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 (similar to 2 nm) protected by L-cysteine [HOCOCH(NH2)CH2SH] ligands (Pd-n(L-Cys)(m)) and supported on the surfaces of CeO2, TiO2, Fe3O4, and ZnO nanoparticles for CO catalytic oxidation. The Pd-n(L-Cys)(m) nanoclusters supported on the reducible metal oxides CeO2, TiO2 and Fe3O4 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-n(L-Cys)(m) is observed on the three reducible oxides where 100 % CO conversion occurs at 93-110 degrees 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 TiO2 and Fe3O4 supports, complete removal of the ligands from the Pd-n(L-Cys)(m) clusters leads to a slight decrease in activity where the T-100% CO conversion occurs at 99 degrees C and 107 degrees C, respectively. The high porosity of the TiO2 and Fe3O4 supports appears to aid in efficient encapsulation of the bare Pd-n nanoclusters within the mesoporous pores of the support.