A NEW APPROACH TO THE MECHANISM OF HETEROGENEOUSLY CATALYZED-REACTIONS - THE OXYDEHYDROGENATION OF AMMONIA AT A CU(111) SURFACE

The oxydehydrogenation of ammonia at a Cu(111) surface is a highly efficient process at 295 K, with the selectivity sensitive to the dioxygen-ammonia ratio. However, there is no evidence from either XPS or HREELS for surface oxygen being present during the reaction and, in effect, catalysis occurs at a clean Cu(111) surface. The rate of NHx(a) formation is indistinguishable from the rate of the dissociative chemisorption of oxygen at close to zero coverage suggesting that the reactive oxygen species are the hot transients O- (s). The chemisorbed oxygen overlayer, the O2- (a)-like species are, by comparison. unreactive. The reaction is, therefore, not characteristic of either Eley-Rideal or Langmuir-Hinshelwood mechanisms but involves the interaction of rapidly diffusing ammonia molecules and hot transient O- (s)-like species. Models for this type of reaction have been discussed previously, while very recent studies by scanning tunnelling microscopy have provided further evidence for such oxygen transients.