Metal phosphides (MPs) are a potential alternative for ethane dehydrogenation (EDH) catalysts. They can present metal ensembles similar to those of metal alloys, potentially offering similar selectivity and coke resistance advantages, and tend to be strongly ordering, thus resisting loss of those ensembles at high temperatures. Previously, we showed Ni2P(001) has much improved ethylene selectivity and coke resistance than Ni(111) by means of density functional theory (DFT). Our model revealed that (1) bindings of sp2 and sp3 C to surface P atoms inhibit ethylene overactivation, and (2) weaker binding of small hydrocarbons on Ni2P(001) prevents the coke formation. We proposed these two features provide remarkable product selectivity to Ni2P(001). We are currently trying to perform a screening of a variety of MP series to find a new selective catalyst based on those two features. However, a diverse structural difference of MPs makes it difficult to predict their catalysis in a consistent way. We thus constructed geometrically consistent representations of M2P materials, M = Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, Pt, including both hexagonal (Ni2P, Fe2P) and orthorhombic (Co2P, Ru2P) structures that are common to a subset of these materials. We constructed a consistent series of close-packed planes, hexagonal M2P(001) and orthorhombic M2P(010), and explored adsorptions of EDH intermediates across those surfaces. DFT calculations showed rough correlations of binding energies (BEs) at the metallic site of M2P(001) and parent metals. P incorporation enhances or suppresses binding strength depending on the type of metal. We then investigated CH3 binding, a probe species for P site preferences of sp2 and sp3 C, at M- and P-centered sites. CH3 prefers binding to P atom only on Ni, Ag, Cu and Pd phosphides. Next we repeated the same calculations on M2P(010) to see the sensitivity of our observation to surface structure. Except H binding, there is a weak influence of surface structure on BEs, and metal vs P site preference of CH3 binding. Both surface structures and metal elements have an influence on H binding. Lastly, we suggested metal-rich Pd-P as a potential EDH catalyst through the feature-based screening. Our result demonstrates phosphide surface chemistry is mainly sensitive to metal elements, and it could be predicted from the isostructural model. We will develop a generally transferable approach to modeling phosphide surface chemistry for selective EDH.

Trends in small hydrocarbon adsorption on metal phosphide surfaces and implications for ethane dehydrogenation

ACS Spring Meeting 2021

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