High Hydrogen Partial Pressure Research Articles

Microcrystalline Silicon by Dc Magnetron Sputtering: Growth Mechanisms

ABSTRACTWe have used real-time, in situ spectroscopie ellipsometry (SE) and infrared reflectance (IR) to study microcrystalline silicon (μc-Si) formation in reactive magnetron sputtering (RMS). μc-Si growth occurs at high hydrogen partial pressures and moderate substrate temperatures. We use IR studies and SE studies of film growth on rough surfaces, respectively, to show that these conditions lead to high hydrogen coverage of the film surface and high effective surface diffusivity. The interface region is amorphous and its thickness decreases with deposition rate. For a fixed growth flux, we observe a 30% decrease in the deposition rate of μc-Si relative to the amorphous interface region. This could be due to increased etching or decreased sticking coefficients during microcrystalline growth.

Fatty acid oxidation in anoxic marine sediments: the importance of hydrogen sensitive reactions

In anoxic marine sediments fatty acids may be oxidized directly by sulfate reducing bacteria, or may be oxidized by pathways which result in hydrogen production. Some of these latter reactions are quite sensitive to hydrogen concentrations ... in other words if hydrogen concentrations become elevated, fatty acid oxidation will cease. Thus sulfate reducers may actually play two important roles in the metabolism of fatty acids in marine sediments. The sulfate reducers both can utilize fatty acids directly, and also can oxidize hydrogen and thus control hydrogen partial pressures in the sediments. Therefore sulfate reducers may act indirectly to facilitate fatty acid oxidation by hydrogen-producing pathways. We carried out a series of incubations of slurried salt marsh sediment under high and low hydrogen partial pressures and in the presence and absence of molybdate to investigate the relative importance of sulfate reducers and other bacteria mediating hydrogen-sensitive reactions. Our results suggest that both classes of bacteria contribute significantly to fatty acid turnover in marine sediments. Studies of low molecular weight fatty acid turnover in sediment must explicitly recognize that manipulation of sediment (including addition of molydbate to inhibit sulfate reducers) may have a large impact on hydrogen partial pressures in sediment, and may thus significantly alter the pathways and/or rates of fatty acid turnover.

Deoxidatlon rate of copper droplet levitated in Ar-H2 gas stream

Levitated copper droplets, 5 mm in diameter with initial oxygen contents of 0.036 to 1.9 wt pct, were deoxidized at about 1970 K in an Ar-H2 gas stream. The Ar-H2 gas mixture having hydrogen partial pressure less than 4 kPa was introduced into a silica reaction tube of 11-mm ID at gas flow rates up to 2 x 10-4 Nm3s-1. The effects of initial oxygen content of the droplets, hydrogen partial pressure, and gas flow rate on the deoxidation process were examined. A mixed control model for the deoxidation rate involving both gas and liquid film mass-transfer resistances was combined with a thermodynamic relationship for the dissolved species in molten copper. The value of 2 × 10-4 m 73x00D7; s-1 was assigned to the liquid film mass-transfer coefficient of dissolved oxygen throughout all experimental conditions. Under the experimental conditions of low initial oxygen content and high hydrogen partial pressure, the liquid film mass-transfer resistance was significant. When a droplet of high initial oxygen content was deoxidized, transition phenomena from gas to liquid film mass-transfer control were noticed in the later stage of reaction. It was deduced from the present model that the accumulation of dissolved hydrogen was indispensable to these phenomena.

The reduction behaviour of silica-supported copper catalysts prepared by deposition-precipitation

The reduction behaviour of silica-supported copper catalysts prepared by homogeneous deposition-precipitation has been studied by temperature-programmed reduction (TPR), differential thermogravimetry (DTG), in-situ diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy, and evolved gas analysis (EGA). Preparation by homogeneous deposition-precipitation produces catalyst precursors in which the copper ions are highly dispersed over the silica support, and which show reduction behaviour similar to that of the mineral copper hydrosilicate, chrysocolla. Calcination of the catalyst precursors at 700 K leads to decomposition of the copper hydrosilicate, causing subsequent reduction to occur at a lower temperature. During initial reduction of the chrysocolla-like precursors, the consumption of hydrogen and the release of water are not synchronous. Water formed by the reduction is retained in the catalyst, probably due to participation of water in a reorganization of the silica support. During the second reduction stage of silica-supported copper oxide particles, formed by previous reduction and re-oxidation of the chrysocolla-like precursor, the consumption of hydrogen and the release of water do occur simultaneously. Finally, the influence of the composition of the reducing gas atmosphere on the reduction behaviour of the catalysts has been investigated. Increasing the hydrogen concentration in the reducing gas mixture from 5 to 50 vol.% does not affect the on-set of the reduction reaction, whereas addition of about 2.4 vol.% H 2O to the reducing gas mixture leads to significantly higher on-set temperatures. Once initiated, the reduction proceeds more rapidly at higher hydrogen partial pressures.

The effect of the H2 partial pressure on the metabolite pattern ofLactobacillus casei, Escherichia coli andClostridium butyricum

The metabolite pattern of batch cultures ofLactobacilluscasei LMG 6400,Clostridiumbutyricum LMG 1213t1 andEscherichiacoli LMG 2093 was effected only for the latter organism when the H2 partial pressure was below 1 atmosphere: high hydrogen partial pressures increased the formate formation, low pressures gave rise to increased acetate production and higher cell yields.

Utilization of vegetable oils as an alternative source for diesel-type fuel: hydrocracking on reduced Ni/SiO 2 and sulphided Ni-Mo/γ-Al 2O 3

Vegetable oil hydrocracking was studied in a batch reactor under high hydrogen partial pressure (10–200 bars) at 623–673 K, catalyzed by either reduced Ni/SiO 2 or sulphided Ni-Mo/γ-Al 2O 3. We have established the sequence of reactions which transform a vegetable oil into a diesel-type fuel. We have demonstrated the role of the catalyst in each of these reactions, and the occurrence of thermodynamic equilibria restricting the completion of the transformation. The displacement of these equilibria was achieved by an increase in hydrogen pressure, where molar yields close to 100% were attained. The corresponding product is a mixture of hydrocarbons (essentially normal alkanes in the diesel fraction).

The effects of hydrogen and segregation on fatigue crack nucleation at defined grain boundaries in nickel bicrystals

In order to explore the role of impurity segregation in intergranular fatigue crack initiation and propagation, tests have been made on nickel bicrystals, variously heat-treated to induce increasingly severe degrees of sulfur segregation, and using air, vacuum and hydrogen as the environment. The grains of the bicrystals were oriented to yield two types of boundaries: type I, with strong elastic-plastic incompatibility and type II, a compatible tilt boundary. If the boundaries were clean, persistent slip band cracking in the grains occurred in preference to intergranular cracking. Although equal degrees of sulfur segregation, as measured by Auger-spectroscopy, could be produced at the two types of boundaries by the heat treatments, the incompatible one was much more susceptible to intergranular cracking than the other, which could be made to crack intergranularly only by high partial pressures of hydrogen. The results show that high stresses associated with incompatibility coupled with a lowering of cohesive forces at the boundary produced by the segregant are the main factors controlling intergranular fracture.

Scanning tunneling microscopy and (scanning) tunneling spectroscopy on stepped Ni(111)/H

Topographic and electronic properties of both the (111) flat surface and atomic steps of nickel have been measured with scanning tunneling microscopy (STM) and (scanning) tunneling spectroscopy in UHV. Micrographs of the (111) plane show a hexagonal pattern with a dimension of two times the unit structure and a corrugation of 1 Å which increases towards a step. This suggests a p(2 × 2)2H monolayer, induced by the high partial pressure of hydrogen. Micrographs of atomic steps show protrusions of 1–3 Å at the corners of the steps. Several explanations are discussed. Spectroscopic images of these areas show the same hexagonal pattern on the (111) surface and indicate an increase in the density of states at the adsorption sites and at the step edges. dI dV versus V spectra were also recorded. Various peaks in the density of states, at different energies below 1 eV above E F showed up, their intensity strongly dependent on the lateral position of the tip. These local changes in the density of states might indicate the presence of adsorbate-induced surface states.

PRODUCTION OF ACETIC ACID FROM HYDROGEN AND CARBON DIOXIDE BY CLOSTRIDIUM SPECIES ATCC 29797

Abstract Clostridium sp. ATCC 29797 converts H2 and CO2 to acetic acid in stationary phase in almost 100% yield. The resting cells are stable for up to 300 h and produce acetate to a final product concentration of 221 mM. The specific productivity of acetate during the stationary phase appears limited by the specific activity of hydrogenase. A high pressure fermentor was constructed to investigate the use of pressure, as a general strategy, for increasing the rates of microbial syngas conversions. Fermentations were conducted at pressures up to 1000 psig using various mixtures of H2 and CO2. The use of high pressure proved to be an effective method for eliminating gas transfer resistances and increasing the volumetric productivity. Hydrostatic pressure had little effect on the specific productivity of acetate, however, high partial pressures of hydrogen caused the specific productivity and total acetate accumulated to decrease. High pressures of hydrogen also caused a decrease in the specific activity of ...

Pressure vessel, piping, and welding needs for coal conversion systems

The dissolution /hydrogenation areas of direct coal liquefaction processes, as well as some types of coal gasifiers, impose severe materials performance requirements due to the critical combination of high operating temperatures and high hydrogen partial pressures. Careful consideration must be given to selecting the materials of construction and fabrication procedures for the reactor vessels, heat exchangers, and transfer piping to assure reliability and safety of operation.

Microcrystalline formation in sputtered a-Si: H films

Microcrystalline formation in hydrogenated silicon films deposited under high hydrogen partial pressures by reactive r.f. sputtering method has been studied by means of small angle scattering of X-ray. The existence of microcrystalline regions are detected in samples deposited under high hydrogen partial pressure above 40% at the total gas pressure of H 2 and Ar mixture of 2 × 10 -2 torr. The mean radius of microcrystalline regions increases with increasing hydrogen partial pressure. Relation between the presence of microcrystalline state and the electric properties of films is briefly discussed.

Effect of hydrogen of conversions of C6 hydrocarbons in the presence of alumina-platinum catalysts

This study investigated the effects of H/sub 2/ on conversion of alkanes, olefins, and cyclanes over platinum- alumina catalysts possessing substantial acidic characteristics. Results indicated that the activity, selectivity, and mechanism of conversions of C/sub 6/-hydrocarbons over alumina-platinum catalysts with substantial acidic function (Pt/Al/sub 2/O/sub 3/ and Pt-Re/Al/sub 2/O/sub 3/-F) are dependent on the partial pressure of hydrogen in the gas phase. Also, at relatively low hydrogen concentrations, C/sub 5/- and C/sub 6/-dehydrocyclization and skeletal isomerization of alkanes, and the hydrogenolysis of methylcyclopentane, are promoted, owing to its direct involvement in the reactions and to the removal of firmly adsorbed hydrocarbon forms from the surface. At high partial pressures of hydrogen, the competitive nature of the adsorption of the hydrocarbon and hydrogen results in the suppession of these reactions. Results also indicated that skeletal isomerization of alkanes proceeds preferentially by the C/sub 6/-cyclic mechanism at high hydrogen concentrations, and by the bifunctional mechanism at low concentrations.

Reactivities of carbon to steam and hydrogen and applications to technical gasification processes—A review

Experimental results of reactivity measurements on coal and chars to steam, hydrogen and steamhydrogen mixtures are compared and their application to technical processes is discussed. The change of surface area as a function of burn-off has a minor significance for chars made from coal with high initial accessible surface areas. In this case the reaction rate under constant steam pressure is first order with respect to the amount of carbon being gasified. The reaction rate of hydrogasification markedly decreases with burn-off, since the activation energy increases with burn-off due to the consumption of reactive carbon during the reaction. The reaction of carbon with steam hydrogen mixtures can be described as a superposition of carbon steam reaction and hydrogasification. The overall rate relative to that with hydrogen or steam alone can increase if a high amount of reactive carbon is present and immediately reacts with hydrogen. It can decrease if the inhibition of the steam carbon reaction by hydrogen predominates. Some consequences for the technical performance of coal gasification are: The overall reaction rate in a fluid bed decreases with increasing bed height, since hydrogen-steam ratios and therefore the inhibition by hydrogen increase. The overall reaction rate in a moving bed with countercurrent flow of carbon and reacting gases increases with increasing overall pressure since highly reactive carbon, formed in the pyrolysis zone, can immediately react with high partial pressures of hydrogen.

Effects of hydrogen incorporation in some deposited metallic thin films

Incorporation of gases in thin films, either added purposely or inherently present due to deposition system or process, can have significant effects on thin film stress and electromigration. Aluminum thin films which effects on thin film stress and electromigration. Aluminum thin films which were deposited in high partial pressures of hydrogen mostly in the atomic state were observed to have unusually long mean times to failure for current densities above 1×106 A/cm2. The role of hydrogen in increasing film conductivity and decreasing grain boundary diffusion is discussed.

Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO$z.sbnd;MoO3/$gamma;-Al2O3: Low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Hydrogenolysis and isomerization of n-pentane on group VIII transition metals

The reaction of n-pentane in the presence of hydrogen was studied using supported Group VIII transition metals as catalyst. The specific activity for hydrogenolysis referred to unit surface area of the metal was examined and the following sequence was obtained: Ru > Rh > Pt > Ni > Co > Ir > Pd ~ Fe. Hydrogenolysis on nickel proceeded via successive demethylation at the terminal carbon-carbon bond of adsorbed hydrocarbon, whereas on cobalt and iron, the desorption step of surface residues was so slow that the exact mode of hydrogenolysis could not be determined. On Group VIII noble metals, all the carbon-carbon bonds of n-pentane cracked almost statistically. On ruthenium, rhodium, and iridium, multiple bond breaking (extensive degradation) occurred at low partial pressure of hydrogen, although at high partial pressure of hydrogen, the amounts of methane and ethane were equal, respectively, to those of butane and propane. The main features of the product distributions of hydrogenolysis on palladium and on platinum catalysts could be accounted for by assuming that only one carbon-carbon bond was ruptured at the residence of a molecule on the surface. Carbon-supported catalysts produced methane and butane in higher proportion than silica gel supported catalysts. Among all the Group VIII transition metals with the exception of osmium, only palladium and platinum were found to be active for isomerization of n-pentane to isopentane. The selectivity of platinum for isomerization was found to decrease with increasing partial pressure of hydrogen.

Hydrogenolysis of n-pentane on nickel catalyst

The reaction of normal pentane and hydrogen on a nickel-silica gel catalyst was studied in a flow system under a hydrogen pressure of less than 50 atm. Under the range of temperatures studied (300–400 °C), the predominant reaction was the consecutive removal of terminal methyl groups as methane, and the formation of hydrocarbons containing one less carbon atom. The product distributions varied widely with both variation of the partial pressure of hydrogen and with the reaction temperature. At high partial pressures of hydrogen or low temperatures mainly butane and corresponding amounts of methane were produced. At low partial pressures of hydrogen or high temperatures the pentane was more extensively degraded. Velocity of pentane hydrogenolysis increased with partial pressure of pentane and was inversely proportional to partial pressure of H 2. The results could be explained by the mechanism involving cracking of terminal carbon-carbon bonds of adsorbed hydrocarbon radicals.

Selectivity in Hydrogenolysis of Paraffinic Hydrocarbons on Nickel Catalyst

Hydrogenolysis of several paraffinic hydrocarbons, straight (n-heptane, n-decane) and branched (i-pentane, 3-methylpentane, i-octane), was carried out over a nickel-silica gel catalyst (Ni 8.0 wt%) in the temperature range of 300 to 400°C. Under high partial pressures of hydrogen, predominant reaction was successive degradation to methane and a hydrocarbon containing one carbon atom less than the reactant hydrocarbon. So, primary products of heptane hydrogenolysis were methane and hexane. In the case of decane hydrogenolysis, the selectivity in splitting of terminal C-C bonds was somewhat lower than in the hydrogenolysis of heptane.In the case of i-paraffinic hydrocarbons, methyl groups not attached to quarternary carbon atoms were selectively removed. Methyl groups attached to quarternary carbon of isooctane were stable and scarecely removed to leave neopentane. The methyl groups bonded to secondary carbon atoms were found to be somewhat reactive than those bonded to tertiary carbon atoms.

Selectivity of gold for hydrogenation and dehydrogenation of cyclohexene

Rates of reaction of cyclohexene over a gold powder catalyst, in the presence or absence of hydrogen, were measured in a batch recirculation system between 203° and 285°C. After activation of the powder, consisting of a treatment with hydrogen first and then with oxygen, the measured rates are in excellent agreement with those previously reported on gold evaporated films. It has now been shown that, on gold, cyclohexene reacts irreversibly to form either cyclohexane or benzene. The selective production of benzene can be controlled within wide limits at a fixed temperature. It is depressed by high partial pressures of hydrogen, enhanced in the absence of hydrogen, while benzene becomes practically the sole product in the presence of small quantities of oxygen. Thus, at 203°C, the selectivity as measured by the ratio of benzene to cyclohexane is increased by a factor of 3000 when a partial pressure of hydrogen equal to 0.74 atm is replaced by a partial pressure of oxygen equal to 0.041 atm.