Time-dependent chemiluminescence from the surface-catalysed recombination of O and NO on polycrystalline Cu, Au and Ag

Abstract

The chemiluminescence produced by the surface-catalysed recombination of nitric oxide with a supersonic beam of atomic oxygen on clean but otherwise uncharacterised polycrystalline copper, gold and silver has been studied. The dependence of the luminescence on time, substrate temperature and pre-exposure to the O-atom beam has been investigated. On the Cu and Au samples, an initial luminescent signal was observed which decayed with time to a steady state. The intensity of the initial luminescent signal observed on Cu was much less than that observed on Ni under corresponding conditions. The initial intensity on Cu at 195 K was similar to that previously obtained on Pt at the same temperature but was lower at higher temperatures. The initial intensity on Au was much less than that observed on Ni, Pt and Cu. The magnitude of the steady-state luminescence on Cu was similar to that observed in the corresponding reactions on Ni and Pt. Au exhibited only a very weak steady-state chemiluminescence. The Ag sample exhibited different behaviour, showing luminescence only at 195 K. A fast decay was observed only if exposure to the nitric oxide beam occurred prior to admittance of the O-atom beam, i.e. an unoxidised surface. If the surface was then subject to prolonged exposure of the beams, the observed signal showed a slow increase to a maximum, before again falling. The mechanism previously proposed has been used to determine the effective rate constants for the reactions on the Cu and Au substrates, which are compared to the values obtained previously for Ni and Pt.