Li Yang,Yong Jiao,Dongyan Jia,Yanzhi Li,Chuanhua Liao,*

1 School of Energy Science and Engineering,Nanjing Tech University,Nanjing 211816,China

2 School of Mechanical and Power Engineering,Nanjing Tech University,Nanjing 211816,China

Keywords:Perovskite Catalysis Activation of peroxymonosulfate Active oxygen species Degradation of organic compounds

ABSTRACT Metal-based perovskite oxides have contributed significantly to the advanced oxidation processes(AOPs)due to their diverse active sites and excellent compositional/structural flexibility.In this study,we specially designed a perovskite oxide with abundant oxygen vacancies,SrCo0.8Fe0.2O3 (SCF),and firstly applied it as a catalyst in peroxymonosulfate (PMS) activation towards organic pollutants degradation.The result revealed that the prepared SCF catalyst exhibited excellent performance on organic compounds degradation.Besides,SCF showed much better activity than La0.5Sr0.5Co0.8Fe0.2O3(LSCF)in terms of reaction rate and stability for the degradation of the organic compounds.Based on the analysis of scanning electron microscope,transmission electron microscope,X-ray diffraction,N2 adsorption-desorption,X-ray photoelectron spectroscopy and electron paramagnetic resonance,it was confirmed that the perovskite catalysts with high content of Sr doping at A-site could effectively create a defect-rich surface and optimize its physicochemical properties,which was responsible for the excellent heterogeneous catalytic activity of SCF.SCF can generate three highly active species:1O2,SO-4· and ·OH in PMS activation,revealing the degradation process of organic compounds was a coupled multiple active species in both radical and nonradical pathway.Moreover,it was mainly in a radical pathway in the degradation through PMS activation on SCF and SO-4· radicals produced were the dominant species in SCF/PMS system.This study demonstrated that perovskite-type catalysts could enrich OVs efficiently by doping strategy and regulate the PMS activation towards sulfate radical-based AOPs.

1.Introduction

Advanced oxidation processes (AOPs) have emerged as advanced water treatment technologies widely used to convert the recalcitrant and hazardous compounds from contaminated water into nontoxic and mineralized products [1-3].The highly active species,such as hydroxyl radicals (·OH),sulfate radicals(SO-4·) and other reactive oxygen species (ROS),can be produced in AOPsviaheat,light,electricity,ultraviolet (UV),or various catalysts(such as single oxides,carbon-based materials,metal nitride,peroxide,composite oxides,and sulfides),etc.[4,5].Among them,sulfate radical-based AOPs(SR-AOPs),which mainly involves the generation of SO-4·,have attracted great interest due to their versatile and eco-friendly nature [6].SO-4· radicals can be produced primarily by activating peroxydisulfate (PS) or peroxymonosulfate (PMS).Both PMS and PS are stable at room temperature and have mild oxidation.However,PMS or PS tends to be activatedviaUV,transition metals,heat,alkali,and activated carbon to generate SO-4· (lifetime is 30-40 μs).The redox potential of SO-4· possess so high (E0=2.5-3.1 V)that it is close to or even exceeds ·OH (E0=1.9-2.7 V,lifetime is 20 ns) [7].Since SO-4· possess high redox potentials,long lifetime,wide pH adaptation,and selective oxidation to most organic contaminants in aqueous solution [8],SR-AOPShas been widely considered a highly potential water treatment technology towards aqueous organic pollutants.

Recently,SO-4· was discovered that it can be easily generated on activation of PMS or PS by transition metals because of the excellent oxygen storage capacity and outstanding reactivity [9].Considering the advantages of stable structure,easy separation,and wide pH range of heterogeneous catalysis [10,11],it has gained more attention than the homogeneous system in SR-AOPs.In heterogeneous systems,catalysts with high activity and stable structure are the key to activate PMS effectively.Perovskite-type oxides,as a particular structure of transition metal oxide,can be flexibly tuned by changing the type and proportion of chemical compositions to be easily designed.Meanwhile,their merits of unique catalytic activity,photovoltaic performance,oxygen ions,protons,electronics and cationic diffusion performance have made them widely used in the fields of electrocatalysis,selective catalytic reduction,catalytic oxidation of hydrocarbons,solar cells and sensors [12,13].In recent years,perovskite oxides have also developed rapidly in the heterogeneous PMS activation process for organic compounds degradation.

For a typical ABO3perovskite,its A-site cations are usually lanthanide or alkaline earth metals,which have 12-fold coordination,and B-site cations are typically transition metal with 6-fold coordination with oxygen anions.A perovskite structure can form under the condition that the tolerance factor,defined ast,is in the range of 0.75-1.0.The merits of perovskite oxides as catalysts in AOPs are that transition metal elements’ oxidation state and redox capability in their lattice structure can be well-regulated through the doping strategy.Consequently,the catalytic activity may well be manipulated through component design [14].In the past,it has been found that A-site or B-site cation tuning can significantly improve the catalytic activity and structural stability of ABO3perovskites.Furthermore,the high oxygen vacancies (OVs) content can be introduced into catalysts by fascinating through the doping strategy.The surface OVs was found to play an essential role in PMS activation for water remediation,which can effectively promote surface oxygen mobility and electron transfer,thus enhanced the catalytic activity [15,16].

Herein,we synthesized SCF and LSCF as potential catalysts for catalytic oxidation of organic contaminants.This work applied perovskites as PMS activators to generate reactive oxygen species(ROS) for organic compounds degradation.Owing to the study of catalytic performance,perovskite SCF exhibited a much higher catalytic activity than LSCF.The results of electron paramagnetic resonance (EPR) and radical quenching tests proved that in addition to SO-4· and ·OH radicals,there was also a large amount of1O2,which identified PMS activation on perovskite catalysts in both radical and nonradical pathways.It is important to highlight that more OVs of SCF induced from Sr-doping in A-sites than LSCF,which made SO-4· radicals played a dominant role in SCF/PMS systems towards the degradation of aqueous organic pollutants.Our study illustrated in detail a strategy in design perovskite-type catalysts with high performance in radical pathways in applying PMSbased AOPs.

2.Experimental

2.1.Chemicals and materials

2.2.Catalyst preparation

Both SCF and LSCF perovskite materials were fabricated using the citrate sol-gel method and subsequent calcination,as reported previously [17].In the preparation procedure,the metal nitrates(Sr(NO3)2,Co(NO3)2·6H2O and Fe(NO3)3·9H2O) of SCF were at a molar ratio of 5:4:1,while the metal nitrates (La(NO3)2·6H2O,Sr(NO3)2,Co(NO3)2·6H2O and Fe(NO3)3·9H2O) of LSCF were at a molar ratio of 5:5:8:2.

2.3.Activation procedures and analytical methods

The catalytic performance of SCF and LSCF was evaluated by the degradation of phenol model wastewater.In a typical process,an appropriate amount of SCF and LSCF perovskite catalysts were added into 200 ml solution of 20 mg·L-1phenol solution at 25°C,respectively.Then the mixture was stirred for 10 min at a speed of 250 r·min-1to create the adsorption-desorption equilibrium on the surface of catalysts.Afterward,a certain amount of PMS was added into the mixture quickly and started the reaction.At regular intervals,about 1.5 ml of the solution was transferred and filtered by 0.22 μm filter-membrane and then mixed with methanol(Vfiltrate:Vmethanol=2:1)to terminate the reaction.The phenol concentration was analyzed by high-performance liquid chromatography (HPLC,Agilent1260) with a C18 column (4 × 150 mm,5 μm).To survey the stability of catalysts,catalysts were collected after reaction by pumping filtration (0.22 μm membrane) and washed with deionized water three times and then dried at 60 °C for 24 h to obtain reacted catalysts (1st run).The reacted catalysts(2nd run,3rd run and,4th run) were recycled by repeating the above test steps.The stability of SCF and LSCF catalysts after reuse was evaluated in terms of the degradation efficiency of phenol.All the tests were operated in triplicates and the average was obtained for analysis.The metal leaching was measured with inductively coupled plasma atomic emission spectroscopy (ICP-AES,Aglient 720ES).

The reaction kinetics of phenol degradation followed the firstorder kinetic model,as shown in Eq.(1) [7]:

Wherekis the reaction rate constant of catalytic oxidation of phenol(min-1),C0andCtare the initial concentration of phenol at 0 andtmin,respectively (mg·L-1).To explore the catalytic performance on activation of PMS towards the degradation of phenol,the observed kinetic curves of phenol degradation under various reaction conditions were plotted and obtained by kinetic fitting.

2.4.Characterization

X-ray diffraction(XRD)patterns of both as-prepared perovskite oxides were monitored through an X-ray diffractometer (Bruker D8) with CuKα radiation (λ=0.15 nm) from 10° to 90°.The raw data obtained were analyzed with MDI Jade version 6.5 to obtain its diffraction pattern and information such as crystal type,cell parameters and unit cell size.Scanning electron microscopy(SEM) and transmission electron microscopy (TEM) were utilized to perform the surface morphology and crystalline structure of perovskite oxides.The specific surface areas (SSA),pore volumes and pore diameters distribution of perovskite oxides were surveyed from a nitrogen adsorption-desorption apparatus (Quanta Autosorb-iQ,USA) at -196 °C and estimated using the Brunauer Emmett Teller (BET) equations.Surface elemental analysis for all catalysts was performed using X-ray photoelectron spectroscopy(Thermo Scientific) with the AlKα-type of source gun.The data were fitted by the public software package XPSPEAK Version 4.1 software using a Shirley background.The morphology of as-prepared samples was examined by scanning electron microscope (SEM-EDS,SU-8010,Hitachi) and transmission electron microscopy (TEM,FEI Talos F200x,FEI Company,USA).The element compositions of the sample were analyzed using energy disperse spectroscopy (EDS).

2.5.Identification of ROS species produced by activating PMS

The dominant active species during the degradation process were identified by comparing the degradation efficiency of phenol with and without quenching agents.MeOH,TBA,and NaN3were selected as the quenchers of SO-4·,·OH and singlet oxygen species(1O2) to identify the dominant ROS in the reaction,respectively.Moreover,the electron spin resonance paramagnetic spectrometer(EPR,EMX-E,Bruker) was used to monitor the amounts and types of ROS generated in the reactions at the 323.3 mT center field,9056 MHz microwave frequency and 1 min sweep time.

3.Results and Discussion

3.1.Phase and microstructural structures of catalysts

Powder XRD patterns of SCF and LSCF perovskite oxides are displayed in Fig.1.According to the standard anatase XRD card in the database,SCF and LSCF materials match well with JCPDS 82-2445 and JCPDS 48-0124,respectively,validating the single-phase of the SCF and LSCF materials with well-crystallized structure.As can be seen from Fig.1,XRD results of SCF showed that the more Sr2+doping in A-site made the characteristic diffraction peaks of SCF shift at a slight angle compared with JCPDS 82-2445.The unit cell parameters of the two materials were refined by Jade 6.5 and listed in Table 1.Based on the structural refinement of SCF and LSCF,it was revealed that SCF materials belonged to cubic perovskite crystal withPm3mspace group,while LSCF belonged to rhombohedral perovskite crystal withR-3cspace group.As can be seen in Table 1,the substitution of La3+(0.13 nm)by large ionic radius Sr2+(0.14 nm)at A site reduces the cell volume and parameter c of catalyst,resulting in the transformation of lattice and crystal phase.The results of Brunauer-Emmett-Teller (BET) also showed that two catalysts exhibit close SSA listed in Table 1.Due to their small and similar specific surface area,it can be ascertained that morphological structures are unlikely to contribute significantly to the difference in the catalytic performance of both catalysts.

Fig.1. XRD patterns of SCF and LSCF samples.

Table 1 Lattice parameters and specific surface area of SCF and LSCF catalysts

As shown in Fig.2a-b(SCF)and Fig.2c-d(LSCF),the morphology of both materials was observed to be agglomerated structures after high-temperature sintering(950°C).In addition,SCF nanoparticles displayed sheet-like agglomerated in Fig.2a,while LSCF exhibited more dispersed than SCF in Fig.2c.The local magnification in Fig.2b and Fig.2d showed the single-particle size of SCF was smaller than that of LSCF,which was consistent with the XRD analysis results (Table 1).The La,Sr,Co,Fe and O peaks were detected in EDS spectra (see Supplementary Material Fig.S1),and the atomic ratio of La,Sr,Co,Fe and O in Table S1 was close to the nominal ratio,suggesting high purity of prepared catalysts.As can be seen in Fig.2e-h,these nanoparticles displayed aggregated,agreeing well with the SEM observation.The interplanar spacings of SCF and LSCF were found to be 0.26 nm (Fig.2f) and 0.27 nm(Fig.2h),respectively,which matched well the distance of (110)crystal plane for the SCF and LSCF.The above characterization proved the high crystallinity of as-prepared SCF and LSCF materials and all the metal elements were successfully doped into the perovskite structure.

3.2.Catalytic performance and stability

The catalytic performance of catalysts was comparatively evaluated by phenol removal in the various reaction system.As shown in Fig.3a,hardly phenol removal was detected in the effects of selfdecomposition of PMS and catalyst self-absorption during organic degradation,implying both the PMS and catalysts were necessary for the oxidation process.On the other hand,the degradation effect of phenol was greatly improved in SCF/PMS and LSCF/PMS systems.The 100% phenol removal rate in SCF/PMS and LSCF/PMS system could achieve in 15 min and 20 min,respectively.Both SCF and LSCF perovskite catalysts displayed a significant effect on PMS activation,and SCF showed higher catalytic activity than LSCF for promoting the PMS activation.

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Furthermore,to elucidate the effect of such factors as catalyst dosage,PMS dosage,initial pH,and reaction temperature on PMS activation,relevant tests of SCF and LSCF catalysts were carried out under these factors at varied conditions.The effects of these factors on the catalytic performance in SCF/PMS and LSCF/PMS were discussed in detail in the Figs.S4-S7 of Supplementary Material.Moreover,in order to provide an in-depth evaluation of the adaptability of SCF,in addition to phenol (PN),sulfamethoxazole (SMX),acetaminophen (ACT) and benzoic acid (BA) were chosen to identify the catalytic degradation efficiencies of such contaminants in SCF/PMS.The degradation efficiencies of PN,SMX,ACT and BA in Fig.3b reached up to 100% within 20 min,15 min,15 min,and 40 min,respectively.Almost all contaminants could be ultimately degraded by SCF/PMS system within 40 min,indicating that SCF is indeed a heterogeneous catalyst with excellent adaptability for the degradation of different contaminants.

Fig.2. (a) TEM and (b) HR-TEM images of SCF catalyst,(c) TEM and (d) HR-TEM images of LSCF catalyst.,SEM images of (e,f) SCF and (g,h) LSCF catalysts.

The reusability and stability of SCF and LSCF catalysts for phenol degradation were investigated.As shown in Fig.4a and Fig.4b,the degradation efficiency of organic matter of both SCF (Fig.4a) and LSCF (Fig.4b) showed a gradual decrease with the increasing number of recycling tests.However,the organic matter in water could be decomposed entirely in 60 min at all recycle runs,indicating that both material structures are stable for PMS activation in organic pollutants degradation process.In addition,the leached Co,Fe from SCF and LSCF after reaction were detected by an inductively coupled plasma optical emission spectroscopy(ICP-OES).As shown in Table S2 (Supplementary Material),the leached metal concentrations of SCF were close to LSCF in this study.The above results indicated that metal leach shows no significant effects on the catalytic performance between SCF/PMS and LSCF/PMS systems.

3.3.Catalytic reaction mechanism

Fig.3. Degradation effects of (a) organics in various catalytic systems and (b) various contaminants in SCF/PMS systems.Conditions:[phenol]=20 mg·L-1,initial pH=6.8,[catalysts]=0.1 g·L-1,[PMS]=0.5 g·L-1, T=25 °C.

Fig.4. Stability analysis of reusability of(a) SCF and(b) LSCF catalysts.Conditions:[phenol]=20 mg·L-1,initial pH=6.8,[catalysts]=0.1 g·L-1,[PMS]=0.5 g·L-1,T=25 °C.

The XPS wide-scan spectra of SCF and Spent SCF in Fig.S2(Supplementary Material) indicate the presence of all elements(Sr,Co,Fe,and O).The high-resolution XPS spectra of Co 2p before and after reaction for SCF are illustrated in Fig.5a and 5b.As can be seen in Fig.5a and 5b,two main peaks at 780 eV and 795 eV belonged to Co 2p3/2and Co 2p1/2,respectively.Shirley background was utilized for peak fitting on the spectra and a series of peaks could be separated.It could be seen that three main peaks in the Co 2p3/2spectrum were observed and belonged to Co0,Co2+and Co3+,respectively,indicating that the mixed-valence state of Co in both SCF catalyst [18,19].The proportion of Co3+and Co0of SCF decreased with Co2+increased after reaction (Table S3(Supplementary Material)).All of the Co0species in SCF was disappeared and exsolved to form Co2+,while Co3+species were reduced to Co2+in Spent SCF,which explained the formation of more OVs than LSCF to enhance catalytic performance.As reported in previous studies,Co-based catalysts could activate PMS to produce free radicals and its catalytic ability mainly depended on the redox of Co3+/Co2+[20].These surface properties were beneficial to the electron transfer processes for PMS activation.As for iron at B-site,the proportions of different Fe species were listed in Table S4 and Fig.S3 (Supplementary Material).It was found that the SCF owns the high Fe4+content of 48% and the proportion of Fe4+decreased after reaction (in Spent SCF),implying Fe participated in the catalytic reaction.Therefore,iron at B-site benefited from creating a widespread covalent network to improve activity and stability.

The high-resolution XPS spectra of O1s for SCF and Spent SCF are shown in Fig.5c and 5d.As displayed in Fig.5c and 5d,the asymmetrical main peak of O1s can be deconvoluted into four peaks,which were attributed to lattice oxygen ion (O2-),chemisorbed oxygen speciessurface adsorbed oxygen (—OH/O2),molecular water and carbonatesAccording to the O 1s spectra curve fitting results of SCF and Spent SCF,the contents of O2-,,—OH/O2,andare shown in Table S5.Note that the high proportion of O2-and,resulting from OVs on the surface of SCF material,facilitated its excellent catalytic activity.Moreover,O2-was involved in the redox reaction of Co3+/ Co2+and hence Co3+could accept electrons from O2-and reduced to Co2+to balance the charge on the surface of catalysts.Thus,more content of O2-was conducive to electron transfer and promoted the catalytic activity [21].Due to the abundant OVs caused by A-site doping into SCF oxides,a large number of ROS appeared during heterogeneous catalysis reactions on the surface of SCF,which were responsible for the excellent catalytic performance of SCF for PMS activation and organic compounds degradation.

Sharp and symmetrical peak signals have been obtained using EPR detection of the OVs content of catalysts (Fig.5e).As shown in Fig.5e,the OVs signal peak intensity of SCF was much stronger than that of LSCF,proving that the more substitution of Sr doped at A-site of catalyst,the more proportion of OVs obtained on the surface of catalysts.The adsorbed oxygen molecules can attach to the OVs and convert tothrough electron charge transfer[7].As a result,the catalytic performance of SCF catalyst on PMS activation was extensively promoted due to more OVs allowed moreto generate than LSCF [22,23],well agreeing with the data of XPS analysis.

Fig.5. XPS spectra of (a,b) Co 2p and (c,d) O 1s of SCF catalysts before and after reaction,(e) EPR analysis of oxygen vacancies of SCF and LSCF catalysts.

The previous reports found that both MeOH (k·OH=9.7 × 108and TBA (k·OH=6 × 108could quench both ·OH and· radicals [22].Because of the different reaction rates of·OH and· radicals with the quenchers,MeOH was used as species scavengers for both ·OH and·,while TBA was considered as the scavenger especially for ·OH [24-26].As can be seen in Fig.6a,whether in SCF/PMS or LSCF/PMS systems,almost no effect on the degradation of organic matter can be observed at the same dosage of 0.3 mol·L-1TBA,suggesting that ·OH performed a trivial impact during the process of organic matter degradation.However,noticeable inhibition effects were observed at the dosage of 0.3 mol·L-1MeOH in both SCF/PMS and LSCF/PMS systems.These results indicated that·was a much more dominant species than·OH towards organic matter degradation in the radical pathway,which means SCF was easier to activate PMS to generate· than LSCF.Furthermore,as displayed in Fig.6b,the degradation of organic matter in SCF/PMS and LSCF/PMS systems was significantly inhibited at the dosage of 10 mmol·L-1NaN3,revealing that abundant1O2were generated during the reactions and composed the organic compounds in the nonradical pathway.The above tests indicated the phenol degradation resulted in both radical and nonradical processes.

Fig.6. Degradation effect of organics (a) with MeOH and TBA and (b) with NaN3 regent;EPR spectrum using (c) DMPO and (d) TEMP as a spin trap.Conditions:[phenol]=20 mg·L-1,initial pH=6.8,[catalysts]=0.1 g·L-1,[PMS]=0.5 g·L-1, T=25°C.

In situEPR was carried out to trap ROS and identify the contribution of the electron-transfer process in the degradation of phenol.As shown in Fig.6c,it can be seen that the stronger signals of DMPO-OH and DMPO-adducts in SCF/PMS system than that in LSCF/PMS system,proving more ·OH and· radicals were generated by PMS activated on SCF compared to LSCF.Then TEMP was applied as a spin trapping agent for the1O2capture in the EPR analysis [27,28].The typical three-line EPR spectrums of TEMP-1O2in Fig.6d appeared in both SCF/PMS and LSCF/PMS systems,confirming that1O2existed in both SCF/PMS and LSCF/PMS systems.The signal intensity in SCF/PMS system was slightly weaker than that of LSCF/PMS,revealing that less1O2was produced by SCF than LSCF through PMS activation.These results were coincident with that of the quenching tests.Hence,the catalytic reaction mechanism makes us conclude that SCF and LSCF mineralize organic compounds by PMS activation in both radical and nonradical pathways.Theradicals generated in SCF/PMS played much more roles than1O2nonradicals,leading to the more excellent catalytic performance of SCF than LSCF.It is disclosed that fascinating abundant OVs on the surface of perovskite catalysts can regulate PMS activation towards the degradation of aqueous organic pollutants in SR-AOPSpathway.

According to the above results and analysis,the specific mechanisms of PMS activation on SCF were proposed as follows(Eqs.(2)-(8)).The high catalytic activity of SCF catalysts for aqueous organic pollutants could be attributed to the synergetic involvement of·OH and1O2.The SCF catalyst first generates·OH free radicals and1O2through PMS activation.Then,·OH and1O2synergistically contributed to the decomposing organic matter in the aqueous phase,which was a coupling mechanism of multiple ROS.

Scheme 1. Proposed activation mechanism of PMS activation towards pollutant oxidation on SCF catalyst.

Table 2 PMS activation by the perovskite catalysts to degrade phenol

The metals at B-site and OVs on SCF as main active sites activatedto generateand ·OH.Meanwhile,the OVs could be converted to oxygen ions (O*) through electron transfer (Eqs.(2)-(3)).By the reduction of B site metals withO* could also be oxidized to produce OVs(Eq.(4))and then O*reacted withto form1O2(Eq.(5)).In addition,B site metals could also directly activate PMS to produce peroxymonosulfate radicalsand thenreacted with H2O to generate1O2(Eqs.(6)-(7)).Furthermore,the higher proportion of Sr doping at A-site of SCF material leads to a relatively more content of the OVs in the structure of SCF than LSCF,which was conducive to produce abundant·OH,and1O2by PMS activation for the mineralization of aqueous organic pollutants (Eq.(8)).Therefore,the SCF exhibited efficient activation of PMS towards the oxidation of aqueous organic pollutants.The above mechanism of PMS activation in the phenol degradation processes is described in Scheme 1.Compared the experimental results of SCF with the reported PMS-base perovskite catalysts for phenol degradation,SCF/PMS system could entirely degrade organic matter within 20 min and the degradation efficiency was highest as compared to those perovskite catalysts listed in Table 2,which further proved that the remarkable performance on peroxymonosulfate activation towards the degradation of aqueous organic pollutants.

4.Conclusions

In this work,two different oxides with similar compositions,SCF and LSCF,were successfully synthesized and tested for PMS activated degradation of phenol.Compared with LSCF,SCF showed excellent performance and stability in PMS activation and organic compound degradation.Based upon the results of free radical quenching tests and EPR detection,it was proved that·OH and1O2synergistically contributed to oxidize and degrade organic pollutants in the degradation reactions,among whichradicals was the dominant species in SCF/PMS system.It can be inferred that the high proportion of Sr doping at A-site of SCF could effectively improve the defect degree and from more OVs,leading to promote the electrontransfer capability created from the heterostructure of SCF materials.Our research illustrates that the SR-AOPs pathway can be rationally controlled by tuning the properties of the perovskitetype catalysts and provides a designation strategy of catalysts in water treatment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (Project No.2018YFB1502903).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.10.015.