Boosting the activation of molecular oxygen and the degradation of tetracycline over high loading Ag single atomic catalyst

Photocatalytic activation of molecular oxygen (O2) is a promising way in oxidative degradation of organic pollutants. However, it suffers from low efficiency mainly due to the limited active sites for O2 activation over traditional photocatalysts. Therefore, we established a single atomic Ag-g-C3N4 (SAACN) catalyst with 10 wt% loading of Ag single sites for boosting the O2 activation during the degradation of tetracycline (TC), and 10 wt% loading of nanoparticle Ag-g-C3N4 (NPACN) was studied as a comparison. When using SAACN, the accumulative concentration of superoxide (•O2−), hydroxyl radical (•OH), singlet oxygen (1O2) reached up to 0.66, 0.19, 0.33 mmol L−1h−1, respectively, within 120 min, 11.7, 5.7 and 4.9 times compared with those using NPACN, representing 17.24% of dissolved O2 was converted to reactive oxygen species (ROS). When additionally feeding air or O2, the accumulative concentrations of •O2−, •OH, 1O2 were even higher (air: 4.21, 0.97, 2.02 mmol L−1 h−1; O2: 17.13, 1.32, 9.00 mmol L−1 h−1). The rate constants (k) for degrading the TC were 0.0409 min−1 over SAACN and 0.00880 min−1 over NPACN, respectively (mineralization rate: 95.7% vs. 59.9% after 3 h of degradation). Moreover, the degradation ability of SAACN did not decrease in a wide range of pH value (4–10) or under low temperature (10 °C). Besides the high exposure of Ag single sites, other advances of SAACN were: 1(O2 was more energetic favorable to adsorb on single atomic Ag sites; 2) Positive Ag single sites were easier to obtain the electrons from the surrounding N atoms, and facilitated electron transfer towards adsorbed O2.