Density functional study of hypophosphite adsorption on Ni (1 1 1) and Cu (1 1 1) surfaces

Surface structures and electronic properties of hypophosphite, H2PO2−, molecularly adsorbed on Ni(1 1 1) and Cu(1 1 1) surfaces are investigated in this work by density functional theory at B3LYP/6-31++g(d, p) level. We employ a four-metal-atom cluster as the simplified model for the surface and have fully optimized the geometry and orientation of H2PO2− on the metal cluster. Six stable orientations have been discovered on both Ni (1 1 1) and Cu (1 1 1) surfaces. The most stable orientation of H2PO2− was found to have its two oxygen atoms interact the surface with two PO bonds pointing downward. Results of the Mulliken population analysis showed that the back donation from 3d orbitals of the transition metal substrate to the unfilled 3d orbital of the phosphorus atom in H2PO2− and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H2PO2− play very important roles in the H2PO2− adsorption on the transition metals. The averaged electron configuration of Ni in Ni4 cluster is 4s0.634p0.023d9.35 and that of Cu in Cu4 cluster is 4s1.004p0.033d9.97. Because of this subtle difference of electron configuration, the adsorption energy is larger on the Ni surface than on the Cu surface. The amount of charge transfers due to above two donations is larger from H2PO2− to the Ni surface than to the Cu surface, leading to a more positively charged P atom in NinH2PO2− than in CunH2PO2−. These results indicate that the phosphorus atom in NinH2PO2− complex is easier to be attacked by a nucleophile such as OH− and subsequent oxidation of H2PO2− can take place more favorably on Ni substrate than on Cu substrate.