C-H Activation Chemistry at Metal Phosphide Surfaces

Metal phosphides have the potential to combine the intrinsic catalytic reactivity of noble metals with the thermal robustness of ceramic materials. Further, like intermetallics, metal phosphides tend to be ordered and thus their surfaces present well defined and potentially tunable reactive sites. In this work, we combine computational models and experimental synthesis and characterization to evaluate the potential of metal phosphides for catalytic dehydrogenations of light hydrocarbons. Density functional theory (DFT) methods are used to compare ethane dehydrogenation pathways over Ni2P, a representative phosphide known to be active for dehydrogenations, with Ni itself, a material well known to be highly susceptible to coking. Calculations show that the Ni ensembles at the Ni2P surface are metallic and that the adsorption energies of dehydrogenation intermediates are similar to those on Ni. However, computed reaction barriers show that site isolation imposed by the presence of surface phosphorus inhibits reaction pathways that lead toward coke precursors. Ni2P catalysts are prepared on an SiO2 support and are found to exhibit higher selectivity to desired olefin products than do similarly prepared Ni catalysts in both ethane and propane dehydrogenation. The results highlight the promise of these materials and point the way towards further catalyst optimization by control of support, particle size, and composition.