The interaction of NO and CO with Cu(111)

Abstract

NO interaction with Cu(111) is shown to be dissociative for temperatures varying from 320 to 570 K and for exposures up to 150 Pa·s. Upon adsorption a saturation surface coverage (0.5 monolayer) of adsorbed oxygen and nitrogen is obtained. After an incubation period penetration of oxygen in subsurface layers is observed, leaving the surface coverage almost unchanged. Both the adsorption and the penetration are first order in the NO pressure, the reactions are not activated. The reduction of the adsorbate at 570 K proceeds in two stages, i.e. the net reduction of subsurface oxygen and the subsequent reduction of adsorbed oxygen. The rate of reduction strongly depends on the amount of adsorbed nitrogen and on the presence of subsurface oxygen. At a CO pressure exceeding 0.1 Pa reduction is followed by the desorption of nitrogen, probably via an isocyanate intermediate. From the reduction experiments and from experiments in which the isothermal desorption rate of nitrogen was measured it is concluded that adsorbed nitrogen interacts with adsorbed oxygen, resulting in a lower reactivity of oxygen and, possibly, nitrogen. This interaction is considerably weakened when also penetrated oxygen is present.