Effect of Bulk Properties on the Rehydration Behavior of Aluminas

Coster, D.; Fripiat, J. J.; Muscas, M.; Auroux, A.

doi:10.1021/la00007a047

Cited by 36 publications

(28 citation statements)

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“…The differential heat of H 2 O adsorption on ␥-Al 2 O 3 (Fig. 2) decreases logarithmically with increasing coverage (Freundlich behavior), which is consistent with previous studies of this material (15,16) is higher than that of ␥-Al 2 O 3 .…”

Section: Syntheses Of Nanocrystalline Metal Oxidessupporting

confidence: 91%

Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas

McHale

Auroux

Perrotta

et al. 1997

Science

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875

677

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is the thermodynamically stable phase of coarsely crystalline aluminum oxide, but syntheses of nanocrystalline Al 2 O 3 usually result in ␥-Al 2 O 3. Adsorption microcalorimetry, thermogravimetric analyses, and Brunauer-Emmett-Teller adsorption experiments, coupled with recently reported high-temperature solution calorimetry data, prove that ␥-Al 2 O 3 has a lower surface energy than ␣-Al 2 O 3 and becomes energetically stable at surface areas greater than 125 square meters per gram and thermodynamically stable at even smaller surface areas (for example, 75 square meters per gram at 800 kelvin). The results are in agreement with recent molecular dynamics simulations and provide conclusive experimental evidence that differences in surface energy can favor the formation of a particular polymorph.

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Section: Syntheses Of Nanocrystalline Metal Oxidessupporting

confidence: 91%

Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas

McHale

Auroux

Perrotta

et al. 1997

Science

Self Cite

875

677

View full text Add to dashboard Cite

show abstract

“…The amorphous material obtained via calcination at 973 K contained approximately 10% of the Al 3+ in 5-fold coordination and a higher percentage of tetrahedral aluminum than octahedral aluminum. Pentacoordinated Al 3+ has been previously detected in nanocrystalline γ-Al 2 O 3 , which suggests that the amorphous material is very similar to a transitional alumina . The percentage of octahedrally coordinated Al 3+ increased fairly regularly with increasing calcination temperature (see Table ).…”

Section: Resultsmentioning

confidence: 69%

Nanocrystalline Spinel from Freeze-Dried Nitrates: Synthesis, Energetics of Product Formation, and Cation Distribution

1998

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The formation of MgAl 2 O 4 from a freeze-dried nitrate precursor was studied by thermogravimetric analysis, differential thermal analysis, powder X-ray diffraction, transmission electron microscopy, 27 Al magic angle spinning NMR, and high-temperature solution calorimetry. A single phase, slightly alumina-rich spinel of composition Mg 0.957 Al 2.028 O 4 was obtained from the precursor by calcination at temperatures g 1073 K. Transmission electron microscopy revealed that material calcined at 1073 K was nanocrystalline, with grain sizes on the order of 20 nm. 27 Al NMR revealed that this material had an unusually high degree of cation disorder, with an order parameter of 0.59 at room temperature. This degree of disorder, which has previously only been achieved in MgAl 2 O 4 via neutron bombardment, provides strong thermodynamic evidence that the freeze-dried precursor contained a highly disordered and probably close to random mixture of cations. Significant levels of fivecoordinated Al 3+ were detected in amorphous samples calcined at 973 K. Increasing calcination temperatures resulted in a decrease in the percentage of tetrahedral Al 3+ and a simultaneous increase in the average particle size of the material. Drop solution calorimetry in 2PbO‚B 2 O 3 at 975 K revealed an enthalpy difference of 39.9 ( 7.4 kJ mol -1 between the disordered nanophase MgAl 2 O 4 synthesized at 1073 K and the well-crystallized material synthesized at 1773 K. Particle size, cation distribution, and adsorbed H 2 O affect the energetics, with the surface energy term dominant.

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“…The peak located at around 200~ has been attributed by us in our previous papers [7,8] to charge transfer effects induced by desorption of a reversible chemisorbed hydrogen on platinum, while the higher temperature peak has been associated with the desorption of spilt-over hydrogen species. This feature is characteristic for desorption spectra of water from alumina [12]. In our case, we must to note that the low peak is nearly symmetrical in all cases and this fact can be proof that the weakly adsorbed species were removed from the surface by purging with N2 at the adsorption temperature.…”

Section: Resultsmentioning

confidence: 99%