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 Ni 2 P(0001) and Ni 5 P 4 (0001)/(0001̅) 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 Ni 3 P 2 termination of Ni 2 P(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni 5 P 4 are found to be the Ni 3 P 3 and Ni 4 P 3 bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P 3 site, which is present on Ni 5 P 4, and the Ni 3 site, which is present on both Ni 2 P and Ni 5 P 4, strongly bind excess P. Additionally, we predict the presence of stable P n ( n = 2, 4) agglomerates on Ni 5 P 4 at the P 3 -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.