Palladium Single-Atom Catalysts Supported on C@C3N4 for Electrochemical Reactions

Single atom catalysts (SACs) maximize the utilization of noble metal whereas nanoparticle catalysts have inner metal atoms unavailable. In this study, various electrocatalytic reactions were investigated for Pd and Pt SACs. The single atoms were immobilized on thin layers of graphitic carbon nitride with carbon black (for simplicity, C@C3N4) to produce an electrochemically efficient and stable SACs. Single atomic structure was confirmed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and extended X-ray absorption fine structure (EXAFS) analyses. Oxygen reduction reaction (ORR) and CO stripping experiments were conducted, and the results were compared with the corresponding nanoparticle catalysts. Lack of ensemble sites in the SACs resulted in two-electron pathway for ORR; single atomic Pd on C@C3N4 (C@C3N4-Pd-1) showed high activity and selectivity for H2O2 formation. DFT calculations showed that C@C3N4-Pd-1 follows a downhill path for H2O2 formation unlike single atomic Pt on C@C3N4 (C@C3N4-Pt-1), resulting in enhanced H2O2 selectivity. Weaker adsorption of oxygen intermediates on C@C3N4-Pd-1 resulted in enhanced ORR activity. The SACs showed no interaction with CO as confirmed by no CO stripping peak. This resulted in no activity for formic acid oxidation following indirect pathway or methanol oxidation, which necessitates COads as reaction intermediates. SACs can be efficient electrocatalysts with high activity and unique selectivity.