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 (CUiO-66/CUiO-66-NH2/CUiO-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 (EBA/ELA) and areas (SBA/SLA). CUiO-66 exhibited the highest ELA and lowest EBA, which are favorable for endothermic H2O2 distortion, whereas CUiO-66-SO3H exhibited the lowest ELA and highest EBA, with only EBA being favorable for endothermic •OH desorption, while leaving the other elementary steps exothermic. Kinetic analysis and DFT calculations revealed that CUiO-66-SO3H possessed the lowest energy barrier (EBARRIER), demonstrating •OH desorption was the rate-determining step alongside with the significance of high EBA for reducing EBARRIER. Meanwhile, the highest pre-factor was observed for CUiO-66 with the largest SLA, corroborating the significance of large SLA for escalating the collision frequency between Zr4+ and H2O2/•OH. These results boost to adjust EBA/SLA for promoting •OH productivity via catalytic H2O2 homolysis.