Improved H_2 Storage in Zeolitic Imidazolate Frameworks Using Li^+, Na^+, and K^+ Dopants, with an Emphasis on Delivery H_2 Uptake

We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li^+, Na^+, and K^+)-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H_2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H_2, Na-ZIF-70 to 2.19 wt % H_2, and K-ZIF-70 to 1.62 wt % H_2, much higher than 0.74 wt % H_2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H_2 binding energies to the dopants. Moreover, the total H_2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H_2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H_2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H_2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H_2 binding energy needs not always lead to higher delivery H_2 uptake

Zeolitic Imidazolate Frameworks as H_2 Adsorbents: Ab Initio Based Grand Canonical Monte Carlo Simulation

We report the H_2 uptake behavior of 10 zeolitic−imidazolate frameworks (ZIFs), based on grand canonical Monte Carlo (GCMC) simulations. The force fields (FFs) describing the interactions between H_2 and ZIF in the GCMC were based on ab initio quantum mechanical (QM) calculations (MP2) aimed at correctly describing London dispersion (van der Waals attraction). Thus these predictions of H_2 uptake are based on first principles (non empirical) and hence applicable to new framework materials for which there is no empirical data. For each of these 10 ZIFs we report the total and excess H_2 adsorption isotherms up to 100 bar at both 77 and 300 K. We report the hydrogen adsorption sites in the ZIFs and the relationships between H_2 uptake amount, isosteric heat of adsorption (Q_(st)), surface area, and free volume. Our simulation shows that various ZIFs lead to a variety of H_2 adsorption behaviors in contrast to the metal−organic frameworks (MOFs). This is because ZIFs leads to greater diversity in the adsorption sites (depending on both organic linkers and zeolite topologies) than in MOFs. In particular, the ZIFs uptake larger amounts of H_2 at low pressure because of the high H_2 adsorption energy, and ZIFs have a variety of H_2 adsorption sites. For example, ZIF-11 has an initial Q_(st) value of ~15 kJ/mol, which is higher than observed for MOFs. Moreover, the preferential H_2 adsorption site in ZIFs is onto the organic linker, not nearby the metallic joint as is the case for MOFs

Negative tunneling magneto-resistance in quantum wires with strong spin-orbit coupling

© 2015 IOP Publishing Ltd. We consider a two-dimensional magnetic tunnel junction of the FM/I/QW(FM+SO)/I/N structure, where FM, I and QW(FM+SO) stand for a ferromagnet, an insulator and a quantum wire with both magnetic ordering and Rashba spin-orbit (SOC), respectively. The tunneling magneto-resistance (TMR) exhibits strong anisotropy and switches sign as the polarization direction varies relative to the quantum-wire axis, due to interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the quantum wire.This work was supported by the BK21 Plus Project from the Korean Government and by MINECO (Spain) Grant FIS2011-23526.Peer Reviewe

Overexpression of phospholipase D enhances Bcl-2 expression by activating STAT3 through independent activation of ERK and p38MAPK in HeLa cells

AbstractThe purpose of this study was to identify the role of phospholipase D (PLD) isozymes in Bcl-2 expression. Overexpression of PLD1 or PLD2 increased Bcl-2 expression and phosphatidic acid (PA), the product of PLDs, also upregulated Bcl-2 expression. Treatment with PA activated the phospholipase A2 (PLA2)/Gi/ERK1/2, RhoA/Rho-associated kinase (ROCK)/p38 MAPK, and Rac1/p38 MAPK pathways. PA-induced phosphorylation of ERK1/2 was attenuated by a PLA2 inhibitor (mepacrine) and, a Gi protein inhibitor (pertussis toxin, PTX). On the other hand, p38 MAPK phosphorylation was attenuated by a dominant negative Rac1 and a specific Rho-kinase inhibitor (Y-27632). These results suggest that PLA2/Gi acts at the upstream of ERK1/2, while Rac1 and RhoA/ROCK act upstream of p38 MAPK. We next, tried to determine which transcription factor is involved in PLD-related Bcl-2 expression. When signal transducer and activator of transcription 3 (STAT3) activity was blocked by a STAT3 specific siRNA, PA-induced Bcl-2 expression was remarkably decreased, suggesting that STAT3 is an essential transcription factor linking PLD to Bcl-2 upregulation. Taken together, these findings indicate that PLD acts as an important regulator in Bcl-2 expression by activating STAT3 involving the phosphorylation of Ser727 through the PLA2/Gi/ERK1/2, RhoA/ROCK/p38 MAPK, and Rac1/p38 MAPK pathways

Total Reflection and Negative Refraction of Dipole-Exchange Spin Waves at Magnetic Interfaces: Micromagnetic Modeling Study

We demonstrated that dipole-exchange spin waves traveling in geometrically restricted magnetic thin films satisfy the same laws of reflection and refraction as light waves. Moreover, we found for the first time novel wave behaviors of dipole-exchange spin waves such as total reflection and negative refraction. The total reflection in laterally inhomogeneous thin films composed of two different magnetic materials is associated with the forbidden modes of refracted dipole-exchange spin waves. The negative refraction occurs at a 90 degree domain-wall magnetic interface that is introduced by a cubic magnetic anisotropy in the media, through the anisotropic dispersion of dipole-exchange spin waves.Comment: 13 pages, 5 figure