ABSTRACT
The Mg(NH2)2/2LiH mixture is considered as one of the most valuable reversible hydrogen storage systems for feeding PEM fuel cells. In this paper, we investigate the mechanochemical synthesis in the Li-Mg-N-H system under deuterium gas, using Li3N and Mg as reactants, and the structural and sorption properties of the intermediate and final products mainly by means of neutron powder diffraction. Mechanochemistry leads to the end formation of amorphous Mg(ND2)2, which crystallizes upon heating above 425 K. During synthesis, a novel cation-mixed nitride/imide phase of simplified composition Li3MgN2D has been unveiled as the intermediate phase. It crystallizes in the cubic disordered anti-fluorite type structure (S.G. Fm3[combining macron]m) with a lattice parameter of 4.996 Å at room temperature. Deuterium absorption in this compound occurs through an original solid solution type mechanism ending with the imide compound ß-Li2MgN2D2. The conjoint use of mechanochemistry under deuterium gas and in situ neutron diffraction techniques offers new avenues for better characterization of the efficient hydrogen storage materials. In particular, this work highlights the unexpected role of intermediate nitride/imide phases in the Li-Mg-N-H system.
ABSTRACT
Hydrogen uptake during the mechanochemistry of lithium nitride under 9 MPa hydrogen pressure has been analyzed by means of in situ solid-gas absorption and ex situ X-ray diffraction (XRD) measurements. In situ hydrogenation curves show two H-sorption steps leading to an overall hydrogen uptake of 9.8 wt% H after 3 hours of milling. The milled end-products consist of nanocrystalline (â¼10 nm) LiNH2 and LiH phases. The first reaction step comprises the transformation of the polymorph α-Li3N (S.G. P6/mmm) into the ß-Li3N (S.G. P63/mmc) metastable phase and the reaction of the latter with hydrogen to form lithium imide: ß-Li3N + H2â Li2NH + LiH. Reaction kinetics of the first step is zero-order. Its rate-limiting control is assigned to the collision frequency between milling balls and Li3N powder. In the second absorption step, lithium imide converts to lithium amide following the reaction scheme Li2NH + H2â LiNH2 + LiH. Reaction kinetics is here limited by one-dimensional nucleation and the growth mechanism, which, in light of structural data, is assigned to the occurrence of lithium vacancies in the imide compound. This study provides new insights into the reaction paths and chemical kinetics of light hydrogen storage materials during their mechanochemical synthesis.
ABSTRACT
The hydrogen sorption properties of oxide-supported Ir-Pd nanoalloys have been determined for the first time, and correlated with their catalytic behavior. The addition of Ir to Pd suppresses hydride formation and leads to improved catalytic performances with respect to pure metals in the preferential oxidation of CO in H2 excess (PROX).
ABSTRACT
YMn2 forms either interstitial YMn2Hx hydrides for x < or = 4.5 or a complex YMn2H6 hydride when submitted to high hydrogen pressure. These compounds have been studied by inelastic neutron scattering (INS) in order to clarify the different modes of H vibration. The INS spectra of YMn2Hx hydrides are strongly dependent on the H content. YMn2H6 and YMn2D6 show broad bands, also observed by Raman and IR spectroscopy, assigned to H-Mn-H (or D) and Mn-H bending and stretching modes. Both ErMn2D6 and ErMn1.8Fe0.2D6 show, in addition to the H vibration mode, an intense band at 215 cm(-1) which has been attributed to a magnetic excitation of Er3+ in view of its momentum transfer dependence.
