Nb-H System Research Articles

Highly efficient reduction of p-Nitrophenol by sodium borohydride over binary ZIF-67/g-C3N4 heterojunction catalyst

In this study, a facile coprecipitation based synthesis scheme was used to prepare a non-noble metal doped graphitic carbon nitride-based catalyst ZIF-67/g-C3N4 (abbreviated as ZIF-CN) for the reduction of nitrophenols (NPs) in presence of sodium borohydride (NBH). The binary nanocomposite involves Co-nanoparticles (diameter: 5–10 nm), embedded in the conductive g-C3N4 (gCN) layer. The synthesized catalyst exhibits excellent reduction activities towards various nitrophenols (NPs) including 4-nitrophenol (PNP), 2,4-dinitrophenol (DNP) and 2,4,6-trinitrophenol (TNP). The rate constant of PNP reduction in ZIF-CN(5)/NBH system was 3.8 times and 7.3 times that of pristine ZIF-67 and gCN, respectively, in presence of 0.15 g/l catalyst and 10.0 mM NBH, at pH 7. The presence of anions (at 0.1 mM), i.e.,H2PO4−,HCO3−,SO42−andNO3−decreases the rate of PNP reduction, whereas, Cl- marginally influences the process. The continuous, conductive gCN sheets introduce large number of active sites for adsorption of BH4−and NP and subsequent electron transfer from the π-complex to the Co-nodes, forming active hydrogen species, accelerating the reduction. Moreover, the synthesized ZIF-CN catalyst can be reused for at least 10 consecutive cycles, without considerable loss of catalytic activity. Therefore, the synthesized ZIF-CN catalyst has enormous potential towards NP reduction, at ambient conditions.

Nb-H system at high pressures and temperatures

We studied the Nb-H system over extended pressure and temperature ranges to establish the highest level of hydrogen abundance we could achieve from the resulting alloy. We probed the Nb-H system with laser heating and x-ray diffraction complemented by numerical density functional theory-based simulations. New quenched double hexagonal close-packed (hcp) $\mathrm{Nb}{\mathrm{H}}_{2.5}$ appears under 46 GPa, and above 56 GPa cubic $\mathrm{Nb}{\mathrm{H}}_{3}$ is formed as theoretically predicted. Nb atoms are arranged in close-packed lattices which are martensitically transformed in the sequence: face-centered cubic (fcc) \ensuremath{\rightarrow} hcp \ensuremath{\rightarrow} double hcp (dhcp) \ensuremath{\rightarrow} distorted body-centered cubic (bcc) as pressure increases. The appearance of fcc $\mathrm{Nb}{\mathrm{H}}_{2.5\ensuremath{-}3}$ and dhcp $\mathrm{Nb}{\mathrm{H}}_{2.5}$ cannot be understood in terms of enthalpic stability, but can be rationalized when finite temperatures are taken into account. The structural and compressional behavior of $\mathrm{Nb}{\mathrm{H}}_{xg2}$ is similar to that of NbH. Nevertheless, a direct H-H interaction emerges with hydrogen concentration increases, which manifests itself via a reduction in the lattice expansion induced by hydrogen dissolution.

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients Dt of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbHxTy and α-NbDxTy (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on Dt show typical hydrogen concentration dependence: Dt of tritium for both α-NbHxTy, and α-NbDxTy, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of Dt of tritium differs from that of Dt of protium in α-NbHx or of deuterium in α-NbHxTy. On the other hand, no appreciable differences in the concentration dependence of Dt of tritium between α-NbHxTy, and α-NbDxTy, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that Dt of tritium for a tritiated niobium (α-NbTx) is not very different from that for α-NbHxTy and α-NbDxTy. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained Dt of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.

Hydrogen Concentration Dependence of Tritium Tracer Diffusion Coefficient in Alpha Phase of Niobium

In order to examine influences of coexistent hydrogen isotopes on diffusion behavior of tritium in niobium, tracer diffusion coefficients Dt of tritium in alpha phase of hydrogenated and deuterized niobium (α-NbHxTy and α-NbDxTy (x<0,8,y<<x)) have been measured at 473 K, 493 K and 553 K. The data on Dt show typical hydrogen concentration dependence: Dt of tritium for both α-NbHxTy, and α-NbDxTy, decreases with hydrogen concentration under all experimental conditions. The obtained concentration dependence of Dt of tritium differs from that of Dt of protium in α-NbHx or of deuterium in α-NbHxTy. On the other hand, no appreciable differences in the concentration dependence of Dt of tritium between α-NbHxTy, and α-NbDxTy, are observed: there are no definite isotope effects due to the coexisting hydrogen isotopes. This result suggests that Dt of tritium for a tritiated niobium (α-NbTx) is not very different from that for α-NbHxTy and α-NbDxTy. The chemical diffusion coefficient D* of tritium is also evaluated on the basis of the obtained Dt of tritium and of a literature value of a thermodynamic factor F for Nb-H and Nb-D systems.

Hydrogen-Induced Defects in Niobium Studied by Positron Annihilation

Changes of the defect structure of niobium induced by hydrogen loading are presented in this work. It was found that annealing of virgin bulk Nb (99.9%) at 1000 °C for 1h leads to a complete recovery of defects. Subsequently, the defect-free samples were step-by-step electrochemically loaded with hydrogen up to xH = 0.06 (H/Nb atom ratio), i.e. in the �-phase region, where the Nb-H system represents a single- phase solid solution. The evolution of the microstructure with increasing hydrogen concentration was studied by X-ray diffraction and two complementary techniques of positron annihilation spectroscopy (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with measurement of Doppler broadening. It was found that new defects were created due to hydrogen loading. The concentration of these hydrogen-induced defects increases with increasing hydrogen concentration. A comparison of PAS results with theoretical calculations revealed that complexes consisting of a vacancy, surrounded likely by four hydrogen atoms, were introduced into the samples due to hydrogen loading.

Alloying Effects on the Hydriding Properties of Niobium Metal

Niobium metal forms two kind of hydrides, NbH and NbH 2 , by hydrogenation, and the corresponding plateau region appears at low and high hydrogen permeable regions in the PCT curve of the Nb-H system. However, alloying effects have never been investigated in this system. In the present study, alloying effects were firstly investigated with various binary Nb-3 mol%M and Nb-5 mol%M alloys. Here M's were 4d transition metals, Zr, Mo, Ru, Rh and Pd, all of which were soluble in bcc Nb. The stability of each hydride was found to be varied with the addition of a small amount of alloying elements into niobium metal. For example, the β (NbH) phase became most unstable by the addition of Ru, whereas δ (NbH 2 ) phase became most unstable by the addition of Mo. These data will be useful for the design of hydrogen permeable niobium-based membranes.

Finite element evaluation of elastic accommodation energies during room temperature α-β hydrogen transformations in the niobium-hydrogen system: Plane stress formulation

A plane stress finite element model, based on the initial strain approach, has been proposed to evaluate elastic accommodation energies during solid state hydrogen transformations. Elastic accommodation energies as a function of transformation progress have been obtained for the forward and reverse hydrogen α-β transformations in the Nb-H system at 313K. The evaluated elastic accommodation energies are comparable to those obtained by continuum mathematical models and to the published experimental values. The finite element model can successfully account for relaxation of stresses due to the presence of free surfaces.

Self-organized criticality of the fracture processes associated with hydrogen precipitation in niobium by acoustic emission.

The statistical analysis of the acoustic signals, emitted by hydrogenated Nb samples during hydrogen precipitation from solid solution, reveals that the distribution of their amplitudes follows the Gutenberg-Richter power law, N(A'gA)\ensuremath{\sim}${\mathit{A}}^{\mathrm{\ensuremath{-}}\mathit{c}}$ with c=0.9 over more than 2 orders of magnitude. This behavior can be considered as evidence that fracture processes due to hydride precipitation in the Nb-H system determine a self-orignized critical state.

Low Frequency Peak of Internal Friction at α + α'&lt;====&gt; α + β Peretectical Reaction in Nb-H Systems

Low Frequency Peak of Internal Friction at α + α'&lt;====&gt; α + β Peretectical Reaction in Nb-H Systems

Cross relaxation between proton and quadrupolar nuclear spins in metal-hydrogen systems.

Enhanced proton-NMR spin-lattice-relaxation rates ${\mathit{R}}_{1}$ measured over a range of frequencies and temperatures in the Nb-H, Nb-V-H, and Ta-H metal-hydrogen systems are attributed to dipolar cross relaxation between the Zeeman energy levels of the proton spin and the combined Zeeman-quadrupole levels of the metal nuclear spin. This cross-relaxation mechanism can dominate the total ${\mathit{R}}_{1}$ at low temperatures even in static (nonrotating) polycrystalline samples, resulting in relaxation rates up to 400 times greater than those expected from electronic relaxation alone. At low temperatures, the relaxation rates decreased roughly linearly with temperature, in some cases extrapolating to zero at 0 K, and in other cases having a finite intercept at 0 K. We calculate the cross-relaxation rate for the cases of Nb-H and Ta-H and compare the results with experimental data. A crucial feature of our interpretation is that, even in the ordered hydrides, there is substantial structural disorder. Although experiments were limited to proton relaxation in metal-hydrogen systems, our conclusions about the effects of cross relaxation on the total spin-lattice-relaxation rate should apply equally well to other heteronuclear spin systems in solids having some degree of structural disorder.

Phase equilibria in the niobium-vanadium-hydrogen system

The effect of vanadium additions to niobium on the metal-hydrogen phase equilibria has been studied. Measurements of the equilibrium H 2(D 2) pressure-composition-temperature isotherms for Nb 1 − x V x alloys with 0 ⩽ x < 0.2 were used to determine the depression of the α — α' critical temperature with increasing vanadium concentration. A simple lattice-fluid model guided reduction of the data. Changes in the triple point temperature as well as the shift of the ζ → ϵ phase transition were determined by differential scanning calorimetry measurements. A rapid overall depression was found, of the order of 7 K (at.% substituted V) −1, for the metal-hydrogen (deuterium) phase boundary structure when compared with the Nb-H system in the hydrogen concentration range of interest. The results explain the enhanced terminal solubility of hydrogen in this system found by Miller and Westlake. The changes in the phase equilibria are discussed in terms of the effect of hydrogen trapping and compared with the results of a cluster-variation calculation for random-field systems of MacGillivray et al., taking into account a distribution of H-site energies due to alloying.

Absence of phase separation in ion-implanted hydrides: the case of Nb-H

Monocrystalline thin Nb films ( approximately 600 AA) were implanted at 15 K with hydrogen ions (proton energy: 14 keV) up to H/Nb approximately 1.5. In situ TEM studies were performed, which show a linear increase of the lattice parameter with the implantation fluence. About six ordered hydride phases are found between H/Nb=0 and 1.5 when Nb is charged with H under quasi-equilibrium conditions (e.g., high pressure or electrolytic charging); the present low-temperature-implanted Nb-H system is single phased throughout the entire concentration range. Specific effects due to the implantation-induced disorder and to the film thickness are discussed.

Mössbauer and time-differential perturbed angular correlation studies of the hydrides and deuterides of vanadium

Hydrides and deuterides of vanadium were studied by source and absorber Mössbauer experiments with the resonances of 99Ru, 193Ir and 197Au and by perturbed angular correlation experiments with the 353 — 90 keV γ-γ cascade in 99Ru. The observed isomer shifts and electric quadrupole interactions are discussed in terms of the local environment of the probes in the different hydride and deuteride phases and compared with similar results for the Nb-H system.

On some measurements of the propagation properties of thermal waves in the Nb-H and Ta-H systems

By means of an experimental technique based on a nonsteady-state method,i.e. on the propagation of thermal waves, we have measured at room temperature for oscillation frequencies between 6 and 30 mHz the thermal diffusivity, the lateral thermal-loss coefficient and the «fractional heat loss» in Nb and Ta wires as a function of hydrogen doping. The appearance of hydride formation and precipitation notably changes the behaviour of these quantities. From these measurements it was also possible to calculate the propagation velocity and attenuation coefficient of thermal waves. It was found that the increase of the hydrogen concentration in the solid solution produces a gradual decrease in the velocity and increase in the attenuation coefficient until the solubility limit is reached. For larger hydrogen concentrations, the velocity showed a tendency to increase towards the value of the pure metal, while the attenuation coefficient decreased below the value of the pure metal. These results as a whole appear quite promising for studying the properties of hydrogenated systems.

Elastic constants and ultrasonic attenuation of the α-α' phase of the Nb-H(D) system. II. Interpretation of results

For pt.I see ibid., vol.15, p.507 (1985). An analysis of the variation of the elastic moduli and attenuation in the Nb-H(D) system with composition and temperature is carried out. The composition-dependent changes in elastic moduli are shown to consist of a temperature-independent part and a temperature-dependent part. The temperature-independent changes are analysed on the basis of contributions due to changes in volume and in the electronic structure of the alloy. The isotopic dependence of these contributions is shown to be very small. The temperature-dependent contributions to the changes in elastic moduli are each associated with an acoustic attenuation and exhibit a significant isotopic dependence. These temperature-dependent contributions are analysed to determine the critical temperatures and relaxation strengths as functions of composition. The role of phase transitions and stress-induced changes in short-range ordering is considered in interpreting the temperature-dependent changes in modulus and the attenuation. It is shown that the results are consistent with a relaxation due to stress-induced changes in the short-range ordering of the hydrogen interstitials, i.e. a Zener relaxation.

Elastic constants and ultrasonic attenuation of the α-α' phase of the Nb-H(D) system. I. Results

High-frequency acoustic measurements have been made of the elastic moduli and attenuation of single crystals of the Nb-H(D) system in the alpha - alpha ' phase. A novel transducer bonding method is described which allows accommodation of the volume change which accompanies alloying of Nb with H(D). This technique allows the acoustic measurements to be made throughout the alpha - alpha ' solid solution range up to H(D)/Nb=0.8 without interference from the phase transitions. The moduli and attenuation are determined as functions of temperature and frequency as the phase boundaries are approached, and the relaxations which precede the phase transitions are determined. The isotopic dependences of the moduli are determined throughout the alpha - alpha ' solid solution range.

Hysteresis in the Nb-V-H system

Studies of pressure-composition hysteresis in the Nb-V alloy hydrogen system revealed that the extent of hysteresis depends dramatically on the alloy composition. The hysteresis for the Nb-H system was greater by a factor of 4 than that for the V-H system. Also, a hysteresis maximum was observed at about 80% Nb with the absorption-to-desorption pressure ratio exceeding 20. A slight decrease in hysteresis effect was observed with increasing temperature. These results are discussed in terms of existing theories of hysteresis.

Localized mode energies and hydrogen potential in refractory metals

Recent experimental results on local vibrations of hydrogen atoms in niobium and tantalum are reviewed. Firstly, high resolution measurements of pure Nb-H(D, T) and Ta-H(D) systems where well-defined second harmonics have been resolved are examined and interpreted in terms of a weakly anharmonic potential common to all hydrogen isotopes. Secondly, the first observations by spectroscopic methods of the trapping of hydrogen atoms in niobium are surveyed. The measurements revealed detailed microscopic information, e.g. on the symmetry of the trapping sites and their degree of disturbance. Interstitial oxygen and nitrogen impurities as well as substitutional titanium were found to be strong traps which suppressed the precipitation of hydride phases at low temperature, whereas in the case of vanadium the easily recognizable vibrational pattern of ordered hydride phases was observed.

A resistometric and neutron diffraction investigation of the Nb-H system at high hydrogen concentrations

The electrical resistivity of various NbHx samples has been measured in the temperature range 80-420K for hydrogen concentrations x varying between 0 and 1.1. These measurements were supplemented by a neutron diffraction study of a sample with x=0.825. The singularities in the temperature dependence of the resistivity and the superstructure reflections are used to identify phase transitions. The concentration dependence of the resistivity is discussed on the basis of a model of the Fermi surface which states that the sheet associated with the N symmetry points disappears on hydrogenation. Similarities between the behaviour of the resistivity and the intensity of the superstructure reflections of the hydride are pointed out and interpreted as pre-transitional effects.

The low-temperature stable states of some transition metal hydrides

Ordering of interstitial solid solutions with solute atoms distributed among the sites of no more than two interstitial sublattices are considered at T = 0 K. A correlation between the set of values of energy constants and the type of the stable ordered state is indicated. Prom this point of view Ta-H and Nb-H systems are briefly analysed. [Russian Text Ignored].