Dissociation Of Hydrogen Molecules Research Articles

Hydrogen permeability for oxide-metal multilayered films

The hydrogen permeability of multilayered films consisting of various oxide and metal films, e.g. V 2O 5/Cu, was investigated using the colouring phenomenon of amorphous WO 3 and was found to be associated with the capability of dissociation of hydrogen molecules and the hydrogen densities of the oxide layers. Hydrogen separation has been performed using V 2O 5/Cu multilayered films formed on a polyimide membrane. The hydrogen permeability coefficient of the films in the temperature range 343–368 K was found to be about 10 times as large as that of copper films owing to the capability of hydrogen dissociation at the V 2O 5/Cu double layer and owing to the large amount of hydrogen taken up by V 2O 5. Mixtures of H 2-CO and H 2-Ar with 50 mol% H 2 had a concentration of H 2 as high as 97 mol% after permeation through the membrane. When an LaCu 5 film was formed between the copper and polyimide film, the hydrogen permeability coefficient of the film increased owing to enhanced recombination of H atoms.

Energy transfer and vibrational effects in the dissociation and scattering of D2 from Cu (111)

ACTIVATED surface dissociation of adsorbed molecules is of basic importance to surface chemistry, but in only a few systems are details of the molecule–surface potential-energy surface well understood. Central to this problem are the processes of energy disposal and transfer during the molecule–surface collision and the manner in which molecular motions (translational, rotational and vibrational) are channelled into the dissociative coordinate. The dissociation of hydrogen and deuterium molecules on a copper surface provides a prototypical system for studying these processes. Here we describe measurements of the scattering of D2 from a Cu(111) surface as a function of incident translational energy, angle and vibrational state of the incoming molecules. Molecules in the ground and first excited vibrational levels show very different translational-energy thresholds for removal (0.6 and 0.35 eV respectively). At higher incident energies, efficient transfer of translational to vibrational energy occurs in those ground-state D2 molecules that survive the collision without dissociating. These data will allow a better test of the various potential-energy curves that have been proposed for this system in the dissociative region.

Fano plots for the slow and fast groups of excited hydrogen atoms produced in e-H 2 collisions

Excited hydrogen atoms ( n = 3, 4, 5) produced in the electron-impact dissociation of hydrogen molecules have two groups; slow and fast. The Balmer α, β, and γ lines, obtained at high optical resolution, have been separated into two groups and the emission cross sections of each group have been determined for electron energies of 30–2000 eV. Their Fano plots indicate that the slow group is produced through optically allowed transitions and that the fast group is produced through optically forbidden transitions.

High-resolution multiphoton ionization and dissociation of H2 and D2 molecules in intense laser fields.

We report on the multiphoton ionization and dissociation of hydrogen and deuterium molecules in intense (8\ifmmode\times\else\texttimes\fi{}${10}^{11--}$4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ W/${\mathrm{cm}}^{2}$) laser fields. The investigations were performed with the second harmonics of two different laser systems, a 50-ps neodymium-doped yttrium lithium fluoride and a 10-ns neodymium-doped yttrium aluminum garnet laser. Measurements included energy-resolved photoelectron and mass spectroscopy. In addition, we report on a measurement of the dynamics of different above-threshold dissociation branching ratios for ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{D}}_{2}^{+}$ over a wide range of laser intensities. Our results support the dressed-molecule picture at least qualitatively, but differences do exist with the quantitative predictions.

Compact radio-frequency glow-discharge atomic hydrogen beam source

An atomic hydrogen beam source, compatible with ultrahigh vacuum and clean surface conditions, has been developed. Hydrogen atoms (H0) of thermal energy distribution are produced by dissociation of hydrogen molecules in a radio-frequency (rf) glow discharge. The resulting mixture of atoms and molecules is allowed to flow from the discharge vessel to the vacuum test chamber through a capillary, resulting in the formation of a beam. Dissociation fractions [ 1/2 H0/(H2+ 1/2 H0)] of >0.5 and H0 fluxes of >2×1017 H0/s were achieved. Locating the rf power source adjacent to the discharge vessel resulted in a compact device which is easy to maintain and operate.

Surface hydrogen–palladium studies using a new photopyroelectric detector

A new hydrogen gas trace detection principle based on a photopyroelectric solid-state sensor has been developed. The sensor was made of thin commercially available polyvinylidene fluoride (PVDF) pyroelectric film, sputter coated with palladium (Pd). An infrared laser beam (800 nm) served to produce ac voltages due to the photopyroelectric effect. Exposure to hydrogen gas was shown to produce a differential signal between the Pd and reference electrodes. Accumulated experimental evidence has led us to tentatively attribute the detection principle to the adsorption, dissociation, and absorption of hydrogen molecules on the Pd surface, which caused a shift on the PVDF pyroelectric coefficient, due to electrostatic interactions of H+ ions with the PVDF polar molecular system at the Pd–PVDF interface. A quantitative interpretation of the hydrogen partial pressure dependence of the differential signal has been achieved using simple gas-solid interaction theory and the combination of the Langmuir isotherm with photopyroelectric theory. The new detector could become an excellent tool for surface science studies under ultrahigh vacuum, especially at low temperatures where other sensors do not exhibit good performance.

Cross Sections and Related Data for Electron Collisions with Hydrogen Molecules and Molecular Ions

Data are compiled and evaluated for collision processes of excitation, dissociation, ionization, attachment, and recombination of hydrogen molecules and molecular ions (H+2, H+3) by electron impact as well as for properties of their collision products.

Reaction at traps involving two diffusing species: Application to atomic–molecular hydrogen conversion at defects

Kinetics of diffusion-limited reactions are derived for the case in which there are two mobile species A and M which can convert to one another at a trap or catalysis site. At the same time the standard reaction of A getting trapped and detrapped is allowed to compete with the A⇄M reaction. Nonlinearities associated with trap saturation and the model where M is a diatomic molecule of A are included. Rate equations are derived for the bulk concentrations of A and M. Excellent agreement is obtained with simulation data when account is taken of the correlation between trap occupation and concentration immediately outside the trap. The equations are specifically designed for trapping, recombination, and dissociation of hydrogen atoms and molecules at defects; but they should have a broad range of applicability so that problems with several species and reactions can be handled with the same degree of accuracy as the Smoluchowski theory for trapping and detrapping of a single species. Results are accurate and general enough to include the nonlinearities introduced by trap saturation and trap-concentration-dependent rate constants.

チタン薄膜と水素・酸素の反応カイネティクス

Thin oxide layers are formed on metal surfaces at ambient temperatures even at low oxygen residual gas pressures in vacuum systems or in inert gas atmospheres. Such oxide layers impede or prevent hydrogen absorption at ambient temperatures. To gain a fundamental understanding of the reaction mechanisms, reaction kinetics of hydrogen and oxygen absorption by titanium films were measured by using the Wagener volumetric method. Theoretical approaches to explain the effects observed are discussed. At the initial stage of the oxidation, ion transport by the Mott potential produced by electron tunneling controls the reaction probability. The pressure dependence of the hydrogen absorption rate indicates that the rate determining step of the reaction is either the dissociation of hydrogen molecules on the surface or the permeation or diffusion of hydrogen atoms in the film.

Hydrogen Absorption in Alkali-Metal-Graphite Intercalation Compounds and Their Solid State Properties

Alkali-metal-graphite intercalation compounds absorb hydrogen between graphitic layers through two kinds of absorption mechanisms; physisorption andchemisorption. Below about 200K, large amount of hydrogen molecules are occluded physisorptively, while, above this temperature, chemisorption takes place where hydrogen is accommodated in the gallaries of graphitic sheets through dissociation of hydrogen molecule and charge transfer from the host material. In this article, we discuss the electronic properties and structures of alkali-metal-hydrogen-graphite intercalation compounds prepared through hydrogen chemisorption. Hydrogen chemisorption behaviors depend on the kinds of alkali-metals among K, Rb and Cs. The introduction of hydrogen induces charge transfer from the host alkali-metal-graphite intercalation compounds to hydrogen leading to the enhancement of ionicity in the intercalate lattice consisting of alkali-metal and hydrogen and the reduction in the number of graphitic π electron carriers. Especially, in potassium-hydrogen-graphite ternary systems, two-dimensional metallic hydrogen lattice with novel electronic structure is found to exist between graphitic layers.

Operating characteristics and comparison of photopyroelectric and piezoelectric sensors for trace hydrogen gas detection. I. Development of a new photopyroelectric sensor

A new type of solid-state sensor for the detection of minute concentrations of hydrogen gas has been developed. The sensor was made of thin, commercially available polyvinylidene fluoride (PVDF) pyroelectric film, sputter coated with Pd. An infrared laser beam served to produce alternating temperature gradients on the Pd-PVDF and on reference Al-Ni-PVDF films, which, in turn, generated ac voltages due to the photopyroelectric (P2E) effect. Exposure to hydrogen gas was shown to produce an increased differential signal between the Pd and reference electrodes; this was tentatively attributed to the adsorption and dissociation of hydrogen molecules on the Pd surface, which caused a shift on the Pd-PVDF pyroelectric coefficient, due to interactions at the Pd-PVDF interface. The differential signal was found to be proportional to the square root of the hydrogen partial pressure at very low concentrations (<1000 ppm). A semiquantitative interpretation of the differential signal has been achieved using simple gas-solid interaction theory and the combination of the Langmuir isotherm with the photopyroelectric theory in the range of 4–200 Pa. For high pressures (>200 Pa) the paper is limited only to a phenomenological description. The thickness of the palladium layer has been found to play an important role with respect to the signal response. Presently, hydrogen concentrations as small as 40 ppm, in a flowing H2+N2 mixture, have been detected. The influence of gas flow rate has also been studied. Other characteristics such as the response times, the reversibility, and the durability of the Pd-PVDF-P2E hydrogen detector will also be presented.

Dissociation of hydrogen molecules by a first-stage graphite-caesium intercalation compound

A caesium-intercalated graphite compound, CsC 8, which has been identified with the first-stage through the observation of an in situ Raman spectrum, is found to dissociate hydrogen molecules on its surface with the aid of a hydrogen-deuterium equilibrium reaction. The specific rate constants for the reaction over alkali-metal-graphite intercalation compounds are found to be in the order CsC 8>RbC 24>KC 24, RbC 8, CsC 24>KC 8 at around room temperature.

Chemisorption and interaction of hydrogen and oxygen on Ni/SiO 2 catalysts

The interaction at room temperature of hydrogen with preadsorbed oxygen on the surface of a Ni/SiO 2 catalyst was studied using volumetric, low-field magnetic, and infrared spectroscopic measurements. Oxygen was adsorbed ( T = 22 °C) by dissociative chemisorption of O 2 or by decomposition of N 2O according to N 2O(g) → N 2(g) + O(ads). The chemisorption of O 2 always exceeded the monolayer coverage, which was defined as the amount of hydrogen taken up at 22 °C and p H 2 = 30 Torr. With N 2O the monolayer coverage could be surpassed with sufficiently large exposures. After admission of hydrogen at room temperature to samples that previously had taken up more than a monolayer of oxygen, a surface reaction that led to formation of physisorbed H 2O proceeded. Adsorption of H 2 and subsequent surface reaction were found to occur only when sites that could dissociate hydrogen molecules were present; on completely oxidized NiO no hydrogen adsorption was observed at 30 °C.

Hydrogenation of CO over FeTi 1.14O 0.03 after activation

The hydrogenation of CO over the oxygen-stabilized hydrogen-storage alloy FeTi 1.14O 0.03, catalytically activated by thermal treatment in pure hydrogen, was investigated by gas chromatography, X-ray diffractometry, X-ray photoelectron spectroscopy and pressure differential scanning calorimetry, paying attention to active sites and changes in the alloy before and after hydrogenation. It is concluded that hydrogen atoms available when the α solid solution is formed or when the β hydride is formed and decomposed, play a governing role in the hydrogenation, producing hydrocarbons such as CH 4 (predominant), C 2H 4 and n-C 4H 10 above the catalytically active alloy. However, it was also found that the catalytically active alloy is deactivated on repeated hydrogenation. This is caused by the re-formation at the alloy surface of an oxide layer(s) which progressively blocks the more active sites for hydrogenmolecule dissociation.

Surface phenomena in hydrogen absorption kinetics of metals and intermetallic compounds

Oxide layers on metal surfaces impede or prevent hydrogen absorption at temperatures below 400 °C. Passivation can be caused by slow dissociation of hydrogen molecules on the surface, slow transfer of chemisorbed hydrogen atoms into the oxide or slow permeation of hydrogen atoms through the oxide layer. Experiments on the hydrogen absorption kinetics of film samples with known oxygen precoverage yield direct information on reaction models if relevant results of other techniques are taken into account together with inherent thermochemical and atomistic limits. Typical examples are discussed to elucidate this approach for the investigation of complex reaction mechanisms in hydrogen absorption kinetics at room temperature. They indicate that hydrogen dissociation on the surface is a very important partial step in the reaction and plays a substantial role in activation and passivation of hydrogen absorption reactions at room temperature.

A Theoretical Study of Condensation Processes in OMCVD of GaAs

Kinetics of epitaxial growth of GaAs from trimethylgallium (TMG) and arsine in organometallic chemical vapor deposition (OMCVD) have been suggested in the past to occur according to the Langmuir-Hinshelwood (L-H) or Langmuir-Rideal (L-R) mechanism [1–3], where competitive chemisorption of the Ga- and As-containing species is assumed. In contrast, formation of sp3 bonds on the GaAs growth front suggests that the Ga-containing species are less likely to chemisorb onto Assites, while the As-containing species are less likely to chemisorb onto Ga-sites. In addition, an analysis of probable chemical reactions and the unlikely event of homogeneous dissociation of hydrogen molecules indicate that the chemisorption of hydrogen must be included in the growth kinetics. Since H-As and H-Ga bonds have similar characteristics, such hydrogen chemisorption probably occurs on all sites. Thus, a mix of selective (Ga, As) and competitive (H2) chemisorption processes is likely to be present in practice. Furthermore, the presence of chemisorbed hydrogen will alter the surface As bonds, which, in the absence of hydrogen, are known to dehybridize and dimerize[4]. These basic issues have not been addressed in existing OMCVD growth models. Therefore, an analysis of the adsorption and growth processes, in the epitaxy of (100)GaAs is presented in this paper, with particular attention to the above issues.

The role of gas phase reactions, electron impact, and collisional energy transfer processes relevant to plasma etching of polysilicon with H2 and Cl2

Plasma etching of silicon with H2 and Cl2 was simulated with mixtures of H2, Cl2, and SiCl4. The gas phase chemistry was elucidated by varying power and pressure. The cross sections for production of H (3p2P) atoms were probed through the emission response to pressure variations. Addition of rare gases to a hydrogen plasma identified qualitatively major energy transfer paths between the rare gas metastables and the H2/H system. We conclude that dissociation of hydrogen molecules and excitation of hydrogen atoms are separate electron impact collisional events that constitute the main excitation route in a hydrogen plasma. We find further that the role of metastable H atoms in the 2s2S state at 10.19 eV is minor, and that the direct dissociative excitation in a single electron impact at 16.57 eV is insignificant. The occurrence of state specific collisional energy exchange has limiting consequences for any actinometric method.

Study of the velocity distribution in an intense pulsed hydrogen beam

We present the results of measurements of the velocity distributions of particles in a pulsed hydrogen beam obtained from a dissociator with a radio frequency discharge (duration 1.0 ms, repetition rate 1 Hz). It is shown that the hydrogen inside the dissociator is heated up to ∼2800 K, so the thermal dissociation of hydrogen molecules is essential. In order to cool the atoms, the gas was let through a pyrex channel 5 mm in diameter. The cooling channel walls being at room temperature and the channel having a length of 50 mm, we have obtained a supersonic beam of hydrogen atoms with a Mach number M | = 2.7±0.25. When the channel walls were cooled by the flowing liquid nitrogen and the channel was 70 mm long we obtained a beam of cooled atoms with a Mach number M | = 4.14±0.35. The velocity distribution of atoms depends on the power of the rf discharge inside the dissociator and on the gas consumption per pulse, and varies during the discharge pulse. For a temperature of the cooling channel walls T ch = 77 K, a gas consumption N = 3.3×10 17 molecules per pulse and a discharge power of 0.23 kW cm −3, we have obtained an atomic beam with intensity I(0) = (2.8±0.8)×10 20 atoms sr −1 s −1 and a most probable velocity ν MP = (1.97±0.07)×10 5 cm s −1.

RF plasma-driven hydrogen permeation through a biased iron membrane

The steady-state RF plasma-driven hydrogen permeation through an electrically biased iron membrane has been investigated as a function of the bias potential VM for membrane temperatures in the range of 150–400°C. VM has been gradually increased positively from the floating potential of the membrane. The permeation flux decreases when VM increases at low voltages: positive hydrogen ions are repelled. The membrane temperature does not influence this effect measurably.The permeation flux starts to increase when VM is raised higher, i.e. when energetic electrons strike the surface. This phenomenon shows a pronounced temperature dependence — the enhancement is largest for the lowest temperatures. The effect is interpreted in terms of an electron-induced dissociation of hydrogen molecules on the membrane surface.

Kinetics of the reaction H + C2H6 → H2 + C2H5 in the temperature region of 1000 K

A system for the measurement of rate constants for elementary reactions of hydrogen atoms in the temperature region of 1000 K is described. The concentration of hydrogen atoms is controlled by the equilibrium constant for dissociation of hydrogen molecules. The kinetics of the rate of conversion of ethane to ethylene in this system has been studied over the temperature range 876–1016 K. The results show that the rate-controlling step is[Formula: see text]and the value obtained for the rate constant is[Formula: see text](R = 1.987 cal mol−1 deg−1). This value is compared with values obtained from other methods over the temperature range 300–1400 K. Combination with a recent measurement of the rate constant for the reverse reaction yields an experimental value for the equilibrium constant for the reaction.