Magnesium Hydrogen Research Articles

Kinetic investigation of the effect of milling time on the hydrogen sorption reaction of magnesium catalyzed with different Nb 2O 5 contents

The effect of milling time and Nb 2O 5 content on the magnesium hydrogen sorption reaction is investigated in this paper. The absorption and desorption kinetics of magnesium at 300 °C, with 0.05, 0.1, 0.2, 0.5, 1 mol% Nb 2O 5 contents and after 2, 5, 10, 20, 50, 100 h milling were measured. Reaction kinetics can be enhanced up to a certain limit by increasing the milling time or increasing the Nb 2O 5 content. The kinetic rate constants for different milling times and Nb 2O 5 contents were calculated and compared. Furthermore the rate-limiting step of the absorption and the desorption reactions was determined by fitting the respective kinetic equations. While the absorption is diffusion controlled, the rate-limiting step for desorption changes, with increasing of milling time or Nb 2O 5 content, from chemisorption to interface velocity of the transformed phase, demonstrating the effectiveness of the Nb 2O 5 catalyst.

Thermal Effect of Phosphate Salt Additives on Poly(L‐Lactic Acid) and the Coordination Structure

Poly(L‐lactic acid) (PLLA) was mixed with a few weight percent of magnesium or calcium hydrogen phosphate to improve the thermal property. Calcium hydrogen phosphate as an additive to PLLA was found to be more effective in increasing the glass transition point (Tg) than magnesium hydrogen phosphate, which was investigated by the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Studies on the coordination structure for the polymer complex (ca‐(plla)) confirmed by infrared (IR) spectroscopy and extended X‐ray absorption fine structure (EXAFS) indicated that calcium ion radius would be suitable for the coordination with carbonyl groups of polymer chains.

Preparation of alkaline earth phosphates with sol containing sodium alginate and sodium diphosphate

Magnesium hydrogen phosphate, calcium hydroxyapatite, and strontium hydroxyapatite were successfully prepared from sol consisting of sodium alginate and Na 4P 2O 7 with Mg 2+, Ca 2+, and Sr 2+ in the corresponding nitrates, respectively. It is revealed that the order of the addition of those substrates and the role of sodium alginate are important factors for the preparation of desired phosphate compounds. According to the previous paper on the preparation of calcium hydroxyapatite, sodium alginate was mixed with aqueous Na 4P 2O 7, followed by the addition of the aqueous divalent cations, resulting in the poor formation of the target phosphates. However, as a revised sol–gel technique, sodium alginate was added to the mixture of Na 4P 2O 7 and aqueous Mg 2+ and Sr 2+, resulting in a rather favorable formation of MgHPO 4 and strontium hydroxyapatite, respectively, while the sol thus obtained was stable within a few days. However for aqueous Ca 2+, calcium hydroxyapatite could not be obtained through the revised sol–gel technique. In the preparation of magnesium hydrogen phosphate, sodium alginate contributes mainly to the sol formation of the precursor. The ion exchange between Na + in sodium alginate and aqueous Ca 2+ was important for the preparation of calcium hydroxyapatite. In contrast, the reaction of sodium alginate with the mixture of Na 4P 2O 7 and aqueous Sr 2+ afforded strontium hydroxyapatite at the specific ratio of those three substrates. The structure of calcium and strontium phosphates prepared from the revised sol–gel process evidently depended on the amount of sodium alginate introduced into the mixture of Na 4P 2O 7 and the corresponding divalent cations.

Removal of aqueous ammonium with magnesium phosphates obtained from the ammonium-elimination of magnesium ammonium phosphate

In order to recycle magnesium ammonium phosphate (MgNH 4PO 4⋅6H 2O: MAP) obtained from MAP process, which is one of the attractive processes for removal of aqueous ammonium and phosphate from wastewater, ammonium elimination from MAP to magnesium phosphates and ammonium incorporation into the magnesium phosphates have been investigated in the present study. It is confirmed that magnesium hydrogen phosphate (MgHPO 4) is favorably obtained from the ammonium elimination from MAP at temperatures greater than 353 K, although magnesium phosphate (Mg 3(PO 4) 2) and magnesium pyrophosphate (Mg 2P 2O 7) have been suggested as possible candidates. Based on the dissolution–precipitation mechanism for the removal of aqueous ammonium with magnesium phosphates, three magnesium phosphates were employed for the removal of aqueous ammonium. The order of the removal rate of the aqueous ammonium was MgHPO 4 > Mg 3(PO 4) 2 > Mg 2P 2O 7, as expected from the solubility of those magnesium phosphates. The removability of the solid obtained from ammonium elimination of MAP is also confirmed. The present results show that MAP can be employed as an advanced material for the removal/recovery of ammonium, although it is generally accepted that an excess of MAP obtained from the wastewater treatment can be only used as a slow-acting fertilizer.

Calcium Phosphates as Fillers in Struvite Cements

Struvite or magnesium ammonium phosphate MgNH4PO4 has been proposed as active component in setting surgical cements. The usual formulation is one in which the magnesium component in the powder is either magnesium hydrogen phosphate trihydrate or trimagnesium phosphate or a mixture of these two compounds. As the cement liquid a concentrated solution of diammonium phosphate is taken. To make the cement attractive as a bone substitute material a calcium phosphate filler is generally incorporated. Thus such materials are a type of pseudo calcium phosphate cements. This study was intended to find out which calcium phosphate and which magnesium compound are the most suitable. In the first series of experiments a mixture of 12 g Mg3(PO4)2 and 4 g MgHPO4.3H2O was used as the magnesium component in the powder. To that powder 30 g of either precipitated hydroxyapatite PHA or CaHPO4 or CaHPO4.H2O or b-TCP or a-TCP was added. The cement liquid was a 3.5 M solution of (NH4)2HPO4. At specific liquid/powder ratios L/P suitable setting times were obtained for the different formulations. However, the compressive strengths after immersion of the cements in 0.9% saline solution at 37°C varied over a large range. The best formulation was that with a-TCP which reached a compressive strength of 57 MPa after 18 h of immersion. In the second series of experiments 20 g of Mg3(PO4)2 was used as the magnesium component in the powder. Again 30 g of either of the above mentioned calcium phosphates was used as filler and again a 3.5 M solution of (NH4)2HPO4 was used as the cement liquid. At the appropriate L/P ratios the respective setting times were longer than in the first series of experiments but all five formulations appeared to result in good compressive strengths varying from 41 MPa for the formulation with b-TCP to 67 MPa for the formulation with PHA. In the third series of experiments 30 g a-TCP was taken as the calcium phosphate in the powder. As magnesium components mixtures of Mg3(PO4)2.8H2O and MgHPO4.3H2O and Mg3(PO4)2 were used. Again the cement liquid consisted of a 3.5 M solution of (NH4)2HPO4. The formulations with Mg3(PO4)2.8H2O had the shortest setting times and the lowest compressive strengths, whereas those with Mg3(PO4)2 had the longest setting times and the highest compressive strengths. Therefore, it is advantageous to use Mg3(PO4)2 as the magnesium component.

Preliminary results on the Immobilisation of Radionuclides from Waters with Specific Adsorbers Based on Phosphate Salts

The present paper is focused on the ability of aluminium phosphate (ALPC), magnesium ammonium phosphate (MGPC), magnesium hydrogen phosphate (MGHPC), and calcium hydrogenphosphate (CAHPC), adsorbed onto charcoal, to immobilise actinides by adsorption from natural waters. The objective of this process is to evaluate the environmental pollution due to the actinides. Europium, thorium, protactinium, neptunyl, and uranyl ions were chosen to simulate actinides in the +3, +4, +5 and +6 oxidation state. The adsorbers were tested using natural waters samples. The adsorption trends and capacities were analysed. ALPC and MGPC exhibited a similar behaviour and adsorbed demonstrating that the +5, +4 and +3 actinide ions can be easily immobilised from natural waters and may be successfully used at pH 7-8. MGHPC may be used at a higher pH, whereas CAHPC is effective in the whole pH range. In all cases, thorium, protactinium and europium were strongly

MAGNESIUM HYDROGEN SULFATE POWDER CATALYZED STEREOSELECTIVE CONVERSION OF DIALKYL 2-(IMIDO-N -YL)-3-(TRIPHENYLPHOSPHORANYLIDENE)- BUTANEDIOATES TO ELECTRON-POOR (Z)-N -VINYLIMIDES IN SOLVENT-FREE CONDITIONS

Magnesium hydrogen sulfate powder was found to catalyze stereoselective conversion of dialkyl 2-(imido-N-yl)-3-(triphenylphosphorany-lidene)butanedioates to electron-poor (Z)-N-vinylimides in solvent-free conditions at 95°C 1 h in high conversions. Microwave also was found to catalyze the same reactions in the presence of magnesium hydrogen sulfate powder in solvent-free conditions in 3 min.

Effect of Nb2O5 Content on Hydrogen Reaction Kinetics of Mg.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effect of Nb 2O 5 content on hydrogen reaction kinetics of Mg

The effect of Nb 2O 5 concentration on the kinetics of the magnesium hydrogen sorption reaction at 300 °C and 250 °C is investigated. Absorption and desorption kinetics of nanocrystalline magnesium catalyzed with 0.05, 0.1, 0.2, 0.5, and 1 mole% Nb 2O 5 are determined at 300 and 250 °C. Fastest kinetics are obtained using 0.5 mole% Nb 2O 5. At 300 °C, absorption and desorption of 7 wt.% of hydrogen are facilitated in 60 and 90 s, respectively. At 250 °C, more than 6 wt.% are absorbed in 60 s and desorbed again in 500 s. Respective reaction rates and activation enthalpies are calculated. The activation energy for desorption varies exponentially with Nb 2O 5 concentration and reaches a limit of 61 kJ/mole at 1 mole%. The rate-limiting step for a particular parameter set is deduced by fitting the experimentally obtained transformed phase fraction and time data with analytical kinetics rate expressions. The results show that there is a change in the rate-limiting step with catalyst content.

Excessively bright hydrogenstrontium discharge light source due to energy resonance of strontium with hydrogen

A plasma formed with a low field (1 V cm $^{-1}$ ) at low temperatures (e.g. $\approx{10}^3$ K) from atomic hydrogen generated at a tungsten filament and strontium which was vaporized by heating the metal. Strong extreme ultraviolet emission was observed, but not when sodium, magnesium or barium replaced strontium or with hydrogen or strontium alone. Characteristic emission was observed which supported a resonant energy-transfer mechanism. Significant Balmer $\alpha$ line broadening corresponding to an average hydrogen atom temperature of 23–45 eV was observed for glow discharges of strontium–hydrogen, helium–hydrogen, argon–hydrogen, strontium–helium–hydrogen and strontium–argon–hydrogen compared with $\approx$ 3 eV for pure hydrogen, krypton–hydrogen, xenon–hydrogen and magnesium–hydrogen. To achieve the same optically measured light output power, hydrogen–sodium, hydrogen–magnesium and hydrogen–barium mixtures required 4000, 7000 and 6500 times the power of the hydrogen–strontium mixture, respectively. A glow discharge plasma formed for hydrogen–strontium mixtures at an extremely low voltage of about 2 V compared with 250 V for hydrogen alone and sodium–hydrogen mixtures, and 140–150 V for hydrogen–magnesium and hydrogen–barium mixtures.

Comparison of excessive Balmer α line broadening of glow discharge and microwave hydrogen plasmas with certain catalysts

From the width of the 656.3 nm Balmer α line emitted from microwave and glow discharge plasmas, it was found that a strontium–hydrogen microwave plasma showed a broadening similar to that observed in the glow discharge cell of 27–33 eV; whereas, in both sources, no broadening was observed for magnesium–hydrogen. Microwave helium–hydrogen and argon–hydrogen plasmas showed extraordinary broadening corresponding to an average hydrogen atom temperature of 180–210 eV and 110–130 eV, respectively. The corresponding results from the glow discharge plasmas were 33–38 eV and 30–35 eV respectively, compared to ≈4 eV for plasmas of pure hydrogen, neon–hydrogen, krypton–hydrogen, and xenon–hydrogen maintained in either source. Similarly, the average electron temperature Te for helium–hydrogen and argon–hydrogen microwave plasmas were high, 30 500±5% K and 13 700±5% K, respectively; compared to 7400±5% K and 5700±5% K for helium and argon alone, respectively. External Stark broadening or acceleration of charged species due to high fields can not explain the microwave results since no high field was present, and the electron density was orders of magnitude too low for the corresponding Stark effect. Rather, a resonant energy transfer mechanism is proposed.

Calorimetric investigations on kinetics of the dehydrogenation process in the system lanthanum–magnesium–hydrogen

A kinetic study of the dehydrogenation of several magnesium-based alloys/compounds with lanthanum is presented and the results are referred to the kinetics of the dehydrogenation process of pure magnesium hydride. Measurements were performed in a differential scanning calorimeter (DSC) by applying various pressure steps to the samples at isothermal conditions. Different thermokinetic models were fitted to the experimental obtained DSC curves as well as the power curves, calculated from the DSC curves, to evaluate the kinetic parameters. The derived dehydrogenation rate is discussed in terms of their dependence on hydrogen pressure and temperature. Under certain conditions, it is possible to visualise individual rate-determining steps. Considering these results first mechanistic considerations on the dehydrogenation reaction of the magnesium-based alloys/compounds can be drawn.

Measurement of hydrogen Balmer α line broadening and thermal power balances of noble gas-hydrogen discharge plasmas

Line broadening of the hydrogen Balmer lines provides a sensitive measure of the number and energy of excited hydrogen atoms in a glow discharge plasma. The width of the 656.5 nm Balmer α line emitted from glow discharge plasmas having atomized hydrogen from pure hydrogen alone, hydrogen with magnesium or strontium, a mixture of 10% hydrogen and helium, argon, krypton, or xenon, and a mixture of 10% hydrogen and helium or argon with strontium was measured with a high resolution (±0.025 nm) visible spectrometer. It was found that strontium–hydrogen, helium–hydrogen, argon–hydrogen, strontium–helium–hydrogen, and strontium–argon–hydrogen plasmas showed significant broadening corresponding to an average hydrogen atom temperature of 25– 45 eV ; whereas, pure hydrogen, krypton–hydrogen, xenon–hydrogen, and magnesium–hydrogen showed no excessive broadening corresponding to an average hydrogen atom temperature of ≈3 eV . Since line broadening is a measure of the plasma temperature, and a significant difference was observed between these noble gases, the power balances of glow discharge plasmas of (1) pure krypton alone, (2) a mixture of hydrogen with argon or krypton and (3) a mixture of hydrogen and helium or argon with vaporized strontium were measured. The power emitted for power supplied to the glow discharge increased by 35– 83 W depending on the presence of helium or argon and less than 1% partial pressure of strontium metal in noble gas–hydrogen mixtures. Whereas, the chemically similar noble gas krypton alone or with hydrogen had no effect on the power balance. Catalyst atoms or ions which ionize at integer multiples of the potential energy of atomic hydrogen ( Sr, He + , or Ar +) caused an increase in power; whereas, no excess power was observed in the case of krypton which does not provide a reaction with a net enthalpy of a multiple of the potential energy of atomic hydrogen under these conditions. For a power input to the glow discharge of 110 W , the excess output power of mixtures of strontium with argon–hydrogen (95/5%), strontium with hydrogen, strontium with helium–hydrogen (95/5%), and argon–hydrogen (95/5%) was 75, 58, 50, and 28 W , respectively, based on a comparison of the temperature rise of the cell with krypton–hydrogen mixture (95/5%) and krypton alone. The input power was varied to find conditions that resulted in the optimal output for the strontium–hydrogen plasma. At 136 W input, the excess power significantly increased to 184 W . These studies provide a useful comparison of catalysts for the optimization of a catalytic reaction of atomic hydrogen which represents an important new power source.

Development and characterization of a novel carbon fiber based cathode for semi-fuel cell applications

Fabrication of a microfibre carbon electrode (MCE) using a textile science flocking technique is described. Carbon fibers, 0.5 mm long and 10 μm in diameter are aligned perpendicular to the surface of a conductive carbon epoxy coated carbon paper substrate in a high voltage (typically 70 kV) field. SEM images are used to confirm high carbon fiber density and associated high volumetric surface area. Coverage of the carbon fiber surface with stable Pd–Ir clusters is shown. Performance of the MCE in a magnesium–hydrogen peroxide semi-fuel cell (SFC) under optimum conditions and at reduced concentration of H 2O 2 is reported upon.

ChemInform Abstract: Preparation of Calcium and Magnesium Hydrogen Phosphates from Natural Dolomite and Their Sorptive Properties.

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Acceptor activation of Mg-doped GaN by microwave treatment

A microwave treatment method different from thermal annealing and low-energy electron beam irradiation was proposed to activate Mg dopants in p-type GaN epitaxial layer. From photoluminescence spectra and Hall effect measurements, it was shown that microwave treatment is a very effective way to activate the acceptors in Mg-doped p-type GaN layer. The activation of Mg dopant in p-type GaN layer may be explained as the breaking of magnesium–hydrogen bonding due to the microwave energy absorption.

Preparation of calcium and magnesium hydrogen phosphates from natural dolomite and their sorptive properties

The reaction of dolomite with 10–20% phosphoric acid was studied by x-ray diffraction and chemical analysis in a wide range of dolomite concentrations in the reaction system. The structure, composition, and sorption capacity of the reaction products were determined. The reaction was shown to yield a mixture of calcium hydrogen phosphate, CaHPO4 · 2H2O, with the brushite structure and magnesium hydrogen phosphate, MgHPO4 · 3H2O, with the newberyite structure. The sorption capacity of the reaction product for Pb ions was up to 19 mg-equiv/g. With 10% H3PO4 at an initial dolomite concentration of 0.17 g/ml, a near-theoretical reaction yield was attained.

Thermodynamic optimization of the system magnesium–hydrogen

On the basis of our own calorimetric data and of known equilibria data the phase equilibria in the system Mg–H have been completely calculated by means of an optimization. Both for α-MgH x and MgH 2 optimized free enthalpy functions are obtained.

Thermodynamic investigation of the magnesium–hydrogen system

Thermodynamic properties of the magnesium hydride–magnesium system have been investigated using both calorimetric and equilibrium pressure measurements. Based on calorimetric measurements, on average a satisfactory agreement between experimental and calculated enthalpy and entropy values for the formation of MgH 2 is achieved; systematic deviations are found only at higher temperatures (∼480°C). Experimental determination of PCIs (absorption and desorption mode) has been carried out in the temperature range 403–520°C, using Mg powders of different origin and particle size. According to measurements at 526°C, a minor dependence of H 2 equilibrium pressure over MgH 2 on the particle size of Mg powders used exists.

Characterization by XRD, DRIFT, and MAS NMR Spectroscopies of a Mg2P2O7Catalyst

The dehydration–dehydrogenation of 2-hexanol in the presence of a catalyst obtained from magnesium hydrogen phosphate was studied. The catalyst activity was found to be related to the nature of surface OH sites. Examination of the catalyst by the XRD, DRIFT,1H and31P MAS NMR, and UV–visible absorption techniques allowed the nature of the catalytically active sites to be established and the catalytic activity results to be explained.