IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM1
CO Oxidation on Au Crystals: Studies on the Nanoscale

Wednesday, October 31, 2001, 8:20 am, Room 121

Session: Surface Reactions on Metals
Presenter: N. Kruse, Free University of Brussels, Belgium
Authors: T. Bär, Free University of Brussels, Belgium
T. Visart de Bocarmé, Free University of Brussels, Belgium
B.E. Nieuwenhuys, Leiden University, The Netherlands
N. Kruse, Free University of Brussels, Belgium
Correspondent: Click to Email
Until recently gold was regarded as a metal of little interest for applications in heterogeneous catalysis. The discovery that supported gold catalysts are among the most active ones in CO oxidation has completely changed this picture. The mechanism of the reaction is, however, still under debate. In particular, the question for the role of the support material and for possible impurity effects must be asked. With this background we have studied oxygen activation and reaction with CO on pure Au single crystal specimens, in the absence of any oxidic support material, using Field Ion Microscopy (FIM) and atom-probe techniques. FIM-images (using H@sub 2@ and Ne as imaging gas at 55 K and at field strengths of up to 35 V/nm) indicated clean, nearly hemispherically shaped Au crystals of (111) orientation. Exposure of such a specimen to O@sub 2@ gas at 100 mbar and temperatures between 300 and 450 K, in the absence of an electric field, led to oxygen chemisorption and formation of a "surface oxide". In particular, layer edges of (111) and (100) planes were decorated by oxygen atoms. In situ studies of surface oxide formation performed in an O@sub 2@ gas atmosphere of 1,0*10@super -4@ mbar showed that the presence of an electric field of 12-15 V/nm significantly enhances oxygen adsorption. Exposure to CO gas at 300 K led to the removal of the surface oxide, i.e. CO@sub 2@ formation. This was associated with the occurrence of a reaction front which started in the apex centre, i.e. the (111) pole, and extended in nearly concentric circles to the outskirts of the Au specimen. Using video-FIM several cycles of oxide build-up and titration with carbon monoxide were performed to demonstrate the reversibility of the surface processes. Using atom-probe techniques during the ongoing reaction AuO and AuCO ionic species could be observed in variable amounts. We conclude that pure gold crystals are active catalysts for the CO oxidation at 300 K.