Regulable metal-oxo-bridge configurations as electron transfer bridge to promote Cu(II)/Cu(I) redox behavior for efficient peroxymonosulfate activation

Highlights

• Regulable Cu–O–C configuration was constructed as electron transfer bridge.

• Doped oxygen resulted in high dispersed Cu atoms joint with oxygen in p-CN matrix.

• Rapid redox behavior of Cu(II)/Cu(I) in 2:1 CuAl@p-CN for excellent PMS activation.

• Porous structure of p-CN was in favor of active species migration for catalysis.

• Efficient removal of mirco-polluted veterinary antibiotics in water was achieved.

Abstract

Constructing metal-oxo-bridge coordination as electron transfer bridge is benefit for the catalysis acceleration towards water pollutants treatment. In this work, regulable Cu–O–C configuration was constructed through tunable ratio of copper aluminum oxide (CuAl_xO_y) and porous g-C₃N₄ (p-CN), employed as electron transfer bridge for Cu^(II)/Cu^(I) redox behavior modulation to realize sufficient peroxymonosulfate (PMS) activation. A series of characterization demonstrated that the composites presents porous structure for p-CN contribution, and the oxygen content plays crucial role in Cu–O–C bonds fabrication. The proper ratio of CuAl_xO_y and p-CN is conducive to create an oxygen-rich environment, resulting in high dispersed copper atoms connect with oxygen in p-CN matrix. As a result, 2:1 CuAl@p-CN possesses appropriate Cu–O–C configuration, has favorable electron transfer environment for rapid redox behavior of Cu^(II)/Cu^(I), leading to excellent PMS activation for mirco-polluted veterinary antibiotics elimination. The activated PMS produced active species and followed the priority order of ¹O₂ > SO₄•⁻ > •OH for contaminants degradation, and the specific porous structure reduce the migration distance of active species for efficient catalysis. This study offers a deeper insight into the construction of regulable metal-oxo-bridge configurations for electron transfer towards PMS activation and contributes to an efficient strategy for wastewater remediation.

Introduction

Heterogeneous catalysis of peroxymonosulfate (PMS) activation has received considerable attention in recent years for its asymmetric structure as efficient electron acceptor. Produced sulfate radicals (SO₄•⁻) from the activation of PMS possess much higher redox potential (E₀(SO₄•⁻/SO₄•²⁻) = 2.5–3.1 eV) and greater adaptive capability contrast to •OH (E₀(•OH/H₂O) = 1.9–2.7 eV) (Duan et al., 2018, Zhang et al., 2019b), resulting in superior degradation performance for refractory organic pollutants (Gao et al., 2018). The homolytic cleavage of the peroxide bond (O–O) in PMS can be realized by applying high energy or using chemical activators. Numerous exciting advances including UV irradiation (Zeng et al., 2020), electrochemistry-assisted (Li et al., 2018b, Yan et al., 2017) and ultrasound irradiation (Liu et al., 2017) have been developed to effectively activate PMS. In addition, some research about transition metal oxides (Co, Cu, and Mn, etc.) are generally reported as activators for PMS activation (Huang et al., 2017, Ren et al., 2015). Nevertheless, these methods suffered from the consumption of additional energy input or released toxic ions problems, depressed the development of enhanced catalysts with adequate stability and suppressed the practical application (Jaafarzadeh et al., 2017, Wang et al., 2018). Thus, approaches to develop stabilized catalysts with efficient catalysis need to be conducted to accelerate the interfacial electron transfer for the radical release from PMS cleavage.

Previous reports manifested that minimizing the migration distance of electron-migration channel and the reactive radicals to the target organic molecules are desirable for maximizing the catalysis, and the coordination environment is crucial for the distributions of dispersive electrons of the active components in the catalyst (Li et al., 2018c). In such transition metal-activated reaction, the rate-limiting step is the reduction of M^(n+m)+ to Mnⁿ⁺ due to the low transportation rate of electrons (Xu and Wang, 2012). Therefore, aiming at acceleration of electron transfer rate to realize the rapid redox cycle of such multivalent transition metals are meaningful for PMS activation enhancement (Jiang et al., 2018). Zang et al. (2019) reported Cu-N₂ coordination, which identified as efficient active sites to achieve highly efficient electrocatalysis. Ma et al. (2017) synthesized heterogeneous Fe-N complexes and the Fe^(III)/Fe^(II) redox cycle was accelerated by enhancing electron transfer via Fe-N active sites. Lyu et al. (2017) successfully incorporated Cu and Co into g-Al₂O₃ to form the δ-type Cu–O–C bridge, which can establish an electron-rich area around Cu ascribed to the enhanced Cu-π interaction to activate H₂O₂. To summarize, these catalytic processes evidenced that constructing metal-involving coordination as electron-migration bridge is benefit for the catalysis acceleration. Not only that, the stability could be facilitated by building a bond bridge for the superior metal riveting on the superficial of the catalyst (Yu et al., 2020). However, how to achieve effective regulation of these metal-involving bond bridges to enforce accurate and effective rapid redox behavior of metal species has rarely been researched.

To realize regulable metal-involving configuration, it is crucial to select solid matrix and suitable metal species. Two-dimensional (2D) materials possess unique structure and electronic properties. The interactions in 2D materials are one of the most important intermolecular binding forces, which are relevant for aromatic recognition in chemical process (Lyu et al., 2017). Moreover, the considerable changes in the electronic structure of 2D materials, as well as the possibility of chemical and structural modifications, can provide new opportunities in catalysis (Deng et al., 2016). Copper, commonly utilized as an efficient active metal, presents excellent performance in heterogeneous catalysis (Du et al., 2015, Li et al., 2018a). It could be possible to anchor Cu on the surface of 2D material for the fabrication of copper-containing bridge. Besides, inserting Al into the lattice oxygen of copper oxide can lead to uneven electrons distribution on the surface, derived from the much lower electro-negativity of Al than Cu. More than that, a large electron density around Cu could be formed for the decrease in the coordination of Cu due to Al addition (Lyu et al., 2018). For 2D materials selection, g-C₃N₄ consists of tri-s-triazine with typical π-conjugated graphitic planes, possesses tunable electronic properties and excellent chemical stability (Xu et al., 2018b), which is a decent choice to incorporate metallic elements into g-C₃N₄ framework for inducing delocalized electrons (Qu et al., 2017). The construction of unique porous structure of g-C₃N₄ can provide readily accessible channels and more surface-active sites for reaction process (Liang et al., 2015). Recent reports illustrated that the addition of (NH₄)₂S₂O₈ in the dicyandiamide calcination process can successfully achieve effective simultaneous control of oxygen dopants and nitrogen defects on the porous g-C₃N₄ surface (Jiang et al., 2019). The (NH₄)₂S₂O₈ could separate dicyandiamide crystal grains and dicyandiamide derived intermediates in space and suppressing the formation of integrated tri-s-triazine structure, and the decomposed O₂ would deep into the framework, resulting in morphological and structural variation in contrast to bulk g-C₃N₄. The doped oxygen could be in favor of the porous structure construction and the metals complexed with oxygen ligands, adjustable tune the electronic properties of catalysts to promote catalytic activity. Moreover, moderate nitrogen defects would form a unique chemical environment, benefited to disperse metal particles on the ligand (Fu et al., 2017, Yu et al., 2017). Inspired by above consideration, it is desirable to make use of the porous oxygen-doped g-C₃N₄ as a polymeric ligand, and introduce copper aluminum oxide into it for complexation with oxygen to build a metal-oxo-bridge for electron transfer acceleration.

Hence, in this work, we developed a facile method for the fabrication of Cu–O–C configuration. Regulable Cu–O–C bonds construction was achieved by tunable the composition ratio of copper aluminum oxide (CuAl_xO_y) and oxygen-doped porous g-C₃N₄ (p-CN). We suppose to use the constructed Cu–O–C bonds as electron transfer bridge to achieve rapid redox behavior of Cu^(II)/Cu^(I) towards PMS activation for pollutants contamination. Through morphological and structural characteristic, we compared the influence about composition and surface chemical properties of the synthesized samples to get clear the architecture of Cu anchoring on the p-CN surface and search reasons for the regulable of Cu–O–C configurations formation. Besides, Cu^(II)/Cu^(I) redox behavior of different proportion samples was studied and discussed through solid-ESR, H₂-TPR, EIS and XPS analysis. To evaluate PMS activation efficiency, chloramphenicol (CAP), thiamphenicol (TAP), florfenicol (FF) (Chemical properties and textural characteristics are shown in Table S1) were selected to as targeted pollutants, which can be accumulated by food chain in surface water and further result in more potential hazards to aquatic ecosystems and human health for the transmission of antibiotic resistance genes (ARG) to pathogenic microbes (Martinez, 2009, Sapkota et al., 2008). The stability and reusability of prepared samples were also explored. The possible potential mechanisms of enhanced catalysis by rapid electron transfer via regulable Cu–O–C configuration for efficient Cu(II)/Cu(I) redox circulation were proposed. We hope that this study can afford a new direction of PMS activation from the perspective of metal-oxo-bridge construction as electron transportation to achieve excellent Cu(II)/Cu(I) redox behavior.