Rational design of copper encapsulated within nitrogen-doped carbon core-shell nanosphere for efficiently photocatalytic peroxymonosulfate activation

Abstract

Photocatalysis utilizing solar energy is a promising strategy for mitigating energy crisis and environmental pollution. Exploring a cost-effective, stable, eco-friendly, and efficient photocatalytic system is extremely urgent. Herein, copper encapsulated within nitrogen-doped carbon nanosphere (Cu@NC) possessing a unique core-shell structure with high catalytic activity was prepared by ion-exchange and pyrolysis using resin as support. The protective carbon shell can prevent the leaching of metal ions and deactivation of the catalyst. Benefiting from the special structure, Cu@NC exhibited excellent activity and durability toward the degradation of tetracycline by the activation of peroxymonosulfate (PMS). The radical trapping experiments and electron spin resonance analyses were applied to elucidate the main reactive species. This work highlights the great potential of Cu@NC core-shell nanosphere as photocatalyst, which provides a new opportunity for the remediation of environmental pollution.

Introduction

Photocatalytic technology plays a vital role in realizing counteracting energy storage and environmental remediation via solar energy. Up to now, the enormous researches trigger interests to explore efficient photocatalysts for renewable photocatalytic treatment [1], [2], [3], [4], [5], [6]. However, the sufficient utilizing of solar energy and elucidating relevant reaction mechanism are still grand challenges [7], [8], [9]. Therefore, it is crucial to explore novel-structure photocatalysts with competitive or even superior performance for environmental applications.

Semiconductor-based photocatalytic is a promising strategy to eliminate refractory contamination in wastewater [10], [11], [12], [13], [14]. This challenging technique requires photosensitive semiconductors to achieve visible-light driven high charge transfer kinetics. As a novel class of semiconductor photocatalyst, metal nanoparticles supported on carbon-based materials have been regarded as the appealing candidate on account of the tunable surface chemistry, modified electronic structure, and optimal photocatalytic activity [15], [16]. The surface carbon layer in the unique encapsulation structure can prevent the leaching of metal ions and avoid the deactivation of catalyst in the catalytic process [17]. Moreover, the addition of carbon can accelerate the transfer and separation of electrons and boost the reduction of high valence transition metal sites in the reaction process [18]. Accordingly, encapsulating metal or oxide in the carbon support is an ideal strategy for designing advanced photocatalysts.

In recent years, advanced oxidation processes (AOPs) as a fully decomposition process of recalcitrant organic pollutants have garnered tremendous attentions [19], [20], [21], [22], [23], [24], [25]. It utilizes superoxide or oxidant (H₂O₂, S₂O₈²⁻, HSO₅⁻ and ozone) to generate reactive oxygen species (ROS) for achieving complete decomposition of organic pollutants. In this regard, the hydroxyl radical (^∙OH) generated from H₂O₂ through Fenton reaction, as the classic reactive species in AOPs, has been applied to degrade various organic contaminants [26], [27], [28]. However, some drawbacks, such as relatively low efficiency of reaction and poor stability of H₂O₂, still limit its practical applications. Compared with H₂O₂, sulfate radical (SO₄^∙−) via the activation of PMS have the remarkable advantages for degrading pollutants including lower price, better chemical stability, higher redox potential, and longer life-time period [29], [30], [31]. Zhu et al. reported Co₃O₄ nanoparticles modified g-C₃N₄ composites (Co₃O₄-g-C₃N₄) as visible-light responsive photocatalyst, which displays a synergetic effect for PMS activation to degrade diclofenac sodium [32]. Suib et al. proposed a photo-assisted PMS activation approach using cobalt doped mesoporous iron oxide with exceptional activity and stability up to 7 cycles [33]. Although metals such as Co and Fe have high catalytic activity, their practical application is still hindered due to toxicity and difficulty of recycle [34]. Copper-based materials are one of the most prominent catalysts for PMS activation because of low-cost, availability, and low toxicity compared with many other transition metals [35]. However, there still remain some challenges to further increase the activity and stability through a strong interaction with the support, which is crucial to obtain highly efficient catalysts.

The emerging carbon-based catalysts have been extensively studied owing to the large surface area and high conductivity, and regarded as ideal alternatives for PMS activation [36], [37], [38], [39]. In particular, metal@carbon hybrids bear the properties of each component and can produce persulfate activators that enable outstanding performance independent of the precursor type. Further, the doping of N atom in the carbon matrix can significantly improve the catalytic performance of PMS activation, which increases the alkalinity of PMS surface adsorption and facilitates electron transfer reaction by activating the adjacent sp² carbon atoms [36]. Thus, the development of a low-cost and effective strategy to synthesize well-defined metal@nitrogen-doped carbon for PMS activation is highly desirable but still a huge challenge.

Herein, using low-cost and widely available resin as the precursor, N-doped graphite carbon structure anchoring metals was synthesized, which has the merits of simple preparation and easy accessibility. Notably, the unique core-shell structure can provide enhanced contact area and abundant metallic Cu active sites. The encapsulation structure of the surface carbon layer prevents the leaching of metals in the catalytic process, which endows the catalyst with excellent activity and durability. Owing to the above advantages, Cu@NC can be regarded as promising heterogeneous catalyst. The catalytic performance of Cu@NC by the activation of PMS was assessed via taking a typical antibiotic tetracycline as the target pollutant, and the photocatalytic degradation efficiency was up to 97% within 15 min. Based on the radical quenching experiments and electron spin-resonance (ESR) analyses, photogenerated holes (h⁺), hydroxyl radical (^∙OH), sulfate radical (SO₄^∙−) and superoxide radical (^∙O₂⁻) are identified as the critical active species. This work presents a promising strategy for the degradation of pollutants through PMS activation.