Effects of increased surface area and chemisorbed H₂O on the relative stability of nanocrystalline γ-Al₂O₃ and α-Al₂O₃

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.