Hydrogen Uptake Rate Research Articles

The Uptake and Production of Molecular Hydrogen by Unicellular Cyanobacteria

Summary: Nine strains of unicellular non-nitrogen-fixing cyanobacteria had high rates of hydrogen evolution and uptake compared with filamentous nitrogen-fixing strains. Maximal hydrogen uptake activities were observed in assays including oxygen under both light and dark conditions suggesting the participation of the ‘Knallgas’ reaction. Nitrogenase activity, measured as acetylene reduction, was not found in aerobic cultures of any of the unicellular strains tested, and the capacity to synthesize nitrogenase under anaerobic conditions was absent in all five of the strains tested under these conditions. Thus the capacity to synthesize nitrogenase is not a prerequisite for hydrogenase synthesis and hydrogen evolution by these unicellular cyanobacteria. Rates of hydrogen evolution and uptake by six of the unicellular strains were increased following adaptation with CO2/H2 gas mixtures.

ChemInform Abstract: SOLUTION OF HYDROGEN AND DEUTERIUM IN PALLADIUM PHOSPHIDE (PD6P)

AbstractPd6P löst H2 unter Bildung einer interstitiellen Lösung, die bei l at und 298 K die Zusammensetzung Pd6PH0.15 aufweist.

Observation of a first-order structural phase transition in a monolayer of Pd on Nb(110) and its relationship to electronic structure

It is shown that a commensurate 1\ifmmode\times\else\texttimes\fi{}1 monolayer of Pd on a Nb(110) surface undergoes a first-order structural phase transition to an incommensurate monolayer Pd(111) structure when \ensuremath{\approx} 18% more Pd atoms are added to the surface. We find, however, that this phase transition has no substantial effect on the surface electronic structure. A steady shift toward the Fermi level of the local density of states at the surface is found as the Pd coverage is increased beyond about one monolayer. This shift correlates quite well with a previously observed increase in the hydrogen uptake rate.

Solution of hydrogen and deuterium in palladium phosphide, Pd6P

Pd6P dissolves hydrogen to form an interstitial solution which has the composition Pd6PH0.15 at 1 atm and 298 K. The hydrogen solubilities follow Sieverts' law of ideal solubility at low hydrogen contents showing that molecular hydrogen dissociates upon entering the phosphide lattice. The equilibration of hydrogen with the phosphide is rapid at 273 K indicating that the H2→ 2H(surface) reaction occurs readily. Pre-exposure of Pd6P to CO(g) does not significantly inhibit the rate of hydrogen uptake. This suggests that phosphorus, which is a known poison for hydrogenation catalysts, does not act as such when chemically combined with palladium.Pressure–composition–temperature data have been obtained for the Pd6P–H system from which thermodynamic parameters for the solution of hydrogen have been derived. It is known from neutron diffraction studies that deuterium enters both the octahedral and square pyramidal interstices in Pd6P. A statistical thermodynamic model is proposed to describe the pressure–composition data for this system and it is compared with experimental data.

Hydrogen uptake by the nitrogen-starved cyanobacterium Anabaena cylindrica

The role of the oxyhydrogen reaction in the nitrogen metabolism of Anabaena cylin-drica, particularly under conditions of dinitrogen starvation, was investigated. It was shown that although this reaction supports nitrogenase activity in the dark, when the cells are deprived of nitrogen the rate of hydrogen uptake is little changed. Measurements of ammonia excretion into the medium in the presence of methionine sulfoximine under such conditions indicated that hydrogen uptake supported the turnover of cell protein as an alternative source of nitrogen. In the absence of H 2 and O 2 in the dark, nitrogenase activity was negligible but protein turnover continued. In their presence nitrogenase activity was greatly stimulated; turnover was also stimulated but to a greater extent in the absence of nitrogenase substrates. The oxyhydrogen reaction also stimulated uptake of ammonium ions by intact filaments in argon in the dark. Only at very low hydrogen tensions can net hydrogen formation be obtained in argon/CO 2 in the light, casting considerable doubt on the suitability of hydrogenase-containing organisms for biophotolytic hydrogen formation. Addition of exogenous ammonia to the cultures incubated in argon resulted in a pronounced stimulation of H 2 uptake; nitrate and its derivatives had no such effect, nor did various amino acid derivatives of ammonia.

Relationship between Electronic Structure and Hydrogen-Uptake Kinetics

Striking changes have been observed in the photoemission energy-distribution curves from two phases of Pd overlayers on (110) Nb surfaces. In the commensurate (110) phase the Pd $4d$ band is found to have an edge about 1.5 eV below ${E}_{\mathrm{F}}$, similar to that of a noble metal. When the transition to the incommensurate Pd(111) phase occurs, the Pd $4d$ bands broaden and cross the Fermi level. As the density of Pd $4d$ states increases near ${E}_{\mathrm{F}}$, a dramatic increase in the hydrogen-uptake rate through the Pd is observed.

Uptake rates for hydrogen by niobium and tantalum: effect of thin metallic overlayers

The hydrogen uptake rates of clean niobium and tantalum foils were measured as a function of temperature and pressure. The “sticking coefficient” (the number of hydrogen atoms entering the bulk per twice the number of hydrogen molecules hitting the surface) of hydrogen on clean niobium and tantalum at about 2.2 × 10 −5 Torr hydrogen and room temperature was found to be surprisingly low, approximately 0.005 and 0.05 respectively, resulting in very low uptake rates under these conditions. This result is explained on the basis of a recently proposed model. It was found that these uptake rates can be significantly enhanced or decreased by the surface deposition of a thin layer of palladium, platinum or nickel. The hydrogen uptake rates, i.e. the sticking coefficients, were measured as a function of the overlayer thickness. In the case of palladium it was found that a layer less than one monolayer thick either will not change or will even inhibit the hydrogen uptake, whereas a layer over one monolayer thick will dramatically enhance the sticking coefficient to a value of unity. It was found that the change at one monolayer is related to a phase transition of the palladium surface layer at that coverage. Neither platinum nor nickel exhibits this effect but both influence the uptake rate by an amount depending on their thicknesses. Thick overlayers of platinum, nickel and palladium inhibit uptake because of a limited permeation rate through the layer.

Hydride formation in La 1− xMg xNi 2

A single-phase ternary compound with the C15 structure exists from LaNi 2 to La 0.33Mg 0.67Ni 2. The resulting compounds form hydrides at room temperature. Whilst LaNi 2 forms a pseudo-amorphous hydride, partial magnesium substitution results in well-crystallized hydrides. This has to do with the diminishing rate of hydrogen uptake in magnesium-substituted materials providing slower heat evolution in the hydriding process. In desorption experiments at elevated temperatures the hydrides show a tendency to decompose into LaH 3, MgH 2 and nickel. Magnesium-rich specimens appear to evolve hydrogen at room temperature; however, desorption rates are slow. The implications of a local model for understanding the hydriding behavior in disordered (positional entropy) materials are discussed.

Enhancement of Hydrogen Uptake Rates for Nb and Ta by Thin Surface Overlayers

It is shown that thin layers of Pd or Pt enhance the uptake rate of hydrogen into bulk Nb with almost every hydrogen atom that hits the surface passing into the bulk. A model for the kinetics is formulated showing that in a composite-system large uptake rates are achieved by combining the surface properties of Pd with the bulk properties of Nb, thereby resulting in a smaller barrier for migration into the bulk.

Corrosion and Electrochemical Behavior of Titanium in aqueous sulfidic solutions

AbstractThis paper summarizes the results of an investigation of the corrosion and electrochemical behavior, including cathodic hydrogen absorption, of titanium in acidic and alkaline sulfidic solutions. The effect of chloride and cyanide ions on the corrosion behaviour of titanium in sulfidic solutions was also studied. Titanium in acidic and alkaline sulfidic solutions exhibits a stable passive corrosion potential and negligible corrosion. Chloride and cyanide ions do not affect the corrosion behavior of titanium in sulfidic solutions. Cyanide ions under conditions of cathodic polarization tend to promote activation of titanium. Cathodic polarization at temperatures higher than 72 °C promotes hydrogen uptake in titanium and formation of titanium hydride. The rate of hydrogen uptake increases with decreasing pH and increasing temperature. Cyanide ions tend to promote the rate of hydrogen uptake, whereas chloride ions have essentially no effect.

Selective catalytic behaviour of tungsten carbide in the liquid-phase hydrogenation of organic compounds

The catalytic activity of tungsten carbide for liquid-phase hydrogenation has been studied in acidic (H2SO4) media at 40–90 °C. Tungsten carbide proved to be a selective catalyst, easily promoting the reduction of aromatic nitro-compounds and quinones. However, the saturation of double and triple bonds, aromatic nuclei, and oxo-groups did not occur under these conditions. The effect of temperature and the composition of the liquid phase on the rate of hydrogen uptake have been studied. Some problems concerning the mechanism of hydrogenation are discussed.

Peculiarities in the hydrogenation of alkenylfurans with a double bond situated in the 2, 3 position to the ring

Up to the present there has been no doubt about the possibility of selective hydrogenation of the side chain double bond in alkenylfurans [1]. In particular, catalysts consisting of palladium on carriers (SrCO3 [2], BaSO4 [3, 4]) have been used. They have been used to hydrogenate compounds with the double bond at various positions in the side chain [2–4], but not alkenylfurans with the double bond at the 2, 3 position. Having synthesized compounds of this latter type [5], the present authors attempted to hydrogenate selectively 2-(3-methylbuten-2-yl) furan and 2-(2,3-dimethylbuten-2-yl) furan, but it was unexpectedly found that with Pd/BaSO4 and at atmospheric pressure hydrogenation was non-selective, and gave the corresponding alkyltetrahydrofurans. Hydrogen uptake rate fell uniformly, and slowed down greatly when hydrogenation was complete.

Untersuchung des wasserstoffaustausches an Raney-Nickel mit hilfe einer magnet-elektrode

A cell assembly is described in which finely grained Raney nickel catalyst is brought into contact with an indifferent electrode by magnetic force. The contact can be made continuous or intermittent. This offers a convenient means for the control of the charge transfer at the electrode. The aluminium and hydrogen content of the Raney nickel catalyst has been determined as a function of pre-treatment. Freshly prepared catalyst contains 0·2–0·3 H per 1 Ni, considerably less than prevailing opinion would suggest. The hydrogen exchange proceeds reversibly only up to an overpotential of ca +150 mV. At higher overpotentials oxidation of nickel occurs, followed by inactivation of the catalyst. The formerly described different kinds of hydrogen bound at the surface may partly be explained by different oxidation states of nickel. The hydrogen concentration as function of polarization, the rate of hydrogen uptake and the current of potentiostatic discharge must be known if the process is to be optimized. The initial current at an overpotential of 120 mV with Raney nickel of 42–50 μm grain size and an electrode coverage with 20 mg catalyst for 1 cm 2 is 38 A/g at 25°C. During a contact time of 1 s the average current decreases to one half of the initial current. In addition to the investigations of the hydrogen electrode, preliminary experiments of oxygen reduction and alcohol oxidation at magnet electrodes are described.