Platinum nanoparticles (Pt NPs) are one of the most efficient cocatalysts in photocatalysis, and their size determines the activity and the selectivity of the catalytic reaction. Nevertheless, an in-depth understanding of the platinum’s size effect in the carbon dioxide photocatalytic reduction is still lacking. Through analyses of the geometric features and electronic properties with variable-sized Pt NPs, here we show a prominent size effect of Pt NPs in both the activity and selectivity of carbon dioxide photocatalytic reduction. Decreasing the size of Pt NPs promotes the charge transfer efficiency, and thus enhances both the carbon dioxide photocatalytic reduction and hydrogen evolution reaction (HER) activity, but leads to higher selectivity towards hydrogen over methane. Combining experimental results and theoretical calculations, in Pt NPs, the terrace sites are revealed as the active sites for methane generation; meanwhile, the low-coordinated sites are more favorable in the competing HER.

Different from isolated metal atoms and large metal nanoparticles (NPs), supported metal clusters (SMCs) possess distinct geometric and electronic structures and thus exhibit enhanced activity and designated selectivity in catalysis. So far, with the development in synthetic methodologies and characterization techniques, SMCs with fine structures could be constructed and well-defined at the atomic level. In addition, based on computational modeling of SMCs, theoretical calculations corroborated well with experimental results, providing in-depth insights into the structure–property relationship for SMCs in catalysis. In this Review, classic synthetic strategies and key characterization techniques of SMCs are summarized. Subsequently, the applications of SMCs in important catalytic reactions based on recent studies are discussed, including aerobic oxidation, hydrogenation, dehydrogenation, water–gas shift (WGS) reaction, and photocatalytic reactions. In particular, the importance of the cluster size-effect and metal–support interactions in determining the catalytic performance of SMCs is highlighted. Lastly, challenges and prospects in SMCs’ catalysis are illustrated.

Increasing attention has been paid to single-atom catalysts (SACs) in heterogeneous catalysis because of their unique electronic properties, maximized atomic utilization efficiency, and potential to serve as a bridge between the heterogeneous and homogeneous catalysis. However, SACs can have limited advantages or even constrained applications for the reactions that require designated metallic states with multiple atoms or surface sites with metal–metal bonds. As a cross-dimensional extension to the concept of SACs, fully exposed cluster catalysts (FECCs) offer diverse surface sites formed by an ensemble of metal atoms, for the adsorption and transformation of reactants/intermediates. More importantly, FECCs have the advantage of maximized atom utilization efficiency. Thus, FECCs provide a novel platform to design effective and efficient catalysts for certain chemical processes. This outlook summarizes recent advances and proposes prospective research directions in the design of catalysts and characterizations of FECCs, together with potential challenges.