A green edge-hosted zinc single-site heterogeneous catalyst for superior Fenton-like activity

Abstract

Developing green heterogeneous catalysts with excellent Fenton-like activity is critical for water remediation technologies. However, current catalysts often rely on toxic transitional metals, and their catalytic performance is far from satisfactory as alternatives of homogeneous Fenton-like catalysts. In this study, a green catalyst based on Zn single-atom was prepared in an ammonium atmosphere using ZIF-8 as a precursor. Multiple characterization analyses provided evidence that abundant intrinsic defects due to the edge sites were created, leading to the formation of a thermally stable edge-hosted Zn-N₄ single-atom catalyst (ZnN₄-Edge). Density functional theory calculations revealed that the edge sites equipped the single-atom Zn with a super catalytic performance, which not only promoted decomposition of peroxide molecule (HSO₅^-) but also greatly lowered the activation barrier for ^•OH generation. Consequently, the as-prepared ZnN₄-Edge exhibited extremely high Fenton-like performance in oxidation and mineralization of phenol as a representative organic contaminant in a wide range of pH, realizing its quick detoxification. The atom-utilization efficiency of the ZnN₄-Edge was ~10⁴ higher than an equivalent amount of the control sample without edge sites (ZnN₄), and the turnover frequency was ~10³ times of the typical benchmark of homogeneous catalyst (Co²⁺). This study opens up a revolutionary way to rationally design and optimize heterogeneous catalysts to homogeneous catalytic performance for Fenton-like application.

Keywords: Fenton-like process; coordination environment; edge sites; peroxymonosulfate; single-atom catalysts.

Fig. 1.

Synthetic scheme and morphology characterizations of the ZnN₄–Edge catalysts. (A) Illustration of the preparation process of ZnN₄–Edge. (B) TEM image; (C) AC HAADF-STEM and the enlarged image (D) of ZnN₄–Edge. (E–H) HAADF-STEM image (E) and corresponding EDS maps of C (F), N (G), and Zn (H) of the area (scale bar, 50 nm).

Fig. 2.

Characterization of porous structure and defects of Zn–N₄ catalysts. (A) The DFT pore-size distribution of Zn/NC and Zn–N₄ catalysts fitted with N₂ adsorption and desorption isotherms. (B) The Raman spectrum of ZnN₄ and ZnN₄–Edge exhibited broad D and G peaks, revealing disordered carbon structures. (C) EPR spectra of ZnN₄ and ZnN₄–Edge samples.

Fig. 3.

Atomic structural analysis of Zn–N₄ catalysts. (A) Fourier-transformed magnitudes, FT|k³χ(k)|, of the experimental Zn K-edge EXAFS signals of Zn foil, ZnO, and Zn–N₄ catalysts. (B) WT curves of the ZnN₄–Edge, Zn foil, and ZnO. The color in the contour figure indicates the moduli of the Morlet WT. (C) The corresponding EXAFS fitting curves in R-space. The fitting curve signal (red line), and the experimental signal (gray circle). (D) Zn K-edge XANES spectra of the Zn foil, ZnO, and Zn–N₄ catalysts.

Fig. 4.

Catalyst performance and correlation between ROS concentration and phenol removal efficacy. (A) The removal rate against organic contaminants of Zn/NC, Zn–N₄ nanocatalysts. (B) Comparison of the kinetics of organic contaminants removal based on metal active sites. [Inset: TOF, per metal atom basis. (Co²⁺) = 0.268 μM was added to achieve the same metal concentration]. (C) Experimental EPR spectra of DMPO radical adducts formed in aqueous solutions over the ZnN₄-Edge with different reaction times. Both spectra in the figure are ROS radicals trapped by DMPO forming a DMPO-SO₄ adduct (* in the figure), as well as a DMPO-OH adduct (circle symbols in the figure). (D) Steady-state concentrations of different radicals in different PMS systems. (E) Correlation analysis between the removal of phenol through the reactor system and the relative amount of OH radicals (to Co₃O₄) with Zn/DNC nanocatalysts; the shaded area represents the 90% confidence band. (F) Removal of multiple pollutants by ZnN₄-Edge after 2 and 5 min reactions ([Pollutant]₀= 0.1 mM). Reaction conditions for A–F: Nanocatalysts dosage = 50 mg L^–1 (if any), [PMS]₀ = 3 mM, [Phenol]₀ = 1 mM (if any), pH₀ = 7.2.

Fig. 5.

DFT studies on the Fenton-like activity of ZnN₄−Edge, ZnN₄, and Co₃O₄ nanocatalysts. The energy profile diagram shows the most favorable paths of PMS dissociation into OH radical in neutral conditions. Color code: Zn, pink; C, gray; N, blue; S, yellow; O, red; H, white.