Tenhanced performance of ceria with surface sulfation for selective catalytic reduction of NO by NH3

Short CommunicationThe enhanced performance of ceria with surface sulfation for selective catalytic reduction of NO by NH 3Tingting Gu,Yue Liu,Xiaole Weng ⁎,Haiqiang Wang,Zhongbiao WuDepartment of Environmental Engineering,Zhejiang University,Hangzhou 310027,ChinaZhejiang Provincial Engineering Research Center of Industrial Boiler &Furnace Flue Gas Pollution Control,Hangzhou 310027,Chinaa b s t r a c ta r t i c l e i n f o Article history:Received 13July 2010Received in revised form 25September 2010Accepted 4October 2010Available online 1November 2010Keywords:SCRSurface sulfation CeO 2NO NH 3The selective catalytic reduction (SCR)of NO with NH 3over fresh and sulfated CeO 2catalysts had been investigated in this study.Experimental results showed that the sulfated CeO 2sample had signi ficantly higher NO conversion than the fresh one and revealed an excellent selectivity to N 2within the temperature range of 200–570°C.By characterizations via BET,XRD,XPS,EDS and TPD,it was concluded that the improvement in SCR activity by sulfation might be originated from the increase of active oxygen species and the enhancement of NH 3chemisorption after surface sulfation,both of which were conducive to NH 3activation.©2010Elsevier B.V.All rights reserved.1.IntroductionSelective catalytic reduction (SCR)of NO x with ammonia is the mainstream technique for the abatement of NO x emission from stationary source [1].Catalysts used in SCR reaction,e.g.WO 3–V 2O 5/TiO 2[2]and MoO 3–V 2O 5/TiO 2[3],have been extensively commer-cialized for industrial application.However,these vanadium-based catalysts are very expensive and not environmental friendly as the vanadium is very harmful to ecological environment.As such,recent efforts have been made to develop relatively “green ”catalysts to replace the V-based catalysts.Ceria (CeO 2)is cheap and accounts for a large part of rare earth oxide market.It possesses high oxygen storage/release capacity with very good redox property [4].This owns to its excellent reducibility from Ce 4+to Ce 3+,which is originated from the high mobility O 2−in ceria fluorite lattice [5].As such,ceria has been widely used as a crucial component in three-way catalysts (TWCs)for automotive emission-control [6],and also been considered as a promising catalyst in the application of selective catalytic reduction (SCR)for NO x abatement,such as Ce/TiO 2[7],Ce/Al 2O 3[8],Ce –Mn –O x [9]and CeO 2–zeolite [10].In particular,ceria based catalysts have shown a great resistance to SO 2poisoning in SCR reaction [11].Although the SO 2poisoning effect on SCR activity has been reported in many papers [12,13],some researchers af firmed just theopposite.For example,Notoya et al.[14]had indicated that SO 2might have a contribution to the production of active surface oxygen,which was bene ficial for reduction of NO by NH 3;Xie et al.[15]and Amiridis et al.[16]had found that SO 2could promote the SCR activity of CuO/Al 2O 3(≥T300°C)and V 2O 5/TiO 2(≥T350°C)catalysts,respectively.A similar phenomenon was also observed at 180°C for V 2O 5/AC [17]and the promoting role of SO 2was assumed resulting from the increased acidity on the surface after sulfation.Therefore,the effect of SO 2on SCR activity still needs further investigation.Furthermore,there are documents reported that either pure CeO 2[18]or it mixed with alumina [19]was an effective SO 2sorbent and Wu et al.[11]had found that Ce doping was effective to enhance SO 2resistance for Mn/TiO 2catalysts.However,the work did not reveal any insight into the interaction between ceria and SO 2and the sulfation effect in SCR reaction was also not clari fied.In this work,we studied the in fluences of SO 2on SCR reaction behaviors over pure ceria.The CeO 2samples,prepared by thermal decomposition method,with or without pretreatment by SO 2,were used as catalysts in SCR of NO by ammonia.Moreover,the surface properties and crystalline structures of the catalysts were character-ized by BET,XRD,XPS,EDS and TPD methods to investigate the sulfation effect.2.Experimental 2.1.Catalyst preparationThe fresh CeO 2sample was prepared by thermal decomposition of Ce(NO 3)3.6H 2O (SCRC,99%)at 550°C under air atmosphere in tubeCatalysis Communications 12(2010)310–313⁎Corresponding author.Department of Environmental Engineering,Zhejiang University,Hangzhou 310027,China.Tel.:+8657187951571;fax:+8657187953088.E-mail address:xlweng@ (X.Weng).1566-7367/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.catcom.2010.10.003Contents lists available at ScienceDirectCatalysis Communicationsj o u r n a l h o m e p a g e :w ww.e l s ev i e r.c o m /l o c a t e /c a tc o mfurnace for 4h.The sulfated CeO 2sample was obtained by treating the as-prepared ceria with N 2containing 3vol.%O 2and 300ppm SO 2at 300°C for 1h.All the catalysts were sieved to 40–60meshes.2.2.Catalytic activity measurementSCR activity measurements were carried out in a fixed-bed quartz tubular flow reactor (i.d.9mm)within the temperature range of 200–570°C.The feed gas consisted of 1000ppm NO,1000ppm NH 3,3vol.%O 2and balanced N 2.In all runs,the catalysts load in the reactor was of 2ml and the gas hourly space velocity (GHSV)was kept at 60,000h −1.The concentrations of NO and NO 2were monitored by a NO –NO 2–NO x analyzer (Thermo,Model 42i-HL),while the formation of N 2O was monitored using a Gasmet FTIR gas analyzer (DX-4000).2.3.Catalyst characterizationsThe speci fic surface areas were measured by N 2adsorption at 77K by the BET method using a Micromeritics ASAP 2020.The pore size distributions were measured from the N 2desorption isotherm using the cylindrical pore model (BJH method).X-ray diffraction patterns (XRD)of the samples were recorded on a Rigaku D/Max-RA powder diffractometer using Cu K αradiation (40kV and 150mA).X-ray photoelectron spectroscopy (XPS)was acquired at room temperature using a Thermal ESCALAB 250spectrometer Al K αX-rays.The bulk atomic concentrations were determined by X-ray energy dispersive spectroscopy (EDS)linked with transmission electron microscopy (TEM)in JEM 2100F system at a voltage of 200kV.TPD of NH 3experiments were performed using 100mg samples.After pretreatment in He environment at 400°C for 1h,samples were cooled to room temperature and then saturated with anhydrous NH 3(4%in He)at a flow rate of 30ml/min for about 30min.Desorption was carried out by heating the samples in He environment (40ml/min)from 100°C to 800°C with a heating rate of 5°C/min.3.Results and discussion 3.1.Catalytic activity testsFig.1illustrated the NO conversion within the temperature range of 200–570°C over fresh and sulfated CeO 2samples.As expected,the fresh CeO 2sample was observed with very poor activity,where its NO conversion was lower than 50%within the temperature range investigated.The catalytic ef ficiency was signi ficantly improved by sulfation treatment,where more than 95%NO conversion wasachieved in the temperature range of 270–520°C.The high temper-ature deactivation (≥520°C)observed for sulfated sample could be mainly attributed to the oxidation of NH 3by gaseous oxygen,which signi ficantly reduced the amount of activated NH 3[20].The N 2O,a potential by-product in SCR reaction,had also been monitored over sulfated sample and was observed less than 10ppm within the whole temperature range.This indicated that the sulfated sample had a signi ficantly high N 2selectivity (≥99%).3.2.XRD and BET measurementsXRD was used to investigate the crystal structures of fresh and sulfated CeO 2samples.As shown in Fig.2,the diffraction peaks of these samples were both consistent with cubic fluorite structure (PDF-ICDD34-0394),but the intensity of sulfated sample was slightly decreased compared with that of the fresh sample,indicating that the sulfation would lead to a decrease in CeO 2crystallinity.Furthermore,no extra peaks ascribed to sulfated phase were observed in the XRD results,suggesting that the sulfated species might exist as either surface sulfate or amorphous bulk sulfate.BET values for the samples are present in Table 1.It was observed that the surface area and pore volume were decreased after sulfation.This could be caused by surface shielding/doping of sulfated species to the sample,which blocked the micropores in CeO 2lattice and increased average pore size.3.3.EDS and XPS analysisTo obtain a better understanding of chemical states for all elements on the surface,the samples were then subjected to XPS analysis.The XPS spectra (Fig.3(I))revealed that Ce 3d was composed of two multiplets (v and u),which corresponded to the spin orbit split 3d 5/2and 3d 3/2core holes,respectively.Based on previous studies [21–23],the u ‴,u ″,u,v ‴,v ″,v peaks were attributed to Ce 4+state while the u ′and v ′peaks were assigned to Ce 3+state.It can be clearly seen that the intensities of Ce 4+characteristic peaks decreased after sulfation,accompanying with an increase of Ce 3+peaks.ThisisFig.1.NO conversions over fresh and sulfated CeO 2catalysts.([NH 3]=[NO]=1000ppm,[O 2]=3vol.%,N 2balance,GHSV=60,000h −1).Fig.2.XRD spectra of fresh and sulfated CeO 2catalysts.Table 1BET measurements for the catalysts.Samples BET surface area (m²/g)Pore volume (×10−2cm³/g)Pore diameter (nm)Fresh CeO 255.3920.6012.43Sulfated CeO 236.7216.9714.82311T.Gu et al./Catalysis Communications 12(2010)310–313unsurprising as Waqif et al.[24]had reported that the SO 2could act as a reducing agent,thereby inducing a reduction from Ce 4+to Ce 3+on sample surface.A similar phenomenon was also observed by Smirnov ea al.[25].The spectra also showed a lower binding energy shift of Ce 4+characteristic peaks after sulfation,indicating an expansion of CeO 2fluorite lattice.Again,this was caused by the increase of Ce 3+,whose effective ionic radius is about 14%larger than that of Ce 4+in the same coordination [26].It was widely accepted that the migration of oxygen in ceria took place mainly via vacancy hopping [27].Therefore,since more oxygen vacancies were generated in the sulfated sample (due to the increase of Ce 3+),the migration of oxygen from bulk to the surface would become much easier.Furthermore,the increase of oxygen vacancies could also lead to more gaseous oxygen being replenished on the sample surface [28].Both of these could facilitate the activation and transportation of active oxygen species in SCR reaction,leading to a signi ficant increase in SCR activity for the sulfated sample (see Section 3.1).Fig.3(II)revealed the fitted O1s peaks for lattice oxygen O α(529.0–530.0eV),chemisorbed oxygen O β(531.3–531.9eV)and hydroxyl groups O γ(532.7–533.5eV)[29].Noticeably,the chemi-sorbed oxygen content was greatly increased after sulfation treatment (see Table 2).This is a promising result as the surface chemisorbed oxygen was the most active oxygen in SCR reaction [30].The presence of such high surface chemisorbed oxygen in the sulfated CeO 2sample could also increase its SCR activity.Fig.3(III)revealed the S 2p spectra of sulfated CeO 2sample.The binding energy of S 2p equal to 169.3eV demonstrated that the S 2p feature was consistent with the S (VI)oxidation state and assigned to the sulfate species [31,32].Compared with the bulk concentration of S as measured by EDS (see Table 2),a signi ficant reduction of S amount was observed.This implied that the sulfate species might mainly accumulate on sample surface rather than entering into the CeO 2lattice.3.4.NH 3-TPD analysisThe surface acid properties of the catalysts were analyzed by NH 3-TPD.As shown in Fig.4,the total acidity of fresh CeO 2was observed lower than that of sulfated CeO 2.The TPD pro files evidenced the presence of acid sites of different strength on each sample.ThefreshFig.3.XPS (I)Ce3d (II)O 1s (III)S 2p states photoemission spectra of (a)fresh CeO 2and (b)sulfated CeO 2.Table 2Surface and bulk atomic concentration of samples before and after the sulfation.SamplesSurface atomic concentration (%)Bulk atomicconcentration (%)CeSO CeSOO αO βO γFresh CeO 234.85–42.5315.117.5135.44–64.55Sulfated CeO 226.544.2235.6224.638.9934.240.1565.59Fig.4.NH 3-TPD pro files for (a)fresh CeO 2and (b)sulfated CeO 2.312T.Gu et al./Catalysis Communications 12(2010)310–313CeO2catalyst had two major desorption peaks,one at about500°C (moderate acid strength),the other at around630–680°C(strong acid strength)whilst the sulfated CeO2only exhibited one desorption peak at about680°C(strong acid strength).This suggested that more acid sites of higher energy existed on sulfated CeO2compared with fresh CeO2.According to the reference[33],the strongest acid sites released NH3at temperature as high as above627°C(see peakβand γin Fig.4),which were shown to be of Lewis-acid nature and the moderated acid sites at lower temperature was ascribed to Brønsted-acid.Above all,by sulfating,the Brønsted acid sites of CeO2 disappeared,while the Lewis acid sites increased greatly.This was due to that sulfate groups could generate strong acidity,and sulfate species themselves are Lewis acids or by attracting electrons they generate Lewis acid centers on the oxide surfaces[34].The strong acidity after sulfation would lead to a significant increase in adsorption capacity of NH3for the sample,which,to some extent, favored the SCR reaction.NO-TPD results(not shown)of sulfated CeO2demonstrated that there was no adsorbed NO on sample surface.Similar phenomena were also observed in other catalysts,such as V2O5/TiO2[35]and MnCeO x[36].Kapteijn et al.[37]suggested that the SCR reaction might take place via an Eley–Rideal type mechanism in the absence of NO adsorption where adsorbed ammonia(on Lewis acid sites)reacts with NO in the gas phase or weakly bined with the previous reports[1,38–41],detailed reaction steps can be listed as follows:NH3ðgÞ→NH3ðaÞð1ÞNH3ðaÞþOðaÞ→NH2ðaÞþOHðaÞð2ÞNH2ðaÞþNOðgÞ→NH2NOðaÞ→N2ðgÞþH2OðgÞð3Þwhere the reaction step(2)was the key step for NH3-SCR process,i.e. reacting between adsorbed ammonia and activated oxygen.Through the TPD and XPS results,we found that the sulfation treatment not only significantly improved the strength and concentration of NH3 adsorption,but also enhanced the formation of oxygen vacancies over ceria samples(resulting in the increase of chemisorbed oxygen),both of which promoted the SCR reaction[41].4.ConclusionThe NO conversion for SCR reaction with NH3over ceria catalyst was greatly improved by surface sulfation.From XPS results,it was observed that sulfation could result in an enrichment of Ce3+on sample surface(leading to an increase in active oxygen content)and induced a presence of strong acid sites in the sample(favoring the NH3chemisorptions and activation),both of which were conducive to SCR activity.The results present herein may give a new thought for the preparation of SCR catalysts,i.e.via a sulfation pretreatment.This is an approach that we are currently pursuing,the results of which will be reported in due course.AcknowledgmentsThe project isfinancially supported by the National Natural Science Foundation of China(NSFC-50878190),Changjiang Scholar Incentive Program,Science Foundation of Chinese University and Natural Science Foundation of Zhejiang Province(Y5090053). 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