A dehydrogenation mechanism of metal hydrides based on interactions between Hdelta+ and H-

This paper describes a reaction mechanism that explains the dehydrogenation reactions of alkali and alkaline-earth metal hydrides. These light metal hydrides, e.g., lithium-based compounds such as LiH, LiAlH4, and LiNH2, are the focus of intense research recently as the most promising candidate materials for on-board hydrogen storage applications. Although several interesting and promising reactions and materials have been reported, most of these reported reactions and materials have been discovered by empirical means because of a general lack of understanding of any underlying principles. This paper describes an understanding of the dehydrogenation reactions on the basis of the interaction between negatively charged hydrogen (H-, electron donor) and positively charged hydrogen (Hdelta+, electron acceptor) and experimental evidence that captures and explains many observations that have been reported to date. This reaction mechanism can be used as a guidance for screening new material systems for hydrogen storage.     

Compensation effect in heterogeneous non-isothermal kinetics

A number of 1145 sets of kinetic parameters derived in our earlier papers from TG curves have been worked up. The apparent activation energy and pre-exponential factor values have been found to obey a linear compensation law (isokinetic relation) if the thermal decomposition begins in the same temperature interval, irrespective of the nature of the chemical reaction. The isokinetic temperatureT _i has been found to be very close to the mean value of the temperaturesT _0.1 at which the conversion becomes equal to 0.1 and atT _i the rate constant has been found to be approximately equal to 10^?3s^?1 in allT _0.1 intervals investigated. It is concluded that the kinetic compensation effect observed in heterogeneous non isothermal TG kinetics is not a true one.     

Electrochemical behavior of metal hydrides

Metal hydride electrodes are of particular interest owing to their potential and practical application in batteries. A large number of hydrogen storage materials has been characterized so far. This paper deals with the effect of the chemical nature and stoichiometry of specific alloy families (AB₅, A₂B, AB/AB₂ and AB₂) on the hydride stability, hydrogen storage capacity and kinetics of hydrogen sorption-desorption in the solid phase/gas and solid phase/electrolyte solution systems. Special attention has been paid towards the electrochemical properties of metal hydrides in terms of their performance in Ni-MH rechargeable alkaline cells. Electronic Publication     

Anomalous site disorder in metal hydrides

We point out that existing measurements in fcc metal hydrides LaH_x and YH_x of intrinsic disorder x₀(2) (octahedral site occupation at the stoichiometric composition x = 2) appear to be highly inconsistent with one another by thermodynamic and spectroscopic techniques. The high-temperature thermodynamic data give x₀(2) ～ 0.01, while NMR, ESR, and neutron scattering have x₀(2) an order of magnitude larger at room temperature, where conventional wisdom indicates it should be less. This is explained by a model in which large-amplitude hydrogen vibrations at high temperature prevent occupation of both a tetrahedral and neighbor octahedral sites. Hence x₀(2) decreases at high temperature, and good agreement is found with reasonable values of the parameters.     

Mechanical properties of metal hydrides

The role of metal hydrides in the hydrogen embrittlement of a number of metal-hydrogen systems is reviewed. The stress-induced formation and cleavage of hydrides at crack tips and the thermodynamics of the hydrides in the stress field of the crack tip are discussed.The mechanical response of the hydrides to applied stresses is described. Many of the hydrides exhibit some plastic deformation under favorable stress systems with the amount of this deformation increasing as the temperature of testing is increased. Phonon properties and the elastic moduli of the hydrides are briefly discussed and it is suggested that these parameters do not provide a basis for understanding the mechanical properties of the hydrides. The deformation response of the hydrides is consistent with the expected behavior of dislocations in the ordered hydride structures.     

The properties of the metal complex hydrides

The essential features (geometries of the minima and of the saddle points, energy barriers) of the potential energy surface of the four hydrides YXH₄ mentioned in the title have been determined with two basis sets, of minimal and DZ quality respectively. The importance of the different extent of the deformation of the XH₄ group in the different structures of the four hydrides is brought out and discussed. The aspects of charge distribution and bonding are examined drawing on population analysis, comparison of the electrostatic molecular potentials and decomposition of the interaction energy (this last referred to the Y⁺ + XH ₄ ^– YXH₄ process). The capability of XH₃ in effecting the etherolytic disruption of the Y-H bond is finally brought out.     

Note on the kinetics of dehydrogenation of cyclohexane

Kinetic data available for Pt and Ni suggest doubts about the acceptance of the method applied by V.I. Anikeev et al. [1] as basis for Kinetic Laws for cyclohexane dehydrogenation on Pd/C catalysts.

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Pt Ni , . . . [1] Pd/C.

     

Solvent effect in the kinetics of cobaloxime(II)-catalyzed oxidative dehydrogenation of 3,5-di-tert-butylcatechol by O2

In the presence of triphenylphosphinecobaloxime(II), 3,5-di-tert-butylcatechol undergoes catalytic oxidative dehydrogenation to the corresponding 1,2-benzoquinone (DTBQ) at room temperature and atmospheric dioxygen pressure. The semiquinone anion radical (DBS^•−) and its cobaloxime(III), complex Co^III(DBSQ)^•−) have been detected as intermediates by ESR spectroscopy. The kinetics were followed in benzene by measuring the dioxygen uptake as a function of time. The reaction is somewhat faster in MeOH, which is due to the greater stability of the hydrogen-bonded intermediate (X) formed from superoxocobaloxime (Co^IIIO₂) and the catechol. H-atom abstraction occurs in the rate-determining decomposition of X. The system investigated is a functional model of catecholase (oxidase) activity, based on free-radical intermediates, a possibility recently demonstrated for certain oxidoreductases.     

Dimethyl ether as circular hydrogen carrier: Catalytic aspects of hydrogenation/dehydrogenation steps

The intermittent nature of renewable resources requires for most applications the development of efficient and cost-effective technologies for steady supply of ...     

Chemistry of metal hydrides : XXI. Insertion of acetylenes into palladium hydrides

trans-Pd(NO₃)H(PCy₃)₂ (I) when treated with Et₄NCl forms the hydride trans-PdClH(PCy₃)₂ which in turn reacts with AgPF₆ in acetonitrile to give trans-[PdH(CH₃CN)(PCy₃)₂]PF₆ (III). Both I and III react smoothly with acetylenes containing one electron-withdrawing group to give alkenyl products. The geometry of the resulting alkenyl ligand implies that cis addition has occurred and that the hydridic hydrogen adds to the acetylenic carbon containing the electron-withdrawing group.Acetylenes containing two electron-withdrawing groups give mixtures from which both alkenyl and zerovalent acetylene compounds can sometimes be isolated. In the presence of proton sponge, monosubstituted acetylenes still give alkenyl products while those substituted with two electron-withdrawing groups give the zerovalent products in good yield. The relevance of these results to an understanding of the nature of the migratory insertion reaction is discussed.     

The multifarious world of transition metal hydrides

Transition metal (TM) hydrides display a remarkable range of bonding types, encompassing classical M-H moieties, dihydrogen complexes containing the eta 2-H2 ligand, and trihydrides which display quantum mechanical site exchange. Furthermore, C-H, Si-H and B-H moieties can bind to TM centres in an eta 2-manner, to give sigma-bond complexes with a spectrum of M...H contributions. In addition to these primary bonding modes, TM complexes also indulge in a wide spectrum of hydrogen-bonding interactions, including both M...H-X and the unique type M-H...H-X. This review begins with a historical perspective of the development of TM hydride chemistry, and proceeds to focus on three significant developments of the past two decades: the discovery of sigma-bond and dihydrogen complexes, the involvement of TM hydrides in hydrogen bonding, and the role played by quantum mechanical phenomena in the chemistry and dynamics of TM hydrides. The account concludes with an overview of the inter-relationship between these apparently disparate novel aspects of TM hydride chemistry.     

Superconductivity of light‐metal hydrides

Hydrogen‐rich materials are potential high‐temperature superconductors at pressures lower than metal hydrogen, mainly because hydrogen atoms can provide strong electron–phonon coupling and high phonon frequencies in hydrogen‐rich materials. This review provides a systematic overview of the crystal type, stability, pressure‐induced transition, metallization and superconductivity of binary light‐metal hydrides under high pressure.     

First-principles study of superabundant vacancy formation in metal hydrides

Recent experiments have established the generality of superabundant vacancies (SAV) formation in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations, we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible. Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal alloy.     

Electron transfer in the reduction of primary halides by metal hydrides

The reactions of various main-group metal hydrides with 1-halo-5-hexenes and with 1-halo-2,2-dlmethyl-5-hexenes produce both straight chain and cyclized reduction products. The formation of cyclic hydrocarbons clearly indicates the presence of radical intermediates during the course of these reactions.     

Rhodium catalyzed dehydrogenation of triorganotin hydrides and mercaptans,preparation of triorganotin mercaptides

Triorganotin hydrides react with a variety of aliphatic and aromatic mercaptans in the presence of a catalytic amount of chlorotris(triphenolphosphine)rhodium to produce triorganotin mercaptides.     

Influence of the fractal nature and dipersity of deposited metal catalysts on the kinetics of the hydrogenation of acetone

The influence of the morphology of deposited catalysts on some kinetic parameters of a heterogeneous catalytic process has been studied. The interconnection between the pre-exponential factor of the rate constant of the limiting stage of the reaction and the fractal dimension of the distribution of the active component on the surface of the carrier has been established. The relation has been illustrated using the heterogeneous catalytic hydrogenation of acetone to isopropanol on deposited transition metals. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 102–106, March–April, 2007.     

Homogeneous catalytic hydrogenation of free carboxylic acids in the presence of cluster ruthenium carbonyl hydrides

Saturated monocarboxylic acids up to C₆, several bicarboxylic acids and some of the corresponding anhydrides are hydrogenated in the homogeneous phase with H₄Ru₄(CO)₈(PBu₃)₄ as catalyst to give the corresponding alcohols (present among the reaction products as esters) or lactones at 100–200°C under a pressure of 100–200 atm of hydrogen. Anhydrides react at temperatures lower than those needed for acids. Esters are not reduced. Only δ-valerolactone is hydrogenated to 1,5-pentanediol. Ruthenium carbonyl carboxylates have been recovered at the end of the reaction and appear to be catalytically active intermediates.