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Molecular Hydrogen (molecular + hydrogen)
Selected AbstractsBooks and Multimedia ReviewMETEORITICS & PLANETARY SCIENCE, Issue 10 2001Article first published online: 4 FEB 2010 Book reviewed in this article: The Oxford Companion to the Earth edited by Paul L. Hancock, Brian J. Skinner and David L. Dineley. Molecular Hydrogen in Space edited by F. Combes and G. Pineau des Forets. The Cambridge Handbook of Physics Formulas by G. Woan. From Mountains to Meteorites by Brian Mason and Simon Nathan. [source] DFT/CCSD(T) Investigation of the Interaction of Molecular Hydrogen with Carbon NanostructuresCHEMPHYSCHEM, Issue 11 2009Miroslav Rube Abstract The interaction of molecular hydrogen with carbon nanostructures is investigated within the DFT/CC correction scheme. The DFT/CC results are compared with the benchmark calculations at the CCSD(T) level of theory for benzene and naphthalene, and at the MP2 level for the more extended systems. The DFT/CC method offers a reliable alternative to the highly correlated ab initio calculations at a cost comparable to the standard DFT method. The results for H2 adsorbed on graphene as well as single-wall carbon nanotubes (SWCNT) are presented. The DFT/CC binding energy on graphene of 5.4 kJ,mol,1 is in good agreement with experiment (5.00±0.05 kJ,mol,1). For (10,10)-SWCNT, the H2 molecule is mostly stabilized inside the tube with an estimated binding energy of 7.2 kJ,mol,1. [source] A Simple Pore Water Hydrogen Diffusion Syringe SamplerGROUND WATER, Issue 6 2007Don A. Vroblesky Molecular hydrogen (H2) is an important intermediate product and electron donor in microbial metabolism. Concentrations of dissolved H2 are often diagnostic of the predominant terminal electron-accepting processes in ground water systems or aquatic sediments. H2 concentrations are routinely measured in ground water monitoring wells but are rarely measured in saturated aquatic sediments due to a lack of simple and practical sampling methods. This report describes the design and development (including laboratory and field testing) of a simple, syringe-based H2 sampler in (1) saturated, riparian sediments, (2) surface water bed sediments, and (3) packed intervals of a fractured bedrock borehole that are inaccessible by standard pumped methods. [source] Reactivity Pattern in the Room-Temperature Activation of NH3 by the Main-Group Atomic Ions Ga+, Ge+, As+ and Se+EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 10 2010Gregory K. Koyanagi Abstract The activation of ammonia by the main-group cations Ga+, Ge+, As+ and Se+ has been explored both experimentally and theoretically. ICP/SIFT tandem mass spectrometer measurements of room-temperature kinetics have revealed a substantial variation in rates and product distributions across the Periodic Table of Elements. The main features of the observed primary chemistry include H-atom elimination, ammonia addition and a cation-assisted proton transfer to yield NH4+ that is second order in ammonia. These observations are shown to be completely consistent with computed potential energy surfaces for the reactions of each of the four atomic cations. Dehydrogenation by the elimination of molecular hydrogen, not observed experimentally, is shown by the calculations to be inhibited by the presence of a kinetic barrier. [source] A Density Functional Study of the Hydrogenation of Ketones Catalysed by Neutral Rhodium-Diphosphane ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 21 2006Francine Agbossou-Niedercorn Abstract The potential energy profile of RhI -catalysed hydrogenation of ketones has been computed for the simple model system [Rh{H3POCH2CH2N(H)PH3}(Cl)] using DFT calculations. The general sequence of the catalytic cycle involves coordination of the carbonyl derivative to the neutral RhI complex followed by oxidative addition of molecular hydrogen providing rhodium dihydride intermediates. The latter are converted into alkoxy hydrides by a migratory insertion reaction. Reductive elimination of the alcohol and substitution of the latter by the incoming substrate completes the catalyticcycle. Intermediates and transition states of all catalyticsteps have been located. Two isomeric derivatives bearingthe model substrate have been found for the [Rh{H3POCH2CH2N(H)PH3}(Cl)(H2CO)] complex. Eight diastereomeric pathways have been followed for the cis addition of molecular hydrogen to [Rh{H3POCH2CH2N(H)PH3}(Cl)(H2CO)] leading to eight distinct isomeric dihydride intermediates. Four dihydride complexes can be considered as the more accessible compounds. The site preference for migratory insertion and transition states discriminates the main path of the catalytic reaction. Migratory insertion to form the alkoxy hydride constitute the turn over limiting step of the process. The potential energy profile has been found to be smooth without excessive activation barriers. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] [FeFe]-hydrogenase-like gene is involved in the regulation of sensitivity to oxygen in yeast and nematodeGENES TO CELLS, Issue 4 2009Michihiko Fujii Oxygen is essential for the life of aerobic organisms, but reactive oxygen species (ROS) derived from oxygen can be a threat for it. Many genes are involved in generation of ROS, but not much attention has been focused on the reactions from which ROS are generated. We therefore screened for mutants that showed an increased sensitivity to oxidative stress in the nematode Caenorhabditis elegans, and isolated a novel mutant, oxy-4(qa5001). This mutant showed an increased sensitivity to a high concentration of oxygen, and decreased longevity at 20 °C but not at 26 °C. The genetic analysis has revealed that oxy-4 had a causative mutation in an [FeFe]-hydrogenase-like gene (Y54H5A.4). In the yeast Saccharomyces cerevisiae, a deletion of NAR1, a possible homologue of oxy-4, also caused a similar increased sensitivity to oxygen. [FeFe]-hydrogenases are enzymes that catalyze both the formation and the splitting of molecular hydrogen, and function in anaerobic respiration in anaerobes. In contrast, [FeFe]-hydrogenase-like genes identified in aerobic eukaryotes do not generate hydrogen, and its functional roles are less understood. Our results suggested that [FeFe]-hydrogenase-like genes were involved in the regulation of sensitivity to oxygen in S. cerevisiae and C. elegans. [source] The formation of molecular clouds in spiral galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006C. L. Dobbs ABSTRACT We present smoothed particle hydrodynamics simulations of molecular cloud formation in spiral galaxies. These simulations model the response of a non-self-gravitating gaseous disc to a galactic potential. The spiral shock induces high densities in the gas, and considerable structure in the spiral arms, which we identify as molecular clouds. We regard the formation of these structures as due to the dynamics of clumpy shocks, which perturb the flow of gas through the spiral arms. In addition, the spiral shocks induce a large velocity dispersion in the spiral arms, comparable with the magnitude of the velocity dispersion observed in molecular clouds. We estimate the formation of molecular hydrogen, by post-processing our results and assuming the gas is isothermal. Provided the gas is cold (T, 100 K), the gas is compressed sufficiently in the spiral shock for molecular hydrogen formation to occur in the dense spiral arm clumps. These molecular clouds are largely confined to the spiral arms, since most molecular gas is photodissociated to atomic hydrogen upon leaving the arms. [source] H i kinematics in a massive spiral galaxy at z= 0.89MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2005L. V. E. Koopmans ABSTRACT We present a kinematic model of the neutral hydrogen in the spiral galaxy of the lens system PKS 1830,211, based on a Multi-Element Radio-Linked Interferometer Network (MERLIN) 1.4-GHz radio map and the integrated and redshifted 21-cm hydrogen absorption-line profile as measured with the Westerbork Synthesis Radio Telescope (WSRT). Degeneracies in the models do not allow a unique determination of the kinematic centre, and forthcoming deeper Hubble Space Telescope observations with the Advanced Camera for Surveys (ACS) are required to break this degeneracy. Even so, we measure the inclination of the hydrogen disc: i= 17°,32°, indicating a close to face-on spiral galaxy. The optical depth increases with radius over the extent of the Einstein ring, suggesting H i depletion towards the lens centre. The latter could be due to star formation or conversion of H i into molecular hydrogen because of a higher metalicity/dust content in the galaxy centre. The neutral hydrogen optical depth gives NH I= 2 × 1021 cm,2 at r= 5.0 h,170 kpc in the disc (Ts= 100 K), comparable to local spiral galaxies. [source] Light-driven Hydrogen Production by a Hybrid Complex of a [NiFe]-Hydrogenase and the Cyanobacterial Photosystem IPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 3 2006Masaki Ihara ABSTRACT In order to generate renewable and clean fuels, increasing efforts are focused on the exploitation of photosynthetic microorganisms for the production of molecular hydrogen from water and light. In this study we engineered a ,hard-wired' protein complex consisting of a hydrogenase and photosystem I (hydrogenase-PSI complex) as a direct light-to-hydrogen conversion system. The key component was an artificial fusion protein composed of the membrane-bound [NiFe] hydrogenase from the ,-proteobacterium Ralstonia eutropha H16 and the peripheral PSI subunit PsaE of the cyanobacterium Thermosy-nechococcus elongatus. The resulting hydrogenase-PsaE fusion protein associated with PsaE-free PSI spontaneously, thereby forming a hydrogenase-PSI complex as confirmed by sucrosegradient ultracentrifuge and immunoblot analysis. The hydrogenase-PSI complex displayed light-driven hydrogen production at a rate of 0.58 ,mol H2· mg chlorophyll,1· h,1. The complex maintained its accessibility to the native electron acceptor ferredoxin. This study provides the first example of a light-driven enzymatic reaction by an artificial complex between a redox enzyme and photosystem I and represents an important step on the way to design a photosynthetic organism that efficiently converts solar energy and water into hydrogen. [source] Effect of palladium addition on the electrochemical properties of amorphous 2Mg + 3d alloys doped by nickel atoms (3d = Fe, Ni)PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 5 2010Maciej Jarzebski Abstract Amorphous 2Mg+ 3d/x wt% Ni materials were prepared by mechanical alloying (MA) of Mg and 3d elemental powders (3d = Fe, Ni x =0, 100 and 200 wt.%) under high purity argon atmosphere in SPEX 8000 Mixer Mill. The effect of the Ni addition on the electrochemical properties of the synthesized nanostructured alloys was investigated in details. The discharge capacity of amorphous 2Mg+Fe (Ni) high energy ball milled with Ni was improved. With increasing nickel content in the studied 2Mg+Fe (Ni) materials, at first cycle, the discharge capacity increases first and then decreases, and for example, for x = 100 in 2Mg+Fe reaches a maximum value of 155 mAh g,1. Additionally, when coated with palladium, the discharge capacity of an amorphous 2Mg+Fe (Ni) powders was increased as well. The catalytic elements (Ni, Pd) were distributed on the surface of ball milled alloy particles homogenously and role of these particles is to catalyze the dissociation of molecular hydrogen on the surface of studied alloy. Mechanical coating with palladium effectively reduced the degradation rate of the studied electrode materials. Compared to that of the uncoated powders, the degradation of the coated was suppressed. The amorphous Mg-based hydrides offer a breakthrough in prospects for practical applications (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Plasma-Enhanced Atomic Layer Deposition of Palladium on a Polymer Substrate,CHEMICAL VAPOR DEPOSITION, Issue 6-7 2007A. Ten, Eyck Abstract In this paper, a method for the plasma-enhanced (PE) atomic layer deposition (ALD) of palladium on air-exposed, annealed poly(p -xylylene) (Parylene-N, or PPX) is presented. Palladium is successfully deposited on PPX at 80,°C using a remote, inductively coupled, hydrogen/nitrogen plasma with palladium (II) hexafluoroacetylacetonate (PdII(hfac)2) as the precursor. By optimizing the mixture of hydrogen and nitrogen, the polymer surface is modified to introduce active sites allowing the chemisorption of the PdII(hfac)2. In addition, enough free hydrogen atoms are available at the surface for ligand removal and Pd reduction, while at the same time, enough hydrogen atoms are consumed in the plasma to ensure there is no visible degradation of the PPX. X-ray photoelectron spectroscopy (XPS) measurements of the substrate after hydrogen/nitrogen plasma treatment at 50,W clearly show the presence of nitrogen bound to the substrate surface. XPS measurements of the deposited Pd films indicate good quality for both substrates, suggesting that the substrate temperature was low enough to prevent dissociation of the hfac ligand and adequate scavenging of the hfac ligand by the available atomic hydrogen. The remote hydrogen/nitrogen plasma enables Pd film deposition on polymer surfaces, which do not typically react with the Pd precursor, and are not catalysts for the dissociation of molecular hydrogen. [source] Mapping the Transformation [{RuII(CO)3Cl2}2],[RuI2(CO)4]2+: Implications in Binuclear Water,Gas Shift ChemistryCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2010Moumita Majumdar Abstract The complete sequence of reactions in the base-promoted reduction of [{RuII(CO)3Cl2}2] to [RuI2(CO)4]2+ has been unraveled. Several ,-OH, ,:,2 -CO2H-bridged diruthenium(II) complexes have been synthesized; they are the direct results of the nucleophilic activation of metal-coordinated carbonyls by hydroxides. The isolated compounds are [Ru2(CO)4(,:,2 - C,O -CO2H)2(,-OH)(NPF -Am)2][PF6] (1; NPF -Am=2-amino-5,7-trifluoromethyl-1,8-naphthyridine) and [Ru2(CO)4(,:,2 - C,O -CO2H)(,-OH)(NP-Me2)2][BF4]2 (2), secured by the applications of naphthyridine derivatives. In the absence of any capping ligand, a tetranuclear complex [Ru4(CO)8(H2O)2(,3 -OH)2(,:,2 - C,O -CO2H)4][CF3SO3]2 (3) is isolated. The bridging hydroxido ligand in 1 is readily replaced by a ,-donor chlorido ligand, which results in [Ru2(CO)4(,:,2 - C,O -CO2H)2(,-Cl)(NP-PhOMe)2][BF4] (4). The production of [Ru2(CO)4]2+ has been attributed to the thermally induced decarboxylation of a bis(hydroxycarbonyl),diruthenium(II) complex to a dihydrido,diruthenium(II) species, followed by dinuclear reductive elimination of molecular hydrogen with the concomitant formation of the RuIRuI single bond. This work was originally instituted to find a reliable synthetic protocol for the [Ru2(CO)4(CH3CN)6]2+ precursor. It is herein prescribed that at least four equivalents of base, complete removal of chlorido ligands by TlI salts, and heating at reflux in acetonitrile for a period of four hours are the conditions for the optimal conversion. Premature quenching of the reaction resulted in the isolation of a trinuclear RuI2RuII complex [{Ru(NP-Am)2(CO)}{Ru2(NP-Am)2(CO)2(,-CO)2}(,3:,3 - C,O,O, -CO2)][BF4]2 (6). These unprecedented diruthenium compounds are the dinuclear congeners of the water,gas shift (WGS) intermediates. The possibility of a dinuclear pathway eliminates the inherent contradiction of pH demands in the WGS catalytic cycle in an alkaline medium. A cooperative binuclear elimination could be a viable route for hydrogen production in WGS chemistry. [source] Gaseous HgH2, CdH2, and ZnH2CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2005Alireza Shayesteh Abstract Gaseous HgH2, CdH2, and ZnH2 molecules were synthesized by the direct gas-phase reaction of excited mercury, cadmium, and zinc atoms with molecular hydrogen. The molecules were identified by their high-resolution infrared emission spectra, and the metal,hydrogen bond lengths were determined from the rotational analysis of the antisymmetric stretching fundamental bands. [source] DFT/CCSD(T) Investigation of the Interaction of Molecular Hydrogen with Carbon NanostructuresCHEMPHYSCHEM, Issue 11 2009Miroslav Rube Abstract The interaction of molecular hydrogen with carbon nanostructures is investigated within the DFT/CC correction scheme. The DFT/CC results are compared with the benchmark calculations at the CCSD(T) level of theory for benzene and naphthalene, and at the MP2 level for the more extended systems. The DFT/CC method offers a reliable alternative to the highly correlated ab initio calculations at a cost comparable to the standard DFT method. The results for H2 adsorbed on graphene as well as single-wall carbon nanotubes (SWCNT) are presented. The DFT/CC binding energy on graphene of 5.4 kJ,mol,1 is in good agreement with experiment (5.00±0.05 kJ,mol,1). For (10,10)-SWCNT, the H2 molecule is mostly stabilized inside the tube with an estimated binding energy of 7.2 kJ,mol,1. [source] Radiolysis of Confined Water: Hydrogen Production at a High Dose RateCHEMPHYSCHEM, Issue 12 2005Sophie Le Caër Dr. Abstract The production of molecular hydrogen in the radiolysis of dried or hydrated nanoporous controlled-pore glasses (CPG) has been carefully studied using 10 MeV electron irraditation at high dose rate. In all cases, the H2 yield increases when the pore size decreases. Moreover, the yields measured in dried materials are two orders of magnitude smaller than those obtained in hydrated glasses. This proves that the part of the H2 coming from the surface of the material is negligible in the hydrated case. Thus, the measured yields correspond to those of nanoconfined water. Moreover, these yields are not modified by the presence of potassium bromide, which is a hydroxyl radical scavenger. This experimental observation shows that the back reaction between H2 and HO. does not take place in such confined environments. These porous materials have been characterized before and after irradiation by means of Fourier-transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) techniques, which helps to understand the elementary processes taking place in this type of environment, especially the protective effect of water on the surface in the case of hydrated glasses. [source] | ||||||||||||||||