Field Desorption Pathways of Water during the H2- Oxidation on a Pt Field Emitter

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 H30+. less H20+ and to some extent field and coverage sensitive 02+ species. The product ion spectrum qualitatively coincides with that of pure water field desorbed at 300 K. On the oxygen side of the H2/02/Pt system, H30+ is formed by the field induced disproportionation of two water molecules leaving behind an OHad. For the system H20/Pt, the observed emission of 02+ 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 02+ and H30+ ions from the same site was detected. We propose the following dispropoaionation 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 02+ and H30+ 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 + H 2 0 + a d ~ H 3 0 + is predominant.