Effects of increased surface area and chemisorbed H2O on the relative stability of nanocrystalline gamma-Al2O3 and alpha-Al2O3

Corundum, alpha-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 gamma-Al2O3 becomes the thermodynamically stable polymorph when a critical surface area is exceeded. High-temperature solution calorimetry was performed on several samples of nanocrystalline gamma-Al2O3 and alpha-Al2O3. The aluminas adsorbed atmospheric H2O which could not be completely removed without coarsening (particularly for alpha-Al2O3). Samples of gamma-Al2O3 with <21 mg/(100 m(2)) and alpha-Al2O3 with <29 mg/(100 m(2)) 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 alpha-Al2O3 and gamma-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 gamma-Al2O3 is surface energy stabilized with respect to alpha-Al2O3. However, if this is the case, the high-energy sites on the alpha-Al2O3 surface are relatively few, and effectively stabilized at low temperatures by adsorbed H2O. The enthalpy of hypothetical coarse grained gamma-Al2O3 was also explored and found to be +13.4 +/- 2.0 kJ/mol relative to coarse grained alpha-Al2O3.