Explore the vast range of titles available.

Nanoscaling interstitial metal hydrides offers opportunities for hydrogenation applications by enhancing kinetics, increasing surface area, and allowing for tunable properties. The introduction of interfaces impacts hydrogen absorption properties and distribution heterogeneously, making it, however, challenging to examine the multiple concurrent mechanisms, especially at the atomic level. Here, the effect of proximity on interstitial hydrogen in ultrathin single‐crystalline vanadium films is demonstrated by comparing hydride formation in identically strained Fe/V‐ and Cr/V‐superlattices. Pressure concentration and excess resistivity isotherms show higher absolute solubility of hydrogen, higher critical temperature, and concentration in a Cr/V‐superlattice. Direct measurements of hydrogen site location and thermal vibrations show identical site occupation of octahedral z at room temperature with a vibrational amplitude of 0.20–0.25 Å over a wide range of hydrogen concentrations. These findings are consistent with a more extended region of hydrogen depletion in the vicinity of Fe compared to Cr, which showcases an inverse of the hydrogen spillover effect. Advancing the understanding of interface effects resolves previously puzzling differences in the hydrogen loading of Fe/V‐ and Cr/V‐superlattices and is relevant for advancing both catalysis and storage. Interstitial hydrogen behaves distinctly at interfaces compared to the bulk. Vanadium films show proximity‐dependent hydrogen depletion near interfaces–an inverse of hydrogen spillover. Ion beam and resistivity measurements reveal Fe/V‐superlattices have lower hydrogen solubility and higher critical temperature than Cr/V due to proximity‐induced finite size effects, with similar site occupation.

Recent reports show [FeFe] hydrogenase mimics are active for the electrochemical reduction of CO2 to formate (HCOO−). Herein, the electrochemical reduction of CO2 with the [FeFe] hydrogenase mimic [Fe2(μ‐pdt)(CO)6, 1, where pdt = propane‐1,3‐dithiolate] in acetonitrile is reported. In the presence of the weak acid, methanol (MeOH), 1 reduces CO2 to both CO (Faradaic Efficiency maximum [FEmax] of 16 ± 6%) and HCOO− (FEmax = 20%) and produces H2 (FEmax = 56 ± 4%). Without added MeOH, 1 reacts with adventitious water to form H2 (FEmax = 85 ± 1%), HCOO− (FEmax = 7.8%), and CO (FEmax = 7 ± 3%) with CO32− being detected by infrared spectroscopy.  Product formation is potential dependent: more negative potentials increases selectivity for HCOO− over CO. The first reduction of 1 forms a pdt‐bridged dimer, 2. However, the reduction of 2 at the potentials required for electrochemical CO2 reduction leads to two new species. Using density functional theory, and infrared spectroelectrochemistry (IR‐SEC), these structures are identified to be [Fe(CO)4]2− (3) and a trinuclear Fe3 species (4). While these species can reduce CO2 to CO and HCOO−, the predominant formation of H2 reveals kinetic issues in  CO2 reduction. The work offers to consider alternate competing mechanistic pathways and explains the lack of product selectivity when using hydrogen evolution reaction catalyst for CO2 reduction to HCOO−. Combining cyclic voltammetry, infrared spectroelectrochemistry, and density functional theory methods, the reduction of CO2 by an [FeFe] hydrogenase mimic was examined. Despite its ability to form stable reduced hydride species, this catalyst does not selectively reduce CO2 to HCOO− under the tested conditions. This suggests a more complex pathway to formate involving mononuclear or trinuclear Fe species may be involved.

Hirshfeld atom refinement (HAR) is a method which enables the user to obtain more accurate positions of hydrogen atoms bonded to light chemical elements using X-ray data. When data quality permits, this method can be extended to hydrogen-bonded transition metals (TMs), as in hydride complexes. However, addressing hydrogen thermal motions with HAR, particularly in TM hydrides, presents a challenge. At the same time, proper description of thermal vibrations can be vital for determining hydrogen positions correctly. In this study, we employ tools such as SHADE3 and Normal Mode Refinement (NoMoRe) to estimate anisotropic displacement parameters (ADPs) for hydrogen atoms during HAR and IAM refinements performed for seven structures of TM (Fe, Ni, Cr, Nb, Rh and Os) and metalloid (Sb) hydride complexes for which both the neutron and the X-ray structures have been determined. A direct comparison between neutron and HAR/SHADE3/NoMoRe ADPs reveals that the similarity between neutron hydrogen ADPs and those estimated with NoMoRe or SHADE3 is significantly higher than when hydrogen ADPs are refined with HAR. Regarding TM—H bond lengths, traditional HAR exhibits a slight advantage over the other methods. However, combining NoMoRe/SHADE3 with HAR results in a minor decrease in agreement with neutron TM—H bond lengths. For the Cr complex, for which high-resolution X-ray data were collected, an investigation of resolution-related effects was possible.

This study aims to shed light on the unusual trend in the stabilities of Zr(MoxFe1−x)2, 0 ≤ x ≤ 1, hydrides. Both the rule of reversed stability and the crystal volume criterion correlate with the increased hydride stabilities from x = 0 to x = 0.5, but are in contrast with the destabilization of the end member ZrMo2 hydride. The pressure-composition isotherms of ZrMo2-H2 exhibit very wide solid solubility regions, which may be associated with diminished H–H elastic interaction, uelas. In order to discern this possibility, we measured the elastic moduli of Zr(MoxFe1−x)2, x = 0, 0.5, 1. The shear modulus, G, shows a moderate variation in this composition range, while the bulk modulus, B, increases significantly and monotonically from 148.2 GPa in ZrFe2 to 200.4 GPa in ZrMo2. The H–H elastic interaction is proportional to B and therefore its increase cannot directly account for a decrease in uelas. Therefore, we turn our attention to the volume of the hydrogen atom, vH, which usually varies in a limited range. Two coexisting phases, a Laves cubic (a = 7.826 Å) and a tetragonal (a = 5.603 Å, c = 8.081 Å) hydride phase are identified in ZrMo2H3.5, obtained by cooling to liquid nitrogen temperature at about 50 atm. The volume of the hydrogen atom in these two hydrides is estimated to be 2.2 Å3/(H atom). Some very low vH values, have been reported by other investigators. The low vH values, as well as the one derived in this work, significantly reduce uelas for ZrMo2-H2, and thus reduce the corresponding critical temperature for the metal-to-hydride phase transition, and the heat of hydride formation. We suggest that the bulk stiffening in ZrMo2 confines the corresponding hydride expansion and thus reduces the H-H elastic interaction.

This contribution is devoted to hydrogen bonds and other weak interactions, which play an important role in the chemistry of transition metal hydrides determining their stability and reactivity as proton or hydride donors. The formation of M‐H δ‐⋯δ+ HX or M‐H δ+ ⋯Y hydrogen bonds entails a stretching of M‐H bond and its additional polarization (repolarization). The dihydrogen bond formation activates not only the M‐H, but also the X‐H bond, preparing them for the subsequent reaction. Ultimate extension of this concept is dihydrogen bonding between two hydride complexes of different polarity, M‐H δ−⋯δ+ H‐M', which activates two M‐H bonds in one step. Proton transfer within such dihydrogen bonded complex yields unusual ionic complex with µ,η 1:1 end‐on coordinated H 2 . The reactivity of hydride and dihydrogen complexes can be further tuned using weak solute‐solvent interactions.

A new subfield of bioorganometallic chemistry is evolving, which derives from hydrogenase enzymes: metalloproteins containing low‐valent, first‐row transition metals within metal–metal binding distance and stabilized by the most classical of organometallic ligands, carbon monoxide. The review of the structures, mechanisms, and synthetic analogs of the active sites of [NiFe]‐, [FeFe]‐, and [Fe]‐hydrogenase enzymes recalls the discovery and characterization of such organometallics, well buried within proteins and required by some of the most ancient of organisms. Historically, the role of synthetic chemists in the study of enzymes has been largely directed toward providing structural and spectroscopic references for biochemists. The same is true with hydrogenases; however, with the obvious structures and reactivity in the active sites, organometallic chemists are moving forward with enzyme active site‐inspired compounds that display catalytic function, both as small molecules in their own right and through incorporation into active enzymes. These are described in this chapter.

Environmentally benign synthesis of graphite at low temperatures is a great challenge in the absence of transition metal catalysts. Herein, we report a green and efficient approach of synthesizing graphite from carbon dioxide at ultralow temperatures in the absence of transition metal catalysts. Carbon dioxide is converted into graphite submicroflakes in the seconds timescale via reacting with lithium aluminum hydride as the mixture of carbon dioxide and lithium aluminum hydride is heated to as low as 126 °C. Gas pressure-dependent kinetic barriers for synthesizing graphite is demonstrated to be the major reason for our synthesis of graphite without the graphitization process of amorphous carbon. When serving as lithium storage materials, graphite submicroflakes exhibit excellent rate capability and cycling performance with a reversible capacity of ~320 mAh g –1 after 1500 cycles at 1.0 A g –1 . This study provides an avenue to synthesize graphite from greenhouse gases at low temperatures. Green synthesis of graphite is a great challenge in the absence of the graphitization of amorphous carbon at high temperatures. Here, the authors report a green approach of synthesizing graphite from carbon dioxide at low temperature in seconds timescale.

With the discovery 1 of superconductivity at 203 kelvin in H 3 S, attention returned to conventional superconductors with properties that can be described by the Bardeen–Cooper–Schrieffer and the Migdal–Eliashberg theories. Although these theories predict the possibility of room-temperature superconductivity in metals that have certain favourable properties—such as lattice vibrations at high frequencies—they are not sufficient to guide the design or predict the properties of new superconducting materials. First-principles calculations based on density functional theory have enabled such predictions, and have suggested a new family of superconducting hydrides that possess a clathrate-like structure in which the host atom (calcium, yttrium, lanthanum) is at the centre of a cage formed by hydrogen atoms 2 – 4 . For LaH 10 and YH 10 , the onset of superconductivity is predicted to occur at critical temperatures between 240 and 320 kelvin at megabar pressures 3 – 6 . Here we report superconductivity with a critical temperature of around 250 kelvin within the F m 3 ¯ m structure of LaH 10 at a pressure of about 170 gigapascals. This is, to our knowledge, the highest critical temperature that has been confirmed so far in a superconducting material. Superconductivity was evidenced by the observation of zero resistance, an isotope effect, and a decrease in critical temperature under an external magnetic field, which suggested an upper critical magnetic field of about 136 tesla at zero temperature. The increase of around 50 kelvin compared with the previous highest critical temperature 1 is an encouraging step towards the goal of achieving room-temperature superconductivity in the near future. A lanthanum hydride compound at a pressure of around 170 gigapascals is found to exhibit superconductivity with a critical temperature of 250 kelvin.

Due to the high energy needed to break the N ≡ N bond (945 kJ mol −1 ), a key step in ammonia production is the activation of dinitrogen, which in industry requires the use of transition metal catalysts such as iron (Fe) or ruthenium (Ru), in combination with high temperatures and pressures. Here we demonstrate a transition-metal-free catalyst—potassium hydride-intercalated graphite (KH 0.19 C 24 )—that can activate dinitrogen at very moderate temperatures and pressures. The catalyst catalyses NH 3 synthesis at atmospheric pressure and achieves NH 3 productivity (µmol NH3 g cat −1  h −1 ) comparable to the classical noble metal catalyst Ru/MgO at temperatures of 250–400 °C and 1 MPa. Both experimental and computational calculation results demonstrate that nanoconfinement of potassium hydride between the graphene layers is crucial for the activation and conversion of dinitrogen. Hydride in the catalyst participates in the hydrogenation step to form NH 3 . This work shows the promise of light metal hydride materials in the catalysis of ammonia synthesis. Ammonia is industrially synthesized through an established process based on iron or ruthenium transition metal catalysts, although the quest for alternative and more sustainable processes is still ongoing. Here, the authors show that potassium hydride confined between graphene layers can reduce dinitrogen and catalyse ammonia synthesis under mild conditions.

Mg-based materials have been widely studied as potential hydrogen storage media due to their high theoretical hydrogen capacity, low cost, and abundant reserves. However, the sluggish hydrogen absorption/desorption kinetics and high thermodynamic stability of Mg-based hydrides have hindered their practical application. Ball milling has emerged as a versatile and effective technique to synthesize and modify nanostructured Mg-based hydrides with enhanced hydrogen storage properties. This review provides a comprehensive summary of the state-of-the-art progress in the ball milling of Mg-based hydrogen storage materials. The synthesis mechanisms, microstructural evolution, and hydrogen storage properties of nanocrystalline and amorphous Mg-based hydrides prepared via ball milling are systematically reviewed. The effects of various catalytic additives, including transition metals, metal oxides, carbon materials, and metal halides, on the kinetics and thermodynamics of Mg-based hydrides are discussed in detail. Furthermore, the strategies for synthesizing nanocomposite Mg-based hydrides via ball milling with other hydrides, MOFs, and carbon scaffolds are highlighted, with an emphasis on the importance of nanoconfinement and interfacial effects. Finally, the challenges and future perspectives of ball-milled Mg-based hydrides for practical on-board hydrogen storage applications are outlined. This review aims to provide valuable insights and guidance for the development of advanced Mg-based hydrogen storage materials with superior performance.