The promotion of surface-catalysed reactions by gaseous additives. The role of a surface oxygen transient

Abstract

The activation of N—H bonds in ammonia by ‘oxygen’ has been investigated at Mg(0001) surfaces through studies of coadsorption of dioxygen, nitrous oxide and nitric oxide. Time-and temperature-dependent photoelectron spectroscopy has enabled quantitative information to be obtained on the molecular events involved.

N—H activation is shown to depend on the dynamics of the individual surface–molecule interactions. A distinction is made between coadsorption and preadsorption in that the chemisorbed oxygen overlayer (MgO) and bulk magnesium oxide are both relatively unreactive compared with the surface ‘oxygen’ transient generated during the dissociative chemisorption of the coadsorbed ‘promoter’ molecules, O₂, NO and N₂O. Although the ammonia surface coverage is negligible at 295 K (ΔH≈ 40 kJ mol^–1) the transient ‘oxygen’ O^–(s) is active in H-abstraction, leading to strongly chemisorbed NH₂ and OH species. A kinetic model is developed for the O^–(s)–ammonia interaction from which an expression for τ_O^–(s), the surface lifetime of the oxygen transient, is derived during the initial stages of interaction. At 295 K the estimated value of τ_{O^–(s)→ O^2–(s)} is ca. 10^–8s.

The conclusions are of general significance to selective oxidation catalysis at metal surfaces, analogous activation of C—H bonds having also been observed recently in this laboratory with propene and also to the mechanism of metal oxidation.