Formation Of Metal Hydrides Research Articles

Single Particle Plasmonics for Materials Science and Single Particle Catalysis

Single particle nanoplasmonic sensing and spectroscopy is a powerful and at the same time relatively easy-to-implement research method that allows monitoring of changes in the structure and properties of metal nanoparticles in real time and with only few restrictions in terms of surrounding medium, temperature and pressure. Consequently, it has been successfully used in materials science applications to, for instance, reveal the impact of size and shape of single metal nanoparticles on the thermodynamics of metal hydride formation and decomposition. In this Perspective, we review and discuss the research efforts that have spurred key advances in the development of single particle nanoplasmonic sensing and spectroscopy as a research tool in materials science. On this background we then assess the prospects and challenges toward its application in single particle catalysis, with the aim to enable operando studies of the relationship between metal nanoparticle structure or oxidation state and catalytic perfo...

Electrocatalytic hydrogen evolution with gallium hydride and ligand-centered reduction.

GaIII chloride tetrakis(pentafluorophenyl)porphyrin (1) was synthesized and shown to be a highly active and stable post-transition metal-based electrocatalyst for the hydrogen evolution reaction (HER). Electrochemical and spectroscopic studies indicate that both the first and second reduction events of 1 are ligand-centered. The 2e-reduced form 1 2- reacts with a proton to give GaIII-H species (1-H), which undergoes protonolysis with Brønsted acids to produce H2. The identification of key intermediates 1 -, 1 2-, and 1-H leads to a catalytic cycle, which is valuable for the fundamental understanding of the HER. This study presents a rare but highly active HER electrocatalyst with unusual features, including ligand-centered electron transfer and formation of post-transition metal hydride.

Hydrogen Activation by Silica-Supported Metal Ion Catalysts: Catalytic Properties of Metals and Performance of DFT Functionals.

Single-site heterogeneous catalysts (SSHC) have received increasing attention due to their well-defined active sites and potentially high specific activity. Detailed computational studies were carried out on a set of potential SSHC's, i.e., silica-supported metal ions, to investigate the reactivity of these catalysts with H2 as well as to evaluate the performance of density functional theory (DFT) methods in conjunction with triple-ζ quality basis sets (i.e., cc-pVTZ) on reaction energetics. The ions considered include 4d and 5d metals as well as several post-transition metal ions. A representative cluster model of silica is used to calculate reaction free energies of the metal hydride formation that results from the heterolytic cleavage of H2 on the M-O bond. The hydride formation free energy is previously shown to be strongly correlated with the catalytic activity of such catalysts for alkene hydrogenation. ONIOM calculations (CCSD(T)//MP2) are used to assess the accuracy and reliability of the MP2 results and it is found that MP2 is a suitable level of theory for gauging the performance of DFT functionals. The performance of various DFT functionals is assessed relative to MP2 results and it is found that the wB97xd and PBE0 functionals have the lowest standard deviation (STD) value while the MN12SX and PBE functionals have the lowest mean absolute deviation (MAD) values. The B3LYP functional is shown to have similar MAD and STD values as the top performing functionals. Potential active SSHC's for exergonic hydrogen activation predicted in this study include mostly late and post transition metal ions, i.e., Au3+, Pd2+, Pt4+, Pd4+, Ir4+, Hg2+, Rh3+, Pb4+, Tl3+, In3+, Ir3+, Os4+, Cd2+, Ru2+, and Ga3+. This study provides important guidance to future computational studies of such catalyst systems.

The effect of hydrogen on the properties of polymorphic metals during thermal cycling near the polymorphism temperature

The effect of hydrogen on the numerous properties of metals is known. In general, it is associated with the structure of its atom, which consists of a proton and an electron. Getting into the metal, it turns into an elementary particle, which has its own special properties. The paper considers the case of the interaction of hydrogen with polymorphic metals during their thermal cycling around the temperature of polymorphism. The main metal of this study is iron. It was revealed that during thermocyclic treatment in a hydrogen medium in iron during cooling, the yield strength decreases catastrophically by 3–4 orders of magnitude (up to a certain time, it was considered one order). In the absence of hydrogen (thermal cycling in an inert atmosphere), such changes did not occur. It was determined that when the crystal lattice restructuring from the γ phase to the α phase, the solubility of hydrogen decreases and it accumulates at the transformation boundary. Depending on the process parameters (cooling rate, saturation pressure, overheating temperature, symmetry relatively to the transformation point, etc.), various types of involuntary metal flow can be obtained. Besides the iron, other polymorphic metals - manganese, cobalt, titanium, zirconium - have been investigated. The deformation effect was observed only for manganese. Cobalt, due to another mechanism of polymorphic transformation, can not be deformed. Hydride-forming metals under normal conditions of cycling are severely cracked (the formation of hydrides), their small deformation is possible under very low hydrogen pressure. The reason for the formation of such properties is the creation of a special zone on the boundary of two phases – the H-layer. The concentration of hydrogen in the H-layer can reach tens of atomic percentages. This is confirmed by numerous experiments on fixing a new phase (N-martensite) and change the physical and mechanical properties of iron. In the deformation process, protons actively interact with defects in the lattice, which causes the pores formation inside the metal. These studies were awarded a diploma for the opening No. 313 authors V.Yu. Karpov and V.I Shapovalov.

Competitive interplay of deposition and etching processes in atomic layer growth of cobalt and nickel metal films

Abstract

Nickel Complexes with Non‐innocent Ligands as Highly Active Electrocatalysts for Hydrogen Evolution

AbstractRational design of a molecular catalyst for a hydrogen evolution reaction (HER) with both high rates and low overpotential remains a challenge. Here we report a series of Ni‐based catalysts incorporating non‐innocent ligands and aimed at reduction of the overpotential by reserving electrons in the ligand, which could decouple the correlation between metal center reduction and metal hydride formation. The third‐order rate constant of our catalysts can be increased by 2 orders of magnitude without increase of overpotential by altering the substituents in the imide since the electronic effect of substituents can change the kinetics of metal hydride formation dramatically. Furthermore, constructing a proton shuttle in the ligand further improves the activity. The catalyst with both of these features can catalyze HER at a turnover frequency of 2572 s–1 with low concentrations of acid and an extremely high third‐order rate constant of 2.7×106 M–2·s–1 at a low overpotential.

Investigation of TiCr Hydrogen Storage Alloy

A new reversible hydrogen storage material, based on TiCr metal alloy, is proposed. Cr and Ti were mixed and melted in a final atomic ratio of 1,78. Chemical-physical characterisations, in terms of XRD and SEM-EDX, were performed. The quantification of Laves phases was performed through Rietveld refinements. The atomic Cr/Ti ratio was determined by EDX analysis and 1,71 was obtained. The H2 sorption/desorption measurements by Sievert apparatus were carried out. After different tests varying temperature and pressure, a protocol measurement was established; and a H2 sorption value of 0,4 wt% at 200 °C/10 bar with a fast kinetic at 5 bar (Dwt% of about 0,3 wt%) were obtained. Hydrogen desorption measurements performed in the same conditions of T confirmed a totally reversible trend. A confirm of metal hydride formation was recorded by XRD, in fact, comparing X-Ray patterns before and after volumetric tests a notable difference was recorded.

Universal Scaling and Design Rules of Hydrogen-Induced Optical Properties in Pd and Pd-Alloy Nanoparticles.

Hydride-forming metal nanoparticles sustaining localized surface plasmon resonance have emerged as prototypical material to study the fundamentals of hydrogen-induced phase transformations. They have also been proposed as signal transducers in next-generation hydrogen sensors. However, despite high current interest in hydrogen sorption by nanomaterials in general and such sensors in particular, the correlations between nanoparticle size, shape, and composition, the amount of hydrogen absorbed, and the obtained optical response have not been systematically experimentally studied. Focusing on hydrogenated Pd, PdAu- and PdCu-alloy nanoparticles, which are of particular interest in hysteresis-free plasmonic hydrogen sensing, we find that at practically important Au/Pd and Cu/Pd ratios the optical response to hydrogen concentration is linear and, more interestingly, can be described by a single universal linear trend if constructed as a function of the H/Pd ratio, independent of alloy composition. In addition to this correlation, we establish that the amplitude of optical signal change is defined solely by the spectral plasmon resonance position in the non-hydrogenated state for a specific nanoparticle composition. Thus, it can be maximized by red-shifting the LSPR into the NIR spectral range via tailoring the particle size and shape. These findings further establish plasmonic sensing as an effective tool for studying metal-hydrogen interactions in nanoparticles of complex chemical composition. They also represent universal design rules for metal-hydride-based plasmonic hydrogen sensors, and our theoretical analysis predicts that they are applicable not only to the H/Pd/Au or H/Pd/Cu system investigated here but also to other H/Pd/Metal combinations.

Hydrogen storage on LaNi5−xSnx. Experimental and phenomenological Model-based analysis

Three hydride-forming metals (LaNi5, LaNi4.73Sn0.27, and LaNi4.55Sn0.45) have been studied as solid phase hydrogen storage material in batch experiments using pure hydrogen and temperatures ranging from 300 K to 340 K. This process mainly involves: physisorption of hydrogen gas molecules; chemisorption and dissociation of hydrogen molecules; surface penetration of hydrogen atoms; hydride formation; and diffusion of hydrogen atoms through hydride-forming metal. In case the material is fully hydrided, hydride formation ceases and diffusion proceeds on the fully hydrided material.A phenomenological model was developed by aggregating the first four mechanisms in a single sorption kinetic term involving a first-order driving force, the remaining mechanism being the atomic diffusion in the hydride-forming material. The driving force is computed between external partial pressure and equilibrium pressure according to the Pressure-Composition-Temperature model (PCT). The corresponding parameters for an empirical PCT were estimated from equilibrium data. This equation is more suitable for process engineering optimization due to the smoothness in its concentration domain. Specific sorption rate and diffusion coefficients of the process were also estimated from dynamic data. From a sensitivity analysis, productivity proved to be related to particle diameter. In the frame of batch processes, the global rate is dominated by the sorption kinetic term at the beginning of the experiments with the material being free from hydride, whereas with more than 5–10% of the material being hydrided, diffusion dominates the process. LaNi5 shows higher hydrogen storage capacity than LaNi4.73Sn0.27 and LaNi4.55Sn0.45 within the investigated temperature and pressure ranges. Diffusion and sorption kinetic limited regions were identified from a sensitivity analysis of process productivity and normalized marginal values. The present work is oriented to modeling, designing, and optimizing storage and purification devices.

In-operando stress measurement and neutron imaging of metal hydride composites for solid-state hydrogen storage

Solid-state hydrogen storage in metal hydrides offers highest volumetric energy storage densities and low working gas pressures at the same time. Recently developed metal hydride composites (MHC) consist of a hydride-forming metal alloy and a secondary phase, typically graphite, to realize form-stable composites and short loading and unloading times (<5 min). Hydride formation causes a volume expansion of the storage material. Thus, it is mandatory to characterize this behavior for the sake of system safety.This work focuses on the in-operando characterization of the volume expansion of MHC that could trigger mechanical stresses acting on the walls and internal assemblies of the storage container. MHC with different metal particle shapes (flakes and powder) were studied. In-operando neutron imaging of axially freely expanding MHC was applied to analyze the time-resolved and spatial concentration of hydrogen, reaction fronts and the evolution of volume expansion and stability of the MHC. Stress measurements revealed that stresses of up to 330% of the respective operating gas pressure occur for confined MHC. Both techniques combined deliver crucial information and implications for the design of safe (according to ISO 16111), efficient and dynamic metal hydride storage systems.

Density Functional Approach Toward the Adsorption of Molecular Hydrogen as Well as the Formation of Metal Hydride on Bare and Activated Carbon-Supported Rhodium Clusters

A systematic density functional theory (DFT) investigation has been performed to understand adsorption phenomenon as well as the mechanism of hydrogen molecule dissociation to form metal hydride, catalyzed by bare and activated carbon-supported small rhodium clusters. H–H bond length of hydrogen molecule adsorbed on activated carbon-supported rhodium cluster is found to be higher. Five- as well as six-member rings of the activated carbon act as the support for hydrogen-adsorbed rhodium clusters. Rhodium cluster linked with five-member ring of activated carbon has the enhanced ability to activate hydrogen molecule. DFT-evaluated transition states show lower activation energies in the dissociation of hydrogen molecule catalyzed by activated carbon-supported rhodium clusters, whereas dissociation of hydrogen molecule catalyzed by Rhn, supported on activated carbon, follows two pathways. First, hydrogen atoms stay on Rhn after dissociation, and in the second, hydrogen is dislodged from Rhn to carbon of the ac...

Development of activity–descriptor relationships for supported metal ion hydrogenation catalysts on silica

Single-site heterogeneous catalysts receive increasing attention due to their unique catalytic properties and well-defined active sites. We report a combined computational and experimental study on a series of silica-supported metal ion hydrogenation catalysts (i.e., In3+, Ga3+, Zn2+, Mn2+, and Ti4+/SiO2). These catalysts were synthesized, characterized, and evaluated for gas-phase propylene hydrogenation. Computational studies were carried out on active-site structures and reaction mechanisms. An activity–descriptor relationship was established, which correlates a computational quantity (reaction free energy of the metal hydride formation) with the experimental reaction rate, as a function of the metal. Microkinetic modeling provided qualitative kinetic insights into the activity–descriptor relationship. This relationship was used to predict the trend of activities in a variety of M/SiO2 catalysts. These fundamental studies and the developed activity–descriptor relationship open up new opportunities for rational design of hydrogenation catalysts.

CFD modelling of a membrane reactor for hydrogen production from ammonia

Despite the growing use of hydrogen (H2) as a transport fuel, one of the major barriers still remaining is efficient and inexpensive fuel distribution and storage. Current approaches, such as compression, liquefaction or metal hydride formation, incur a significant energy penalty. Ammonia (NH3) has long been considered a prospective H2 medium, exhibiting a higher volumetric H2 density than liquid H2, through liquid-phase storage at mild pressure. Decomposition of NH3 into H2 and N2 can be achieved via use of catalytic reactors and fuel-cell-grade H2 can be produced using metal membranes at H2 distribution sites.In this study, a 3-Dimensional (3D) Computational Fluid Dynamics (CFD) model has been developed to understand the performance of the H2 separation process in gas mixtures derived from an NH3-cracking reaction. The reactor consists of 19 tubular membrane tubes, each 470 mm long, inside a tubular shell with an inner diameter of 130 mm. Standard transport and energy equations governing a 3D, pressure-based, steady-state model were derived from the laws of conservation of mass, momentum and energy. The governing equations were solved using commercial CFD software ANSYS Fluent 18.0. Gas flow and mixing were modelled by the two-equation standard k-epsilon model for closure. Coupled solver was used for pressure-velocity coupling, enabling a pseudo-transient option with pseudo time steps of 0.01 s. To estimate H2 permeation through the metal membrane, a constant H2 permeability of 3.0E–07 mol.m−1 s−1 Pa−0.5 derived from series of experiments tested under a range of industrial conditions, was used. Model simulations were conducted for an adiabatic temperature of 300 °C, a feed-side pressure of 7.8 bara and a permeate side pressure of 0.1 bara. A parametric analysis was carried out to explore the effects of variation in total feed-gas flow and effects of changes in NH3-cracking efficiency on H2 production rates and H2 yields. The model estimated that 4.6–11.6 kg H2/day can be produced from a 30-70 L min−1 NH3 inlet flow with 80-90% NH3-cracking efficiency. At lower NH3 inlet flow rates, higher H2 yields can be obtained within a shorter distance of the membrane tubes due to relatively slower velocities and longer residence times. At high inlet flow rates, H2 yields were significantly lower due to their faster velocities and shorter resident times, but high yields (>95%) were still observed at the membrane reactor outlet. A sensitivity analysis of the model showed that even if metal membranes functioned at only 50% of the maximum permeability, a high H2 yield similar to that estimated using 100% permeability can still be achieved at the H2 outlets.

Effects Of Measurement Conditions On Pressure-Concentration Isotherm And Thermodynamic Properties Of Lani4.6al0.4 Hydride

Pressure-concentration isotherms (PCIs) of LaNi4.6Al0.4 metal hydride are measured using a Sievert’s Apparatus at different temperatures to study the phenomenon of the formation of metal hydrides. PCI properties (plateau pressures, plateau slope, hysteresis, etc.) and thermodynamic properties (reaction enthalpy and entropy) of LaNi4.6Al0.4 metal hydride are calculated. To understand the effects of measurement parameters on PCI and thermodynamic properties the experimental conditions are varied. During the PCI measurements, the available pressure differences between supply volume and reactor volume are varied from 1 bar to 4 bar, then effects of the pressure interval (pressure difference) during each step of the absorption/desorption PCI measurement on the hydrogen storage capacity, equilibrium pressure, plateau slope, metal hydride bed temperature and hysteresis are studied. All of these properties showed a significant variation with pressure intervals. Later, to study the effect on thermodynamic properties, van’t Hoff plots are constructed for wide temperature range and reaction enthalpies are calculated for different temperature points. The effect of the temperature range on the enthalpy of formation is observed to be dependent on the experimental temperature range.

Surface-limited permeation regime in the study of hydrogen interactions with metals

The application of permeation technique in the surface-limited regime for determining the rates of hydrogen absorption and desorption and terminal solid solubility for precipitation in hydride-forming metals is discussed in detail. The validity of various approximate expressions used to process the experimental data is verified by comparison with the results of computer simulation. Examples of the processing of the experimental curves obtained in the study of hydrogen permeation are presented, and the main attention is paid to possible limitations of the approximations used.

Hydrogen effects in non-ferrous alloys: discussion.

This is a transcript of the discussion session on the effects of hydrogen in the non-ferrous alloys of zirconium and titanium, which are anisotropic hydride-forming metals. The four talks focus on the hydrogen embrittlement mechanisms that affect zirconium and titanium components, which are respectively used in the nuclear and aerospace industries. Two specific mechanisms are delayed hydride cracking and stress corrosion cracking.This article is part of the themed issue 'The challenges of hydrogen and metals'.

Stress development of metal hydride composites for high density hydrogen storage applications

Abstract Strong mechanical forces can appear from gas absorbing solids if a volume expansion occurs during the chemical reaction. In particular for metal hydride reactors, the evolution of mechanical stresses on the reactor wall or internal assemblies over many hydrogen absorption/desorption cycles needs to be considered for reactor safety. In this work, we report on a recently developed in-situ measuring principle that allows the determination of mechanical stresses that originate from a metal hydride formation. For this purpose, a unique lab-scale reactor, equipped with a measuring cell, has been designed and tested with recently developed metal hydride composites (MHC). For spatially confined MHC we found that mechanical stresses were developed two and a half times higher than the hydrogen gas pressure applied for hydride formation.

Study of the ternary system Al–H–RE (RE = Er, La and Y) in liquid state

Hydrogen is the only gas able to dissolve in aluminum. The solubility S of hydrogen in Al obeys the Sieverts' law and S is much larger in the liquid phase (above 660.4 °C) than in the solid one. This might lead to the formation of porosity during aluminum casting. In the present work, the ternary system Al–H–RE (RE = Er, La, Y) is investigated. The equilibria between the different phases are determined in the presence of liquid Al, RE and H2 gas by both experimental measurements and phase diagram calculations. The possible formation of metallic hydrides and the change in the hydrogen solubility S are discussed regarding pressure and temperature determining the thermodynamics of the system.

In-situ Pd–Pt nanoalloys growth in confined carbon spaces and their interactions with hydrogen

Optimized synthesis of Pd–Pt nanoalloys confined in mesoporous carbons by a simple and fast one-pot microwave assisted approach is reported herein. The influence of several synthetic parameters (Pd–Pt composition, cross-linker type, Pd precursor type and its addition time) on the carbon framework and Pd–Pt nanoparticles formation and characteristics was investigated. Small and uniform distributed nanoparticles on tailored mesoporous carbons are obtained in short time compared to the classical approaches. The metallic composition has a great influence on the nanoparticle size and an important effect on the carbon pore size distribution. When Pt content increases (from 10 to 90 at.%), an increase in the particle size (from 6.5 to 18 nm) and in the pore size distribution of the carbon support (from 3 to 13 nm) is observed. Bulk immiscible Pd–Pt alloys were formed in the whole composition range as highlighted by the linear relationship between the lattice parameter and the metal content. The Pd precursor, the cross-linker and the addition time proved to have a significant effect in the final size/shape of the Pd–Pt nanoparticles.The optimized synthetic method can successfully tailor the size and the confinement of nanoparticles into the carbon matrix. Consequently, the hydrogen absorption properties and hydride formation can be tuned by the particle size for the richest Pd-composition nanoalloy (Pd90Pt10). The smaller Pd–Pt nanoparticles (6 to 20 nm) are confined in the carbon matrix and are surrounded by a graphitic layer preventing the hydrogen absorption while larger particles (50 nm) absorb hydrogen with metal hydride formation.

Dehydrogenative Coupling of Hydrosilanes and Alcohols by Alkali Metal Catalysts for Facile Synthesis of Silyl Ethers

Cross-dehydrogenative coupling (CDC) of hydrosilanes with hydroxyl groups, using alkali metal hexamethyldisilazide as a single-component catalyst for the formation of Si–O bonds under mild condition, is reported. The potassium salt [KN(SiMe3)2] is highly efficient and chemoselective for a wide range of functionalized alcohols (99 % conversion) under solvent-free conditions. The CDC reaction of alcohols with silanes exhibits first-order kinetics with respect to both catalyst and substrate concentrations. The most plausible mechanism for this reaction suggests that the initial step most likely involves the formation of an alkoxide followed by the formation of metal hydride as active species.