SCANNING-TUNNELING-MICROSCOPY STUDY OF MODEL CATALYSTS OBTAINED BY CLUSTER BEAM DEPOSITION OF PALLADIUM ONTO HIGHLY ORIENTED PYROLITHIC GRAPHITE

Cluster beam deposition is used to obtain model catalysts (palladium o n highly oriented pyrolitic graphite). The supported particles are cha racterized by transmission electron microscopy (TEM) and scanning tunn eling microscopy. Decoration of graphite steps and formation of groups of palladium particles on the graphite surface are imaged by TEM. Hig h spatial resolution observations show periodic (square-root 3 X squar e-root 3)R30-degrees charge density modulations of graphite around the supported particles. Such modulations are explained following the wor k of Mizes and Foster [Science 244, 559 (1989)] that take into account the fact that the particles act as perturbations to the graphite wave functions. On the basis of this explanation, the interactions of pall adium and platinum (Xhie et al., Phys. Rev. B 43, 8917 (1991)] with th e HOPG are compared. The results of this comparison are in complete ag reement with the different catalytic behavior of the two metals in pre sence of reactions of hydrogenation of unsaturated hydrocarbons.