Insight into the suppressive effects of sulphoxy species on the NO–D 2 reaction on platinum surfaces: A surface science approach

Abstract

The influence of S 18O 2, O 2 and their reaction products, surface sulphoxy species, on NO–D 2 reactions on the surface of stepped Pt(332) was studied. Oxidation of S 18O 2 by O 2 proceeds to a very slight extent on Pt(332), taking place as the surface temperature is increased to 300 K and higher. The oxidation reaction gives rise to surface sulphoxy species (S 18O x, x > 2) that persist on the surface up to 500 K but desorb completely at 600 K. The surface sulphoxy species desorb mainly as S 18O 2 (predominantly) and S 18O 3. On the other hand, co-adsorbed S 18O 2 molecules render the desorption of O atoms, (which otherwise occurs at 700 K and higher from a clean Pt(332) surface), undetectable. Such oxygen desorption reappears and gains a considerable intensity as the sulphoxy species covered Pt(332) surface was further exposed to NO molecules. However, NO dissociation is suppressed in the presence of S 18O 2, O atoms and surface sulphoxy species; the suppressive effect from O atoms and surface sulphoxy species is much more significant than that from S 18O 2. No N 2 desorption resulting from NO dissociation is detected under some conditions. The suppressive effect exerted by O atoms and surface sulphoxy species also holds for the NO–D reaction, but is highly dependent on the exposures of O 2 and S 18O 2. The NO–D reaction is not suppressed on the Pt(332) surface which has been pre-exposed to 0.4 L O 2 at 90 K and then annealed to 200 K, due to rapid removal of O atoms (including those from NO dissociation) as a result of the facile reaction between O and D. On the Pt(332) surface, pre-exposed to O 2 and S 18O 2 and then annealed to 400 K, the efficiency of the NO–D reaction (manifested by N 2 production) is obviously lower than that on a clean surface (without surface sulphoxy species); however, the suppressive effect becomes significant only as the exposures of O 2 and S 18O 2 are ≥ 0.4 L. Surface sulphoxy species-induced suppression of the NO–D reaction on Pt(332) mainly results from NO–Pt interactions being weakened and a lack of D atom supply at the surface temperatures where NO dissociation becomes significant. D 2 desorption from the Pt(332) surface finishes at ~ 350 K, at which temperature surface sulphoxy species and O atoms persist and NO dissociation just becomes appreciable. As such, the NO–D reaction evolves into NO dissociation on the surface sulphoxy species covered Pt(332). The present results also suggest that the site blocking effect of surface sulphoxy species and O atoms does not evidently contribute to their suppression of NO–D reaction.