The Critical Impacts of Ligands on Heterogeneous Nanocatalysis: A Review

Ligand utilization is a necessary and powerful technique for the colloidal synthesis of nanoparticles (NPs) with controllable sizes and regulated morphologies. For catalysis applications, it is commonly believed that surface ligands on metal NPs block the active catalytic sites and reduce the catalytic activity. Nevertheless, since 2010, an increasing number of research groups have demonstrated the unexpected benefits of ligands that improve catalytic activity and/or selectivity. These benefits can be ascribed to the construction of an inorganic-organic interface, through which a series of factors, such as steric, electronic, and solubility effects, can be utilized to produce favorable changes to the interfacial environment. Considering the tremendous number of developments in this emerging research field, it is necessary to compile a comprehensive and systematic overview of recent advances. In this Review, we summarize the critical impacts of ligands on heterogeneous nanocatalysis. First, we introduce the vital roles of ligands in colloid syntheses for controllable sizes and regulated shapes. Second, the detrimental effects of ligands for nanocatalysis are described on the basis of traditional views. Third, a series of strategies for ligand removal are reviewed and compared. Fourth, on the basis of research that has been conducted in the past decade, the three main beneficial ligand effects (steric, electronic, and solubility) on heterogeneous nanocatalysis are classified and discussed. For each effect, the possible corresponding beneficial mechanism is presented, and typical examples are provided. Recent advances regarding density functional theory (DFT) calculations and the regulation of ligand surface coverage have been dedicated to explaining the ligand-promotion mechanism in nanocatalysis and searching for optimal nanocatalysts. Fifth, the stabilities of cutting-edge ligand-capped nanocatalysts before and after catalytic reactions are discussed. Finally, we highlight the remaining challenges and propose future perspectives. Although much progress has been achieved, the impacts of ligands on the catalytic activities of nanocatalysts are multifaceted and still debatable. We hope this Review will deepen readers' understanding of the actual impacts ligands have on heterogeneous catalysis.