Interaction of NH₃ with oxygen‐pretreated Ni at 600 K

Abstract

The aim of the present study was to investigate the reaction between ammonia and pre-adsorbed oxygen on Ni at 600 K using metastable impact electron spectroscopy (MIES) and UPS techniques. For a given oxygen-pretreated nickel surface, the changes in the MIES spectra along with ammonia adsorption can be decomposed into three different steps: a first domain without significant changes, a second domain corresponding to drastic changes in the intensity of a signal at 15.8 eV; and a third domain where no more change is detected. The first domain may correspond to the ammonia chemisorption on the surface, whereas the changes in the second domain would be attributed to surface dehydroxylation. The existence of NH x groups on the surface has been detected by UPS; they result from ammonia chemisorption. The changes in the MIES spectra have also been studied versus the initial oxygen uptake on Ni. It emerged that the first domain is larger with increasing initial oxygen exposure: this may be explained by the poorer surface reactivity of NiO, contrary to the oxygen-adsorbed phase.

Thermal Chemistry of NH3 on Oxygen-Pretreated Ni(111) Studied by Metastable Induced Electron Spectroscopy and Ultraviolet Photoelectron Spectroscopy

“…The subtraction of the latter spectrum from the NiO spectrum highlights three weak peaks at 6.8, 9.4, and 12.3 eV BE, respectively. These structures have already been observed at high temperature and attributed to NH x ( x = 1, 2) compounds . Let us note that, at 600 K, the superposition of poorly intense structures leads to a structureless and slight plateau …”

“…Over a recent study about NH 3 exposure on oxygen-pretreated polycrystalline Ni at high temperature (600 K), we observed that surface-adsorbed oxygen atoms react with ammonia, and that, finally, water is desorbed while atomic nitrogen partly covers the surface. However, because of the high temperature, neither the intermediate adspecies nor the phases containing oxygen on the surface O-adsorbed and/or NiO islands were identified.…”

“…These structures have already been observed at high temperature and attributed to NH x (x ) 1, 2) compounds. 8 Let us note that, at 600 K, the superposition of poorly intense structures leads to a structureless and slight plateau. 8 The peak at 12.3 eV BE was attributed to NH 2 , 29,30 and the other ones were related to NH.…”

“…8 Let us note that, at 600 K, the superposition of poorly intense structures leads to a structureless and slight plateau. 8 The peak at 12.3 eV BE was attributed to NH 2 , 29,30 and the other ones were related to NH. 29 The lack of structures at about 5 eV BE indicates that no atomic nitrogen is present on the surface; adsorbed ammonia decomposition is thus incomplete.…”

“…4 This result is consistent with scanning tunneling microscopy (STM) data on Ni(110); moreover, it demonstrates that ammonia reactivity depends on the O coverage and is more efficient at low O coverage. 5 However, open questions remain about adsorption and dissociation mechanisms on oxygen-pretreated Ni surfaces above 300 K. Following previous studies, oxygen pretreatment of Ni surface is assumed to greatly enhance NH 3 decomposition 6 or to stabilize NH 3 against decomposition to NH or N. 7 Over a recent study about NH 3 exposure on oxygen-pretreated polycrystalline Ni at high temperature (600 K), 8 we observed that surface-adsorbed oxygen atoms react with ammonia, and that, finally, water is desorbed while atomic nitrogen partly covers the surface. However, because of the high temperature, neither the intermediate adspecies nor the phases containing oxygen on the surface O-adsorbed and/or NiO islands were identified.…”