Experimental Evidence of Na2[B12H12] and Na Formation in the Desorption Pathway of the 2NaBH4+ MgH2System (original) (raw)
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In the present work we report the desorption pathway of the 2NaBH4 + MgH2 system. Ex-situ X-ray powder diffraction (XRPD) and solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) measurements have been performed on samples heat treated up to 450 °C for different times. Ex-situ X-ray powder diffraction experiments conducted on fully desorbed samples allowed us to identify nanocrystalline MgB2 and metallic Na as dehydrogenation products. 11B and 23Na NMR analyses have been also carried out in order to evaluate the structural evolution of decomposed materials. Our measurements show that the local structure of MgB2 is influenced by replacement of Mg with Na atoms in the Mg sites. Moreover, amorphous Na2[B12H12] was detected in the partially desorbed sample and in the final products of the decomposition reaction. The presence of the [B12H12]2– anion was confirmed by both direct comparison with the 11B{1H} NMR spectrum of pure Na2[B12H12] and dynamic cross-polarization experiments.
Hydrogen Desorption Reactions of the Na-Mg-B-H System
Advances in Science and Technology, 2011
Hydrogen storage in the solid state has shown increasing research and development, and recently an approach in mixing two hydride systems together by ball milling (reactive hydride composites) has been investigated in more detail, e.g. NaBH 4 plus MgH 2. Thermodynamic destabilization may occur by new compounds formation during dehydrogenation, e.g. MgB 2. A study of the the role of O 2 /H 2 O contamination for the reaction 2NaBH 4 + MgH 2 ↔ 2NaH + MgB 2 + 4H 2 was conducted using in-situ X-ray powder diffraction. Desorption reaction is observed to begin by a competition of MgH 2 and NaBH 4 decomposition due to higher reactivity promoted by ball milling processing summed to O 2 /H 2 O contamination. Oxidation of NaBH 4 into NaBO 2 is observed to happen in higher degree than MgH 2 /Mg into MgO for the Na-Mg-B-H system.
Structural analysis of activated Mg(Nb)H2
Journal of Alloys and Compounds, 2005
A metastable niobium hydride, believed to trigger rapid H desorption from the MgH 2 matrix, was detected in the course of real time synchrotron diffraction studies of the desorption kinetics of hydrogen in MgH 2 with about 5% NbH nanoparticles. In order to completely characterize this metastable phase, we performed in situ neutron diffraction experiments during hydrogenation/dehydrogenation of a magnesiumniobium nanocomposite. At 250 • C (under 1 bar of H pressure) a hydride phase NbH x appears. Unfortunately, the available low pressure together with the low q-range did not allow to propose a structural arrangement for this phase. We thus switched to standard neutron diffraction (that is at constant pressure and room temperature) of ball-milled Mg-Nb-H nanocomposites for which various NbH hydrides were detected including the solid solution. It turns out that the formation of these niobium hydrides (metastable or not) is closely related to the milling time and depends on the quality of the starting metal.
Hydrogen desorption mechanism of 2NaBH4+MgH2 composite prepared by high-energy ball milling
Scripta Materialia, 2009
The desorption mechanism of 2NaBH 4 + MgH 2 composite has been investigated. The results show that the chemical interaction between MgH 2 and NaBH 4 is capable of decreasing the dehydrogenation temperature with respect to the single compounds. The dehydriding reaction starts at around 320°C with the desorption of the MgH 2 to Mg and proceeds via the chemical dismutation of NaBH 4 in NaH and an intermediate specie. In situ synchrotron X-ray powder diffraction indicates that MgB 2 is one of the final reaction products.
The Journal of Physical Chemistry C, 2015
A combined in situ X-ray diffraction (XRD) and Xray absorption spectroscopy (XAS) experiment was carried out to monitor hydrogen desorption from NbH 0.9 nanoclusters embedded into MgH 2. Just after the MgH 2 → Mg transition, a NbH x bcc nanophase is detected, whose lattice parameter measured by XRD is significantly longer than the one inferred from XAS. This difference is explained considering the broad niobium hydride cluster size distribution and in particular the fact that the XRD signal, differently from the XAS one, is dominated by larger NbH x crystalline structures. The results indicate that, during the hydride to metal phase transformation of the matrix, the NbH x cluster composition depends on the cluster size. It is shown for the first time for embedded nanoparticles that faster (and complete) Nb dehydrogenation is favored for small (1.5−4 nm) clusters with respect to larger (∼20 nm) ones. The role of the matrix and of the annealing atmosphere in the stability of the Nb-related nanophases is discussed.
Microstructural study of hydrogen desorption in 2NaBH4 + MgH2 reactive hydride composite
International Journal of Hydrogen Energy, 2012
The desorption mechanism of as-milled 2NaBH 4 þ MgH 2 was investigated by volumetric analysis, X-ray diffraction and electron microscopy. Hydrogen desorption was carried out in 0.1 bar hydrogen pressure from room temperature up to 450 C at a heating rate of 3 C min À1. Complete dehydrogenation was achieved in two steps releasing 7.84 wt.% hydrogen. Desorption reaction in this system is kinetically restricted and limited by the growth of MgB 2 at the Mg/Na 2 B 12 H 12 interface where the intermediate product phases form a barrier to diffusion. During desorption, MgB 2 particles are observed to grow as plates around NaH particles.
Hydrogen desorption mechanism inMgH2−Nbnanocomposites
Physical Review B, 2001
In situ time-resolved x-ray scattering measurements of hydrogen desorption in MgH 2-Nb nanocomposites using synchrotron radiation are presented. Results show that niobium acts like a catalyst for hydrogen absorption and desorption. It is found that hydrogen desorption involves the formation of a short-lived metastable niobium-hydride phase which acts as a gateway through which hydrogen released from MgH 2 is flowing. The desorption kinetics of this system is revealed through temporal profiles showing the lifetimes of the different phases involved.
Int J Hydrogen Energ, 2009
Nanostructured MgH2-Ni/Nb2O5 nanocomposite was synthesized by high-energy mechanical alloying. The effect of MgH2 structure, i.e. crystallite size and lattice strain, and the presence of 0.5 mol% Ni and Nb2O5 on the hydrogen-desorption kinetics was investigated. It is shown that the dehydrogenation temperature of MgH2 decreases from 426 °C to 327 °C after 4 h mechanical alloying. Here, the average crystallite size and accumulated lattice strain are 20 nm and 0.9%, respectively. Further improvement in the hydrogen desorption is attained in the presence of Ni and Nb2O5, i.e. the dehydrogenation temperature of MgH2/Ni and MgH2/Nb2O5 is measured to be 230 °C and 220 °C, respectively. Meanwhile, the dehydrogenation starts at 200 °C in MgH2–Ni/Nb2O5 system, revealing synergetic effect of Ni and Nb2O5. The mechanism of the catalytic effect is presented.
H-2 sorption performance of NaBH4-MgH2 composites prepared by mechanical activation
Ecosystems and Sustainable Development Vii, 2009
The current research on solid state hydrogen storage materials for on-board applications is focused on reactive hydrides composites (RHC), i.e. systems based on the improvement of the dehydrogenation thermodynamic of a complex hydride when one (generally the light hydride MgH 2) or more hydrides take part to the reaction. The extent of the destabilization, as well as the sorption characteristics of the composites, strongly depends on the structural and nanostructural properties of the constituent hydrides, which are in turn affected by the preparation route. The aim of this work is to evaluate the influence of different mechanical activation conditions on the storage properties of NaBH 4-MgH 2 composites, up to now scarcely explored in literature. The first results regard composites with 2:1 and 1:2 stoichiometry milled under different atmosphere (Ar or H 2). X-ray powders diffraction analysis shows that milling does not lead to the formation of any new phase, but it reduces the average crystallite size of the powders down to nanometric scale. All the mixtures release an H 2 amount close to the theoretical value expected for the full dissociation of both the hydrides and much higher than the target fixed by the US Department of Energy for on-board application. The thermal programmed desorption profiles of the mixtures clearly show two steps, with MgH 2 dissociating first and with higher rate and NaBH 4 gradually dehydrogenating at temperatures close to 400°C. Concerning the 2:1 stoichiometry, when the samples are processed under Ar the two dehydrogenation processes are characterized by a lower starting temperature but also by a lower average rate with respect to the sample milled in H 2. The 1:2 sample milled under Ar shows the best kinetic performance. Unfortunately, also for this mixture more than 10 h are required to obtain full desorption at a temperature as high as 450°C.