Spectro-kinetic modelling of photocatalytic oxidation of heterocyclic compounds in a continuous-flow packed bed reactor

Abstract

Mechanistically derived predictive kinetic models for photocatalytic processes have inherent challenges due to short lived charge carriers and free radical intermediate species. In this work, we present a spectro-kinetic model for the photocatalytic oxidation of carbazole (CAB), a nitrogen-containing heterocyclic compound, using a visible-light-active Ag/AgBr/TiO₂ photocatalyst immobilized on glass beads. Experiments were performed in a continuous-flow packed bed photoreactor in industrial wastewater matrix under visible light. Reactive oxygen species (ROS) were monitored using electron paramagnetic resonance (EPR) spectroscopy, identifying hydroxyl radicals (˙OH) as the predominant ROS, with superoxide (O₂˙⁻). and H₂O₂ acting as intermediates. The model incorporates key pathways including direct hole oxidation and ˙OH-mediated oxidation of CAB, as well as charge recombination and radical quenching. Quenching of (˙OH) by intermediate CAB-derived ˙CR radicals emerged as the rate-determining step, exerting the greatest impact on apparent quantum efficiency (AQE). The model showed excellent agreement with experimental results (R² = 0.99), accurately predicting CAB degradation kinetics. Parametric analysis confirmed ˙OH radical mediated oxidation as the primary pathway, followed by secondary contribution from direct h⁺ attack. The system achieved 54% CAB removal with an AQE of 75%. This study demonstrates the value of integrating spectroscopic measurements with mechanistic modeling to guide photocatalytic process development.