How a gold substrate can increase the reactivity of a Pt overlayer

The growth and chemical reactivity of Pt on Au(111) have been studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Deposition of Pt at coverages from 0.02 ML up to 2.5 ML on Au(111) a t room temperature initially leads to the formation of a surface alloy, in which 3% of the Au atoms are replaced by Pt. Subsequent Pt evaporation lead s to island growth with a mixed Pt-Au island composition. The reactivity of the Pt/Au system is studied using CO as a probe molecule. We show that a s tronger bonding of CO to the first layer of Pt on Au(111) exists compared w ith the binding of CO on clean Pt. The Au substrate therefore very surprisi ngly increases the Pt overlayer reactivity. The results can be understood i n a simple model, in which the change in the CO binding energy is directly proportional to the shift of the d-band center of the metal overlayer. Acco rding to this model, the increased reactivity of the Pt/Au(111) system shou ld hold for other adsorbates and reactions as well. (C) 1999 Elsevier Scien ce B.V. All rights reserved.