Concentration Of Molecular Hydrogen Research Articles

Inelastic neutron scattering of molecular hydrogen in amorphous hydrogenated carbon

We have, by use of inelastic neutron scattering detected the presence of molecular hydrogen in amorphous hydrogenated carbon. We have found the hydrogen to be in a high-pressure, asymmetric environment formed by the compressive stresses in the a-C:H films. On comparing two samples, we have also found that the sample with higher molecular hydrogen concentration has a lower total hydrogen composition. This is caused by a higher network density, trapping the molecular hydrogen during network growth.

Vacuum Operation for Overloaded Anaerobic Treatment Systems

Overloaded anaerobic treatment systems are characterized by high concentrations of volatile fatty acids and molecular hydrogen and poor conversion of primary substrates to methane. Previous experiments with fixed–film reactors indicated that operation with reduced headspace pressures enhanced anaerobic treatment. For these studies, four suspended culture, anaerobic reactors were operated with headspace pressures maintained between 0.5 and 1.0 atm and a solids retention time of 15 days. For lightly loaded systems (0.4 g SCOD/g VSS-day) vacuum operation provided minor treatment improvements. For shock organic loads, vacuum operation proved to be more stable and to support quicker recovery from upset conditions. Based on these studies and a companion set of bioassay tests, it was concluded that: (a) a loading rate of about 1.0 g SCOD/g VSS-day represents a practical loading limit for successful anaerobic treatment, (b) a headspace pressure of approximately 0.75 atm appears to be an optimum operating pressure for anaerobic systems and (c) simple modification to existing systems may provide relief for organically overloaded systems.

Hydrogen incorporation in undoped microcrystalline silicon

Hydrogen in undoped, unalloyed microcrystalline silicon (μc-Si:H) has been investigated with secondary-ion mass spectrometry (SIMS), Raman spectroscopy, infrared absorption spectroscopy, and nuclear magnetic resonance (NMR). The samples were grown by plasma-enhanced chemical vapor deposition with hydrogen to silane dilution ratios (H2:SiH4) ranging from 0:1 to 98:1. Microcrystallinity is obtained for dilution ratios of 20:1 and greater. The hydrogen concentration is shown to depend nonmonotonically on the degree of hydrogen dilution. The H concentration in the films decreases with dilution for ratios from 0:1 to 10:1 and then increases with greater dilution. This dependence on dilution is established with both NMR and SIMS and suggests the existence of competing processes in the incorporation of hydrogen during deposition. It is further observed that the formation of microcrystallites is accompanied by the appearance of both higher order silicon hydrides and large concentrations of unbound molecular hydrogen.

Mathematical modelling of the electrochemistry in corrosion fatigue cracks in structural steel cathodically protected in sea water

A mathematical model has been developed to describe the mass transport and electrochemical conditions in a corrosion fatigue crack in steel cathodically protected in sea water. Mass transport by advection (fluid flow induced by the cyclic displacement of the crack walls), diffusion and ion migration was considered, and the principal electrochemical reaction was taken to be the reduction of water. To simulate the buffering action of sea water the following processes were considered to occur within the crack: (a) equilibrium between bicarbonate and carbonate ions with precipitation of calcium carbonate as aragonite and (b) precipitation of magnesium hydroxide. The effects on crack tip pH, potential and molecular hydrogen concentration of varying the external potential, ΔK (range of the stress intensity factor), R value (minimum load/maximum load), crack depth and frequency at a temperature of 5°C were investigated, and the relative importance of the different buffering reactions was assessed. For a wide range of parameters, the crack tip pH was considerably alkaline with the pH greater than 10.0 for potentials in the range −800 mV to −1100 mV(SCE). Precipitation of magnesium hydroxide was the more effective buffering process provided the solubility limit ( pH ⋍ 10.6 ) was exceeded. The buffering reaction of the bicarbonate/carbonate system depended on the rate of replenishment of bicarbonate ions and was only partially effective in magnesium-free sea water. The two processes in combination provided a more efficient buffering of the crack solution than either process individually. Comparison of the model predictions with experimental measurements indicated good agreement with respect to crack tip potential while the experimental pH measurements fell roughly between the predictions of models based on sea water with and without magnesium. The latter observation is considered to reflect competitive or interactive precipitation in the real situation. Estimates of crack tip reaction rates based on scratching electrode experiments in simulated crack environments suggest that at sufficiently negative potentials generation of hydrogen atoms on the external surface will be the driving force for corrosion fatigue crack growth rates at long times. Using experimental measurements of crack tip pH and potential it appears that bulk charging may become dominant at potentials more negative than about −1000 mV(SCE). The model predictions suggest that the significance of bulk charging is likely to be enhanced at low R values and for deep cracks for which the potential drop is greatest.

Development of a four population model of the anaerobic degradation of carbohydrates

Abstract A four population model of anaerobic degradation of carbohydrates is presented. Acidogens, acetogens, CO2 reducing methanogens and acetoclastic methanogens are considered, the kinetics related to the first three populations depending on molecular hydrogen concentration. The model takes into account the complex interactions between the physico‐chemical and the biological components and can simulate digester operation both in steady state and transient conditions. The former are obtained by a simple analytical solution.

The Time Dependence of Interface State Production

A model for the reaction kinetics of interface state generation is developed. The reaction kinetics lie between third and first order for high and low concentrations of radiolytic molecular hydrogen, respectively. The application of these equations to the treatments of post irradiation effects is discussed.

Diurnal variability of dissolved molecular hydrogen in the tropical South Atlantic Ocean

In oligotrophic surface waters between 800 and 1600 km off Salvador, Brazil, molecular hydrogen concentrations were highly correlated with solar radiance levels, increasing during the morning to twice the night-time level and declining after noon. Regions to the south showed no evidence of diurnal periodicity, and profiles observed in the same region as the surface diurnal effect revealed a H 2 gradient within the mixed layer. Production of H 2 may be due to the nitrogenase activity of N 2-fixers, while the afternoon decline of H 2 levels may be explained by downward transport into the mixed layer accompanied by loss to the near-surface atmosphere. Observations of H 2 in the oceans show a strong relationship to in situ biological activity, particularly photosynthetic nitrogen fixation. Yet the dynamics of the relationships have up to now been masked by inadequate sampling and the long time constants of H 2 processes. The diurnal H 2 cycle is thus a phenomenon that may prove H 2 to be a sensitive chemical tracer of microbiological activity.

Choice of optimum parameters in stabilizing combustion in a flow with a forechamber jet of H2 combustion products

In this paper, the authors examine the effects of the basic parameters in a jet of combustion products from hydrogen-air mixtures as regards the concentration limits for the stabilization of the flame from previously mixed propane-air mixtures in a turbulent flow. Also determined were the simulation criteria for the stabilization with the use of forechamber devices. The results show that the stabilization effect is greatest when the excess air coefficient in the forechamber approx. = 0.7. Results indicate that chemical and aerodynamic factors have important effects in the forechamber method of stabilizing combustion in a flow. Theoretical analysis of the stabilization conditions is based on an ignition model for a turbulent flow containing a hot gas jet. In the simulation, it is assumed that ignition by the forechamber method is due to the temperature rise in the flow of the main mixture because the hot combustion products from the forechamber mix in with it. Calculations of the concentrations show that there is a logarithmic relationship for the effect of the initial activecenter concentration on the ignition for isothermal conditions. The active-center concentration is not quantitatively increased by enriching the mixture with fuel, nor is the chemical activity in the flamemore » raised. One can explain results in which there was some improvement in the performance of a forechamber device when the forechamber mixture was rich by saying that there was an increased concentration of molecular hydrogen in the forechamber flame.« less

Collective dissociative desorption of water molecules from glass produced by IFP-2000 flash tubes

When the radiation from an IFP-2000 lamp interacts with the surface crazed layer of glass, there is a weak spectral absorption not only in the vacuum ultraviolet but also in the range 420 >λ > 290 nm. The adsorbed H2O molecules are desorbed mainly by the dissociation, for the concentration of molecular hydrogen in the products is 80–84%. At intensities E≥600 W/cm2 and E≥4600 W/ cm2, in accordance with the working conditions, the desorption becomes collective. The rate of integral photon absorption then attains 1019 photons/cm2·sec. The transition to collective desorption in the first case (at relatively low intensity) is due to the production of a plasma in the material as a result of a sliding discharge struck at the surface of the glass.

CORRELATION BETWEEN OUTPUT POWER AND COMPOSITION OF DISCHARGE PRODUCTS IN A WATER VAPOR LASER

Experiments on continuous-flow water-vapor gas-discharge lasers indicated that the maximum of the output power at λ = 27.9 μ corresponded to the maximum of the concentration of molecular hydrogen and oxygen formed as a result of decomposition of water vapor. Laser emission from water vapor occurred when the degree of decomposition at the exit from the discharge tube reached at least 70%. The influence of the products of decomposition and of the discharge current on the output power was analyzed.

Diffusional effects in premixed laminar flames of hydrogen, oxygen, and nitrogen

Fuel-rich laminar flames of H2, O2, and N2 have been burned on a Meker type of burner (diameter 10 mm). The concentration of molecular hydrogen in them was determined by direct sampling and was found to decrease with distance along the axis of each flame. This is attributed to H2 diffusing from the bulk of a flame and being consumed at the flame's edges by oxidation with O2 from the surrounding atmosphere. A mathematical model of this process, based on the diffusion of H2 in these flames being of a pseudo-binary type, as well as being unaffected by any chemical reactions of H2, is tested and found to hold well. Such a diffusion process is characterized by a dispersion coefficient, D, which depends on flame temperature and composition and on the separate binary diffusion coefficients (DN2H2 and DH2OH2) of H2 in N2 and H2O, respectively, according to 1 − x H 2 D = x N 2 D N 2 H 2 + x H 2 O D H 2 OH 2 In this relation the x's are the mole fractions of the species denoted by the subscripts. The values indicated here for the two binary diffusion coefficients (i.e., DN2H2 and DH2OH2) are found to be slightly different from previous determinations, and these differences are attributed to the participation of chemical reactions of H2 and to the simplifications involved in treating a multicomponent diffusion process as a pseudo-binary one.

Water vapor, molecular hydrogen, methane, and tritium concentrations near the stratopause

Molecular hydrogen, methane, water vapor and tritium concentrations near stratopause from air samples collected on Aerobee flight with liquid hydrogen cooled cryocondenser

Radical recombination in rich premixed hydrogen/oxygen flames

The lithium/lithium hydroxide technique, employing atomic absorption spectroscopy for atomic lithium determination, has been used to study the decay of hydrogen-atom concentrations in fuel-rich, atmospheric-pressure hydrogen/oxygen/diluent flames. The observed second-order rate constants for hydrogen-atom decay are interpreted in terms of the reactions, H+H+M→H2+M, Rate constant=kH, M H+OH+M→H2O+M, Rate constant=kOH,M where M represents any of the bulk species present in the burnt-gas region of the flame. Analyses of the dependence of the observed second-order rate constant on molecular hydrogen concentration in over one hundred and fifty flames containing as diluent, nitrogen, argon, helium, carbon dioxide or steam have led to the following values (in ml2 molecule−2 sec−1): kH,N2=1.0×10−32, kH,Ar=0.6×10−32, kH,He=1.3×10−32, kOH,N2=1.3×10−32, kOH,Ar=1.8×10−32, kOH,He=2.0×10−32, kOH,H2O=2.4×10−32, all at 1900° K. The rate constants kH,H2O and kOH,H2 cannot be determined individually because of the balanced reaction, H+H2OH2+OH, Equilibrium constant, K and the value (kH,H2O+KkOH,H2)=2.1×10−32 ml2 molecule−2 sec−1 at 1900° K, was obtained. The algebraic complexity of the rate of recombination in flames diluted with carbon dioxide (when partial reduction to carbon monoxide occurs) prohibits the determination of kH,CO, kH,CO2, kOH,CO, and kOH,CO2. A simplified procedure based on the assumption of equal thirdbody efficiencies for carbon dioxide and carbon monoxide leads to an effective rate constant for Reaction (1) of 1.4×10−32 ml2 molecule−2 sec−1 and an upper limit for the rate constant of Reaction (2) of 0.5×10−32 ml2 molecule−2 sec−1, with these species as third bodies.

Further examination of the hydrogen content in the upper atmosphere of Venus

In a recent paper, Earth et al. [1968] computed the molecular hydrogen concentration in the upper atmosphere of Venus from the component of the Lyman-alpha emission attributed to photodissociation of molecular hydrogen as measured by Mariner 5. The outcome of these computations, as seen in Figure 3 of Barth et al. [1968], resulted in part from the assumptions of a molecular hydrogen density of 2.8 × 109 molecules cm−3 at a planetocentric distance of 6500 km, with an isothermal region of temperature 650°K above this level. As a consequence, the optical thickness of molecular hydrogen at 6500 km, based upon the photoionization cross sections of Lee et al. [1952], would result in significant ultraviolet energy in the wavelength region below 804 A being absorbed above 6500 km. Correspondingly, the rapid settling of the heavy CO2 molecule in the lower thermosphere would severely limit the effectiveness of CO2 as a cooling agent at these high altitudes. Under such circumstances, it is difficult to understand both the isothermal region above 6500 km and the relatively low exospheric temperature of Venus. To verify these points the author reconstructed the upper atmosphere of Venus by solving numerically the heat conduction equation in conjunction with separate diffusion equations for each constituent similar to the method outlined in Chamberlain [1962], Chamberlain et al. [1966], and McGovern [1967].

An Upper Limit of the Concentration of Molecular Hydrogen in Interstellar Space

An Upper Limit of the Concentration of Molecular Hydrogen in Interstellar Space

Atomic and molecular hydrogen in the thermosphere

Calculations have been performed to determine the altitude distribution of atomic and molecular hydrogen in the Earth's atmosphere following a suggestion by Bates and Nicolet that molecular hydrogen may be considerably more abundant than atomic hydrogen at the 100-km level. It is shown in this paper, on the basis of the Lyman-α absorption measurements of Purcell and Tousey, that the molecular hydrogen concentration at 100 km may only be comparable to the atomic hydrogen concentration there.

The Concentration of Molecular Hydrogen on the Platinum Cathode

Abstract The concentration of molecular hydrogen in the vicinity of the bright platinum cathode during electrolysis was determined as a function of current density by means of two different methods, the potential-time curve on the reversal of current from cathodic to anodic, and the concentration overpotential obtained by extrapolating the decay curve of the overpotential to zero time, free from other overpotentials. The agreement between the results for the concentration derived from the potential-time curve and from the concentration overpotential was satisfactory. It was found that the concentration of hydrogen increased with the increasing ik value and approached a limiting value of 11.6×10−2 mol./l. for cathodic currents exceeding about ik=300 mamp./cm2.

Passivating effect of chemisorbed oxygen on the anodic oxidation of molecular hydrogen at platinum

The oxygen coverage was determined by cathodic charging curves at different potentials under the conditions of periodic, low-speed, potentiostatic current potential curves in 1 N HClO4, with vigorous hydrogen or argon stirring between +0·6Vand +1·5V. In the case of hydrogen stirring the net current is practically due to the anodic oxidation of molecular hydrogen in the potential range of passivation. Fick's law is applicable to the hydrogen diffusion between + 0·8 V and + 1·1 V daring the anodic sweep. The decrease of the concentration of molecular hydrogen in the diffusion layer can be corrected for. The correlation between reaction rate and oxygen coverage shows that most of the passivation is caused by a small oxygen coverage (10 per cent). After the very active centres for hydrogen oxidation have been covered by oxygen, the inhibition increases little with potential above + 1·1 V during the anodic sweep.

KINETICS OF THE HOMOGENEOUS REACTION BETWEEN CUPRIC ACETATE AND MOLECULAR HYDROGEN IN AQUEOUS SOLUTION

Cupric acetate was found to react homogeneously with molecular hydrogen in aqueous solution according to the following equation:[Formula: see text]The paper describes a kinetic study of this reaction. Rates were determined at temperatures between 80 and 140 °C and hydrogen partial pressures between 6.8 and 34.0 atm. The reaction was found to be of second order, the rate being proportional to the concentrations of cupric acetate and molecular hydrogen. It was established that the rate was independent of the surface of the reaction vessel, the cuprous oxide product and of the concentrations of sodium acetate and acetic acid in the solution. The reaction has an activation energy of 24200 cal. per mole. The kinetic results are discussed and a mechanism is proposed. This appears to be one of the few known homogeneous reactions of molecular hydrogen in solution.