Investigation of the Relationship between Metal Loading and Acidic Oxygen Evolution Reaction Activity in Single-Atom Catalysts

Revealing the underlying relationship between the loading of metal atoms and catalytic behavior is crucial for guiding the design of optimal single-atom catalysts (SACs) in heterogeneous catalysis. However, the apparent catalytic behavior controlled by interactions between individual active sites remains elusive. Here, we successfully synthesized a series of atomically dispersed Ir SACs with the Ir-PN3 configuration through a straightforward P atom anchoring strategy, which effectively prevents metal aggregation due to its strong metal-coordination capabilities. These SACs are loaded at varying percentages, ranging from 5% to 21 wt %, and exhibited an unforeseen "volcano-type" relationship between the Ir loadings and their acidic oxygen evolution reaction activity. Notably, the highest activity is observed at a moderate loading level of 14%. This phenomenon can be attributed to the competition between the number and electronic structure of Ir active sites. Electronic structural characterizations and theoretical calculations reveal that the key for establishing the "volcano-type" relationship is the strong electronic interactions among Ir active sites at high densities, leading to charge rearrangement. This study highlights the importance of the relationship between the metal loading and activity of SACs, which may not exhibit a positive linear regression.