Mechanically Induced Phase Transformation of γ-Al2O3 into α-Al2O3. Access to Structurally Disordered γ-Al2O3 with a Controllable Amount of Pentacoordinated Al Sites

One of the most important goals in materials science is the modification and design of solids to obtain functionalized materials with tailored properties. However, in many cases the structure-property relationships are unknown or turn out to be highly complex and difficult to bring under control. In the present paper we show how the atomic-scale structure of a technically important oxide can be modified by mechanical rather than by chemical treatment. We comprehensively investigated the phase transformation of gamma-Al2O3 into alpha-Al2O3 which was mechanically initiated by treatment of various samples in a high-energy ball mill. The progress of the transformation is followed on an atomic scale by (27)AlMAS NMR spectroscopy carried out at a very high magnetic field of 17.6 T. Depending on the kind of milling, unsaturated, i.e., pentacoordinated, Al ions are formed to an unexpectedly large number fraction as high as 20%. The progress of the phase transformation turns out to depend on a number of parameters such as the initial morphology and surface area of the samples as well as the milling conditions. By systematically evaluating and varying these parameters, several ways have been found to easily manipulate the phase transformation and, more importantly, to ultimately control both the formation and amount of pentacoordinated Al centers. These have been shown to act as anchoring sites for catalytically active materials such as widely used Pt. Finally, the mechanical preparation route found might establish a basis for the design of catalysts whose activity can be thoroughly tailored.