OXYGEN-INDUCED RECONSTRUCTION AND SURFACE OXIDATION OF RHODIUM

The reaction of oxygen with Rh single-crystal tips was investigated us ing field ion microscopy (FIM). Emphasis was laid on revealing the ato mic structure of individual tip surface planes along with their influe nce on the early stages of the reaction, i.e., chemisorption and nucle ation. Under field-free conditions, the interaction of oxygen with a ( 001) oriented Rh tip was found to lead to major reconstructions at tem peratures above 400 K. While the original shape of the tip was nearly hemispherical before the reaction, it was polyhedral thereafter. In pa rticular, the {011} and {113} planes were seen to adopt a ''missing ro w'' type reconstruction in the presence of oxygen adsorbate. On the Rh {113} plane, a (1 x 3) reconstruction prevailed for oxygen exposures l arger than 60 L (1 L = 1.3 x 10(-4) Pa.s) and temperatures T greater t han or equal to 550 K. In this type of missing-row reconstruction two adjacent dense-packed chains of atoms are absent. Surface oxidation wa s found to be promoted by the presence of an external electric field o f similar to 15 V/nm. Studies as a function of the surface temperature were performed in real time by video-FIM leading to the observation o f 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 rath er inactive at temperatures between 350 and 483 K. The formation of su rface granules with sizes of similar to 5-10 nm in areas of high oxida tion activity was interpreted as being due to a nucleation process for ming RhxRhy precipitates. Surface granules could be easily removed by reaction with CO gas, meaning that only the topmost layers of the Rh t ip were involved in surface oxidation.