Iron Hydride Research Articles

Microstructural evolution of ferritic steel powder during mechanical alloying with iron oxide

Abstract Mechanical alloying of mixed powders is of great importance for preparing oxide dispersion strengthened ferritic steels. In this study, the microstructual evolution of ferritic steel powder mixed with TiHx, YH2 and Fe2O3 in the process of mechanical alloying is systematically investigated by using X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy and microhardness tests. It is found that titanium, yttrium hydrides and iron oxide are completely dissolved during milling, and homogeneous element distribution can be achieved after milling for 12 h. The disintegration of the composite powder particles occurs at 24 h and reaches the balance of welding and fracturing after 36 h. The oxygen content increases sharply with the disintegration of powder particles due to the absorption of oxygen at the solid/gas interface from the milling atmosphere, which is the main source of extra oxygen in the milled powder. Grain refinement down to nanometer level occurs due to the severe plastic deformation of particles; however, the grain size does not change much with further disintegration of particles. The hardness increases with milling time and then becomes stable during further milling. The study indicates that the addition of iron oxide and hydrides may be more beneficial for the dispersion and homogenization of chemical compositions in the powder mixture, thus shortening the mechanical alloying process.

Structural and vibrational properties of Ca 2FeH 6 and Sr 2RuH 6

The structural and vibrational properties of the isostructural compounds Ca 2FeH 6 and Sr 2RuH 6 are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A 2TH 6 (A=Mg, Ca, Sr; T=Fe, Ru) in order to identify correlations between selected frequencies and the T–H bond length. The bulk moduli of Ca 2FeH 6 and Sr 2RuH 6 have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg 2FeH 6 and several other compounds of this structure.

Reactivity studies on [Cp′FeI]2: From iron hydrides to P4-activation

Metathesis of [Cp′FeI]2 (1) with KHBEt3 affords the polyhydride iron complexes [Cp′FeH2]2 (2) and [Cp′2Fe2H3] (3). The ratio in which both 2 and 3 are obtained correlates to the applied H2 pressure during synthesis. Complex 2 activates CH- or CD- bonds in aromatic compounds and shows slow H/D exchange in the presence of D2 at room temperature in cyclohexane solvent. [Cp′FeH2]2 acts as a Cp′Fe(I)-synthon when reacted with white phosphorus (P4) to give [Cp′Fe]2(μ-P4) (4) as the only P-containing product. This complex is best described as a triple-decker complex with a planar arrangement of a severely distorted kite-like cyclo-P4 unit. This distortion persists in solution and solid state as evidenced by a small PP coupling constant in the 31P{1H} NMR spectrum and a long P–P distance of 2.53 A. Complex 4 is an isomer to the long-known [{Cp′Fe}2(μ-η4:η4-P4)] (5) with a cis-tetraphosphabutadiene moiety and it thermally rearranges to 5, [{Cp′Fe}2(μ-η3:η3-P3)] and [Cp′Fe(P5)]. All complexes described in this paper have been completely characterized including X-ray crystallography, variable temperature NMR studies and DFT calculations. Relaxed force constants (inverse compliance constants) are used as bond strength descriptors.

Effect of hydrogen on the melting temperature of FeS at high pressure: Implications for the core of Ganymede

We have carried out in situ X-ray diffraction experiments on the FeS–H system up to 16.5GPa and 1723K using a Kawai-type multianvil high-pressure apparatus employing synchrotron X-ray radiation. Hydrogen was supplied to FeS from the thermal decomposition of LiAlH4, and FeSHx was formed at high pressures and temperatures. The melting temperature and phase relationships of FeSHx were determined based on in situ powder X-ray diffraction data. The melting temperature of FeSHx was reduced by 150–250K comparing with that of pure FeS. The hydrogen concentration in FeSHx was determined to be x=0.2–0.4 just before melting occurred between 3.0 and 16.5GPa. It is considered that sulfur is the major light element in the core of Ganymede, one of the Galilean satellites of Jupiter. Although the interior of Ganymede is differentiated today, the silicate rock and the iron alloy mixed with H2O, and the iron alloy could react with H2O (as ice or water) or the hydrous silicate before the differentiation occurred in an early period, resulting in a formation of iron hydride. Therefore, Ganymede's core may be composed of an Fe–S–H system. According to our results, hydrogen dissolved in Ganymede's core lowers the melting temperature of the core composition, and so today, the core could have solid FeSHx inner core and liquid FeHx–FeSHx outer core and the present core temperature is considered to be relatively low.

Iron(II) and Ruthenium(II) Complexes Containing P, N, and H Ligands: Structure, Spectroscopy, Electrochemistry, and Reactivity

The purpose of this work was to explore the possibility of using iron(II) hydrides in CO(2) reduction and to compare their reactivity to that of their ruthenium analogues. Fe(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(P(OEt)(3))(3)H(+) do not react with CO(2) in acetonitrile, but the one-electron-reduction products of Ru(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(2)(P(OEt)(3))H(+) and the two-electron-reduction product of Fe(bpy)(P(OEt)(3))(3)H(+) do. Ru(bpy)(2)(P(OEt)(3))H(+) also reacts slowly with CO(2) to give a formate complex [as reported previously by Albertin et al. (Inorg. Chem. 2004, 43, 1336)] with a second-order rate constant of ∼4 × 10(-3) M(-1) s(-1) in methanol. The structures for the hydride complexes [Fe(bpy)(P(OEt)(3))(3)H](+) and [Ru(bpy)(2)(P(OEt)(3))H](+) and for the (η(5)-Cp)bis- and -tris-PTA complexes (PTA = 1,3,5-triaza-7-phosphatricyclo[3.3.1.13.7]decane) of iron(II) are reported. These and the CpFe(CO)(bpy)(+) and Fe(II)PNNP compounds have been subjected to electrochemical and UV-vis spectroscopic characterization. Fe(bpy)(P(OEt)(3))(3)H(+) exhibits a quasi-reversible oxidation at +0.42 V vs AgCl/Ag in acetonitrile; Ru(bpy)(P(OEt)(3))(3)H(+) and Ru(bpy)(2)(P(OEt)(3))H(+) are oxidized irreversibly at +0.90 and +0.55 V, respectively, vs AgCl/Ag. The reduction site for Fe(bpy)(P(OEt)(3))(3)H(+) and Fe(bpy)(P(OEt)(3))(3)(CH(3)CN)(2+) appears to be the metal and gives rise to a two-electron process. The bpy-centered reductions are negatively shifted in the ruthenium(II) hydride complexes, compared to the acetonitrile complexes. The results of attempts to prepare other iron(II) hydrides are summarized.

ChemInform Abstract: High-Pressure Chemistry of Hydrogen in Metals: In situ Study of Iron Hydride.

Optical observations and x-ray diffraction measurements of the reaction between iron and hydrogen at high pressure to form iron hydride are described. The reaction is associated with a sudden pressure-induced expansion at 3.5 gigapascals of iron samples immersed in fluid hydrogen. Synchrotron x-ray diffraction measurements carried out to 62 gigapascals demonstrate that iron hydride has a double hexagonal close-packed structure, a cell volume up to 17% larger than pure iron, and a stoichiometry close to FeH. These results greatly extend the pressure range over which the technologically important iron-hydrogen phase diagram has been characterized and have implications for problems ranging from hydrogen degradation and embrittlement of ferrous metals to the presence of hydrogen in Earth9s metallic core.

Ironing Out Formic Acid

Chemistry Some biomass transformations, such as pathways that deoxygenate sugars, produce formic acid as a by-product, and the options for downstream use of large amounts of it have been limited. Boddien et al. now report a photocatalytic route for liberating hydrogen from formic acid. Previous catalysts for this reaction used noble metals, but after screening a range of more abundant transition metals, the authors discovered that effective catalysts formed in situ from an iron carbonyl cluster [Fe3(CO)12], a polydentate nitrogen-donating ligand, and triphenylphosphine. Under irradiation with the visible light from a 300-W xenon arc lamp, these catalysts generated hydrogen from formic acid solutions stabilized with triethylamine; the best examples had turnover frequencies up to 200 per hour and turnover numbers exceeding 100. Extensive spectroscopic studies implicated photogenerated iron hydrides as active species. J. Am. Chem. Soc. 132 , 10.1021/ja100925n (2010).

Iron-Catalyzed Hydrogen Production from Formic Acid

Hydrogen represents a clean energy source, which can be efficiently used in fuel cells generating electricity with water as the only byproduct. However, hydrogen generation from renewables under mild conditions and efficient hydrogen storage in a safe and reversible manner constitute important challenges. In this respect formic acid (HCO(2)H) represents a convenient hydrogen storage material, because it is one of the major products from biomass and can undergo selective decomposition to hydrogen and carbon dioxide in the presence of suitable catalysts. Here, the first light-driven iron-based catalytic system for hydrogen generation from formic acid is reported. By application of a catalyst formed in situ from inexpensive Fe(3)(CO)(12), 2,2':6'2''-terpyridine or 1,10-phenanthroline, and triphenylphosphine, hydrogen generation is possible under visible light irradiation and ambient temperature. Depending on the kind of N-ligands significant catalyst turnover numbers (>100) and turnover frequencies (up to 200 h(-1)) are observed, which are the highest known to date for nonprecious metal catalyzed hydrogen generation from formic acid. NMR, IR studies, and DFT calculations of iron complexes, which are formed under reaction conditions, confirm that PPh(3) plays an active role in the catalytic cycle and that N-ligands enhance the stability of the system. It is shown that the reaction mechanism includes iron hydride species which are generated exclusively under irradiation with visible light.

Iron‐Catalyzed Oppenauer‐Type Oxidation of Alcohols

AbstractA (hydroxycyclopentadienyl)iron dicarbonyl hydride catalyzes the Oppenauer‐type oxidation of alcohols with acetone as the hydrogen acceptor. Many functional groups are tolerant to the oxidation conditions. The same complex also catalyzes the dehydrogenation of diols to lactones. A mechanism involving the formation of iron‐alcohol complexes and their rapid ligand exchange with free alcohols is proposed. The trimethylsilyl groups on the cyclopentadienyl ligand of the catalyst play a critical role in stabilizing the iron hydride and increasing the catalyst lifetime.

Magnetic State in Iron Hydride Under Pressure Studied by X-ray Magnetic Circular Dichroism at the FeK-edge

AbstractIn order to study magnetic states in Fe-hydride under pressure, X-ray magnetic circular dichroism (XMCD) at the FeK-edge has been measured up to 27.5 GPa. As a result, hydrogenation from bcc-Fe to dhcp-FeH occurs within a narrow region of 3.2-3.8 GPa, which is clearly observed by the dichroic profile in dhcp-FeH differing from that in bcc-Fe. Influence of H atoms on Fe 3dand 4pelectronic states is discussed using the pressure-dependent XMCD and the first-principles calculation.

Ultrahigh-pressure study on the magnetic state of iron hydride using an energy domain synchrotron radiation 57Fe Mössbauer spectrometer

AbstractPressure induced magnetic phase transition of iron hydride was investigated with an in-situ Mössbauer spectrometer using synchrotron radiation (SR). The spectrometer is composed of a high resolution monochromator, an X-ray focusing device, a variable frequency nuclear monochromator and a diamond anvil cell. The optical system, advantages of the spectrometer and the observed high pressure magnetic phases of iron hydride are described.

Hydrogen partitioning between iron and ringwoodite: Implications for water transport into the Martian core

We determined the exchange partition coefficients of hydrogen between solid iron and ringwoodite between 16.6 and 20.9 GPa at temperatures up to 1273 K using a Kawai-type multianvil high-pressure apparatus with synchrotron X-ray radiation at the BL04B1 beamline at SPring-8, Japan. The hydrogen concentration in iron hydride was estimated from the volume expansion of iron caused by hydrogenation determined by in situ X-ray diffractions at high pressure and high temperature, and the water content of ringwoodite in the recovered samples was estimated using the Fourier transform infrared spectroscopy (FTIR). According to our results, the exchange partition coefficients of hydrogen between the solid iron and ringwoodite were almost constant, 26, with pressure between 16.6 and 20.9 GPa and 1273 K. These results revealed that hydrogen was strongly partitioned to metallic iron and that iron hydride formed, coexisting with dry ringwoodite under the experimental pressures. Ringwoodite, found in the Martian core–mantle boundary region, is an important hydrogen reservoir. The pattern of quasi-parallel bands of uniformly magnetized crust with alternating positive and negative polarity measured by the Mars Global Surveyor spacecraft strongly shows that a magnetic field did exist in ancient Mars suggesting a possible plate tectonic activity on ancient Mars. Thus, water could have been transported to the deep Martian interior by hydrous minerals during the plate subduction process and stored in ringwoodite in the deep Martian slabs, as is suggested on the Earth today. Our experiments suggested that hydrogen stored in ringwoodite was absorbed by the Martian core at the Martian core–mantle boundary. Thus, water from the ancient Martian ocean may be stored now in the Martian core.

Synthesis, Characterization, and Electrochemistry of a Series of Iron(II) Complexes Containing Self-Assembled 1,5-Diaza-3,7-diphosphabicyclo[3.3.1]nonane Ligands

The reaction between PPh(CH(2)OH)(2), iron(II) sulfate, ammonium sulfate, and formaldehyde in aqueous solution gives the iron(II) complex [Fe(kappa(2)-O(2)SO(2))L(2)] (1), where L is the bidentate phosphine ligand 3,7-diphenyl-1,5-diaza-3,7-diphosphabicyclo[3.3.1]nonane. During the course of the reaction, the ligand L self-assembles on the metal center. The reaction between PPh(CH(2)OH)(2), iron(II) chloride, ammonium chloride, and formaldehyde under similar conditions gives cis-[FeCl(2)L(2)] (cis-2). The complex cis-2 is converted into trans-2 in Et(2)O, whereas in water it is converted into cis-[Fe(OH(2))(2)L(2)](2+), though both of these interconversions are reversible. The chloro ligands in cis-2 are readily displaced by reaction with thiocyanate, azide, and carbonate to give cis- and trans-[Fe(NCS)(2)L(2)] (cis- and trans-3), cis- and trans-[Fe(N(3))(2)L(2)] (cis- and trans-4), and [Fe(kappa(2)-O(2)CO)L(2)] (5), respectively. The complex cis-2 reacts with CO in water to give trans-[FeCl(CO)L(2)]Cl (trans-6), whereas trans-2 reacts with CO in diethyl ether to give cis-[FeCl(CO)L(2)]Cl (cis-6), though cis-6 isomerizes in water to form trans-6. The reaction of cis-2 with sodium borohydride gives the hydride chloride complex trans-[FeCl(H)L(2)] (7). Electrochemical studies have been undertaken on complexes 1, cis-2, and 7. These reveal reversible oxidations for cis-2 and 7, with the latter giving rise to an unusual 17-electron iron(III) hydride chloride complex. Crystal structures have been obtained for 1, trans-2, trans-3, 5, and 7.

Iron/carbon-black composite nanoparticles as an iron electrode material in a paste type rechargeable alkaline battery

Iron/carbon-black composite nanoparticles were synthesized by chemically reducing the iron salt mixing with carbon black by adding NaBH 4 in the aqueous solution. Carbon-black particles, with a mean particle size of approximately 40 nm, function as the nucleation cores for iron deposition. Additionally, core–shell iron composite particles are observed to be 30–100 nm with spherical sharp. At the first time discharge, the iron/carbon-black composite nanoparticle discharged 1200 mAh g −1(Fe) at plateau one and 400 mAh g −1at plateau two at a high current density of 200 mA g −1(Fe). The capacity is larger than the theoretical value, which is attributed to the formation of iron hydride (FeH x ) while the iron was reduced by NaBH 4, followed the hydrogen reaction as an active material while the battery discharge occurs. In further cycles, the iron/carbon-black composite iron electrode shows a good reversibility of about 600 mAh g −1(Fe) when the nickel–iron battery operated between 1.65 and 0.8 V. XRD analysis results indicate that the carbon black in the core of the iron/carbon-black composite enhances the reduction/oxidation reactions of iron, as achieved by the carbon black forming an enhanced electronic conductive network while iron is discharged as the insulator species such as Fe(OH) 2 and Fe 3O 4. SEM images reveal that the iron/carbon-black composite keeps particle sizes smaller than 300 nm during the electrode cycling, indicating that carbon black also acts as the nucleation cores for the dissolution–deposition of iron.

New routes to low-coordinate iron hydride complexes: The binuclear oxidative addition of H 2

The oxidative addition and reductive elimination reactions of H 2 on unsaturated transition-metal complexes are crucial in utilizing this important molecule. Both biological and man-made iron catalysts use iron to perform H 2 transformations, and highly unsaturated iron complexes in unusual geometries (tetrahedral and trigonal planar) are anticipated to give unusual or novel reactions. In this paper, two new synthetic routes to the low-coordinate iron hydride complex [L tBuFe(μ-H)] 2 are reported. Et 3SiH was used as the hydride source in one route by taking advantage of the silaphilicity of the fluoride ligand in three-coordinate L tBuFeF. The other synthetic method proceeded through the binuclear oxidative addition of H 2 or D 2 to a putative Fe(I) intermediate. Deuteration was verified through reduction of an alkyne and release of the deuterated alkene product. Mössbauer spectra of [L tBuFe(μ-H)] 2 indicate that the samples are pure, and that the iron(II) centers are high-spin.

Cyclopentadienone iron alcohol complexes: synthesis, reactivity, and implications for the mechanism of iron-catalyzed hydrogenation of aldehydes.

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(5)-C(4)COH)]Fe(CO)(2)H (3) and aldehydes were characterized by (1)H NMR, (13)C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe(3))(2)-3,4-(CH(2))(4)(eta(4)-C(4)CO)]Fe(CO)(2)(HOCH(2)C(6)H(5)) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh(3). Dissociation of the alcohol ligand in the presence of H(2) leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD(2))C(6)H(4)CHO, which possesses a potential intramolecular alcohol trapping agent, afforded two alcohol complexes, one with the newly formed alcohol coordinated to iron and the other with the trapping alcohol coordinated. The intramolecular trapping experiments support a mechanism involving concerted transfer of a proton from OH and hydride from Fe of 3 to aldehydes. The kinetics and mechanism of the hydrogenation of benzaldehyde catalyzed by 3 are presented.

Electronic Structure and Reactivity of Three-Coordinate Iron Complexes

[Reaction: see text]. The identity and oxidation state of the metal in a coordination compound are typically thought to be the most important determinants of its reactivity. However, the coordination number (the number of bonds to the metal) can be equally influential. This Account describes iron complexes with a coordination number of only three, which differ greatly from iron complexes with octahedral (six-coordinate) geometries with respect to their magnetism, electronic structure, preference for ligands, and reactivity. Three-coordinate complexes with a trigonal-planar geometry are accessible using bulky, anionic, bidentate ligands (beta-diketiminates) that steer a monodentate ligand into the plane of their two nitrogen donors. This strategy has led to a variety of three-coordinate iron complexes in which iron is in the +1, +2, and +3 oxidation states. Systematic studies on the electronic structures of these complexes have been useful in interpreting their properties. The iron ions are generally high spin, with singly occupied orbitals available for pi interactions with ligands. Trends in sigma-bonding show that iron(II) complexes favor electronegative ligands (O, N donors) over electropositive ligands (hydride). The combination of electrostatic sigma-bonding and the availability of pi-interactions stabilizes iron(II) fluoride and oxo complexes. The same factors destabilize iron(II) hydride complexes, which are reactive enough to add the hydrogen atom to unsaturated organic molecules and to take part in radical reactions. Iron(I) complexes use strong pi-backbonding to transfer charge from iron into coordinated alkynes and N 2, whereas iron(III) accepts charge from a pi-donating imido ligand. Though the imidoiron(III) complex is stabilized by pi-bonding in the trigonal-planar geometry, addition of pyridine as a fourth donor weakens the pi-bonding, which enables abstraction of H atoms from hydrocarbons. The unusual bonding and reactivity patterns of three-coordinate iron compounds may lead to new catalysts for oxidation and reduction reactions and may be used by nature in transient intermediates of nitrogenase enzymes.

Melting phase relation of FeH x up to 20 GPa: Implication for the temperature of the Earth's core

High-pressure and high-temperature X-ray diffraction experiments on FeH x up to 20 GPa and 1598 K were performed using a Kawai-type multi-anvil apparatus, SPEED-MkII, installed at BL04B1 beam line of SPring-8 synchrotron facility. Iron powder was packed in a container made of NaCl, a very efficient sealing material for hydrogen under high pressure, together with a hydrogen source, LiAlH 4. The temperature of hydrogenation, transition of iron hydride phases, and melting of γ-FeH x were all determined in situ in the pressure range between 10 and 20 GPa. Hydrogen concentration in both ɛ′- and γ-FeH x phases reached x = 1.0 above 10 GPa. Melting temperatures of γ-FeH were determined to be 1473, 1448 ± 25, 1538 ± 15, 1548 ± 25 and 1585 ± 13 K at 10, 11.5, 15, 18 and 20 GPa, respectively. These temperatures are nearly 700 K lower than that of pure iron under the corresponding pressures. The Clapeyron-slope (d T/d P slope) of the melting curve of γ-FeH is 13 K/GPa, which is significantly smaller than those of other possible core constituents (Fe, FeO, FeS). By extrapolating the ɛ′–γ phase boundary linearly and the melting curve of γ-FeH based on Lindemann's melting law, the triple point of ɛ′- and γ-FeH and iron hydride melt is located at P = 60 GPa and T = 2000 K. Beyond the triple point, an attempt to construct a melting curve of ɛ′-FeH by the Lindemann's law using estimated thermal equation of state of ɛ′-FeH was unsuccessful. Therefore, we decided, instead, to extrapolate the melting curve of γ-FeH beyond the triple point to 135 GPa yielding the melting temperature of FeH ∼ 2600 K at core mantle boundary (CMB). Based on these results, we propose that the temperature of the Earth's outer core could be much lower than current estimates, if the Earth's outer core incorporated significant amounts of hydrogen.

Molecular modelling and experimental studies on steam gasification of low-rank coals catalysed by iron species

Pyrolysis and catalytic steam gasification of brown coal containing iron hydroxyl complexes have been investigated experimentally and with semi-empirical and density functional theory molecular modelling. Pyrolysis yielded mainly CO2, CO, and reduced iron species. Catalytic steam gasification at 900 °C after 15 min, consumed 20 wt.% additional char and a higher than expected yield of H2 due to post-gasification reactions; inorganic and organic oxygen in char increased compared to pyrolysis. Apparent turnover numbers for catalytic gasification were 12–22 mole of carbon per mole of iron. The distribution of iron species in brown coal indicated small iron clusters are likely to form on heating; pyrolysis was thus modelled using molecules of char with [Fe3], [Fe5] and [Fe3O], and the active site for gasification was shown to be [Fe–C]. The mechanism of catalytic gasification involved H2O chemi-adsorbed on [Fe–C], formation of the [Fe ← OH2] coordination bond, with H2 produced via iron hydride complexes. Formation of CO was via oxygen insertion into [Fe–C] to form [Fe–O–C] that decomposed into CO and another [Fe–C] site. Lower activation barriers were obtained for concerted chemistry involving iron-hydrides. Active sites in char were accessible to H2O as pores had developed around iron species; large sized iron species were not catalytically active but caused large pores to form in char.

A Chain to Break Nitrogen

CHEMISTRY Synthetic chemists continue to puzzle over how bacteria manage the feat of reducing triply bonded nitrogen to ammonia without the help of extreme temperatures or pressures. Among the clues teased out of nitrogenase enzyme studies is the possible involvement of paramagnetic iron hydride