Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal-organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a "double solvents" method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation.
The size-dependent structure of CdSe nanoparticles, with diameters ranging from 2 to 4 nm, has been studied using the atomic pair distribution function (PDF) method. The core structure of the measured CdSe nanoparticles can be described in terms of the wurtzite atomic structure with extensive stacking faults. The density of faults in the nanoparticles ∼ 50% . The diameter of the core region was extracted directly from the PDF data and is in good agreement with the diameter obtained from standard characterization methods suggesting that there is little surface amorphous region. A compressive strain was measured in the Cd-Se bond length that increases with decreasing particle size being 0.5% with respect to bulk CdSe for the 2 nm diameter particles. This study demonstrates the size-dependent quantitative structural information that can be obtained even from very small nanoparticles using the PDF approach.
Thermal decomposition of magnesium borohydride, Mg(BH(4))(2), in the solid state was studied by a combination of PCT, TGA/MS and NMR spectroscopy. Dehydrogenation of Mg(BH(4))(2) at 200 °C en vacuo results in the highly selective formation of magnesium triborane, Mg(B(3)H(8))(2). This process is reversible at 250 °C under 120 atm H(2). Dehydrogenation at higher temperature, >300 °C under a constant argon flow of 1 atm, produces a complex mixture of polyborane species. A borohydride condensation mechanism involving metal hydride formation is proposed.
The thermal decomposition of ammonia borane (AB) in the absence and presence of chemical additives was investigated to develop an approach for reducing the induction period for hydrogen release in the solid state. Gas chromatography techniques were used to measure the yield of hydrogen as a function of time under isothermal conditions between 75 and 90 °C to set the baseline. Solid-state 11B-NMR spectroscopy of the products produced after 1 mol equiv of hydrogen had been desorbed from AB (i.e., polyaminoborane) showed a complex mixture of sp3 boron species. Raman microscopy was used to follow the transformation of crystalline AB to amorphous AB upon heating and the subsequent formation of the diammoniate of diborane (DADB). A gas buret was used to monitor the time-dependent release of hydrogen from AB in the presence of chemical additives. The combination of these approaches provides insight into the mechanism of hydrogen release from solid AB. The release of molecular hydrogen is described by a process involving sequential induction (disruption of dihydrogen bonds), nucleation (formation of DADB), and growth (hydrogen release through dehydrocoupling). Addition of DADB or ammonium chloride to neat AB significantly reduces the induction time for hydrogen release. These results provide approaches to improve the hydrogen storage properties of AB.
Sustainable energy generation calls
for a shift away from centralized,
high-temperature, energy-intensive processes to decentralized, low-temperature
conversions that can be powered by electricity produced from renewable
sources. Electrocatalytic conversion of biomass-derived feedstocks
would allow carbon recycling of distributed, energy-poor resources
in the absence of sinks and sources of high-grade heat. Selective,
efficient electrocatalysts that operate at low temperatures are needed
for electrocatalytic hydrogenation (ECH) to upgrade the feedstocks.
For effective generation of energy-dense chemicals and fuels, two
design criteria must be met: (i) a high H:C ratio via ECH to allow
for high-quality fuels and blends and (ii) a lower O:C ratio in the
target molecules via electrochemical decarboxylation/deoxygenation
to improve the stability of fuels and chemicals. The goal of this
review is to determine whether the following questions have been sufficiently
answered in the open literature, and if not, what additional information
is required:
What organic functionalities are accessible
for electrocatalytic hydrogenation under a set of reaction conditions?
How do substitutions and functionalities impact the activity and selectivity
of ECH?
What material
properties cause an
electrocatalyst to be active for ECH? Can general trends in ECH be
formulated based on the type of electrocatalyst?
What are the impacts of reaction conditions
(electrolyte concentration, pH, operating potential) and reactor types?
Electrocatalytic
hydrogenation is increasingly studied as an alternative
to integrate the use of recycled carbon feedstocks with renewable
energy sources. However, the abundant empiric observations available
have not been correlated with fundamental properties of substrates
and catalysts. In this study, we investigated electrocatalytic hydrogenation
of a homologues series of carboxylic acids, ketones, phenolics, and
aldehydes on a variety of metals (Pd, Rh, Ru, Cu, Ni, Zn, and Co).
We found that the rates of carbonyl reduction in aldehydes correlate
with the corresponding binding energies between the aldehydes and
the metals according to the Sabatier principle. That is, the highest
rates are obtained at intermediate binding energies. The rates of
H2 evolution that occur in parallel to hydrogenation also
correlate with the H-metal binding energies, following the same volcano-type
behavior. Within the boundaries of this model (e.g., compounds reactive
at room temperature and without important steric effects over the
carbonyl group), the reported correlations help to explain the complex
trends derived from the experimental observations, allowing for the
correlation of rates with binding energies and the differentiation
of mechanistic routes.
Magnesium amidoborane monoammoniate (Mg(NH(2)BH(3))(2) x NH(3)) which crystallizes in a monoclinic structure (space group P2(1)/a) has been synthesized by reacting MgNH with NH(3)BH(3). Dihydrogen bonds are established between coordinated NH(3) and BH(3) of [NH(2)BH(3)](-) in the structure, promoting stoichiometric conversion of NH(3) to H(2).
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