Reduced graphene oxide-supported metal organic framework as a synergistic catalyst for enhanced performance on persulfate induced degradation of trichlorophenol

Highlights

• MIL-101(Fe) and RGO had synergistic catalytic effect on degradation of pollutants.

• Defects as the catalytic sites on RGO were retained during the reduction process.

• The large π conjugate plane structure was repaired for efficient electron transport.

• Non-radical pathway was induced in this system except for free radical process.

Abstract

In this work, reduced graphene oxide/metal organic framework composites (RGO/MOF) have been fabricated for the purpose of activating persulfate (PS) successfully first time. Benefiting from the abundant active sites of composites and the excellent electron conductivity arising from repaired large π conjugate plane structure, RGO/MIL-101(Fe) performed better than RGO and MIL-101(Fe) for PS activation and organic compounds degradation from aqueous. The physical-chemical properties of composite catalysts were fully characterized and the applications to the catalytic degradation of trichlorophenol (TCP) were evaluated. The results showed that RGO/MIL-101(Fe) could effectively degrade TCP, under the reaction conditions of pH 3.0, 20 mg/L TCP, 20 mM PS, 0.5 g/L catalyst, and the removal efficiency is 92% in 180 min. Furthermore, chemical reduction and thermal process played key role in regulating defect levels and electron transfer channels. The obtained adsorptive and conductive graphene allow rapid electron transport between free radicals and enriched contaminants. These advancements of the structure and chemical properties were beneficial to improve the catalytic activity in the activation of PS. Finally, a possible activation mechanism was also investigated, which involved the prevailing free radical pathway and recessive non-radical pathway.

Introduction

Sulfate radical-based advanced oxidation process (SR-AOP) has been widely investigated in recent years, which uses PS as the initiator to generate sulfate radicals ($\text{S}{\mathrm{O}}_4^{.-}$) for fully decomposing emerging pollutants in aqueous (Sun et al., 2016). The hydroxyl radical (∙OH) can destroy the structure of organic pollutants with the reduction potential of 1.9–2.7Vvs.NHE. Compared with ∙OH, ${\text{SO}}_4^{.-}$ has equal or even higher reduction potential (2.5–3.1Vvs.NHE) (Wang and Wang, 2018). Apart from this, ${\text{SO}}_4^{.-}$ possesses the high selectivity to organics with unsaturated bond and aromatic structure (Olmez-Hanci and Arslan-Alaton, 2013). It illustrates more competent performance than ∙OH in degrading organic compounds into CO₂ and H₂O. Moreover, ${\text{SO}}_4^{.-}$ exhibits a more stable structure, comparatively long half-life, and higher effectiveness for destructing harmful contaminants over a wide pH range of 2.0–8.0 (Oh et al., 2009). Therefore, SR-AOP has received increasing attention. ${\text{SO}}_4^{.-}$ is mainly generated by the activation of PS using coupling transition metal ions (Anipsitakis et al., 2006), and iron ions turns out to be best activator. However, since homogeneous system exists the disadvantages including the instability of generating radicals and the adversity in catalyst recycling, researchers develop heterogeneous iron-based catalysts, such as zero-valent iron and iron oxide (Li et al., 2014; Ding et al., 2013). However, for currently popular catalysts iron-containing oxides, the control of producing free radicals, iron leaching and timely use of produced free radicals are far from satisfactory for actual application. The innovation of Fe-based catalysts for generating free radicals controllably while exhibiting excellent catalytic degradation ability is timely and particularly important.

Iron-based metal-organic frameworks (Fe-MOFs) can be treated as promising heterogeneous catalysts applied in SR-AOP field, such as MIL-53(Fe)、MIL-88A and MIL-100(Fe) (Pu et al., 2017; Li et al., 2016; Liu et al., 2017). To date, they can be promising heterogeneous catalysts for the degradation of pollutants, owing to the dispersed active sites and chemical composition stability (Yang and Gates, 2019). MIL-101(Fe) has been proved to have dispersed metal sites tied in frameworks and chemical stability, these properties can make up radicals generating uncontrollably and iron leaching defects (Li et al., 2016). However, compared with other heterogeneous materials such as zero-valent iron (Ezzatahmadi et al., 2017), Fe₃O₄ (Xu and Wang, 2012) and CuFe₂O₄ (Ding et al., 2013), pure MIL catalysts contain a low content of iron-containing unsaturated active sites, which possess only ${\mathrm{Fe}}^{\mathrm{Ⅲ}}$ sites with weak activity (Yue et al., 2016); compared with carbon materials such as activated carbon, graphene and carbon nanotubes, Fe-MOFs are facing the limitations with single channel structure and easy aggregation resulting in problems on reactive oxygen species transmission and utilization (Jabbari et al., 2016; Wu et al., 2017). In order to overcome difficulties on the catalytic activity of MOFs, the key problems need to be solved are increasing the active sites of pure MOFs for generating radicals and improving the utilization of radicals to accelerate the catalytic degradation process.

In order to improve the utilization of free radicals, they should be transported to the vicinity of target molecules immediately after the generation process, which can reduce radicals quenching and achieve efficient degradation of contaminants. Graphene is used in combination with MOFs to obtain a highly efficient photocatalyst because of its good electrical conductivity (Shen et al., 2014). The negatively charged surface of RGO can be formed during the synthesis process and the introduction of epoxy and hydroxyl groups allow RGO to participate in a wide range of bonding interactions. MOFs skeleton possess the uniformly distributed catalytic active sites and quite a good range of combined manners between metal ions and organic ligands (Ai et al., 2014). These characteristics make MOFs as the potential component of composite materials (Liu et al., 2017). Also, the pore structure is adjustable and the specific surface area can be modified after combining with 2D species. Previous studies have shown that the RGO and MOFs exhibit semiconducting behavior, which can activate PS to generate free radicals respectively (Kang et al., 2016). However, the combination of MOFs with RGO as a selective catalyst applied in SR-AOP has not been explored until now. Therefore, the incorporation of extra active sites in pure MOFs while improving the electron transport between pollutant molecules and radicals seems to be a feasible strategy. It may solve the problems of free radicals release caused by insufficient active sites and efficient radicals utilization in degrading pollutants rapidly.

Herein, we first focus on the fabrication of RGO/MIL-101(Fe) composites and, specifically, their application in the environment restoration. We fabricated RGO/MIL-101(Fe) via one-pot solvothermal approach, and studied the synergistic effects of the composite catalyst on enhanced catalytic activity. The physical-chemical characters of RGO/MIL-101(Fe) and the key role of surface interaction between MOFs and RGO sheet on catalytic activities were evaluated by various methods. We explored the effect of the incorporation of graphene sheet used as the support on MOFs crystal growth and the electron transport of RGO/MIL-101(Fe) to present the superiority of graphene. Subsequently, the catalytic performances of the composite were studied with TCP as the target pollutant, as well as the influence of reaction parameters on catalytic activity. The effective reactive species involved in the PS/RGO/MIL-101(Fe) system were identified by quenching experiments and electron paramagnetic resonance (EPR) spectroscopy to explain the PS activation mechanism.