Hydride Dissociation Research Articles

SYNTHESIS OF THE Ti-Zr-Ni ALLOYS BY THE “HYDRIDE CYCLE” METHOD

A comprehensive study of the technical parameters and conditions for the synthesis of ternary alloys in the Ti-Zr-Ni system by the “hydride cycle” method was carried out. The influence on the synthesis process of such parameters as: temperature and annealing time, heating rate, cooling conditions, material composition, dispersion, hydrogen content in the hydrides used, the presence of impurities, mixing and pressing methods, and the degree of pressing of the starting components was determined. The alloys of the following compositions were synthesized and investigated: Ti40.5Zr31.9Ni27.6, Ti41.5Zr41.5Ni17, Ti40Zr40Ni20, Ti44Zr40Ni16. The optimal technological parameters and conditions for the synthesis of ternary alloys are determined. It has been established that the key factors in the process of compound formation during hydride dissociation are the dispersion and homogeneity of the initial compacted components. It was found that the synthesis of ternary alloys in the Ti-Zr-Ni system occurs during a short-term exothermic reaction in the “thermal explosion” mode, which begins in the temperature region corresponding to the α↔β polymorphic transformation of zirconium and titanium.

The Trajectory of the (η 5-Cyclopentadienyl)cobalt-Mediated Cycloisomerization of Ene-Yne-Ene-Type Allyl Propargylic Ethers to Furans: A DFT Appraisal

AbstractThe mechanisms by which the complexes CpCoL2 (Cp = C5H5; L = CO or CH2=CH2) mediate the cycloisomerizations of α,δ,ω-enynenes containing allylic ether linkages are probed by DFT methods. The outcomes corroborate experimental results and provide energetic and structural details of the trajectories leading to 3-(oxacyclopentyl or cycloalkyl)furans via the intermediacy of isolable CpCo-η 4-dienes. They comprise initial stereoselective complexation of one of the double bonds and the triple bond, rate-determining oxidative coupling to a triplet 16e cobalta-2-cyclopentene, and terminal double bond docking, followed by stereocontrolled insertion to assemble intermediate cis- and trans-fused triplet cobalta-4-cycloheptenes. A common indicator of the energetic facility of the latter is the extent of parallel alignment of the alkene moiety and its target Co–Cα bond. The cobalta-4-cycloheptenes transform further by β-hydride elimination–reductive elimination to furnish CpCo-η 4-dienes, which are sufficiently kinetically protected to allow for their experimental observation. The cascade continues through cobalt-mediated hydride shifts and dissociation of the aromatic furan ring. The findings in silico with respect to the stereo-, regio-, and chemoselectivity are in consonance with those obtained in vitro.

Hydrogen desorption from alloys Mg–Cu(–KCl): Cu catalysis in detail

Effect of chemical composition of Mg-xCu based alloys (x = 9.94–58.00 wt %) modified by KCl upon their hydrogen storage performance was studied. Kinetic curves and pressure-concentration isotherms were measured in the ranges up to 60 bar and 388 °C, respectively. It was observed that desorption rate dc/dt is not significantly influenced by the composition. Unknown Cu-rich phase was detected that has shown a catalytic effect on desorption from a mixture with other phases. Activation energy of hydrogen desorption decreased with increasing x from 180 kJ/mol down to 98 kJ/mol. Average hydride dissociation enthalpy, ΔH, for the lowest plateau was 75 kJ/mol which is equal to literature value for pure Mg. Slightly lover average value, 67 kJ/mol was obtained for the second plateau and ΔH for the third one decreased from 70 kJ/mol for the lowest to 49 kJ/mol for the highest x.

Stoichiometric and Catalytic (η 5-Cyclopentadienyl)cobalt-Mediated Cycloisomerizations of Ene-Yne-Ene Type Allyl Propargyl Ethers

The complexes CpCoL2 (Cp = C5H5; L = CO or CH2=CH2) mediate the cycloisomerizations of α,δ,ω-enynenes containing allylic ether linkages to 3-(oxacyclopentyl or cycloalkyl)furans via the intermediacy of isolable CpCo-η 4-dienes. A suggested mechanism comprises initial complexation of the triple bond and one of the double bonds, then oxidative coupling to a cobalt-2-cyclopentene, terminal double bond insertion to assemble a cobalta-4-cycloheptene, β-hydride elimination, and reductive elimination to furnish a CpCo-η 4-diene. When possible, the cascade continues through cobalt-mediated hydride shifts and dissociation of the aromatic furan ring. The outcome of a deuterium labeling experiment supports this hypothesis. The reaction exhibits variable stereoselectivity with a preference for the trans-product (or, when arrested, its syn-Me CpCo-η 4-diene precursor), but is completely regioselective in cases in which the two alkyne substituents are differentiated electronically by the presence or absence of an embedded oxygen. Regioselectivity is also attained by steric discrimination or blocking one of the two possible β-hydride elimination pathways. When furan formation is obviated by such regiocontrol, the sequence terminates in a stable CpCo-η 4-diene complex. The conversion of the cyclohexane-fused substrate methylidene-2-[5-(2-propenyloxy)-3-pentynyl]cyclohexane into mainly 1-[(1R*,3aS*,7aS*)-7a-methyloctahydroinden-1-yl]-1-ethanone demonstrates the potential utility of the method in complex synthesis.

Using SCF metadynamics to extend density matrix embedding theory to excited states.

A new framework based on density matrix embedding theory (DMET) capable of directly targeting excited electronic states is proposed and implemented. DMET has previously been shown to be an effective method of calculating the ground state energies of systems exhibiting strong static correlation but has never been applied to calculate excited state energies. In this work, the Schmidt decomposition is applied directly on excited states, approximated by higher lying self-consistent field solutions. The DMET prescription is applied following this Schmidt decomposition allowing for a direct embedding of excited states. Initial results are obtained for a system of multiple hydrogen dimers and the lithium hydride dissociation. We analyze the nature of each part of the excited state DMET calculation and identify challenges. These challenges to the implementation of excited state DMET are discussed, and potential suggestions moving forward are recommended.

Enhanced hydrogen sorption kinetics of co-doped MgH2 hydrides

Double substituted MgH2 sorption kinetic is studied by combining density functional theory (DFT) and Kinetic Monte Carlo (KMC) simulations. The DFT calculations were performed to compute the energy barriers of the relevant elementary processes, which will be used as a database for the KMC simulations. The study provides a discussion of the mechanism behind the hydrogen reaction path on magnesium hydride (adsorption, dissociation, surface migration, penetration, and diffusion) as we took into account the density distribution of hydrogen atoms as well as the filling ratios, the diffusion time and finally the temperature. Based on the obtained results, the double substitution not only improves MgH2 stability and desorption temperature but also its adsorption and desorption kinetics. Among the studied systems Mg14ZnLiH32 exhibits the best results with the fastest ab/desorption kinetic, in addition to its ideal heat of formation ΔH=-38.27 kJ/mol and high gravimetric capacity 7.25 wt%.

Hydrogen accumulation and distribution during the saturation of a VT1-0 titanium alloy by an electrolytic method and from a gas atmosphere

The accumulation, distribution, and thermally stimulated release of hydrogen in a VT1-0 titanium alloy during electrolytic saturation and gas-phase saturation are studied. After electrolytic saturation, a 0.4-μm-thick surface layer consisting of δ hydrides with a binding energy of 108 kJ/mol forms in the alloy. The hydride dissociation after electrolytic saturation in heating occurs in the temperature range 320–370°C. After saturation from a gas atmosphere, δ hydrides with a binding energy of 102 kJ/mol form throughout the alloy volume. The dissociation of the hydrides formed during gas-phase saturation in heating occurs in the temperature range 520–530°C. A further increase in the temperature is accompanied by the transformation of titanium from the α into the β modification. At 690–720°C, the phase transformation is completed, and another hydrogen desorption peak appears in a thermally stimulated hydrogen desorption spectrum.

Hydride Dissociation Energies of Six-Membered Heterocyclic Organic Hydrides Predicted by ONIOM-G4Method

Hydride dissociation energy is of great importance in understanding the hydride-donating abilities of organic hydrides. Although the hydride dissociation energies of some organic hydrides have been experimentally measured, much less attention has been focused on the investigation of these quantities from the first principles of physics. Herein, we developed an ONIOM-G4 method and carefully benchmarked this new method against 48 experimental hydride dissociation energies of diverse bulky molecules. It was found that with the combined methods of the HF/6-31+G(d,p)//IEFPCM/Bondi1.15 solvation model, the ONIOM-G4 method can predict the hydride dissociation energies with an error bar of only 1.7 kcal/mol. With the newly developed ONIOM-G4 method, we then systematically studied the hydride dissociation energies of six categories of biologically and pharmaceutically important six-membered heterocyclic organic hydrides, namely, the organic hydrides containing 1,4-dihydropyridine, 1,4-dihydropyrazine, 1,4-oxazine, 1,4-thiazine, 4H-pyran, and 4H-thiopyran ring structures. An extensive hydride dissociation energy scale containing over 100 six-memebered heterocyclic organic hydrides has been established, which may find applications in both synthetic organic chemistry and mechanistic studies of various chemical or biological processes involving transferring of the hydride anion.

Infrared spectra of CH3–MH, CH3–M, and CH3–MH− prepared via methane activation by laser-ablated Au, Ag, and Cu atoms

Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to generation of CH(3)-MH, CH(3)-M, and CH(3)-MH(-), which are identified in the product infrared spectra on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal that C-H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or electron addition also follows. Each type of product has similar photochemical properties regardless of the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the insertion complexes. The methyl metal species have the shortest C-M bonds, consistent with their highest calculated effective bond order, and the CH(3)-MH complexes have higher electron affinities than the metal atoms.

Conversion of 1-tetralone over HY zeolite: An indicator of the extent of hydrogen transfer

The conversion of pure 1-tetralone and its mixtures with n-decane, decalin, tetralin, or 1,5-dimethyl tetralin (DMT) has been investigated over HY zeolite. The dominant reactions undergone by 1-tetralone are the dehydrogenation to 1-naphthol and the subsequent isomerization to 2-naphthol. In the presence of hydrocarbons, the hydrogen transfer/dehydration of naphthols is accelerated, and naphthalene is formed in different amounts, depending on the nature of the co-fed hydrocarbon. In this contribution, it is demonstrated how the product distribution from the tetralone conversion can be used as an indicator of the hydrogen transfer ability of a particular hydrocarbon, or mixture of hydrocarbons. The relative order of hydrogen transfer ability of the various hydrogen donating compounds, as inferred from the naphthalene-to-naphthol product ratio, is DMT > tetralin ≈ decalin > n-decane. This trend agrees well with the hydride dissociation energy of individual donors calculated by DFT.

A DFT and ab Initio Benchmarking Study of Metal−Alkane Interactions and the Activation of Carbon−Hydrogen Bonds

Density functional theory and ab initio methods have been used to calculate the structures and energies of minima and transition states for the reactions of methane coordinated to a transition metal. The reactions studied are reversible C-H bond activation of the coordinated methane ligand to form a transition metal methyl hydride complex and dissociation of the coordinated methane ligand. The reaction sequence can be summarized as L(x)M(CH(3))H <==> L(x)M(CH(4)) <==> L(x)M + CH(4), where L(x)M is the osmium-containing fragment (C(5)H(5))Os(R(2)PCH(2)PR(2))(+) and R is H or CH(3). Three-center metal-carbon-hydrogen interactions play an important role in this system. Both basis sets and functionals have been benchmarked in this work, including new correlation consistent basis sets for a third transition series element, osmium. Double zeta quality correlation consistent basis sets yield energies close to those from calculations with quadruple-zeta basis sets, with variations that are smaller than the differences between functionals. The energies of important species on the potential energy surface, calculated by using 10 DFT functionals, are compared both to experimental values and to CCSD(T) single point calculations. Kohn-Sham natural bond orbital descriptions are used to understand the differences between functionals. Older functionals favor electrostatic interactions over weak donor-acceptor interactions and, therefore, are not particularly well suited for describing systems--such as sigma-complexes--in which the latter are dominant. Newer kinetic and dispersion-corrected functionals such as MPW1K and M05-2X provide significantly better descriptions of the bonding interactions, as judged by their ability to predict energies closer to CCSD(T) values. Kohn-Sham and natural bond orbitals are used to differentiate between bonding descriptions. Our evaluations of these basis sets and DFT functionals lead us to recommend the use of dispersion corrected functionals in conjunction with double-zeta or larger basis sets with polarization functions for calculations involving weak interactions, such as those found in sigma-complexes with transition metals.

Kinetic Monte Carlo simulations of surface growth during plasma deposition of silicon thin films

Based on an atomically detailed surface growth model, we have performed kinetic Monte Carlo (KMC) simulations to determine the surface chemical composition of plasma deposited hydrogenated amorphous silicon (a-Si:H) thin films as a function of substrate temperature. Our surface growth kinetic model consists of a combination of various surface rate processes, including silyl (SiH(3)) radical chemisorption onto surface dangling bonds or insertion into Si-Si surface bonds, SiH(3) physisorption, SiH(3) surface diffusion, abstraction of surface H by SiH(3) radicals, surface hydride dissociation reactions, as well as desorption of SiH(3), SiH(4), and Si(2)H(6) species into the gas phase. Transition rates for the adsorption, surface reaction and diffusion, and desorption processes accounted for in the KMC simulations are based on first-principles density-functional-theory computations of the corresponding optimal pathways on the H-terminated Si(001)-(2x1) surface. Results are reported for two types of KMC simulations. The first employs a fully ab initio database of activation energy barriers for the surface rate processes involved and is appropriate for modeling the early stages of growth. The second uses approximate rates for all the relevant processes to account properly for the effects on the activation energetics of interactions between species adsorbed at neighboring surface sites and is appropriate to model later stages of growth toward a steady state of the surface composition. The KMC predictions for the temperature dependence of the surface concentration of SiH(x(s)) (x = 1,2,3) species, the surface hydrogen content, and the surface dangling-bond coverage are compared to experimental measurements on a-Si:H films deposited under operating conditions for which the SiH(3) radical is the dominant deposition precursor. The predictions of both KMC simulation types are consistent with the reported experimental data, which are based on in situ attenuated total reflection Fourier transformed infrared spectroscopy.

On the growth mechanism of plasma deposited amorphous silicon thin films

We propose a comprehensive mechanism of amorphous silicon thin film growth by plasma deposition based on results of kinetic Monte Carlo simulations according to a database constructed by first-principles density functional theory calculations. The growth mechanism consists of various surface kinetic events including radical-surface and adsorbed radical-radical interactions, radical-surface diffusion, and surface hydride dissociation reactions. Of particular importance is the radical dissociative adsorption mediated by Si over-coordination defects along the reaction pathway. The proposed mechanism explains fully the experimentally measured surface composition of plasma deposited films under conditions that make the silyl radical the dominant deposition precursor.

Mechanisms and energetics of hydride dissociation reactions on surfaces of plasma-deposited silicon thin films

We report results from a detailed analysis of the fundamental silicon hydride dissociation processes on silicon surfaces and discuss their implications for the surface chemical composition of plasma-deposited hydrogenated amorphous silicon (a-Si:H) thin films. The analysis is based on a synergistic combination of first-principles density functional theory (DFT) calculations of hydride dissociation on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of adsorbed SiH(3) radical precursor dissociation on surfaces of MD-grown a-Si:H films. Our DFT calculations reveal that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.40-0.55 eV. The presence of dangling bonds (DBs) results in lowering the activation barrier for hydride dissociation to 0.15-0.20 eV, but such DB-mediated reactions are infrequent. Our MD simulations on a-Si:H film growth surfaces indicate that surface hydride dissociation reactions are predominantly mediated by fivefold coordinated surface Si atoms, with resulting activation barriers of 0.35-0.50 eV. The results are consistent with experimental measurements of a-Si:H film surface composition using in situ attenuated total reflection Fourier transform infrared spectroscopy, which indicate that the a-Si:H surface is predominantly covered with the higher hydrides at low temperatures, while the surface monohydride, SiH((s)), becomes increasingly more dominant as the temperature is increased.

First-principles theoretical analysis of sequential hydride dissociation on surfaces of silicon thin films

We report results from first-principles density functional theory calculations for the mechanism and activation barrier of sequential hydride dissociation on crystalline silicon surfaces and discuss their implications for the surface chemical composition of hydrogenated amorphous silicon (a-Si:H) thin films. We find that, in the presence of fivefold coordinated surface Si atoms, surface trihydride species dissociate sequentially to form surface dihydrides and surface monohydrides via thermally activated pathways with reaction barriers of 0.15–0.55eV. The results are consistent with our molecular-dynamics simulations on a-Si:H film growth surfaces and with experimental measurements of a-Si:H film surface composition.

Generation, Characterization, and Electrochemical Behavior of the Palladium–Hydride Cluster [Pd3(dppm)3(μ3‐CO)(μ3‐H)]+ (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd(3)(dppm)(3)(mu(3)-CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+). This new palladium-hydride cluster has been characterised by (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO(2) (-) and [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd(3)(dppm)(3)(mu(3)-CO)](2+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd--Pd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd--H bond length remains relatively the same.

Formation and dissociation of MgH2 in epitaxial Mg thin films

We have studied hydride formation and dissociation in epitaxial and textured Mg films capped with Pd. We grew epitaxial Mg in the (001) orientation on (001) Al2O3 and in the (110) orientation on (320) LiGaO2; and as a reference, textured (001) Mg on glass. Using x-ray diffraction techniques, we studied the formation of the hydride at 373 K in 0.6 MPa of H2 gas, and the dissociation of the hydride in air at 355 K. We show that the hydride forms epitaxially relative to the Mg; we find the epitaxial relationship to be MgH2(110)[001]∕∕Mg(001)[100] on Al2O3 and MgH2(200)[001]∕∕Mg(110)[1¯11] on LiGaO2. We provide evidence showing that during hydride dissociation a portion of Mg recrystallization begins at the Mg∕MgH2 interface, as well as at the MgH2∕Pd interface. Finally, we find that the kinetics of hydrogen absorption and desorption depends on the surface orientation and degree of crystallinity of the original Mg, and that hydrogen diffusion is an important factor governing desorption kinetics.

New high concentration–high temperature hydrogenation method for slow strain rate tensile tests

The aim of the present paper is to propose an innovative hydrogenation method based on metal hydride dissociation. By heating metal hydride, high pressure of molecular hydrogen is generated, which can be used to promote diffusion through the steel, resulting in very high atomic hydrogen concentration. This new hydrogenation method was tested on a F82H steel, a Reduced Activation Ferritic/Martensitic (RAFM) steel developed for nuclear fusion application, where high temperature Hydrogen Embrittlement (HE) is a matter of serious concern. The main goal of this study is to propose a methodology to perform tensile tests on steel at high temperature and high hydrogen contents. The technique is easy and cheap. Furthermore, by using different metal hydrides and tuning temperatures, different hydrogen concentrations can be tested.

Hydrogenation properties of ThZr2Hx

High-level radioactive wastes generated after reprocessing spent nuclear ftiels from nuclear reactors include long-lived radioactive nuclides. Recently, the transmutation method using actinide hydride targets has been proposed to reduce the amount of actinide in nuclear wastes. One of the major R&D items to establish the transmutation method is development of the actinide hydride target, which can be stably irradiated in reactors. The actinide hydride targets tend to decompose at high temperature and increase the internal pressure of target element. The study of hydride dissociation is important for development of hydride target. The desorption of hydrogen from ThZr2Hx was studied by Thermo Gravimetry (TG) and Differential Thermal Analysis (DTA). The several major endothermic peaks were observed in DTA curve of ThZr2Hx in temperature range from 900 to 1200K. In this paper, these peaks are discussed in relation to stabilities of particular hydrogen occupying sites in crystal structure of ThZr2Hx.

Hydrogen evolution from cathodically charged titanium aluminide alloy Ti–24Al–11Nb

A Ti 3Al-based titanium aluminide alloy, Ti–24Al–11Nb, was cathodically charged with hydrogen in a 5% H 2SO 4 aqueous solution for various charging times, and the formation and dissociation of the hydride, the hydrogen evolution behavior and the total hydrogen uptake were investigated mainly by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). The same kind of hydride phase as observed previously in Ti–25Al alloy (hexagonal hydride) was presumably formed in the Ti–24Al–11Nb alloy after cathodic charging. No damage, such as cracks, was induced by hydrogen charging. Two kinds of TDS peaks, one probably corresponding to hydride dissociation and the other to hydrogen dissolution in the normal lattice site, were found after longer hydrogen charging. It is suggested that niobium addition to Ti 3Al-based titanium aluminide alloy may reduce hydrogen susceptibility during cathodic charging.