The Structure and Reactivity of Al₂O₃-Supported Cobalt−Palladium Particles:  A CO-TPD, STM, and XPS Study

Bimetallic Co−Pd Fischer−Tropsch catalysts have shown increased activity toward CO-hydrogenation and methane conversion. A model for these Fischer−Tropsch catalysts has been constructed and studied under ultrahigh vacuum conditions. The morphology, growth modes, exposed binding sites, and electronic properties of Co−Pd bimetallic particles supported on a thin alumina film have been studied using scanning tunneling microscopy (STM), temperature-programmed desorption (TPD) of CO, and X-ray photoelectron spectroscopy (XPS). Sequential deposition of Co and Pd results in a core−shell structure. Three-fold hollow sites are easily dismantled, whereas atop sites are statistically resilient upon the formation of a bimetallic shell. The binding energy of CO to each of the metals is lowered by the presence of the other. A shift in the Pd 3d level to higher binding energy concurrent with a shift in the Co 2p level to lower binding energy suggests a net polarization of charge or redistribution of d-band states.