Energetics of formation of lamellar porous microstructures in ?-Al2O3

Wilson, Stephen J.; McConnell, J. D. C.; Stacey, M. H.

doi:10.1007/bf00550379

Cited by 22 publications

(12 citation statements)

References 11 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…No change in pore size with temperature was observed for the goethites of this study. Wilson et al (1980) observe a dependence of pore size with water-vapour pressure for boehmite but no change in size with temperature, and a similar relationship may be true for goethite. Although a change in microstructure may occur with Pn2o, thus affecting reaction rates, the transformation mechanism may remain the same.…”

Section: Tem and Xrdsupporting

confidence: 57%

The kinetics and reaction mechanism of the goethite to hematite transformation

Goss

1987

Mineral. mag.

146

View full text Add to dashboard Cite

A complete mechanism for the transformation goethite to hematite based upon the results of thermogravimetric, transmission electron microscope and X-ray diffraction investigations is presented. A porous microstructure and hematite crystallites in twin orientation are found to develop during transformation. For the main part of the transformation, and at higher temperatures, the reaction is controlled by a two-dimensional phase boundary. Activation energies of 169 +_ 8 k J/mole (for an ore mineral) and 154_+ 15 kJ/mole (for a recent sedimentary goethite) were obtained for this part of the transformation. At early stages and lower temperatures, the mechanism is one of proton/iron transfer across the reaction interface. Important goethite characteristics are grain size, shape, crystallinity and excess water content. The activation energy is found to depend upon temperature and degree of dehydration.

show abstract

Section: Tem and Xrdsupporting

confidence: 57%

The kinetics and reaction mechanism of the goethite to hematite transformation

Goss

1987

Mineral. mag.

146

View full text Add to dashboard Cite

show abstract

“…In order to understand this variability, the structural rearrangements of bulk boehmite at low temperatures have been extensively studied. ,− Recently, theoretical calculations have developed new models for γ-alumina, documenting variations in the spinel structure as well as the non-spinel structure. − Dehydration models based on both spinel and non-spinel γ-Al 2 O 3 report fine-scale structural changes and predict porosity around and below 1 nm. Porosity below 1 nm was also seen experimentally by SAXS analysis in dehydrated boehmite materials. , Further theoretical work has hypothesized that fine-scale porosity (around 1 nm in dimension) is characteristic of the γ-Al 2 O 3 crystal structure. ,− Despite intensive research, the distribution of vacancies and atomic structure of experimental γ-Al 2 O 3 remain the subjects of controversy.…”

Section: Introductionmentioning

confidence: 99%

Understanding Pore Formation in ALD Alumina Overcoats

George

Littlewood

Stair

2020

ACS Appl. Mater. Interfaces

View full text Add to dashboard Cite

AlO X thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosity enables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2–7 nm AlO X by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlO X deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlO X ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlO X ALD coatings.

show abstract

“…The topotactic transformation of boehmite to γ-Al 2 O 3 produces a well-developed pore structure, resulting in a high surface area. − The subsequent phase transformations to δ- and θ-Al 2 O 3 are also topotactic and are accompanied by changes in the porous microstructure such that each phase has a characteristic microstructure. Each subsequent phase has an increased mean pore size and decreased surface area.…”

Section: Introductionmentioning

confidence: 99%

“…It is logical that hydrogen, whether in the form of water, hydroxide, or isolated protons, is likely to be present in these regions. However, the lamellar porous microstructure of γ-Al 2 O 3 , with reported mean pore widths of 8−10 Å and spacing between 35 and 40 Å, − creates ample surface for hydrogen species attachment. This can make it difficult to discern between surface hydrogen-containing species and possible hydrogen-containing species within the bulk.…”

Section: Introductionmentioning

confidence: 99%

Boehmite-Derived γ-Alumina System. 2. Consideration of Hydrogen and Surface Effects

et al. 2004

View full text Add to dashboard Cite

A boehmite-derived gamma-alumina (γ-Al 2 O 3 ) system was studied using various complementary techniques to examine surface area and pore size, the amount of hydrogencontaining species, the nature of hydrogen bonding environments, and the location of these species. Using small-angle X-ray scattering, the material examined was shown to have a significantly higher surface area than that typically expected for highly crystalline boehmitederived γ-Al 2 O 3 . This higher surface area was associated with the presence of closed nanopores, the size of which was found to complement observations from transmission electron microscopy. More hydrogen was determined to be in the structure when measured using prompt-gamma activation analysis than indicated by loss on ignition experiments, suggesting that hydrogen-containing species other than water were also present. Neutron vibrational spectroscopy and infrared spectroscopy showed a reduction in signals associated with water and hydroxide species as the calcination temperature increased. Measurements from small-angle X-ray scattering and prompt-gamma activation analysis show that the surface area and the amount of hydrogen present reduce with increasing temperature treatment. This is associated with a reduction in the amount of amorphous material in the structure and an increase in pore and crystallite size. The evidence obtained suggests that the bulk crystalline structure is relatively well-ordered and contains no interstitial hydrogen. Hydrogen is therefore located at the pore surfaces and within amorphous regions, which themselves are located near pore surfaces.

show abstract

Energetics of formation of lamellar porous microstructures in ?-Al2O3

Cited by 22 publications

References 11 publications

The kinetics and reaction mechanism of the goethite to hematite transformation

The kinetics and reaction mechanism of the goethite to hematite transformation

Understanding Pore Formation in ALD Alumina Overcoats

Boehmite-Derived γ-Alumina System. 2. Consideration of Hydrogen and Surface Effects

Contact Info

Product

Resources

About