Promotional effect for SCR of NO with CO over MnOx-doped Fe3O4 nanoparticles derived from metal-organic frameworks

Yu Zhang ,Ling Zhao,3, *,Ziang Chen ,Xinyong Li

1 School of Ecology and Environment,Inner Mongolia University,China

2 School of Environmental Science &Technology,Dalian University of Technology,China

3 College of Veterinary Medicine,University of Florida,United States

Keywords: Metal-organic framework MnOx-doped Fe3O4 NO reduction In situ FT-IR Reaction mechanism

ABSTRACT MnOx-Fe3O4 nanomaterials were fabricated by using the innovative scheme of pyrolyzing manganesedoped iron-based metal organic framework in inert atmosphere and exhibited extraordinary performance of NO reduction by CO(CO-SCR).Multi-technology characterizations were conducted to ascertain the properties of fabricated materials (e.g.,TGA,XRD,SEM,FT-IR,XPS,BET,H2-TPR and O2-TPD).Moreover,the interaction between reactants and catalysts was ascertained by in situ FT-IR.Experimental results demonstrated that Mn was an ideal promoter for iron oxides,resulting in decrease of crystallite size,improve reducibility property,enhance the mobility and the amount of lattice O2-species,as well as strength the adsorption ability of active NO and CO to form multiple species (e.g.,nitrate and carbonate).The unprecedented enhancement of CO-SCR activity over Mn-Fe nanomaterials follows the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction pathway.

1.Introduction

As fueled by the rapid deepening of industrialization and urbanization in China,air pollution has been increasingly considered as rigorous menace and has aroused widespread concern.Similar to most industrialized countries,China is now experiencing the air pollution shifting from SO2-dominated to NOx-and O3-dominated [1].Excessive nitrogen oxides in the atmosphere account for serious problems (e.g.,lung bronchial disease,photochemical smog,as well as global warming).Note that nitrogen oxides were found considerably promoting the conversion of sulfur dioxide to sulfates,limiting the environmental capacity of sulfur dioxide and facilitating the formation of haze.Thus,the de-NOxfrom the sources has become a hot spot in environmental catalysis.Vehicle exhaust emissions have noticeably contributed to national and local emission inventories for NOx.In 2018,China had 327 million motor vehicles,a data 5.5% higher than that in 2017.Meanwhile,these vehicle engines emitted 562.9 Mt of nitrogen oxides and 3089.4 Mt of carbon monoxide.For this reason,it has been a long-term challenge to eliminate undesired auto-exhaust gases[2–4].

The evermore stringent emission legislation has been stipulated globally for NOxencourage scholars to delve into and degrade NOxmore effectively.CO selective catalytic reduction (CO-SCR) is the leading technology for the eradication of two major harmful pollutants (NOxand CO) generated by vehicles simultaneously [5–7].During the reaction,ecotoxic NOxand CO will be transformed to nontoxic N2and CO2.Accordingly,this reaction system is critical to environmental protection.

Catalysts are vital to the CO-SCR technology,and their exploration remains in a high demand but still challenging.Metalorganic frameworks (MOFs) refers to an emerging class of porous crystalline materials fabricated by alternatively linking the metal cations or metallic clusters to organic ligands in a 3D porous structure,it has become a research hotspot since its large surface area,high void volume,noticeably low density and flexible structures for manifold applications (e.g.,catalysis,gas storage,adsorption and separation) [8,9].Thus far,some studies have adopted MOFs as a potential candidate for high performance medium-low temperature SCR catalysts,and commendable evidence have made.Qinet al.reported a metal–organic framework by adding different ratio Ag atoms(Agx-Cu-BTC)to be applied in CO-SCR reaction,and the NO conversion reached 100% before 280°C[10].Wang and his co-workers prepared Mn-,Fe-,Al-or Zn-doped ZIF-67@CuOxcatalysts,and further confirmed the deNOxcapacities were over 90% at 200°C[11].Nonetheless,it has been rarely reported that MOFs acts as a catalyst at elevated temperatures since it is largely unstable at high temperature,and its skeleton will collapse at 300–400 °C.

Since numerous MOFs exhibit limited thermal stability,which is typically not suitable for high-temperature applications,MOFs has been adopted as feasible self-sacrificing template or precursor to construct porous metal oxide nanostructures and carbon-based materials [12–14].Note that many new MOF-derived catalysts synthesized by pyrolysis under an inert gas exhibiting more stable structures.The nanoparticles prepared based on MOFs template will be more uniform in size,exhibit good dispersion and maintain the morphology and porosity of the template.For instance,Jianget al.synthesized MnOxnanoparticlesviathe thermolysis of Mn-MOF-74,which almost kept the spherical shape and particle size of the initial Mn-MOF-74 template [15,16].However,the application of MOF derivatives in automobile exhaust purification by CO-SCR technology is scarce.Herein,a demand of extensive researches for this subject is imperative.

Recently,iron-containing catalysts are highly-profiled for their prominent SCR performance,low cost,high selectivity to N2and eco-friendly property.In the previous study,considerable mixed transition metal oxides (e.g.,Cu-Fe,Fe-Ti,Fe-W and Fe-Co) acted as active components to facilitate the SCR reaction.It has been reported that bimetallic catalysts exhibit higher activity than monometallic.This finding is reasonably explained as the coexistence of two metal atoms in the same unit cell leads to lattice defect.The lattice oxygen is converted to adsorbed oxygen and generates numerous oxygen vacancies.Meantime,the electron arrangement for the lattice defect is unstable and can increase Lewis/Br?nsted acid sites [17–20].This type of structural characteristics is conducive to improving the NO reduction efficiency of the catalysts.To the best of our knowledge,abundant researches have focused on stable Fe2O3(hematite),but the recoverability of Fe2O3is poor that may affect the reusability of the material.Based on this,the magnetic recoverability of nano materials has gained popularity.Magnetite(Fe3O4)enable an effortless catalyst recovery at the end of a chemical reaction [21].Likewise,Mn had been demonstrated as a remarkable low-temperature SCR component owning to variable valence,excellent redox capabilities and affluent Lewis acid sites,as well as less expensive and easilyobtained,the strong interactions between Mn and Fe cations greatly inhibit aggregation of Mn particles,and easier generation of abundant reactive oxygen species [22,23].

Given that the development of denitration technology requires a detailed understanding of the surface materials formed during the catalytic reaction.For this end,a series of Mn–Fe metallic oxides materials with different Mn loadings derived from metal organic framework was preparedviaa simple conventional solvothermal method.The synthetic mechanism of materials is shown in Fig.1.Comprehensive analysis of various characterization results was combined within situFT-IR results to explain the specific circumstance of NO and CO adsorption on catalysts as well as explore the potential mechanism of de-NOxreaction using CO-SCR technology.

2.Experimental

2.1.Reagents and chemicals

All of the chemicals and reagents were received from their respective vendors and used as received without further purification process.N,N-dimethyl-formamide (DMF,HCON(CH3)2,99.5%),Iron (III) chloride hexahydrate (FeCl3?6H2O,>99.0%),1,4-benzenedicarboxylic acid (1,4-H2BDC,C8H6O4,99.0%),manganese(II) acetate tetrahydrate (Mn(CH3COO)2?4H2O,>99.0%),urea (H2-NCONH2,99.0%),ethanol (CH3CH2OH,99.7%).All the gas mixtures were supplied by Beijing:NO/Ar(1.0% (vol)),CO/Ar(2.0% (vol)),H2/Ar (10.0% (vol)),and O2/He (10.0% (vol)).

2.2.MOF precursor and derived oxides preparation

MIL-53(Fe) precursor was prepared by solvothermal method based on the previously reported procedure.More precisely,using the mixed solvent of DMF and distilled water (v/v=4/1,50 ml total) to dissolve the FeCl3?6H2O (2 mmol),1,4-H2BDC (3 mmol)and urea(5 mmol),then subjected to make it fully dissolved.Soon afterwards,the resulting solution was loaded and sealed into a polytetrafluoroethylene-lined hydrothermal autoclave (100 ml)and reacted at 180 °C for 3 h.After cooling the resulting products were collected,washed with an ethanol–water mixture and finally dehydrated under 80°C overnight in an oven.Mn-MIL-53(Fe)with varying Mn:Fe synthesis molar ratios of 0.5:1,1:1,2:1 were prepared through a similar process with addition of Mn(CH3COO)2?4H2O.

A series of Fe3O4and MnOx-doped Fe3O4catalysts was prepared by annealing MOF precursor in a tube furnace at 550 °C for 2 h in N2atmosphere,and the final products were labeled asyMnOx-Fe3O4(where “y”stands for the molar ratio of Mn/Fe).

2.3.Catalytic performance evaluation

The catalytic activity was evaluated in a fixed-bed tubular reactor system (7 mm i.d.).The experiment was implemented with a steam contained 1.0% (vol) NO/Ar,2.0% (vol) CO/Ar and were balanced by Ar with a total flow rate was 150 ml?min-1.Prior to performing the CO-SCR,each sample (200 mg) was pretreated in Ar flow at 100 °C for 1 h.Subsequently,the reaction gas mixture was switched.Data was collected with a flue gas analyzer (Kane,British) when the airstream was stabled.The NO conversion percentage and N2selectivity are calculated as follows:

Fig.1.The synthetic mechanism of Mn-MIL-53(Fe)-derived MnOx-Fe3O4 catalysts.

2.4.Physicochemical characterization

Thermogravimetric (TG) curves were checked on SDT Q600 thermal analyzer under N2flow,over the range from room temperature to 800 °C.The X-ray diffraction analysis for crystallographic structures were obtained on D8/ADVANCE diffractometer(Bruker)using Cu Kα radiation source (0.15406 nm) over the 2θ range of 10°–80°.Inductively coupled plasma optical emission spectrometry (ICP-OES) (ICP-OES-5110) was employed to determine the actual Fe and Mn.SEM images were studied by scanning electron microscope(S-4800).The FT-IR spectrum of the powders were performed on a Bruker VERTEX 70 FT-IR spectrometer using KBr as dispersing medium.The specific surface area and pore volume of the series of materials were estimated on a Micromeritics ASAP 2020 apparatus.The relative atomic compositions and elementary oxidation states were monitored by X-ray photoelectron spectroscopy (XPS) using ESCALABXI photoelectron spectrometer.H2-TPR and O2-TPD data were conducted on a chemisorption analyzer(ChemBET TPR/TPD).Prior to the experiment,the sample (50 mg)was placed in a quartz U-type reactor and flushed with N2at 350 °C for 30 min to yield a clean surface followed by decreasing to target temperature.For TPR investigation,switching on 10% (vol) H2/Ar mixture for 30 min at normal temperature,and then carried out with a ramp of 10 °C?min-1,to 900 °C.In O2-TPD runs,the catalyst was exposed to 5% (vol) O2/He and kept constant for 60 min at 100°C,converted to N2for 30 min to purge superfluous O2on the samples.After that,the temperature was elevated to 800 °C at 10 °C?min-1.

In situFT-IR experiments were conducted at a FTIR spectrometer (Bruker VERTEX 70).Before the measurements,the sample wafers were mounted in the IR cell and activated at 350 °C for 30 min,then naturally cooled to 50 °C under high-purity Ar(50 ml?min-1) gas atmosphere.Collecting background spectrum of the solid and automatically deducted from the sample spectrum.The feed gas composition of CO/Ar(2.0%)or/and NO/Ar(1.0%)was then passed over the sample and the temperature raised from 50 to 400 °C.

3.Results and Discussion

3.1.Catalytic activity analysis

Some scholars reported the application of MOF derivatives in the field of NH3-SCR and H2-SCR denitration and exhibiting excellent catalytic activity,as shown in Table 1.But there is no report about CO-SCR reaction.In this part,we tested the CO-SCR performance of synthetic MOF derivatives,the schematic diagram of the laboratory-scale apparatus was presented in Fig.2.The COSCR reaction tests of the Fe3O4and MnOx-Fe3O4catalysts prepared by annealing(Mn)-MIL-53(Fe)were performed in the temperature range of 100–500 °C (shown in Fig.3(a)).The NO conversion of all samples enhanced with the increase of temperature.However,we can see that the catalytic activities of all these catalysts are relatively poor in the low-temperature region (T<300 °C).Notably,with the elevation of temperature,the NO conversion of the Mndoped catalysts was facilitated significantly in comparison with that of the single oxides Fe3O4.Moreover,2MnOx-Fe3O4sample achieved the greatest NO conversion rate of about 97.5% at 500°C.Meanwhile,Fig.3(b)exhibits the N2selectivity of catalysts in CO-SCR.As we can see in Fig.3(b),the MIL-53(Fe)-derived Fe3O4achieved the maximum N2yield of 17.6% at 300 °C,while the N2selectivity of the Mn-doped catalysts was enhanced significantly at a whole temperature range (150–500 °C).The 2MnOx-Fe3O4exhibited outstanding catalytic performances,with a maximum N2yield of 71.5% .It is therefore speculated that Mn doping could enhance the catalytic efficiency of Fe3O4nanoparticles prepared from (Mn)-MIL-53(Fe) precursor.In addition,the optimal sample(2MnOx-Fe3O4) was selected to conduct three cycle experiments(Fig.4(a))and found that the catalytic effect has slightly decreased.Combined with the XRD after the test,we can find that the structure of the catalyst remains stable after the reaction.

Table 1 Catalytic activity on MOF derivatives with reported studies

3.2.TGA results

MIL-53(Fe) is unstable and easily decomposes into iron oxide during heat treatment.To study the decomposition behavior of MOF precursor,the samples of MIL-53(Fe) and (Mn)-MIL-53(Fe)were examined on TGA instrument,and the result was presented in Fig.5.It could be concluded from plots that the prominent weight loss can be split into two stages in the range of 30–600 °C.The first step of weight loss stage below 270 °C was associated with the loss of solvents in the samples.Subsequent weight loss above 270 °C resulted from the collapse of MOF structure caused by the disappearance of organic ligands from their backbone [28].It is easy to see that (Mn)-MIL-53(Fe) precursor had a large weight loss than MIL-53(Fe),indicating the existence of strong organic coordination in (Mn)-MIL-53(Fe).When the temperature exceeded 540 °C,the substance tends to be stable,confirming the decomposition of MOF precursor is expected to take place completely.

3.3.XRD results

The crystalline structure of MIL-53(Fe) was characterized by XRD,as evidenced by the pattern displayed in Fig.4(b),the MOF precursor was in good agreement with the previous work,indicating successful formation of MIL-53(Fe) [28].To further determine the optimal calcination temperature,we studied the change of the calcined MIL-53(Fe) crystal phase at various temperatures(Fig.4(c)).When the calcination temperature was lower than 500 °C,two kinds of iron oxides can be clearly detected,namely,αFe2O3and Fe3O4.The primary diffraction peaks at 18.33°,30.05°,35.38°,37.05°,43.06°,53.43°,56.92° and 62.43° wellfitting the (111),(220),(311),(222),(400),(422),(511) and (440)crystal planes of Fe3O4,respectively.But beyond that,the peaks at 24.14°,33.15°,40.78°,49.48°,54.00°,63.93° was attributable to αFe2O3[29–31].Under higher annealing temperature (550 °C and 650 °C),the characteristic peaks of αFe2O3disappeared and the products transformed into a single Fe3O4species.Compared to calcination at 650°C,the crystallinity of 550°C was better.In order to obtain single Fe3O4oxides,combining TGA and XRD data,our follow-up experiment selected 550 °C as the calcination temperature of the catalyst.

Fig.2.Schematic of the fixed bed reactor for selective catalytic reduction of NO with CO.

Fig.3.(a)NO Conversion of prepared catalysts under the condition of 1.0% (vol)NO,2.0% (vol)CO,Ar balance,GHSV=23,000 h-1.(b)Results of N2 selectivity(%)as a function of reaction temperatures.

In view of the XRD pattern of the calcined products at 550 °C demonstrated in Fig.4(d),the diffraction peaks of all MnOx-Fe3O4catalysts shifted to a lower angle direction as compared with the pure Fe3O4material,such finding revealed that manganese ions replaced iron ions on the equivalent crystallographic position.The average crystallite sizes of the samples were obtained based on the XRD patterns according to the half-peak width of the(311) plane using Scherrer equation.As illustrated in Table 2,the crystallite sizes were estimated as 23,18,17 and 17 nm for Fe3O4,0.5MnOx-Fe3O4,1MnOx-Fe3O4and 2MnOx-Fe3O4,respectively.The addition of Mn caused the crystallite size decreased,further demonstrating that manganese ions were incorporated into the crystal lattice of Fe3O4.Several studies highlighted that the decrease in crystallite size could lead to an increase in the BET surface area,thereby facilitating full access between the catalysts and the reactants[32].In addition,the atomic ratios of Mn/Fe detected by ICP in as synthesized samples are very close to the theoretical value (Table 3).

3.4.Morphology

The morphology of the as-fabricated catalysts was presented in Fig.6.As showed in Fig.6(a) and Fig.6(b),the solvothermal reaction yielded MIL-53(Fe) and Mn-doped MIL-53(Fe) with a spherelike particles and rougher surface with particle size distribution of 100–300 nm.Fig.6(c) and d showed the SEM images of Fe3O4and MnOx-Fe3O4,we can clearly see that the metal oxide obtained by annealing well retains the morphology of the MOF precursor.

3.5.FT-IR results

FT-IR test was performed to delve into the molecular structure and determine the functional groups of samples,as well as the measured results are exhibited in Fig.7.For Fe3O4catalyst,the peak at 572 cm-1corresponds to the Fe-O bond vibration of Fe3O4nanoparticles [33].Obviously,this band shifted toward lower wavenumbers after Mn doped.Based on the results of previous researches,the characteristic peaks of the (Mn-O) mode appear at 500–600 cm-1[34].Hence,this transformation originates from replacing iron with manganese.The signal at 748 cm-1belongs to C-H bending vibrations.Moreover,the peaks at 1377 and 1587 cm-1originate from the stretching vibrations of C-O and C=O,respectively [8].The signal at approximately 3431 cm-1was harvested and linked to the stretching vibrations of O-H of water molecules adsorbed on the surface [35].

Fig.4.(a) The catalytic activity of the 2MnOx-Fe3O4 catalyst after 3 cycles;XRD patterns of the products (b) MIL-53(Fe),(c) calcination of MIL-53(Fe) at different temperatures,(d) Fe3O4 and MnOx-Fe3O4 catalysts obtained by calcination at 550 °C.

Fig.5.TGA patterns of MIL-53(Fe) and (Mn)-MIL-53(Fe) MOFs.

3.6.BET results

The N2adsorption–desorption isothermal plots and the corresponding BJH pore size distribution curves (inset) are related to prepared composites as presented in Fig.8,as suggested from the figure,all these catalysts showed type-IV isotherm with H3hysteresis loop,demonstrating the presence of mesoporous structure [36].Furthermore,the data related to textural parameters were listed in Table 2.The specific surface areas of the Fe3O4,0.5MnOx-Fe3O4and 1MnOx-Fe3O4catalysts were 24.3,37.2 and 39.1 m2?g-1,indicating that the Mn-doped catalyst displayed an upward trend in a certain range.The 0.5MnOx-Fe3O4and 1MnOx-Fe3O4catalysts could possess more surface active sites exposed to participate in the catalytic reaction owing to their higher specific surface area,which were in good agreement with CO-SCR reaction tests (Fig.3).However,the surface area of the catalyst evidentlydecreased to 10.4 m2?g-1when the Mn:Fe synthesis molar ratios were 2:1,probably due to the excessive deposition of Mn leading to the agglomeration of particles.

Table 2 Textural properties of the Fe3O4 and MnOx-Fe3O4 catalysts

Fig.6.SEM images of (a) MIL-53(Fe),(b) (Mn)-MIL-53(Fe),(c) Fe3O4,(d) 0.5MnOx-Fe3O4.

Fig.7.FT-IR spectra of fresh catalysts.

3.7.XPS results

Fig.9 presents the results of XPS analysis of fabricated catalysts.As shown in Fig.9(a),the C 1s spectra were recorded in the energy range of 283–289 eV.Three characteristic peaks can be achieved by fitting the curves,where centered around 284.6 eV corresponding to C-C bonding and that at 285.7 eV was primarily associated with C-O bonding,the peak located at 288.5 eV resulted from C=O bonding [37].

Fig.9(b)displayed the XPS spectra for O 1s of the four catalysts.The asymmetrical O 1s spectra of the samples could be deconvoluted into three components at 532.6–533.1 eV,531.4–531.8 eV and 529.8–530.3 eV,attributable to the chemisorbed adsorbed oxygen species(e.g.,,O-,designated asOα′,Oα)and surface lattice oxygen(e.g.,O2-,designated as Oβ),respectively[38].It is generally known that the chemisorbed adsorbed oxygen species possess better mobility and reactivity,being more active and valuable than Oβspecies,which is important for promoting the selective catalytic reduction of NO reaction [39].As displayed in Table 3,on the basis of the deconvolution results,the (Oα+Oα′)/(Oα+Oα′+Oβ) ratio were declined as follows,2MnOx-Fe3O4(59.5%) >1MnOx-Fe3O4(55.3%) >0.5MnOx-Fe3O4(49.0%) >Fe3O4(48.1%),as evidenced by the MnOxpromoter can generate more oxygen vacancies.The above trend is highly in accordance with the catalytic activity (see Fig.3).

The Fe 2p spectra were also studied by XPS,as presented in Fig.9(c),the signals of Fe 2p3/2and Fe 2p1/2were centered around 710.8 eV and 724.3 eV,respectively.By a peak-fitting deconvolution,the peaks corresponding to Fe2+(710.2 eV,723.5 eV) and Fe3+(711.2 eV,725.3 eV) could be identified,respectively [40,41],demonstrating the presence of these two iron oxidation states in the catalysts.The percentage contents of Fe2+for the four samples were calculated by respective peak area in Fe 2p spectrum.As shown in the Table 3,the proportion increased in the order of Fe3O4(36.7%) <0.5MnOx-Fe3O4(42.1%) <1MnOx-Fe3O4(45.4%) <2MnOx-Fe3O4(46.6%),indicating more Fe2+ions exist in the structure by adding an appropriate amount of Mn elements.

Fig.8.N2 adsorption-desorption isotherms and BJH pore size distribution curves of catalysts:(a) Fe3O4,(b) 0.5MnOx-Fe3O4,(c) 1MnOx-Fe3O4,(d) 2MnOx-Fe3O4.

Fig.9(d)gives Mn 2pprofiles,and the peaks cover the Mn 2p1/2spectrum related to the binding energy of 655.3–652.3 eV and the Mn 2p3/2spectrum associated with the binding energy of 643.6–640.6 eV.To be more specific,the peaks at 640.7 eV and 652.4 eV was indicative of the presence of surface Mn2+,the binding energies of 642.0 eV and 653.6 eV corresponded to Mn3+,the peaks located at 643.6 eV and 655.1 eV belong to Mn4+,giving evidence to the existence of Mn2+,Mn3+and Mn4+[38].According to literature,the catalytic performance of MnOxspecies can be attributed to the potential of Mn to form multiple oxides and selectively provide oxygen from its crystalline lattice [42].

3.8.H2-TPR results

It has been extensively accepted that the oxidation reduction performance was the critical point affecting catalytic properties.The reducibility of these catalysts containing different amounts of manganese was assessed by H2-TPR,and the relevant profiles as presented in Fig.10.These H2-TPR curves can be divided into the low temperature region(labeled as H1)and the high temperature region(labeled as H2).As for the fabricated Fe3O4catalyst,the first peak at 380°C(H1)belongs to the reduction of Fe2O3→Fe3O4promoted by surface lattice oxygen,and the wide peak at 642 °C(H2) is related to the bulk lattice oxygen,which stimulates the transformation of Fe3O4to FeO,and then to Fe0[43].Importantly,after manganese had been loaded onto the Fe3O4catalyst,the H2reduction peaks shifted to a lower temperature direction.One can see clearly that the H2 peak temperature varies as below:642 °C (for Fe3O4) →581 °C (for 0.5MnOx-Fe3O4) →563 °C (for 1MnOx-Fe3O4)→605°C(for 2MnOx-Fe3O4).It is worth noting that the peak areas of H1+H2 dramatically enhance with an increase in MnOxcontent,indicating that the redox performance of catalyst is improved obviously after the introduction of Mn species.This result indicates that the synergistic effect of Mn species with other active components is beneficial to the generation of several active oxygen species (Fe3+-O–Mn4+and Fe3+-O–Mn3+),which are quickly reduced to form defects (Fe3+-□-Mn4+and Fe3+-□-Mn3+),and further contributes to enhance reducibility and promote the SCR performance of the MnOx-Fe3O4catalysts[38].

3.9.O2-TPD results

Fig.9.XPS spectra of synthesis catalysts:(a) C 1s,(b) O 1s,(c) Fe 2p,(d) Mn 2p.

In order to ascertain the properties of the oxygen species probably bounded up with CO-SCR reaction,O2-TPD experiments over the as-prepared catalysts are conducted,as exhibited in Fig.11.For Fe3O4,no O2desorption peaks could be observed,confirming a limited reducibility of this catalyst.After Mn-doping,one O2desorption peak was detected and attributed to the lattice oxygen species O2-,which could be associated to manganese oxides [6].Meanwhile,the O2desorption peak area was strongly enhanced with the rise in Mn doping,as showed in Table 3.Number of oxygen vacancies is related to the intensity of peak.The easier reduction of Mn4+to Mn3+as well as the abundant surface vacancies and deficiencies generated from the crystal lattice by manganese ions substitution result in an easier diffusion from bulk to the surface,that is,an improved oxygen mobility [44].It is worth noting that 2MnOx-Fe3O4sample has the largest desorption peak area,manifesting that it could possess the highest oxygen mobility,the most oxygen vacancies as well as the best de-NOxactivity.

3.10.In situ FT-IR testing on NO or/and CO interaction with samples

Fig.10.H2-TPR profiles of Fe3O4 and MnOx-Fe3O4 catalysts.

Fig.11.O2-TPD profiles of Fe3O4 and MnOx-Fe3O4 catalysts.

To further verify intermediate species and elucidate possible reaction mechanism between the catalysts and effluent gas components,the in situ technology was employed to reveal the adsorption status of CO and/or NO species on the catalysts surface at different temperatures.Table 4 lists the observed bands over all catalyst samples and their assignments.

Table 4 Assignments of the in situ FT-IR bands formed upon the adsorption of CO or/and NO over catalysts

3.10.1.Single CO adsorption

In situFT-IR spectra of CO adsorption on the samples over the temperature range from 50 to 400 °C were displayed in Fig.12.We can see that all the catalysts covered by identical peaks at 2173 and 2120 cm-1,as derived from the P and R branches of gaseous CO[32].For Fe3O4catalyst(Fig.12(a)),several bands at 1332,1371,1477 and 1546 cm-1were ascertained,belonging to monodentate carbonate species (1332 and 1477 cm-1) [44],bidentate formate (1371 cm-1) [45],and the vibration modes of bidentate carbonate (1546 cm-1) [46],respectively.It is more noteworthy that these above species calculated by the band intensities are observed to increase obviously with the further elevation of temperatures,which is probably due to the high stability of the species formed by CO adsorption.

In the case of MnOx-Fe3O4catalysts (Fig.12(b)-(d)),more species were formed on the surface of these catalysts.For 0.5MnOx-Fe3O4,the peaks associated with bidentate carbonates are at 1012 and 1143 cm-1,while the peaks belonging to monodentate carbonates are at about 1284,1330 and 1494 cm-1[36,47].More important,the hydrogen-carbonate group could be reflected by the band at 1627 cm-1,demonstrating the interaction of adsorbed CO with surface hydroxyls [48].In term of 1MnOx-Fe3O4and 2MnOx-Fe3O4catalysts (Fig.12(c) and (d)),the FT-IR spectra of CO adsorption at lower wavenumber range (below 1600 cm-1)are similar to that of 0.5MnOx-Fe3O4catalyst,whereas in the higher wavenumber range,carbonate groups(1703,1801 and 1930 cm-1)are observed in situ FT-IR profile,which differs from what occurs over the 0.5MnOx-Fe3O4sample.

3.10.2.Single NO adsorption

In situFT-IR spectra of NO adsorption with temperature variation were harvested in Fig.13.Notably,for all the samples,the vibration of the adsorbed NO species in bent and linear coordination modes can be reflected by the two characteristic peaks at 1847 and 1903 cm-1[20].Besides,these peak intensities decreased with elevated temperatures.In terms of Fe3O4sample (Fig.13(a)),the band at 1195 cm-1resulted from v(N–O)stretching vibrations of the monodentate nitrate species [45],and it is weakened and even disappeared with the elevation of temperature owing to their poor stability.The band of N2Oadsspecies(1332 cm-1)can be seen at 200°C[49],acting as an intermediate to N2formation(CO+N2-O →CO2+N2),whereas the band at 1371 cm-1followed typical adsorption of freeion arose when the temperature was raised to 100 °C [50].The bands at 1548 and 1571 cm-1were attributed to the bridged nitro and chelating bidentate nitrates respectively[45,51].

Fig.12.FT-IR spectra of (a) Fe3O4,(b) 0.5MnOx-Fe3O4,(c) 1MnOx-Fe3O4,and (d) 2MnOx-Fe3O4 in a flow of CO (2% in volume).

In the case of the three MnOx-Fe3O4samples (Fig.13(b)-(d)),more various types of NOxspecies are detected compared with Fe3O4,namely bidentate nitrates (1008 and 1082 cm-1) [45,50],vibration of the nitrosyl species (1122 cm-1) [20],monodentate nitrites (1141 cm-1),nitrito species (1164 cm-1),linear nitrite(1230 cm-1),N2Oadsspecies (1284 and 1332 cm-1),monodentate nitrate (1496 and 1475 cm-1) [51],and chemisorbed NO (1703 and 1799 cm-1),respectively,suggesting the changes in the adsorbed NOxspecies distribution were caused by the introduction of Mn.In other words,addition Mn facilitates the adsorption and formation of NOxspecies.It is worth noting that no signal for NO2is observed whereas the peaks for N2Oadsspecies (1284 and 1332 cm-1) are evident,implying that NO is converting into N2O but not into NO2.The results are consistent with the SCR mechanism on account of forming surface N2O species proposed in the literature [49].

By comparing Fig.13(a)–(d),note that all the MnOx-Fe3O4catalysts have the characteristic bands at 1288–1000 cm-1,1475 cm-1,1703 cm-1and 1799 cm-1,belonging to nitrites,nitrates,nitrosyl species and chemisorbed NO,which are not found in Fe3O4sample.We suspect that these absorption peaks are related to Mn species.In addition,according to the peak area in the infrared image,it is clearly see that the total amount of surface adsorbed NOxspecies are enhanced obviously as the Mn content increases,and nitrates are formed at lower temperatures (below 100 °C),indicating that the activation capacity of NO is significantly enhanced,which were closely related to their catalytic performance.The strengthen in NO activation capacity over MnOx-Fe3O4sample caused by rich surface active oxygen species(Fe3+-O–Mn4+and Fe3+-O–Mn3+)generated by the intensification of Mn-O-Fe bond lattice distortions,and a weaker energy barrier for the formation and adsorption of NOxspecies[38].Moreover,further raising the reaction temperature leads to these adsorbed NOxspecies growing remarkably,especially up to 300 °C.

3.10.3.NO and CO co-interaction with the catalysts

To delve into the surface reaction progress,0.5MnOx-Fe3O4sample acted as an example to expose NO+CO stream under simulated reaction conditions,as reflected in Fig.14.Note that the adsorption modes on the catalyst surfaces was identical to that of single NO adsorption,nitrates (1012,1082 and 1496 cm-1),monodentate nitrites (1141 cm-1),N2Oadsspecies (1284,1330 cm-1) can be detected at low temperature.Moreover,more peaks belonging to carbonates generate on the catalyst surface at higher temperature,covering carboxylate-like species(1440 cm-1),bidentate carbonate (1519,1558 cm-1),hydrogencarbonate group (1629 cm-1),P-branch and R-branch of CO in gas phase (2119 and 2173 cm-1),as well as CO2in gas phase(2322 and 2362 cm-1) [48].Importantly,the intensities of the bands of 2322 and 2362 cm-1are increasing accompanied by the peaks of 2119 and 2173 cm-1are somewhat declined with raising reaction temperature,indicating the consumption of CO and enhanced formation of CO2by surface reduction.This occurrence seems to be one significant reason leading to the increase in the reactivity of CO-SCR reaction over MnOx-Fe3O4catalysts,which are in good agreement with the catalytic activity analysis (Fig.3).These results further prove that manganese species plays an important role in CO-SCR reaction.

Fig.13.FT-IR spectra of (a) Fe3O4,(b) 0.5MnOx-Fe3O4,(c) 1MnOx-Fe3O4,and (d) 2MnOx-Fe3O4 in a flow of NO (1.0% in volume).

Fig.14.FT-IR spectra of 0.5MnOx-Fe3O4 catalysts in a flow of NO+CO.

3.11.Possible CO-SCR reaction mechanism

Fig.15.Mechanism on the model reaction NO+CO over catalysts.

The reaction mechanism can reveal the catalytic process and provide guidance for the improvement of catalyst performance.Based on the above information,we temporarily propose a mechanism for CO reduction of NO by MnOx-Fe3O4catalysts prepared by annealing (Mn)-MIL-53(Fe).Specifically,the proper introduction of manganese significantly boosts the catalytic performance.Based on the H2-TPR,O2-TPD and XPS analysis,we can conclude that the synergistic interaction between Mn and other active components can help to produce several active oxygen species (Fe3+--O–Mn4+and Fe3+-O–Mn3+),which are quickly reduced to form defects (Fe3+-□-Mn4+and Fe3+-□-Mn3+),and further improve the reducibility and the mobility of lattice O2-and more O2-desorption.Thus,obviously heighten the catalytic behavior.According to the results of systematical FT-IR characterization,these Mn-Fe catalysts destruction of NOxpollutants takes placeviatwo pathways,namely,the E-R mechanism at low temperature segments(the reaction of gaseous CO with adsorbed NO species to produce CO2and N2) and the L-H reaction pathway at high temperature region (the reaction of adsorbed NO species and adsorbed CO species on the active sites to form CO2and N2),the latter is usually prioritized during NO-CO reactions [52].According to the achieved results,the corresponding schematic illustration is presented in Fig.15.When Mn-Fe materials contacted with the gaseous reactant CO and NO,NO molecules were preferentially attached on the active sites at lower temperature (T＜200 °C),relying on their unpaired electrons.The oxygen species on the catalysts were combined to form surface NOxspecies (e.g.,nitrates,nitrites,nitrito species,nitrosyl species andion),thereby restricting the CO adsorption procedure.Moreover,a small amount of CO2was generated from the reaction of adsorbed species below 200 °C.As the reaction temperature exceeded 200 °C,the absorbed NO species were dissociated into O and N atoms.Previous literature stressed that NO dissociation is a vital step in converting NO and CO into N2and CO2,which is further conducive to stronger adsorption of gaseous CO molecules by releasing active sites [36].Meantime,CO could react with the O atom produced by NO dissociation to form abundant CO2.In short,NO molecules are adsorbed onto catalytic materials and react with adsorbed CO species,leading to various intermediates and eventually the formation of N2and CO2.

4.Conclusions

In brief,MnOx-Fe3O4nanomaterials were fabricated by using the innovative scheme of pyrolyzing manganese-doped ironbased metal organic framework in inert atmosphere.Subsequently,they were applied for the CO-SCR reaction to control NO.With the operating temperature of 500 °C,the NO conversion of 2MnOx-Fe3O4sample was about 97.5% ,which is twice that of pure Fe3O4composite.Characterization results clarified the promotional effects of Mn by decrease the crystallite size,improve reducibility property,as well as enhance the mobility and the amount of lattice O2-species.Moreover,in situFT-IR results suggested that MnOx-Fe3O4catalysts strengthen the adsorption and dissociation of NO,form new active species (nitrosyl,chemisorbed NO and nitrites) compared with Fe3O4.The unprecedented enhancement of CO-SCR activity over MnOx-Fe3O4nanomaterials follows both E-R and L-H reaction pathway.Hopefully,this study can open a new avenue for the design of late-model catalytic materials for vehicle exhaust purification.

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

We acknowledge the financial support of the National Natural Science Foundation of China (Nos.21866022,21567018),Inner Mongolia "Grassland talents" project,Inner Mongolia Key Laboratory of Environmental Pollution Control and Waste Resource Recycle,Key Laboratory of Ecology and Resource Use of the Mongolian Plateau and Collaborative Innovation Center for Grassland Ecological Security,Ministry of Education of China.