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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 Had + H2O+ad→H3O+ is predominant.
