Abstract:
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Metal phosphides have emerged as a new
powerful class of materials that can be employed as
heterogeneous catalysts in transformations mainly to generate
new energy vectors and the valorization of renewables. Synthetic
protocols based on wet techniques are available and are based
on the decomposition of the organic layer decorating the
nanoparticles. For nickel, the phosphine of choice is
trioctylphosphine, and this leads to the formation of NiPx
materials. However, the temperature at which the decomposition
starts has been found to depend on the quality of the
nickel surface. Density functional theory, DFT, holds the key to
analyze the initial steps of the formation of these phosphide
materials. We have found how clean nickel surfaces, either (111)
or (100), readily breaks the ligand P−C bonds. This triggers the
process that leads to the replacement of a surface nickel atom by P and concomintantly forms a Ni adatom on the surface
surrounded by two methyl groups, thus starting the formation of the NiPx phase. The whole process requires low energies, in
agreement with the low temperature found in the experiments, 150 °C. In contrast, if the surface is oxidized, the reaction does
not proceed at low temperatures and oxygen vacancies need to be created first to start the P−C bond breaking on the Ni-clean
patches. Our results show that the cleaner the surface is, the milder the reactions are required for the NiPx formation, and thus
they pave the way for gentler synthetic protocols that can improve the control of these materials. |