Metal-organic framework-derived ZrO₂ on N/S-doped porous carbons for mechanistic and kinetic inspection of catalytic H₂O₂ homolysis

Homolytic and heterolytic H₂O₂ scissions are central to produce ^•OH for aqueous contaminant fragmentation. However, the kinetic, mechanistic, and energetic aspects of homolytic H₂O₂ cleavage remain under-explored, providing impetus for research with the use of difficult-to-degrade phenol as a model pollutant. Herein, UiO-66 and its analogues functionalized with –NH₂/-SO₃H (UiO-66-NH₂/UiO-66-SO₃H) were synthesized to generate ZrO₂ poly-crystallites on N/S-doped carbon catalysts via pyrolysis (C_UiO-66/C_UiO-66-NH2/C_UiO-66-SO3H). The catalyst surfaces contained distinct concentrations of Lewis basic N/S dopants, which donated electrons to adjacent Brönsted acidic –OH (BA) and Lewis acidic Zr⁴⁺ (LA) species dissimilarly, resulting in the catalysts with diverse BA/LA strengths (E_BA/E_LA) and areas (S_BA/S_LA). C_UiO-66 exhibited the highest E_LA and lowest E_BA, which are favorable for endothermic H₂O₂ distortion, whereas C_UiO-66-SO3H exhibited the lowest E_LA and highest E_BA, with only E_BA being favorable for endothermic ^•OH desorption, while leaving the other elementary steps exothermic. Kinetic analysis and DFT calculations revealed that C_UiO-66-SO3H possessed the lowest energy barrier (E_BARRIER), demonstrating ^•OH desorption was the rate-determining step alongside with the significance of high E_BA for reducing E_BARRIER. Meanwhile, the highest pre-factor was observed for C_UiO-66 with the largest S_LA, corroborating the significance of large S_LA for escalating the collision frequency between Zr⁴⁺ and H₂O₂/^•OH. These results boost to adjust E_BA/S_LA for promoting ^•OH productivity via catalytic H₂O₂ homolysis.