Size Sensitivity of Supported Palladium Species on Layered Double Hydroxides for the Electro-oxidation Dehydrogenation of Hydrazine: From Nanoparticles to Nanoclusters and Single Atoms

The hydrazine oxidation reaction (HzOR) is structure-sensitive, and a slight modification of the catalyst structure can generate a significant change in activity. To date, no study on the size effect of the catalyst at a subnanometer scale on HzOR has been reported yet. Herein, we report the fabrication of Pd species with sizes ranging from nanoparticles (NPs) and nanoclusters (NCs) to single atoms (SAs) onto a NiFe-layered double hydroxide by fine-tuning precursors and reduction methods. When applied for the electro-oxidation dehydrogenation of hydrazine, the as-prepared Pd NCs/NiFe exhibit a current density of 4.3 A mgPd–1 at 0.35 V versus RHE. It should be noted that the mass activity of Pd NCs/NiFe was 36 times that of Pd SAs/NiFe and 7 times that of Pd NPs/NiFe, respectively. The in situ electrochemical impedance spectroscopy and density functional theory calculation demonstrate that different from previous studies, both the Pd SAs/NiFe and Pd NPs/NiFe are suboptimal in HzOR due to the isolated active sites of the Pd SAs/NiFe and large steric hindrance of Pd NPs/NiFe, respectively. Instead, the strong intracluster interactions of Pd NCs/NiFe lift the d-band center closer to the Fermi level, leading to stronger hybridization of Pd d-orbitals and the σ orbitals of N2H4 molecules. Consequently, the excellent performance of Pd NCs/NiFe can be attributed to its multiple neighboring metal sites, high d-band center, and small steric hindrance.