Item

Abstract

Using the field ion microscope as a flow reactor the catalytic oxidation of hydrogen by O₂ on Pt has been studied by the ToF coincidence technique. The "imaging ions" consist mainly of H₃O⁺. less H₂O⁺ and to some extent field and coverage sensitive O₂⁺ species. The production spectrum qualitatively coincides with that of pure water field desorbed at 300 K. On the oxygen side of the H₂/O₂/Pt system, H₃O⁺ is formed by the field induced disproportionation of two water molecules leaving behind an OHad. For the system H₂/O₂/Pt, the observed emission of O₂⁺ during field ionisation is striking in so far as Oad recombination at 300 K is improbable. To clarify the nature of the precursor of this reaction channel a coincidence experiment was performed and a simultaneous emission of O₂⁺and H₃O⁺ ions from the same site was detected. We propose the following d'isproportionation for this step: a hydroxyl forms a transition complex with a peroxy-surface species, which then desintegrates into molecular oxygen and hydronium, which emit as O₂⁺ and H₃O⁺ in the field. During the ongoing hydrogen oxidation this coincidence is also observed on the oxygen side of the reaction; on the hydrogen side of the reaction no coincidence is observed, as in this case the pathway H_ad + H₂O⁺_ad→H₃O⁺ is predominant.