碳烟催化燃烧MnOx-CeO2 mixed oxides for the low-temperature oxidation of diel soot

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MnO x-CeO2mixed oxides for the low-temperature
oxidation of diel soot
Kirill Tikhomirov,Oliver Kro¨cher*,Martin Elner,Alexander Wokaun
Paul Scherrer Institute,CH-5232Villigen PSI,Switzerland
Received26September2005;received in revid form9November2005;accepted10November2005
Available online19December2005
Abstract
TG-FTIR andflow reactor experiments were performed to study soot oxidation on MnO x-CeO2mixed oxide catalysts in model diel exhaust gas(10%O2,5%H2O,1000ppm NO,balance N2).The ignition temperature in TG-FTIR experiments for a1:20mixture of soot and catalyst was found to be$2808C,which is significantly lower than for the individual oxides.
It was found that NO is oxidized over the catalyst to NO2,which is stored on the catalyst as nitrate at low temperatures.At higher temperatures the nitrates decompo,releasing NO2to the gas pha which acts as the oxidizing agent for soot.The nitrate storage capacity of MnO x-CeO2is three tofive
times higher than that of the individual oxides resulting in a major contribution of the relead NO2to the soot oxidation process.This explains the strong synergistic effect of mangane and cerium in the mixed oxide on the soot oxidation.
大疆精灵4Adding SO2to the model gas resulted in a deactivation of the catalyst,which is traced to the loss of the NO oxidation activity.The sulphates could be decompod by heating the catalyst under reducing as well as oxidizing conditions.However,the initial activity of the catalyst could not be restored.
#2005Elvier B.V.All rights rerved.
梦见刀是什么意思Keywords:Diel soot oxidation;Mangane cerium mixed oxides;NO x storage catalyst
1.Introduction
Diel engines have been ud for a long time in heavy duty vehicles and for long-range transports.In the last two decades, their high efficiency and hence better fuel economy in comparison to spark ignition engines led to an unprecedented growth of their share of the pasnger car market as well.With this expansion,the environmental pollution by diel exhaust incread dramatically.Especially nitrogen oxides and parti-culate matter emissions from diel engines po major health hazards.
As exhaust after treatment techniques are going to be indispensable to fulfil the future emission standards and decrea the environmental stress,much effort has been put on rearch in thisfield in the last10years.For soot removal,the u of a catalytic trap performing bothfiltration and catalytic combustion of soot appears to be an effective solution.Many catalysts have been propod for such traps,with mixed metal oxides,noble metals and perovskites being the outnumbering materials[1–3].Platinum-bad catalysts em to reprent the best solution up-to-date,providing NO to NO2oxidation,which allows soot oxidation at quite low temperatures[4–6]. However,noble metal catalysts are expensive and are troubled with poisoning problems.A cheap and efficient substitute is therefore still desired.
In this work MnO x-CeO2mixed oxides have been investigated for the NO2-supported soot oxidation in the low-temperature range.
2.Experimental
2.1.Catalyst preparation
MnO x-CeO2mixed oxide catalysts were prepared by coprecipitation using acetates of Ce(III)and Mn(II)as starting materials.Catalysts with molar ratios Mn:Ce=0:100,25:75, 50:50,75:25and100:0wer
e synthesized,further being referred as Mn0Ce100,Mn25Ce75,Mn50Ce50,Mn75Ce25and Mn100Ce0,accordingly.For the preparation of Mn50Ce50,
/locate/apcatb
Applied Catalysis B:Environmental64(2006)72–78
*Corresponding author.Tel.:+41563102066;fax:+41563102323.
E-mail address:Oliver.Kroecher@psi.ch(O.Kro¨cher).
0926-3373/$–e front matter#2005Elvier B.V.All rights rerved.
doi:10.1016/j.apcatb.2005.11.003
0.05mol of Ce(OOCCH3)3Áx H2O and0.05mol of Mn(OOCCH3)2Á4H2O were dissolved in400ml H2O at 458C and60ml ammonia solution(25%)was added dropwi. Subquently,100ml3M H2O2solution was added to the suspension of metal hydroxide precipitates in order to complete the transfer of the metals to the higher oxidation states. Afterwards,the precipitate wasfiltered,washed with water, dried at1208C overnight and calcined at6508C for5h.
2.2.Catalyst characterization
The specific surface area(S BET)of each prepared sample was determined by nitrogen physisorption at77K(Micro-meritics ASAP2001).Prior to the measurement,the samples were degad at2508C.X-ray analysis of the crystalline structure of the samples was performed with Fe K a radiation on a Philips X’Pert-MPD diffractometer.The molar compositions of the prepared catalysts were quantified by ICP-AES in a Varian Vista AX spectrometer.Fifty milligrams of each sample were dissolved in50ml of a50%aqueous H2SO4solution followed by dropwi addition of3%aqueous H2O2till the solution was colourless.Subquently,the solution was diluted with water in the ratio1:10and H3BO3was added prior to analysis.
2.3.Soot oxidation in TG-FTIR
Soot oxidation experiments were performed by means of a TGA-DTA/DSC device(Netzsch STA449C Jupiter)coupled with an FTIR-spectrometer(Bruker Tensor27DGTS).The soot was collected from the exhaust of a EURO II diel engine downstream of a diel oxidation catalyst to ensure the removal of adsorbed hydrocarbons[7].The specific surface area(BET)of the soot was measured to be110m2/g.
研究生满分多少分
5.0mg of soot were mixed with100.0mg of the catalyst in the crucible with a spatula in order to achieve‘‘loo’’soot-catalyst contact being reprentation for the conditions in a diel particulatefilter[8,9].The mixture was heated from30 to7008C at a heating rate of108C/min in a140ml/min model gasflow of10%O2,3%H2O and1000ppm NO in N2. Mass loss of the sample,the CO and CO2concentrations downstream of the reaction chamber and the DTA signal were measured by FTIR spectroscopy.Soot oxidation rates were calculated on the basis of CO2formation rate,being in good agreement with the reaction rates calculated from the sample mass loss.The CO yield was negligible for all catalysts.
2.4.NO oxidation,NO x storage and SO2poisoning in the
flow reactor
NO oxidation,NO x storage and SO2poisoning were investigated in aflow reactor,in which1.00g of catalyst powder wasfixed between two quartz wool plugs.The reactor was heated with a heating coil,whereas the temperature was regulated by a PID controller using a thermocouplefixed on the outer surface of the reactor.Moreover,the temperature inside of the catalyst sample was measured with a cond thermocouple. The composition of the feed gas(150l N/h)was adapted to a typical diel exhaust gas,containing10%O2and5%H2O with balance N2.For the NO oxidation,NO adsorption and SO2 poisoning experiments,1000ppm NO were added to the feed, in SO2poisoning experiments additionally50ppm SO2.Flow rates were regulated using massflow controllers(Brooks 5850S,5881).A Nicolet Magna-IR560FTIR spectrometer was ud to quantify the NO and NO2concentrations and to ensure the abnce of side products downstream of the reactor.For regeneration,the SO2poisoned catalyst was treated for1h with a150l N/hflow of5%H2in N2at6508C,followed by a treatment with150l N/h of10%O2in N2for1h.The conditions were chon,as preliminary tests in the TG-FTIR instrument showed that sulphates are stable under oxidative conditions up to9508C,whereas they are quantitatively decompod at6508C when treated with a hydrogen contain-ingflow.
2.5.DRIFT investigation of the NO adsorption
FTIR analysis of adsorbed species was performed at1508C using a Nicolet FTIR spectrometer Nexus670with a DRIFT cell.Prior to the adsorption experiments the IR spectrum of the fresh Mn25Ce75catalyst was recorded as reference.Sub-quently,the sample was treated with10%O2and5000ppm NO in N2for30min at1508C and another measurement was made in order to determine the IR spectrum of the adsorbed species in the range from2000to800cmÀ1.
3.Results
3.1.Catalyst characterization
The BET surface areas(S BET)of the different metal oxides are given in Table1.For all mixed oxides the specific surface area is between79and89m2/g,whereas it is much smaller for the individual oxides.In Fig.1,the X-ray powder diffraction patterns of the catalysts are shown.For all mixed oxides reflections at the same positions as for Mn0Ce100could be found.However,the signals were much broader than for the individual oxide.Only for Mn75Ce25weak signals corre-sponding to mangane oxide could be obrved.The Mn:Ce molar ratios of the catalysts quantified by ICP-AES were found to be Mn25Ce75=3.10:1,Mn50Ce50=1.04:1,Mn75Ce25= 0.38:1,which is in good agreement with the ratios of the starting acetates.
K.Tikhomirov et al./Applied Catalysis B:Environmental64(2006)72–7873
Table1
Specific surface area(BET)of MnO x-CeO2catalysts
Sample S BET(m2/g)
Mn0Ce10037
Mn25Ce7586
Mn50Ce5079
Mn75Ce2589
Mn100Ce024
3.2.Soot oxidation on various catalysts
In order to compare the soot oxidation activities of the prepared catalysts,carbon oxidation rates wer
e measured in TPO experiments.It can be clearly en from Fig.2that the catalytic activity of the mixed oxides is much higher than for the individual ones.The curve shape of all mixed oxides exhibit two characteristic peaks,the first between 300and 5008C and the cond starting at 5508C till the oxidation is complete.The difference in activity between the catalysts with various Mn:Ce ratios is rather small.Catalysts with higher cerium content show slightly higher maximum oxidation rates for T <4508C (3.8,3.6and    3.5m g C /s for Mn25Ce75,Mn50Ce50and Mn75Ce25,respectively),whereas the ont temperature for soot oxidation is
shifted to higher temperatures (305,295and 2808C for Mn25Ce75,Mn50Ce50and Mn75Ce25,respectively).
The influence of the feed gas components on the soot oxidation was investigated in TPO experiments with Mn25Ce75catalyst using different feed gas compositions (Fig.3).The ont temperature for the NO-free oxidation lies around 4008C,while that for NO assisted oxidation starts at around 1008C lower temperatures.Water does not influence the oxidation temperature,but has a minor positive effect on the oxidation rate.校园春色你我色
In order to detect possible activation effects,the TPO profiles of the fresh and the NO-pretreated Mn2
5Ce75catalyst were measured.Fig.4reveals that NO pretreatment drastically increas the soot oxidation rates up to 5008C.Also the ont temperature of the oxidation peak is lowered for the pretreated catalyst by approximately 508C.No cond peak was obrved for the pretreated catalyst.
The soot oxidation activity of the mixed oxides was compared with that of the active mass of a commercial platinum-bad NO oxidation catalyst from Umicore.The platinum content of the honeycomb catalyst was 90g/ft 3corresponding to approximately 2.5wt.%of platinum in the active mass.The BET surface area of the active mass was determined to be 195m 2/g.The result of the comparison can be en in Fig.5.The soot oxidation starts at significantly lower temperature when using the Mn25Ce75catalyst with a difference in the ont temperature of $608C and proceeds at higher rates over the whole temperature range from 250to 4508C compared to the commercial oxidation catalyst.3.3.NO oxidation and storage
Bad on the assumption that NO 2–and not NO directly –acts as the actual soot oxidizing agent,we investigated the oxidation of NO to NO 2over Mn25Ce75.From Fig.6,it can be en that the catalyst shows remarkable NO oxidation activity
K.Tikhomirov et al./Applied Catalysis B:Environmental 64(2006)72–78
74Fig.1.Powder diffraction patterns of MnO x -CeO 2catalysts:(*)reflections of the C-rare earth type of Mn 2O 3;(*)reflections of the CeO 2fluorite
structure.上海南京东路
Fig.2.TPO profiles for soot oxidation on different MnO x -CeO 2catalysts.Reactant gas:10%O 2+1000ppm NO +3%H 2O in N 2.Catalyst compositions:(*)Mn0Ce100;(&)Mn25Ce75;(&)Mn50Ce50;(~)Mn75Ce25;(~)
Mn100Ce0.
Fig.3.TPO profiles for soot oxidation on the Mn25Ce75catalyst.Reactant gas:(*)10%O 2in N 2;(&)10%O 2+1000ppm NO in N 2;(&)10%O 2+1000ppm NO +3%H 2O in N 2.
already at low temperatures.At 1508C,4%of the NO is converted;at 2008C already 10%is oxidized.The maximum NO 2yield is obrved at 3508C,followed by a decrea due to the thermodynamic equilibrium between NO and NO 2.
The strong effect of the NO-pretreatment on the soot oxidation activity at temperatures <5008C prod to a NO x storage effect,which was further investigated by treating Mn25Ce75catalyst with NO.During the treatment,consump-tion of NO was obrved (Fig.7a).The NO x adsorption was highest at the beginning and decread to 0after 30min.However,after 5min already 90%of the total integrated amount of NO x was adsorbed.Subquently,the catalyst was purged with 150l N /h of N 2at 1508C for 15min and then heated to 4508C at a heating rate of 108C/min.During the thermal regeneration almost only NO 2was desorbed from the catalyst,with only a small peak of NO between 300and 3508C (Fig.7b).The results were similar for all mixed oxides and the individual oxides showed only very poor storage performance.The amounts of NO x stored on the different catalysts were calculated from the adsorption and desorption profiles and are
summarized in Table 2.The adsorption and desorption results are in a good agreement.
DRIFT spectra of the adsorbed species on Mn25Ce75were measured to gain insight into the type of NO x storage on the catalyst (Fig.8).The spectrum contains a strong band at 1540cm À1and a weak band at 1440cm À1,which are assigned to bidentate and monodentate covalently bonded nitrates,respectively.Strong bands at 1280and 1030cm À1are ascribed to ionic nitrate NO 3À.During the treatment of the individual oxides none of the bands could be obrved at the positions mentioned.
K.Tikhomirov et al./Applied Catalysis B:Environmental 64(2006)72–78
75
Fig.4.TPO profiles for soot oxidation catalyst:(&)fresh catalyst;(~)catalyst pretreated with 10%O 2+1000ppm NO +3%H 2O in N 2at 1508C for 1
h.
Fig.5.Comparison of the TPO profiles for soot oxidation on the Mn25Ce75catalyst (&)and a commercial platinum catalyst (~).Reactant gas:10%O 2+1000ppm NO +3%H 2O in N 2
制片人是做什么的.
Fig.  6.NO oxidation over the Mn25Ce75catalyst.Feed gas:10%O 2+1000ppm NO +5%H 2O in N 2,GHSV =52,000h À1
.
Fig.7.(a)NO adsorption on Mn25Ce75at 1508C.Feed gas:10%O 2+1000ppm NO +5%H 2O in N 2,W/F =0.025g s cm À3(b)NO x -TPD profile.Feed gas:10%O 2+5%H 2O in N 2.
3.4.Sulphur poisoning
As NO 2formation plays a key role in the soot oxidation process,we investigated the influence of sulphur poisoning on the NO oxidation performance of Mn25Ce75.In Fig.9,the NO oxidation activity of the Mn25Ce75catalyst upon SO 2-treatment is shown.The NO 2yield decread continuously when SO 2was added to the feed.After 30min of treatment the NO 2yield was reduced from initially 63–12%.The catalyst activity did not recover after the SO 2dosage was stopped.Also no recovery was obrved after the regenera-tion of the catalyst under reducing conditions at 6508C (results not shown).
Additionally,the influence of sulphur poisoning on the soot oxidation and the possibility of regeneration was investigated.The soot oxidation activity of the fresh Mn25Ce75catalyst was compared with tho of the SO 2-treated and the regenerated sample (Fig.10).The catalytic activity was almost completely lost after the sulphur treatment,as the TPO profile is similar to the one for non-
catalytic soot oxidation (not shown).The regeneration under reducing conditions followed by calcination as described in Section    2.4only slightly recovered the soot oxidation activity,discernible from a small increa in the soot combustion rate at temperatures over 4008C.However,the ont temperature of the oxidation peak was not affected.
4.Discussion
知母的功效
4.1.Catalyst characterization
qq英文名BET measurements showed that the specific surface areas of the mixed oxides are significantly larger than tho for the individual oxides (Table 1).However,no clear dependency on the Mn:Ce ratio could be noticed.The XRD diffraction patterns (Fig.1)of all mixed oxides show only the signals of the CeO 2cubic fluorite structure,whereas broadening of the signals for the mixed oxides can be ascribed to lattice distortion due to introduction of Mn 3+ions.Only for Mn75Ce25some reflections characteristic for a C-rare earth type of Mn 2O 3were obrved.An extensive study of MnO x -CeO 2structures and properties was performed by Machida et al.[10].They have shown that a solid solution between Mn 2O 3and CeO 2is formed,in which Mn 3+ions replace Ce 4+in the CeO 2fluorite structure.The findings are in good agreement with the results of our characterization.
K.Tikhomirov et al./Applied Catalysis B:Environmental 64(2006)72–78
76Table 2
Adsorbed and desorbed NO x amounts on different MnO x -CeO 2catalysts Sample NO x adsorbed (10À4mol/g)NO x desorbed (10À4mol/g)Mn0Ce1000.630.48Mn25Ce75  2.39  2.41Mn50Ce50  2.19  2.27Mn75Ce25  2.10  2.14Mn100Ce0
0.82
0.97
Fig.8.DRIFT differential spectrum of Mn25Ce75after exposure to 10%O 2and 5000ppm NO in N 2at 1508
C.
Fig.9.Influence of the SO 2treatment on the NO oxidation over the Mn25Ce75catalyst at 3508C.Feed gas:10%O 2+1000ppm NO +5%H 2O in N 2,50ppm SO 2applied for 30min,GHSV =52,000h À1
.
Fig.10.TPO profiles for soot oxidation on the Mn25Ce75catalyst:(*)fresh catalyst;(&)catalyst pretreated with 10%O 2+1000ppm NO +50ppm SO 2+5%H 2O in N 2for 10h at GHSV =52,000h À1;(~)catalyst pretreated and reduced in 5%H 2in N 2at 6508C for 1h at GHSV =52,000h À1.

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