AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM7
CO and Ethylene Dissociation on Ni(111): Influence of Steps

Monday, November 3, 2003, 10:20 am, Room 327

Session: Catalysis I: Adsorption and Reactions of Small Molecules at Surfaces
Presenter: R.T. Vang, University of Aarhus, Denmark
Authors: R.T. Vang, University of Aarhus, Denmark
E.K. Vestergaard, University of Aarhus, Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
Steps on metal surfaces are well known to be more reactive than terraces and often provide the important active sites for catalytic processes. From high-resolution STM experiments we have found direct atomic-scale evidence that CO and C@sub 2@H@sub 4@ (ethylene) dissociate on the step edges of Ni(111) with a much higher rate than on the terraces. When CO is dosed at 400 K onto a clean Ni(111) surface small carbon islands are nucleated at the step edges. The coverage of these carbon islands saturates as soon as a thin brim of carbon is formed along the steps, indicating that adsorbed carbon prevents further CO dissociation, thus blocking the step sites. Concerning ethylene adsorption we observe the same mechanism of carbon growth at steps saturating after the formation of a thin carbon brim, when the dosing is performed at room temperature (RT). At higher temperatures (above 350 K), however, ethylene dosing leads to a continuous growth of carbon islands, which is interpreted as dissociation on terrace sites. When the step sites are blocked by small amounts of Ag, Au or S, we find that the dissociation rate of the two molecules is dramatically changed. No carbon is seen along the modified step edges after exposure to CO at 400 K or ethylene at RT. It is, however, still possible to form carbon islands by dosing ethylene at elevated temperatures owing to dissociation at terrace sites. These findings exemplify the concept of active sites in catalytic reactions and provide new directions for designing selective catalysts from fundamental surface science studies.