L. Kovarik, K. Khivantsev, J. Szanyi
Jul 22, 2022
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Microscopy and Microanalysis
Abstract
Transition aluminas are used in catalytic applications as catalysts and catalytic supports due to their excellent thermal stability, ability to maintain high surface area, and most importantly their unique surface properties [1]. Understanding the surface properties of transition aluminas has been the subject of extensive research, and while much progress has been made, the actual surface structure of transition aluminas remains only conceptually understood [1,2,3]. A detailed nature of bonding environment has been mostly inaccessible to surface science techniques because of high degree of structural disorder, as well as inherent difficulties associated with studying high surface-area materials. In this work, we will present results from atomic resolution STEM imaging of surface structures of transition aluminas as enabled by segmented and pixelated (EMPAD) detectors. Our observations focus on important low index surfaces, as well as irrational surfaces, of -Al 2 O 3 and -Al 2 O 3. These are high temperature phases of transition aluminas. The microstructural observations were performed with aberration corrected Thermo-Fisher Scientific Themis Z 30-300, operated at 300kV, and using complementary HAADF, Differential Phase Contrast techniques (iDPC, dDPC) and Center of Mass (iCoM, dCOM) imaging techniques. Low index (100) surfaces represent one of the main terminations of transition aluminas. An example of (100) surface of -Al 2 O 3 as obtained with iDPC and dDPC imaging is shown in Figure 1. The images reveal the details of oxygen and aluminum sublattice and based on the analysis of symmetry and relative intensities, it will be shown how the (100) surface undergoes complex reconstructions. The key feature of (100) surfaces are densely populated rows of pentacoordinated Al 3+ sites. At subsurface region,