Influence of operational parameters on the photocatalytic performance of DE-NOx process Via MIL-101(Fe)

1.Introduction

Metal organic frameworks are crystalline porous materials constructed by nodes(as metallic ions or clusters)and linkers(as organic ligands),generating infinite networks with various shapes and sizes of pores and windows.The versatility in the composition of the organic linkers and metal clusters,together with the large surface area and high porosity that facilitate the incorporation of guest molecules into the framework are some characteristics of MOFs that make them potential materials for various applications,such as drug delivery,gas storage,separation,and catalysis[1–5].

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In recent years,MOF-based materials have been extensively investigated for the abatement of industrial gaseous pollutants(COx,SOx,NOx,volatile organic compounds and so on)due to not only the extremely high porosity but also the existence of open metal sites(or coordinatively unsaturated metal sites)that has been highlighted in some reports.Wang et al.investigated the CO2 reduction of Fe-containing MOFs to give formate form(HCOO-)under visible light irradiation[6]and found out that MIL-101(Fe)showed the best performance among three investigated Fe-based MOFs,attributed to the existence of the coordinatively unsaturated Fe sites in its structure.Eubank et al.[7]reported the high adsorption capacity of nitric oxide(NO)in MIL-100 and MIL-127(MIL:Material Institute Lavoisier)of Cr(III)and Fe(III)whilst A.C.Mckinlay et al.[8]investigated the NO adsorption-desorption of MIL-88(Fe)series.They discovered that a significant amount of NO is adsorbed due to not only the coordination bond between NO and the Lewis acid sites(Cr and Fe nodes)but also a stronger interaction between NO and additional iron(II)sites.Both of two groups have shown that NO adsorption capability was limited due to the closed pores of MILs preventing the accessibility of NO gas to metal active sides.Thus,some robust MOFs,such as MIL-101 or MIL-100,were expected to get better adsorption capability.Particularly,the later report figured out that NO partly desorbed in the presence of high concentration of H2O(wet condition).

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Fe-based MOFs have been investigated in photocatalytic fields,including wastewater treatment[9–11],water splitting[12,13],and CO2 reduction[6].Among them,MIL-101 is one of the most interesting candidates due to the regular distribution of semiconductor-like Fe3-μ3-oxo clusters in the MOF network and the existence of opened metal sites[6].Together with high porosity nature,MIL-101(Fe)is a potential candidate for gaseous pollutants treatment.In this respect,we synthesized and tested the photocatalytic performance of Fe-based MIL-101,and its photocatalytic ability was investigated using nitrogen oxides(NOx),a family of compounds consisting of nitrogen and oxygen,that has been considered as one of the most fatal pollutants nowadays.In the presence of moisture,these oxides dissolve ready giving acidic compounds(HNO2,HNO3),then neutralized to form salts,which can cause acid rain,photochemical smog,ozone layer depletion,or human and animal health[14].The main source of NOx in air is from the combustion of fuels,with the major components of NO.Since NO possesses no real threat,but it is the intermediate product for the formation of more toxic NO2 gas.Also,NO is only slightly soluble in water,whilst NO2 is highly solute in water and forms nitric acid[15].

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Among numerous methods established for NOx emission control,photocatalytic NOx-oxidation without any chemical supports has shown great advantages.Particularly,photocatalysis is thought to be one of the best methods for removal of NOx in ppb level,therefore could be a promising solution for air purification in an urban environment.The fundamental mechanism of de-NOx photo-oxidation is shown below[16]:

To date,the interaction between Fe sites in iron-based MOFs and NOx gases was figured out by many research[7,8,17]but the use of Febased MOFs as photocatalysts for de-NOx oxidation process has not respect to examine yet.In this work,the de-NOx performance of MIL-101 was determined with some operating parameters,such as initial NO concentration,substrates,the quantity of photocatalytic powder,light intensity and light spectrum,humidity.The parameter optimization for the optimum photo-deNOx process for MIL-101(Fe)thus helps in understanding more about the mechanism of NOx photo-oxidation for this new candidate.Since the optimization of operating parameters is very important as it gives the clear concept about the competition between NO and water adsorbed on the photocatalyst surface,which mainly influences the efficiency of de-NOx process.

2.Experimental

Iron (III) chloride (FeCl3·6H2O) from Aldrich, Benzene-1,4-dicarboxylic acid(C6H4(COOH)2)from Junsei,N,N-Dimethylformamide(DMF)from Daejung,were used without further purification.Nitrogen monoxide(NO)2.1 ppm in nitrogen(N2)from AirKorea,commercial zero-air(21%O2/89%N2)were used for de-NOx reaction performance.Ethanol,acetone and other chemicals were analytical grade and used without further purification.Doubly distilled water was used for all synthesis and treatment process.

2.1.Synthesis of MIL-101(Fe)

MIL-101(Fe)was synthesized using the microwave-solvothermal method according to the literature with a bit modification.Therefore,5 mmol of H2BDC and 5 mmol of FeCl3·6H2O were dissolved one-by-one in 70 mL of DMF under magnetic stirring for 15 min to form a transparent solution.The resulting mixture was transferred to a sealed 120 mL-volume vessels,then heated by microwave at 300 W.MIL-101(Fe)was formed at 150°C after 30 min of reaction time.This work has been done by a microwave synthesizer(Wave Magic MWO-1000S,EYELA Co.).

Fig.4 described the decrease of NOx concentration for various quantities of MIL-101 powder under solar irradiation.The NO conversion was observed above 70%in all experiments.The NO2 concentration released very small for all experiments.When increasing the amount of MIL-101 sample from 0.05 to 0.1 g,the efficiency of de-NOx increased by 5%,from 72%to 77%.However,it was only 76%when the amount of the photocatalyst was increased to 0.2 g.This phenomenon was attributed to the penetration of excited photons into sample bulk.The number of excited active sites when using 0.1 g of the sample was higher than that of 0.05 g of the sample,resulting in the increase of de-NOx efficiency.For 0.2 g of the sample,the efficiency is likely the same as for 0.1 g of the sample,probably because of the limitation of the number of photons harvested when the thickness of the powder layer increases.Thus,0.1 g of MIL-101(Fe)was found to be the amount of sample needed for de-NOx performance in this study.

2.2.Characterization of MIL-101(Fe)

As shown in Fig.9,after illumination to the light,NO concentration decreased instantly for both MIL-101(Fe)and P25.After 5 min of irradiation,the maximum NO conversion was obtained to 69%for MIL-101(Fe)and 61%for P25.The concentration of NO was maintained steadily for 1 h of reaction for both samples.The release of NO2 was rather small in the experiments.At the first 5 min irradiation,the concentration of NO2 increased a bit,then decreased with the reaction time.Notably,there was a gradual increase of NO2 concentration releasing during photo-oxidation process via P25 sample.

2.3.De-NOx photocatalytic procedure

(a)Photo-reactor:The photocatalytic performance for NOx removal by as-synthesized samples was carried out at room temperature in a continuous flow reactor.The de-NOx system was designed following the standard ISO 22197–1 as a basic reference.The volume of the reactor which was made by alumina ceramics was about 300 cm3.A quartz window(d=5 cm)was set at the center of the cap of the reactor.

SEM images of after-reacted samples were investigated to demonstrate the effect of water vapor on photocatalytic activity of MIL-101(Fe).As shown in Fig.S3,under 30%and 50%of RH,there was no significant change in particle morphology of MIL-101(Fe).However,MIL-101 crystals partly decomposed after reaction at 90%of RH,explaining the decrease of de-NOx efficiency at this condition.

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For photocatalytic powder pre-treatment,the calculated amount of the sample was dispersed in 2 mL of acetone,sonicated for 5 min,then drop-by-drop coated on the sample dish.The dish was dried at 80°C for 1 h to remove completely acetone.After cooled to room temperature,the dish was introduced into the reactor.

It was predicted that the increase in incident light intensity would increase photocatalytic efficiency due to the increase in the number of harvested photons.Nonetheless,the influence of light intensity on de-NOx performance is complicated,especially under low concentration of pollutants and high light intensity.During irradiation,there is a competition between the generation rate and the recombination rate of electron-hole pairs.The first rate is enhanced by photocatalytic reaction whilst the later rate is dominant when using a high-intensity light source[25].Moreover,in term of gaseous pollutant treatment,the generation of heat during irradiation strongly influences the gas sorption capacity of the material,leading to the decrease of de-NOx effi-ciency by the heat.The low efficiency and non-stability NO conversion process under 150 mW/cm2 of light intensity thus was assigned to the domination of recombination of charges and the formation of heat.

(d)Inlet gases:The photocatalytic performance was carried out as the following literature with some modification.Briefly,the inlet gas flow rates of NO and air were controlled at hundreds of sccm(sccm:standard centimeter cubic per min),by mass flow controllers.The purpose of the research is controlling NOx emission in urban areas,thus initial NO concentration was 0.1–1 ppm for all de-NOx process.

Fig.1.XRD patterns of MIL-101(Fe).

(e)In this work,relative humidity(RH)of the system was controlled by passing an air stream through a humidification chamber,the range of RH was at about 30–90%.

After the adsorption-desorption equilibrium among gases and photocatalyst was achieved,the light was turned on.All de-NOx photocatalytic tests were performed at room temperature(from 20 to 25°C).The concentration of NOout,NO2out,and NOx(standing for a total of NOout and NO2out)was detected by a NOx analyzer(Horiba,Ambient NOx monitor APNA-370)after a given period of time.

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3.Results and discussion

The successful synthesis of MIL-101(Fe)was confirmed by XRD patterns and FT-IR spectra.As shown in Fig.1,the characteristic peaks of MIL-101(Fe)was at 2θ=3.3,5.1,5.8,8.1,8.9,and well matched with the spectra reported as[6,18].This result demonstrated the formation of the pure phase of MIL-101(Fe).Also,the shape peaks represent the high crystallinity of the synthesized sample. Peaks at 1594 cm-1 and 1388 cm-1 in FT-IR spectra(Fig.S1)were assigned to the asymmetrical and symmetrical vibration of O-C-O bonding in carboxylate groups.In addition,the peak at 1502 cm-1 corresponds to C=C stretching vibration of aromatic rings and the weak peak at 554 cm-1 was contributed to the coordination Fe-O bond in the samples[19].To examine photoresponsive properties of the as-synthesized sample,UV–Vis DRS measurement was carried out(Fig.S2).A strong absorption band in 200–400 nm range is assigned to ligand-to-metal charge transfer(LMCT)from terephthalate to iron clusters[6,20].Besides,a weak band in the visible region extending from 450 nm to 600 nm,was contributed to the light absorption by iron clusters in the MIL-101 net,corresponding to the d-d transition of Fe3+[19].Apparently,MIL-101(Fe)could be excited by both UV and visible light,thus hence could become an efficient photocatalyst for the de-NOx process under solar irradiation.The morphologies of as-obtained MIL-101(Fe)sample were studied by SEM images and shown in Fig.S3.The MIL-101 particles possess octahedral shape with particle size ranging from submicrometer to 1µm.The smaller particle size of MIL-101(Fe)was expected to enhance the efficiency of gas molecules-photocatalyst interaction.

Fig.2.Photocatalytic de-NOx performance for 0.1 g of MIL-101(Fe)coated on different substrates:aluminum ceramics,FTO glass,and n-glass under solar simulated irradiation,at 400–450 ppb of initial NO,Troom,RH=50%.

3.1.Effect of the substrates

The photocatalytic performance is strongly influenced by the nature of the substrates and the technique used to immobilise photocatalytic powder onto the substrates[15,21,22].Fig.2 depict the effect of different non-porous substrates for NO conversion.The conversion effi-ciency for MIL-101(Fe)powder coated on Al2O3 ceramics,FTO and nglass reached to 69%,54%and 47%,respectively.For Al2O3 substrate,after decreasing to 150 ppb,the NO concentration maintained during 1 h irradiation.In contrast,during the same time of irradiation,NO concentration dramatically increased in the cases of two glass-by substrates,from 197 to 325 ppb for FTO and from 221 to 373 ppb for nglass.Clearly,the efficiency of NO conversion for MIL-101(Fe)did not keep stable when using glass substrates.This phenomenon was attributed to the wettability properties of the substrates that can be observed in Fig.3.Al2O3 substrate is hydrophobic while glass substrates are hydrophilic,thus prohibited the condensation of un-reacted H2O vapor as well as HNO3 product on the surface of the sample-coated substrates.Notably,the high condensation of H2O and HNO3 might lead to limiting the number of active sites on the surface of the photocatalyst,hence,decreasing NOx conversion efficiency[23,24].

For all three substrates,the release of NO2 gas was very small,demonstrating the high ionization of NO for as-synthesized MIL-101(Fe)samples. Interestingly, NO2 concentration decreased for the Al2O3 substrate,whilst it slightly increased for FTO substrate after irradiation and maintained at a higher level in comparison to the others.The difference probably comes from conducting property of FTO glass surface.The conductive properties of a substrate,in fact,improve the charge separation of a photocatalyst[18].The ease of electron transfer on FTO conducting substrate thus promotes the fast formation of NO2,as following reaction(7).Similarly,the low water-adsorption capacity of the Al2O3 ceramic substrate proved that MIL-101(Fe)can absorb enough water from air flow to supply for de-NOx photocatalysis without using any porous substrate.

3.2.Effect of catalytic doses

Fig.3.Images of the contact angle of(a)aluminum ceramics-,(b)FTO glass-and(c)normal glass substrates.

Fig. 4.Photocatalytic de-NOx performance with various amounts of MIL-101(Fe)under solar simulated irradiation,at about 300 ppb of initial NO,Troom,RH=50%.Inset is the table describing the percentage of NO conversion.

After-reaction powders were centrifuged,dispersed in ethanol at 60°C in 3 h,then washed several times with ethanol and doubly distilled water,finally dried in an oven at 80°C for at least 6 h and stored in a vacuum box.

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3.3.Effect of initial NO concentration

The initial concentration of NO has critical effects on NOx photocatalytic removal.So,the inlet concentration of NO was controlled to get the range of 200–1000 ppb,standing by C200 to C1000,which is similar to the level of NO in urban environments.De-NOx performance of MIL-101(Fe)via different inlet concentration of NO has been shown in Fig.5 and the percentage of NO conversion have been calculated and presented in Fig.6.

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During the photocatalytic process,electron-hole pairs are separated when a photocatalyst absorbes enough photon energy provided by light sources.These pairs of charges participate in the redox process that degrades pollutants into harmless products.The efficiency of the pollutant degradation thus is affected by the efficiency of electron-hole separation that related to the way materials harvest photon from light sources.MIL-101(Fe)has been demonstrated its absorption capacity inboth UV and visible range,as shown in Fig.S2,thus it can be excited by UV and/or visible light.The result in Fig.7-a shows that the efficiency of de-NOx performance mainly came from the visible spectrum,whilst the contribution of UV spectrum is quite small and that of IR spectrum can be neglected.Obviously,due to 2.7 eV of band gap energy of MIL-101(Fe),visible light must be the most suitable excitation source.The photon energy from UV light accounting for only 5%of solar spectrum is considered to have a contribution to de-NOx performance,and energy provided by IR light was not sufficient for charge excitation but the formation of heat in the system.The unsteady-state of NO concentration under UV cut-offirradiation was probably due to the occupation of unreacted H2O molecules on active sites of MIL-101(Fe),inhibiting de-NOx photocatalytic activity.

Fig.5.Photocatalytic NOx removal performance via MIL-101(Fe)at various concentration of initial NO under solar simulated irradiation,Troom,RH=50%.

Fig.6.The percentage of NOx concentration converted via MIL-101(Fe)for various initial concentrations of NO.

It is noteworthy that photocatalytic activity is strongly influenced by the number of active centers that were limited by the nature of various photocatalysts and the way photons were harvested.Herein,active centers for MIL-101(Fe)are metallic nodes that are responsible for the interaction between the photocatalyst and gaseous molecules.During the de-NOx reaction,there was a competition between H2O,NO,O2 to link to the active centers of the photocatalyst.A higher NO concentration will increase the probability of NO molecules interacting with the active sites,leading to the increase of the amount of NO converted.However,the increase of NO concentration also exhibited H2O and O2 adsorbed on photocatalyst's surface,hence the formation of active radicals(OH.,)was limited.As a result,NO conversion rate decreased when initial NO concentration increased.Moreover,the higher concentration of NO will generate more HNO2 and HNO3.Such by-products might condense and cover active sites,even deconstruct the photocatalytic surface,leading to the decrease of NO conversion efficiency.Indeed,there was a slight decrease of NO conversion for experiments performed at 1000,850 and 700 ppb of initial NO concentration,as shown in Fig.5.However,a too washy initial NO concentration might restrict the interaction between photocatalyst and NO molecules,thus reducing the efficiency of NO conversation,as seen in the case of C200.

3.4.Effect of light spectrum and light intensity

To investigate the influence of light sources on the photocatalytic activity of MIL-101(Fe),de-NOx performance was conducted under various conditions of light spectrum and light intensity.As shown in Fig.7-a,under the full range of solar irradiation,NO conversion effi-ciency reached to 69,3%.This efficiency slightly decreased to 64.5%and 65%when excluding the effect of UV light and IR light,respectively.For UV cut-offlight,the efficiency of NO conversion decreased from 64.5%to 54.3%after 1 h of irradiation.Varying the light intensity by controlling the distance between light source and sample-coated dishes,the result of the de-NOx process was described in Fig.7-b and Table 1.The highest de-NOx efficiency was obtained for 100 mW/cm2 of solar irradiation intensity. Moreover, the steady-state NO concentration during 1 h of irradiation only occurred in this condition.The decrease of light intensity resulted in the decrease of de-NOx efficiency as well as the stability of de-NOx efficiency.Particularly,when increasing the light intensity to 151 mW/cm2,de-NOx efficiency also decreased unexpectedly.There was no high NO2 release for all such experiments.

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The amount of NO converted during 1 h of irradiation was the highest for C1000,from 1000 ppb to 550 ppb,and gradually decreased on decreasing the inlet concentration of NO.Nevertheless,in term of reaction rate,NO conversion rate was only 48.8%for C1000,as shown in Fig.6.Obviously,the de-NOx reaction rate increased when decreased the initial NO concentration,and obtain the highest value of 75.9%for C350.At high concentration of initial NO(C550 to C1000),the change in NO conversion rate seems not much different,whilst at the lower initial NO concentration,this difference tended to be bigger.For C200,the conversion rate was not higher but lower than C450 and C300.

Table 1 The maximum and minimum efficiency of photocatalytic NOx conversion process for MIL-101(Fe)at the different intensity of the excitation light.

151 mW/cm2 100 mW/cm2 83 mW/cm2 61 mW/cm2 Max.Eff 46.8 69.3 62.3 64.0 Min.Eff 0.08 66.5 52.4 51.7

(c)Light source: A 150 W commercial xenon arc lamp (Abet Technologies,LS-150)was used as the simulated solar light source.An optical UV cut-offfilter and an IR cut-offfilter were used to vary wavelength range of solar irradiation that was beyond 400 nm and below 700 nm,respectively.The lamp was vertically placed above the reactor's window with the control of distance to vary incident light intensity on the surface.The illuminance of the light source was first calibrated by a solar-light reference(BS-520BK)to obtain 100 mW/cm2 of incident light.

3.5.Effect of humidity

The photocatalytic NO conversion via as-synthesized MIL-101(Fe)and the change of photocatalytic efficiency after 1 h reaction was shown in Fig.8.

Fig.7.Photocatalytic de-NOx performance of MIL-101(Fe)under various conditions of the light source.The experiments were carried out at Troom,RH=50%with 0.1 g of samples.

Fig.8.(a)Photocatalytic de-NOx performance of MIL-101(Fe)and(b)The change of de-NOx efficiency after 1 h of solar irradiation under various relative humidity.The experiments were carried out at Troom,with 0.1 g of samples.

It is seen that,for initial NO concentration at 350 ppb,the increase of RH,from 40%to 90%,led to the decrease of de-NOx efficiency.At 40–60%of RH range,there was no significant change of the maximum value of de-NOx efficiency(Max.eff)that was obtained to about 75%after 5 min of irradiation.Max.effdramatically reduced to 60%under 90%of RH.The steady state of de-NOx efficiency also decreased following the increase of RH.At 40%of RH,the de-NOx conversion was maintained to 77%of efficiency.De-NOx efficiency reduced by 4%at 50%of RH and by 10%at 60%and 70%of RH.The highest decrease in de-NOx efficiency was from 59%to 25%under 90%of RH.Particularly,at 30%of RH,the de-NOx efficiency was 71%and was lower than even the efficiency at 70%of RH.The steady state of the de-NOx process at this RH was also low,that decreased to 58%of conversion efficiency.

Water,a reactant for de-NOx photocatalysis as a source of hydroxyl radicals,might reduce the efficiency of this process on the other hand.It is due to the competition between water and pollutants on occupying active sites of photocatalysts[23].For MIL-101(Fe),iron clusters are active sites that can link to both water and NO molecules[7],thus the de-NOx photocatalytic ability of MIL-101(Fe)is greatly affected by this competition.Obviously,at 40%of RH,the amount of water provided was optimal for the conversion of water to active radicals and promoted the de-NOx process.When the relative humidity increased,the amount of water participated to the system increased,thus the occupation of water on active sites of MIL-101(Fe)was dominant,reducing the access to active sites of NO,resulting in the decrease of de-NOx efficiency.During the de-NOx performance under high relative humidity,iron active sites continued forming links to water molecules,therefore inhibiting pollutant adsorption of MIL-101(Fe).Consequently,the steady state of de-NOx efficiency decreased when increasing the relative humidity.Moreover,with excess water,ionic by-products(HNO3 and HNO2)from photo-oxidation process cannot be removed from photocatalyst surface.Such kinds of acids would condense on the surface,together with non-reacted water,cover active sites and decompose photocatalysts,contributing to the reduction of de-NOx efficiency.In contrast,at low relative humidity,the de-NOx efficiency was lower due to the lack of hydroxyl radicals from water,leading to the decrease of NO conversion. At this condition, the de-NOx efficiency was not maintained,attributing to the shortage of water happening during reaction time.

(b)Substrate and immobility of samples:To study the effect of substrates on the de-NOx process,3 types of sample dishes,aluminum ceramic,FTO glass and normal Soda-lime glass(n-glass),were used.All substrates were used in this work is non-porous to take advantages of the porosity of MIL-101(Fe),from that the potential of this material was highlighted.For the first substrate,an aluminum ceramic dish containing photocatalytic powders was put inside the reactor.The area containing photocatalytic powders is a roundshaped hole(d=5 cm)fixed at the center of the dish to be fully exposed under an exciting light source.For two later substrates,square-shaped glasses (5×5 cm2) were also introduced at the center of the reactor.The distance between the paralleled sample's surface and the quartz window was 5 mm.

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From the obtained results,it was concluded that MIL-101(Fe)performed de-NOx photocatalytic ability best at 40–50%RH.

3.6.A comparison to commercial TiO2

To evaluate the efficiency of MIL-101(Fe)for photocatalytic de-NOx abatement, a comparison reaction was made with respect to P25 Degussa.The experiments were carried out at the optimal condition for MIL-101(Fe),based on the previous results.

The products were identified by X-ray diffraction analysis using Rigaku X-Ray Diffractometer at a scan rate of 4°min-1 in 2θ ranging from 2°to 30°with Cu Kα radiation(λ=1.54Å).Morphology and particle size of samples were examined with a scanning electron microscope(SEM,JEOL-2020).Infrared spectra were obtained with an FTNIR spectrometer(Bruker Optik,Vertex 70)within the wavenumber interval of 400–4000 cm-1.The light absorption ability was recorded by UV–Vis diffuse reflectance spectrophotometer(DRS,UV–Vis spectrophotometer NIR JASCO 570).The Brunauer-Emmett-Teller surface area and pore volume were analyzed by nitrogen gas absorption-desorption method using Micromeritics ASAP 2020 system.MIL-101 sample was degassed at 150°C for at least 12 h under vacuum condition,N2 adsorption-desorption isotherms were measured at 77 K.

Fig.9.Photocatalytic de-NOx performance of MIL-101(Fe)and TiO2-P25.The experiments were carried out for under 100 mW/cm2 of incident intensity of solar-light,at Troom,50%of RH,with 0.1 g of samples.

The result in Fig.9 has shown that the de-NOx efficiency of MIL-101(Fe)was 8%higher than that of TiO2-P25 at the ambient condition.Moreover,compared to P25,MIL-101(Fe)seems to release less NO2 gas,a very important property required for de-NOx photocatalysts.This phenomenon was assigned to the difference in the adsorption ability of two samples.As shown in Fig.S4,BET surface area of MIL-101(Fe)sample was 1642 m2/g and quite higher than that of P25(49 m2/g),equivalent to the higher storage capacity for gases and vapor of MIL-101(Fe).It should be noted that the formation of NO2 is influenced by the amount of adsorbed water on the surface of photocatalysts.NO2 formed on the surface of photocatalysts requires sufficient·OH radicals to be converted to HNO3,otherwise,it would be released to the environment[26].At optimum condition of humidity,50%of RH in this experiment,enough water was adsorbed by MIL-101(Fe)to supply hydroxyl radicals that oxidize NOx gas to ionic products,minimizing the release of NO2.The high porosity of MIL-101(Fe)also improved the interaction time between the material and gaseous molecules,thus extending the possibility of pollutants conversion.

The difference in de-NOx efficiency of MIL-101(Fe)is nothing of great interest because the influences of some operating factors,such as light source,optimal initial NO concentration,on P25 and MIL-101(Fe)were not the same.However,due to the versatility in the composition of MOF and the huge possibility of post-modification strategy,this result has shown the potential of MILs(Fe)in gaseous pollutants photo-oxidation.

3.7.Proposed mechanism

At the first stage,NO,O2 and H2O are adsorbed on the surface of MIL-101 and come in contact with Fe active sites of the MOF.When MIL-101 is exposed to the suitable light,electric charges are generated at Fe3-μ3-oxo clusters or at BDC-linkers,then oxidize and reduce H2O and O2 to·OH and radicals.After that,the radicals react with adsorbed NO to form HNO2,NO2 as intermediate products,and HNO3 as a final product(Table S1).Finally,such by-products are swept away by the flow of air and water vapor.Herein,the competition of guest molecules(H2O,NO,and O2)with the limitation of Fe active sites during irradiation plays a key role in the de-NOx performance.

根据WHO疼痛缓解标准，术后3 d，30例患者中完全缓解8例，部分缓解15例，轻度缓解5例，无缓解2，缓解率76.67%（23／30）；术后1年时，完全缓解7例，部分缓解15例，轻度缓解5例，无缓解3，缓解率73.33%（22／30）。

4.Conclusion

In this work,photocatalytic NOx removal performance of as-synthesized MIL-101(Fe)has been investigated with varying operating conditions of an in-situ system for de-NOx photocatalysis.A continuous flow reactor with 300 cm3 of free volume was designed for NO photooxidation.The study has shown the influences of operating parameters,such as substrates,the quantity of samples,initial NO concentration,light intensity and light spectrum,and humidity on the de-NOx performance of MIL-101(Fe).In the optimum condition with aluminum ceramic substrate,50%of relative humidity,0.1 g of photocatalyst dose,the de-NOx efficiency was up to 76%during 1 h of solar irradiation.

Similarly,a comparison between TiO2-P25 and as-synthesized MIL-101(Fe)was also carried out.The efficiency of the de-NOx process for MIL-101(Fe)was 8%higher than P25.The release of toxic NO2 was smaller than that of P25 as well.Then the results shows the potentiality of MIL-101(Fe)in the field of NOx treatment.

Acknowledgements

This research was supported by Global Research Laboratory Program of the National Research Foundation of Korea(NRF)funded by the Ministry of Education,Science and Technology(MEST)of Korea(Grant No.:2010-00339)and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01052893).

以联合体作为经营主体符合农村产业融合中关于要素融通、产业联结、收益共享、风险共担等基本要求。通过将联合体作为政策扶持项目的实施主体，能够切实提高农村产业融合发展扶持政策的精准度，确保农业发展质量提升与农民利益实现。尽管联合体已形成可复制、可推广、可借鉴的成熟经验，并在部分地区有力地推动了农村产业融合发展，但仍有完善空间。

利用稳定渗流法测定混凝土试件的水渗透系数[17]。测试过程为先将混凝土试件进行21 h的真空饱水，之后将试件的侧面进行封蜡处理(消除水的侧漏以及保证水的单轴渗透)，在中心位置留80 mm×80 mm的过水(气体)面积；最后装入压力桶中进行水渗透试验。稳定渗流的水压力设置为2 MPa，由H4-S型抗渗仪从0逐级加压至2 MPa，并保持恒定。记录图4中移液管液面的初始读数，之后每隔4 h记录一次读数。当前后两次读数的增量趋于稳定时，可认为水的渗流已达到稳定状态，再记录3次读数，得到通过混凝土试件的平均流量Q。

Appendix A.Supplementary material

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.pnsc.2018.11.006.

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