Regulating electron distribution of Fe/Ni-N₄P₂ single sites for efficient photo-Fenton process

Highlights

• CN-FeNi-P-induced photo-Fenton system reaches 95.9 % TOC removal in MOX degradation.

• More •OH is formed with the aid of the photogenerated electron.

• P-bridging single-atoms make Ni atoms serve as the optimal sites.

• Bi-single-atom structure and enhanced electron transfer lower the reaction barrier.

Abstract

Regulating local electron density by introducing single-atom is an effective strategy to improve the activity of heterogeneous photo-Fenton processes. Here N, P coordinated Fe and Ni single-atom catalysts on carbon nitrides (CN-FeNi-P) were prepared to activate H₂O₂ for contaminant mineralization under visible light irradiation. The as-prepared CN-FeNi-P presented a higher moxifloxacin degradation activity in photo-Fenton system, which was up to 3.7 times that of pristine CN, meanwhile, its TOC removal reached to 95.9 % in 60 min. Based on density functional theory calculations, the Ni single-atoms serve as the optimal reactive sites to produce •OH. The strong interaction between Fe and Ni single-atoms by P-bridging and the modulated local electron structure after introducing P into coordination environment can lower •OH formation energy. This study provides new doping strategies to design single-atom catalysts and expands the family of the Fenton-like system for advanced oxidation technologies.

Introduction

Photocatalytic degradation is a promising technology in environmental remediation because of its inexhaustible driving force from solar light and green process (Chen et al., 2020a, Li et al., 2021, Yu et al., 2021, Zhou et al., 2021, Zou et al., 2016). However, slow catalytic efficiency and low mineralization degree of photocatalytic technology are key challenges that limit its practical development and application. Recently, heterogeneous Fenton-like reactions are promising alternative strategies to address the above-mentioned issue. Compared to traditional (homogeneous) Fenton systems and photocatalytic degradation, heterogeneous Fenton-like reactions could avoid the production of massive sludge for post-treatment, and present an outstanding cyclic stability and wide applicability on the basis of ensuring the catalytic rates. For conventional heterogeneous Fenton-like reactions, it is mainly driven by the complexing transition metal (Fe, Cu Co, and Ni) species on the surface of catalysts via activation of H₂O₂ (Miao et al., 2020, Zhang et al., 2022, Zhu et al., 2019, Thomas et al., 2021, Li et al., 2022b), which are described as follows using iron as an example. In the reactions, the low regeneration efficiency of Fe(III) species, the rate-limiting step, limit its application.

Fe(II) + H₂O₂ → Fe(III) + •OH + OH^-

Fe(III) + H₂O₂ → Fe(II) + HO₂• + H⁺

Photo-driven heterogeneous Fenton catalysis is capable to excite photogenerated electrons to activate H₂O₂. The introduced optical irradiation could enhance the production of •OH by direct and indirect H₂O₂ activation ((3), (4), (5)). Previous studies show that multi-active sites, such as ZnO@FePc (Qian et al., 2021) and Cu-Fe bi-metal oxide quantum dots/g-C₃N₄ nanosheets (Liu et al., 2022), are formed in heterogeneous photo-Fenton systems and synergistically improve catalytic activity. The formed photocatalytic sites and Fenton-like sites could contribute to the adsorption and activation of H₂O₂ and lead to multiple synergies of photocatalytic degradation and Fenton oxidation.

H₂O₂ + hυ → •OH

Catalyst + hυ → h⁺ + e⁻

h⁺ + H₂O/OH⁻ → •OH

Recently, single-atom catalysts are attracting increasing attention in catalytic fields, owing to its utmost utilization and formation reactive centers with high electron density. Inspired by the outstanding characteristics of atomically dispersed catalysts, single-atom-mediated photo-Fenton systems are widely investigated (Zhang et al., 2018, Zhang et al., 2021, Li et al., 2020, Li et al., 2022a, Liu et al., 2020, Jiang et al., 2020). Introducing of Fe species into graphitic carbon nitride (g-C₃N₄) is a common strategy to construct Fe-N₄ coordination sites in heterogeneous Fenton-like reactions. Previous studies show that the Fe single-atoms could serve as the electron centers to improve the H₂O₂ conversion efficiency (Su et al., 2021). Further modifications by strong π-conjugated system and nitrogen vacancies could promote the generation and transportation of the charge carriers as well as the redox circulation of the Fe sites. Moreover, various transition metal single-atom sites (Cr, Mn, Fe, Co and Cu) coordinated with N atoms are reported. Among them, Cr single-atom catalysts exhibit excellent performances for H₂O₂ activation and bisphenol degradation attributed to the synergy of photocatalysis and single-atom catalysis (Chen et al., 2021). Therefore, it is crucial to develop a facile and scale-up approach to synthesize transition metal single-atom catalysts for high-effective contaminant degradation.

In our previous study, N and P coordinated bimetal single-atom catalysts on carbon nitrides (CN-FeNi-P) were prepared and presented highly catalytic degradation for enoxacin under visible light irradiation (Zhan et al., 2022). To furtherly improve catalytic efficiency, in this work, a feasible strategy was designed to construct a photo-Fenton-induced system using the as-prepared CN-FeNi-P catalysts. The effects of H₂O₂ dosage, catalyst dosage and initial concentration on the catalytic performance were systematically investigated. The active species were identified by quenching experiments and electron spin resonance (ESR) spectroscopy. The degradation mechanism of CN-FeNi-P-induced photo-Fenton was revealed by density functional theory (DFT) calculations. Moreover, the application performance, including various solution pH, anion, cation and catalytic stability were studied.