Formation and hydrogen storage properties of in situ prepared Mg–Cu alloy nanoparticles by arc discharge

Mg–Cu alloy nanoparticles were in situ prepared by a physical vapor condensation method (arc discharge) in a mixture of argon and hydrogen. Four crystalline phases, Mg, Mg₂Cu, MgCu₂ and MgO, were formed simultaneously during the arc-discharge evaporation. Detailed experiments revealed that nanostructured hydrogen-active phases of Mg₂Cu and Mg exhibit enhanced hydrogen absorption kinetics possibly due to the small grain size and surface defects. The maximal hydrogen storage contents of Mg–Cu alloy nanoparticles can reach 2.05 ± 0.10 wt% at 623 K.     

Non-isothermal kinetics and in situ SR XRD studies of hydrogen desorption from dihydrides of binary Ti–V alloys

Phase transformations during dynamic dehydrogenation of Ti_1−xV_xH₂ (x = 0.1; 0.2; 0.3) were studied using in situ Synchrotron X-Ray Diffraction (SR XRD) and non-isothermal kinetics experiments. The main dehydrogenation path for γ-Ti_1−xV_xH₂ was found to be γ → δ → β → β_alloy. Body-centred tetragonal δ-hydride was found to be an intermediate phase of the γ → β transformation in Ti_0.8–0.9V_0.1–0.2H₂. TDS, in situ SR XRD and isoconversional kinetics studies showed that hydrogen desorption from Ti_1−xV_xH₂ is composed of simultaneous reactions taking place between 300 and 600 °C. The effective activation energy of hydrogen desorption depends on the vanadium contents and the reaction pathway, increasing from 21 kJ/mol H₂ (γ → δ) to 60–110 kJ/mol H₂ (δ → β).     

The effect of reduction of pretreated NiO–ZnO catalysts on the water–gas shift reaction for hydrogen production as studied by in situ DRIFTS/MS

The water–gas shift (WGS) reaction on co-precipitated NiO–ZnO catalysts at different reduction temperatures has been studied by a temperature-programmed reaction using in situ diffuse reflectance infrared Fourier Transform Spectroscopy, coupled with mass spectroscopic (in situ DRIFTS/MS) techniques. The results reveal that a catalyst reduced at 493 K (labeled H220) showed higher activity than one reduced at 673 K (labeled H400) due to the ability of NiO on the H220 catalyst to promote CO conversion of the WGS reaction. In situ DRIFTS/MS studies show that there are three adsorbed species over the H220 catalyst at room temperature: adsorbed CO bands, molecularly adsorbed H₂O and carboxyl species. Increasing the temperature to 423 K led to the emergence of CO₂ and H₂ and the disappearance of carboxyl species. However, the low catalytic activity of the H400 catalyst could be attributed to the conversion of the NiO sites to reduced Ni metal sites, which (i) adsorbed CO as the strong linearly bonded CO on the catalyst surface, slowing down the CO reaction, and (ii) showed a lower H₂O uptake.     

Kinetics of hydrogen in Zr–H and Zr–D systems

We measured dependences of the electrical resistance on time of isothermal annealing for Zr rods saturated electrolytically by hydrogen or deuterium. The annealing of samples was carried out at temperatures 305–498 K. The resistance of inhomogeneously saturated samples increased with the time of annealing. The model of diffusion of the hydrogen from the surface of the sample into its volume described this increase adequately. The resistance of homogeneously saturated samples had a minimum at some time of annealing. We showed that the decrease of the resistance during annealing obeyed the exponential law, and that the characteristic time of the decrease obeyed the Arrhenius law with the activation energy about 0.16 eV. We supposed that the resistance decreases due to the formation of the hydride in the saturated layer or on the boundaries of grains.     

Effect of para–ortho conversion on hydrogen storage system performance

We present a study of the effects of para–ortho conversion on performance of an adsorption-based hydrogen storage system using finite element methods implemented in COMSOL Multiphysics 4.3a platform. The base model which does not take into account the para–ortho conversion is validated using the experimental data of Maxsorb activated carbon measured with a test bench at room and cryogenic temperatures. The validated model is subsequently applied to simulate the storage system filled with MOF-5 and then extended to investigate the effects of endothermic para–ortho conversion of hydrogen isomers on storage and thermal performances during hydrogen charging/discharging cycle for four inlet temperatures, 35, 50, 77 and 100 K. Our results show that the endothermic conversion reduces the system temperature and increases the net storage capacity. The temperature changes due to the different heat sources are used to investigate the effect of conversion on the temperature reduction. The adsorbed and gas phase masses in the storage system with and without conversion at the end of the charging time are used to determine the effect of conversion on the storage system capacity. Even though the conversion is more significant at low temperature (35 K), the gains are larger at high temperature (100 K).     

MgH2–Nb2O5 investigated by in situ synchrotron X-ray diffraction

In situ real time synchrotron radiation powder X-ray diffraction (SR-PXD) experiments are utilized to study changes in the crystalline compounds under dynamic hydrogenation and dehydrogenation reactions of MgH₂ ball milled with 8 mol% Nb₂O₅. The ball milling conditions were systematically varied to prepare three samples with different reactivity. Up to eight full cycles of hydrogen release and uptake were investigated for each sample, which reveal that Nb₂O₅ reacts with Mg forming a ternary oxide, Mg_xNb_1−xO. The PXD data for the ternary oxide is similar to that observed for the isostructural compounds MgO and NbO although shifted to lower Bragg diffraction angles revealing an expansion of the unit cell. Rietveld refinements suggest that Mg_xNb_1−xO has a limiting composition of x ∼ 0.6 after eight cycles of hydrogen release and uptake. At elevated temperatures Nb(II) is reduced to metallic Nb(0) and extracted from the ternary oxide and forms in a reaction with Mg. This work suggests that a ternary solid solution Mg_xNb_1−xO is the active material responsible for the prolific kinetic properties for the additive Nb₂O₅. Mg_0.6Nb_0.4O has a ∼4.6% larger unit cell volume as compared to the binary oxides, MgO and NbO, which may lead to formation of cracks and hydrogen diffusion pathways in dense magnesium oxide surface layers.     

Kinetics of methane decomposition to COx-free hydrogen and carbon nanofiber over Ni–Cu/MgO catalyst

Kinetic modeling of methane decomposition to CO_x-free hydrogen and carbon nanofiber has been carried out in the temperature range 550–650 °C over Ni–Cu/MgO catalyst from CH₄–H₂ mixtures at atmospheric pressure. Assuming the different mechanisms of the reaction, several kinetic models were derived based on Langmuir–Hinshelwood type. The optimum value of kinetic parameters has been obtained by Genetic Algorithm and statistical analysis has been used for the model discrimination. The suggested kinetic model relates to the mechanism when the dissociative adsorption of methane molecule is the rate-determining stage and the estimated activation energy is 50.4 kJ/mol in agreement with the literature. The catalyst deactivation was found to be dependent on the time, reaction temperature, and partial pressures of methane and hydrogen. Inspection of the behavior of the catalyst activity in relation to time, led to a model of second order for catalyst deactivation.     

Nanostructured rapidly solidified LaMg11Ni alloy. II. In situ synchrotron X-ray diffraction studies of hydrogen absorption–desorption behaviours

Recently, the present authors [17] have reported dramatic improvements in the hydrogenation behaviours of nanostructured LaMg₁₁Ni prepared by Rapid Solidification, caused by modifications of the microstructure and crystal structure. The aim of the present work was to study the mechanism and kinetics of the hydrogen interaction with rapidly solidified LaMg₁₁Ni by employing in situ synchrotron X-Ray diffraction studies of hydrogen absorption–desorption processes in hydrogen gas or in vacuum.     

CFD analysis and experimental measurements of the liquid aluminum spray formation for an Al–H2O based hydrogen production system

The paper proposes a combined approach between numerical modeling and experimental measurements for the analysis of a cogeneration system based on the reaction of liquid aluminum and water steam. Scrap aluminum is used for hydrogen production and the primary one is employed as an energy carrier to transport the energy from the alumina reduction system to the site of the suggested plant. The analysis focuses on the liquid aluminum injection phase immediately downstream the nozzle.High frequency thermo-cameras are employed to qualitatively assess the thermal behaviour the liquid aluminum jet. Fast imaging techniques are used to capture the multiphase flow pattern of the liquid metal jet during the injection phase.The experimental results are used to validate a 2D multi-phase CFD approach. The computational fluid dynamics model of the injection phase is created and used to extend the measurements and deepen the understanding of the thermo-fluid dynamics behaviour of the system. In particular, the influence of different nozzles diameters and different injection pressures on the liquid aluminum jet is investigated.A modular approach is adopted for the domain subdivision in order to represent accurately all the geometrical features, while the volume of fluid approach is used to model the multi-phase flow distribution in the real geometry under actual operating conditions. Finally, a good agreement between the measurements and the calculations is found.     

Porous Ni–Co surface formation and analysis of hydrogen generation by gas sensor

A Ni–Co alloy was used as the test piece. The porous Ni–Co alloy surface was prepared by Al electrodeposition at ?1.4 V and ?1.8 V and Al dissolution at ?0.5 V in a NaCl–KCl-3.5 mol% AlF₃ molten salt. The bath temperature was 750 °C and 900 °C. As a result, a porous Ni–Co alloy could be prepared by Al electrodeposition and Al dissolution on the Ni–Co alloy in the molten salt. It was clarified that a denser surface was formed at the bath temperature of 750 °C than at the bath temperature of 900 °C. Furthermore, it was clarified that the porous layer became thicker when the electrodeposition potential was ?1.8 V than when it was ?1.4 V. The formed porous Ni–Co alloy was evaluated for cathode performance in a 10 mass% KOH solution. Furthermore, the amount of generated hydrogen was measured by a constant voltage and constant current test with a gas sensor using a solid electrolyte. In the cathode polarization curve, the porous Ni–Co alloy showed a higher current density at a lower potential than the untreated Ni–Co alloy. It was shown that the Ni–Co alloy formed under the electrodeposition conditions at the electrodeposition potential of ?1.4 V and bath temperature of 750 °C is a very excellent cathode material. Furthermore, based on the constant voltage test, it was revealed that the porous treated sample generates a higher amount of hydrogen than the untreated sample.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Kinetics of non-premixed hydrogen–oxygen catalytic recombination reaction at ambient temperature

This study proposes the use of the hydrogen–oxygen catalytic recombination reaction to safely eliminate the leaked hydrogen in a confined environment. Experiments on the hydrogen–oxygen reaction catalyzed by using Pt/C as a catalyst are conducted at ambient temperature in a small cylindrical vessel. The macroscopic kinetic process of the hydrogen–oxygen recombination reaction is investigated, and the effects of the reaction parameters, such as the initial hydrogen volume fraction and catalyst layer position, on the reaction temperature and hydrogen conversion are examined. The reaction temperature and temperature rise rate are shown to reach the maximum values when the initial hydrogen fraction is 70 vol%. When the initial hydrogen fraction is ≤ 67 vol%, the hydrogen conversion reaches 100%. After the initial hydrogen fraction is > 67 vol%, the hydrogen conversion decreases significantly, and the hydrogen conversion is only 53% for the initial hydrogen fraction is up to 80 vol%. Moreover, the position of the catalyst layer has a significant effect on the reaction rate and heat distribution inside the vessel. When the catalyst layer is near the bottom of the reaction vessel, the reaction rate is accelerated and the released heat accumulates at the bottom of the vessel. The influence law of the aforementioned factors can provide a technical reference for applications of the hydrogen–oxygen catalytic reaction.     

Combined particle mass spectrometer – Quartz crystal microbalance apparatus for in situ nanoparticle monitoring during flame assisted synthesis

A newly developed apparatus for simultaneous in situ monitoring of size, mass and number concentration of a flame synthesised particle aerosol is presented. The diagnostics methodology is relying on combination of particle mass spectrometry and quartz crystal microbalance techniques. The analysis protocol includes molecular beam sampling, sorting the charged particles constituent according to mass/charge ratio (m/ze) via electrostatic deflection, and detection of the neutral particles by monitoring the variation of the quartz crystal oscillation frequency upon exposure to the particle – laden molecular beam. From these measurements, the total mass concentration and probability density distribution of m/ze could be directly determined, allowing to deduce the particle number concentration in the aerosol, at least in relative units. The feasibility of the method was demonstrated on the example of iron oxide nanoparticles (NPs) forming in CH₄/O₂/N₂ dual stage flame doped with iron pentacarbonyl, Fe (CO)₅ as a precursor of iron oxide NPs. Sensitivities of 6 × 10⁴ NP/s and 8 × 10⁶ NP/s were achieved for charged and neutral particles, respectively. Possible implications of the obtained results on the elucidation of iron oxide NP formation mechanism are discussed.     

An efficient ZnS-UV photocatalysts generated in situ from ZnS(en)0.5 hybrid during the H2 production in methanol–water solution

Highly active ZnS-UV was obtained in situ from ZnS(en)_0.5 hybrid during the hydrogen formation using a methanol–water solution under UV irradiation. X-ray diffraction patterns and UV spectroscopy for both ZnS-UV and ZnS-400 obtained from the calcination of the ZnS(en)_0.5 hybrid showed similar structural and photophysical properties; however, the efficiency of the ZnS-UV semiconductor was 7 times higher (4825 μmol h⁻¹ g⁻¹) compared to the ZnS-400. The highest H₂ production was obtained using a UV lamp of very low intensity (2.2 mW cm⁻¹) and it is attributed to a quantum size effect caused by the slow elimination of ethylenediamine (en) in the structural ZnS layer during the UV irradiation.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.     

Effect of chemisorbed CO on MoOx–Pt/C electrode on the kinetics of hydrogen oxidation reaction

The influence of poisoning of MoO_x–Pt catalyst by CO on the kinetics of H₂ oxidation reaction (HOR) at MoO_x–Pt electrode in 0.5 mol dm⁻³ HClO₄ saturated with H₂ containing 100 ppm CO, was examined on rotating disc electrode (RDE) at 25 °C. MoO_x–Pt nano-catalyst prepared by the polyole method combined with MoO_x post-deposition was supported on commercial carbon black, Vulcan XC-72. The MoO_x–Pt/C catalyst was characterized by TEM technique. The catalyst composition is very similar to the nominal one and post-deposited MoO_x species block only a small fraction of the active Pt particle surface area. MoO_x deposition on the carbon support can be ruled out from the EDAX results and from the low mobility of these oxides under used conditions. Based on Tafel–Heyrovsky–Volmer mechanism the corresponding kinetic equations from a dual-pathway model were derived to describe oxidation current–potential behavior on RDE over entire potential range, at various CO coverages. The polarization RDE curves were fitted with derived polarization equations according to the proposed model. The fitting showed that the HOR proceeded most likely via the Tafel–Volmer (TV) pathway. A very high electrocatalytic activity observed at MoO_x–Pt catalyst for the hydrogen oxidation reaction in the presence of 100 ppm CO is achieved through chemical surface reaction of adsorbed CO with Mo surface oxides.     

Research into the formation process of hydrogen–air mixture in hydrogen fueled engines based on CFD

The density of hydrogen is much smaller than that of air, so it is hard for hydrogen and air to form high grade mixture. Furthermore, the diffusing speed of hydrogen is so high that the formation state of mixture changes rapidly. Therefore it will become more difficult to carry through the further research of mixture space–time distributing rule. In order to investigate the formation rule of hydrogen–air mixture and improve the mixture quality, in this paper, computation fluid dynamics (CFD) mode is adopted to carry through three-dimensional numerical simulation research of flow field in hydrogen fueled engine cylinder. The numerical simulation is done in a two-stroke hydrogen fueled engine, and the mixture forming state at different hydrogen-injecting time is contrasted. The evolvement rule of flow field in cylinder and mixture forming state is shown in the result. The simulation results show that, when hydrogen-injecting begins at 260 °CA, the forming quality of the mixture is better than other two states, this is the same as the experimental results. It indicates that CFD is one of the effective methods to analyze the formation of mixture in hydrogen fueled engine.