Mechanically Induced Phase Transformation of γ-Al₂O₃into α-Al₂O₃. Access to Structurally Disordered γ-Al₂O₃with a Controllable Amount of Pentacoordinated Al Sites

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 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 γ-Al2O3 into α-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 27Al 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.

Structure and ion dynamics of mechanosynthesized oxides and fluorides

“…A systematic study of the influence of the various milling conditions on the phase transformation induced by mechanical action showed that the underlying mechanism is presumably similar to that reported for the thermally induced transformation [82,83], i.e. the growth of α-Al 2 O 3 starts from the unsaturated Al centers on the surface of a metastable γ-Al 2 O 3 crystallite and propagates into the inner regions [73]. Worth noting, when the initially formed penta-coordinated Al centers are immediately trapped after they have formed, e.g.…”

“…Reflections assigned to ZrO 2 originate from abrasion during milling in zirconia vials. The formation of crystalline α-Al 2 O 3 is hardly seen via X-ray diffraction; 27 Al MAS NMR helps identify amorphous phases formed, see (b) [73]. The 27 Al MAS NMR spectra of the starting material (shown at the bottom) reveal octahedrally and tetrahedrally coordinated Al cations as expected.…”

“…Worth noting, when the initially formed penta-coordinated Al centers are immediately trapped after they have formed, e.g. by adding CaF 2 , the initialization of the transformation can be largely suppressed [73]. This clearly illustrates the important role of unsaturated Al ions against, e.g.…”

“…The 27 Al MAS NMR spectra of the starting material (shown at the bottom) reveal octahedrally and tetrahedrally coordinated Al cations as expected. With increasing milling time of up to 10 h the signal at 70 ppm remains almost unaffected pointing to Al ions subjected to local environments being identical or resembling that of γ-Al 2 O 3 [73]. Most interestingly, mechanical treatment using short milling times does not significantly induce the γ-α phase transformation but leads to the generation of penta-coordinated, unsaturated Al cations that may act as anchoring sites for catalytically active materials such as Pt [78,79].…”

“…The last example presented focuses on the transformation of γ-Al 2 O 3 into the thermodynamically more stable α-modification through mechanical action [73]. The transformation can be induced by mechanical treatment in planetary or shaker mills.…”