Effects of Increased Surface Area and Chemisorbed H₂O on the Relative Stability of Nanocrystalline γ-Al₂O₃ and α-Al₂O₃

Corundum, α-Al 2 O 3, appears to be the thermodynamically stable phase of aluminum oxide at all common pressure and temperature conditions, but attempted syntheses of nanocrystalline Al 2 O 3 usually result in other polymorphs of the oxide (transition aluminas). Herein we explore the possibility that γ-Al 2 O 3 becomes the thermodynamically stable polymorph when a critical surface area is exceeded. High-temperature solution calorimetry was performed on several samples of nanocrystalline γ-Al 2 O 3 and α-Al 2 O 3 . The aluminas adsorbed atmospheric H 2 O which could not be completely removed without coarsening (particularly for α-Al 2 O 3 ). Samples of γ-Al 2 O 3 with <21 mg/(100 m 2 ) and α-Al 2 O 3 with <29 mg/(100 m 2 ) coverages of adsorbed H 2 O lied at equal enthalpies with respect to corundum and H 2 O(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 H 2 O revealed that increased surface area of nanocrystalline α-Al 2 O 3 and γ-Al 2 O 3 significantly destabilized the materials with respect to coarse grained samples. However, down to the lowest attainable coverages of H 2 O the experimental “surface energies” of the two phases were nearly equal. Our results cannot definitely rule out the assumption that γ-Al 2 O 3 is surface energy stabilized with respect to α-Al 2 O 3 . However, if this is the case, the high-energy sites on the α-Al 2 O 3 surface are relatively few, and effectively stabilized at low temperatures by adsorbed H 2 O. The enthalpy of hypothetical coarse grained γ-Al 2 O 3 was also explored and found to be +13.4 ± 2.0 kJ/mol relative to coarse grained α-Al 2 O 3 .