A facile preparation of hausmannite as a high-performance catalyst for toluene combustion

Qi Liu,Gao Cheng,Ming Sun,Weixiong Yu,Xiaohong Zeng,Shichang Tang,Yongfeng li,Lin Yu

School of Chemical Engineering and Light Industry,Key Laboratory of Clean Chemistry Technology of Guangdong Regular Higher Education,Guangzhou Key Laboratory of Clean Transportation Energy Chemistry,Guangdong University of Technology,Guangzhou 510006,China

Keywords:Catalysis Nanomaterials Environment Toluene Solvents Mn3O4

ABSTRACT Mesoporous transition metal oxide catalysts are well-used in the elimination of volatile organic compounds.In this study,we developed an efficient method for the preparation of mesoporous-Mn3O4 (m-Mn3O4) without the use of templates or surfactants.In this method,KCl protects oxygen defects on the surface of fresh Mn3O4 crystallites.m-Mn3O4 shows higher ameliorative catalytic activity than bulk-Mn3O4 (b-Mn3O4) and calcined-Mn3O4 (c-Mn3O4),achieving toluene catalytic oxidation of T10 and T90 (the temperature at a conversion rate of about 10% and 90%) at 191 ℃and 230 ℃,respectively(WHSV=40,000 ml?g-1?h-1).Based on various characterizations,the prepared m-Mn3O4 has large specific surface area and abundant oxygen defects,and thus can provide more surface active sites,which give it superior toluene combustion activity.

1.Introduction

Volatile organic compounds (VOCs) pose great harm to human health and the environment [1,2].Hence,it is imperative to develop efficient approaches for VOCs abatement.Catalytic combustion is considered to be an efficient solution for VOCs treatment because of its high efficiency,low energy requirements,and relatively low volume of by-products [3].In general,two basic kinds of catalysts have been used:supported noble metal catalysts(e.g.,Pt/TiO2[4],Ag/UiO-66 [5],Pt/Al2O3[6],Pt/K-SiO2[7]) and transition metal oxide catalysts[1].But,the high cost,easy poisoning and sintering of the noble metal catalysts limits their application [8].Therefore,transition mental oxides (e.g.,Mn3O4,Co3O4,NiO,Fe2O3and CeO2) are becoming increasingly popular in view of advantages such as low cost,poison resistance,and thermal stability [9].

Among the transition metal oxides,Mn3O4is reported to be highly active for catalytic combustion of VOCs such as ethanol,ethyl acetate,formaldehyde,benzene,toluene,and ethylbenzene[10–13].Pulet al.reported that a 3D framework of mesoporous Mn3O4synthesized through the reduction method exhibited higher activity for toluene and carbon monoxide complete oxidation,compared with frameworks synthesized through precipitation and solution combustion [14].Liaoet al.found that hollow Mn3O4exhibited better activity and thermal stability because of its hollow structure and abundant active oxygen [15].Furthermore,mesoporous Mn3O4exhibited better performance for VOCs combustion than nonporous Mn3O4[16].Mesoporous catalysts which have large pore volume and high specific surface area usually improve catalytic activity for VOCs oxidation because they expose more reactive sites [17].Mesoporous structures can also efficiently enhance mass transfer of gas phase reactants and accelerate their adsorption and diffusion [18].

Many reported methods for the preparation of mesoporous Mn3O4were based on template synthesis (hard template [19] or soft template [20]),hydrothermal Ostwald ripening synthesis[21] and precursor synthesis [22].However,these methods are often energy-intensive and laborious,and few studies have reported facile methods of fabricating porous metal oxides at room temperature.Therefore,the search for an easy template-free strategy for synthesis of mesoporous metal oxides,or even one in which the template can be easily removed,is of great importance.

In this study,we developed a facile method of preparing mesoporous Mn3O4with abundant oxygen defects at normal temperature without the use of templates or surfactants.During the preparation,KCl played an important role in the formation of pores structure and oxygen defects.Furthermore,we found that the amount of H2O and KOH would also influence the crystal and pore structures of products.As-prepared catalyst have been performed to catalyze the toluene combustion,and the m-Mn3O4exhibits superior catalytic oxidation activity,ascribing to its mesoporous structure and rich oxygen defects.

2.Methods

2.1.Catalyst preparation

Mesoporous Mn3O4was prepared by a simple roomtemperature synthetic route as follows:first,30 ml of KOH ethanol solution (0.8 mol?L-1) was poured into 20 ml of MnCl2ethanol solution (0.8 mol?L-1),and 5 ml of deionized water was dropped into the mixed solution.Then the mixture was reacted under ultrasonic conditions for 30 min.Finally,the as-obtained powder was washed with water several times,and dried at 60 ℃overnight.The product was denoted as m-Mn3O4.

The bulk Mn3O4was synthesized using a similar process.30 ml of KOH aqueous solution was poured into 20 ml of MnCl2aqueous solution.Then the mixture was reacted for 30 min.The powder was washed alternately with water and ethanol several times,and finally dried at 60 ℃overnight;the product was denoted as b-Mn3O4.

For comparison,commercial MnO2was calcined at 800 ℃in an Ar gas atmosphere,and was denoted as c-Mn3O4.

To probe into the formation and effect of mesoporous characteristics,Mn3O4-KCl was prepared using a similar process to that used for m-Mn3O4.30 ml of KOH ethanol solution was poured into 20 ml of MnCl2ethanol solution under continuous stirring,and 5 ml of deionized water was dropped into the mixed solution.Then the mixture was reacted in an ultrasonic environment for 30 min.The as-obtained powder was washed with ethanol several times,and dried at 60 ℃overnight.

2.2.Materials characterization

XRD (X-ray powder diffraction) patterns were collected on a Panalytical Aries X-ray diffractometer under Cu Kα radiation.The data were obtained at a scan speed of 5 (°)?min-1from 10° to 80°.The Raman spectra were obtained from a LabRAM HR800 UV Laser Raman spectrometer with a wavelength of 632.8 nm.FESEM(field emission scanning electron microscopy)was performed with an SU8220 microstructure.FE-TEM (field emission transmission electron microscopy)of the microstructure was performed on an FEI Themis Z instrument at 300 kV.N2adsorption–desorption experiments were performed with an Autosorb iQ2 (ANTON PAAR QUANTATEC INC) and all samples were degassed at 250 ℃for 2 h at vacuum ambiance.Specific surface area was measured with the BET method,and average pore size distribution with the BJH method.H2-TPR (hydrogen temperature-programmed reduction)experiments were conducted with the Micro.AutoChem Ⅱ2920.During H2-TPR,30 mg of the sample were preprocessed in Ar at 250 ℃and then heated to 600 ℃,in 5% H2/Ar combination gas.In addition,standard Ag2O samples were used to correct the hydrogen consumptions.XPS (X-ray photoelectron spectroscopy) data was acquired with an Escalab 250Xi device employing an Al Kα source.

2.3.Activity investigation

Catalytic activity tests of toluene combustion were carried out in a fixed bed reactor.Catalyst (0.25-0.38 mm) mixed with an equal amount of quartz sand was placed in the fixed bed reactor center supported by silica wool.The gas flow rate was set as 33.4 ml?min-1,and achieved a concentration of 0.1% toluene.Toluene was generated from a bubbler and the off gas was detected by an on-line gas chromatograph (Agilent 6820) with a flame ionization detector (FID).Only CO2and H2O were observed as final oxidation products.The catalytic activity test for weight hourly space velocity was done in triplicate,and the error bars of average toluene conversion were computed.The following equations were used to compute the reaction rate (r) and apparent activation energy (Ea):

whereCiis the toluene concentration in the inlet andmcatis the mass of catalyst.Whenris obtained,apparent activation energy can be calculated as follows:

3.Results and Discussion

3.1.Structure analysis

In order to recognize the crystalline phase of the as-prepared samples,we conducted XRD analyses.The XRD patterns of three catalysts are shown in Fig.1(a),and all characteristic peaks are well indexed with pure Mn3O4(JCPDS No.24-0734),where the 2θ values of 18°,28.88°,31.01°,32.31°,36.08°,36.45°,58.51°,59.84°,and 64.65° correspond to the (101),(112),(200),(103),(211),(202),(321),(224) and (400) planes of Mn3O4[23],respectively.Moreover,m-Mn3O4displays weaker and wider diffraction peaks than b-Mn3O4and c-Mn3O4,indicating its lower crystallinity and probable rich oxygen defects.The results for Mn3O4-KCl are indexed with Mn3O4(JCPDS No.24-0734) and KCl (JCPDS No.41-1476) (Fig.1(c)).

The Raman spectra were determined to acquire more structural information for the three samples.As shown in Fig.1(b),only one peak at 645 cm-1is observed for m-Mn3O4and b-Mn3O4,while three distinct peaks at 310,361 and 652 cm-1are observed for c-Mn3O4,in accordance with the literature [24].This is ascribed to the pure Mn3O4phase.The Raman peak at 652 cm-1is ascribed to the A1gmode in spinel structure,which further corresponds to the Mn-O stretching vibration of Mn2+in tetrahedral coordination.Meanwhile,the other two Raman peaks at 361 and 310 cm-1can be ascribed to the combined vibrations of oxygen atoms located at tetrahedron and octahedron sites [22].The Raman peaks of m-Mn3O4are clearly weaker than others,implying low crystallinity;this is confirmed by the XRD results.Moreover,a red band-shift phenomenon appeared in m-Mn3O4(645 cm-1),suggesting that there are more defects in its structure in m-Mn3O4compared to c-Mn3O4(652 cm-1) [25].

Fig.2 shows the N2adsorption–desorption isotherms and pore size distribution curves of the prepared products,and the results are listed in Table 1.Accordingly,m-Mn3O4displays a clear V-type isotherm and H1-type hysteresis ring (Fig.2(a)),implying mesoporous characteristic[26].As shown in pore size distribution curves (Fig.2(b)),m-Mn3O4displays an average pore size of～17.3 nm.The c-Mn3O4samples display a type III isotherm,suggesting that they have no pores;this conjecture is supported by pore size distribution curves.As seen in Table 1,the reducing degrees of specific surface area were as follows:m-Mn3O4>b-Mn3O4>c-Mn3O4.This does correspond to the result of catalytic activity.Furthermore,Mn3O4-KCl exhibits specific a nonporous structure and surface area of 21.7 m2?g-1,dramatically lower than that of m-Mn3O4,suggesting the formation of mesoporous structure after being washed with water.Generally,nano-catalysts possess high specific surface area and mesoporous structure can provide rich active sites,which is especially beneficial especially to heterogeneous catalysis [27].Hence the m-Mn3O4,which had both high specific surface area (108.2 m2?g-1) and mesoporous structure (～17.3 nm) showed excellent catalytic activity for toluene catalytic oxidation.

Fig.1.(a)XRD patterns of three Mn3O4 samples;(b)Raman spectra of three Mn3O4 samples;(c)wide-angle XRD patterns of Mn3O4-KCl sample.①The×3 symbol indicates that the ordinate was three times greater than for other samples.

Fig.2.(a) N2 adsorption–desorption curves of Mn3O4 samples;(b) BJH pore size distributions curves of Mn3O4 samples.

Fig.3.FE-SEM images of (a) m-Mn3O4,(b) b-Mn3O4,and (c) c-Mn3O4.

Fig.3 displays the FE-SEM images of the m-Mn3O4,b-Mn3O4and c-Mn3O4catalysts.It is evident that m-Mn3O4showed a nearly spherical particle morphology with a diameter of～15 nm,and rich pores stacked by these particles are observed.Comparably,b-Mn3O4showed a nearly spherical particle morphology with a diameter of～60 nm,suggesting that ultrasonic reactions facilitate a decrease in particle diameter.Furthermore,b-Mn3O4exhibited a poreless morphology,due to KCl being used as a template to fabricate the pore structure.For c-Mn3O4sample,it shows a sintered morphology,which is one of the reason for its low activity.

We further observed the microcosmic framework of asprepared products with FE-TEM.As seen in Fig.4(a),the mesoporous frameworks of m-Mn3O4consist of uniformly sized nanoparticles which formed irregular pores with an average pore size of～17 nm,corresponding to the results of the N2-adsorption–desorption experiments.For comparison,Fig.4(b)and (c) show TEM images of b-Mn3O4and Mn3O4-KCl,which both exhibit tightly bound accumulate crystals and nonporous structures.In Fig.4(d) and (e),interplanar spacing of 0.239 nm is evident,corresponding to the (004) plane of Mn3O4.Fig.4(d) also shows oxygen defects formed by KCl removal [28,29].In the HRTEM image(Fig.4(f)),two lattice spacings are observed,indicating that KCl and Mn3O4deposited nearly.Hence,the diverse microtopography of the samples before and after removing KCl(Fig.4(a)and c) indicates the formation mechanism of m-Mn3O4.

During the preparation process,since the solubility of KCl is much higher than that of Mn3O4,the Mn3O4crystal nucleus crystalized first.According to the heterogeneous nucleation,KCl tended to crystallize on the defects of fresh Mn3O4surface,thereby reducing the surface Gibbs free energy.KCl was not only a template for mesoporous formation,but also prevented the disappearance of oxygen defects caused by the crystallization of Mn3O4and Ostwald ripening [30].Finally,the sample was washed several times to remove KCl and successfully prepared mesoporous Mn3O4with a large number of surface oxygen defects.

Fig.5.XRD patterns of m-Mn3O4 with different amount of water.

We successfully prepared the mesoporous Mn3O4sample under optimal conditions.Its structure was indeed influenced by the amount of water and the concentration of KOH.In order to study the influence of the amount of water,we performed amountdependent experiments by XRD (Fig.5).When 5 ml of water was added,the characteristic peaks were well indexed with pure Mn3O4(JCPDS No.24-0734).However when 3 ml of water was added,the (002) diffraction peak of MnOOH (JCPDS No.18-0804)appeared,indicating that a new crystalline phase had been produced.Meanwhile,the intensity of all the diffraction peaks indexed with Mn3O4began to decrease significantly.When adding 1 ml of water,all characteristic peaks decreased continuously.In general,according to the reaction formula below,when the water added in reaction was inadequate,the concentration of OH-was insufficient and MnOOH appeared in the product.Thus,with the rise of the amount of water,the concentration of OH-also rose and MnOOH was completely oxidized to Mn3O4.Therefore,we concluded that during the synthesis of mesoporous Mn3O4,the amount of water has an important influence on the formation of the pure Mn3O4crystalline phase.

Fig.6.XRD patterns of Mn3O4 samples with different concentration of KOH.

Regarding the concentration of KOH,the crystalline phase showed no change when the concentration of KOH increased from 0.4 to 1.2 mol?L-1(Fig.6).However,the sample for low concentration of KOH (0.4 mol?L-1) exhibited a structure with fewer pores and large crystalline particles,as shown in Fig.7(a).This is due to the Ostwald ripening of Mn3O4crystal when KCl sediment is scanty [30].A high concentration of KOH (1.2 mol?L-1) induced aggregation of KCl instead of homogeneous fabrication of Mn3O4.This is the reason for the almost total lack of pore structure in Fig.7(c).Moreover,when the concentration of KOH decreased from 0.8 to 0.4 mol?L-1,the specific surface area decreased to 68.7 m2?g-1and the average pore size decreased to 12.99 nm(Fig.8).This result can be ascribed to the lack of KCl sediment,which led to a decreasing trend in pores and specific surface area.When the concentration of KOH increased to 1.2 mol?L-1,the specific surface area decreased dramatically to 7.2 m2?g-1and the average pore size increased to 27.88 nm.This resulted from superabundant sediment and aggregation of KCl.Hence,the pore structure of Mn3O4can be affected by the concentration of KOH.Based on the above conclusions,we were able to identify optimal solution.

3.2.Catalytic performance and discussion

Fig.7.FE-SEM images of Mn3O4 samples with different concentration of KOH.(a) 0.4 mol?L-1 KOH;(B) 0.8 mol?L-1 KOH;(c) 1.2 mol?L-1 KOH.

Fig.8.(a) N2 adsorption–desorption curves;(b) BJH pore size distributions curves of Mn3O4 samples with different concentrations of KOH.

Fig.9.(a) Toluene conversion over the Mn3O4 samples;and (b) Arrhenius plots of Mn3O4 samples at WHSV of 40000 ml?g-1?h-1.

As seen in Fig.9(a),the toluene catalytic oxidation tests over m-Mn3O4,b-Mn3O4and c-Mn3O4were measured at a WHSV of 40,000 ml?g-1?h-1.It is clear that the catalytic activities display the following sequence:m-Mn3O4>b-Mn3O4>c-Mn3O4.In detail,the temperature at conversion of about 10%of toluene oxidation on m-Mn3O4,b-Mn3O4,and c-Mn3O4is found to be 191 ℃,217 ℃and 291 ℃,respectively.Moreover,the m-Mn3O4catalyst exhibit high toluene conversion (90%) at 230 ℃,which is ≈2.6 and ≈69 times higher than the toluene conversions achieved on b-Mn3O4(34.8%) and c-Mn3O4(1.3%) catalysts,respectively.Thus,the m-Mn3O4catalyst demonstrate higher catalytic activity than the b-Mn3O4and c-Mn3O4catalysts.The apparent activation energy(Ea) was further used to appraise the catalytic activities of the Mn3O4samples,and was calculated from the linearity relationship between lnkand 1000/T(Fig.9(b)).The activation energy value(Table 1) of the m-Mn3O4sample (36.4 kJ?mol-1) is far lower than those of the b-Mn3O4(85.7 kJ?mol-1) and c-Mn3O4(89.5 kJ?mol-1)samples,illustrating that m-Mn3O4was more efficient for catalytic combustion of toluene.The catalytic performances of manyrecorded catalysts for toluene combustion are shown in Table 2 for comparison,indicated of an advanced catalytic activity of m-Mn3O4sample.

Table 1 Activity tests,structural properties,and kinetic parameters of Mn3O4 samples.

Fig.10.(a) Different WHSVs for toluene combustion over m-Mn3O4 sample.(b)Catalytic stability measurement of toluene combustion over m-Mn3O4 at a WHSV of 40,000 ml?g-1?h-1 and 230 ℃.(c) Wide-angle XRD patterns of used m-Mn3O4.

Fig.11.XPS of (a) Mn 2p,(b) Mn 3 s and (c) O 1 s for Mn3O4 samples.

Catalytic oxidation of toluene was also performed at two other WHSVs (20000 and 60000 ml?g-1?h-1),and the catalytic oxidation trend is given in Fig.10(a).It can be clearly seen that there is an increase in catalytic activity with the drop of WHSV,because of the limited contact time between reactant molecules and the surface active sites of the catalyst.Moreover,we evaluated stability through a continuous reaction experiment over m-Mn3O4catalyst for a total running time of～50 h at a temperature of 230 ℃(Fig.10(b)).Toluene conversion was almost constant during the entire experiment.Then,we characterized the used m-Mn3O4by XRD and find no crystal structure change in the catalyst (Fig.10(c)).In consequence,we can state that m-Mn3O4performs with favorable stability for toluene catalysis.

The surface elemental states of the m-Mn3O4,b-Mn3O4and c-Mn3O4samples were acquired by the XPS technique.Fig.11 presents the high-resolution spectra (Mn 2p,Mn 3s and O 1s) of the three Mn3O4samples,and the corresponding results are listed in Table 3.The BE (binding energy) of Mn 2p1/2and Mn 2p3/2(Fig.11(a)) are centered at～653.5 and～641.7 eV,respectively,and the splitting is 11.8 eV,which corresponds to recorded Mn3O4samples[39].Accordingly,the oxidation state of Mn is identified accurately by the Mn 3s spectra.An equation (AOS=89.5–1.13ΔE3s) can be used to calculate the AOS (average oxidation state) of Mn [40].Fig.11(b) shows the Mn 3s spectra of the asprepared Mn3O4samples.The interval in BE (ΔE3s) between the two Mn 3s peaks of the three samples is 5.7,5.6,and 5.5 eV.The AOS of Mn for m-Mn3O4is calculated to be 2.51,which is lower than the AOS for b-Mn3O4(2.65)or c-Mn3O4(2.74).This result suggests that the m-Mn3O4sample has the most surface oxygen defects,which is confirmed by the electroneutrality principle[41].

Fig.11(c) presents the O 1s spectra of the Mn3O4samples.Deconvolution of two regions located at～529.9–530.1 and～530.6–531.6 eV corresponds to the surface lattice oxygen species(OL) and surface absorbed oxygen species (OA),respectively [42].The OA/OLratio of the Mn3O4samples in Table 3 decreases in the following order:m-Mn3O4>b-Mn3O4>c-Mn3O4,corresponding to the result of catalytic activity.These results imply that the predominant OAexists on the m-Mn3O4surface.In an unsaturated coordination environment,m-Mn3O4possesses surface oxygen species chained to metal cations,which is involved in the formation of OA[43].Our findings confirm that the absorption and activation of gas phase oxygen is dependent on surface oxygen defects in the catalyst [44].Therefore,m-Mn3O4with rich surface oxygen defects has an abundance of OA.

The reducibility of m-Mn3O4,b-Mn3O4and c-Mn3O4was proved by H2-TPR measurement(Fig.12).Generally,the reduction of these samples can be divided into two stages (Table 3).The lowtemperature peak is ascribed to the reduction of Mn3+to Mn2+in the tetrahedral site of the hausmannite lattice and the hightemperature peak can be ascribed to the reduction of hausmannite[45].Clearly,m-Mn3O4exhibits the best reducibility,with two reduction peaks at 145 ℃and 346 ℃.For b-Mn3O4,the lower reduction peak is located at 195 ℃,while the higher reduction peak centers at 406 ℃.For c-Mn3O4,it exhibits the worst reducibility,with two reduction peaks at 457 ℃and 651 ℃.Total H2consumption is 4.22 mmol?g-1for m-Mn3O4,3.87 mmol?g-1for b-Mn3O4,and 4.30 mmol?g-1for c-Mn3O4,which is in agreement with theoretical values[46].These results show that the reducibility follows the order m-Mn3O4>b-Mn3O4>c-Mn3O4,indicating the best reducibility for m-Mn3O4.Better reducibility of m-Mn3O4means that it has relatively higher mobility of OLspecies,which contributes significantly to improving catalytic activity for toluene combustion [47].

Table 3 Surface information and H2-TPR profiles of Mn3O4 samples.

Commonly,this toluene catalytic oxidation follows a Mars-van Krevelen(MVK)mechanism.The reactants(toluene molecules)are absorbed on the oxygen defects which are active sites.Activated toluene molecules react with lattice oxygen species to form CO2and water.Meanwhile,reduced lattice oxygen sites are reoxidized by O2molecules from the air[29,48,49].Therefore,abundant surface oxygen defects and excellent mobility of lattice oxygen species of m-Mn3O4sample are the reasons for its superior catalytic performance of m-Mn3O4toluene elimination.

Fig.12.H2-TPR outlines of Mn3O4 samples.

4.Conclusions

We propose a facile and green method of fabricating m-Mn3O4with rich oxygen defects at room temperature,without any templates or surfactants.Of the Mn3O4samples tested,m-Mn3O4presents superior catalytic activity and stability for toluene combustion.The experimental results indicate that the excellent catalytic performance of m-Mn3O4is due to its high specific surface area,mesoporous structure and abundant oxygen defects.

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 Natural Science Foundation of China(Nos.21306026,21576054),Natural Science Foundation of Guangdong Province (No.2018A030310563),and the Foundation of Higher Education of Guangdong Province(2018KZDXM031).