Classification and Identification of Facet- and Edge-Specific γ-Al <sub>2</sub> O <sub>3</sub> Surface Sites from <sup>1</sup> H/ <sup>27</sup> Al NMR Cross-Signatures and DFT Modeling
Domenico Gioffrè, Pierre Florian, Thomas Pigeon, Pascal Raybaud, Céline Chizallet, Christophe Copéret
Abstract
γ-Al 2 O 3 is used as both a catalyst and a support for catalytic active phases. The properties of γ-Al 2 O 3 have been ascribed to specific surface sites, with varying Al coordination number, acidity, and basicity, depending on the morphology of the material. Here, we combine surface-specific 27 Al{ 1 H} 2D high-field NMR (28.2T, 1.2 GHz for 1 H frequency) at fast MAS (50 kHz) to observe four main distinct families of surface Al–OH sites. Comparing the measured NMR signatures ( 27 Al δ iso, C Q, and 1 H δ iso ) to computed values from a large range of structural DFT models enables to identify specific edge and facet Al–OH surface sites in γ-Al 2 O 3, including a distinct [4] Al–OH site with an unprecedented C Q approaching 18 MHz. This molecular-level description of alumina surfaces opens new opportunities to understand its unique properties, such as the stabilization of small particles down to single atoms, central to catalytic processes.