Insights into the role of singlet oxygen in the photocatalytic hydrogen peroxide production over polyoxometalates-derived metal oxides incorporated into graphitic carbon nitride framework

To develop a new strategy of enhancing the photoinduced holes (h+) consumption to promote the photoinduced electrons (e−) utilization for O2 reduction to H2O2 and maintaining the chemical stability of g-C3N4-based catalysts, the hybrid catalyst of g-C3N4-CoWO has been prepared through the calcination of the graphitic carbon nitride (g-C3N4) precursor of 3-amino 1, 2, 4-triazole (3-AT) and the polyoxometalates (POMs) precursor of (NH4)8Co2W12O42 (NH4-Co2W12). The hybrid catalyst of g-C3N4-CoWO with well-defined and stable structure exhibits efficient catalytic performance (9.7 μmol h-1) for photocatalytic H2O2 production in the absence of organic electron donor under visible light. The value of electrons transfer during the oxygen reduction reaction (ORR) process obtained from the Koutecky-Levich plot for g-C3N4-CoWO (n = 1.95) is higher than that for g-C3N4 (n = 1.18), suggesting that the CoWO incorporated into g-C3N4 framework can generate more e− for O2 reduction. The superoxide radicals (O2-) quantitative and scavenger experiments combined with the electron spin resonance (ESR) results reveal that the negative shifts of the conduction band (CB) level from g-C3N4 to g-C3N4-CoWO can enhance the single-electron reduction of O2 to O2-. The h+ and 1O2 scavenger experiments results combined with the ESR results demonstrate that the CoWO incorporated into g-C3N4 framework can promote the oxidation of O2- to 1O2 by h+. The 1O2 quantitative experiments results indicate that the 1O2 can proceed two-electron reduction to H2O2. The enhanced h+ consumption and the 1O2 transferred from O2- can promote the photocatalytic H2O2 production over g-C3N4-CoWO. In addition, the recycle experiment results reveal that the heterogeneous g-C3N4-CoWO is catalytic stable.