Single and dual metal atom catalysts for enhanced singlet oxygen generation and oxygen reduction reaction

We demonstrate rational design of graphene-supported single and dual metal atom catalysts (SACs and DACs) for photocatalytic applications, such as singlet oxygen (¹O₂) sensitization and H₂O₂ production. Here we combine density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations with experimental verifications. We found a synergistic effect between triplet sensitization and triplet-triplet (Dexter) energy transfer; both play a role in the photocatalytic activity through the volcano plot of 3d transition metal SACs. More specifically, FeN₄-SAC exhibits a low ISC energy gap (ΔE_ISC) of 0.039 eV, compared with 0.108 eV for FeNiN8-DACs, both possessing a high Bader charge transfer of 0.366 e⁻ and 0.405 e⁻, respectively. Guided by these computational results, we synthesized a series of SACs and a DAC and confirmed their structures with scanning transmission electron microscopy (STEM) along with the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). We then confirm their band structures with low-energy inverse photoemission spectroscopy (LEIPS) and UV-vis-NIR. Subsequently, we synthesized the catalysts for the photooxygenation of anthracene and two-electron oxygen reduction reaction (ORR) to measure their photocatalytic activity. We found that H₂O₂ production through the two-electron ORR competes with the ¹O₂ generation through Dexter energy transfer. FeN₄-SAC demonstrates a high photooxygenation conversion of 86% and a high ¹O₂ quantum yield of 1.04, obtained from electron spin resonance (ESR) spectroscopy, with low H₂O₂ production. In contrast, NiN₄-SAC exhibits a low ¹O₂ generation and a high H₂O₂ production mainly because of the high Gibbs free energy of the OOH* intermediate. This work proposes an effective DFT-guided strategy for designing SACs and DACs for various photocatalytic applications.