Proceedings of the 43rd International Field Emission Symposium
J. Phys. IV France 06 (1996) C5-31-C5-36
Field Desorption Pathways of Water during the H2- Oxidation on a Pt Field EmitterW. Drachsel1, C. Wesseling1 and V. Gorodetskii2
1 Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
2 Boreskov Institute of Catalysis, 630090 Novosibirsk, Russia
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.
© EDP Sciences 1996