A new potassium-based intermediate and its role in the desorption properties of the K–Mg–N–H system (original) (raw)
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Synthesis. KH (30 wt% dispersion in mineral oil) was purchased from Sigma-Aldrich. Before usage the oil was removed by washing 3 times with hexane in vacuum filtration. KNH 2 was synthesized by reactive ball milling of the dry potassium hydride powder in a Fritsch Pulverisette 6, at 400 RPM, with a BPR ca. 20:1, under 7 bar of ammonia atmosphere. The reaction vessel (a high pressure vial from Evicomagnetics) was evacuated and refilled with ammonia 4 times, for a total milling time of 18 h. Both the starting materials (KH and KNH 2 ) were then independently ball milled in a SPEX 8000 mill for 600 minutes to obtain a finer powder. The mixtures of KNH 2 and KH were prepared grinding the two reactants for 5 minutes in an agate mortar. The annealed samples were prepared in a thermal reactor from Parr Instruments, heating at 270 °C under Argon atmosphere for 1 h. The ball milled samples were prepared using a SPEX 8000 Mill, milling the powder for 5h with a BPR of 10:1. Hardened steel vials and balls were used. Potassium deuteramide (KND 2 ) was synthesized by thermal treatment of metallic potassium under 5 bar of deuterated ammonia (ND 3 ) for 16 h at 300 °C in an autoclave from Parr Instruments. The reaction can be written as: K + ND 3 → KND 2 + 1 / 2 D 2 Potassium deuteride was synthesize ball milling pure potassium under 50 bar of deuterium (D 2 ) for 36 h with a rotational speed of 600 rpm and a ball to powder ratio of 60:1. Potassium (98 % purity) under mineral oil was purchased as commercial product from Sigma Aldrich. A cube of 1.1 g approximately was cut. Before use, the oil was removed washing with hexane. In addition the material surface was polished with a sharp blade. The diffractograms on the ball milled samples ( ) were collected with a Bruker D8 Advance diffractometer in Bragg-Brentano geometry using a General Area detector and a Cu X-ray source. The sample was investigated using an airtight sample holder from Bruker. The incoherent scattering of the Poly (methyl methacrylate) dome is responsible for the bump observed between 1 and 2 Å -1 in all the diffractograms ( ). The diffractograms on the annealed samples ( ) were performed in the 2θ range 2° -90° (step size of 0.017°, time per step 200 s) using a laboratory diffractometer (Panalytical X'Pert Pro Multipurpose Diffractometer) equipped with Ni filtered Cu source in Debye-Scherrer geometry. Samples were sealed into boron silica glass capillaries of internal diameter 0.8 mm in a protected atmosphere. The sample handling was performed in an Argon circulation glove box, with oxygen and moisture concentrations lower than 2 ppm. The scattering images obtained by in situ synchrotron X-ray diffraction were integrated with the program Fit2D. 1 Rietveld refinement was performed by means of MAUD program (Material Analysis Using Diffraction) 2 on the diffractograms selected for phase identification and determination of the cell parameters ( , S7, S8, S9). In any case structural models from the literature were used for the known phases of KNH 2 and KH. For the new K(NH 2 ) x H (1-x) phases a general structural model was obtained using FOX software 3 on the pattern obtained by in situ powder neutron diffraction on the sample with nominal composition x = 0.5 and then adapted for the other different compositions. The occupancies for the amide and hydride anions were calculated and fixed in order to be in agreement with the refined composition of the room temperature pattern. The Rietveld refinement of the neutron diffraction pattern was performed with GSAS software. 4
KNH2 KH: A metal amide hydride solid solution
KNH2 KH: A metal amide hydride solid solution, 2016
We report for the first time the formation of a metal amidehydride solid solution. The dissolution of KH into KNH2 leads to an anionic substitution, which decreases the interaction among NH2-ions. The rotational properties of the high temperature polymorphs of KNH2 are thereby retained down to room temperature.
Inorganic chemistry, 2018
The crystal structure of a mixed amide-imide phase, RbMgNDND, has been solved in the orthorhombic space group Pnma ( a = 9.55256(31), b = 3.70772(11) and c = 10.08308(32) Å). A new metal amide-hydride solid solution, Rb(NH) H, has been isolated and characterized in the entire compositional range. The profound analogies, as well as the subtle differences, with the crystal chemistry of KMgNDND and K(NH) H are thoroughly discussed. This approach suggests that the comparable performances obtained using K- and Rb-based additives for the Mg(NH)- 2LiH and 2LiN H-MgH hydrogen storage systems are likely to depend on the structural similarities of possible reaction products and intermediates.
Influence of K2TiF6 additive on the hydrogen sorption properties of MgH2
In this study, the hydrogen storage properties of MgH2 with the addition of K2TiF6 were investigated for the first time. The temperature-programmed desorption results showed that the addition of 10 wt% K2TiF6 to the MgH2 exhibited a lower onset desorption temperature of 245 C, which was a decrease of about 105 C and 205 C compared with the asmilled and as-received MgH2, respectively. The dehydrogenation and rehydrogenation kinetics of 10 wt% K2TiF6-doped MgH2 were also significantly improved compared to the undoped MgH2. The results of the Arrhenius plot showed that the activation energy for the hydrogen desorption of MgH2 was reduced from 164 kJ/mol to 132 kJ/mol after the addition of 10 wt% K2TiF6. Meanwhile, the X-ray diffraction analysis showed the formation of a new phase of potassium hydride and titanium hydride together with magnesium fluoride and titanium in the doped MgH2 after the dehydrogenation and rehydrogenation process. It is reasonable to conclude that the K2TiF6 additive doped with MgH2 played a catalytic role through the formation of active species of KH, TiH2, MgF2 and Ti during the ball milling or heating process. It is therefore proposed that this newly developed product works as a real catalyst for improving the hydrogen sorption properties of MgH2.
IOP Conference Series: Materials Science and Engineering
The development of high capacity materials for hydrogen storage such as MOFs to support the utilization of hydrogen as a renewable energy source is an attractive area for further investigation. In this research, a well-known type of MOF, specifically HKUST-1, was synthesized using the solvothermal method and ex-situ modified with magnesium(II) ions at 3, 5, and 10 wt% to serve as additional binding sites to form Mg 2+ /HKUST-1. The resulting materials were analysed via XRD, FTIR, SEM-EDX, and a nitrogen sorption isotherm. Hydrogen sorption measurements were conducted using a Sievert system. The structure and morphology of HKUST-1 remained stable after it was embedded with Mg 2+ according to X-ray diffraction, FTIR, and SEM analysis. Nitrogen sorption isotherm revealed that before and after modification with Mg 2+ , the materials could be classified as microporous and the surface area increased significantly from 713.283 to 1200.211 m 2 /g. The presence of Mg 2+ in the HKUST-1 enhanced its hydrogen sorption capacity up to 0.292 wt% at 1.2 bar under an operational temperature of 30°C and 0.48 wt% at 80°C (1.4 bar).
The Journal of Physical Chemistry B, 2006
The present paper quantifies and develops the kinetic aspects involved in the mechanism of interplay between electron and ions presented elsewhere 1 for K h Fe k [Fe(CN) 6 ] l ‚mH 2 O (Prussian Blue) host materials. Accordingly, there are three different electrochemical processes involved in the PB host materials: H 3 O + , K + , and H + insertion/extraction mechanisms which here were fully kinetically studied by means of the use of combined electronic and mass transfer functions as a tool to separate all the processes. The use of combined electronic and mass transfer functions was very important to validate and confirm the proposed mechanism. This mechanism allows the electrochemical and chemical processes involved in the K h Fe k [Fe(CN) 6 ] l ‚mH 2 O host and Prussian Blue derivatives to be understood. In addition, a formalism was also developed to consider superficial oxygen reduction. From the analysis of the kinetic processes involved in the model, it was possible to demonstrate that the processes associated with K + and H + exchanges are reversible whereas the H 3 O + insertion process was shown not to present a reversible pattern. This irreversible pattern is very peculiar and was shown to be related to the catalytic proton reduction reaction. Furthermore, from the model, it was possible to calculate the number density of available sites for each intercalation/deintercalation processes and infer that they are very similar for K + and H + . Hence, the high prominence of the K + exchange observed in the voltammetric responses has a kinetic origin and is not related to the amount of sites available for intercalation/ deintercalation of the ions.
Crystal structure solution of KMg(ND)(ND2): An ordered mixed amide/imide compound
International Journal of Hydrogen Energy, 2014
Crystal structure solution Ab-initio methods Powder diffraction a b s t r a c t An ordered mixed deuterated amide/imide potassiumemagnesium compound was synthesized with the intent of solving its structure using neutron diffraction technique with help of "ab-initio" methods. Obtained powder diffraction patterns were compatible with the orthorhombic P2 1 2 1 2 1 space group, and lattice parameters a ¼ 9.8896(3) A; b ¼ 9.3496(3) A; c ¼ 3.6630(1) A, respectively.