Insight into the atomic-level structure of γ-alumina using a multinuclear NMR crystallographic approach

Area of Science:

• Materials Science
• Solid-State Chemistry
• Spectroscopy

Background:

• Gamma-alumina (γ-Al₂O₃) is a widely used catalyst support, but its precise bulk and surface structures remain incompletely understood.
• Distinguishing between various oxygen species and their coordination environments is crucial for understanding γ-Al₂O₃'s catalytic properties.

Purpose of the Study:

• To elucidate the detailed bulk and surface structural characteristics of γ-Al₂O₃.
• To assign and quantify different oxygen species and their coordination environments in both bulk and surface sites.
• To investigate the nature and interactions of surface hydroxyl groups (aluminols) on γ-Al₂O₃.

Main Methods:

• Multinuclear Nuclear Magnetic Resonance (NMR) spectroscopy utilizing ¹⁷O isotopic enrichment.
• Density Functional Theory (DFT) calculations for predicting NMR parameters and structural models.
• Advanced NMR techniques including Variable Temperature experiments, Cross-Polarization (CP), TRAPDOR, RESPDOR, D-HMQC, and DQ MAS.

Main Results:

• Bulk γ-Al₂O₃ features aluminum cations in spinel-like sites with alternating Al^VI and Al^IV vacancies. No significant oxygen vacancies or hydrogen were detected in the bulk.
• Surface analysis revealed diverse protonated and non-protonated oxygen species. Two types of protonated species were assigned to accessible and less accessible aluminol sites.
• ¹H NMR parameters confirmed shifts depend on denticity and neighboring aluminum coordination. RESPDOR and D-HMQC experiments characterized μ₁, μ₂, and μ₃ aluminols, detailing their aluminum coordination preferences.

Conclusions:

• The study provides a comprehensive structural model for γ-Al₂O₃, resolving ambiguities in previous spectral assignments.
• Detailed characterization of surface aluminol species and their interactions with aluminum sites offers critical insights for catalyst design.
• The combination of isotopic enrichment, advanced NMR, and DFT is a powerful approach for investigating complex oxide structures.