Structural heterogeneity of single-atom catalysts and true active site generation via ligand exchange during electrochemical H2O2 production

Although often overlooked, single-atom catalysts inevitably contain heterogeneous active sites and their structures can further change under reaction conditions. Here, we conduct rigorous in situ X-ray absorption spectroscopy with 1–10 wt% Pt on S-doped zeolite-templated carbon (SZTC) to elucidate their remarkable electrocatalytic H₂O₂ synthesis properties. SZTC with a curved polyaromatic framework and abundant sulfur functional groups can stabilize isolated Pt sites with up to 10 wt% Pt loading. Although all catalysts contain apparently identical Pt–S₄ sites in their as-synthesized form, the Pt-specific activity increases rapidly with increasing Pt loading. It appears that Pt first forms inert Pt–S₄ in SZTC and then forms labile Pt–S₄ with increasing Pt loading. Upon contact with aqueous electrolytes, only the labile sites are converted to true active sites, Pt–S₂(H₂O)₂, via ligand exchange with H₂O. The results provide invaluable insights for understanding the heterogeneity and dynamic structural changes of single-atom catalysts.