adsorption sites, which reduces the amount of hydrogen sulfide that can be adsorbed. Our findings provide important insights into the material design for biogas purification. A schematic representation of molecular interactions between adsorbates and the wall for biogas mixtures (comprising CH4, CO2, H2O, H2S, and H2) inside graphitic nanopores.

Capture of acidic gas molecules in metallic nanopillar array surfaces.

To effectively capture the acidic fluid molecules in industrial exhaust, this study employed molecular dynamics to simulate the dynamic adsorption behavior of a mixture of carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), and water (H2O) molecules in gold (Au) nanoslits. We systematically examined the self-diffusion coefficient (DZ), average adsorption energy (Ea,av), and static adsorption amount (Nsa) of individual ingredients and a mixture of the adsorbates under various temperatures (T), concentrations (c), and array slit widths (d). The simulation results indicate that Au(110) has better capture capabilities with regard to H2O and H2S, followed by CO2 and then finally CO. Among the various slit structures, the design of array structures with slit widths 8.15 × 5.76 Å (case C) resulted in the highest average adsorption energy and static adsorption amount for all of the adsorbates. This is due to the fact that an appropriate slit width can increase the self-diffusion coefficient of the gas molecule and provide more stable adsorption sites to capture the adsorbates. Compared to the smooth surface structure, the nanopillar array structures significantly increased the self-diffusion coefficients and the adsorption energy of specific molecules. The comprehensive molecular model is helpful to predict atomistic-level adsorption behaviors for acidic gas molecules.

Molecular dynamics investigation of separation of hydrogen sulfide from acidic gas mixtures inside metal-doped graphite micropores.

The separation of poisonous compounds from various process fluids has long been highly intractable, motivating the present study on the dynamic separation of H2S in acidic-gas-mixture-filled micropores. The molecular dynamics approach, coupled with the isothermal-isochoric ensemble, was used to model the molecular interactions and adsorption of H2S/CO2/CO/H2O mixtures inside metal-doped graphite slits. Due to the difference in the adsorption characteristics between the two distinct adsorbent materials, the metal dopant in the graphitic micropores leads to competitive adsorption, i.e. the Au and graphite walls compete to capture free adsorbates. The effects of competitive adsorption, coupled with changes in the gas temperature, concentration, constituent ratio and slit width on the constituent separation of mixtures were systematically studied. The molecule-wall binding energies calculated in this work (those of H2S, H2O and CO on Au walls and those of H2O, CO and CO2 on graphite walls) show good agreement with those obtained using density functional theory (DFT) and experimental results. The z-directional self-diffusivities (Dz) for adsorbates inside the slit ranged from 10(-9) to 10(-7) m(2) s(-1) as the temperature was increased from 10 to 500 K. The values are comparable with those for a typical microporous fluid (10(-8)-10(-9) m(2) s(-1) in a condensed phase and 10(-6)-10(-7) m(2) s(-1) in the gaseous state). The formation of H-bonding networks and hydrates of H2S is disadvantageous for the separation of mixtures. The results indicate that H2S can be efficiently separated from acidic gas mixtures onto the Au(111) surface by (i) reducing the mole fraction of H2S and H2O in the mixtures, (ii) raising the gas temperature to the high temperature limit (≥400 K), and (iii) lowering the slit width to below the threshold dimension (≤23.26 Å).

Effects of carbonization parameters of Moso-bamboo-based porous charcoal on capturing carbon dioxide.

This study experimentally analyzed the carbon dioxide adsorption capacity of Moso-bamboo- (Phyllostachys edulis-) based porous charcoal. The porous charcoal was prepared at various carbonization temperatures and ground into powders with 60, 100, and 170 meshes, respectively. In order to understand the adsorption characteristics of porous charcoal, its fundamental properties, namely, charcoal yield, ash content, pH value, Brunauer-Emmett-Teller (BET) surface area, iodine number, pore volume, and powder size, were analyzed. The results show that when the carbonization temperature was increased, the charcoal yield decreased and the pH value increased. Moreover, the bamboo carbonized at a temperature of 1000(°)C for 2 h had the highest iodine sorption value and BET surface area. In the experiments, charcoal powders prepared at various carbonization temperatures were used to adsorb 1.854% CO2 for 120 h. The results show that the bamboo charcoal carbonized at 1000(°)C and ground with a 170 mesh had the best adsorpt on capacity, significantly decreasing the CO2 concentration to 0.836%. At room temperature and atmospheric pressure, the Moso-bamboo-based porous charcoal exhibited much better CO2 adsorption capacity compared to that of commercially available 350-mesh activated carbon.

Molecular dynamics investigations of liquid-vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores.

Formaldehyde exposure has been associated with several human cancers, including leukemia and nasopharyngeal carcinoma, motivating the present investigation on the microscopic adsorption behaviors of formaldehyde in multi-component-mixture-filled micropores. Molecular dynamics (MD) simulation was used to investigate the liquid-vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores. The effects of the slit width, system temperature, concentration, and the constituent ratio of the mixture on the diffusion and adsorption properties are studied. As a result of interactions between the components, the z-directional self-diffusivity (D(z)) in the mixture substantially decreased by about one order of magnitude as compared with that of pure (single-constituent) adsorbates. When the concentration exceeds a certain threshold, the D(z) values dramatically decrease due to over-saturation inducing barriers to diffusion. The binding energy between the adsorbate and graphite at the first adsorption monolayer is calculated to be 3.99, 2.01, 3.49, and 2.67 kcal mol(-1) for CO2, CO, CH2O, and H2O, respectively. These values agree well with those calculated using the density functional theory coupled cluster method and experimental results. A low solubility of CO2 in water and water preferring to react with CH2O, forming hydrated methanediol clusters, are observed. Because the cohesion in a hydrated methanediol cluster is much higher than the adhesion between clusters and the graphitic surface, the hydrated methanediol clusters were hydrophobic, exhibiting a large contact angle on graphite.

Effects of vacancy cluster defects on electrical and thermodynamic properties of silicon crystals.

A first-principle plane-wave pseudopotential method based on the density function theory (DFT) was employed to investigate the effects of vacancy cluster (VC) defects on the band structure and thermoelectric properties of silicon (Si) crystals. Simulation results showed that various VC defects changed the energy band and localized electron density distribution of Si crystals and caused the band gap to decrease with increasing VC size. The results can be ascribed to the formation of a defect level produced by the dangling bonds, floating bonds, or high-strain atoms surrounding the VC defects. The appearance of imaginary frequencies in the phonon spectrum of defective Si crystals indicates that the defect-region structure is dynamically unstable and demonstrates phase changes. The phonon dispersion relation and phonon density of state were also investigated using density functional perturbation theory. The obtained Debye temperature ( θ D ) for a perfect Si crystal had a minimum value of 448 K at T = 42 K and a maximum value of 671 K at the high-temperature limit, which is consistent with the experimental results reported by Flubacher. Moreover, the Debye temperature decreased with increases in the VC size. VC defects had minimal effects on the heat capacity (C v ) value when temperatures were below 150 K. As the temperature was higher than 150 K, the heat capacity gradually increased with increasing temperature until it achieved a constant value of 11.8 cal/cell · K. The heat capacity significantly decreased as the VC size increased. For a 2 × 2 × 2 superlattice Si crystal containing a hexagonal ring VC (HRVC10), the heat capacity decreased by approximately 17%.

Pressure-induced solid-state lattice mending of nanopores by pulse laser annealing.

This paper presents the pressure-induced solid-state lattice mending of nanopores in single-crystal copper by femtosecond laser annealing processes. The microscopic mechanism of lattice mending is investigated by a modified continuum-atomistic modeling approach. Three typical lattice mending phases, including (i) the incubation of dislocation nucleation, (ii) plastic deformation under the combined effect of pressure and atomic thermal diffusion, and (iii) lattice recovery and reconstruction, are characterized via the microscopic structure changes and transient thermodynamic trajectories. The simulation results reveal that the structural mending of a pore is originated in heterogeneous nucleation of dislocations from the pore surface. The shear-induced multiple lattice glides are found to significantly contribute to the mending of a nanopore in the process of solid-state structural mending. The mending rates of two different modes, the pressure-induced and the classical unsteady-state atomic diffusion, are estimated and found to be very different from each other, by an order of 10(4). In addition, the location of the pore is also found to significantly influence the annealing threshold. Since the largest amplitude of the pressure wave is built up at a characteristic depth of approximately 45 nm below the irradiated surface, the shock wave will directly impinge on the pore and induce a fast solid-state lattice mending if the pore is located within the lower limit of the range of the characteristic depth. Furthermore, it is also interesting to note that the mending of a nanopore close to the characteristic depth by annealing fluence is generally lower than that of a pore near the surface. These results provide vital insights of photomechanical interactions with the microstructure of metallic solid, and the proposed approach can be further considered and enhanced to predict the mending depth for various defects in the future.

Mice deficient in collapsin response mediator protein-1 exhibit impaired long-term potentiation and impaired spatial learning and memory.

Collapsing response mediator protein-1 (CRMP-1) was initially identified in brain and has been implicated in plexin-dependent neuronal function. The high amino acid sequence identity among the five CRMPs has hindered determination of the functions of each individual CRMP. We generated viable and fertile CRMP-1 knock-out (CRMP-1(-/-)) mice with no evidence of gross abnormality in the major organs. CRMP-1(-/-) mice exhibited intense microtubule-associated protein 2 (MAP2) staining in the proximal portion of the dendrites, but reduced and disorganized MAP2 staining in the distal dendrites of hippocampal CA1 pyramidal cells. Immunoreactivity to GAP-43 (growth-associated protein-43) and PSD95 (postsynaptic density-95) (a postsynaptic membrane adherent cytoskeletal protein) was also decreased in the CA1 region of the knock-out mice. These changes were consistent with the mutant mice showing a reduction in long-term potentiation (LTP) in the CA1 region and impaired performance in hippocampal-dependent spatial learning and memory tests. CRMP-1(-/-) mice showed a normal synapsin I labeling pattern in CA1 and normal paired-pulse facilitation. These findings provide the first evidence suggesting that CRMP-1 may be involved in proper neurite outgrowth in the adult hippocampus and that loss of CRMP-1 may affect LTP maintenance and spatial learning and memory.

TXAS-deleted mice exhibit normal thrombopoiesis, defective hemostasis, and resistance to arachidonate-induced death.

Besides its well-recognized role in hemostasis and thrombosis, thromboxane A(2) synthase (TXAS) is proposed to be involved in thrombopoiesis and lymphocyte differentiation. To evaluate its various physiologic roles, we generated TXAS-deleted mice by gene targeting. TXAS(-/-) mice had normal bone marrow megakaryocytes, normal blood platelet counts, and normal CD4 and CD8 lymphocyte counts in thymus and spleen. Platelets from TXAS(-/-) mice failed to aggregate or generate thromboxane B(2) in response to arachidonic acid (AA) but produced increased prostaglandin-E(2) (PGE(2)), PGD(2), and PGF(2 alpha). AA infusion caused a progressive drop of mean arterial pressure (MAP), cardiac arrest, and death in wild-type (WT) mice but did not induce shock in TXAS(-/-) mice or in WT and TXAS(-/-) mice treated with antagonist to the thromboxane-prostanoid (TP) receptor. The TXAS(-/-) mice were able to maintain normal MAP upon AA insult when TP was present but were unable to do so when TP was blocked by an antagonist, suggesting a role of endoperoxide accumulation in influencing MAP. We conclude that TXAS is not essential for thrombopoiesis and lymphocyte differentiation. Its deficiency causes a mild hemostatic defect and protects mice against arachidonate-induced shock and death. The TXAS-deleted mice will be valuable for investigating the roles of arachidonate metabolic shunt in various pathophysiologic processes.