Anchoring Effect of Exfoliated-Montmorillonite-Supported Pd Catalyst for the Oxygen Reduction Reaction

We report a combined experimental and computational study on exfoliated montmorillonite (ex-MMT) nanoplatelet supported palladium catalysts. The experimental results revealed that the Pd/ex-MMT catalysts were more stable than Pd/C catalysts in an acidic environment. The results from the density functional theory (DFT) calculations revealed that the O atoms from the AlO6 octahedra in ex-MMT act as anchoring sites for Pd nanoparticles (NPs) that are strongly bound to the ex-MMT support. The partial density of state calculations indicated that the Pd-d states and the O(AlO6)-p states have similar energy. Therefore, it is very easy for electrons to transfer between these states and to form Pd–O(AlO6) bonds. In addition, the defective or destroyed SiO4 tetrahedra can be restructured directly by the O atoms from O2 molecules or indirectly by the intermediates from the reduction of O2. The DFT calculations also revealed that the defects in the ex-MMT supports provide anchor sites for the Pd catalysts to tightly bind on the ex-MMT supports. For Pd5/ex-MMT, with the attacks of Oads, the Pd atoms in the Pd5 cluster remain linked together through a Pd–Pd bond and are anchored tightly on the ex-MMT. However, for Pd/C, the Pd atoms in the Pd5 cluster are not only oxidized but also separated from the carbon bulk.