Hydrogenation Dehydrogenation Research Articles

Electrochemical Corrosion Behavior of Air-Exposed Zr–Mn–Cr–Ni–V Alloy

Scanning electron microscopy and electron microprobe analysis show that the ZrMnCrNiV alloy has a dendritic structure and is chemically inhomogeneous. The corrosion mechanism for the unexposed alloy and the alloy exposed in air for 7 and 15 days, followed by aging in a 30% KOH solution, is the same: corrosion originates at the interphase boundary and propagates along it, which is typical of pitting corrosion. If the alloy is preliminary exposed in air, its surface has a greater number of pittings, but all of them are smaller in area and depth, making the corrosion process more uniform. In hydrogenation–dehydrogenation of this alloy, even more uniform distribution of smaller corrosion areas is observed. Studies of the corrosion resistance of this alloy in a KOH solution carried out by atomic adsorption spectrometry show that the alloy powder exposed in air has higher corrosion resistance compared to the unexposed powder. Electrochemical corrosion studies of the alloy conducted in the anodic region using the method of polarization curves indicate that the corrosion rate for the unexposed and exposed alloys is controlled by the rate at which passivating films form. The most extensive passivation region is observed in the alloy exposed in air for 15 days. It shows adequate corrosion resistance in a 30% KOH solution. The cyclic resistance studies for the electrodes produced from the alloy powder exposed for 10 days, at a discharge to potential difference E = –1.0 V and E = –0.8 V, demonstrate that oxidation in the hydrogenation-dehydrogenation process affects the cyclic resistance. It is found that there is liming time for exposing the alloy in air (as an ingot and/or powder) after which the cyclic resistance deteriorates.

N‐Substituted Hydrazones by Manganese‐Catalyzed Coupling of Alcohols with Hydrazine: Borrowing Hydrogen and Acceptorless Dehydrogenation in One System

AbstractAn unprecedented one‐step synthesis of N‐substituted hydrazones by coupling of alcohols with hydrazine is reported. This partial hydrogen‐borrowing reaction is catalyzed by a new manganese pincer complex under mild reaction conditions, thus liberating water and dihydrogen as the only byproducts. Mechanistic insight, based on the observation of intermediates, is provided.

N-Substituted Hydrazones by Manganese-Catalyzed Coupling of Alcohols with Hydrazine: Borrowing Hydrogen and Acceptorless Dehydrogenation in One System.

An unprecedented one-step synthesis of N-substituted hydrazones by coupling of alcohols with hydrazine is reported. This partial hydrogen-borrowing reaction is catalyzed by a new manganese pincer complex under mild reaction conditions, thus liberating water and dihydrogen as the only byproducts. Mechanistic insight, based on the observation of intermediates, is provided.

Theoretical Study of the Metal-Controlled Dehydrogenation Mechanism of MN2H3BH3 (M = Li, Na, K): A New Family of Hydrogen Storage Material.

Metal hydrazineboranes (MHBs), as a kind of new hydrogen storage materials, show excellent hydrogen storage performance and dehydrogenation properties. Herein, we designed multiple dehydrogenation pathways to compare the metal-controlled effect. Quantum chemistry theory is used to calculate the crystal structure for determining the molecular structure. With an increase of the metal radius, the energy difference of the isomers also increases. The dehydrogenation pathways of lithium hydrazineborane (path A) and sodium hydrazineborane (path B) appear totally similar to each other in the dehydrogenation process despite the energy barrier, as well as the comparison paths A' (for LiHB) and B' (for NaHB). In contrast with LiHB and NaHB, the tautomeric reaction occurs in the potassium hydrazineborane (KHB) first, and the following dehydrogenation path is similar to that of the LiHB and NaHB. It explores the hydrogen-release properties of the different metal hydrazineboranes and also indcates the affection of the metal in the metal hydrazineboranes hydrogen-storage system.

Mechanistic Studies on NaHCO3 Hydrogenation and HCOOH Dehydrogenation Reactions Catalysed by a FeII Linear Tetraphosphine Complex.

We present a theoretical extension of the previously published bicarbonate hydrogenation to formate and formic acid dehydrogenation catalysed by FeII complexes bearing the linear tetraphosphine ligand tetraphos-1. The hydrogenation reaction was found to proceed at the singlet surface with two competing pathways: A) H2 association to the Fe-H species followed by deprotonation to give a Fe(H)2 intermediate, which then reacts with CO2 to give formate. B) CO2 insertion into the Fe-H bond, followed by H2 association and subsequent deprotonation. B was found to be slightly preferred with an activation energy of 22.8 kcal mol-1 , compared to 25.3 for A. Further we have reassigned the Fe-H complex, as a Fe(H)(H2 ), which undergoes extremely rapid hydrogen exchange.

BINAP-copper supported by hydrotalcite as an efficient catalyst for the borrowing hydrogen reaction and dehydrogenation cyclization under water or solvent-free conditions

A BINAP-Cu system supported by hydrotalcite has been developed and proved to be a highly efficient catalyst for the atom-efficient and green borrowing hydrogen reaction and dehydrogenative cyclization.

Hydrogen transfer reactions relevant to Guerbet coupling of alcohols over hydroxyapatite and magnesium oxide catalysts

The rates of double bond hydrogenation, deuterium exchange, and benzyl alcohol dehydrogenation were compared to those of ethanol coupling.

Charge effects regulate reversible CO2 reduction catalysis

Modular but geometrically constrained ligands were used to investigate the impact of key ligand design parameters (charge and bite angle) on CO2 hydrogenation and formic acid dehydrogenation activity. These studies yielded an optimized catalyst that achieved over 118 000 turnovers in CO2 hydrogenation, 247 000 turnovers in HCO2H dehydrogenation, was applied in a hydrogen storage device used for 6 cycles of hydrogen storage/release without requiring changes in pH or solvent, and generated H2/CO2 gas at a pressure of 190 atm from formic acid.

Where does Au coordinate to N-(2-pyridiyl)benzotriazole: gold-catalyzed chemoselective dehydrogenation and borrowing hydrogen reactions

Pyridyltriazole gold(i) complexes proved to be an efficient precatalyst for the most challenging gold-catalyzed borrowing hydrogen reaction and dehydrogenation of alcohols and amines.

Electrochemical Properties of Zr–Mn–Cr–Ni–V Alloy in Long-Term Cycling After Air Oxidation

A zirconium-containing alloy was subjected to air oxidation to examine its influence on the lattice parameters and cyclic resistance. The samples made of freshly prepared alloy powder and of powder oxidized in air for 7, 15, and 30 days were studied. Cyclic voltammetry was used to analyze in detail the current–voltage characteristics and X-ray diffraction was employed to determine the lattice parameters. The freshly prepared alloy was found to be very unstable and significantly lose its activity in the hydrogenation–dehydrogenation process. This results from oxidation of the alloy, which induces interfacial stresses and causes mechanical damage of the electrodes. Air oxidation of the alloy powder followed by compaction of the electrodes stabilizes its electrochemical properties and, as a result, increases the cyclic resistance. According to X-ray diffraction, the phase ratio after air oxidation remains practically unchanged, and the lattice expansion (within 1%) can be attributed to measurement errors.

Reversible hydrogenation—dehydrogenation reactions of meta-terphenyl on catalysts with various supports

For developing new composite systems (substrate—catalyst) for hydrogen storage, the activities of Pt and Pd catalysts on various supports were compared in reversible meta-terphenyl hydrogenation and perhydro-meta-terphenyl dehydrogenation. The microstructure of the catalysts was studied. Carbon-supported catalysts are more efficient in both reversible reactions than alumina-supported systems.

Perovskite-based mixed protonic–electronic conducting membranes for hydrogen separation: Recent status and advances

Hydrogen share in the energy market has increased significantly in line with greater demand for zero emission fuel and the development of novel production routes via renewable resources. The application of mixed protonic–electronic conducting (MPEC) ceramic membrane within the hydrogen production process is an innovative route that enables high purity hydrogen production with low cost. This review provides readers a brief summary of the research efforts on MPEC ceramic membrane for hydrogen separation as well as the membrane reactor for hydrogen production and dehydrogenation or hydrogenation reactions. Most of the existing MPEC ceramic membranes come from either a single-phase or a dual-phase membrane. We discuss the working principles, the performances, the advantages and disadvantages, and the main issues of all these membranes. Major emphasis of the review is to cover the literature published in the last ten years since the earlier progress has been well documented by the previously existing reviews. We also put forward recommendations for future research direction in this topic.

Microwave-activated dehydrogenation of perhydro-N-ethylcarbazol over bimetallic Pd-M/TiO2 catalysts as the second stage of hydrogen storage in liquid substrates

Reversible catalytic hydrogenation–dehydrogenation of aromatic and heterocyclic compounds is considered as a promising approach to hydrogen storage. While exploring the microwave-assisted catalytic processes, the Pd/TiO2 catalysts were shown to be most effective. The catalytic activity of the Pd/TiO2 system can be enhanced by introduction of a second metal after the formation of the supported Pd nanoparticles, with Pt and Ru acting as most efficient promoters. The studies of the catalytic behavior of the supported bimetallic Pd–Ru/TiO2 and Pd–Pt/TiO2 catalysts in perhydro-N-ethylcarbazole (H-NEC) dehydrogenation revealed that the catalytic activity under microwave activation is higher than that in a conventional thermal mode at the same reaction temperatures. Complete dehydrogenation occurred only under microwave activation, with a negligible by-product formation. This feature of the catalysts makes possible their application in the hydrogen storage systems based on the reversible hydrogenation–dehydrogenation cycles under microwave activation.

Microstructural Evolution during Pressureless Sintering of Blended Elemental Ti-Al-V-Fe Titanium Alloys from Fine Hydrogenated-Dehydrogenated Titanium Powder

A comprehensive study was conducted on microstructural evolution of sintered Ti-Al-V-Fe titanium alloys utilizing very fine hydrogenation-dehydrogenation (HDH) titanium powder with a median particle size of 8.84 μm. Both micropores (5–15 μm) and macropores (50–200 μm) were identified in sintered titanium alloys. Spherical micropores were observed in Ti-6Al-4V sintered with fine Ti at the lowest temperature of 1150 °C. The addition of iron can help reduce microporosity and improve microstructural and compositional homogenization. A theoretical calculation of evaporation based on the Miedema model and Langmuir equation indicates that the evaporation of aluminum could be responsible for the formation of the macropores. Although reasonable densification was achieved at low sintering temperatures (93–96% relative density) the samples had poor mechanical properties due mainly to the presence of the macroporosity and the high inherent oxygen content in the as-received fine powders.

Understanding the Lewis Acidity of Co(II) Sites on a Silica Surface.

Heterogeneous catalysts consisting of isolated transition-metal sites dispersed on the surface of metal oxide supports are commonly used in the chemical industry. Often their reactivity relies on the Lewis acidity of the active sites on the surface of the catalyst. A recent report from our group showed that silica-supported Co(II) sites, prepared via surface organometallic chemistry, are active in both alkene hydrogenation and alkane dehydrogenation, possibly linked to the Lewis acidity of the Co(II) sites. Here we use molecular probes and analogues to both qualitatively and quantitatively model the Lewis acidity of the surface sites. Some sites do not bind probe molecules like carbon monoxide, tetrahydrofuran, and olefins, while others exhibit a continuum of Lewis acidities. This is consistent with variations in the coordination environment of Co. These results suggest that only the most Lewis acidic sites are involved in dehydrogenation and hydrogenation, consistent with catalyst poisoning studies.

Ruthenium (II) and iridium (III) complexes of N-heterocyclic carbene and pyridinol derived bidentate chelates: Synthesis, characterization, and reactivity

We report the synthesis and characterization of new ruthenium(II) and iridium(III) complexes of a new bidentate chelate, NHCR′-pyOR (OR=OMe, OtBu, OH and R′=Me, Et). Synthesis and characterization studies were done on the following compounds: four ligand precursors (1–4); two silver complexes of these NHCR′-pyOR ligands (5–7); six ruthenium complexes of the type [η6-(p-cymene)Ru(NHCR′-pyOR)Cl]X with R′=Me, Et and R=Me, tBu, H and X=OTf−, PF6− and PO2F2− (8–13); and two iridium complexes, [Cp∗Ir(NHCMe-pyOtBu)Cl]PF6 (14) and [Cp∗Ir(NHCMe-pyOH)Cl]PO2F2 (15). The complexes are air stable and were isolated in moderate yield. However, for the PF6− salts, hydrolysis of the PF6− counter anion to PO2F2− during t-butyl ether deprotection was observed. Most of the complexes were characterized by 1H and 13C NMR, MS, IR, and X-ray diffraction. The ruthenium complexes [η6-(p-cymene)Ru(NHCMe-pyOR)Cl]OTf (R=Me (8) and tBu (9)) were tested for their ability to accelerate CO2 hydrogenation and formic acid dehydrogenation. However, our studies show that the complexes transform during the reaction and these complexes are best thought of as pre-catalysts.

Tailoring the Thermodynamics and Kinetics of Mg–Li Alloy for a MgH2‐Based Anode for Lithium‐Ion Batteries

AbstractUnderstanding the lithiation–delithiation of the Mg–Li alloy during the absorption–desorption of hydrogen is essential for the development of Li‐ion batteries with MgH2 as a negative electrode. Tuning the hydrogenation–dehydrogenation kinetics and thermodynamics of the Mg–Li alloy could also be helpful to develop a lightweight material for on‐board hydrogen‐storage applications. Single‐phase Li3Mg7 (the highest % of lithium compound) was prepared by ball milling of LiH and MgH2 as precursors of Li and Mg followed by dehydrogenation at 400 °C under dynamic vacuum conditions. The cyclic hydrogenation–dehydrogenation behavior of the intermetallic was studied in detail. During hydrogenation, Li3Mg7 was delithiated to LiH and MgH2, whereas during dehydrogenation, LiH and MgH2 were lithiated to form the Li3Mg7 phase along with a Mg–Li solid solution. The hydrogenation–dehydrogenation kinetics of pristine Li3Mg7 were found to be slow. The hydrogenation–dehydrogenation kinetics were remarkably improved by doping with ZrCl4 as a catalyst. The activation energy and the thermodynamic parameters of the uncatalyzed and catalyzed alloy were evaluated, and the results were compared.

Alkali metal silanides α-MSiH3: A family of complex hydrides for solid-state hydrogen storage

The alkali metal silanides α-MSiH3 appear to be a promising family of complex hydrides for solid-state hydrogen storage. Herein the structural, energetic and electronic properties of α-MSiH3 silanides (M = Li, Na, K, Rb, Cs) and MSi Zintl phases are systematically investigated for the first time by using first-principles calculations method based on density functional theory. The structural parameters of α-MSiH3 and MSi including lattice constants and atomic positions are determined through geometry optimization. The obtained results are close to the experimental data analysed from X-ray and neutron powder diffraction. The calculations of formation enthalpy show that α-KSiH3, α-RbSiH3 and α-CsSiH3 silanides are easier to be synthetized relative to α-LiSiH3 and α-NaSiH3, which interprets well the lower thermostabilities of experimental α-LiSiH3 and α-NaSiH3. Nevertheless, LiSi, KSi and CsSi phases are easier to be formed relative to NaSi and RbSi. The calculations of hydrogen desorption enthalpy reveal that the dehydrogenation abilities of α-MSiH3 silanides along the decomposition path of α-MSiH3→MSi + H2 are gradually enhanced in the order of α-CsSiH3, α-RbSiH3, α-KSiH3, α-NaSiH3 and α-LiSiH3, which may be originated from their decreasing thermostabilities. From a comprehensive point of view including hydrogen storage capacity, thermostability and dehydrogenation ability, α-KSiH3 (∼4.29 wt%) is identified as the most promising alkali metal silanide for reversible hydrogen storage. Analysis of electronic structures indicates that a significant charge transfer leads to positively charged M ions and negatively charged SiH3 complex, which constitutes the ionic bonding between them. The bonding within SiH3 complex not only involves the covalent hybridization between Si (3s) (3p) and H (1s) orbitals, but also exhibits some ionic bond characteristics due to the partial charge transfer from Si to H. The covalent bonding interactions between H and Si atoms within SiH3 mainly dominate the thermostabilities and dehydrogenation properties of α-MSiH3 silanides.

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal.

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features and for metal–ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1–5. In general, NHC-pyOR complexes 1–3 were modest precatalysts for both reactions. NHC complexes 1–3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5–8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.

Porosity, Microstructure, and Mechanical Properties of Ti-6Al-4V Alloy Parts Fabricated by Powder Compact Forging

Ti-6Al-4V alloy powders produced using a hydrogenation–dehydrogenation process and a gas atomization process, respectively, were rapidly consolidated into near-net-shaped parts by powder compact forging. The porosity, microstructure, and tensile mechanical properties of specimens cut from regions at different distances from the side surfaces of the forged parts were examined. The regions near the side surfaces contained a fraction of pores due to the circumferential tensile strain arising during the powder compact forging process, and the porosity level decreased rapidly to zero with increasing the distance from the side surface. The forged parts had a fully lamellar structure with the α + β colony sizes and α lamella thickness changing little with the distance from the side surface. The specimens cut from the regions near the side surfaces had a lower yield strength and tensile strength. The correlation of porosity with the yield strength of the specimens suggested that the reduction of load bearing areas due to the porosity and unbonded or weakly bonded interparticle boundaries was not the only reason for the lower strength, and the stress concentration at the pores and associated with their geometry also played an important role in this. It is likely that the effect of stress concentration on yield strength reduction of the forged part increases with oxygen content. The Hall–Petch relationship of the yield strength and the average α lamella thickness suggested that the strength of the fully dense and fully consolidated forged parts was increased by oxygen solution strengthening.