Probing Interactions of gamma-Alumina with Water via Multinuclear Solid-State NMR Spectroscopy

摘要

Interactions of gamma-alumina with water are important in controlling its structure and catalytic properties. We apply solid-state multinuclear NMR spectroscopy to investigate these interactions by monitoring H-1 and O-17 spectra in real-time. Surface-selective detection is made possible by adsorbing O-17-enriched water on gamma-alumina nanorods. Structural evolution on the surface was selectively probed by H-1/O-17 double resonance NMR and Al-27 NMR at ultrahigh 35.2 T magnetic field. Formation of hydroxyl species on the surface of nanorods is rapid upon the exposure of water, which involves low coordinated aluminum ions with doubly bridging and isolated hydroxyl species being generated first. Fast exchange occurs between oxygen atoms in the water molecules and bare surface sites, indicating high reactivity of these oxygen species. These results provide new insights into the structure and dynamics on the surface of gamma-alumina and the methods applied here can be extended to study the interaction of other oxides with water.