Selective Catalytic Reduction of Nitric Oxide with Propylene over One-Step Synthesized Cu-SAPO-34 Catalysts

ZHOU HaoYANG DiWANG Cong-YingZHAO Hui-ShuangWU Shi-GuoSU Ya-Xin＊,

(1Changzhou Institute of Engineering Technology,Changzhou,Jiangsu 213164,China)

(2School of Environmental Science and Engineering,Donghua University,Shanghai 201620,China)

Abstract:To investigate the performance of Cu-chabazite for selective catalytic reduction of NO with propylene(C3H6-SCR),a series of Cu-SAPO-34 catalysts with varied Cu/Al ratio have been synthesized by the one-pot hydrothermal method.These catalysts were characterized using N2adsorption-desorption,X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),temperature programmed reduction by H2(H2-TPR),and in-situ diffuse reflectance infrared Fourier transform spectroscopy(in-situ DRIFTS)techniques.The effect of Cu component on physicochemical properties and C3H6-SCR activity were investigated.Cu-SAPO-34 catalysts with Cu loading(22.76%～4.12%(w/w))exhibited attractive activities,with nearly 100% NOxconversion and 100% N2selectivity at the temperatures(300～400℃)in the excess of oxygen.Based on in situ DRIFT studies,the formation of NO2-/NO3-intermediates requires the presence of isolated Cu2+ions in Cu-SAPO-34,which undergo a periodic Cu2+/Cu+redox cycle during C3H6-SCR.

Keywords:Cu-SAPO-34;selective catalytic reduction;NO;C3H6;Cu2+ions

0 Introduction

The efficient and sustainable removal of NOxreleased from diesel engines remains a significant challenge.Recently,copper zeolites with chabazite(CHA)structure,such as Cu-SAPO-34 and Cu-SSZ-13,have received much attention due to their excellent activities and remarkable hydrothermal stabilities in the selective catalytic reduction of NOxwith ammonia(NH3-SCR)[1-3].However,there are still some problems related to the use of NH3as reductant(e.g.,corrosion,toxicity).Hydrocarbons including alkanes and alkenes are a promising alternative reductant for SCR(HC-SCR)[4-5].Considering that the sizes of some hydrocarbon molecules(e.g.,CH40.38 nm,C2H40.39 nm and C3H60.4 nm)are close to the pore of CHA structure(0.38 nm),it is possible to carry out HC-SCR reaction on CHA zeolites[6-8].In addition,the zeolite framework is always in a vibration state,then the reactant molecules(larger than 20% pore diameter)can enter the zeolite channel at high reaction temperatures(＞ 100℃)[9].

Up to now,the active centers and reaction pathway in Cu-CHA for NH3-SCR remain an unsettled debate.More recent studies have shown that isolated copper species in CHA zeolites act as active sites and the SCR reaction follows a redox mechanism[10].Ma et al.[11]proposed that copper species were necessary to form nitrates at low temperatures and NO2at high temperatures.Gao et al.[12]further reported reaction kinetics and transient intermediates in the periodic cycle of Cu ions(CuⅡ→CuⅠ→CuⅡ)instandardNH3-SCR.However,little is known about the nature of Cu species over Cu-CHA for C3H6-SCR.

In the present study,a series of Cu-SAPO-34 catalysts with varied Cu/Al molar ratio were synthesized and their applications in C3H6-SCR have been investigated.Moreover,these samples were characterized using inductively coupled plasma optical emission spectrometer(ICP-AES),N2adsorption-desorption,X-ray diffraction(XRD),temperature-programmed reduction using hydrogen(H2-TPR),X-ray photoelectron spectra(XPS)and in situ diffuse reflectance infrared Fourier transform infrared(in situ DRIFTS)technologies to reveal the role of copper species on their catalytic properties.

1 Experimental

1.1 Preparation

The Cu-SAPO-34 catalysts were synthesized by the one-pot hydrothermal method.The original gel molar composition of Cu-TEPA(tetraethylenepentamine),Al2O3,P2O5,SiO2,DEA(diethylamine)and H2O was x∶1.0∶0.8∶0.72∶1.8∶36(x=0.08,0.12,0.16,0.20,0.24 and 0.28),and the resulting products with different Cu/Al molar ratio were denoted as Cux-SAPO-34.Typically,the CuSO4·3H2O was dissolved in deionized water to prepare a 20%(w/w)cupric sulfate solution,followed TEPA and phosphoric acid were added gradually.Then the pseudoboehmite,silica gel and DEA were introduced successively to the mixture.After stirring for 2 h,SAPO-34 seeds were added into the gel and stirred continuously.Then the resultant mixture was transferred into a 25 mL stainless-steel autoclave,which maintained 175℃for 6 days under the autogenous pressure without stirring.Finally,the obtained products were separated,filtered,washed,and dried at 100℃for 12 h,then calcined in air at 550℃for 8 h to remove the template.The chemical contents,wCu/wAland(wSi+wP)/wAlratios of Cu-SAPO-34 catalysts are listed in Table 1.Moreover,the ICP-AES analysis showed that content of element S in Cu-SAPO-34 catalysts was very low(about 0.005%(w/w)),indicating most of sulfur species were removed after high temperature calcination.

Table 1 Elemental analysis of Cux-SAPO-34 catalysts

Continued Table 1

1.2 Activity measurements

C3H6-SCR activity was carried out in a fixed-bed quartz flow reactor containing 400 mg catalyst[13].The total flow rate of reactive gases was 100 mL·min-1,which typically contained 0.05%(V/V)NO,0.05%(V/V)C3H6,10%(V/V)O2and the balance of N2,corresponding to a gas firing hourly space velocity(GHSV)of 15 000 mL·g-1·h-1.An online FTIR spectrometer(Thermo Nicolet IS10)was used to continuously analyse the effluent gases from the microreactor,including C3H6,NO,NO2and N2O at atmospheric pressure.NOxconversion and N2selectivity were defined as follows:

1.3 Characterization

The copper and other chemical elements contents of the as-synthesized Cu-SAPO-34 were determined by Agilent 730 inductively coupled plasma(ICP)optical emission spectrometer.XRD patterns were recorded with a Riguku D/max-2550 instrument using Cu Kα radiation(λ=0.154 nm)at 40 kV and 200 mA.The scanning range was 5°～80°(2θ)at a scan rate of 2(°)·min-1with a step size of 0.02°.The textural properties of the catalysts were investigated by N2adsorptiondesorption at-196℃ using a TriStarⅡ3020 gas adsorption analyzer.XPS experiments were performed on a ThermoFisher K-Alpha spectrometer equipped with monochromatic Al Kα X-ray source(1 486.68 eV),the binding energies were referenced to the C1s of adventitious carbon at 284.8 eV.H2-TPR experiments were performed on an Autosorb-iQ-C chemisorption analyser with a thermal conductivity detector.50 mg sample was pre-treated in flowing He at 300℃for 30 min,then the TPR was performed in flowing 10%(V/V)H2/Ar from ambient temperature to 700℃at a rate of 10℃·min-1.

In situ DRIFTS spectroscopy measurements were carried out on an FTIR spectrometer(Thermo Nicolet IS50)with a reaction chamber.Typically,the sample was pre-treated at 500℃in flowing N2for 60 min to remove any adsorbed impurities.Then the mixed gases with a composition of 0.1%(V/V)NO,10%(V/V)O2with or without 0.1%(V/V)C3H6in N2balance,were introduced to the chamber at a flow rate of 20 mL·min-1.Each spectrum was recorded with a resolution of 4 cm-1using a number of 64 scans with the MCT/A detector.

2 Results and discussion

2.1 C3H6-SCR activity

Cux-SAPO-34 catalysts with different Cu/Al molar ratios and SAPO-34 were tested for C3H6-SCR,and NOxconversion,C3H6conversion and N2selectivity are showed in Fig.1.The bare SAPO-34 exhibited inert NOxconversion in the temperature range of 100～400℃.In contrast,the activities of Cu-SAPO-34 catalysts were improved remarkably when the temperature exceeded 200℃due to the copper loading.For example,the NOxand C3H6conversion were 39.8% and 15.7% over SAPO-34,while they reached 97.2% and 95% over Cu0.20-SAPO-34 at 300℃.However,the conversion of NOxand C3H6did not match exactly in Fig.1,indicating that there existed C3H6combustion even in the low temperature.Moreover,the C3H6-SCR activities of Cu-SAPO-34 catalysts decreased drastically,while propylene conversion remained nearly 100% with increasing temperature over 400℃.It implies that the C3H6combustion would dominate at high temperatures,resulting in the low selectivity of propylene toward NOxreduction.

Fig.1 NOxconversion(a),C3H6conversion(b)and N2selectivity(c)over series Cux-SAPO-34 catalysts and SAPO-34

Among all Cux-SAPO-34 catalysts,Cu0.20-SAPO-34 exhibited high C3H6-SCR activity with nearly 100% NOxconversion and 100% N2selectivity in the range of 300～400 ℃ ,which appeared to be more attractive results under similar reaction conditions[6-7,14].Further increasing the copper loading of Cu-SAPO-34,the NOxconversion at low temperatures(＜300 ℃)increased significantly.However,the NOxconversion and N2selectivity(due to the low NO2production)at high temperatures(＞400 ℃)decreased with the increase of Cu/Al ratio.

2.2 Textures of Cu-SAPO-34 catalysts

The crystalline structure of the series of Cux-SAPO-34 catalysts were investigated by XRD measurements.In Fig.2a,typical chabazite structure with high crystallinity could be observed in the synthetic Cu-SAPO-34,which are in accordance with those of commercial SAPO-34(Nankai University Catalyst Co.,Ltd).The lower intensity of the commercial SAPO-34 may be due to the hydrolysis of Si-O-Al bonds under ambient temperature for months[15].Although Cu-SAPO-34 samples were loaded with up to 5.68%(w/w)Cu(Table 1),it is difficult to identify the diffraction peaks related to crystalline copper species(e.g.,Cu,CuO,Cu2O),indicating Cu species were incorporated as isolated ions or highly dispersed on zeolite support.Furthermore,a slight shift to lower angles of the SAPO diffraction peaks with Cu/Al ratio increasing could be clearly observed in the enlarged XRD pattern(Fig.2b).The main diffraction peak(100)shifted from 20.70°of Cu0.08-SAPO-34 to 20.66°of Cu0.28-SAPO-34 alone with the decrease of the reflection intensity.These behaviors were previously explained by a lattice expansion due to the integration of metal ions into the zeolite framework[16-17].Therefore,the copper ions mostly might be incorporated into the SAPO-34 framework,namely some Al3+ions(0.053 nm)were substituted with Cu2+ions(0.072 nm)in the synthesis of Cu-SAPO-34,and the differences in the sizes of two ions caused small changes in the crystallinity of SAPO-34.

Fig.2 XRD patterns of series Cux-SAPO-34 catalysts and SAPO-34

The N2adsorption-desorption isotherms of Cux-SAPO-34 catalysts are shown in Fig.3,and their surface areas and textural properties are summarized in Table 2.Obviously,the Cux-SAPO-34 samples mainly exhibited type-Ⅰ isotherms(IUPAC classification)with a hysteresis loop of type H4,which were usually found in the crystalline aggregate of zeolite.It can be observed that BET surface areas(SBET)decreased gradually with the decrease of pore volumes,and the surface areas of all samples were more than 600 m2·g-1while the pore volumes were approximately 0.23 cm3·g-1.However,the activities of catalysts(Fig.1)were not positively correlated with their textural properties.For example,Cu0.20-SAPO-34 sample exhibited relatively higher activity and N2selectivity with lower surface area and pore volume,indicating there may be some other factors that affect the C3H6-SCR activity of Cu-SAPO-34 catalysts.

Fig.3 N2adsorption-desorption isotherms of series Cux-SAPO-34 catalysts

2.3 Redox behaviour of Cu species in C3H6-SCR

The Cu0.20-SAPO-34 sample exhibited the excellent C3H6-SCR activity among the Cux-SAPO-34 catalysts.Hence,Cu0.20-SAPO-34 sample was chose to investigate the redox behaviour of Cu species in C3H6-SCR by H2-TPR.The TPR profiles of three pretreatment samples(fresh sample after calcination in air,the sample after reaction in NO+C3H6+O2and the sample after reducing in C3H6)are presented in Fig.4.According to the literature[18-20],the single peak at around 215℃of fresh sample(Fig.4a)was assigned to the reduction of isolated Cu2+to Cu+,and the broad peak at around 600℃was attributed to the continues reduction of Cu+to Cu0.The result shows the main copper species in fresh Cu-SAPO-34 were Cu2+ions,which were in accord with the results of XRD.After the sample(Fig.4c)was exposed in 0.1%(V/V)C3H6at 300°C for 2 h,the peak corresponding to the reduction of isolated Cu2+almost disappeared due to the reduction of Cu（Ⅱ） to Cu（Ⅰ） by propylene.However,the similar peak at around 215℃still appeared on the sample(Fig.4b)quenched after reaction,suggesting the reduction/reoxidation process occurred on Cu2+ions during C3H6-SCR.

Table 2 Textural properties of series Cux-SAPO-34 catalysts

The chemical state changes of Cu species were further characterized by XPS,as shown in Fig.5.The Cu2p3/2peak at about 936.0 eV alone with the satellite peak around 941～948 eV was used as a characteristic to determine Cu2+[21-22].Compared to the fresh sample,the concentration of Cu2+on the surface of the sample after C3H6reducing decreased significantly,indicating Cu2+in SAPO-34 was reduced to Cu+by C3H6.Furthermore,little virtually changes of Cu2+on the sample after reaction were detected due to the redox cycle of Cu ions kept the average steady-state Cu valence at+2.The XPS results agree well with the H2-TPR results,revealing the monovalent to divalent redox of isolated Cu ions in C3H6-SCR.

Fig.4 H2-TPR profiles of Cu0.20-SAPO-34 catalysts after different treatments

Fig.5 XPS spectra of Cu2p on Cu0.20-SAPO-34 catalysts after different treatments

2.4 In situ DRIFTS analysis

To gain more information about the role of Cu species in C3H6-SCR,in situ DRIFTS on the reactive gases(0.1% NO+10% O2,followed by 0.1% C3H6)adsorption were carried out over Cu0.24-SAPO-34 catalyst as well as the bare SAPO-34.In Fig.6,some weak banks located at 1 593,1 628 and 1 732 cm-1associated with NOxadsorbed species were detected on SAPO-34(Fig.6a)after flowing in the NO+O2mixture for 45 min at 250℃.When C3H6was subsequently introduced,several new banks at 1 217,1 475,1 532,1 696 and 1 302 cm-1appeared gradually,which could be ascribed to adsorbed propylene,acetate,formate and carbonate[21,23].

During the same process of NO+O2adsorption,some obviousdifference on the FTIR spectrum appeared overCu-SAPO-34 catalyst(Fig.6b).The strong bands due to adsorbed NO2(1 665 cm-1),monodentate nitrate(1 604 cm-1),bidentate nitrate(1 571 cm-1)and nitrite species(1 485 cm-1)were observed on Cu-SAPO-34 catalyst[11,24].However,those nitrates and nitrite species were present in lower amounts on bare SAPO-34,suggesting NO molecules interacted more strongly with Cu ions and formed Cu-(NOx)like complexes.Ruggeri et al.[25]proposed the oxidation of NO on Cu2+site over Cu-CHA catalyst involves Cu2+reduction to Cu+,NO2leaving the Cu2+sites in high oxidation state,and the reoxidation of Cu+sites by oxygen.Therefore,active sites for NO chemisorption and activation should be isolated Cu2+.With the flowing of C3H6,the species of acetate(1 589 cm-1),formate(1 502 cm-1)and carbonate(1 282 cm-1)were gradually formed on the sample[14,23],and they were similar to those of the SAPO-34.Hence C3H6adsorption on Cu-SAPO-34 may occur mainly on Brønsted acid sites of CHA zeolite,which can be active oxidized by the adsorbed oxygen and/or lattice oxygen on adjacent copper sites[8].Meanwhile,the surface NO2-/NO3-species disappeared gradually with carbonate increased,and adsorbed NO2(1 665 cm-1)[24]on Cu-SAPO-34 and adsorbed NO(1 732 cm-1)[26]on SAPO-34 were the main NOxdesorption product at the end of adsorption,respectively.These phenomena indicate that NO2-/NO3-species were the reactive intermediates during C3H6-SCR reaction.Moreover,according to the previous TPR and XPS analysis,the reduction of Cu2+would occur at the same time of propylene adsorption,which might be through an electron transfer from C3H6to Cu2+with the formation of Cu+[12].

Fig.6 Dynamic changes in in situ DRIFT spectra for the adsorbed species over SAPO-34(a)and Cu-SAPO-34(b)at 250℃exposed to a flow of 0.1%(V/V)NO+10%(V/V)O2followed by 0.1%(V/V)C3H6

Furthermore,the bands at 2 270 and 2 168 cm-1are attributed to isocyanate(-NCO)species,commonly agreed as active intermediates for C3H6-SCR,which arise from the reaction of adsorbed NO2-/NO3-and CxHyOzspecies[27-29].By comparison,those bands were not clearly observed on SAPO-34,suggesting this reaction required the presence of a sufficient amount of active copper species,probably for the NOadsorption and activation in higher oxidation states.It is noteworthy that an additional band at 1 437 cm-1appeared on Cu-SAPO-34,may result from deformation vibrations of ammonium(R-NH2)intermediates,which are favorable for HC-SCR reaction[27-28].Interestingly,the intensity of R-NH2of SAPO-34 was obviously lower than that of Cu-SAPO-34,which might be related to the intensity of-NCO intermediates.As shown in Fig.6,the intensity of-NCO on Cu-SAPO-34 catalyst was significantly higher than that of SAPO-34,then more R-NH2intermediatescould comefrom thehydrolysisof-NCO[30].

Following the analysis above,a possible reaction pathway of C3H6-SCR over Cu-SAPO-34 catalyst could be proposed in Scheme 1.The formation of NO-/NO-23species by NO molecules requires the presence of the isolated Cu2+ions in Cu-SAPO-34,which undergo a periodic Cu2+/Cu+redox cycle during C3H6-SCR.Meanwhile,propylene can be adsorbed on Brønsted acid sites and activated to CxHyOzspecies including HCOO-and CH3COO-.Then HCOO-/CH3COO-species react with NO2-/NO3-species and transform to-NCO and RNH2intermediates.On the other hand,these CxHyOzspecies may be directly converted into CO2and H2O at high temperatures in the excess of oxygen,resulting in the decrease of C3H6-SCR activity.Finally,-NCO and R-NH2species react with adsorbed NO2to from the final products of N2,H2O and CO2.

Scheme 1 Proposed reaction pathway of C3H6-SCR reaction over Cu-SAPO-34 catalyst

3 Conclusions

In this work,the one-pot synthesized Cu-SAPO-34 catalysts with varied Cu/Al ratio were applied for C3H6-SCR.Cu-SAPO-34 catalysts with Cu loading(2.76%～4.12%(w/w))exhibited high catalytic activity,with nearly 100% NOxconversion and 100% N2selectivity at the temperatures(300～400℃)in the excess of oxygen.Based on a combination of characterizations,copper species in Cu-SAPO-34 exist mostly incorporated as isolated Cu2+ions,which are active sites for NO chemisorption and activation.In situ DRIFTS reveal that the formation of NO2-/NO3-intermediates requires the presence of isolated Cu2+ions,which undergo a periodic Cu2+/Cu+redox cycle during C3H6-SCR.