Atomic-Scale Imaging and Electronic Structure Determination of Catalytic Sites on Pd/Cu Near Surface Alloys

Water-gas shift chemistry provides a useful method for producing hydrogen from coal; however, fuel cell applications demand that this hydrogen be free of impurities. Due to their unique properties, Pd/Cu alloys represent an import class of materials used for H purification membranes and also serve as the active metals in many heterogeneous catalysts. Little is known about how Pd and Cu interact electronically in these mixed systems and there is debate in the literature over the direction of charge transfer between the two species. This study used the differential conductance (dI/dV) spectroscopy capabilities of a low-temperature scanning tunneling microscope (STM) to investigate the atomic-scale electronic structure of Pd/Cu surface alloys. dI/dV spectroscopy gives a direct measure of the local density of states of surface sites with subnanometer precision. Results from this work demonstrate that individual, isolated Pd atoms in a Cu lattice are almost electronically identical to their host atoms. Over an energy range that spans 1 eV on either side of the Fermi level, the only significant electronic difference between isolated Pd and their host Cu atoms is that Pd atoms have a very slightly depleted electron density in the region of the Cu surface state maximum.