Reaction of NO on Ni-Pt bimetallic surfaces investigated with theoretical calculations

We applied periodic density-functional theory to investigate the adsorption and dissociation of NO on bimetallic surfaces, including the xNi@Pt(111), Ni_xPt_4-x(111), and (4-x)Pt@Ni(111) surfaces (x = 0-4). For all bimetallic surfaces, NO is preferentially adsorbed on Ni-rich sites, and the adsorption energies increase with an increasing number of top-layer Ni atoms on the surface. When the top-layer compositions are equal (but with varied composition of inner layers), the adsorption energy of NO on these surfaces decreases in the order xNi@Pt(111) > Ni_xPt_4-x(111) > (4-x)Pt@Ni(111), whereas the NO dissociation barriers increase in the opposite order; a larger adsorption energy of NO leads to a smaller NO dissociation barrier. We employed the local density of states to investigate the inner-layer effect of the various surfaces; the inner-layer Pt atoms of the 4Ni@Pt(111) surface caused the greatest upshift of the d-band center (of top-layer Ni atoms) toward the Fermi energy, so that the 4Ni@Pt(111) surface exhibited the greatest NO adsorption energy, -2.97 eV, and the smallest NO dissociation barrier, 1.20 eV. In contrast, the inner-layer Ni atoms of the (4-x)Pt@Ni(111) surfaces cause a downshift of the d-band center from the Fermi energy and show much smaller NO adsorption energies and larger NO dissociation barriers. The order of reactivity for dissociation of NO (4Ni@Pt(111) > Ni(111) > Ni_xPt _4-x(111) > Pt(111) > 4Pt@Ni(111)) indicates that various combinations of Ni and Pt would produce varied catalytic effects.