Insight into the Structure–Activity Relationship of Delafossite Catalysts for Enhanced Peroxymonosulfate Activation and Pollutant Degradation

Abstract

Delafossite compounds (general formula ABO₂) have gained attention as promising catalysts for advanced oxidation processes (AOPs) due to their excellent substrate compatibility. However, the specific role of redox-active B-site metal centers in their catalytic mechanisms remains insufficiently elucidated. In this work, delafossite-type CuFeO₂ and CuMnO₂ catalysts were successfully synthesized via a one-step hydrothermal method and applied to peroxymonosulfate (PMS) activation to degrade ofloxacin (OFX) and methylene blue (MB). Catalytic performance assessments demonstrated that Mn substitution at the B site markedly enhanced the removal efficiency of OFX. Mechanistic studies involving structural characterization (XRD, BET, XPS) and quenching tests revealed that the enhanced activity originates from the promoted self-decomposition of PMS facilitated by the incorporated Mn, which boosts the formation of radical species (^•OH and SO₄^•−) due to the improved electrical conductivity resulting from manganese’s multivalent nature. By clarifying the influence of the B-site metal electronic structure, this study provides fundamental insights into PMS activation mechanisms and informs the rational design of highly efficient and stable delafossite-based catalysts.

1. Introduction

The serious environmental pollution issue has been one of the topics of concern as people become more aware of environmental protection [1,2,3,4]. To address this challenge, various remediation approaches, including biodegradation, adsorption, and chemical oxidation, have been employed [5]. Among these, peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) have garnered significant attention owing to their exceptional oxidative capacity for organic pollutants (like antibiotics, microplastics, and dyes) and remarkable selectivity in complex environmental matrices [6,7]. Heterogeneous catalysis represents an economical and efficient strategy for PMS activation, enabling the generation of diverse reactive oxygen species (SO₄^•−, ^•OH, ¹O₂, etc.) without requiring external energy input [8,9,10]. Additionally, the catalytic activity intensively depends on the properties of the catalyst itself. Hence, preparing effective catalysts and revealing the corresponding mechanism are crucial for the development and application of this technology.

To date, numerous studies have sought high-performance heterogeneous catalysts for efficient activation of PMS. Transition metals (e.g., Co, Cr) and their oxides are the primary catalysts for activating peroxymonosulfate (PMS). However, they suffer from several drawbacks, including high energy consumption (often requiring light irradiation), potential environmental toxicity, and low oxidation efficiency [11,12,13,14]. For example, Fu et al. utilized a glucose-modified CuFeO₂ (Glu/CFO) catalyst to activate PMS and degrade oxytetracycline. Because of the narrowed band gap and increased photogenerated electrons transfer, Glu/CFO showed superior reuse performance and high material stability under long-term operation [15], while the addition of light sources increased the system cost. In addition, CuCoO₂ facilitates the generation of reactive oxygen species via high-spin-state mechanisms, whereas CuCrO₂ enhances electron transfer through non-radical pathways [16]. Nevertheless, although conventional Co- and Cr-based catalysts exhibit high catalytic activity, metal leaching poses a significant risk of secondary pollution. Thus, designing energy-efficient, stable, and eco-friendly catalysts with high activity is key to achieving efficient PMS activation.

Up to now, some studies on PMS activation over ABO₂ catalysts have been reported. Delafossite (ABO₂, where A usually represents a monovalent metal and B is a positive trivalent transition metal) has been extensively investigated in the catalytic field due to its unique physicochemical properties with adjustable crystal structure and electronic properties [17,18,19]. Specifically, most B-site metal ions are environmentally benign and can effectively activate PMS without the need for light irradiation. Additionally, the synergistic effect of the A-site (such as Cu⁺) and B-site (such as Mn ³⁺, Fe³⁺) in copper iron ore can optimize the electron transport path, and its stable crystal structure helps to suppress metal leaching, which is beneficial to optimize its catalytic activity and overcome the dissolution of transition metal ions during PMS activation [20,21]. For example, it was reported by Zhao et al. that the activation pathway of PMS can be regulated through altering the d-orbital electronic configuration of ABO₂ [22]. Therefore, delafossite catalysts have shown great potential in the PMS activation process. An in-depth understanding of the mechanism through B-site modulation of ABO₂ will lay the foundation for the rational design of precision catalysts.

Herein, Cu-based ABO₂ (CuFeO₂ and CuMnO₂) were successfully synthesized by adjusting the composition of B-site metal elements. The crystal structure of the synthesized catalyst was systematically characterized using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The catalytic activity was subsequently evaluated through PMS activation to remove a series of refractory organic pollutants. Furthermore, the influence of key operational parameters—including PMS concentration and catalyst loading—was systematically evaluated. The underlying reaction mechanism was elucidated through radical quenching experiments. We believe this study elucidates the fundamental mechanisms of PMS activation and establishes a foundation for the rational design of highly efficient and stable delafossite-based catalysts through precise regulation of the B-site metal electronic structure.

2. Results and Discussion

2.1. Structural Analysis of CuMnO₂ and CuFeO₂ Catalysts

Figure 1a,b depicts the crystal structures of CuMnO₂ and CuFeO₂ catalysts. In the scanning range of 10–80°, the diffraction peaks located at 31.18°, 35.68°, 40.17°, 55.26°, 60.92°, and 64.76° matched well with the CuFeO₂ standard card (PDF card No.75-2146), indicating the successful synthesis of CuFeO₂ [23,24]. Similarity, the peak values of 31.40°, 32.92°, 35.32°, 36.95°, 39.96°, 40.46°, 51.93°, 57.01°, and 59.11° can be assigned to CuMnO₂ (PDF card No.50-0860) [25,26]. This result suggests the successful preparation of delafossite with different B-site metal ions (CuFeO₂ and CuMnO₂) by the one-step hydrothermal method. N₂ adsorption and desorption isotherms of CuFeO₂ and CuMnO₂ are shown in Figure 1c,d. CuFeO₂ and CuMnO₂ are classified according to type-IV hysteresis (IUPAC) classification, which represents the relatively strong interaction between the surface of samples and nitrogen, characteristic of a mesoporous material. It has been shown that H3 hysteresis loops are presented. The type-H3 hysteresis loop may possibly be associated with slit-shaped pores between parallel layers [27,28]. According to the above test, the BET surface areas of CuMnO₂ and CuFeO₂ are 4.56 and 15.20 m²/g. The larger surface area may benefit the enrichment of pollutants on the surface of catalysts.

Chemical composition analysis of these materials was further investigated by XPS. The three elements Cu, Fe (Mn), and O were detected in the full-scale CuFeO₂(CuMnO₂) XPS spectrum (Figure 2a,d). Here, in Figure 2b,d, two characteristic peaks arising from Cu 2p_3/2 and Cu 2p_1/2 orbitals were located at 932.2 eV and 952.1 eV, respectively, confirming that copper is mainly in its Cu(I) state in both CuMnO₂ and CuFeO₂ [29,30]. By comparison, the binding energies of Fe 2p_3/2 and Fe 2p_1/2 are 711.5 eV and 724.8 eV, respectively, revealing the chemical state of Fe(III) [31,32]. The Mn 2p XPS spectrum is presented in Figure 2f, in which the peaks at 640.5, 641.8, and 642.8 eV can be assigned to Mn(II), Mn(III), and Mn(IV), respectively [33,34]. The existence of multivalent manganese may be attributed to the spontaneous oxidation-reduction reaction of manganese during the hydrothermal reaction process.

2.2. Catalytic Performance Analysis

2.2.1. Performance Tests of CuMnO₂ and CuFeO₂ Catalysts

To evaluate the catalytic performance of CuFeO₂ and CuMnO₂, OFX was employed as the model pollutant, and their degradation efficiency was systematically compared under identical reaction conditions. As shown in Figure 3a, CuMnO₂ demonstrated significantly enhanced catalytic activity, achieving an OFX removal efficiency of 82%, compared to only 48% for CuFeO₂ under identical reaction conditions (catalyst dose: 0.45 g/L, PMS concentration: 0.45 g/L, reaction time: 60 min). After 45 min, the OFX removal efficiency of CuFeO₂ exhibited a gradual decline, while CuMnO₂ sustained its high degradation efficiency. Furthermore, kinetic analysis based on the first-order model revealed that the reaction rate constant for OFX degradation by CuMnO₂ (0.030 min⁻¹) was substantially higher than that of CuFeO₂ (0.011 min⁻¹), as summarized in Figure 3b. These results clearly demonstrate the superior catalytic activity of CuMnO₂ under the same reaction conditions.

To verify the above conclusion, the catalytic activities of delafossite-type oxides—CuFeO₂ and CuMnO₂—regulated by different B-site elements were compared and investigated through the degradation of MB under identical conditions. As shown in Figure 4a,b, the degradation efficiency of MB varies with the type of B-site element in the delafossite catalysts. Specifically, CuMnO₂ exhibits more pronounced degradation performance compared to CuFeO₂. According to the removal efficiency results in Figure 4c, CuMnO₂ achieves over 90% MB removal within 180 min, whereas CuFeO₂ reaches only 80% in the same period. Similar to the degradation of OFX, CuMnO₂ also maintained sustained catalytic activity at 90 min during MB degradation. These findings suggest that both CuFeO₂ and CuMnO₂ are effective in degrading MB, with CuMnO₂ demonstrating superior catalytic activity. Hence, such B-site-dependent behavior may extend to the degradation of other pollutants as well.

2.2.2. Effect of Catalyst Amount

In the PMS activation process, the reaction conditions exhibited a significant influence on OFX degradation efficiency. The effect of catalyst concentration (0.30, 0.45, and 0.60 g/L) on OFX removal, at a fixed PMS dosage of 0.45 g/L, is shown in Figure 5. As illustrated in Figure 5a, the degradation efficiency reached a plateau when the catalyst concentration increased from 0.30 to 0.45 g/L, with corresponding first-order kinetic rate constants of 0.033 and 0.034 min⁻¹, indicating only marginal enhancement. This limited improvement is likely attributable to an insufficient quantity of CuMnO₂, which provided too few active sites to effectively promote OFX degradation. However, when the catalyst loading increased to 0.60 g/L, a remarkable OFX removal efficiency exceeding 90% was achieved within 45 min, accompanied by a pronounced acceleration in degradation kinetics. The first-order rate constantly increased substantially from 0.035 to 0.045 min⁻¹, representing a 28.5% enhancement in reaction rate. This improvement can be ascribed to the greater availability of active sites at higher catalyst loadings, facilitating more efficient activation of PMS and thereby significantly promoting the degradation of organic pollutants [35,36]. Based on these findings, a catalyst dosage of 0.60 g/L was identified as the optimal reaction condition.

2.2.3. Effect of PMS Dosage

Under the optimal CuMnO₂ concentration (0.60 g/L), we further explored the effects of PMS dosage (0.30 g/L, 0.45 g/L, and 0.60 g/L) on the removal efficiency of OFX, and the experimental results of the corresponding OFX residual concentration with time, as well as the first-order kinetic reaction rate constants, are presented in Figure 6. As PMS dosage increased from 0.30 to 0.45 g/L under a fixed CuMnO₂ load of 0.60 g/L, a significant improvement was observed: OFX degradation efficiency rose from 83% to 92%, while the corresponding rate constant increased from 0.028 to 0.045 min⁻¹. This improvement can be attributed to the increased collision probability between CuMnO₂ and PMS when sufficient catalyst is present, thereby promoting OFX removal. However, when the PMS concentration was further increased to 0.60 g/L, the OFX removal rate decreased, accompanied by a reduction in the first-order kinetic rate constant to only 0.030 min⁻¹. This decline may result from excessive PMS adsorption onto the catalyst surface, which could block active sites and impede the diffusion of reactive oxygen species, ultimately reducing their utilization efficiency and limiting OFX degradation [37,38]. Therefore, 0.45 g/L of PMS was selected as the optimal reaction condition.

2.3. Catalytic Mechanism Analysis

To further elucidate the reaction mechanisms, the reactive oxygen species (ROS) involved in these systems were investigated. As shown in Figure 7, the effects of CuFeO₂ and CuMnO₂ on OFX removal efficiency were compared in the presence and absence of TBA, a highly hydrophobic compound widely used as a scavenger for hydroxyl (6.0 × 10⁸ M⁻¹ s⁻¹) and sulfate radicals ((4.0–9.1) × 10⁵ M⁻¹ s⁻¹) [39,40]. According to the results in Figure 7, the addition of TBA led to a slight decrease in OFX removal in the CuFeO₂ system, with efficiency declining from 35% to 28% within 60 min. In contrast, a significant inhibition was observed in the CuMnO₂ system, where OFX degradation decreased from 92% to 73% over the same period. A comparison of the first-order rate constants (Figure 7c,d) clearly reveals that the addition of TBA had a negligible effect on the CuFeO₂ system (0.006 vs. 0.004 min⁻¹), whereas it severely inhibited the reaction in the CuMnO₂ system (0.045 vs. 0.005 min⁻¹). These results suggest that a greater number of radical species (SO₄^•−/^•OH) was generated in the CuMnO₂ system. The substitution of the B-site metal from Fe to Mn alters the internal electronic structure of the ABO₂ material, resulting in enhanced electrical conductivity of CuMnO₂. This improvement facilitates interfacial electron transfer, thereby promoting faster PMS activation and leading to the production of more reactive oxygen radicals.

Based on the above research, the mechanism by which B-site metal regulation in delafossite catalysts enhances the reactivity of PMS activation can be attributed to the improved electrical conductivity resulting from manganese’s multivalent nature. Specifically, in the CuMnO₂ catalytic system, the B-site transition metal (Mn) exhibits a less-than-half-filled 3d orbital electronic configuration (Mn³⁺, 3d⁴), which promotes PMS decomposition to generate sulfate radicals through 3d orbital electron transfer. In contrast, the CuFeO₂ system, featuring Fe³⁺ (3d⁵) in an intermediate half-to-full filled state, predominantly yields non-reactive oxygen species [41]. The relevant reaction equations for both CuFeO₂ and CuMnO₂ catalytic processes are presented in Equations (1)–(9). In general, low-valence metal species such as Fe²⁺, Mn³⁺, and Cu⁺ can react with PMS to generate SO₄^•−/^•OH radicals [42]. By comparison, high-valence metals like Fe³⁺ tend to react with PMS to generate SO₅^•− radicals, which may further react to produce the non-radical singlet oxygen (¹O₂) [43]. Thus, in the CuFeO₂ system, the non-radical pathway dominated by ¹O₂ is predominant due to the presence of high-valence Fe³⁺. In contrast, the multivalent nature of manganese in CuMnO₂ facilitates the continuous generation of low-valence Mn species, enabling a radical-dominated pathway that efficiently produces SO₄^•− and ^•OH. The proposed reaction mechanisms for PMS activation in CuFeO₂ and CuMnO₂ catalysts are shown in Figure 8.

$$\equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_4^{\bullet -}+\mathrm{O}{\mathrm{H}}^{-}$$

(1)

$$\equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_4^{2-}+{}^{\bullet }\mathrm{O}\mathrm{H}$$

(2)

$$\equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{F}\mathrm{e}\left(\mathrm{I}\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_5^{\bullet -}+{\mathrm{H}}^{+}$$

(3)

$$\equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{V}\right)+\mathrm{S}{\mathrm{O}}_4^{\bullet -}+\mathrm{O}{\mathrm{H}}^{-}$$

(4)

$$\equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{V}\right)+\mathrm{S}{\mathrm{O}}_4^{2-}+{}^{\bullet }\mathrm{O}\mathrm{H}$$

(5)

$$\equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{V}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{M}\mathrm{n}\left(\mathrm{I}\mathrm{I}\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_5^{\bullet -}+{\mathrm{H}}^{+}$$

(6)

$$\mathrm{S}{\mathrm{O}}_5^{\bullet -}+\mathrm{S}{\mathrm{O}}_5^{\bullet -}\to \mathrm{S}{\mathrm{O}}_4^{2-}+{}^1{\mathrm{O}}_2$$

(7)

$$\equiv \mathrm{C}\mathrm{u}\left(\mathrm{I}\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv \mathrm{C}\mathrm{u}\left(\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_5^{\bullet -}+{\mathrm{H}}^{+}$$

(8)

$$\equiv \mathrm{C}\mathrm{u}\left(\mathrm{I}\right)+\mathrm{H}\mathrm{S}{\mathrm{O}}_5^{-}\to \equiv 2\mathrm{C}\mathrm{u}\left(\mathrm{I}\mathrm{I}\right)+\mathrm{S}{\mathrm{O}}_4^{\bullet -}+\mathrm{O}{\mathrm{H}}^{-}$$

(9)

2.4. Potential Application of CuMnO₂ Catalyst

The cycling stability and high mineralization efficiency demonstrated by the catalyst underline its strong potential for practical environmental applications. As shown in Figure 9a, the CuMnO₂ catalyst maintains consistent performance over four consecutive cycles with minimal activity loss (92% vs. 88%), indicating robust structural integrity and operational durability. Such stability is critical for long-term use in continuous flow water treatment systems, where catalyst reuse is essential for economic feasibility and reduced secondary waste generation. Furthermore, the comparative TOC removal results (Figure 9b) reveal that CuMnO₂ achieves significantly higher mineralization efficiency than CuFeO₂. This ability to thoroughly oxidize organic pollutants into harmless end-products (e.g., CO₂ and H₂O) is a crucial advantage in preventing the formation of potentially toxic by-products. These properties suggest that CuMnO₂ is a promising candidate for applications in industrial wastewater treatment.

3. Materials and Methods

3.1. Materials

The chemical reagents used in this study, including catalyst precursors and compounds for activity evaluation, were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). These include metal salts Cu(NO₃)₂·3H₂O, Fe(NO₃)₃·9H₂O, and Mn(CH₃COO)₂·4H₂O for catalyst synthesis; NaOH as a precipitant; peroxymonosulfate (PMS, 2KHSO₅·KHSO₄·K₂SO₄, KHSO₅ ≥ 42.8%) as the oxidant; and organic compounds including ethanol, ofloxacin (OFX, C₁₈H₂₀FN₃O₄, ≥98%), methylene blue (MB, C₁₆H₂₄ClN₃O₃S, BS), and tert-butanol (TBA, C₄H₁₀O, ≥98%) for quenching experiments and pollutant analysis.

3.2. Preparation of CuMnO₂ and CuFeO₂

The synthesis process for CuFeO₂ is described as follows: firstly, 0.1 mol/L aqueous solutions of Cu(NO₃)₂·3H₂O and Fe(NO₃)₃·9H₂O were added dropwise into NaOH solution (0.4 mol/L) to achieve complete precipitation. Then the precipitation was aged for 6 h, washed with distilled water to neutral pH, and redispersed into 50 mL of deionized water. Next, 15 mL of ethylene glycol was added to the well-dispersed pellet. Finally, the above solution was transferred to a 100 mL autoclave, where it was placed in a 200 °C oven for 12 h. The synthesized product was purified by repeated washing with distilled water and ethanol, followed by drying at 60 °C for 6 h to yield the final catalyst. CuMnO₂ can be prepared similarly to CuFeO₂, with 0.1 mol/L of Cu(NO₃)₂·3H₂O and Mn(CH₃COO)₂·4H₂O being added dropwise to NaOH (0.4 mol/L) solution before being reacted in the oven.

3.3. Characterization of the Prepared Catalysts

The crystal structures were initially evaluated by X-ray diffraction (XRD, Bruker AXS D8, Bruker Corporation, Billerica, MA, USA) with Cu Kα radiation (λ = 1.540598 Å) at the 2 thetas range from 10 to 80 degrees. The oxidation states of Cu, Fe/Mn, O, and C were analyzed by X-ray photoelectron spectroscopy (XPS, MULTILAB2000, Thermo Fisher Scientific, Waltham, MA, USA) and calibrated with the C 1 s hydrocarbon peak at 284.8 eV. The Specific Surface Area and Pore Size Distribution were analyzed through BET (V-Sorb4800, Beijing Instrument Co., Ltd., Beijing, China).

3.4. Catalytic Experiments and Analysis Methods

Typically, the related catalytic experiments were conducted at room temperature (25 ± 2 °C) with 10 mg/L OFX or 0.1 g/L MB in the presence of 0.45 g/L PMS and 0.45 g/L catalyst (CuMnO₂ or CuFeO₂). At specified time intervals, samples were withdrawn, centrifuged, and analyzed to evaluate the degradation efficiency. Furthermore, radical quenching experiments were performed by adding TBA to the reaction system. TBA reacts with ^•OH and SO₄^•− at rate constants of 6.0 × 10⁸ M⁻¹ s⁻¹ and (4.0–9.1) × 10⁵ M⁻¹ s⁻¹, respectively, to identify the dominant reactive oxygen species involved in the catalytic process.

The catalytic performance was further evaluated by monitoring the degradation process via ultraviolet–visible (UV–Vis, UV-2600, SHIMADZU, Kyoto, Japan) spectroscopy. The detection wavelengths were set at 290 nm for OFX and 664 nm for MB, corresponding to their characteristic absorption peaks. Moreover, the reaction kinetics for the degradation of organic pollutants were determined based on the following equation:

ln (C_t/C₀) = kt

(10)

where C₀ and C_t were OFX or MB concentration in solution at time 0 and t, respectively; k is the apparent first-order rate constant.

4. Conclusions

In conclusion, two delafossite-structured catalysts (CuFeO₂ and CuMnO₂) were successfully synthesized via a one-step hydrothermal method. Comparative analysis revealed that CuMnO₂ exhibited superior catalytic activity for activating PMS to degrade organic pollutants like OFX (82% vs. 48%) and MB (90% vs. 80%), achieving higher removal efficiencies and reaction rate constants than CuFeO₂. This enhanced performance is attributed to the multivalent nature of manganese (Mn²⁺/Mn³⁺/Mn⁴⁺) in CuMnO₂, which facilitates more efficient electron transfer and generates a greater abundance of reactive oxygen species (SO₄^•− and ^•OH), as confirmed by radical quenching experiments. Optimal degradation conditions were identified as a catalyst dosage of 0.60 g/L and a PMS concentration of 0.45 g/L. The study elucidates that the choice of B-site metal in the delafossite structure critically determines the catalytic mechanism and efficiency, providing valuable insights for designing highly effective catalysts for advanced oxidation processes in water treatment.