Exploring the mechanism of ZrO₂ structure features on H₂O₂ activation in Zr–Fe bimetallic catalyst

Highlights

• Environment-friendly Fe–Zr bimetallic catalyst has been synthesized.

• Surface hydroxyl of amorphous ZrO₂ could promote HO₂^•/O₂^•- formation.

• High-temperature calcination induces heterojunction formation in Fe–Zr bimetals.

• Iron sites at (1 1 0)-(1 0 0) interface exhibit strongest H₂O₂ activation ability.

Abstract

Bimetallic Fenton catalyst has attracted widespread attention in refractory organics removal. Herein, we firstly investigated the influence of ZrO₂ structure features on H₂O₂ activation in Zr-Fe bimetallic catalyst. The results show that the heterojunction structure will be formed after high temperature calcination, which makes the ability of Zr-Fe bimetallic catalyst to activate H₂O₂ for bisphenol A degradation is 3.1 times higher than that of α-Fe₂O₃ without Zr doping. Through characterization and density functional theory, it was identified that compared to (1 1 0) interface, the adsorption energy of H₂O₂ on iron sites at (1 1 0)-(1 0 0) interface reduced by 1.27 kJ mol⁻¹, while the Fe–O bond length in the stable configuration of Fe-OOH increased by 0.16 Å, which was beneficial to the association and dissociation of H₂O₂ on iron sites. Besides, the surface -OH of amorphous ZrO₂ in Zr-Fe bimetallic catalyst synthesized under low temperature conditions could promote HO₂^•/O₂^•- formation through the surface electron transfer, accelerating the Fe(III) reduction. In conclusion, this study reveals the influence of environment-friendly ZrO₂ structure features on H₂O₂ activation, proposes a new insight into strengthening the synergistic effect between bimetals, and provides a reference for the structural optimization of bimetallic catalysts.

Introduction

Hydroxyl radicals (HO^•) with high oxidation ability (E⁰ = 2.73 V) and strong electrophilic addition properties, are one of the most powerful oxidizing agents, which have a significant effect on the removal of refractory organics in wastewater treatment [1], [2], [3]. Activation of H₂O₂ by transition metals is an effective way to generate HO^• [4], [5], [6]. Among them, the Fenton reaction catalyzed by Fe(II) is considered to be the most economical and effective choice for HO^• generation [7], [8], [9]. However, the traditional Fenton process still faces bottlenecks, such as narrow working pH range and secondary pollution caused by iron sludge, which need to be further improved [10], [11], [12]. In recent years, a large number of researches have been devoted to the development of heterogeneous Fenton processes. But, the cycling rate between Fe(II) and Fe(III) (k = 0.02 M⁻¹ s⁻¹) in the Haber-Weiss process is much lower than that of Fe(II) and H₂O₂ (k = 76 M⁻¹ s⁻¹), which still limits the generation of HO^• [13], [14].

The bimetallic catalytic system can realize the rapid Fe(III) reduction by coordinating the spontaneous electron transfer process inside the catalyst, without external energy supply [15], [16]. In addition to the activation of H₂O₂ by low valence transition metals such as Cu(I), Co(I) and Mn(II), the thermodynamic spontaneous process based on the energy difference between redox potentials can also accelerate Fe(III) reduction, thus promoting the formation of HO^• in the bimetallic catalysts systems [17], [18]. However, the dissolution of heavy metals in this type of bimetallic catalysts can reach several milligrams per liter [19], [20], [21]. And this will not only threaten the safety of water quality, but also increase the cost of subsequent treatment.

Good biocompatibility and stability of zirconium make it can be used in the synthesis of environment-friendly bimetallic catalysts [22]. Since Zr does not possess the characteristics of variable valence state similar to Cu, Mn and Co etc., which has unsaturated 3d orbital, ZrO₂ is only used as a stable carrier to enhance the dispersion of iron oxides in heterogeneous Fenton processes [23], [24]. The dispersing effect and low dissolution rate of ZrO₂ enable the Zr-Fe bimetallic catalyst to efficiently activate H₂O₂ while avoiding the problem of secondary pollution caused by the dissolution of heavy metals [25]. Nevertheless, Zr with fully occupied 3d orbital may also realize the rapid electron transfer inside the catalyst through d⁰ surface electron transfer process [26]. Besides, direct electron supply can also achieve the reduction of H₂O₂, thereby generating active oxygen species [27], [28]. It is inferred that Zr might be able to activate H₂O₂ through the surface electron transfer.

To our knowledge, the activation of H₂O₂ is an interface reaction on the iron site in heterogeneous Fenton processes [29], [30]. And the density functional theory (DFT) calculations show that the interface energies at different crystal planes of iron oxide are different [31], [32]. It means that the association of H₂O₂ on the iron surface will be affected by the exposed crystal faces of the catalyst. But there are few basic researches on the structure features of Zr-Fe bimetallic catalysts. Therefore, the synergistic effect between Zr-Fe bimetals has not been fully understood.

Herein, amorphous-ZrO₂ (a-ZrO₂) and monoclinic-ZrO₂ (c-ZrO₂) were prepared and used as the components of Zr-Fe bimetals to explore the different structure features of zirconium oxides in heterogeneous Fenton processes. This study reveals the relationship between Zr-Fe bimetals structure features and H₂O₂ activation, and provides a new insight into strengthening the synergistic effect between bimetals. In addition, ZrO₂ has a stable structure, and zirconium, as an environment-friendly metal, can also effectively avoid potential secondary pollution, which further expands the application of heterogeneous Fenton catalysts in the field of environment remediation.