(563e) Structure and Chemistry Of ‘Clean' and Modified Transition Al2O3 Surfaces

Transition Al₂O₃ represent a group of materials that have very attractive structural, surface and dielectric properties, which makes them a material of choice in a range of applications such as adsorbents, catalysts, catalytic supports, hard protective coatings, abrasives or membrane materials.¹  The structural stability and the ability to maintain high surface area at elevated temperatures is one of the key properties that makes these materials highly relevant for catalytic applications.  The most prominent polymorph for catalytic application is g-Al₂O₃, which can be readily prepared with a surface area in excess of 200 m²/g, and which can maintain structural stability up to temperatures of 700-800°C.  However, after prolonged exposure to higher temperatures, structural transformations to d-Al₂O₃ and q-Al₂O₃, and then a‑Al₂O₃ at ~1100-1200°C occur leading to a loss of surface area and, thus, often destabilization and deterioration of catalytic properties.²

Recently, we described the use of ethanol temperature programmed desorption (TPD) as a sensitive method to follow changes in the g-Al₂O₃ surface during thermal dehydration/dehydroxylation,³ demonstrating that the temperature of ethylene (the primary product during ethanol TPD) desorption was sensitive to the surface structure and/or surface electronic properties.

In this presentation, we report new results from high-resolution TEM (HRTEM) and ²⁷Al NMR spectroscopy that provide new insights into the bulk and surface structures of transition aluminas.⁴  In addition, we will describe ethanol TPD results of ‘clean’ and chemically modified (by addition of oxides of Pt, Pd, Cu, Ln, Zn, Ba and K) where the ethylene desorption temperatures show a strong correlation with the electronegativities of the added metal atoms.⁵

References

1. L.K. Hudson, C. Misra, A.J. Perrotta, K. Wefers, and F.S. Williams, Aluminum Oxide. (Wiley-VCH Verlag GmbH & Co. KGaA, 2000).  doi:10.1002/14356007.a01_557
2. J. M. McHale, “Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas.”  Science 277, 788–791 (1997).
3. J.H. Kwak, C.H.F. Peden, and J.  Szanyi, “Using a Surface Sensitive Chemical Probe and a Bulk Structure Technique to Monitor the g‑ to q-Al₂O₃ Phase Transformation.”  J. Phys. Chem. C 115, 12575-12579 (2011).
4. L. Kovarik, A. Genc, C.M. Wang, A. Qiu, C.H.F. Peden, J. Szanyi, and J.H. Kwak, “Tomography and High Resolution Electron Microscopy Study of Surfaces and Porosity in a Plate-like g-Al₂O₃.”  J. Phys. Chem. C 117 179-186 (2013).
5. J.H. Kwak, C.H.F. Peden, and J.  Szanyi, “The Use of Ethanol TPD to Probe Modifications of the Acid/Base Properties g‑Al₂O₃ following addition of oxides.”  In preparation.