Novel Preparation of Palladium Nanoclusters Using Metal Nitrates and Their Catalysis for Oxidative Acetoxylation of Toluene in the Presence of Molecular Oxygen

Anion ligands-preserved cationic giant Pd clusters were prepared by treatment of palladium carbonyl acetate cluster, Pd 4 (CO) 4 (OAc) 4 ·2AcOH (PCA), with metal nitrates, e.g. Cu(NO 3 ) 2, and Fe(NO 3 ) 3, in the presence of 1,10-phenanthroline (phen). Extensive characterization of the Pd clusters was made by XRD, XPS, XAFS, TPD, FE-SEM, ion-chromatography, and CO adsorption. It was revealed that the inner part of the clusters was an exclusively metallic form, whereas both Pd cations (Pd 2+ and Pd + ) and Pd 0 species were located on the same surface of the clusters to form specific ensemble sites. Further, the particle size and surface fraction of Pd 0, Pd +, and Pd 2+ of the giant Pd clusters could be tuned by changing the amount of the metal nitrate; the largest particle size of about 35 Å and the highest surface fraction of the Pd 2 O could be obtained when the Cu(NO 3 ) 2 /PCA ratio was 0.10. The 8-shell giant Pd clusters of about 35 Å in diameter having a large number of surface Pd cations showed a high catalytic activity for the oxidative acetoxylation of toluene to give benzyl acetates in the presence of molecular oxygen. Furthermore, the Pd cluster could be immobilized on a neutral TiO 2 surface by keeping its original cluster size and high catalytic activity. The prominent catalysis of this Pd cluster can be ascribed to the ensemble sites composed of metallic Pd atoms and neighboring Pd cations on its surface.