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The reaction of oxygen with Rh single-crystal tips was investigated using field ion microscopy (FIM). Emphasis was laid on revealing the atomic structure of individual tip surface planes along with their influence on the early stages of the reaction, i.e., chemisorption and nucleation. Under field-free conditions, the interaction of oxygen with a (001) oriented Rh tip was found to lead to major reconstructions at temperatures above 400 K. While the original shape of the tip was nearly hemispherical before the reaction, it was polyhedral thereafter. In particular, the {011} and {113} planes were seen to adopt a “missing row” type reconstruction in the presence of oxygen adsorbate. On the Rh{113} plane, a (1 × 3) reconstruction prevailed for oxygen exposures larger than 60 L (1 L = 1.3 × 10-4 Pa·s) and temperatures T ≥ 550 K. In this type of missing-row reconstruction two adjacent dense-packed chains of atoms are absent. Surface oxidation was found to be promoted by the presence of an external electric field of ∼15 V/nm. Studies as a function of the surface temperature were performed in real time by video−FIM leading to the observation of a strong local variation in the oxidation activity. While the {113} planes and the vicinals of the (111) pole turned out to undergo rapid oxidation, the flat planes with {001} and {111} symmetry remained rather inactive at temperatures between 350 and 483 K. The formation of surface granules with sizes of ∼5−10 nm in areas of high oxidation activity was interpreted as being due to a nucleation process forming Rh x O y precipitates. Surface granules could be easily removed by reaction with CO gas, meaning that only the topmost layers of the Rh tip were involved in surface oxidation.

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