Classification and Identification of Facet- and Edge-Specific γ-Al2O3 Surface Sites from 1H/27Al NMR Cross-Signatures and DFT Modeling.

Abstract

γ-Al₂O₃ is used as both a catalyst and a support for catalytic active phases. The properties of γ-Al₂O₃ 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 ²⁷Al{¹H} 2D high-field NMR (28.2T, 1.2 GHz for ¹H frequency) at fast MAS (50 kHz) to observe four main distinct families of surface Al-OH sites. Comparing the measured NMR signatures (²⁷Al δ_iso, C_Q, and ¹H 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-Al₂O₃, 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.