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The catalytic oxidation of hydrogen on Rh tips was investigated using field ion microscopy and pulsed field desorption mass spectrometry (PFDMS). The reaction was carried out in the 10 −3 Pa range at 400-600 K.In a first series of experiments, emphasis was laid on revealing tip morphologies during the ongoing water formation. At a reaction temperature of 500 K, remarkable similarities with the previously studied pure oxygen case were found. Accordingly, a polyhedral tip shape with missing-row-type reconstruction on f011g and f113g planes was formed. In a second series of experiments, the gas mixture was varied to demonstrate the occurrence of local structural changes as well as multistability in the surface reaction at temperatures between 400 and 500 K and viewing fields of 10 V nm −1 . A phase diagram was established within this temperature range. At 550 K and 9 V nm −1 , self-sustained kinetic oscillations with cycles of ∼40 s were observed in a continuous flow of O 2 -H 2 (pO 2 = 1.0 × 10 −3 Pa and pH 2 = 1.3 × 10 −3 Pa). Using PFDMS, ∼400 atomic sites close to the central (001) pole of the Rh tip were probed with varying pulse heights. During imaging in a wide range of H 2 -O 2 gas mixtures it was found that water ionization causes formation of the field ion image, leading to the conclusion that brightness analysis is suitable for measuring the local catalytic activity. On the oxygen side of the phase diagram, water molecules were formed on an oxidized surface so that RhO 2+ , RhO + and Rh 2 O + appeared in the mass spectra. On the other hand, by increasing the H 2 pressure, the surface oxide was reacted off within ∼0.1 s, thereby leaving a relatively dark surface (H-side of the diagram). Chemical probing in the latter case demonstrated fast water formation leaving few oxygens on the surface, i.e. negligible Rh x O n+ amounts appeared in the mass spectra.

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