Shiya He,Zhimin You,Xin Jin,Yi Wu,Cheng Chen,He Zhao,Jian Shen,*

1 Department of Environment and Resources,Xiangtan University,Xiangtan 411105,China

2 State Key Laboratory of Heavy Oil Processing,College of Chemical Engineering,China University of Petroleum,Qingdao 266580,China

3 Beijing Engineering Research Center of Process Pollution Control,Division of Environment Technology and Engineering,Key Laboratory of Green Process and Engineering,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

Keywords: Redox engineering Crystal field stability phase Lattice oxygen Toluene degradation

ABSTRACT Excellent performances promoted by lattice oxygen have attracted wide attention for catalytic degradation of volatile organic compounds(VOCs).However,how to control the continuous regeneration of lattice oxygen from the support is seldom reported.In this study,we selected sepiolite supported manganese-cobalt oxides(CoxMn100-xOy)as model catalysts by tuning Co/(Co+Mn)mass ratio(x=3%,10%,15%,and 20%)to enhance toluene degradation efficiency,owing to lattice oxygen regeneration by redox cycle existing at the interface and Mn species with high valence state,initiated by cobalt catalytic performance under the role of crystal field stability phase.The results of activity test show that the sepiolite-Co15Mn85Oy catalyst exhibit outperformances at 193°C with 10,000 h?1 GHSV.In addition,the catalyst existed at the bottom of the“volcano”curve correlated T50 or T90 with Co/(Co+Mn)weight ratio is sepiolite-Co15Mn85Oy,conforming its outperformance.Further characterized by investigating active sites structural and electronic properties,the essential of superior catalytic activity is attributed to the grands of lattice oxygen continuous formation resulted from redox engineering based on the high atomic ratio of surface lattice oxygen with continuous refilled from the support and that of Mn4+/Mn3+ cycle initiated by cobalt catalytic behaviors.All in all,redox engineering,not only promotes grands of active species reversible regeneration,but supplies an alternative catalyst design strategy towards the terrific efficiency-to-cost ratio performance.

1.Introduction

Volatile organic compounds(VOCs),either from natural or anthropogenic processes emissions,not only induce photochemical smokes,but also are harmful for human beings because of their toxicity[1–3].Among them,toluene,as an important kind of benzene-based VOCs,has been widely used in industrial feedstock and solvent due to the fantastic electron-releasing properties of methyl group,intrinsic polarity,and relatively low toxicity as compared with benzene molecule.Although its toxicity is low,long-term exposure to toluene still induces biological toxicological effects[4].Therefore,it is urgent to take actions to control toluene discharge amounts,especially below the present discharging limitations(3.0 mg·m?3).

Presently,grand efforts on the development of VOCs degradation technologies have been conducted,such as adsorption [5],photocatalysis[6],plasma[7],thermal incineration[8],and catalytic oxidation [9].Although high removal efficiency of the individual technology could be potentially achieved,many drawbacks of those technologies still hamper further application,such as secondary pollutants,high treatment cost,and difficulty to scale up.Actually,thermal catalytic oxidation is widely accepted as effective technology for VOCs degradation because of low operation temperature with high degradation efficiency,particularly for the large volumes of diluted VOC contaminated air,and easy to scale-up[10].However,ongoing researches have reported that the issues of thermal catalytic oxidation need to be addressed through(i)decreasing ignition temperatures,(ii) enhancing activity as to the low concentration of VOCs and the large volumes to be treated,(iii)promoting mineralization efficiency,and (iv) increasing mechanical properties of catalysts.The core for addressing the above-mentioned challenges is rational design of the multifunctional catalyst(noble metals and transition metal oxides)[10,11].Compared with noble metal catalysts,transition metal oxides,such as Mn-based oxides,proved its feasibility in the VOCs catalytic oxidation process[12,13],ascribing to the controllable oxidation states as characterized with superb lattice oxygen storage capacity and oxygen mobility[14,15].Meanwhile,low cost is another merit as compared with noble metals.However,low specific surface area and phase transition still restrain its practical application.Therefore,further improving the activity of intrinsic active sites and stabilizing active phase are viewed as two approaches to boost catalytic oxidation performance.

Recent surges suggested that doping multivalent elements could enhance the oxidation states of active species via tuning the local coordinate environment.For example,Zhao et al.prepared a series of CeMnOxcatalysts by redox co-precipitation method,and indicated that the strong interaction between Ce and Mn oxides could lead to more surface adsorbed oxygen and Mn4+species [12].Luo et al.conformed such conception of enhancing manganese oxide performance by inducing copper element into manganese oxide substrates[3].As to toluene degradation,doping approach shows positive effects for enhancing catalytic oxygen performance of MnOx[16].To further promote the catalytic performance and stabilize active phase,selecting porous support are necessary because of its high specific surface area,large pore volume,good dispersibility,and strong metal-support interaction.For example,Xu et al.[17]prepared a carbon-coated ordered mesoporous silica-supported Ni-based catalyst and indicated that the support can improve Ni nanoparticle dispersion,thus improving the catalytic performance.He et al.[18]also proved that the mesoporous structure of the ZSM-5 provides a large specific surface area and pore size,resulting in numerous accessible active sites.Compared with traditional porous materials,sepiolite,as natural and environment benign porous materials,has induced wide attentions.The acidic[SiO4]and alkalescent[MgO6]centers in sepiolite can activate the adsorbed environmental pollutant,further boosting the catalytic reaction process[19–22].For example,Dong et al.prepared sepiolite-supported rare earth oxide catalysts and pointed that the presence of hydroxyl groups on the surface of sepiolite can produce more amount of the Br?nsted acid sites,so that it could improve the catalytic performance[20].Li et al.loaded Bi2O3on sepiolite and successfully prevented agglomeration of Bi2O3powders[23].However,research on the catalytic degradation of toluene by sepiolite-supported catalysts is seldom reported and the corresponding mechanism is needed an in-depth investigation(Fig.1).

Therefore,the hypothesis of this work is to fabricate sepiolite-supported manganese-based oxides as the model catalysts by controlling cobalt doping amount to enhance toluene degradation performance via redox engineering in the interface.The catalytic performances of sepiolite-CoxMn100-xOy(x=3%,10%,15%,and 20%,by mass)were evaluated according to the data of toluene removal efficiency,T90(T50),reuse efficiency,and stability.Furthermore,the structural and electronic properties of the active sites related to catalytic performance were studied by diversity characterization.Finally,the plausible catalytic oxidation mechanism was also presented.Hopefully,the outcome of this work will provide instructions for the boosting thermal catalytic performance in the community of environment pollution control and serve as the redox engineering based general strategy for catalysts design.

2.Materials and Methods

2.1.Materials

Mn(NO3)2(50%,A.R.),Co(NO3)2·6H2O (≥99.0%,A.R.),and NaOH were purchased from Sinopharm Chemical Reagent Co.Ltd.Sepiolite was purchased from Yixian,Hebei province.Deionized water was used throughout the whole experiment process.

2.2.Catalysts synthesis

In a typical synthesis of sepiolite-supported Cox-Mn100-xcatalysts,a mixture of cobalt and manganese precursors(Mn(NO3)2and Co(NO3)2)with different mass ratios x(x=3%,10%,15%,and 20%)are firstly prepared and stirred for half an hour.Then,1 g of sepiolite was added to the mixture and stirred for another 2 h.After that,the suspension pH is adjusted by dropwise adding 0.25 mol·L?1NaOH until pH=10 and the suspension is continuously stirred for 24 h.Subsequently,the pristine samples are washed by DI water several times until the waste liquid pH will no longer change,and then dehydrated at 60°C overnight.The dried samples are further annealed at 550°C for 4 h under a static air atmosphere.Finally,the samples are collected and stored at desiccator for further use.

2.3.Catalyst characterization

Fig.1.Structural illustration of sepiolite which is employed as support in this work[24].

Powder X-ray diffraction(XRD)patterns were recorded collected using a D/max 2500 X-ray powder diffractometer with Cu Ka radiation,and the intensity data was collected over 2θ angle with 5°–80°range.The Brunauer–Emmett–Teller (BET) surface area,pore volume,and pore size distribution of the samples were obtained by an autosorbiQ instrument at ?196 °C.Prior to the measurement,all samples were degassed at 150°C for 6 h.H2-TPR measurements were performed on AutoChem II 2920 using a TCD detector.For each sample,50 mg granules were treated at 300°C in a flow of He(40 ml·min?1)and then cooled down to room temperature.After that,the samples were reduced in a flow of 10%H2/Ar(20 ml·min?1)and the temperature was increased from 50 to 800°C within a heating rate of 10°C min?1.Gemini300 scanning electronic microscopy(SEM)was taken to obtain images of samples.Transmission electron microscopy(TEM)images of the samples were taken by JEOL JEM 2100 instrument.X-ray photoelectron spectroscopy(XPS)measurements were performed on a Kratos XSAM-800 electron spectrophotometer at 13 kV and 20 mA with Al Kα radiation.And all the binding energy was corrected by referring to the binding energy of C 1 s(284.8 eV).Raman spectra were collected on a SENTERRA dispersive Raman microscope from Renishaw equipped with an excitation laser of 532 nm.

2.4.Catalytic activity test

Catalytic activity tests were performed in a conventional fixed-Bed flow reactor of a quartz tube.A loading of 0.25 g of catalyst (60–80 mesh) was placed into a tubular glass reactor (i.d.: 20 mm,length:600 mm).Gaseous toluene was generated by flowing nitrogen through a container with the pure toluene in an ice-water bath,carrying by synthetic air(80/20,volume ratio)with a total flow of 300 ml·min?1,corresponding to a gas hourly space velocity(WHSV)of 10,000 h?1.The inlet velocity of toluene was 300 mg·m?3and the toluene concentration was analyzed by gas chromatography (GC112A,China) with equipped with a flame ionization detector(FID).The schematic of the experimental set-up for catalytic activity tests was shown in Fig.2.To evaluate the catalytic performance,the removal of toluene(Y)was calculated,following the Eq.(1):

3.Results and Discussion

3.1.Catalytic activity

The catalytic activity of the sepiolite-CoxMn100-xOywith mass ratio from 3%to 20%were evaluated by investigating toluene conversion efficiency.It is known that the lower the reaction temperature within high toluene conversion,the higher activity will be obtained[25,26].Fig.3 presents the effects of reaction temperature on conversion for achieving 50%and 90%toluene removal efficiency.Fig.3a shows that the highest toluene removal rate was observed on sepiolite-Co15Mn85Oywithin low temperature of the operation window.Specifically,the 90%toluene conversion of sepiolite-Co15Mn85Oywas obtained at 193°C and maintained at such high removal efficiency within wide reaction temperature,ranging from 193°C to 350°C.However,either decreasing cobalt mass ratio (sepiolite-Co10Mn90Oy,and sepiolite-Co3Mn97Oy)or increasing the corresponding parameter of the catalyst(sepiolite-Co20Mn80Oy)will unfavorably increase the reaction temperature to achieve 90%degradation efficiency.The reaction temperature for 90%toluene conversion of sepiolite-Co10Mn90Oy,sepiolite-Co3Mn97Oy,and sepiolite-Co20Mn80Oyare 217°C,305°C,and 211°C,respectively.

As shown in Fig.3b,both the“volcano”curve of the effects of the cobalt weight ratio on T50and T90further prove sepiolite-Co15Mn85Oyas the best activity in the series of sepiolite-CoxMn100-xOy.Meanwhile,H2-TPR was conducted to investigate the relative reducibility of sepiolite-CoxMn1-xOy(Fig.3c,corresponding H2consumption of each reduction region as inset of Fig.3c).It is clearly observed that all the catalysts exhibited two overlapping reduction peaks,except for sepiolite-Co15Mn85Oywith a dual-step reduction process.The first step occurring in the temperature range 260–320°C is a reduction process for Mn4+to Mn3+[27],while the second reductive step involves from Mn3+to Mn2+,or from Co2+to Co0at a relatively higher temperature(565–640°C)[28].With the introduction of cobalt,the reduction temperature of all the catalysts is much lower than that of single Mn or Co catalysts,as previously reported[29].Such behavior is ascribed to high oxygen mobility from asymmetric metal-oxygen bond extension or compression[27].Taking the total H2consumption into consideration,the descending order as follows: sepiolite-Co15Mn85Oy> sepiolite-Co20Mn80Oy>sepiolite-Co10Mn90Oy>sepiolite-Co3Mn97Oy.Compared with the other catalysts,the H2-TPR profile of sepiolite-Co15Mn85Oydisplays a broad peak centered at 260°C and a small shoulder peak around at 328°C,indicating loss of surface oxygen species[30].Therefore,such phenomenon is strongly ascribed to the fact that the surface oxygen vacancies are generated at low temperature[30,31].Furthermore,catalyst stability and recycle performance also serve as two key factors for its industrial application.Herein,we selected sepiolite-Co15Mn85Oyfor detailed studies.Fig.3d shows that the toluene degradation efficiency of sepiolite-Co15Mn85Oyafter the first and second run are similar.However,compared with the pristine catalyst,the initial temperature window of the highest toluene removal is open near 217°C after the first recycle(Fig.3d),indicating slight deactivation.However,the initial temperature window of the highest toluene removal is still lower than that of the catalyst as previously reported[32].To our excited,further improving recycle time,the curve of toluene conversion vs temperature nearly overlaps without further deactivation.Therefore,sepiolite-Co15Mn85Oydisplays high stability.Moreover,sepiolite-Co15Mn85Oyalso shows excellent performance by evaluating under steady-state conditions at 80%of toluene conversion.Fig.3d(inserted)presents the toluene conversion as a function of reaction time over a stream(48 h)for sepiolite-Co15Mn85Oy.As seen in Fig.3d,sepiolite-Co15Mn85Oyexhibits perfect stability and no obvious deactivation even over a continuous 48-hour test.Therefore,according to the data in Fig.3,sepiolite-Co15Mn85Oycould serve as a potential candidate for catalytic degradation of toluene with excellent performance under milder conditions.The detailed degradation mechanism will be illustrated in the following section.

Fig.2.Experimental set-up of activity test for toluene degradation by sepiolite supported CoxMn100-xOy(x:3,10,15 and 20).

Fig.3.(a)Correlated between toluene conversion with temperature over sepiolite supported CoxMn100-xOy(x:3,10,15,20)under the condition of synthetic air(80/20 vol%)with a total flow of 300 ml min?1;(b)T50 and T90 of sepiolite supported CoxMn100-xOy(x:3,10,15,20);(c)H2-TPR and H2 consumption(inset)profiles of sepiolite supported CoxMn100-xOy(x:3,10,15,20);(d)the effects of cycle times on the catalytic performance of optimal catalyst(sepiolite-supported Co15Mn85Oy)under the condition of synthetic air(80/20 vol%)with a total flow o0183f 300 ml·min?1 and stability test(inset)of sepiolite-supported Co15Mn85Oy conducted at 80%of toluene conversion over 48 h.

3.2.Catalysts characterizations

3.2.1.Morphology characterizations

To identify the morphology of the sepiolite-CoxMn100-xOy(x:3,10,15,and 20) catalysts,scanning electron microscopy (SEM) images were firstly collected.As shown in Fig.4a–d,CoxMn100-xOy(x: 3,10,15,and 20)is obviously coated on the surface of the fibrous sepiolite.The cobalt manganese oxide particles become better distributed with particle size increasing by improving weight ratio Co and Mn precursors increased from 3%to 20%,presented in Fig.S1.Meanwhile,the real Co/Mn atomic stoichiometry of CoxMn100-xOy(x:3,10,15,and 20)based on the EDX data(Table S1)from SEM are 1.10,1.10,1.20,and 1.20,respectively,which means that composition effect is not a key factor for catalytic performance control,and also indicates that the actual Co/Mn stoichiometry of CoxMn100-xOyphase with different precursor weight ratio is closed for each other,indicating CoxMn100-xOyexistence as single phase.Furthermore,correlated with catalytic activity data,sepiolite-Co15Mn85Oyis selected to identify detailed morphology by high-resolution transition electron microscopy(HR-TEM)and element distribution mappings spectrums(EDS).As shown in Fig.4e–g,TEM and HR-TEM images indicate the axial growth direction(110)of sepiolite with(110)surfaces for the sequential Co15Mn85Oyshell coating.Also,EDX mappings(Fig.4h–i)further confirm that the Co15Mn85Oyare evenly distributed over the sepiolite surface.

3.2.2.Structure characterizations

Fig.5.X-ray diffraction patterns(a)and Raman spectrums(b)of sepiolite supported CoxMn100-xOy(x:3,10,15,20).

To obtain the detailed structural information on sepiolite-CoxMn100-xOycatalysts,X-ray diffraction (XRD) study was carried out ranging from 5°to 80°(2θ).As shown in Fig.5a,the characteristic peaks of sepiolite and manganese oxides are co-existing in all catalyst samples,however,no obvious diffraction peaks for cobalt oxides are present in the XRD spectrum,implying that cobalt element interacting with manganese oxide phases.This is consistent with the XRD data of CoMn bimetallic oxides reported by Qu and colleagues [33].Moreover,the diffraction angle of typical facet peaks also show shift with different directions(facet peak assignment as shown in Table S2),thus confirming the variation of lattice spacing.From the view of Jahn-Teller effects,such variation,induced from heteroatom doping,will induce the elongation or contraction of metal–ligand bond either in apical or in-plane direction to stabilize phase via minimizing lattice energy [34].To further identify lattice distortions induced by doping trace cobalt species,Raman bands were also recorded in the frequency range from 100 to 800 cm?1(Fig.5b).In the region of low frequency,the Raman band in the vicinity of 175 cm?1represents Co--O stretching mode in CoO4tetrahedron[35].As cobalt ratio increasing up to 15%,namely sepiolite-Co15Mn85Oy,the Co--O stretching mode starts to disappear,ascribing to a small fraction of Co2+ions on tetrahedral sites[36].In the middle frequency region between 300 and 400 cm?1,it is found that three bands at approximately 315,365,and 375 cm?1should be assigned to oxygen modes in MnO6and/or MnO4(Mn2+and Mn3+)sites,whereas cobalt oxides exhibit no bands within this frequency range.Viewing from Fig.4b,the band intensity in the middle frequency decreases towards to lower values as cobalt precursor,supposing either CoO6and/or CoO4formation or Mn2+and/or Mn3+transformation towards Mn4+.By correlating with the findings at low frequency,it could be inferred that Co may be existing in the form of CoO6.Moreover,oxygen vibrational modes also further confirm the presence of metal-oxygen structure.It is known that the band in high frequency region between 570 and 670 cm?1is viewed as vibrational modes involving the motion of oxygen atoms within the MO6octahedra.Especially,the two bands centered at 578 and 620 cm?1were well agreed with previous reports on Mn4+ions on octahedral sites,and may also arise from the random Mn3+/Co3+mixing on the octahedral sites because of the redistribution of metal ions[35,37].Also,previous work suggested that Mn4+ions give rise to peak broadening of this mode.Additionally,the chemical shift of band peaks is determined by the ionic radii of heteroatoms.Larger ionic radii promotes the band towards blue shift and verse vice [38].As displayed in Fig.5b,the band centered between 620 and 650 cm?1become border as the increment of cobalt precursor weight ratio.Therefore,it is inferred that Mn4+octahedral structure existed in the catalysts,and also that band shifting towards low frequency is attributed to larger ionic radii dopant,such as Co3+,taking up octahedral sites.Collectively,the core structure specie of sepiolite-CoxMn100-xOyshould be existed as Co3+-O and Mn4+-O octahedron.However,the Co and Mn species are violated with precursor valence state.Therefore,it is necessary to further identify the structure evolution mechanism of the catalyst active sites.

3.2.3.Species evolution of catalysts

To further probe the mechanism of structure evolution for catalyst active sites,X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate surface electronic properties.The XPS spectra(Mn2p,Co2p,and O1s)of the catalysts are presented in Fig.S2.As shown in Fig.S2,the Mn2p XPS spectrum is divided into three spin-orbit peaks of Mn 2p3/2 at 643.85 eV,Mn 2p3/2 at 642.03 eV,and Mn 2p3/2 at 640.65 eV,correlating with the presence of Mn4+,Mn3+,and Mn2+,respectively by Gaussian peak-fitting method[39].With the same method,the two spin-orbit peak and two shake-up satellites are obtained[40].In detail,the peak centered at 782.75 eV and the shake-up satellite peak at 798.58 eV(ΔECo2p:15.83 eV)confirm the existence of Co2+species.The other spin-orbit peak at 780.72 eV and the shake-up satellite peak at 796.5 eV(ΔECo2p:15.78 eV)represent the presence of Co3+.The XPS spectrum of O1s could be divided into three peaks located in 530.89 eV,531.95 eV,and 532.95 eV,which are assigned to lattice oxygen (Olatt),surface oxygen (Osur),and chemisorbed oxygen(Oadp),respectively[40].To confirm the mechanism of structure evolution,each specie atomic ratio of Mn,Co,and O element(calculated the areas of the corresponding peaks)varied with the cobalt weight ratio in precursor mixture are presented in Fig.6.Firstly,for the cobalt element,the atomic ratio of Co3+species more than the corresponding Co3+in the low weight ratio of cobalt nitrate (sepiolite-Co3Mn97Oy).To our interest,Co2+transfer to Co3+although without extrareducing agent or non-spontaneous reaction between Co2+and Mn2+was because of ΔG> 0 (detailed discussion is referred to the Supporting Information).Therefore,retrospecting to the conclusion of Raman analysis,we can infer that Co3+takes up octahedral sites.To compare the M-O octahedrite stability,we induce crystal field stabilization energy(CFSE).It is known that CFSE is dependent on the electronic properties(charge/radius ratio,electrons distributions,etc.)of the center element if sharing the same ligand atom.In this work,the Co(III)octahedrite is more stable than Co(II)octahedrite,which is ascribed to fulfill state in the 3d orbit and its higher charge/radius ratio.As the cobalt precursor(Co2+)mass ratio increased,the atomic ratio of Co(III)decreases,apparently agreeing with our expectation.However,the atomic ratio of Mn2+shows obvious decrements,accompanying by the corresponding parts of Mn3+and Mn4+increase,as cobalt nitrate mass ratio increases(Fig.6).Combining the standard electrode potential of Co3+/Co2+,Mn3+/Mn2+,Mn4+/Mn2+,and Mn4+/Mn3+in the alkaline condition,the reaction of Mn2+reduction towards high valence state by Co3+is spontaneous[41].Therefore,it is inferred that high valence state Mn species are induced by synergism effects of Co(III)-O octahedrite transferred from Co(II)-O tetrahedron and redox ability between Co3+and Mn2+.Actually,the cobalt nitrate plays a catalytic role under the effect of the crystal field.As to oxygen species distribution,it is interesting that the atomic ratio of lattice oxygen over the surface shows the same tendency with Mn4+.At the same time,the atomic ratio of surface oxygen decreases with a large margin(Fig.6).As previously reported[27],oxygen evolution in the CoMn2O4detected by insitu TP technologies conforms that oxygen evolution follows both from migration from bulk phase lattice to surface lattice and surface oxygen vacancies produced by surface oxygen spill out.In this work,boosting lattice oxygen could be ascribed to the fact that larger oxidation state,such as Mn4+,induce cationic vacancies formation to balance charge,driving lattice oxygen transferring from bulk phase to surface.Additionally,the atomic ratio of surface oxygen descendant will result in a high concentration of oxygen vacancies[31].

Fig.6.Species atomic ratio of Mn,Co,and O element(calculated the corresponding peaks areas) varied with the cobalt mass ratio in precursor mixture of sepiolite supported CoxMn100-xOy(x:3,10,15,20).

3.3.Plausible toluene degradation mechanism for sepiolite-Co15Mn85Oy

The aforementioned analyses have shown that cobalt species in the precursor mixture induces a significant change in an atomic ratio of eventual lattice oxygen and Mn4+species in catalyst structure.As previously pointed,oxygen species lattice oxygen plays a significant effect on the oxidation activity and selectivity of toluene over transition metalbased catalysts[42]because strong chemisorption takes place between toluene and lattice oxygen,promoting toluene deep degradation[43]as proved by the d-band center of active sites downshift towards Femi level(Fig.7a–b),or act as reversible redox center for lattice oxygen regeneration[44].Moreover,the high atomic ratio of Mn4+accelerate the creation of oxygen vacancies,subsequently driving the lattice oxygen transferred from bulk phase to surface.Therefore,the oxygen vacancies could be refilled with mobile oxygen in the sepiolite structure.Additionally,the redox cycle of Mn4+/Mn3+could easily take place,making lattice oxygen transfer continuously[45](Fig.7c).In all,under the catalytic behaviors of Co dopant,the lattice oxygen and Mn4+/Mn3+could boost toluene degradation performances with high efficiency under mild condition by such cost-effective catalysts.

4.Conclusions

In this work,to degrade toluene with more mild conditions,costeffective sepiolite supported CoxMn100-xOyare developed.The optimal catalytic performance shows that the completely conversion of toluene is achieved by sepiolite supported Co15Mn85Oyat low temperature(193°C),which is superior to the performance of catalyst as previously reported.Additionally,the excellent performance is accompanied by high reuse efficiency,stability,and acceptable mechanical properties.Further by characterized by SEM,TEM,XRD,XPS,Raman,and H2-TPR,the essentials of outperformance are ascribed to the redox engineering induced the high atomic ratio of surface lattice oxygen with continuous refilled from the bulk phase and initiated Mn4+/Mn3+cycle by cobalt catalytic behaviors.Collectively,redox engineered sepiolite-based catalysts not only boost VOCs catalytic oxidation via inducing grands of lattice oxygen from both active components and support,but could provide high efficiency-to-cost ratio catalysts in the related catalytic oxidation technology.

Fig.7.(a)d electrons density of state of sepiolite supported CoxMn100-xOy(x:3,10,15,and 20).(b)d band center of sepiolite supported CoxMn100-xOy(x:3,10,15,and 20).(c)Plausible toluene degradation mechanism for sepiolite-Co15Mn85Oy.

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 is Supported by the National Natural Science Foundation of China(21707023),Provincial Key Research and Development Plan of Hunan Province(2018SK2034),and New Faculty Start-Up Funding from Xiangtan University(18QDZ16).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.07.050.