Role of oxygen transients in the chemistry of dioxygen at atomically clean metal surfaces: the Zn(0001)–dioxygen–ammonia system

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Abstract
The exact sequence of events involving surface trapping or precursor states is not understood even for the dissociative chemisorption of simple diatomic molecules at metal surfaces. It was to search for experimental evidence for such states in the Zn(0001)–dioxygen system that coadsorption studies, using the ammonia molecule as a specific probe for oxygen transient states, were initiated. Using a combination of photoelectron and vibrational spectroscopies it was shown that a dioxygen–ammonia surface complex provides a highly efficient low-energy pathway for dioxygen bond cleavage leading to chemisorbed amide, hydroxide and oxide species. Surface species have been identified by both core-level and electron energy-loss spectroscopies with XPS providing quantitative surface concentration data. The kinetics exhibit all the characteristics of a precursor-mediated reaction including the rate increasing with decreasing temperature. In the absence of ammonia, dioxygen bond cleavage is highly inefficient at a Zn(0001) surface. Central to the model developed is the concept of an ammonia molecule undergoing surface diffusion (hopping) acting as a chemical trap for reactive oxygen transients and leading to the formation of strongly chemisorbed species which can be spectroscopically identified. Although the dynamics of molecule–surface interactions are being pursued by molecular-beam studies the strategy developed here provides an insight to the chemistry associated with short-lived surface oxygen trapping states. The results have implications for the energetics of bond breaking, the efficiency of accommodation of molecules and catalysis at metal surfaces, it also highlights the limitations of the more static approach to unravel reaction mechanisms at surfaces in that the Zn(0001) oxide overlayer is unreactive to ammonia; a clear distinction must be made between ‘preadsorption’ and ‘coadsorption’.

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