Synthetic methods used to produce metal nanoparticles typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with the potential for both steric and chemical stabilization. We report a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions < or =1 nm, with a significant fraction existing as Ag(3) clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible.
With 11.6 wt % H 2 , Ca(BH 4 ) 2 is a promising hydrogen-storage candidate material provided the issue of reversibility can be addressed. We investigate theoretically and experimentally the structure and reversibility of Ca(BH 4 ) 2 in (de)hydrogenation reactions, including an intermediate CaB 12 H 12 phase. From our first-principles calculations, we predict several polymorphs of CaB 12 H 12 that compete in energy in dehydrogenation (within 1 kJ/mol-H 2 ), indicating no long-range ordered state is likely. Our experimental microchemical analysis and structural characterization of the dehydrogenated Ca(BH 4 ) 2 show that the intermediate phase is amorphouslike. Theoretically determined X-ray diffraction patterns for all coexisting Ca(BH 4 ) 2 , CaH 2 , and CaB 12 H 12 polymorphic phases reproduce the observed diffraction peaks. The calculated reaction formation enthalpies versus H-content reveal limited reversibility, as CaB 12 H 12 is energetically very favorable. Our results suggest the (de)hydrogenation process for Ca(BH 4 ) 2 via CaB 12 H 12 intermediates is reversible, but, due to its stability, full release of H 2 between Ca(BH 4 ) 2 and CaH 2 + CaB 6 (or B) is not possible except at high temperature. Thus, Ca(BH 4 ) 2 has limited viability as a reversible on-board storage material for vehicular applications, although it may hold some promise as a chemical hydride. IntroductionHydrogen-powered technologies are one of many being rapidly developed to combat environmental change. Because a large amount of energy is consumed in the transportation industry, technologies must be developed to provide suitable energy-per-volume substitutes. In particular, hydrogen-storage materials must be developed for mobile applications where the weight of the storage system must be minimized but still yield maximum weight-percent hydrogen. Of the many metallic borohydrides that have been studied, Ca(BH 4 ) 2 has emerged as a promising candidate material due to its 11.6 wt % H 2 capacity.Several routes have been pursued to create a reversible Ca(BH 4 ) 2 system, and the related reaction pathways are with P and T the pressure and temperature, respectively. The primary issue is system reversibility. Reversibility has been demonstrated for samples derived from CaB 6 and CaH 2 at >70 MPa and >400°C via reaction 3 with 9.6 wt % H 2 release. 1 Starting with commercially available powders of a Ca(BH 4 ) 2 · 2THF (THF is tetrahydrofuran), reaction 4 with 6.3 wt % H 2 and reaction 2 with 8.7 wt % H 2 are similar to those suggested by Miwa and co-workers 2,3 for LiBH 4 . Reaction 2, as demonstrated by Kim et al.,4 is reversible at a pressure of 9 MPa, and a TiCl 3 catalyst was found to improve the kinetics by about a factor of 3. For other M m (BH 4 ) n complexes, such as Mg(BH 4 ) 2 , the presence of the dodecaborane closomolecule (B 12 H 12 ) via reaction 4 has been observed. 5 The dehydrogenation of Ca(BH 4 ) 2 should show characteristics similar to those of Mg(BH 4 ) 2 , where dodecaborane was found.In this work, we investigate, by comb...
AlH 3 , a metastable binary hydride with a hydrogen content of 10.1 wt % hydrogen and a density of 1.48 g/mL, is a potential lightweight hydrogen storage system for transportation applications. A key component to understanding the discharge and uptake of hydrogen is the role of the Ti dopant in these processes. Here, the morphological and compositional changes caused by the synthesis and dehydriding of the AlH 3 adduct of triethylenediamine was determined by using a combination of scanning electron and scanning transmission electron microscopy as well as X-ray energy dispersive spectroscopy. It is shown that there is significant loss of the added Ti at each step in the synthesis; the Ti agglomerates in the Al particles, causing enrichment, and the amount of Ti needed to catalyze the activity is on the order of 0.4 at.%, which amounts to approximately a 90% reduction from the amount added.
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