Oxygen-Induced Reconstruction and Surface Oxidation of Rhodium V. K. Medvedev, † Yu. Suchorski, ‡ C. Voss, † T. Visart de Bocarme ´, † T. Ba ¨r, † and N. Kruse* ,† Chemical Physics at Surfaces and Heterogeneous Catalysis, Free University of Brussels, Campus Plaine, CP 243, B-1050 Brussels, Belgium, and Institute for Physical Chemistry, University of Hannover, Callinstrasse 3-3a, D-30167, Germany Received May 22, 1998. In Final Form: July 31, 1998 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 g 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 RhxOy 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. 1. Introduction Studies of the early stages of metal oxidation have a long-standing history in surface science. Although there is general agreement that the mechanistic steps leading to oxide formation involve dissociative oxygen chemi- sorption on the metal surface, lattice penetration of atomic oxygen, nucleii formation of suboxides, and their growth to amorphous aggregates finally crystallizing in stoichio- metric oxide phases, no standardized model could be worked out so far. On the basis of kinetic studies it soon became clear that the sequence of events may be complex. Rather than successively, these events may occur simul- taneously, depending on temperature, pressures, and, of course, the chemical nature of the metal itself. A thorough review of the state-of-the-art along with a number of case studies was given some years ago. 1 The chemisorption of oxygen on both Rh single crystals and polycrystalline samples was the subject of intensive research, 2-24 but only a few studies have addressed the question for oxide nucleation and growth. A significant point in the development toward a better understanding of the oxide growth kinetics along with the compositional analysis marks the paper by Kellogg. 17 Using small Rh crystal tips in a combined study by field ion microscopy (FIM) and imaging atom-probe mass spectrometry, Kellogg determined an activation energy as low as 13 kJ/mol for the early stages of oxide growth at temperatures between 400 and 650 K. 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