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 gamma-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 gamma-Al2O3 surfaces for various P surface coverages after drying at 120 degrees C. Thanks to P-31 double- and triple-quantum filtered NMR experiments as well as to Al-27-P-31 dipolar- and scalar-based correlation spectra, we demonstrate the presence of polyphosphates and of Al-O-P connectivities at the exposed facets of gamma-Al2O3. DFT-based thermodynamics shows that phosphates (mono- or di-) are preferentially covalently bonded on the (1 1 0) gamma-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 P-31 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.

Automated summary

By combining DNP SENS techniques and DFT calculations, an accurate molecular description of phosphate speciation on gamma-Al2O3 surfaces for various P surface coverages has been provided, revealing the presence of polyphosphates and Al-O-P connectivities, as well as suggesting preferential covalent bonding modes with high-dentation on the (1 1 0) gamma-Al2O3 facet.