Title：Surface hydroxyls of spinel oxides: A double-edged sword in the peroxymonosulfate activation process
Journal: Applied Catalysis B: Environment and Energy
IF: 20.3
Original link：https://doi.org/10.1016/j.apcatb.2024.124849
Reporter：Xinlei Wang-23-master
The sulfate-based advanced oxidation process (SR-AOP) mediated by spinel oxides has shown great potential in pollutant degradation. However, the role of surface hydroxyls in the reaction process remains unclear, especially in real environments containing salts. Here, we investigated the role of surface hydroxyls of spinel oxides in activating peroxymonosulfate (PMS). By adjusting the intrinsic composition of the metal elements, we synthesized a CoFeMnO4 spinel with high surface hydroxyls using a simple and scalable co-precipitation method. CoFeMnO4 demonstrated unparalleled activation performance and a unique non-radical activation pathway (dominated by high-valence metal-oxo species and singlet oxygen), achieving 100 % sulfamethoxazole degradation within 4 min with a low concentration of PMS. Surface hydroxyls of CoFeMnO4 played a crucial role in the activation process, serving as adsorption sites for PMS and facilitating its activation. Interestingly, we discovered that several common anions (Cl−, HCO3−, SO42− and NO3−) could undergo ligand exchange with surface hydroxyls of CoFeMnO4, occupying some of sites intended for PMS adsorption, which significantly inhibited the PMS decomposition on the catalyst surface. Further, it was finally discovered that the inhibition of the reaction system by these anions was primarily due to their competition for surface hydroxyls, rather than the commonly believed the consumption of reactive oxygen species by anions. Given the inevitable presence of various anions in real water bodies, this finding highlights the dual nature of surface hydroxyls as active sites in SR-AOPs: While they aid in activation process, the competition from anions for them would diminish the overall system performance.
As a broad-spectrum antibiotic, sulfamethoxazole (SMX) is widely used in medicine and livestock industry. Due to extensive use and uncontrolled discharge, large amounts of SMX are released into the natural environment, posing threats to ecosystems and human health. Its stable structure makes it resistant to traditional treatment processes, highlighting the need for an efficient method to remove SMX. Sulfate radical-based advanced oxidation processes (SR-AOPs) are considered a promising water treatment technology capable of effectively removing various persistent pollutants, including SMX. Compared to traditional AOPs, SR-AOPs offer superior oxidation performance, a broader applicable pH range, and a wider array of activation methods.
In contrast, transition metal oxides synthesized via co-precipitation can meet the yield needs for engineering applications. This method allows for low-cost, large-scale synthesis of catalysts under mild condition. Spinel (AB2O4), a typical transition metal oxide, offer significant advantages such as excellent stability, strong performance, and environmental compatibility, and can be easily synthesized via co-precipitation. More importantly, their activation pathways for PMS can be adjusted flexibly by altering metal compositions and controlling crystallinity to meet various application needs.
1. Characterisation
Structure and characterization of the samples: (A) XRD patterns of spinel oxides with different Mn content. (B) FTIR spectra of spinel oxides with different Mn content. (C) Nitrogen adsorption-desorption isotherms of CoFe2O4 and CoFeMnO4. (D) The SEM and TEM image of CoFeMnO4. XPS spectra of Co 2p (E), Fe 2p (F), Mn 2p(3 s) (G) and O 1 s (H) regions for CoFeMnO4 and CoFe2O4. (I) The surface hydroxyl density of CoFeMnO4 and CoFe2O4. (J) Adsorption energy of water on CoFeMnO4 and CoFe2O4. (K) Changes in the adjacent Co−O bond lengths of CoFeMnO4 and CoFe2O4 before and after water adsorption. In the DFT calculation, red balls are oxygen, blue balls are cobalt, orange balls are iron, cyan balls are manganese, yellow balls are sulfur, and white balls are hydrogen.
2. Performance of the reaction system
Catalytic performance of the samples. (A) Performance for SMX degradation of different reaction systems (B) The kinetic fitting of SMX degradation in different reaction systems. (C) Comparison of the yield of CoFeMnO4 using scaled-up synthesis and small-scale synthesis. (D) Comparison of the activities of CoFeMnO4 synthesized on a large scale and on a small scale. (E) Comparison of normalized rate constant of SMX degradation for different catalysts. (F) Cycling performance of CoFeMnO4. Reaction conditions: Temperature = 25°C, pH = 7.0 ± 0.1, [SMX] = 20 μM, [PMS] = 0.1 mM. Catalyst dosage for Fig. 1A is 0.05 g/L, all other catalyst dosages are 0.2 g/L.
3. Reactive oxygen species in the reaction system
Identification of the main ROS in the CoFeMnO4/PMS system. (A) Effects of different scavengers on SMX degradation in the reaction system. (B) EPR spectra with DMPO as the spin trap. (C) EPR spectra with TEMP as the spin trap. (D) I-t and OCP curves of CoFeMnO4 as working electrode. (E) Transformation of PMSO in CoFeMnO4/PMS system. (F) Transformation of PMSO in PMS alone system. (G) EIC spectrum of the 18O isotope labeling experiment on PMSO. (H) MS spectrum of the 18O isotope labeling experiment on PMSO. Reaction conditions: Temperature = 25°C, pH = 7.0 ± 0.1, [SMX] = 20 μM, [PMS] = 0.1 mM, [Catalyst] = 0.2 g/L.
4. The role of surface hydroxyls
Identification of the active sites in the CoFeMnO4/PMS system. (A) Effect of phosphate on SMX degradation. (B) Effect of phosphate on PMS activation. (C) kobs of SMX degradation and PMS decomposition, inset is the correlation of the inhibition rate of phosphate on SMX degradation and PMS decomposition. (D) Effects of phosphate on the ATR-FTIR spectra of CoFeMnO4 in D2O suspension. (E) Effects of PMS on the ATR-FTIR spectra of CoFeMnO4 in D2O suspension. (F) Schematic diagrams of terminal hydroxyls and bridging hydroxyls on CoFeMnO4. (G) Adsorption energy and Bader charge of different hydroxyl sites on PMS. In the DFT calculation, red balls are oxygen, blue balls are cobalt, orange balls are iron, cyan balls are manganese, yellow balls are sulfur, and white balls are hydrogen. Reaction conditions: Temperature = 25°C, pH = 7.0 ± 0.1, [SMX] = 20 μM, [PMS] = 0.1 mM, [Catalyst] = 0.2 g/L.
5. Reaction mechanism
Reaction mechanism of the CoFeMnO4/PMS system. (A) The O 1 s (A), Co 2p (B), Fe 2p (C) and Mn 3 s (D) spectra of CoFeMnO4 before and after the reaction. (E) Differential charge of PMS when activated at different sites. (F) FTIR spectra of CoFeMnO4 before and after the reaction. (G) In situ ATR-FTIR of the activation process of PMS by CoFeMnO4 (in H2O). In the DFT calculation, red balls are oxygen, blue balls are cobalt, orange balls are iron, cyan balls are manganese, yellow balls are sulfur, and white balls are hydrogen.
6. The inhibition mechanism of common anions on the reaction system
The influence of anions on the reaction system. Among them, (A), (B), (C) and (D) represent four anions: Cl–, HCO3–, SO42– and NO3–. (1) is the effect of the corresponding anions on SMX removal. (2) is the effect of the corresponding anions on PMS decomposition. (3) is the kobs of SMX removal and PMS decomposition in the system when the corresponding anions exist. The gray color represents the kobs for SMX degradation, while the colored sections represent the kobs for PMS decomposition. (4) is the correlation between SMX degradation and inhibition of PMS decomposition in the presence of the corresponding anions. (5) is the effect of corresponding anions on the ATR-FTIR spectra of CoFeMnO4 in D2O suspension. Reaction conditions: Temperature = 25°C, pH = 7.0 ± 0.1, [SMX] = 20 μM, [PMS] = 0.1 mM, [Catalyst] = 0.2 g/L.
This study synthesized a CoFeMnO4 spinel oxide with high surface hydroxyls density using a simple co-precipitation strategy, demonstrating excellent catalytic activity for PMS activation. At a concentration of 0.1 mM PMS and 0.1 g/L catalyst, the catalyst achieved complete degradation of SMX within 3 min. High-valence metal-oxo species (such as Co(IV)=O and Fe(IV)=O) and 1O2 are the main ROS in the reaction system, and surface hydroxyls are identified as the key active sites of CoFeMnO4. Unlike previous views, this study found that the four common anions (such as Cl−, HCO3−, SO42−, and NO3−) mainly inhibit the performance of the reaction system by competing with PMS for the surface hydroxyls of CoFeMnO4, rather than by consuming and quenching ROS as traditionally believed. Given the widespread presence of anions in real waters and the crucial role of surface hydroxyls in many metal catalysts, the mechanism of anions inhibiting the performance of the reaction system by occupying surface hydroxyls is likely to be very common. This should be given sufficient attention rather than simply considering the interactions between anions and ROS. Strategies should be developed to reduce or avoid the performance decline caused by the interactions between surface hydroxyls and anions.