Water and the silica surface as studied by variable-temperature high-resolution proton NMR

Kinney, David R.; Chuang, I-Ssuer; Maciel, Gary E.

doi:10.1021/ja00068a041

Cited by 185 publications

(168 citation statements)

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“…As a proof, these protons are characterized by a long T 2 of 17 ms (Table S-1 of the Supporting Information), indicative of very weak dipolar interactions. The assignment of the 1.8-ppm line to isolated silanols is in agreement with previous 1 H NMR studies based on thermal dehydration [60,62] and T 2 relaxation-time measurements. [60,63] The resonance at 2.8 ppm, which exhibits a short T 2 (2.4 ms) is probably due to weakly hydrogen-bonded silanols.…”

Section: H Mas Nmr Spectroscopy Of Dried Glassessupporting

confidence: 91%

“…[63][64][65][66] The assignments of various peaks of water, isolated and/or bound silanols in the 1 H NMR spectra have been elucidated [60][61][62][63][64] despite lines broadening arising from strong 1 H-1 H dipolar couplings and/or chemicalshift anisotropies. As shown in previous studies, [60,63] measurements of the spin-spin T 2 relaxation time allow the confirmation of the attributions of the various protons. Indeed, the spin-spin magnetization echo decay is dependent on the dipolar dephasing, which decreases in the presence of strongly homonuclear coupled protons, meaning that long T 2 times are due to weakly coupled protons.…”

Section: H Mas Nmr Spectroscopy Of Dried Glassesmentioning

confidence: 99%

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Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate

Caër¹,

Rotureau²,

Brunet³

et al. 2005

ChemPhysChem

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The production of molecular hydrogen in the radiolysis of dried or hydrated nanoporous controlled-pore glasses (CPG) has been carefully studied using 10 MeV electron irradiation at high dose rate. In all cases, the H2 yield increases when the pore size decreases. Moreover, the yields measured in dried materials are two orders of magnitude smaller than those obtained in hydrated glasses. This proves that the part of the H2 coming from the surface of the material is negligible in the hydrated case. Thus, the measured yields correspond to those of nanoconfined water. Moreover, these yields are not modified by the presence of potassium bromide, which is a hydroxyl radical scavenger. This experimental observation shows that the back reaction between H2 and HO* does not take place in such confined environments. These porous materials have been characterized before and after irradiation by means of Fourier-transform infrared (FT-IR) spectroscopy, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) techniques, which helps to understand the elementary processes taking place in this type of environment, especially the protective effect of water on the surface in the case of hydrated glasses.

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Section: H Mas Nmr Spectroscopy Of Dried Glassessupporting

confidence: 91%

Section: H Mas Nmr Spectroscopy Of Dried Glassesmentioning

confidence: 99%

Radiolysis of Confined Water: Hydrogen Production at a High Dose Rate

Caër¹,

Rotureau²,

Brunet³

et al. 2005

ChemPhysChem

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“…Several investigators have shown that the surface facets of b-crystoballite are a good model for silica. 23,24,25,26,27,28,29,30,31 The most stable facet of b-crystoballite is the (111) surface, which is made up rings containing six Si atoms. Every other Si atom on the (111) surface of b-crystoballite is terminated by an OH group.…”

Section: Theoretical Methodsmentioning

confidence: 99%

A Density Functional Theory Study of the Oxidation of Methanol to Formaldehyde over Vanadia Supported on Silica, Titania, and Zirconia

2002

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Density functional theory was used to investigate the mechanism and kinetics of methanol oxidation to formaldehyde over vanadia supported on silica, titania, and zirconia. The catalytically active site was modeled as an isolated VO 4 unit attached to the support. The calculated geometry and vibrational frequencies of the active site are in good agreement with experimental measurements both for model compounds and oxidesupported vanadia. Methanol adsorption is found to occur preferentially with the rupture of a V-O-M bond (M = Si, Ti, Zr) and with preferential attachment of a methoxy group to V. The vibrational frequencies of the methoxy group are in good agreement with those observed experimentally as are the calculated isobars. The formation of formaldehyde is assumed to occur via the transfer of an H atom of a methoxy group to the O atom of the V=O group. The activation energy for this process is found to be in the range of 199-214 kJ/mol and apparent activation energies for the overall oxidation of methanol to formaldehyde are predicted to lie in the range of 112-123 kJ/mol, which is significantly higher than that found experimentally. Moreover, the predicted turnover frequency (TOF) for methanol oxidation is found to be essentially independent of support composition, whereas experiments show that the TOF is 10 3 greater for titania-and zirconia-supported vanadia than for silica-supported vanadia. Based on these findings, it is proposed that the formation of formaldehyde from methoxy groups may require pairs of adjacent VO 4 groups or V 2 O 7 dimer structures.3

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“…The study of the water vapor uptake by dehydrated silica reaching the Earth's surface by scattering the incoming radi-has a long history (8,9,(14)(15)(16)(17)(18)(19). Usually silica has been ation (2).…”

Section: Introductionmentioning

confidence: 99%