Many industrial catalysts involve nanoscale metal particles (typically 1-100 nm), and understanding their behavior at the molecular level is a major goal in heterogeneous catalyst research. However, conventional nanocatalysts have a nonuniform particle size distribution, while catalytic activity of nanoparticles is size dependent. This makes it difficult to relate the observed catalytic performance, which represents the average of all particle sizes, to the structure and intrinsic properties of individual catalyst particles. To overcome this obstacle, catalysts with well-defined particle size are highly desirable. In recent years, researchers have made remarkable advances in solution-phase synthesis of atomically precise nanoclusters, notably thiolate-protected gold nanoclusters. Such nanoclusters are composed of a precise number of metal atoms (n) and of ligands (m), denoted as Aun(SR)m, with n ranging up to a few hundred atoms (equivalent size up to 2-3 nm). These protected nanoclusters are well-defined to the atomic level (i.e., to the point of molecular purity), rather than defined based on size as in conventional nanoparticle synthesis. The Aun(SR)m nanoclusters are particularly robust under ambient or thermal conditions (<200 °C). In this Account, we introduce Aun(SR)m nanoclusters as a new, promising class of model catalyst. Research on the catalytic application of Aun(SR)m nanoclusters is still in its infancy, but we use Au₂₅(SR)₁₈ as an example to illustrate the promising catalytic properties of Aun(SR)m nanoclusters. Compared with conventional metallic nanoparticle catalysts, Aun(SR)m nanoclusters possess several distinct features. First of all, while gold nanoparticles typically adopt a face-centered cubic (fcc) structure, Aun(SR)m nanoclusters (<2 nm) tend to adopt different atom-packing structures; for example, Au₂₅(SR)₁₈ (1 nm metal core, Au atomic center to center distance) has an icosahedral structure. Secondly, their ultrasmall size induces strong electron energy quantization, as opposed to the continuous conduction band in metallic gold nanoparticles or bulk gold. Thus, nanoclusters become semiconductors and possess a sizable bandgap (e.g., ~1.3 eV for Au₂₅(SR)₁₈). In addition, Aun(SR)m can be doped with a single atom of other metals, which is of great interest for catalysis, because the catalytic properties of nanoclusters can be truly tuned on an atom-by-atom basis. Overall, atomically precise Aun(SR)m nanoclusters are expected to become a promising class of model catalysts. These well-defined nanoclusters will provide new opportunities for achieving fundamental understanding of metal nanocatalysis, such as insight into size dependence and deep understanding of molecular activation, active centers, and catalytic mechanisms through correlation of behavior with the structures of nanoclusters. Future research on atomically precise nanocluster catalysts will contribute to the fundamental understanding of catalysis and to the new design of highly selective catalysts for specific chemical processes.
