Obtaining an atomic-scale description of the chemical interactions of phosphates with an oxide support, such as γ-Al 2 O 3 , 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-Al 2 O 3 surfaces for various P surface coverages after drying at 120 °C. Thanks to 31 P doubleand triple-quantum filtered NMR experiments as well as to 27 Al− 31 P dipolar-and scalar-based correlation spectra, we demonstrate the presence of polyphosphates and of Al−O−P connectivities at the exposed facets of γ-Al 2 O 3 . DFT-based thermodynamics shows that phosphates (mono-or di-) are preferentially covalently bonded on the (1 1 0) γ-Al 2 O 3 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 31 P 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.
Aqueous phase reforming of alcohols over Pt has been discussed to operate along two pathways, decarbonylation and decarboxylation. To gain a better understanding of the activity of various catalysts for decarboxylation, we examined computationally a key step of this mechanism on the 12 transition metals of groups 8 to 11, namely the formation of a carboxylic acid intermediate via metal-mediated insertion of OH into an acyl group. The trend of the calculated barriers of OH insertion parallels the oxophilicity of the metals. A separation of the reaction into two formal steps isolates OH activation as a major contribution to the barrier and, not unexpectedly, indicates a strong dependence on the OH adsorption energy. A decomposition analysis of the activation energy reveals that weaker OH adsorption also correlates with the interaction energy between the adsorbed fragments in the transition state, thus indirectly lowering the barrier for OH insertion. Metals in the bottom right-hand corner of the transition metal block studied -Pt, Au, and Ag-bind OH relatively weakly, hence feature a high OH insertion activity. We applied these findings to rationalize various experimental results and suggest catalysts for decarboxylation.
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