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

Abstract

The structural changes of γ-Al 2 O 3, Ni/γ-Al 2 O 3, and Pt/γ-Al 2 O 3 catalysts under aqueous phase reforming conditions (liquid water at 200 °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 γ-alumina is converted into a hydrated boehmite (AlOOH) phase with significantly decreased acidity and surface area. For metal-free γ-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 γ-alumina increases, suggesting surface pitting and formation of small boehmite particles on the surface of γ-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 γ-alumina into boehmite is accompanied by sintering of the supported metal particles.