Hydrogenation of NO and NO2 over palladium and platinum nanocrystallites: case studies using field emission techniques

Abstract

In this work, we investigate the catalytic hydrogenation of NO over palladium and platinum and of NO₂ over platinum surfaces. Samples are studied using field emission techniques including field emission/ion microscopies (FEM/FIM). The aim of this study is to obtain detailed information on the non-linear dynamics during NO_x hydrogenation over nanocrystallites at the atomic scale. The interaction between Pd and pure NO has been studied between 450 K and 575 K and shows the dissociative adsorption of NO. After the subsequent addition of hydrogen in the chamber, a surface reaction with the oxygen-adlayer can be observed. This phenomenon is reversible upon variation of the H₂ pressure, exhibits a strong hysteresis behaviour but does not show any unstable regime when control parameters are kept constant. On platinum, NO is dissociated and the resulting O(ads) layer can also react with H₂. Although occurring on both Pd and Pt metals, the reaction mechanism seems to be different. On palladium, NO dissociation takes place on the whole visible surface area leading to a “surface oxide” that can be reacted off by raising the H₂ pressure whereas on Pt, the catalytic reaction is self-sustained and restricted to 〈001〉 zone lines comprising {011} and {012} facets and where self-triggered surface explosions are observed. Two kinetic phase diagrams were established for the NO–H₂ reaction over palladium and platinum samples under similar experimental conditions. Their shapes reflect a different chemical reactivity of metal surfaces towards oxygen species resulting from the dissociation of NO. NO₂ hydrogenation is followed over Pt samples and shows self-sustained kinetic instabilities that are expressed as peaks of brightness that are synchronized over the whole active area (corresponding to the 〈001〉 zone lines as in the NO case) within 40 ms, the time resolution of the video-recorder used for this work.