Paper SS2-WeA8
The Use of High-Field Nuclear Magnetic Resonance Spectroscopy to Study the Surface Structure of Poorly Crystalline γ-Al₂O₃ Surfaces

Wednesday, October 17, 2007, 4:00 pm, Room 611

γ-alumina, one of the metastable ‘transition’ alumina structural polymorphs, is an important catalytic material both as an active phase and as a support for catalytically active phases, with widespread applications ranging from petroleum refining to automotive emission control. As such, the bulk and surface structure of γ-Al₂O₃, and its formation and thermal stability have been and continue to be the subject of a considerable amount of research, including attempts to prepare model surfaces via the controlled oxidation of NiAl alloy single crystals. However, due to the low crystallinity and very fine particle size of γ-Al₂O₃, it is very difficult to apply well-established analytical techniques for determining its surface structures. Of particular importance for understanding the catalytic properties of γ-Al₂O₃, relating its surface structure to the origin of Lewis acidity has been of considerable interest and has been studied by FTIR and solid state NMR spectroscopies, and most recently by theoretical calculations. In this presentation, we report the first use of very high field (21.1T) NMR to identify and quantify surface Al species thought to be responsible for imparting Lewis acidity to the γ-Al₂O₃ surface. In particular, a peak in the NMR spectrum at ~23 ppm with relatively low intensity, can be assigned to 5-coordinated Al³⁺ ions, and can be clearly distinguished from the two other peaks representing Al³⁺ ions in tetra-, and octahedral coordination sites. Spin-lattice ²⁷Al relaxation time measurements clearly show that these penta-coordinated Al³⁺ sites are located on the surface of the γ-Al₂O₃ support. Furthermore, we report the first observation of preferential anchoring of an impregnated catalytic phase onto these penta-coordinated Al³⁺ sites by noting that BaO deposition onto γ-alumina resulted in the loss of intensity of the 23 ppm peak, and that the intensity loss observed was linearly proportional to the amount of BaO deposited. Thus, the results of this study strongly suggest that, at least for BaO, these penta-coordinated Al³⁺ ions are the nucleation sites. The implications of these results, especially with respect to the recent and extensive use of theoretical calculations for determining the γ-Al₂O₃ surface structure, will be discussed.