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

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(OOOl)-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 molecubsurface 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 '.

The role of precursor states in adsorption, surface reactions and catalysis

“…Oxygen adsorption on the clean surface was found to be very slow (the estimated sticking probability was *10 -3 ), yet it was enhanced to a very high value when both oxygen and ammonia were co-dosed, Fig. 8 [1,27]. It was proposed that this was due to a mobile, precursor molecular oxygen ''O 2 d-'' species, interacting with ammonia in the weakly-held state.…”

“…Roberts et al [1,[27][28][29] have reported surface reactions which are of an inverse type, that is, oxygen adsorption itself is enhanced via the precursor state. In these experiments oxygen was not pre-adsorbed, but precursors are pivotal for high reaction rates to be seen.…”

Heterogeneously Catalysed Oxidation Reactions with P450-like Characteristics; Radical Reactions and the ‘Elusive’ Oxygen State

“…Since at the sp metals Zn(0001) and Ag(111), dioxygen bond cleavage is ''slow'' there is a kinetic window to [10][11][12] with decreasing temperature (Fig. 2) and also as the ratio NH 3 :O 2 increases.…”

On the Role of Water in Heterogeneous Catalysis: A Tribute to Professor M. Wyn Roberts

“…The 'catalytic' role of the ammonia in dioxygen bond cleavage is highlighted by comparison with pure dioxygen at 200 K. Adapted from Ref. [44] with permission from the Royal Society of Chemistry …”