Trapping of metastable oxygen species at Cu(111) surfaces

Abstract

The coadsorption of ammonia–nitric oxide and ammonia–dioxygen mixtures at a Cu(111) surface has been studied at temperatures between 170 and 480 K using X-ray photoelectron (XP) and vibrational electron energy loss (VEEL) spectroscopies. At low temperatures (T⩽, 250 K) further evidence has been obtained for the formation of two oxygen states, characterised by O1s binding energies of 531.3 and 528.8 eV and the assignment of the low binding energy state to a metastable atomic species has been confirmed. It is suggested that this state corresponds to small Cu–O transients trapped by the coadsorbed ammonia and prevented from coalescing into larger copper oxide is lands. At 298 K, coadsorption results in a highly efficient oxydehydrogenation of the ammonia. The rate of this reaction, which is shown to be independent of the substrate temperature, is very close to the rate of dioxygen bond cleavage at Cu(111).