Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides

In heterogeneous catalysis, catalyst synthesis precedes operation and, in most cases, is conducted in an altogether different chemical environment. Thus, determination of the structure and composition of the catalyst surface(s) due to fabrication is essential in accurately evaluating their eventual structure(s) during operation, which provides the origin of their catalytic activities and are therefore key to catalyst optimization. We explore the reconstructions of both Ni₂P(0001) and Ni₅P₄(0001)/(0001) surfaces with first-principles density functional theory (DFT). Most of the stable terminations under realistic synthesis conditions are determined to be P-rich on both materials. A P-covered reconstruction of the Ni₃P₂ termination of Ni₂P(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni₅P₄ are found to be the Ni₃P₃ and Ni₄P₃ bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P₃ site, which is present on Ni₅P₄, and the Ni₃ site, which is present on both Ni₂P and Ni₅P₄, strongly bind excess P. Additionally, we predict the presence of stable P_n (n = 2, 4) agglomerates on Ni₅P₄ at the P₃-hollow and Ni-Ni bridge sites. This study highlights the importance of considering the aggregation behavior of nonmetal components in predicting the surface reconstruction of transition metal compounds.