Boosting the activation of molecular oxygen and the degradation of tetracycline over high loading Ag single atomic catalyst
Graphical abstract
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 O2, 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.
Section snippets
Synthesis of catalyst
SAACN was synthesized via facile thermo-polymerization method: First, a reactive comonomer, silver tricyanomethanide (AgTCM) was prepared via the reported method (Chen et al., 2016). Then, 0.6 g of AgTCM was dissolved in 20 mL of ultrapure water, together with 6 g of melamine and 30 g of ammonium chloride. The mixture was stirred at 70 °C until the water was completely removed. Then, the mixture was transferred into a 50 mL porcelain crucible with cover and heated at 550 °C for 3 h in a muffle
Microstructure and chemical nature of catalysts
The morphology of SAACN was observed by transmission electron microscopy (TEM). The TEM image (Figure S2a) indicated that the obtained sample possessed a highly open porous structure, meanwhile Ag nanoparticles were not detected. The well dispersed Ag atoms could be directly monitored by spherical aberration STEM (Fig. 1a, b). The Ag atoms were confirmed by isolated bright dots in the high-magnification HAADF-STEM image. As elucidated in Fig. 1c (zone 1 and 2 are enlarged and shown in Figure
Conclusion
Via advanced oxidation technology, reactive oxygen species (ROS) can be generated, efficiently oxidized and mineralized the degradation-resistant organic compounds (Wang et al., 2018, 2020b; Xiao et al., 2020). Compared with current ROS precursors, molecular oxygen is very potential due to its high abundance, green and economic (Chen et al., 2020; Zhang et al., 2021). However, based on the existing methods, the activation efficiency of molecular oxygen is still too low to be applied. In this
Supporting Information
Additional information about TEM images, XRD patterns, Nitrogen adsorption/desorption isotherms, Elemental mapping and TGA curves of SAACN and NPACN, FT-IR spectra, Raman spectrum, ESR spectra, Steady-state and time-resolved PL spectrum, UV–vis DRS spectrum, Transient photocurrent curves of the SAACN and NPACN and other additional details and experiment methods.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financially support by the Natural Science Foundation of China as general projects (grant Nos. 22076082, 21874099 and 21872102) and the Tianjin Commission of Science and Technology as key technologies R&D projects (grant Nos. 18ZXSZSF00230,19YFZCSF00740 and 20YFZCSN01070).
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