Unveiling the micro-mechanism of H2O2 activation and the selective regulation strategy over single-atom catalysts†

H₂O₂ is an important green oxidant, and activation of H₂O₂ is the key process determining its efficiency in removing environmental pollutants. However, due to complex catalytic sites and diverse active free radical products, the micro-mechanism of H₂O₂ activation and the selective regulation strategy are still ambiguous. Herein, single-atom catalysts (SACs) are selected as the model catalysts to investigate this fundamental mechanism. With a single active site, it is more beneficial to explain the mechanism. In this work, the differences in active free radical products (OH, ·OOH, ¹O₂) of H₂O₂ over three SACs (Fe, Co, Cu) and intrinsic selective regulation strategies are elucidated based on electron paramagnetic resonance (EPR) and density functional theory (DFT) calculation. EPR testing suggests that Co-SAC has the highest production of ·OH radicals, while Cu-SAC surpasses the other two catalysts in generating both ·OOH and ¹O₂ radicals. DFT calculations indicate that among the SACs, the lowest barrier of ·OH radical formation is Co-SAC (0.54 eV), while Cu-SAC demonstrates a notably lower energy barrier for ·OOH formation (0.26 eV) and ¹O₂ generation (0.51 eV), which is consistent with the EPR experimental results. More importantly, our work reveals that there is a linear relationship between charge transfer and the energy barrier of free radical generation. When the charge transfer amount is greater than 1.02, it is more inclined to promote the generation of ·OOH, and it will generate ¹O₂ free radicals when the charge transfer amount is smaller than 1.02. This work provides a predictive mechanism for SACs to selectively regulate the active free radical products, which is of great significance for developing green environmental protection technologies based on H₂O₂.