Corundum, α-Al2O3, appears to be the thermodynamically stable phase of aluminum oxide at all common pressure and temperature conditions, but attempted syntheses of nanocrystalline Al2O3 usually result in other polymorphs of the oxide (transition aluminas). Herein we explore the possibility that γ-Al2O3 becomes the thermodynamically stable polymorph when a critical surface area is exceeded. High-temperature solution calorimetry was performed on several samples of nanocrystalline γ-Al2O3 and α-Al2O3. The aluminas adsorbed atmospheric H2O which could not be completely removed without coarsening (particularly for α-Al2O3). Samples of γ-Al2O3 with <21 mg/(100 m2) and α-Al2O3 with <29 mg/(100 m2) coverages of adsorbed H2O lied at equal enthalpies with respect to corundum and H2O(g, 298 K), independent of surface area. This result provides experimental verification for a direct dependence of the heat of adsorption on the surface energy of the adsorbent. Attempts at correcting the data for heat effects due to adsorbed H2O revealed that increased surface area of nanocrystalline α-Al2O3 and γ-Al2O3 significantly destabilized the materials with respect to coarse grained samples. However, down to the lowest attainable coverages of H2O the experimental "surface energies" of the two phases were nearly equal. Our results cannot definitely rule out the assumption that γ-Al2O3 is surface energy stabilized with respect to α-Al2O3. However, if this is the case, the high-energy sites on the α-Al2O3 surface are relatively few, and effectively stabilized at low temperatures by adsorbed H2O. The enthalpy of hypothetical coarse grained γ-Al2O3 was also explored and found to be + 13.4 ± 2.0 kJ/mol relative to coarse grained α-Al2O3.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry