Experimental and Computational Investigation of the Role of P in Moderating Ethane Dehydrogenation Performance over Ni-Based Catalysts
2020
Ko, Jeonghyun | Muhlenkamp, Jessica A. | Bonita, Yolanda | LiBretto, Nicole J. | Miller, Jeffrey T. | Hicks, Jason C. | Schneider, William F.
We investigated the influence of P incorporation into a Ni catalyst on ethane dehydrogenation (EDH). Density functional theory calculations on model Ni(111) and Ni₂P(001) surfaces reveal that surface P generally decreases adsorption energies of fragments relevant to EDH at surface Ni sites but that P itself participates in binding some of these intermediates. These nonlinear influences of P cause CH₃CH₂–H activation to occur with similar facility on metal and phosphide surfaces, while CH₂CH–H activation, an indicator of coking tendency, has much greater barriers on the phosphide. We prepared Ni and Ni–P catalysts on an SBA-15 support to test these predictions. A Ni–P catalyst with a 2:1 ratio (Ni₂P(2)/SBA-15), corresponding to the Ni₂P phase, showed >80% ethylene selectivity during EDH at 873 K, compared to <1% ethylene selectivity on Ni/SBA-15, and maintained this selectivity up to 4 h time-on-stream. Diffuse reflectance infrared Fourier transform spectroscopy observations following ethylene exposure and heating under an inert flow indicate the appearance of carbon deposits on Ni/SBA-15 compared to ethylene desorption from Ni₂P(2)/SBA-15, consistent with predicted adsorption energy trends. Thermogravimetric analysis of spent EDH catalysts indicates significantly less carbon deposition on Ni₂P(2)/SBA-15 relative to Ni/SBA-15. The results highlight the potential of metal phosphides as selective and robust alkane dehydrogenation catalysts.
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