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

Highlights

• A single atomic Ag-g-C₃N₄ (SAACN) with 10 wt% loading of Ag catalyst is designed.

• When using SAACN, 17.24% of dissolved O₂ is converted to reactive oxygen species.

• The rate constants (k) for degrading the tetracycline are 0.0409 min^-1 over SAACN.

• Abundant ROS generate over SAACN due to O₂ adsorption and electron transfer.

• A potential method for converting O₂ into ROS and degraded pollutants is provided.

Abstract

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 O_2, 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.

Introduction

Advanced oxidation processes (AOPs) have been widely integrated into traditional water and wastewater treatment plants for intensive environmental remediation. Under the action of AOPs, precursors including hydrogen peroxide (H₂O₂) (Guo et al., 2019), ozone (O₃) (Yu et al., 2020), peroxy-monosulfate (PMS) (Chu et al., 2019) etc. were activated and transformed into reactive oxygen species (ROS), e.g. superoxide (•O₂⁻), via direct chemical reactions or catalytic processes. These precursors could provide highly efficient ROS generation pathways and the corresponding valid removal methods toward pollutants. However, using these precursors were still limited by their production cost and difficult storage in the real applications. Therefore, from the viewpoint of sustainable remediation technology, it is necessary to develop some green, rich and more cheap ROS precursors and their corresponding activation technologies. Recently, reports showed that molecular oxygen (O₂) was a quite potential precursor for generating ROS (Chen et al., 2018; Shang et al., 2019; Sun et al., 2019), which widely exists in kinds of environmental media. Generally, O₂ can be catalytically converted into ROS mainly via electron transfer or energy transfer, the former way mainly generated •O₂⁻, H₂O₂ and hydroxyl radicals (•OH), while the latter mainly generated the singlet oxygen (¹O₂) (Nosaka and Nosaka, 2017). Currently, the conversion efficiency of O₂ is greatly limited by the strict dynamics and thermodynamics barriers during the electron or energy transfer processes. And usually, less than 5% O₂ in the medium could be catalytically transformed into ROS, which made the O₂ far away from a good ROS precursor like H₂O₂ or O₃ etc. Therefore, it is quite necessary to develop the novel and efficient catalysts for activating the O₂ into ROS, and to further understand the mechanism overcome those dynamic or thermodynamic barriers.

Recently, single atom catalyst (SACs) have attracted growing attention in environmental remediation owing to its excellent catalytic activity. For example, Ma et al. reported that single cobalt atom could improve the activation of PMS and the oxidation of multiple organic pollutants (Xu et al., 2020). Sun et al. reported that single iron atom could boost fenton-like reactions for degradation of p-hydroxybenzoic acid and phenol (Yin et al., 2019). In these studies, SACs have shown great potential in catalytic AOPs processes, especially due to their atomic dispersion of active centers and special electronic structure. This may make the SACs as good candidates for efficiently activating the O₂ into ROS. However, in the few early reports, the conversion efficiency of O₂ into ROS using SACs is still lower than other ROS precursors, and the productive concentration of ROS is always less than 10⁻¹ mmol•L⁻¹ level (Li et al., 2020a; Zhang et al., 2018). Further analysis showed that this can be mainly ascribed to the small loading amount of single atomic centers (usually less than 1 wt%). Even worse, the unclear activation mechanism of O₂ over single centers also hindered the rational design of the SACs_. Therefore, it is urgent to develop new SACs with high loading of atomic centers for activation of O₂ and to further explore its working mechanism.

In these regards, we established a new type of single atomic Ag-g-C₃N₄ (SAACN) catalyst with a loading of 10 wt% Ag single sites to boost the photocatalytic activation of O₂ for the degradation of tetracycline (TC), a typical antibiotics pollutant (Wang et al., 2016; Xu et al., 2019a, 2019b). 10 wt% loading of nanoparticle Ag-g-C₃N₄ (NPACN) was also synthesized and compared here. For SAACN, the cumulative concentrations of •O₂⁻, •OH, ¹O₂ reached 0.66, 0.19, 0.33 mmol L⁻¹ h⁻¹, respectively, and the conversion efficiency of dissolved O₂ into ROS was 17.24%. While this value is 1.21% when using the NPACN. When additionally feeding air or O_2, the accumulative concentrations of •O₂⁻, •OH, ¹O₂ using SAACN 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). Due to the high concentration of ROS, the SAACN achieved an excellent mineralization rate of 95.7% for TC, which is much higher than that using NPACN (59.9%). Moreover, the mechanism analysis showed that the advances of SAACN can be attributed to the optimized adsorption of O₂ and the facilitated electron transfer pathway surrounding the Ag single sites.