Published: October 05, 2011 r2011 American Chemical Society 22770 dx.doi.org/10.1021/jp206077r | J. Phys. Chem. C 2011, 115, 22770–22780 ARTICLE pubs.acs.org/JPCC Mechanically Induced Phase Transformation of γ-Al 2 O 3 into α-Al 2 O 3 . Access to Structurally Disordered γ-Al 2 O 3 with a Controllable Amount of Pentacoordinated Al Sites A. D€ uvel,* ,† E. Romanova, † M. Sharifi, † D. Freude, ‡ M. Wark, † P. Heitjans, † and M. Wilkening* ,† † Institute of Physical Chemistry and Electrochemistry, and Center for Solid State Chemistry and New Materials (ZFM), Leibniz University Hannover, Callinstr. 3a, D-30167 Hannover, Germany ‡ Institute for Experimental Physics, University of Leipzig, Linn estr. 5, 04103 Leipzig, Germany b S Supporting Information I. INTRODUCTION The study of solid state reactions as well as phase transforma- tions from a microscopic point of view is of great importance to uncover the details of the underlying formation mechanisms. The information gained is highly useful to vary the preparation conditions in order to synthesize materials with tailored proper- ties in a knowledge-based way. In particular, with nonconven- tional preparation techniques such as mechanosynthesis, in many cases nonequilibrium, metastable compounds are accessible. Understanding their formation processes might help improve their properties so that highly functionalized materials can be made available. In this study, we show in detail by using 27 Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, in particular, by which parameters the mechanically induced phase transformation from γ-Al 2 O 3 to its α-modification is influenced. It turned out that the transforma- tion highly depends on the properties of the starting materials as well as the various milling conditions applied. The present study is closely entangled with the preparation of nanocrystalline γ-Al 2 O 3 by high-energy ball milling. Besides the α-modification and γ-phase, six polymorphs of Al 2 O 3 are known, viz., δ, η, θ, k, F, and χ. Whereas the α-phase is the thermodynamically stable one, the other polymorphs are metastable at room temperature. α-Al 2 O 3 is a well-known multipurpose material with applications in many industries. For instance, besides its prevailing use as abrasive it represents a universally applicable high-temperature ceramic due to its hardness and resistance against corrosive materials. 1À7 Further- more, transition metal doped alumina such as Cr:Al 2 O 3 , see ref 8, and Ti:Al 2 O 3 belong to the very first lasing media and continue to be of great importance in this field. 9,10 The γ-phase of Al 2 O 3 is commonly used as a catalyst support material in practical applications. 11À14 Many studies are concerned with the investigation of the so-called metalÀsupport interactions; see, e.g., refs 15À17. Understanding these interactions from an atomic-scale point of view might be very helpful for the target- oriented design of new catalysts as well as improved support materials. Quite recently, it has been proven by high-resolution 27 Al NMR that coordinatively unsaturated, i.e., 5-fold, coordi- nated Al 3+ centers on the γ-Al 2 O 3 surfaces act as binding sites for active catalyst phases such as widely used PtO being the precursor for metallic Pt. 15 Interestingly, 5-fold Al ions 13,18 also show up during the thermally induced irreversible γ f α phase transfor- mation which has been the topic of numerous studies. 12,19À22 Received: June 28, 2011 Revised: September 30, 2011 ABSTRACT: 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 structureÀproperty 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 γ-Al 2 O 3 into α-Al 2 O 3 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 Al MAS 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.