Studies of the hydrogen held by solids IV. Deuterium exchange and NMR investigations of silica, alumina, and silica-alumina catalysts

Hall, W

doi:10.1016/0021-9517(63)90006-x

Cited by 63 publications

(8 citation statements)

References 37 publications

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“…The exchange process occurs mainly between 400 and 800 °C, which proves that hydrogen is very difficult to exchange on silica. The amount of exchanged hydrogen (1.2 at nm -2 ) and the temperature of exchange are in agreement with the results of Hall et al., who found a maximum rate of exchange between 610 and 625 °C and a number of exchanged species between 1.6 and 2.6 at nm -2 . It can be seen in Figure that a very small amount of reactive hydrogen (∼0.1 at nm -2 ) is able to exchange between 100 and 180 °C.…”

Section: Resultssupporting

confidence: 92%

“…By contrast to oxygen, a great number of works dealing with hydrogen exchange on oxides or supported metal catalysts are available in the literature, and three reviews are concerned mainly with hydrogen spillover. − Two techniques have been used to study the hydrogen exchange: analysis of the partial pressure of the hydrogen isotopomers [two isotopomers of one molecule are two isomers in which isomerism is due only to the presence of different isotopes of the same element (for example HD and D 2 are two isotopomers of the H 2 molecule)] in the gas phase generally by mass spectrometry (MS) and analysis of the surface hydroxyl groups (OH and OD) by infrared (IR) spectroscopy. The first technique allows one to carry out quantitative, kinetic studies on the exchangeable hydrogen species, − while the second is well adapted for the determination of the nature of the different hydroxyl groups that could be present and exchanged on the oxides. − In certain works both methods are used. , In our work the temperature range of hydrogen exchange and the number of exchangeable hydrogens will be determined by MS analysis. As for the study of hydrogen spillover on supported metal catalysts, two types of strategy can be employed.…”

Section: Introductionmentioning

confidence: 99%

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Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

and¹,

Duprez²

1997

J. Phys. Chem. B

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Exchange of D2 with the OH groups of several oxides was carried out on the samples on which we investigated the 18O2/16OH exchange (part I, J. Phys. Chem. 1996, 100, 9429). Temperature-programmed isotopic exchange was first carried out on the bare oxides. In every case, the reaction follows a simple heteroexchange mechanism, with HD as a primary product. The maximal rates of exchange are obtained at the following temperature: CeO2 (prereduced), 100 °C > MgO, 120 °C > ZrO2, 145 °C > CeO2 (preoxidized), 160 °C > γ-Al2O3, 190 °C > Cl−Al2O3, 200 °C ≫ SiO2, 540 °C. The first two samples exchange their hydrogen at a significant rate at room temperature. Secondary peaks and shoulders show the multiplicity of the OH groups on most oxides. Compared to oxygen, hydrogen exchange occurs at much lower temperatures, the shift of the temperature ranges of O and H exchange being between 250 and 430 °C. Isothermal isotopic exchange was carried out between 25 and 100 °C over rhodium catalysts supported on these oxides. The presence of rhodium accelerates the hydrogen exchange, at least by 2 orders of magnitude. As with oxygen exchange, three main steps must be considered: (i) adsorption−desorption on the metal, (ii) transfer of D atoms onto the support (spillover), and (iii) hydrogen diffusion. Contrary to what was observed with O2 exchange, step i is never a rate-determining step (rds) for H2 exchange, the rate of H2 + D2 equilibration being much higher than the rate of exchange. The results are in accordance with a model where at T < 75 °C the rds of exchange would be the hydrogen transfer from the metal to the support (spillover step), while at T ≥ 75 °C, the rds would be the surface diffusion. This model is valid for all the supports except silica, for which most of the hydroxyl groups exchange at high temperatures. The rate of exchange depends linearly on the density of the hydroxyl groups and tends toward zero for a fully dehydroxylated support. Coefficients of surface diffusion give the following order for the hydrogen mobility at 75 °C (base 100 for γ-Al2O3): CeO2, 770 > MgO, 230 > γ-Al2O3, 100 > ZrO2, 23 ≫ SiO2, nd. There is no correlation between the hydrogen mobility and the surface acidity of the oxides, the highest hydrogen mobility being found on basic oxides with a very high oxygen mobility. A model of isotropic heterogeneous diffusion is proposed to explain certain discrepancies observed between the present isotopic exchange method and direct IR spectroscopic methods previously published.

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Section: Resultssupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

and¹,

Duprez²

1997

J. Phys. Chem. B

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“…Because partial rehydroxylation was expected during the functionalization steps in the ODNP sample preparation, the unfunctionalized particles were subjected to similar aqueous conditions before the silanol density measurement. The number of surface silanol groups per mass of silica was measured by taking the average between 1 H NMR measurements using deuterium exchange (5,8) and the change in pH upon submerging the particles in a slightly basic (pH ∼9.5) solution of 1 M NaCl (10). The techniques rely on the exchangeability of the silanol protons with deuterium (from D 2 O) and Na + (from the NaCl solution), and the results agreed well between these two complementary methods (SI Appendix, Table S1).…”

Section: Results and Analysismentioning

confidence: 99%

Surface chemical heterogeneity modulates silica surface hydration

Schrader

Monroe

Sheil

et al. 2018

Proc. Natl. Acad. Sci. U.S.A.

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An in-depth knowledge of the interaction of water with amorphous silica is critical to fundamental studies of interfacial hydration water, as well as to industrial processes such as catalysis, nanofabrication, and chromatography. Silica has a tunable surface comprising hydrophilic silanol groups and moderately hydrophobic siloxane groups that can be interchanged through thermal and chemical treatments. Despite extensive studies of silica surfaces, the influence of surface hydrophilicity and chemical topology on the molecular properties of interfacial water is not well understood. In this work, we controllably altered the surface silanol density, and measured surface water diffusivity using Overhauser dynamic nuclear polarization (ODNP) and complementary silica-silica interaction forces across water using a surface forces apparatus (SFA). The results show that increased silanol density generally leads to slower water diffusivity and stronger silica-silica repulsion at short aqueous separations (less than ∼4 nm). Both techniques show sharp changes in hydration properties at intermediate silanol densities (2.0-2.9 nm). Molecular dynamics simulations of model silica-water interfaces corroborate the increase in water diffusivity with silanol density, and furthermore show that even on a smooth and crystalline surface at a fixed silanol density, adjusting the spatial distribution of silanols results in a range of surface water diffusivities spanning ∼10%. We speculate that a critical silanol cluster size or connectivity parameter could explain the sharp transition in our results, and can modulate wettability, colloidal interactions, and surface reactions, and thus is a phenomenon worth further investigation on silica and chemically heterogeneous surfaces.

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“…This contrasts with the silica supports studied previously 1,3 where a relatively narrow peak was observed, in spite of a high surface density of protons. The narrowing in the silica case is due to motion of the surface protons, producing an averaging in the proton environment, but this is thought to involve a local rotation or wagging rather than movement across the silica surface. − Clearly, this motion either does not occur for protons on the MgO surface, possibly because of the more ionic nature of the surface, or else is not effective in causing the proton to sample an average environment.…”

Section: Resultsmentioning

confidence: 99%

In Situ ¹H NMR Investigation of Hydrogen Adsorption on a Cu/MgO Catalyst

et al. 1997

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Hydrogen adsorption on a Cu/MgO catalyst is investigated using in situ 1H NMR. The adsorption is largely homogeneous, demonstrating reasonable agreement with the Langmuir isotherm and only small variations in the frequency shift and spin−lattice relaxation time with increasing coverage. The catalyst is investigated under flowing gas conditions, and the hydrogen/deuterium exchange kinetics are studied. The activation energy for exchange is measured to be 96 kJ mol-1. The 1H variable-temperature NMR probe developed here and used for the in situ adsorption isotherm and the flowing gas experiments is also described.

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Studies of the hydrogen held by solids IV. Deuterium exchange and NMR investigations of silica, alumina, and silica-alumina catalysts

Cited by 63 publications

References 37 publications

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

Surface chemical heterogeneity modulates silica surface hydration

In Situ ¹H NMR Investigation of Hydrogen Adsorption on a Cu/MgO Catalyst

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Studies of the hydrogen held by solids IV. Deuterium exchange and NMR investigations of silica, alumina, and silica-alumina catalysts

Cited by 63 publications

References 37 publications

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D2 with H of SiO2, Al2O3, ZrO2, MgO, and CeO2: Activation by Rhodium and Effect of Chlorine

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D2 with H of SiO2, Al2O3, ZrO2, MgO, and CeO2: Activation by Rhodium and Effect of Chlorine

Surface chemical heterogeneity modulates silica surface hydration

In Situ 1H NMR Investigation of Hydrogen Adsorption on a Cu/MgO Catalyst

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Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

Mobility of Surface Species on Oxides. 2. Isotopic Exchange of D₂ with H of SiO₂, Al₂O₃, ZrO₂, MgO, and CeO₂: Activation by Rhodium and Effect of Chlorine

In Situ ¹H NMR Investigation of Hydrogen Adsorption on a Cu/MgO Catalyst