Recent advances in the synthesis and catalytic applications of ligand-protected, atomically precise metal nanoclusters

Due to their excellent catalytic properties under mild reaction conditions, well-defined, nanosized noble metal catalysts have potential applications in the manufacture of fine chemicals, pharmaceuticals, and food additives. Ligand stabilized MnLm NCs (M: noble metal; n: number of metal atoms, n < 150; L: ligand; m: number of ligands) have a long history in catalysis, but recent advances in synthetic strategies and instrumental characterization have led to a renaissance in the catalytic applications of MnLm NCs. Thus, NCs can serve as model catalysts for understanding fundamental aspects of catalysis, but they also exhibit interesting properties in practical applications. MnLm NCs such as the recently investigated thiolated capped Au NCs have unique geometric structures with ultra-small size and strong quantum confinement, both of which are lacking in larger noble metal nanoparticles (>2 nm). The correlations among the catalytic performance of MnLm NCs with the size, structure, and composition of individual NCs at the atomic level can be demonstrated in realistic ambient reaction conditions, thereby contributing to the rational design of highly active catalysts with novel properties. Our current understanding of these newly emerging catalytic NCs is still in its infancy, but some studies have shown their potential for promoting new types of reactions. This review summarizes recent exciting advances in this field (since 2010), especially the catalytic properties of noble metal NCs in the presence of the ligand shell and after removing the ligand.