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)/(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 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 Pn (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.