The storage of large quantities of hydrogen at safe pressures is a key factor in establishing a hydrogen-based economy. Previous strategies--where hydrogen has been bound chemically, adsorbed in materials with permanent void space or stored in hybrid materials that combine these elements--have problems arising from either technical considerations or materials cost. A recently reported clathrate hydrate of hydrogen exhibiting two different-sized cages does seem to meet the necessary storage requirements; however, the extreme pressures (approximately 2 kbar) required to produce the material make it impractical. The synthesis pressure can be decreased by filling the larger cavity with tetrahydrofuran (THF) to stabilize the material, but the potential storage capacity of the material is compromised with this approach. Here we report that hydrogen storage capacities in THF-containing binary-clathrate hydrates can be increased to approximately 4 wt% at modest pressures by tuning their composition to allow the hydrogen guests to enter both the larger and the smaller cages, while retaining low-pressure stability. The tuning mechanism is quite general and convenient, using water-soluble hydrate promoters and various small gaseous guests.

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Little is known about the induction period before the nucleation and growth of colloidal semiconductor quantum dots. Here, we introduce an approach that allows us to probe intermediates present in the induction period. We show that this induction period itself exhibits distinct stages with the evolution of the intermediates, first without and then with the formation of covalent bonds between metal cations and chalcogenide anions. The intermediates are optically invisible in toluene, while the covalent-bonded intermediates become visible as magic-size clusters when a primary amine is added. Such evolution of magic-size clusters provides indirect but compelling evidence for the presence of the intermediates in the induction period and supports the multi-step nucleation model. Our study reveals that magic-size clusters could be readily engineered in a single-size form, and suggests that the existence of the intermediates during the growth of conventional quantum dots results in low product yield.

The discovery of vast hydrocarbon resources in hydrate form on the continental margins and in permafrost regions has sparked an interest in the possible recovery of relatively clean-burning methane gas, [1] especially in resource-poor national economies. At the same time, it has been proposed that CO 2 , obtained as a by-product of combustion processes, possibly could be stored either in the deep ocean as liquid, or as a solid hydrate, [2] in order to reduce the release of greenhouse gas into the atmosphere. As such, the conversion of methane hydrate to CO 2 hydrate with the net recovery of methane seems quite attractive.[3] The limiting equilibrium composition of the mixed hydrate and the kinetics of conversion then become of primary interest in assessing this potential technology. In this communication we address these various issues, which have been considered so far only from a macroscopic point of view, by following not only kinetics, but also the distribution of guests over different cages using solidstate NMR methods. This combined approach allows the derivation of thermodynamic and kinetic information and also gives a molecular-level rationale for the observations; thus it is a powerful way of studying solid-state reactions.Both CO 2 and CH 4 form a structure I (sI) hydrate, [4] as do mixtures of these gases, [5] with an icelike framework that consists of hydrogen-bonded water molecules. It has eight cagelike guest sites in the unit cell, including two pentagonal dodecahedra (5 12 ) and six tetrakaidecahedra (5 12 6 2 ) consisting of 12 pentagonal and two hexagonal faces. The ideal unit cell can be written as 2 M S ·6 M L ·46 H 2 O with an ideal hydration number M·5 3 = 4 H 2 O if a single type of guest is present. However, sI hydrates are non-stoichiometric, and the actual hydration numbers usually lie somewhere between 6 and 8. [4] The latter can be measured by the careful application of direct methods, [6] and also from cage occupancies obtained from structures derived by single crystal X-ray diffraction, [7] or from spectroscopy [8] where the cage occupancy ratio can be linked to the hydration number by using the van der Waals--Platteeuw, [9] or related, models. To observe favorable exchange between CO 2 and methane hydrate, there must be preferential partitioning of CO 2 and CH 4 between the gas and the hydrate solid phases. The only way this can arise is if CO 2 has a preference for the large cage (5 12 6 2 ) in the hydrate, as the larger cages outnumber the smaller by a factor of three. This premise was investigated by examining the distribution of methane over the two cage sites by MAS 13 C NMR for hydrate samples prepared from gas mixtures (Figure 1). One can see that for pure methane hydrate q L,CH 4 /q S,CH 4 is % 1.26, so that the small cage is occupied to a smaller degree than the larger cage, as reported before for synthetic and natural methane hydrates. [8,10] With increasing CO 2 in the gas mixture, the ratio declines steadily to a value that shows that now fewer than half as many large a...

Three families of colloidal CdSe magic-sized nanocrystals (MSNs) exhibiting bright bandgap photoluminescence (PL) with narrow full width at half-maximum (fwhm) on the order of ca. 10 nm were synthesized in pure form. This noninjection one-pot synthetic approach uses cadmium acetate dihydrate (Cd(OAc)2·2H2O) and elemental selenium as Cd and Se source compounds, respectively, while a fatty acid as surface ligands and 1-octadecene (ODE) as the reaction medium. All of these chemicals were loaded at room temperature in a reaction flask, with low acid-to-Cd and high Cd-to-Se feed molar ratios; the growth of the CdSe MSNs was carried out at 120 −240 °C. This synthetic approach allows long growth/annealing periods at high temperature and thus results in high-quality CdSe MSNs exhibiting strong bandgap PL; furthermore, this ready approach features high synthetic reproducibility and large-scale production. The reason for the low acid-to-Cd feed molar ratio is argued to be related to the low activity of the Cd precursor in the form of Cd(OAc) x (OOC−(CH2) n −CH3)2−x , which releases Cd slowly. The reason for the high Cd-to-Se feed molar ratio is also addressed to help prevent the dissociation of the formed CdSe MSNs. Furthermore, short ligand and low temperature favors the formation of small MSNs, while long ligand and high temperature favors large MSNs. Regarding our synthetic approach, these MSN families can coexist together with one regular nanocrystal ensemble which is larger in size; in such a case, they develop independently from different nuclei; the small MSN families can dissociate into monomers to feed the formation of other MSN families and/or the regular nanocrystal ensemble. The three MSN families exhibit sharp bandgap absorption at 395, 463, and 513 nm, within the size range of 1.7−2.2 nm by diffusion ordered NMR spectroscopy (DOSY).

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Nucleation has been generally acknowledged as a rapid but uncontrollable process that is difficult to decouple from the subsequent growth phase. Here, we report our finding that nucleation of semiconductor magic-size clusters (MSCs) can be well-regulated, without a subsequent evolution in size. Colloidal semiconductor CdS MSCs were synthesized by a two-step approach intentionally designed, without the simultaneous formation of nanocrystals of other sizes. The nuclei MSCs exhibit a sharp optical absorption peaking at 311 nm and are thus denoted by MSC−311. We prepared the immediate precursor for MSC−311 denoted by IP311 which is liquid-like, through a reaction which was normally performed to grow CdS conventional quantum dots (QDs), but at a different temperature (180°C) prior to the nucleation and growth of CdS QDs. We demonstrate that the nucleation of MSC−311 from IP311 followed first order kinetics remarkably well, and the presence of a small amount of methanol accelerated this process effectively. Moreover, the liquid-like prenucleation cluster IP311 and the nuclei MSC−311 have similar masses. Accordingly, we propose that the intramolecular reorganization of IP311 results in the nuclei MSC−311, the formation of which features a two-step nucleation pathway. The present study introduces methodology via absorption spectroscopy to monitor the nucleation kinetics of semiconductor MSCs from their immediate precursors. The repeatable, predictable, and controllable nucleation process investigated here brings a deeper insight into nucleation of other semiconductor nanocrystals and contributes to the foundation for the future development of advanced theoretical models for crystal nucleation.

The structure and composition of CO2 hydrate were determined from single-crystal X-ray diffraction data at 173 K for a crystal grown over heavy water and liquid CO2. Superior diffraction data allowed the derivation of a structural model of unprecedented quality for the hydrate, giving the location of the disordered CO2 molecules in the two cages. In the large cage, the guests are shown to be off-center, with a bimodal distribution of out-of-plane orientations for the long axis of the molecule (173 K). Also, the absolute cage occupancies were determined from the structural model, thus allowing a reliable and direct evaluation of the hydrate composition for this crystal, CO2·6.20(15) D2O. The temperature dependence of the lattice parameters for the single crystal was measured between 123 and 223 K and found to be in good agreement with recent neutron powder diffraction results, and data from all sources were fit to a single polynomial function. The hydrate composition and density are discussed in terms of the information needed for the deep-sea sequestration of CO2. The guest disorder and dynamics are discussed in terms of the model derived from earlier NMR data.