Theoretical studies of molecules on metal surfaces I. Site stability and vibrational frequencies of CO on Ni(111), Ni(110) and Ni(110)

Abstract

Abundant free access to an IBM Supercomputer has facilitated the thousands of calculations necessary to optimize the geometries of arrays of CO molecules chemisorbed on large clusters simulating surfaces of Ni metal, to compute directly from the potential energy surfaces the CO and NiCO vibrational frequencies, and to examine the conditions under which interactions between the molecules alter these frequencies. In this paper we report results for one and two CO molecules on terminal, bridge and hollow sites on the (111), (110) and (100) faces of Ni. Terminal CO stretching frequencies on the three faces at low coverages do not differ much, nor do the bridge frequencies at normal NiNi distances (2.489 Å); however a decidedly lower second frequency is predicted for the long bridge on the (110) face. For three-fold hollow sites on Ni(111), no difference is predicted between sites over Ni atoms and sites over holes. CO molecules occupying the four-fold sites on Ni(100) should have a frequency about 100 cm −1 lower than that observed for three-fold sites on Ni(111). We present strong theoretical evidence that a CO in the rectangular four-fold site on Ni(100) is really a terminal CO bonded to a second-layer atom and is characterized by a frequency only slightly lower than the on-top CO on this surface. Our computed binding energies agree with the trends of previous theoretical calculations, with on-top positions least stable, bridged sites intermediate, and hollow (three-fold or four-fold) sites most stable. However, it is proposed that the increased polarity of the CO molecules observed on going from one-top to bridge to hollow positions causes at high coverage preferential occupation of on-top sites due to intermolecular repulsions. Finally, in contradiction to some recent suggestions, we find that the stretching frequency of NiCO bonds increases as the frequency of the associated CO increases, in apparent agreement with experimental results.