Electrochemical ammonia production on molybdenum nitride nanoclusters.

Theoretical investigations of electrochemical production of ammonia at ambient temperature and pressure on nitrogen covered molybdenum nanoparticles are presented. Density functional theory calculations are used in combination with the computational hydrogen electrode approach to calculate the free energy profile for electrochemical protonation of N2 and N adatoms on cuboctahedral Mo13 nanoparticles. Pathways for electrochemical ammonia production via direct protonation of N adatoms and N2 admolecules with an onset potential as low as -0.5 V and generally lower than -0.8 V on both a nitrogen covered or a clean Mo nanoparticle. Calculations presented here show that nitrogen dissociation at either nitrogen vacancies on a nitrogen covered molybdenum particle or at a clean molybdenum particle is unlikely to occur under ambient conditions due to very high activation barriers of 1.8 eV. The calculations suggest that the nitrogen will be favored at the surface compared to hydrogen even at potentials of -0.8 V and the Faradaic losses due to HER should be low.

DFT based study of transition metal nano-clusters for electrochemical NH3 production.

Theoretical studies of the possibility of producing ammonia electrochemically at ambient temperature and pressure without direct N2 dissociation are presented. Density functional theory calculations were used in combination with the computational standard hydrogen electrode to calculate the free energy profile for the reduction of N2 admolecules and N adatoms on transition metal nanoclusters in contact with an acidic electrolyte. This work has established linear scaling relations for the dissociative reaction intermediates NH, NH2, and NH3. In addition, linear scaling relations for the associative reaction intermediates N2H, N2H2, and N2H3 have been determined. Furthermore, correlations between the adsorption energies of N, N2, and H have been established. These scaling relations and the free energy corrections are used to establish volcanoes describing the onset potential for electrochemical ammonia production and hence describe the potential determining steps for the electrochemical ammonia production. The competing hydrogen evolution reaction has also been analyzed for comparison.

Communications: Elementary oxygen electrode reactions in the aprotic Li-air battery.

We discuss the electrochemical reactions at the oxygen electrode of an aprotic Li-air battery. Using density functional theory to estimate the free energy of intermediates during the discharge and charge of the battery, we introduce a reaction free energy diagram and identify possible origins of the overpotential for both processes. We also address the question of electron conductivity through the Li(2)O(2) electrode and show that in the presence of Li vacancies Li(2)O(2) becomes a conductor.

Structural stability and decomposition of Mg(BH(4))(2) isomorphs-an ab initio free energy study.

We present the first comprehensive comparison between free energies, based on a phonon dispersion calculation within density functional theory, of theoretically predicted structures and the experimentally proposed α (P6(1)) and ß (Fddd) phases of the promising hydrogen storage material Mg(BH(4))(2). The recently proposed low-density [Formula: see text] ground state is found to be thermodynamically unstable, with soft acoustic phonon modes at the Brillouin zone boundary. We show that such acoustic instabilities can be detected by a macroscopic distortion of the unit cell. Following the atomic displacements of the unstable modes, we have obtained a new F 222 structure, which has a lower energy than all previously experimentally and theoretically proposed phases of Mg(BH(4))(2) and is free of imaginary eigenmodes. A new meta-stable high-density I4(1)/amd structure is also derived from the [Formula: see text] phase. Temperatures for the decomposition are found to be in the range of 400-470 K and largely independent of the structural complexity, as long as the primary cation coordination polyhedra are properly represented. This opens a possibility of using simple model structures for screening and prediction of finite temperature stability and decomposition temperatures of novel borohydride systems.

Hydrogen dynamics in Na3AlH6: a combined density functional theory and quasielastic neutron scattering study.

Understanding the elusive catalytic role of titanium-based additives on the reversible hydrogenation of complex hydrides is an essential step toward developing hydrogen storage materials for the transport sector. Improved bulk diffusion of hydrogen is one of the proposed effects of doping sodium alanate with TiCl3, and here we study hydrogen dynamics in doped and undoped Na3AlH6 using a combination of density functional theory calculations and quasielastic neutron scattering. The hydrogen dynamics is found to be vacancy mediated and dominated by localized jump events, whereas long-range bulk diffusion requires significant activation. The fraction of mobile hydrogen is found to be small for both undoped and doped Na3AlH6, even at 350 K, and improved hydrogen diffusion as a result of bulk-substituted titanium is found to be unlikely. We also propose that previously detected low-temperature point defect motion in sodium alanate could result from vacancy-mediated sodium diffusion.

Calculation of quantum tunneling for a spatially extended defect: the dislocation kink in copper has a low effective mass.

Several experiments indicate that there are atomic tunneling defects in plastically deformed metals. How this is possible has not been clear, given the large mass of the metal atoms. Using a classical molecular-dynamics calculation, we determine the structures, energy barriers, effective masses, and quantum tunneling rates for dislocation kinks and jogs in copper screw dislocations. We find that jogs are unlikely to tunnel, but the kinks should have large quantum fluctuations. The kink motion involves hundreds of atoms each shifting a tiny amount, leading to a small effective mass and tunneling barrier.

Determination of the of rate cross slip of screw dislocations

The rate for cross slip of screw dislocations during annihilation of screw dipoles in copper is determined by molecular dynamics simulations. The temperature dependence of the rate is seen to obey an Arrhenius behavior in the investigated temperature range: 225-375 K. The activation energy and the effective attempt frequency can therefore be extracted from the simulations. The transition state energy for the annihilation process is calculated by identifying the transition state using the nudged elastic band path technique. The two activation energies agree very well, indicating that transition state theory is applicable for this type of process.

Computer simulations of the mechanical properties of metals.

Atomic-scale computer simulations can be used to gain a better understanding of the mechanical properties of materials. In this paper we demonstrate how this can be done in the case of nanocrystalline copper, and give a brief overview of how simulations may be extended to larger length scales. Nanocrystline metals are metals with grain sizes in the nanometre range, they have a number of technologically interesting properties such as much increased hardness and yield strength. Our simulations show that the deformation mechanisms are different in these materials than in coarse-grained materials. The main deformation is occurring in the grain boundaries, and only little dislocation activity is seen inside the grains. This leads to a hardening of the material as the grain size is increased, and the volume fraction of grain boundaries is decreased.

Immunoglobulins in granular corneal dystrophy Groenouw type I.

Three patients with granular corneal dystrophy Groenouw type I underwent corneal grafting, and cryostat sections of the corneal buttons were examined immunohistochemically for immunoglobulins. Positive results were obtained for IgG, Kappa-, and Lambda chains with immunofluorescence technique. The reactions were seen exclusively in the same localizations as the Masson trichrome positive deposits.

DNA synthesis in human retinal pigment epithelium in organ culture.

Retinal pigment epithelium from human eyes was maintained in organ culture, and DNA synthesis was visualized by autoradiography of cultures exposed to labelled thymidine. In Ham's F10 medium, labelling of cells was observed after 7 as well as after 14 days in vitro. At both stages the labelled cells were characteristically located adjacent to areas of epithelial damage. Cells remote from such areas did not incorporate thymidine, This pattern was observed in epithelia removed from the posterior pole as well as from the periphery of the fundus. Several types of routine media and media with human as well as foetal bovine serum (FBS) were found to support DNA synthesis in the cultured epithelium, labelled cells were not observed in epithelium maintained in Ham's F10 without serum, or with low serum concentration. Our study demonstrates, that DNA synthesis does take place in organ cultured human retinal pigment epithelium when the appropriate culture conditions are used. Our findings indicate that induction of DNA synthesis in the cells is not promoted by an indiscriminate effect of the medium but related to the presence of epithelial damage.

The effects of horseradish peroxidase on the concentration of 5-hydroxytryptamine and platelet counts in blood of rabbits and guinea pigs.

When HRP was injected i.v. in rabbits and guinea pigs, a rapid fall in the number of circulating platelets and the concentration of 5 HT occurred. Platelet aggregates were trapped in lung capillaries. The aggregation was reversible and the return of platelets to the circulation concomitant with a rise in blood 5 HT concentration.

Vascular reactions to horseradish peroxidase in the guinea pig.

Albino guinea pigs were given intradermal injections of the protein tracer horseradish peroxidase. In a 0.1 mM concentration the tracer did not increase vascular permeability to Evans blue-labelled plasma proteins. In a 1 mM concentration, however, the peroxidase induced a local vascular leakage. This leakage was almost totally inhibited by pretreating the animals with acetylsalicylic acid, while antihistamine had only a weak inhibitory effect. We therefore believe that prostaglandins are important mediators in this HRP-induced vascular reaction.

Ultrastructure and distribution of intercellular junctions in corneal endothelium.

The intercellular junctions of the corneal endothelium has been studied in rabbit, monkey and human eyes. A union of apposing outer leaflets (tight junction) is usually found near the apical end of the intercellular clefts. However, evidence from lanthanum tracer studies indicates that the tight junctions do not seal the intercellular clefts completely. In addition to tight junctions, the presence of gap junctions or nexuses is demonstrated in all three species studied.

Differential reactivity of rabbit iris and ciliary process to topically applied prostaglandin E2 (dinoprostone).

The topical application of prostaglandin E2 (dinoprostone) is followed by massive swelling of the ciliary process, leading to substantial leakage of marker dye into the posterior chamber and directly into the region of the iris, where the primary ciliary processes insert. In contrast, blood vessels of the iris proper retain most of their normal barrier quality and do not leak Evans blue dye. The adrenergic innervation of the anterior segment remains normal in density and in quality after the topical application of PGE.

Adenosine 3',5'-monophosphate increases the outflow of aqueous humor from the rabbit eye.

Direct administration of cyclic-AMP into the anterior chamber increases the outflow facility of the eye for aqueous humor. This is consistent with the hypothesis that catecholamines lower the intraocular pressure of the rabbit eye, at least in part, by a cyclic-AMP-mediated mechanism. This mechanism is active in the outflow channels and increases the rate at which aqueous humor leaves the anterior chamber.

Morphology of the breakdown of the blood-aqueous barrier in the ciliary processes of the rabbit eye after prostaglandin E2.

Protaglandin E2, administered topically to the rabbit eye, causes disruption of the blood-aqueous barrier resulting in a large increase in the protein content of the aqueous humor. The route of plasma proteins into the aqueous humor was studied with the electron microscope, using horseradish peroxidase as a protein tracer. The tracer penetrated the tight junctions of the nonpigmented layer of the ciliary epithelium, filling the lateral intercellular clefts and staining the internal limiting membrane. These morphological studies confirm the prior physiologic demonstration that, in response to prostaglandin, plasma proteins enter the posterior chamber via the intercellular clefts of the nonpigmented epithelium.