(348c) The Use of Ultra High-Field Nuclear Magnetic Resonance Spectroscopy to Study the Surface Structure and Catalytic Properties of Poorly Crystalline γ–Al2O3 Surfaces

Kwak, J. H., Pacific Northwest National Laboratory
Hu, J., Pacific Northwest National Laboratory
Kim, D. H., Pacific Northwest National Laboratory

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 other catalytically active phases. As such, the bulk and surface structure of γ-alumina, and its formation and thermal stability continue to be the subject of a considerable amount of research. However, due to the low crystallinity and very fine particle size of γ-alumina, 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-alumina, relating its surface structure to the origin of Lewis acidity has been of considerable interest and has been studied by solid state NMR and FTIR 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-alumina 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 and Pt deposition onto this γ-alumina sample results in the loss of intensity of the 23 ppm peak linearly proportional to the amount of catalytic phase deposited. Finally, our recent results also suggest an important role for these sites in determining the thermal stability of the γ-Al2O3 phase during high temperature calcination.