Bismuth MOFs based hierarchical Co₃O₄-Bi₂O₃ composite: An efficient heterogeneous peroxymonosulfate activator for azo dyes degradation

Highlights

• Bismuth MOF was first used in the preparation of peroxymonosulfate activator.

• Co₃O₄ nanoparticles on Bi₂O₃ surface show a honeycomb-like morphology.

• The interaction between Bi and Co optimizes the activation performance of Co₃O₄.

• •O₂⁻ and ¹O₂ are proved to be the dominant radical species in the systems.

Abstract

A hierarchical Co₃O₄-Bi₂O₃ composite was first prepared via an adsorption-calcination method using a bismuth-based MOF (Bi-BTC) as the precursor and substrate. In the composite, Co₃O₄ nanoparticles are loaded on the outer layer of Bi₂O₃ rods derived from Bi-BTC and show a uniform honeycomb-like morphology. In dyes degradation tests, the Co₃O₄-Bi₂O₃ composite exhibits much better peroxymonosulfate (PMS) activation performance in RhB degradation than pure Bi₂O₃ or Co₃O₄. The Co₃O₄-Bi₂O₃/PMS system also shows high performance for various azo dyes and maintains good catalytic performance at wide pH range. Besides, the good chemical stability of Co₃O₄-Bi₂O₃ ensures it good reusability in four recycling tests. XPS and electrochemical analysis indicate the interaction of Bi species promotes the activation capacity of Co₃O₄ nanoparticles. The results of radicals quenching tests and ESR analysis reveal the singlet oxygen and superoxide radicals are the dominant functional radical species in Co₃O₄-Bi₂O₃/PMS system and subsequently a reasonable activation mechanism is proposed.

Introduction

The discharge of organic contaminants such as azo dyes and other by-products in chemical industry has posed a great threat to the ecosystem over past decades [1], [2], [3], [4]. Among the various methods handling the organic pollution, Advanced Oxidation Processes (AOPs) shows a clear superiority over other methods on account of its efficient removal capacity for organic pollutants and extensive scope of application [5], [6], [7], [8]. Compared with the H₂O₂ based AOPs systems, the peroxymonosulfate (PMS) shows obvious superiority in transportation and storage, and behaves high oxidation performance in wide pH range [9], [10], [11], [12], so catalysts which can activate PMS in the aqueous solution have attracted extensive interests in recent years. Among the various PMS based catalytic systems, Co₃O₄ nanoparticles-PMS system has shown an excellent activation performance because the Co-OH complex on the surface of Co₃O₄ can effectively activate PMS in the heterogeneous system, and the Co^II/Co^III cycle makes this reaction a continuous and efficient process [13], [14], [15]. However, the Co₃O₄ nanoparticles still show some problems that need to be optimized in practical wastewater treatment. The nano-sized Co₃O₄ particles tend to agglomerate in the synthesis process which would greatly reduce their activation performance due to the mutual coverage of active sites. Besides, their nanoscale size also hinders the solid–liquid separation and reuse in practical process [16], [17]. Loading the functional nanoparticles on appropriate substrates is a promising strategy that can optimize catalytic performance and reusability of nanoscale heterogeneous catalysts [18], [19].

Metal–organic frameworks (MOFs) have aroused increasingly attention in a wide range of applications over the past decades due to their distinct crystalline, uniform morphologies and high specific surface areas [20]. All these features allow MOFs to be ideal substrates to construct novel heterogeneous catalysts. Currently, studies about MOFs are mostly focused on transition metals and lanthanides, including Cu [21], Zn [22], Co [23] species, and so on, while Bismuth which is a typical main group metal showing flexible coordination geometry for MOFs construction is rarely studied. Recently, several researches [24], [25], [26], [27] has proved that Bi species can effectively promote the activity and mobility of lattice oxygen in Co₃O₄ and further enhance the catalytic activity of Co species. Inspired by this, Bismuth MOFs shows its superiorities as the substrate of Co₃O₄ nanoparticles because it can optimize the activation performance for PMS and reusability of Co₃O₄ nanoparticles simultaneously. To date, no Co₃O₄ involved composites based on Bismuth MOFs have been reported.

Herein, a new Bismuth MOF ([Bi(BTC)(DMF)]•DMF•(CH₃OH)₂, termed as Bi-BTC), which has been prepared and used in several fields [28], [29], was used in this work and a novel hierarchical Co₃O₄-Bi₂O₃ composite was synthesized from Bi-BTC for the first time, and used as PMS activator for Azo dyes degradation in wastewater. The Bi-BTC acted as the substrate of the composite and the precursor of Bi₂O₃, and the Co₃O₄ nanoparticles were loaded on the outside of the Bi–BTC through a facile Co (II) ion adsorption process. After calcination simultaneously, Co₃O₄-Bi₂O₃ composite was prepared with a hierarchical honeycomb-like morphology. It was found that the Co₃O₄-Bi₂O₃ composite showed enhanced and stable activation capacity for PMS to degrade azo dyes. In addition to the detailed structural characterizations, the enhancement mechanism of the degradation reaction and the interactions between Co₃O₄ and Bi₂O₃ were also studied.