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.