Structural, morphological and acidic changes of nanocrystalline aluminas caused by a controlled humidity atmosphere

The changes in the structural, morphological and acidity properties of nanocrystalline aluminas provoked by hydration under a controlled humidity atmosphere at room temperature were studied. Besides the physical and chemical adsorption of water on the surface, the hydration process causes the formation of aluminum oxy-hydroxide and trihydroxide (boehmite and bayerite). The recalcination of hydrated γ-Al2O3 cannot recuperate the initial acidic properties of the material. The populations of weak to moderate acid sites strongly decrease under recalcination of hydrated aluminas. On the other hand, the stronger acid site populations are not strongly affected by recalcination.

Besides the multilayer water adsorption on the alumina surface, water vapor can react with transition aluminas, yielding boehmite and bayerite microparticles. Recalcination of these transition aluminas exposed to humidity can only recover their long-range structures, however, is not effective in restoring their initial acidity. Figure optionsDownload as PowerPoint slide