(740b) Computational Investigation of the Metal Phosphide Chemistry for Ethane Dehydrogenation Catalyst

Ethane dehydrogenation (EDH) is highly endothermic, so temperatures approaching 1000 K are necessary to achieve useful conversions at atmospheric pressure. The challenges of EDH are to suppress coke formation and to replace current expensive Pt-based catalysts. Metal phosphides (MPs) are a potential alternative to Pt alloys as EDH catalysts. They can present metal ensembles similar to those of metal alloys, potentially offering similar selectivity and coke resistance advantages.

We employed density functional theory (DFT) to contrast EDH reactivities on Ni(111) and Ni₂P(001). DFT calculations on model Ni(111) and Ni₂P(001) reveal that surface P generally decreases binding energies (BEs) of fragments relevant to EDH at surface Ni sites, but that P itself participates in binding some of these intermediates. These non-linear influences of P cause CH₃CH₂-H activation to occur with the similar facility on metal and phosphide surfaces, while CH₂CH-H activation, an indicator of coking tendency, has much greater barriers on the phosphide. This suggests that MPs could be a new class of EDH catalyst.

We then constructed Ni₂P-like hexagonal M₂P(001) models (M = Fe, Co, Cu, Ru, Rh, Pd, Pt, Ag) to figure out an influence of a metal element in phosphide surface chemistry. DFT calculations showed that rough correlations of BEs at the metallic site of M₂P(001) and parent metals. However, BEs on M₂P(001) generally span a greater range than on parent metals. Further, we observed that adsorbates generally prefer to bind onto the metallic site over the P site on M₂P(001). CH₃ binding is an exception case given that the P site on some M₂P(001) offers stronger bindings than the metallic site. Our result demonstrates phosphide surface chemistry is sensitive to metal elements, and it is complex because of the existence of P site. However, they have a greater tunability than transition metal catalysts.