Mechanism of CO bond scission at alkali promoter sites—IRAS study of the system CO/K/Ni(111)

Abstract

IRAS (infrared reflection absorption spectroscopy) has been employed to study the isotopic mixing reaction: 13C16O(a)+12C18O(a)→13C18O(a)+12C16O(a) on Ni(111) with preadsorbed K adatoms. Under high temperature conditions (T>450 K) where the isotopically mixed CO is being desorbed, it has been shown that the isotopic exchange reaction cannot be detected by IRAS on the surface among CO molecules strongly interacting with K adatoms. This result implies that dissociation of CO occurs at K-promoted Ni sites followed by surface diffusion of C(a) and O(a) away from the K promoter site. Statistical recombination of C(a)+O(a) occurs with concomitant CO desorption on Ni(111) sites some distance from K centers. The empty K-promoted sites may be refilled by surface diffusion of CO from outside. Thus, alkali metal promotion of C–O bond scission may act via a ‘‘feeder-site’’ mechanism connecting promoter atoms to external surface sites via surface diffusion of atomic C and O species.