Palladium Nanoclusters: Preparation and Synthesis
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The chemistry of polynuclear metal clusters in the nanometer scale has become a central issue in materials science because of their unique functions as electronic, optical, magnetic, and catalytic devices, which bring new technologies in many industrial areas. At present, there is some progress being made in the preparation of metal nanoclusters. They have been synthesized by the chemical reduction of metal salts in the presence of stabilizing ligands such as surfactants, organic polymers, and organic bases, which preserve the metal clusters from the agglomeration of the clusters themselves. However, the above methods hardly produce metal nanoclusters with a high degree of size dispersity.
Transition metals have been extensively used as catalysts for many organic reactions, where the oxidation state of the metal plays an important role in attaining highly selective reactions. A multiple interaction of substrate molecules with surface metal atoms of transition metal nanoclusters would offer the possibility of unprecedented catalytic reactions. Furthermore, a creation of specific surface ensemble sites consisting of zero-valent metals and metal cations is a promising strategy for designing highly functionalized nanocluster catalysts based on cooperative action among the multiple metal species.
The purpose of this article is to develop a new synthetic protocol of monodispersed palladium (Pd) nanoclusters at the nanometer-scale precision as high-performance catalysts. We describe here the novel synthesis of Pd nanoclusters with a high degree of size dispersity, where the standard deviation of the mean diameter (σ/d) is less than 10%, by treatment of the small Pd cluster with metal nitrates such as Cu(NO3)2 under an O2 atmosphere. This method also yielded mixed-valence states with Pd0 and cationic Pd species on the cluster surface. An advantage of this synthetic method is that the particle size and the surface oxidation state of monodispersed Pd nanoclusters can be controlled by selecting the amount of the metal nitrates and by varying the preparation time of the clusters. Furthermore, the above Pd nanoclusters are applicable to highly efficient and selective heterogeneous catalysts for liquid-phase oxidations such as acetoxylation of toluene, alcohol oxidation, and the Wacker oxidation under an atmospheric pressure of O2. The unique catalyses of the Pd nanoclusters are attributed to the cooperative action between zero-valent Pd and Pd cations on the surface ensemble Pd sites.