Residual metal single-atom catalysis: enhanced photocatalytic hydrogen production via ligand-modulated d-orbital splitting

This study demonstrates that residual palladium (Pd) species, often considered unintended impurities in organic photocatalysts, can function as highly efficient catalytic sites when atomically dispersed and strategically coordinated within molecular frameworks. A series of pyridine-terminated organic small molecules are synthesized to anchor Pd single atoms via monodentate coordination, which significantly modulates the electronic structure and catalytic behavior. Through a combination of spectroscopic, electrochemical, and computational methods, we reveal that the d _{z ²} orbital of Pd plays a pivotal role in facilitating hydrogen adsorption by optimizing orbital overlap with hydrogen. Furthermore, a low-energy pathway mediated by pyridyl-hydrogen radicals drastically reduces the energy barriers for hydrogen transfer steps. The pyrene core serves as a photochemical charge reservoir, while terminal pyridine groups act as active sites, collectively enhancing exciton dissociation and charge separation. This work provides new insights into the elusive role of residual metals in photocatalysis and introduces the concept of residual metal single-atom catalysis, a strategy for designing efficient metal-organic hybrid systems with high atomic utilization and well-defined mechanisms.