Strong Electron Coupling from the Sub-Nanometer Pd Clusters Confined in Porous Ceria Nanorods for Highly Efficient Electrochemical Hydrogen Evolution Reaction

With the advantage of the high catalytic activity of metal clusters from the abundant uncoordinated metal atoms and the interfacial electron interaction between metal/metal oxides, metal cluster/metal oxide hybrids have great potential in catalysis and have been attracting more and more attention in recent years. In this work, sub-nanometer sized palladium clusters accommodated into porous ceria were successfully synthesized. The characterizations from high-resolution transmission electron microscopy (HRTEM) and extended X-ray absorption fine structure (EXAFS) showed that the synthesized Pd sub-nanoclusters have a size of about 0.5 nm. The Pd clusters confined in CeO2 pores (donated as Pd NCs@CeO2) showed highly efficient electrocatalytic activity and stability for the hydrogen evolution reaction (HER). The electrochemical results indicated that HER on the Pd clusters presents an onset potential of -0.036 V vs RHE and approximately 100 times higher current density than that from the commercial Pt/C at the low-potential region. Further insights from density functional theory suggest that the active centers are the subunit of Pd clusters rather than CeO2 support based on the free energy of absorption for hydrogen atoms (Delta G(H*)). Meanwhile, compared to the free Pd nanoclusters, the electron transfer between Pd and O atoms in Pd NC@CeO2 can effectively modulate the adsorption state of H* on the subunit of the Pd cluster, and this cojoint effect can effectively promote the HER catalytic activity of the composite system. We hope this work can lay the foundation for the application of precious metal nanocluster-transition metal oxide composites in hydrogen energy and other catalysis fields.