Pd model catalysts on clean and modified HOPG: Growth, adsorption properties, and stability

Abstract

Carbon is a common support material for noble metal nanoparticles in heterogeneous catalysis and electrocatalysis. Very few surface-science-based model studies have been performed, however, with noble metal nanoparticles on carbon supports. In this work, we present the first model study on carbon-supported noble metal nanoparticles using infrared reflection absorption spectroscopy (IRAS). The model catalyst is prepared on highly oriented pyrolytic graphite (HOPG) by physical vapor deposition (PVD) of Pd metal. We characterize the particle growth and morphology by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) and probe the adsorption properties by IRAS of adsorbed CO.PVD of Pd on clean HOPG leads to the formation of very large, weakly interacting particles. In order to increase the nucleation density upon Pd deposition, the HOPG surface was modified by Ar+ and O+ bombardment. The defect structures formed were characterized by STM and by IRAS during the ion bombardment. On the pretreated HOPG, Pd nucleates homogeneously and stable Pd nanoparticles (NPs) are formed. We investigated the adsorption properties of the Pd NPs as a function of the particle size and their thermal stability upon annealing. It is shown that carbon species migrate from the HOPG modified by ion bombardment onto the Pd NPs leading to modification of the CO adsorption properties.The work shows that Pd/HOPG is a very well-suited model system for in-situ adsorption and reaction studies by IRAS, in spite of the non-metallicity of the support.