Size-dependent catalytic activity over carbon-supported palladium nanoparticles in dehydrogenation of formic acid

- Decreasing the Pd particle size increases the catalytic activity of Pd/C in FA dehydrogenation.
- The 2.1 nm Pd/C catalyst showed a TOF of 835 h−1 at 25 °C among the highest activities for monometallic Pd catalysts.
- Both low- and high-coordination Pd surface atoms are the active sites in FA dehydrogenation.
- The improved catalytic activity on smaller Pd nanoparticles is likely attributed to the positive charged Pd species.

Hydrogen generation from formic acid (FA) under mild conditions has received significant attention, where Pd-based catalysts have been widely employed due to their superior activities. However, the Pd particle size effect has been much less systematically investigated. In this study, carbon-supported Pd nanoparticles (NPs) with five different Pd particle sizes, ranging from 2.1 to 4.5 nm were synthesized using sodium citrate as the stabilizing agent. The Pd particle sizes were determined by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The FA dehydrogenation reaction was conducted in a FA-sodium formate (SF) aqueous solution in a batch reactor at room temperature. We found that decreasing the Pd particle size from 4.5 ± 0.5 to 2.1 ± 0.3 nm remarkably boosted the catalytic activity by about 3.6 times, resulting in a turnover frequency of 835 h−1, which is among the highest values for supported monometallic Pd catalysts in the literature. Our results suggest that both low- and high-coordination Pd surface atoms participated in the reaction. The remarkably higher activity of smaller Pd NPs was attributed to both higher Pd dispersion and the presence of a larger proportion of Pd species with positive charge, through which the Coulomb interaction between the positive Pd species and negative charged formate ions, the key reaction intermediate, is enhanced. Finally, the deactivation and regeneration of Pd/C catalysts were also discussed.

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