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

Abstract Surface structures and electronic properties of hypophosphite, H 2 PO 2 − , 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 H 2 PO 2 − 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 H 2 PO 2 − was found to have its two oxygen atoms interact the surface with two P O 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 H 2 PO 2 − and 4s orbital's acceptance of electron donation from one lone pair of the oxygen atom in H 2 PO 2 − play very important roles in the H 2 PO 2 − adsorption on the transition metals. The averaged electron configuration of Ni in Ni 4 cluster is 4s 0.63 4p 0.02 3d 9.35 and that of Cu in Cu 4 cluster is 4s 1.00 4p 0.03 3d 9.97 . 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 H 2 PO 2 − to the Ni surface than to the Cu surface, leading to a more positively charged P atom in Ni n H 2 PO 2 − than in Cu n H 2 PO 2 − . These results indicate that the phosphorus atom in Ni n H 2 PO 2 − complex is easier to be attacked by a nucleophile such as OH − and subsequent oxidation of H 2 PO 2 − can take place more favorably on Ni substrate than on Cu substrate.