Investigation of Size Sensitivity in the Hydrogen Production from Formic Acid over Carbon‐Supported Pd Nanoparticles

The size effect in the hydrogen production from formic acid decomposition has been investigated using a series of carbon‐supported Pd NPs with different particle sizes ranging from 2.7 to 5.5 nm. A volcano type relationship between catalytic activity and NPs size was obtained. This result suggests that there is an optimum relative proportion of low‐ and high‐coordination surface atoms within a regular arrangement of the Pd NPs, which was achieved at 3.9 nm in diameter under the present experimental conditions. By performing calculations on the palladium crystallites, decomposition of formic acid is considered as a structure‐sensitive reaction, and high‐coordination sites principally act as catalytically active species in this size range. Kinetic isotope effect using HCOOD and DCOOH was well corresponded to the catalytic tendency. A k H /k D value obtained with Pd/C with optimal size was smallest among investigated, indicating the positive promoting effect in the rate‐determining C−H bond dissociation step from the Pd‐formate intermediate.