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ABSTRACT: The mechanism of thermochemical dehydrogenation of the 1:3 mixture of Li(3)AlH(6) and NH(3)BH(3) (AB) has been studied by the extensive use of solid-state NMR spectroscopy and theoretical calculations. The activation energy for the dehydrogenation is estimated to be 110 kJ mol(-1), which is lower than for pristine AB (184 kJ mol(-1)). The major hydrogen release from the mixture occurs at 60 and 72 °C, which compares favorably with pristine AB and related hydrogen storage materials, such as lithium amidoborane (LiNH(2)BH(3), LiAB). The NMR studies suggest that Li(3)AlH(6) improves the dehydrogenation kinetics of AB by forming an intermediate compound (LiAB)(x)(AB)(1-x). A part of AB in the mixture transforms into LiAB to form this intermediate, which accelerates the subsequent formation of branched polyaminoborane species and further release of hydrogen. The detailed reaction mechanism, in particular the role of lithium, revealed in the present study highlights new opportunities for using ammonia borane and its derivatives as hydrogen storage materials.
Inorganic Chemistry 03/2012; 51(7):4108-15. · 4.60 Impact Factor
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ABSTRACT: Mechanochemical transformations occurring during ball milling of sodium amide (NaNH2) with magnesium hydride (MgH2) taken in 2:3 and 2:1 molar ratios have been investigated using X-ray powder diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) techniques. For the 2NaNH(2)-3MgH(2) system the mechanochemical reaction proceeds via the formation of MgNH as an intermediate, whereas magnesium nitride (Mg3N2), sodium hydride (NaH) and hydrogen (similar to 5 wt%) form as the final products. The overall solid state reaction for this system is 2NaNH(2) + 3MgH(2) -> Mg3N2 + 2NaH + 4H(2). However, the mechanochemical transformation of the 2NaNH(2)-MgH2 system proceeds through the reaction: 2NaNH(2) + MgH2 -> Mg(NH2)(2) + 2NaH, without any hydrogen release. Comparison of the mechanochemical transformations with the previously studied thermochemical transformations reveals that the two approaches lead to the same final products via different reaction pathways.
Journal of Alloys and Compounds 02/2012; 513:324-327. · 2.29 Impact Factor
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ABSTRACT: The use of mixed surfactants in the synthesis of mesoporous silica nanoparticles (MSNs) is of importance in the context of adjusting pore structures, sizes and morphologies. In the present study, the arrangement of molecules in micelles produced from a mixture of two surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) was detailed by solid-state NMR spectroscopy. Proximities of methyl protons in the trimethylammonium headgroup of CTAB and protons in the pyridinium headgroup of CPB were observed under fast magic angle spinning (MAS) by (1)H-(1)H double quantum (DQ) MAS NMR and NOESY. This result suggested that CTAB and CPB co-exist in the pores without forming significant monocomponent domain structures. (1)H-(29)Si heteronuclear correlation (HETCOR) NMR showed that protons in the headgroups of CTAB are in closer proximity to the silica surface than those in the CPB headgroups. The structural information obtained in this investigation leads to better understanding of the mechanisms of self-assembly and their role in determining the structure and morphology of mesoporous materials.
Solid State Nuclear Magnetic Resonance 02/2011; 39(3-4):65-71. · 1.71 Impact Factor
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International Journal of Hydrogen Energy. 01/2011; 36(17):10626-10634.
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Acs Catalysis. 01/2011; 1(7):729-732.
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Energy & Fuels. 01/2011; 25(4):1790-1797.
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ABSTRACT: The conformations of (pentafluorophenyl)propyl groups (-CH(2)-CH(2)-CH(2)-C(6)F(5), abbreviated as PFP), covalently bound to the surface of mesoporous silica nanoparticles (MSNs), were determined by solid-state NMR spectroscopy and further refined by theoretical modeling. Two types of PFP groups were described, including molecules in the prone position with the perfluorinated aromatic rings located above the siloxane bridges (PFP-p) and the PFP groups denoted as upright (PFP-u), whose aromatic rings do not interact with the silica surface. Two-dimensional (2D) (13)C-(1)H, (13)C-(19)F and (19)F-(29)Si heteronuclear correlation (HETCOR) spectra were obtained with high sensitivity on natural abundance samples using fast magic angle spinning (MAS), indirect detection of low-gamma nuclei and signal enhancement by Carr-Purcell-Meiboom-Gill (CPMG) spin-echo sequence. 2D double-quantum (DQ) (19)F MAS NMR spectra and spin-echo measurements provided additional information about the structure and mobility of the pentafluorophenyl rings. Optimization of the PFP geometry, as well as calculations of the interaction energies and (19)F chemical shifts, proved very useful in refining the structural features of PFP-p and PFP-u functional groups on the silica surface. The prospects of using the PFP-functionalized surface to modify its properties (e.g., the interaction with solvents, especially water) and design new types of the heterogeneous catalytic system are discussed.
Journal of the American Chemical Society 09/2010; 132(35):12452-7. · 9.91 Impact Factor
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ABSTRACT: A magnetization storage sequence, ALT-1 (alternating longitudinal and transverse components), is reported. The ALT-1 sequence is a hybrid of two types of storage sequences, the Carr-Purcell type and store-and-restore sequences. During incremental storage periods within the ALT-1 sequence, essentially half of the initially transverse magnetization is stored along the z-axis and the other half is prolonged by an echo-generating pulse. The portions of initial magnetization that are stored as longitudinal components or transverse components are alternated by a pi/2 pulse during the cycle. Both transverse components of the initial magnetization are treated the same in the ALT-1 sequence and orientational (phase) information of the initial magnetization is kept during the storage period. The ALT-1 sequence can preserve magnetization more effectively than a published class of modified Carr-Purcell type sequences, because essentially half of the magnetization during incremental storage periods is not subjected to relaxation from T2 effects.
Solid State Nuclear Magnetic Resonance 12/2009; 36(4):202-8. · 1.71 Impact Factor
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ABSTRACT: The molecular dynamics of the silanols of high surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Deuterated silica gel samples were prepared by replacement of the exchangable silanol protons upon exposure to liquid (2)H(2)O and subsequent dehydration under vacuum at various temperatures that were selected to provide samples with varying populations of specific surface features, including (a) "isolated" silanols, that is, those that are not hydrogen-bonded (as demonstrated by the (1)H NMR chemical shift), (b) hydrogen-bonded silanols with a wide range of hydrogen-bonding strengths (as shown by the broad (1)H NMR peak), and (c) a hydrogen-bonded network of physisorbed, water. On a highly dehydrated surface obtained by dehydration at 500 degrees C and containing only isolated. silanols, analysis of the deuterium line shapes indicates that the "isolated" silanols exhibit a broad, inhomogeneous distribution of librational amplitudes of O-H entities about the internuclear Si-O vector, all in the fast-exchange limit (with reference to a 5 x 10(-6) s time scale defined by deuterium quadrupole interactions). The librational amplitudes depend on the observation temperature: increasing amplitudes with increasing temperature and vice versa. At the lowest observation temperature of 78 K, a small part of the silanols is essentially immobilized, but a sizable fraction remains mobiles At dehydration temperatures of 150 and 300 degrees C, the silica surface contains silanols with a distribution of hydrogen-bond strengths. At each observation temperature, the heterogeneous population of hydrogen-bonded silanols exhibits a broader distribution of librational amplitudes than those exh ibited by the "isolated" silanols at the same observation temperature. Less-aggressive dehydration conditions, for example, dehydration temperatures of 25 and 75 degrees C, creates samples in which, in addition to the silanols, a significant amount of physisorbed (deuterated) water is present. At lower observation temperatures, the effect of this physisorbed water is to narrow the librational amplitude for all of the silanols and water molecules. At the lowest observation temperature of 78 K, most of the surface silanols and water molecules of silica gel dehydrated at 25 and 75 degrees C are essentially immobilized. For silica gel dehydrated at 25 and 75 degrees C, at observation temperatures of 200 K and above, chemical exchange among a fraction of the surface silanols becomes significant, resulting in highly narrowed line shapes, while a substantial fraction of the surface silanols continue to execute the librational motions characteristic of more dehydrated surfaces. Line-shape analysis of the librational motion of the silanols permits the estimation of 121 +/- 1 degrees as the Si-O-(2)H bond angle. The deuterium quadrupole coupling constant of the silanol hydrogens is found to vary with the dehydration conditions and is interpreted as reflecting variations of the ensemble-averaged O-O distance between silanol oxygens and their various hydrogen-bonded partners. These data are interpreted in terms of a distribution of hydrogen-bonding arrangements on the heterogeneous silica surface, as witnessed by the silanols. This work provides an indirect indication that the popular view of a hydrogen bond (e.g., one with an O-O distance of 0.
The Journal of Physical Chemistry C 03/2008; 112(11):4315-4326. · 4.80 Impact Factor
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Chemistry of Materials. 01/2008; 20(7):2444-2454.
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ABSTRACT: We have studied the effects of CO surface coverage on the diffusion rates of CO adsorbed on commercial Pt-black in sulfuric acid media by using C-13 electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy in the temperature range 253-293 K. The temperature range chosen for these measurements was such that the electrolyte is in a liquid-like and liquid environment. For CO coverage between theta = 1.0 and 0.36, the CO diffusion coefficients (D-CO) follow a typical Arrhenius behavior and both the activation energies (E-d) as well as the pre-exponential factors (D-CO(0)) show CO coverage dependence. For partially CO covered samples, E-d decreases linearly with increasing CO coverage, indicating that the repulsive CO-CO interactions exert a stronger influence on the coverage dependence of the activation energy than does the nature of the CO adlayer structure. On the other hand, D-CO(0) shows an exponential decrease with increasing CO coverage, consistent with the free site hopping model [Gomer, R. Rep. Prog. Phys. 1990, 53, 917] as the major mechanism for surface diffusion of CO at partial coverages, unlike the situation found with a fully CO covered surface [ Kobayashi et al., J. Am. Chem. Soc., 2005, 127, 14164]. Overall, these results are of interest since they improve our understanding of the surface dynamics of molecules at electrochemical interfaces, and may help facilitate better control of fuel cell reactions in which the presence of surface CO plays a crucial role in controlling electrocatalytic reaction rates.
The Journal of Physical Chemistry C 05/2007; 111(19):7078-7083. · 4.80 Impact Factor
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Journal of Physical Chemistry C. 01/2007; 111(16):5982-5989.
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ABSTRACT: Oxygen reduction reaction (ORR) measurements and (195)Pt electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy were combined to study a series of carbon-supported platinum nanoparticle electrocatalysts (Pt/CB) with average diameters in the range of roughly 1-5 nm. ORR rate constants and H(2)O(2) yields evaluated from hydrodynamic voltammograms did not show any particle size dependency. The apparent activation energy of 37 kJ mol(-1), obtained for the ORR rate constant, was identical to that obtained for bulk platinum electrodes. Pt/CB catalysts on Nafion produced only 0.7-1% of H(2)O(2), confirming that the direct four-electron reduction of O(2) to H(2)O is the predominant reaction. NMR spectral features showed characteristic size dependence, and the line shapes were reproduced by using the layer-deconvolution model. Namely, the variations in the NMR spectra with particle size can be explained as due to the combined effect of the layer-by-layer variation of the s-type and d-type local density of states. However, the surface peak position of (195)Pt NMR spectra and the spin-lattice relaxation time of surface platinum atoms showed practically no change with the particle size variation. We conclude that there is a negligible difference in the surface electronic properties of these Pt/CB catalysts due to size variations and therefore, the ORR activities are not affected by the differences in the particle size.
Physical Chemistry Chemical Physics 12/2006; 8(42):4932-9. · 3.57 Impact Factor
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ABSTRACT: We report the first direct measurement of CO diffusion on nanoparticle Pt electrocatalysts at the solid/liquid interface, carried out using 13C nuclear magnetic resonance (NMR) with a spin-labeling pulse sequence. Diffusion parameters were measured in the temperature range of 253-293 K for CO adsorbed on commercial Pt-black under saturation coverage. 2H NMR of the same system indicates that the electrolyte remains in the liquid state at temperatures where the CO diffusion experiments were performed. The CO diffusion parameters follow typical Arrhenius behavior with an activation energy of 6.0 +/- 0.4 kcal/mol and a pre-exponential factor of (1.1 +/- 0.6) x 10-8 cm2/s. Exchange between different CO populations, driven by a chemical potential gradient, is suggested to be the main mechanism for CO diffusion. The presence of the electrolyte medium considerably slows down the diffusion of CO as compared to that seen on surfaces of bulk metals under UHV conditions. This work opens up a new approach to the study of surface diffusion of adsorbed molecules on nanoparticle electrode catalysts, including the possibility of correlating diffusion parameters to catalytic activity in real world applications of broad general interest.
Journal of the American Chemical Society 11/2005; 127(41):14164-5. · 9.91 Impact Factor
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ABSTRACT: We report the first observation of the 13C nuclear magnetic resonance spectroscopy (NMR) of 13CO, adsorbed from 13CO saturated 0.5 M sulfuric acid solutions, onto the surfaces of commercial Ru-black nanoparticles. The 13C NMR spectra consist of a symmetrically broadened peak having a large isotropic shift as compared to CO adsorbed onto supported Ru catalysts. The variation of the spin-lattice relaxation rate follows Korringa behavior, indicating the metallic nature of adsorbed CO, in addition to varying across the spectrum in a Korringa-like manner. Motional narrowing of the NMR spectrum at higher temperatures, together with an additional contribution to the spin-lattice relaxation rate, indicate that adsorbed CO undergoes rapid diffusion on the particle surfaces. A two-band model analysis of the NMR results indicates that the CO adsorption bond is weaker on Ru as compared to either Pt or Pd. This is also supported by a reduction in the activation energy for CO diffusion on Ru vs either Pt or Pd nanoparticles.
The Journal of Physical Chemistry B 03/2005; 109(7):2474-7. · 3.70 Impact Factor
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Solar Energy Materials and Solar Cells. 01/2004; 81(4):477-483.
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Journal of Power Sources. 01/2004; 131(1-2):285-288.
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Journal of Electroanalytical Chemistry. 01/2004; 573(1):99-109.
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Energy & Fuels. 01/2004; 18(1):285-286.
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Solid State Ionics. 01/2004; 175(1-4):507-510.
