Hydrogenation Dehydrogenation Research Articles

Microstructural evolution and improved hydrogenation–dehydrogenation kinetics of nanostructured melt-spun Mg–Ni–Mm alloys

The microstructural evolution of as-quenched ribbons and ball-milled hydrides of the Mg–10Ni–2Mm alloy was studied by TEM. These studies showed a refinement of the microstructures during the applied processing and a nucleation of MmMg 12 intermetallic at the grain boundaries of Mg and Mg 2Ni. The interface between MmMg 12 and Mg 2Ni is semi-coherent, with an ordered repetition of the consistent atomic arrangements. The kinetics of H-absorption/desorption is improved due to the fast hydrogen diffusion in the nanograins, thus, providing paths for H-exchange. TEM studies showed (a) stability of the nano-sized grains in the ball-milled Cu-1000 (the surface velocity of the copper wheel: 1000 rpm) sample that underwent cycling of hydrogen desorption and absorption during heating to 350 °C; (b) formation of MmH 3− x hydride from MmMg 12 and its preferential location at grain boundaries of MgH 2. Clearly, MmH 3− x and Mg 2NiH 4 act as nucleation centres to initiate the formation of MgH 2, thus, promoting hydrogen absorption by the Mg alloys. Pressure–composition–temperature diagrams show the presence of two plateaux, Mg–MgH 2 and Mg 2Ni–Mg 2NiH 4. The MgH 2 plateau showed no hysteresis and practically no slope, while the plateau for Mg 2NiH 4 exhibited both a pronounced hysteresis and a slope, particularly for the nanocrystalline sample. The maximum hydrogen storage capacity of the nanocrystalline sample was higher than that of the microcrystalline one.

Catalytic hydrogenation of alkenes on acidic zeolites: Mechanistic connections to monomolecular alkane dehydrogenation reactions

Brønsted acid sites in zeolites (H-FER, H-MFI, H-MOR) selectively hydrogenate alkenes in excess H 2 at high temperatures (>700 K) and at rates proportional to alkene and H 2 pressures. This kinetic behavior and the De Donder equations for non-equilibrium thermodynamics show that, even away from equilibrium, alkene hydrogenation and monomolecular alkane dehydrogenation occur on predominantly uncovered surfaces via microscopically reverse elementary steps, which involve kinetically-relevant (C–H–H) + carbonium-ion-like transition states in both directions. As a result, rate constants, activation energies and activation entropies for these two reactions are related by the thermodynamics of the overall stoichiometric gas-phase reaction. The ratios of rate constants for hydrogenation and dehydrogenation reactions do not depend on the identity or reactivity of active sites; thus, sites within different zeolite structures (or at different locations within a given zeolite) that favor alkane dehydrogenation reactions, because of their ability to stabilize the required transition states, also favor alkene hydrogenation reactions to the exact same extent. These concepts and conclusions also apply to monomolecular alkane cracking and bimolecular alkane–alkene reaction paths on Brønsted acids and, more generally, to any forward and reverse reactions that proceed via the same kinetically-relevant step on vacant surfaces in the two directions, even away from equilibrium. The evidence shown here for the sole involvement of Brønsted acids in the hydrogenation of alkoxides with H 2 is unprecedented in its mechanistic clarity and thermodynamic rigor. The scavenging of alkoxides via direct H-transfer from H 2 indicates that H 2 can be used to control the growth of chains and the formation of unreactive deposits in alkylation, oligomerization, cracking and other acid-catalyzed reactions.

Effective Gasoline Production Strategies by Catalytic Cracking of Rapeseed Vegetable Oil in Refinery Conditions

Effective Gasoline Production Strategies by Catalytic Cracking of Rapeseed Vegetable Oil in Refinery Conditions

Hydrogen storage and structure variation in Mg/Pd multi-layer film

The hydrogen storage properties of the as-deposited Mg/Pd multi-layer films by magnetron sputtering have been studied in this work. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis revealed that the Mg/Pd multi-layer film was composed of fine-crystalline Pd layers and columnar crystalline Mg layers. The cycle hydrogenation and dehydrogenation of the film were performed at different temperatures (305 K, 373 K and 323 K). The film absorbed and desorbed about 2.6 wt.% hydrogen at a low temperature of 373 K, which is close to the theoretical hydrogen absorption value of 2.8 wt.% for Mg 6Pd. When lowering to 323 K, the film can still absorb–desorb about 2.5 wt.% of hydrogen. During hydrogenation–dehydrogenation, Mg 6Pd formed first, followed by MgH 2 and Mg 5Pd 2. After desorption there were only Mg 6Pd and Mg 5Pd 2 phases identified in the film, showing complete dehydrogenation of the thin film at 323 K. It is believed that additional interfacial free energy in the thin film and the catalytic effect of Pd, can be accounted for the low temperature hydrogen absorption–desorption cycles.

Effect of thermo hydrogen treatment on lattice defects and microstructure refinement of Ti6Al4V alloy

An investigation of a rolled Ti6Al4V alloy after thermo hydrogen treatment was performed. The effect of hydrogen content on the types and amount of lattice defects, the microstructure refinement after hydrogenation–dehydrogenation processing and the refining mechanisms were studied. The results show that the types of defects are at first vacancies and dislocations, and then they are mainly dislocations with increasing of hydrogen content. The amount of defects increases gradually with increasing of hydrogen content. After thermo hydrogen treatment, the rolled microstructure of Ti6Al4V alloy is refined.

Property Evaluation of Ti Powder and Its Sintered Compacts Prepared by Ti Scrap

In this study, Ti powders were fabricated from Ti scrap by the Hydrogenation-Dehydrogenation (HDH) method. The Ti powders were prepared from the spark plasma sintering (SPS) and their microstructure was investigated. Hydrogenation reactions of Ti scrap occurred at near <TEX>$450^{\circ}C$</TEX> with a sudden increase in the reaction temperature and the decreasing pressure of hydrogen gas during the hydrogenation process in the furnace. The dehydrogenation process was also carried out at <TEX>$750^{\circ}C$</TEX> for 2 hrs in a vacuum of <TEX>$10^{-4}$</TEX> torr. After the HDH process, deoxidation treatment was carried out with the Ca (purity: 99.5%) at <TEX>$700^{\circ}C$</TEX> for 2 hrs in the vacuum system. It was found that the oxidation content of Ti powder that was deoxidized with Ca showed noticeably lower values, compared to the content obtained by the HDH process. In order to fabricate the Ti compacts, Ti powder was sintered under an applied uniaxial punch pressure of 40 MPa in the range of <TEX>$900-1200^{\circ}C$</TEX> for 5 min under a vacuum of <TEX>$10^{-4}$</TEX> torr. The relative density of the compact was 99.5% at <TEX>$1100^{\circ}C$</TEX> and the tensile strength decreased with increasing sintering temperature. After sintering, all of the Ti compacts showed brittle fracture behavior, which occurred in an elastic range with short plastic yielding up to a peak stress. Ti improved the corrosion resistance of the Ti compacts, and the Pd powders were mixed with the HDH Ti powders.

Comprehensive Study of Melt Infiltration for the Synthesis of NaAlH4/C Nanocomposites

In the search for suitable solid state hydrogen storage systems, NaAlH4 (7.4 wt % H2) holds great promise due to its suitable thermodynamical properties. However, hydrogen release and uptake are hampered by high activation energies, most likely due to solid state mass transfer limitations. A recent strategy to improve the hydrogen de- and rehydrogenation properties of NaAlH4 is to reduce the particle size to the nanometer scale. We prepared high loadings of nanosized NaAlH4 confined in the pores of a carbon support by melt infiltration. XRD, nitrogen physisorption, high pressure DSC and solid-state NMR are used to evidence that the molten NaAlH4 infiltrates the carbon support, and forms a nanosized NaAlH4 phase lacking long-range order. The confined NaAlH4 shows enhanced hydrogen dehydrogenation properties and rehydrogenation under mild conditions that is attributed to the nanosize and close contact to the carbon matrix.

Application of Thermohydrogen Processing for Formation of Ultrafine Equiaxed Grains in Near α Ti600 Alloy

In this article, the grain refinement of the near α Ti600 alloy by thermohydrogen processing (THP) is investigated. The THP processing includes hydrogenation, high-temperature deformation, and vacuum dehydrogenation. The microstructural evolution in each stage of THP processing was carefully investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The experimental results show that face-centered-cubic (fcc) hydride (δ phase) precipitates in the hydrogenated Ti600 alloy at the hydrogen contents of 0.31 wt pct and above. The critical role of hydrides in the processing of grain refinement is clearly demonstrated. The hydrides divided the α grains in hydrogenation treatment, promoted α-phase recrystallization, and facilitated the stabilization of finer α grain size in isothermal compression.

Determination of the Phase Behavior of (LiNH2)c(LiBH4)1−c Quaternary Hydrides through in Situ X-ray Diffraction

Solid hydride materials combining high gravimetric and volumetric hydrogen capacity, rapid hydrogenation and dehydrogenation kinetics, and low operating temperature are highly desirable for hydroge...

Development of the HYD route of hydrodesulfurization of dibenzothiophenes over Pd–Pt/ γ-Al 2O 3 catalysts

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) on a series of Pd–Pt/ γ-Al 2O 3 catalysts was studied as a function of the Pd/(Pd + Pt) molar ratio at different reaction conditions. It was determined that the catalytic performance in HDS is controlled by the Pd/(Pd + Pt) molar ratio. In particular, a synergetic effect exists rather in the development of the hydrogenation (HYD) route of HDS of dibenzothiophene than in the activity, but both can be controlled from the Pd/(Pd + Pt) molar ratio. An analysis of the relationship between the mechanism of HYD, analyzed here from a different perspective as that currently presented in the literature, and the surface properties of the bimetallic Pd–Pt alloyed particles indicated that: (i) the development of HYD is in correspondence with the capacity of Pd–Pt to hydrogenate aromatics under HDS environments; (ii) the dispersion of the active phase is not directly related to the performance in HDS. Consequently, other geometric effects are more important to the development of HYD over Pd–Pt; and (iii) Pd δ+ surface species of the Pd–Pt alloyed particles are responsible for the hydrogenation–dehydrogenation steps during HYD, whereas C–S–C bond scission mainly takes place on Pt sites.

Glycerol as solvent and hydrogen donor in transfer hydrogenation–dehydrogenation reactions

Glycerol is employed successfully as a green solvent and hydrogen donor in catalytic transfer hydrogenation–dehydrogenation reactions. The glycerol donates hydrogen to various unsaturated organic compounds under mild reaction conditions and as a solvent, allows easy separation of products and catalyst recycling.

Technology of High Purity Powder Sintering by Ti Scrap Recycling

In this study, Ti powder was fabricated from Ti scrap by the Hydrogenation-Dehydrogenation (HDH) method. Hydrogenation reactions of Ti scrap occurred at near 450 <TEX>$^{\circ}C$</TEX> with a sudden increase in the reaction temperature and the decreasing pressure of hydrogen gas during the hydrogenation process in the furnace. The dehydrogenation process was also carried out at 750 <TEX>$^{\circ}C$</TEX> for 2hrs in a vacuum of <TEX>$10^{-4}$</TEX> torr. After the HDH process, a deoxidation treatment was carried out with the Ca(purity: 99.5) at 700 <TEX>$^{\circ}C$</TEX> for 2hrs in the vacuum system. It was found that the oxidation content of Ti powder that was deoxidized with Ca showed noticeably lower values, compared to the content obtained by HDH process. In order to fabricate Ti compacts, Ti powder was sintered at <TEX>$1100\sim1400^{\circ}C$</TEX> for 2hrs under a vacuum of <TEX>$10^{-4}$</TEX> torr. The relative density of compact was 94.9% at 1300 <TEX>$^{\circ}C$</TEX>. After sintering, all of the Ti compacts showed brittle fracture behavior, which occurred in an elastic range with short plastic yielding up to a peak stress.

Room-temperature hydrogen-induced resistivity response of Pd/Mg–Ni films

The structure and the response of the effective electrical resistivity 〈ρ〉 in hydrogenation–dehydrogenation processes of palladium-coated/magnesium-nickel (Pd/Mg–Ni) films were investigated as functions of Mg-to-Ni ratio, substrate temperature, and thickness of Pd overcoat. Films of noncrystalline structures with various Mg-to-Ni ratios showed prominent hydrogen-(H-)induced switching effect of 〈ρ〉. A film is supposed to contain segregated noncrystalline regions of different Mg-to-Ni ratios. The regions of an Mg-to-Ni ratio close to 2 are responsible for the switching processes. At room temperature, a dehydrogenation process is much slower than a hydrogenation process. Crystallization hindered the H-induced switching effect of 〈ρ〉. The use of a thicker Pd overcoat accelerated the change of 〈ρ〉 in the initial hydrogenating process but diminished the contrast. Results led to some discussions on the mechanisms governing the switching effects.

Mechanochemical methods for the synthesis of new magnesium-based composite materials for hydrogen accumulation

We present a brief overview of works on the synthesis of magnesium hydrides and alloys by traditional methods and analyze mechanical methods for synthesis of these materials. Advantages of reactive milling for the preparation of new efficient hydrogen sorbents based on magnesium are discussed. It is shown that the reaction rate of the mechanochemical synthesis of MgH 2 increases four times as a result of the introduction of additives of intermetallic compounds based on Ti and Zr. In view of the depletability of fossil fuel and greenhouse effects, which affect negatively the ecological situation of the planet, the development of new approaches to the solution of a global power problem becomes particularly urgent. Hydrogen power is one of the most promising alternative trends. The calorific value of hydrogen is 142 MJ / kg, and in terms of this index, it is three times more efficient than that of gasoline. Since the sole combustion product of hydrogen is water, it is an absolutely pollution-free fuel. However, on our planet, hydrogen practically does not occur naturally in the pure form. The main source of the starting material for hydrogen is water, the resources of which are practically unlimited (though hydrogen is also obtained as a by-product in a number of industrial chemical, metallurgical, and other processes) on the Earth. The extracted gaseous hydrogen must be stored, transported, and burned, efficiently transforming and using the released energy. Compact hydrogen storage is one of the most important problems. In this case, the method of chemical binding of hydrogen in metal hydrides is efficient and safe (in contrast to using compressed hydrogen). In these compounds, large volumes of hydrogen can be accumulated and released back under insignificant changes in temperature and pressure. A substantial disadvantage of metal hydride materials base on alloys of titanium, zirconium, and rare-earth metals (REMs) is an insignificant hydrogen capacity by weight, which is no larger than 2 wt. %, whereas, according to assessments of experts, for hydrogen storage systems in transport, a hydrogen capacity of at least 5 wt. % is required. This is why, in the world, the interest in hydrides of light metals, namely lithium, aluminum, magnesium, and other compositions based on them, has increased. Magnesium hydride, which has a reverse hydrogen capacity of 7.6 wt. % and is much cheaper than other metal hydride materials, is of particular interest. However, the practical use of magnesium hydride in hydrogen storage systems is impossible because of a number of substantial disadvantages. These are high temperatures/pressures (> 300°C and 5 – 10 MPa, respectively) and low sorption–desorption rates, complexity of activation, and degradation of properties in cycling. Investigations performed in the last decade showed that mechanical treatment of magnesium and its hydride in high-energy ball mills is a promising method for the activation and acceleration of hydrogenation–dehydrogenation processes. The use of hydrogen as a process atmosphere for the mechanochemical synthesis of hydride ma

Sintering Characterization of Ti Powder Prepared by HDH Process

In this study, Ti powder was fabricated from Ti scrap by a hydrogenation-dehydrogenation (HDH)method. The Ti powders were compacted by Spark plasma sintering (SPS) and the microstructure andmechanical properties of the powders were investigated. A hydrogenation reaction of Ti scrap occurred attemperatures near 450oC with a sudden increase in the reaction temperature and a decrease in the pressureof the hydrogen gas as measured in a furnace during the hydrogenation process. In addition, a dehydrogenationprocess was carried out at 750oC for 2hrs in a vacuum of 10-4torr. The Ti powder sizes obtained byhydrogenation-dehydrogenation and mechanical milling processes were in the range of 1~90µm and 1~100µm,respectively. To fabricate Ti compacts, Ti powders were sintered under an applied uniaxial punch pressure of40 MPa at in a range of 900~1200oC for 5 min. The relative density of a SPSed compact was 99.6% at 1100oC,and the tensile strength decreased with an increase in the sintering temperature. However, the hardnessincreased as the sintering temperature increased.

Hydrogen storage in Pd capped thermally grown Mg films: Studies by nuclear resonance reaction analysis

The paper reports the studies on reversible hydrogen storage characteristics of Pd/Mg/Si (glass) films by depth profiling hydrogen using 6.44 MeV resonance of 1H( 19F,αγ) 16O nuclear reaction. These bilayered coatings wherein Mg films are prepared by thermal evaporation, exhibit excellent hydrogenation–dehydrogenation characteristics. Mg films absorb 6.0–7.0 wt.% hydrogen on hydrogenation under 0.15 MPa hydrogen gas pressure in 348–423 K temperature range for ≤5 h and undergo complete dehydrogenation in 3 h at 373 K under dynamic vacuum. Pd(40 nm)/Mg(250 nm) films perform satisfactorily up to two cycles of hydrogenation (348 K, 4 h) and dehydrogenation (373 K, 3 h), however, complete hydrogen release is affected in the third cycle. Hydrogen depth profiles suggest that hydriding starts at Pd/Mg interface and subsequently proceeds further into the interiors of the films with the formation of hydrogen deficient non-stoichiometric magnesium hydride as an intermediate step. The hydrogenated films exhibit room-temperature stability that depends on the temperature and duration of hydrogenation. The formation of metastable magnesium hydride may be responsible for the observed (de)hydrogenation characteristics of the films.

Safe handling experience of a tritium storage bed

In an ITER facility, about 3 kg of tritium will be stored in about 50 hydride beds. The safe design and operation of tritium storage beds will be one of the most important points for enhancing overall safety in the facility. In the Tritium Process Laboratory at the Japan Atomic Energy Agency, many tritium storage beds with zirconium–cobalt (ZrCo) and uranium (U) have been used with/without self-accountancy measures, and 20 years of safe handling experience has been accumulated. From this experience, the key issues to be considered for the safety design are the effects of tritium decay, such as decay heat transfer and 3He behavior, which can cause an increase in temperature and pressure in the tritide vessel, along with the normal protection against over-temperature, over-pressure and leaks in the metal-hydride bed. Concerning the safety operation, the key issues are the hydrogenation–dehydrogenation cycle sequence under the requirements of the storage system and the emergency action, such as rapid hydrogen recovery and loss of normal cooling function.

A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl

Noble metal catalysts on mesoporous SiO 2 and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation and tercyclohexane (TCH) dehydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO 2 catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation. In view of a long-term stability in TPh hydrogenation and TCH dehydrogenation, a catalyst with relatively low Pt content on the oxidized graphite coal (Sibunit-ox) was the most promising for a hydrogen storage system because Pt agglomeration on the support was found to be much less than Pd agglomeration in the Pd/Sibunit-ox catalyst.

Formation of nanocrystalline structure of Ti–6Al–4V alloy by cyclic hydrogenation–dehydrogenation treatment

Ti–6Al–4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10–20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of As 2O 3 as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm −2 for 3 h in 1 N H 2SO 4(aq) by adding 0.1 g L −1 As 2O 3; (2) β transformation carried out at 850 °C for 1 h in air furnace, followed by a furnace cooling to 590 °C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 °C and 1.0 × 10 −6 Torr for 10 h; (4) repeated the same processes once more.

Acceptorless, intramolecular, alkyl dehydrogenation in the solid-state in a rhodium phosphine complex; reversible uptake of three equivalents of H2 per molecule

Addition of H2 to the phosphine alkene ligated complex [Rh(dppe)(PCyp2Cyp′)][BArF4] 1 (Cyp = cyclo-C5H9; Cyp′ = cyclo-C5H7, ArF = 3,5-(CF3)2C6H3) in the solid state results in hydrogenation of the alkene and uptake of two further equivalents of H2 to afford the dihydride–dihydrogen complex [Rh(dppe)(PCyp3)(H)2(η2-H2)][BArF4] 2. Placing 2 under a vacuum or argon results in sequential loss of H2 and dehydrogenation of one of the cyclopentyl rings to reform 1. The hydrogenation–dehydrogenation cycle has been repeated 5 times without apparent degradation and has been followed by solid-state 31P{1H} NMR spectroscopy. Intermediates on this process have been trapped using acetonitrile to give stable complexes that have been characterised in solution. We have previously shown that this hydrogenation–dehydrogenation process also happens in the solution-state; and evidence is presented that shows that, apart from a subtle difference, the same overall transformation occurs in the solid-state.