Form Of Hydrogen Molecules Research Articles

Elucidating reaction equations of TiFx (x = 4,3,2) catalysts for hydrogen storage applications

In the never ending quest for clean energy, storing chemical energy in the form of hydrogen molecules in solid state materials is a premier choice. The release of H2 from the H-storage (absorbent) material is hindered by the on-set of reaction mechanism which depends on the energy barrier. In a search for efficient onset (re)hydrogenation temperature, stimuli like catalysts have drawn more attention in the hydrogen fuel economy. It is laborious to down-select the aspirant candidates by screening their material properties and the thermochemical energy which are required for bond breaking. An understanding on perceptible energies involved in breaking/creating bonds has been obtained using the novel “Interface Reaction tool”. To enlighten the underpinning catalytic reaction mechanism to the absorbent, the energetics of interactions between the catalyst and the solid metal hydrides have been derived. For this test study, a leading catalyst TiFx (x = 4,3, and 2) has been added to the well-studied high gravimetric metal hydrides such as MgH2, Mg(BH4)2, and Mg(AlH4)2. The reaction equation at ambient condition has been validated using the total energies from Kohn-Sham density-functional-theory, and the outcome emphasizes the importance of bonding analysis. Hence, we exemplified the bonding states of all the Ti–F derivatives namely TiF4, TiF3 and TiF2 in an effort to elucidate the role of fluorine for the onset of catalytic reaction. Our detailed analysis of reaction pathways indicate the vitality of TiF4 as an additive before and after the H2 release. Our work pioneers the study on often overlooked reaction possibilities at ambient condition. The calculated reaction energy of less than 40 kJ/mol and 4.64 wt % of H2 release for TiF4 and TiF3 underscores their catalytic activity and meet the system targets set by DoE for the year 2020. This work presents “significant disclosures” on the hydrogen decomposition mechanism and provides a platform for the feasibility study of a new material. This proposes an innovative methodology for predicting the suitability of a new material for efficient production of H2 fuel.

Hydrogen existence state of a hydrogenated amorphous carbon coating and its thermal stability

Hydrogenated amorphous carbon (a-C:H) coatings were deposited by plasma-enhanced chemical vapor deposition at a bias voltage of 740 V. The coated 316 L sample was annealed in 5 Pa argon media. The hydrogen content, chemical bond structure, and morphology of the coating and their evolution during annealing were characterised by elastic recoil detection analysis, mass spectrometry, Fourier transformation infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy, respectively. It was found that the hydrogenated a-C:H coating had ~50 at. % hydrogen. However, a large part of the hydrogen probably exists in the form of hydrogen molecules, some of which was encapsulated in some “closed” voids. Obviously, this special structure is an effective way to improve the sp3 CC fraction in the a-C:H with high H content. The molecular hydrogen could stay in the “closed” voids stably during annealing below 490 °C. For these a-C:H coatings with specific structure, when annealed below 330 °C, the transformation of C sp3 to sp2 was controlled by the so-called active CH bond reaction in the a-C:H coating. Annealed above 330 °C, these active CH bonds were depleted out quickly, and carbon clusters started to grow. At annealing temperatures >490 °C, the rapid transformation of C sp3 to C sp2 was attributed to C sp3 bond breakage and atomic rearrangement induced by overcoming energy barrier.

First-principles studies on K-promoted porous iron oxide catalysts

Porous potassium-promoted iron oxide, with the potassium content of 8 wt% can produce dense hydrogen materials as a catalyst. In this letter, we use density functional theory (DFT) and the generalized gradient approximation (GGA) to study this catalyst. Rhombohedral α-Fe2O3 is used to build a porous model. We randomly put potassium atoms in the clean porous structure and iron-vacant defective structure. The results show that potassium atoms eventually exist in the form of clusters in the two samples. To further study the effect of potassium, clean and two K-promoted models are filled with the same quantity of H2. We obtain that the existence of potassium clusters prevents hydrogen molecules to dissociate into atoms in non-defective porous iron oxide. Most of the hydrogen atoms still exist in the form of hydrogen molecules in three models. The structures of potassium clusters for 2 ≤ N ≤ 8 in the porous model are also discussed, in contrast to the isolated clusters.

Observation of Hydrogen Permeation into Fresh Bearing Steel Surface by Thermal Desorption Spectroscopy

This paper describes an experimental study on permeation of hydrogen into bearing steel through fresh surface with thermal desorption spectroscopy. Sliding experiments using a lapping machine with and without lubricant were conducted to generate fresh surfaces. Desorption spectra were taken before and after sliding, and the surfaces were analyzed with X-ray photoelectron spectroscopy. It was found that oxide and/or hydroxide film was formed on the surface, whereby hydrogen was generated and diffused into the bulk steel. When heated, some of hydrogen in the steel reacted with the surface film to be desorbed as water, while some other hydrogen was desorbed in the form of hydrogen molecules. Hydrogen desorption spectra in combination with water desorption spectra thus provides information not only on hydrogen permeated but also on surface oxide/hydroxide film.

Removal of Co-deposited Layers by Excimer Lasers

The JT-60 co-deposited layers with the thickness of nearly 60 μm were clearly and flatly removed by 10 pulses' KrF irradiation at the fluence of 13.3 J/cm2. The effective absorption coefficient and ablation threshold of the TFTR co-deposited layers for KrF laser were 0.4 μm-1and 4.2 J/cm2, which were a bit larger than those of the JT-60 co-deposited layers (0.3 μm-1 and 2.3 J/cm2) for the same laser. In addition, hydrogen (tritium) was found to be released in the form of hydrogen molecules, and the combination of excimer laser and mass spectrometer or tritium monitor could be a promising technique for analyzing the depth profiles in the co-deposited layers and the substrate near the interface. Furthermore, the formation of cones on the JT-60 co-deposited layers due to local surface melting was observed at the fluence of 0.8 J/cm2 for KrF laser irradiation, whereas strong melting was determined to be the mechanism of the ablative removal of the JT-60 co-deposited layers at fluences greater than the ablation threshold of 2.3 J/cm2.

Hydrogen adsorption/desorption properties of mechanically milled activated carbon

We investigated the hydrogen adsorption/desorption properties of activated carbon that was prepared by mechanical milling in a hydrogen atmosphere at an ambient temperature. Although the hydrogen concentration estimated by inert gas fusion-thermal conductivity method increased with milling time up to 10 h, samples milled for longer than 10 h showed little dependence on milling time. Ultimately, the hydrogen concentration reached about 2.6 wt.%. Thermal desorption mass spectroscopy (TDS) revealed that the desorption spectrum of the hydrogen molecules (mass number=2) in a sample milled for 10 h contained peaks at about 500 and 800 °C. The desorption activation energies of hydrogenated activated carbon at these temperatures were calculated to be 124 and 237 kJ/mol, respectively. These values suggest that the hydrogen in activated carbon probably exists as hydrogen molecules and atoms. The mechanism of hydrogen adsorption by activated carbon may consist of hydrogen adsorption into the pores in the carbon in the form of hydrogen molecules, and a combination of dangling bonds created by mechanical milling, with hydrogen atoms.

Hydrogen bulk retention in graphite and kinetics of diffusion

Molecular hydrogen absorption into graphite has been studied in order to obtain information on the ‘true’ hydrogen diffusion coefficient in graphite, oxygen effect and the mechanism of hydrogen trapping, which should be important issues on estimating hydrogen retention and recycling in plasma facing graphite and CFC. Hydrogen may permeate into a filler grain in the form of hydrogen molecules, diffuse through crystallite boundaries, and may finally be trapped as hydrogen atoms at the edge surface of a crystallite. The diffusion coefficient can be given as D ( m 2/ s)=3.3×10 −10(−1.3 eV/kT) , when the trapping effect does not exist. The simulation with mass balance equations can reproduce change of apparent diffusion coefficients.

Computer simulation of the processing of tritium in exhaust gas from the LHD

A new type of tritium cleanup system that differs from conventional systems in its application of a decomposition-processing vessel has been investigated by means of computer simulation. In the decomposition-processing vessel, tritiated compounds are decomposed so that it is possible to extract the tritium in the form of hydrogen molecules while other elements such as carbon are fixed within the vessel. In our computer simulation of the removal of tritium, a single run of processing was assumed to handle 1 N m 3 of a gaseous mixture that contained 1 GBq tritium and was 90% hydrogen, 5% methane and 5% helium. As a result, three pressure curves, one for hydrogen, a ‘both-sum’ (hydrogen+methane) curve and a ‘total-sum’ (hydrogen+methane+helium) curve, showed strong and marked changes in trend at almost the same elapsed time. This time corresponded to the completion of a single cycle of processing. The changes in trend suggest that the three curves had been dominated in the first cycle by the hydrogen initially contained in the exhaust gas and that, after one cycle, they had come to be dominated by the remaining methane and by the hydrogen generated by methane decomposition. It took about 8.7 h for the concentration of tritium in the gas being processed to fall below the relevant limit.

Outgassing from and deuterium retention in beryllium and Be/C mixed-material plasma-facing components

Thermal desorption spectrometry (TDS) is used to examine the outgassing characteristics of as-received beryllium samples and the deuterium retention and desorption behavior of beryllium following plasma exposure. The samples consist of a hot-pressed powder metallurgy product and a plasma-sprayed product. The number of molecules outgassed from a plasma-sprayed sample is ten times that outgassed from a press-sintered sample and a higher desorption temperature is necessary to outgas a plasma-sprayed sample. The majority of the molecules outgassed from each sample type are in the form of hydrogen molecules and water vapor. In addition, the post-plasma-exposure desorption of deuterium from both products are compared. Following identical plasma exposures at close to room temperature, a factor of ten less deuterium is retained in plasma-sprayed samples, presumably due to the columnar structure of the resultant sprayed material that would have enhanced pathways to the sample surface. After higher surface temperature exposure (≈500°C), the retention in the plasma-sprayed material is a factor of three less than the retention in the pressed-powder material. Finally, the impact on deuterium retention of carbon-containing surface layers formed during the plasma exposure is evaluated.

Development of tritium cleanup system for LHD

Energy is vital for humans and we have been consuming a large amount of fossil fuel especially from the beginning of the industrial revolution. Nowadays its huge consumption has however come to threaten our life and we have to prepare nonfossil fuels, for instance solar energy, biomass energy, nuclear energy and so on. Fusion energy is an unlimited resource and one of the strongest candidates of the future energy source. At the National Institute for Fusion Science (referred to as ‘NIFS’ hereafter), we have constructed a new fusion experimental device called large helical device (referred to as ‘LHD’ hereafter) in 1998. The device will generate a small amount of tritium, as a fusion product. In order to remove it from the exhaust gas, we have designed a tritium cleanup system based on a new concept. This system is mainly composed of a palladium permeater, a decomposer and hydrogen absorbing alloys. It may perfectly recover the tritium from exhaust gas without oxidizing it. This system is applicable for the future needs at fusion power plants. In order to remove tritium discharged from fusion experimental facilities, it is usual to employ a system by which tritiated constituents, in various chemical forms, are entirely converted to a form of water vapor by catalytic oxidation. The water vapor containing tritiated form is then absorbed by molecular sieve (referred to as ‘wet system’ hereafter). However, in the case of LHD, it is not rational to deliberately convert the discharged tritium into the water vapor, because the tritium discharged from LHD is almost in a form of hydrogen molecules. Moreover, the tritium in the form of water vapor affects the human body 18 000 times stronger than that of hydrogen molecules. In accordance with these view points, we have developed another type of tritium cleanup system based on a new concept, in which hydrogen molecules including tritiated ones (HT, DT and T 2) found in the exhaust gas of LHD are directly fixed to hydrogen absorbing alloys. Other impurities such as methane and water vapor, parts of which are tritiated, will be decomposed into each elemental form by the decomposition process and hydrogen molecules, including tritiated constituents from the decomposition (referred to as ‘dry system’ hereafter).

Development of a Tritium Cleanup System for a Large Helical Device Using Nonvolatile Getter Materials

A tritium cleanup system has been conceptually developed for the large helical device (LHD) at the National Institute for Fusion Science. The system is a processing device employed to remove tritium from exhaust gas. In the exhaust gas discharged from the LHD in normal operation, the major part of tritium constituents should be in a form of hydrogen molecules because the fuel used in plasma experiments with the LHD is hydrogen molecules. From this viewpoint, we have designed a tritium cleanup system, which is characterized by tritium being removed and stored in a form of hydrogen molecules with less impurities, like oxygen and carbon, and its decomposition and the separation processes are introduced to convert various tritiated compounds into a form of hydrogen molecules of high purity. Besides these, there is another aspect in that getter materials are applied in both decomposition of tritiated compounds and storage of hydrogen molecules containing tritium.The system design is composed of three essential component parts: a hydrogen separator, a hydrogen absorbing vessel, and a decomposition process vessel. The hydrogen separator and the decomposition process vessel make a process loop repeat to remove hydrogen into a form of hydrogen molecules with less impurities. It is important that “less impurities” means having a less bad influence on hydrogen-absorbing materials used in the storage vessel.We think that the hydrogen separator will be manufactured by employing a palladium hydrogen purifier system, which is available in the marketplace, and the hydrogen storage vessel will also be manufactured by using hydrogen-absorbing alloys like titanium. Thus, the serious problem imposed on us is how to realize the decomposition process vessel. To develop the decomposition process vessel, we thought nonvolatile getter materials were promising and carried out performance tests of methane decomposition by the nonvolatile getter materials, where methane was used because it is hardly decomposed and there is little data for a flowing-gas system.The tritium cleanup system that was designed is presented. Also, a methane decomposition curve with ZrNi alloys used as one of the typical nonvolatile getter materials is shown, and the probability of the realization of the decomposition process vessel is examined.

EELS analysis of SiC crystals under hydrogen and helium dual-ion beam irradiation

Electron microscopy and Electron energy loss spectroscopy (EELS) measurement were performed in SiC irradiated with three kinds of irradiation mode, namely, H 2 + ions, He + ions, and a dual beam of He + and H 2 + ions. Amorphization occurred in SiC at a damage density from 0.2 to 0.4 displacement per atom (dpa) irrespective of ion species. High resolution EELS revealed that a large amount of implanted hydrogen is contained in the form of hydrogen molecules and C–H compound in bubbles. It is inferred that some of hydrogen atoms implanted in SiC are trapped in both Si- and C-sites. Most of the helium atoms implanted are considered to exist in bubbles.

The hydrogen effect in copper

The effect of hydrogen on the physical properties of copper is considered with emphasis on determining which form of hydrogen is responsible for the property change. Three situations in which the properties of copper are known to be affected by hydrogen are (1) thermal charging, (2) cathodic charging and (3) electroless copper plating. It is concluded that the hydrogen effect results from two different causes: (1) the development of high pressure of molecular (gaseous) hydrogen in voids, and (2) the formation of defects such as dislocations due to the precipitation of absorbed atomic hydrogen in the form of hydrogen molecules.

The hydrogen effect in copper

The effect of hydrogen on the physical properties of copper is considered with emphasis on determining which form of hydrogen is responsible for the property change. Three situations in which the properties of copper are known to be affected by hydrogen are (1) thermal charging, (2) cathodic charging and (3) electroless copper plating. It is concluded that the hydrogen effect results from two different causes: (1) the development of high pressure of molecular (gaseous) hydrogen in voids, and (2) the formation of defects such as dislocations due to the precipitation of absorbed atomic hydrogen in the form of hydrogen molecules.

Palladium alloys as hydrogen permeable catalysts in hydrogenation and dehydrogenation reactions

The nature and the concentration of the second component of a binary palladium alloy affects its catalytic activity in hydrogen addition and subtraction reactions with organic compounds. Hydrogen permeation across a palladium alloy catalyst enhances the rate and selectivity of hydrogenation compared with what is observed when the compound being hydrogenated is mixed directly with hydrogen. The principal product of diene hydrogenation when the hydrogen permeates through a membrane catalyst is not a saturated hydrocarbon but the corresponding olefin from which polymers can be synthesized. The rate of hydrogen atom addition to the diene molecules on the palladium alloy surface is higher than the rate of hydrogen atom recombination and desorption in the form of hydrogen molecules.

Detection of charge carriers produced in a uniform field vacuum gap

The dark current which flows upon the application of a high voltage to a vacuum gap and the charged particles produced in that spacing were measured. Charged particles were ejected through a cathode hole drilled along the axis and were detected by an electrometer connected to a collector made of an Au-deposited Mo plate placed behind the cathode. A mass analysis by means of quadrupole mass spectrometer was also carried out to identify the flying charged particles. The anode and cathode are made of different materials to allow the determination of the origin of microparticles on the collector. After measurement the collector plate was removed from the vacuum chamber and placed in an electron probe X-ray microanalyzer to find and identify the microparticles coming from a vacuum gap. From these experiments it was concluded that charged particles are ejected from the vacuum gap continuously in the form of hydrogen molecules and anode material.