Palladium nanoparticles (Pd NPs) were deposited on highly oriented pyrolytic graphite (HOPG) substrates by using a potentiostatic double-pulse technique. The NPs possessed a narrow size distribution and wide dispersion. The particle density was in the order of 109 cm−2. The average height of Pd NPs was controlled in a range of 3 to 50 nm by adjusting the duration of growth pulse. The carbon monoxide (CO) stripping at Pd NPs smaller than 14 nm occurred predominantly at a potential above 1.1 V, which is around 0.2 V more positive than that at bulk Pd and larger Pd NPs, due to the small Pd NPs tending to possess well-ordered (111) facets and a high ratio of edge and corner atoms. The high coverage of adsorbed CO (COads) at small Pd NPs can block the formation of adsorbed hydroxyl (OHads) and drive up the oxidation potential. During formic acid oxidation (FAO), small Pd NPs were quickly poisoned by CO, which was formed initially at edges and corner atoms by electrochemical reduction of FAO product CO2 at low potentials. Based on the overall consideration of the low CO tolerance and the high difficulty to remove CO, it must be stated that Pd NPs smaller than 15 nm without strict shape control are not well suited for FAO.
