AVS 50th International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThM

Paper SS2-ThM4
High Pressure STM Studies of Metal and Alloy Surfaces: Adsorption Induced Phase Separation

Thursday, November 6, 2003, 9:20 am, Room 327

Session: Catalysis III: High vs. Low Pressures
Presenter: E.K. Vestergaard, University of Aarhus, Denmark
Authors: E.K. Vestergaard, University of Aarhus, Denmark
R.T. Vang, University of Aarhus, Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
A major issue in the catalytic community concerns the validity of applying surface science data obtained under Ultra-High Vacuum (UHV) conditions to "real" catalytic processes occurring at atmospheric pressure and above. Using high-pressure scanning tunneling microscopy (HP-STM) we have studied the adsorption structures on Pt(111) and Ni(111) at 1 bar of CO, and for these simple systems we demonstrate that the CO adsorption structures at high pressures are similar to structures formed under low pressure and low temperature conditions. Furthermore, we present results concerning the influence of high CO pressures on the stability of a Au/Ni surface alloy. Exposing the Au/Ni(111) surface alloy to 1 bar of CO results in a Ni(111) surface covered with small Au clusters, showing that high CO pressures induce a phase separation of the alloy. When scrutinizing this process at slightly lower pressures (10-20 torr), we have been able to follow the phase separation in real time. STM movies show that the Au cluster formation starts at the step edges of the Ni surface. Ni atoms are removed from the steps, and as the steps thus move over the surface Au clusters are nucleated and left behind in the wake of the moving steps. We propose that the removal of Ni-atoms is caused by Ni-carbonyl formation; CO is known to react with Ni surfaces at elevated pressures to form Ni-carbonyls, which are volatile at room temperature and therefore leave the surface. This latter study provides an example of a pressure gap, where model systems studied under UHV conditions certainly lack important aspects of the processes found at more realistic conditions.