Ligand-Protected Ultrasmall Pd Nanoclusters Supported on Metal Oxide Surfaces for CO Oxidation: Does the Ligand Activate or Passivate the Pd Nanocatalyst?

Herein, we report on the synthesis of ultrasmall Pd nanoclusters (∼2 nm) protected by L-cysteine [HOCOCH(NH )CH SH] ligands (Pd (L-Cys) ) and supported on the surfaces of CeO , TiO , Fe O , and ZnO nanoparticles for CO catalytic oxidation. The Pd (L-Cys) nanoclusters supported on the reducible metal oxides CeO , TiO and Fe O exhibit a remarkable catalytic activity towards CO oxidation, significantly higher than the reported Pd nanoparticle catalysts. The high catalytic activity of the ligand-protected clusters Pd (L-Cys) is observed on the three reducible oxides where 100 % CO conversion occurs at 93-110 °C. The high activity is attributed to the ligand-protected Pd nanoclusters where the L-cysteine ligands aid in achieving monodispersity of the Pd clusters by limiting the cluster size to the active sub-2-nm region and decreasing the tendency of the clusters for agglomeration. In the case of the ceria support, a complete removal of the L-cysteine ligands results in connected agglomerated Pd clusters which are less reactive than the ligand-protected clusters. However, for the TiO and Fe O supports, complete removal of the ligands from the Pd (L-Cys) clusters leads to a slight decrease in activity where the T CO conversion occurs at 99 °C and 107 °C, respectively. The high porosity of the TiO and Fe O supports appears to aid in efficient encapsulation of the bare Pd nanoclusters within the mesoporous pores of the support.