jp503277m_si_001.pdf (2.77 MB)
Visibility of Al Surface Sites of γ‑Alumina: A Combined Computational and Experimental Point of View
journal contribution
posted on 2014-07-17, 00:00 authored by Raphael Wischert, Pierre Florian, Christophe Copéret, Dominique Massiot, Philippe SautetThe
nature of γ-alumina (γ-Al2O3) surface
sites leaves many open questions today, and solid-state
NMR spectroscopy has been proposed and used as a tool for assessing
their structure. Here, we calculated 27Al NMR parameters
from first principles in periodic boundary conditions for a large
number of Al sites with different coordination, potentially present
on the alumina surface. The nature and accordingly the NMR parameters
of these sites change with the level of hydroxylation and thereby
the pretreatment temperature of γ-Al2O3. While the Al chemical shift is little affected by hydroxylation,
the magnitude of the quadrupolar interaction at the Al nucleus is
strongly correlated to hydroxylation, with high to very high quadrupolar
coupling constant (CQ) values (20–34
MHz) on the weakly hydrated major (110) termination, which contains
highly reactive Lewis acidic “defect sites”, and gradual
lowering to bulklike CQ values of around
5 MHz on highly hydrated surfaces. In addition, we studied the effects
of local symmetry on the EFG tensor of Al atoms and the effects of
hydrating neighboring Al sites. These calculation were combined with
{1H}27Al cross-polarization NMR experiments,
carried out at high magnetic field (20 T) and high magic angle spinning
(MAS) frequency (30 kHz) on γ-Al2O3 pretreated
at different temperatures. We clearly show that this method is mainly
sensitive to strongly hydrated surface sites besides the much more
abundant bulk Al atoms, and that catalytically important high-CQ Al centers (Lewis acid sites) and even some
types of hydroxylated Al sites are not visible in the spectrum. Therefore,
even current high-field NMR experiments are not (yet) able to provide
a complete picture of the structure of γ-Al2O3 and much caution should be exercised when interpreting 27Al NMR spectra.