Reconsideration of the contribution of photogenerated ROS in methyl orange degradation on TiO2, Cu2O, WO3, and Bi2O3 under low-intensity simulated solar light: mechanistic understanding of photocatalytic activity

In our study, the effectiveness of photocatalytic degradation of methyl orange (MO) in the presence of several catalysts under simulated solar light was studied. A comparative study was conducted between four different catalysts (TiO2, Cu2O, WO3, and Bi2O3) to assess their photocatalytic activities by varying the illumination time. The photodegradation was examined through quantification of the photogenerated reactive oxygen species (ROS). In situ photogenerated hydroxyl radicals (⦁OH) were detected with terephthalic acid (TA) dosimetry based on fluorescence spectroscopy. Therefore, the concentration of ⦁OH was expressed according to the relative fluorescence intensity. Moreover, proof of a nonlinear relationship connecting the kinetics of the photocatalytic degradation of MO and the generation of ⦁OH radicals was demonstrated using coefficients of determination (R2). This brought new insights regarding the mechanism of the photocatalytic activity. Our findings indicate that the generated ⦁OH radicals from heterogeneous photocatalytic systems are not the unique factor involved in organic pollutant elimination; the double metal structure of metal oxides should also be taken into consideration.