Structural Characterization of Phosphate Species Adsorbed on γ‑Alumina by Combining DNP Surface Enhanced NMR Spectroscopy and DFT Calculations

Obtaining
an atomic-scale description of the chemical interactions
of phosphates with an oxide support, such as γ-Al2O3, is essential to get a rational understanding of the
role of phosphate additives for a great number of heterogeneous catalysts,
as well as to improve the use of this element. Combining cutting-edge
Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP
SENS) techniques with Density Functional Theory (DFT) calculations,
we provide an accurate molecular description of phosphate speciation
on γ-Al2O3 surfaces for various P surface
coverages after drying at 120 °C. Thanks to 31P double-
and triple-quantum filtered NMR experiments as well as to 27Al–31P dipolar- and scalar-based correlation spectra,
we demonstrate the presence of polyphosphates and of Al–O–P
connectivities at the exposed facets of γ-Al2O3. DFT-based thermodynamics shows that phosphates (mono- or
di-) are preferentially covalently bonded on the (1 1 0) γ-Al2O3 facet with high-dentation modes. These high-dentation
modes are favored by entropy gain due to water desorption. We used
the gauge-including projector-augmented wave (GIPAW) DFT method for 31P NMR chemical shifts calculations and propose a systematic
identification of the various types of phosphates covalently or noncovalently
bonded to the alumina surface. The calculations confirm the existence
of polyphosphates as observed experimentally. Since the surface condensation
into polyphosphates is endergonic, the presence of polyphosphates
on the surface is likely to result from their direct adsorption in
impregnation solution. The observed increasing concentration of polyphosphates
with the coverage could be related to a less likely hydrolysis due
to the reduced availability of sites to stabilize the fragmented oligomers.
This understanding opens the way to a better control over the speciation
of phosphate species that are known to be key in the preparation of
supported catalysts over alumina.