Structural Changes of γ-Al2O3-Supported Catalysts in Hot Liquid Water

The structural changes of gamma-Al2O3, Ni/gamma-Al2O3, and Pt/gamma-Al2O3 catalysts under aqueous phase reforming conditions (liquid water at 200 degrees C and autogenic pressure) are examined over the course of 10 h. The changes are characterized by X-ray diffraction, NMR spectroscopy, N-2 physisorption, pyridine adsorption followed by IR spectroscopy, and electron microscopy. It is demonstrated that gamma-alumina is converted into a hydrated boehmite (AlOOH) phase with significantly decreased acidity and surface area. For metal-free gamma-alumina, the transformation is completed within 10 h, whereas the presence of nickel and platinum particles significantly retards the formation of boehmite. In the beginning of-the treatment, the surface area of gamma-alumina increases, suggesting surface pitting and formation of small boehmite particles on the surface of gamma-alumina. This process is followed by the formation of a compact crystalline boehmite phase. It is proposed that the metal particles affect the kinetics of this transformation by blocking specific surface hydroxyl groups that act as initial hydration sites. The transformation of gamma-alumina into boehmite is accompanied by sintering of the supported metal particles.