In-Situ FTIR Study on Effect of Lathanum on Oxidation Mechanism of Methanol Automobile Exhaust on Su

JOURNAL OF RARE EARTHS2002 Vol .20 No.4In-Situ FTIR Study on Effect of Lathanum on OxidationMechanism of Methanol A utomobile Exhaust onSupported Palladium CatalystWang Xingyi(王幸宜)* , Wan Ying(万颖), Lu Guanzhong(卢冠忠), Li Tingchen( 李庭琛)( Institute of Industrial Catalysis ，East China University of Science and Technology ，Shanghai200237 , China )Abstract :The performance of severe oxidation of methanol on 0. 1% Pd supported on alumina was studied by acombined device of chromatograph-micro reactor. The results show that the addition of La into Y-Al2O3 as supportcan affect the performance of Pd catalyst greatly. By using Pd catalyst containing La in methanol oxidation , though ,the ignition temperature is not lower than that by using Pd catalyst , the presence of La does suppress the formationof oxygenic intermediates. The results by in-situ FTIR show that the presence of La in the support affects the ad-sorbed species and hence the mechanism of severe oxidation of methanol on Pd catalyst.Key words : rare earths ; supported palladium catalyst ; methanol oxidation ; lanthanumCLC number : TQ426Document code :A Article ID : 1002 - 0721( 2002 )04 - 0268 - 05Though catalytic converters are applied inconverter , if used in methanol-fueled automo-most gasoline automobiles , the exhaust emis-biles , cannot meet the requirements for practi-sion remains one of the leading causes of urbancal use in respect to the combustion activity atair pollution. Recent attempts to further de-low temperature and the selectivity in severecrease emissions have prompted interest iroxidation. For example ，high conversion ofclean”fuels , among which methanol is fre-methanol can be achieved on Pd and Pt cata-quently considered as priority. Methanol com-. lysts , but selectivity in severe oxidation isbustion produces less amounts of particles , re-poor2~7]. There are reports that rare earth el-active hydrocarbons ，carbon monoxide ，andements have been used as additives to improveoxides of nitrogen than from gasoline . Howev-the performance of the present catalysts iner , high level of formaldehyde concentration ingasoline-fueled automobiles 8] , but such appli-methanol combustion emission is still a prob-cation in the methanol automobile catalysts haslem sought to be solved 1 Therefore , appro-not yet been much reported. The fact is thatpriate catalytic technology is expected to devel-addition of La can modify the performance olop for the complete oxidation of unburnedmetal catalysts used in the catalytic combus-methanol or its partial oxidation products , suchtiort 9 and promote the activity of Ag catalystsas formaldehyde , dimethyl ether and so onf 1.used in methanol automobile10]. In this pa-Supported platinum and palladium catalysts ,per , activity of methanol oxidation was testedwhich are effective for the oxidation of hydro-on the palladium catalyst with the support of Y-carbons and carbon monoxide in currently ap-Al2O3 doped with lanthanum and effects byplied three-way automotive catalysts , are lead-doping were . investioated by in-situ FT-IR oning candidates for methanol -fueled vehicles 2].adsTYH中国煤化工dcNMHGylinkedwiththeac-oxygen species.At present , researchers hold that the catalyticTheReceived date :2001 - 06 - 18 ; revised date :2002 -04- 20.Foundation item : Project supported by Shanghai Academic Foundation for Young Teacher in Advanced SchoolsBiography Wig Xingyi( 1958 - ), Female , ProfessorCorrespondetCE-mail : wangxy @ ecust . edu.cn )No.4In-Situ FTIR Effect of Lathanum on Oxidation Mechanism of Methanol Automnobile Exhaust269tivity and selectivity of CH3OH oxidation .analysis adsorbent at 77.2 K with warm freespace 8. 8558 cm3 and cold free space 23 . 00151 Experimentalcm3 respectively .1.1 Catalysts preparation1.4 FT-IR analysisTwo samples of catalysts Pd/La-Al2O3 andThe FT-IR analyses were carried out onPd/Al2O3 for evaluation were prepared. La-BIO-RAD FTS40 FT-IR spectrometer. The testAl2O3 as support having lanthanum of 5%sample was heated in the chamber into which( mass fraction ) of aluminum , was prepared bygases were allowed to introduce. The catalystimpregnating pseudo boehmite in the solutionsamples ,all in precursor state in fresh condi-of lanthanum nitrate adjusted at pH 2 ~ 3 fol-tion , were first pretreated in N2 at 773 K for 2.lowed by the steps including aging for 24 h ,h , and then the chamber was evacuated to thedrying at 373 K for 12 h and calcining for 4 hpressure of 1.333 X 10- kPa torr , in orderat 773 K. The support with the dimension ofthat adsorption spectra of samples were ob-20 ~ 40 mesh was impregnated again in PdCl2tained as background spectra. Oxygen( 26. 6aqueous solution. The two catalyst precursors ,kPa )at room temperature , was introduced intoboth containing palladium of 0. 1% in weight ,the system for 5 min , before it was evacuatedwere reduced in the mixed gas flow of 10%for 20 min , and under the vacuum state , spec-H2+ 90%N2( 20 ml min-1 )for 3 h at 523 Ktra were measured at 293 and 373 K. At thebefore evaluation .same time , the adsorption spectra of sampleswere recorded at intervals of minutes long .1.2 Catalyst evaluationMoreover , with the same operation proceduresCatalyst evaluation was carried out in afor the gaseous mixture of oxygen and .combined device of chromatograph-micro reac-methanol，spectra at different temperaturestor( 1.2 cmX 40 cm ) packed with 0.5 g cat-were measured as the chamber temperature wasalyst , at a constant space velocity of 15 ,000gradually raised .h-1. The composition of the feed stream ofCH3OH + O2was 0.74% CH3OH ,4.0% O2 in2 Results and DiscussionN2 and the feed stream of CH3OH + O2 + COconsisted of 0. 74% CH3OH , 4.0% O2 and2.1 Catalyst evaluation0.3% CO in N2. The on-line analysis of theTwo samples of Pd catalysts , supportedgaseous mixtures at the inlet and at the outletAl2O3( BET specific surface area 272of reactor was performed by gas chromatogra-m2 g-1 )and La-Al2O5( 258 m2 g-1 ) are herephy. For CH3OH , HCHO，CH3OOCH anddesignated as Pd/Al-O and Pd/La-Al-O re-CH3OCH3 , a column filled with 403 organicspectively . Methanol conversion and selectivitysupport ( coated with sorbitol ) was employedto CO2 as a function of the reaction temperaturewith FID as detector iand for O2 ,N2 ,CO andare listed in Table 1. Activity of both samplesCO2 , two columns were employed in parallelare high ; for Pd/Al-O its Tso and T1oo( Tsoconnected , one filled with molecular sieve 5 Aand. Tiantion tompratures correspondingand the other filld with silicon gel , with TCDtc中国煤化工anol conversion re-as detector .speJTYHCNMH G3 K ,while Tso andT100 on Pd/La-Al-O are 333 and 453 K. Dur-1.3 BET analysising methanol oxidation on Pd/Al-O , in addi-The data of BET specific surface areation , several kinds of oxygenic intermediateswere obt海液数据n TRISTAR 3000 using N2 aswere determined , among which , the amount of270JOURNAL OF RARE EARTHSVol.20methyl formate and formaldehyde both were as electron structure , caused by the addition ofhigh as to 0. 06% at low temperature , whileLa , on oxygen adsorption species 91.the amount of methyl ether up to 0. 032% atIn order to understand the reaction mech-558 K. However , no oxygenic intermediatesanism of CH3OH with O2on Pd/Al-O and Pd/were formed on Pd/La-Al-O within the reactionLa-Al-O，spectra at different temperaturestemperature range. Conclusion can be madewere measured( Fig.2 a ,b ))for adsorption ofthat lanthanum added in the support suppressesCH3OH and O2 on the samples which had beenthe formation of oxygenic intermediates andtreated in vacuum. At 293 K，there werepromotes the selectivity to carbon dioxide inbands at 2950 , 2810 and 1035 cm-1 on themethanol oxidation ，although the activity ofspectra of Pd/La-Al-O , which correspond toPd/ AI-O was higher.Vc-Hand vc-o of CH3-O , due to dissociation2.2 In-situ FT-IR study .adsorption of methanol 13,14,151 and bands o2810 cm-1 and 1035 cm -1 became weaker aSpectra were obtained ( Fig. 1(a))of O2373 K. At the same time , bands at about 2950adsorption over Pd/Al-O sample in the IRcm - 1 became intense and new bands occurredchamber at room temperature. The bands atat 1700， 1600 , 1185 , 1155 and 1025 cm-1.about 1113 cm-1 on the spectra of O2 corre-Bands at 2950 cm-1 correspond to vc-H ofsponded to 02 111 , and when the temperatureCH;OOCH，bands of 1185 ，1155 and 1025was raised to 373 K ,the bands shifted to lowercmto vc-o-c. 1700 cmto vc=o 0wave number. For Pd/La-Al-O in Fig. 1( b),CH3OOCH and HCHO- 16] , and bands at 1600the spectra of O2 adsorption occurred with aand 1585 cm-1 to HCOO- adsorptiont 16]. Atbroad peak at 1105 cm-1 , assigned to O2 dis-373 K , the intensity of bands at 1185 , 1155sociation absorptionat 293 K , but this peakand 1025 cm - 1 reached the maximum and thedid not shift when the temperature rose to 373split of bands in the range from 3000 to 2800K. Generally , 0- or 02- species is favorablecm~ ! occurred ，indicating the presence offor complete oxidation , while 02 species foUc-Hof CH3 ,CH2 or CH. New bands at 1055selective oxidationt12]. From the data in Tablewere assigned to Uc-o-c of CH3OCH，1 ,it can be concluded that the severe oxida-and bands at 1600 cm. , peak of HCOO- be-tion of methanol is related to theoxygencame intense , showing that H2COO( a ) andspecies adsorbing on the catalyst surface .other intermediates containing oxygen wereWhen methanol was oxidized on Pd/Al-0 , aformed from the reaction of CH3-0( a ) with 0great amount of oxygenic intermediates was(a). At 423 K , almost disappeared the bandsformed , while on Pd/La-Al-O , methanol wasat 1185 , 1155，1025 and 1600 cm-1 , butconverted mainly to CO2. This difference in se-bands of H20( 1650 cm-1 ) and COL( 2400 ~lectivity was due to the effect of change in Pd2300 cm -1 ) intensified. That indicates the de-(a}|(h) 37)K373K2Q3 K293 K中国煤化工YHCNMHG12.011.511.010.51.0Wavenumber/ 102 emn-1Wavenumber/ 10: em-'Fig.1 IR spectra of adsorbed O2 on Pd/AI-0( a ) and Pd/La-AI-0 b)No.4In-Situ FTIR Effect of Lathanum on Oxidation Mechanism of Methanol Automnobile Exhaust271Table 1 Effect of La on CH3OH deep oxidation on supported palladium catalyst *Conv.ofConv. ofConv. of CH3OH to partialConv. Of CHzOH to partial oxidation productCatalystsCH20H，CHOH to CO2 oxidation product HC00CHy CatalytsHCHOCHzOCH3Tso/K Tg/K T98/KT。/K T:/K T./K wm/%T./K T./K T。/K wm/% T。/K T./K T。/K wm/%Pd/AI-O348 383 593338 348 383 0.06 Pd/AI-O348 368 403493 5580.0016Pd/La-AI-O 333453 428Pd/La-AI-O32* T。:Appearing temperature ; T。:disappearing temperature ;Wm : maximum concentration ; Tm : temperature corresponding to Wmcomposition of H2COO( a ) into CO2 and H2O.plied that most of the organic species are oxi-At 523 K , bands at 1055 cm-1 of Uc-o-c ofdized into CO2 and H2O. The severe oxidationCH3OCH3 reaches the maximum. At 623 K ,of methanol on Pd/Al-O catalyst can be sug-except the bands for adsorption of H2O andgested following the mechanism below :CO2 , all bands became quite weak. This imCH,OCH (g)O(a)CH,OH(g) > CH,O (Q)厂》 CH,0CH3 (a)刀|CH2O (a)0(日)0(a)HC0OCH, (a)CH,0(a)↓CH,O (g)HC0OCH, (Q)where dash line represented a reaction route at( 1710 cm-1 ) weakened. Bands of H2COO( a )low temperature , real line at high temperature .at 1600，1585 cm - 1 occurred when raisingIR spectra of CH3OH and O2 on Pd/La-temperature rose to higher than 423 K. The in-Al-O at 293 K were shown in Fig. 2( b ) ,wherecrease in the magnitude extent of band intensi-there were adsorption bands of CH30( a ) anal-ty for H2COO( a ) is much less than that forogous to Pd/Al-O , but the intensity of bandsCH2O( a ). From the undetectable amount ofof CH30(a),vc- f 2940 ,2810 cm-1 ),vc=oHCHO( g ) during the reaction , it may infer|(a)that CH30( a ) react with 0( a ) to form CH2O( a ), which , is then oxidized to form H2COO260C( a). At 373 K , the intensities of adsorption of150CUc-o of CH3O(a )and Uc=o of CH20( a ) both80Cdecreased，butintensities of adsorption of20C026.24231612- 11H2COO( a ) increased. At 523 K , in particu-Wavenumber/ 102 cm-1lar , the intensities of adsorption bands of H20向)n 250Cand CO2 are so great that the others seem tooweak. This verified too that H2C00( a ) was100C-formed from 0( a ) and CH3O( a ). Logically ,o0cthe=xidation of methanol= 20个中国煤化工282421716市10o1be suggestedasWavenumber' 102 em-'.JYHCNMHG(a)CH3OH( g)CH3O( a )CH2O(a)Fig.2 In-situ. IR spectra of CH3OH oxidation on Pd/d92H2CO0(a)3H2O+ CO2A20充数凭nd Pd/La-A1-O( b)where 0( a ) is dissociative adsorbed oxygen.272JOURNAL OF RARE EARTHSVol .20oxidation over an alumina- supported palladium. silver3 Conclusioneatalyst[J] J. Catal. , 1987 , 103(2):419.[7] MeCabe R W , MeCready D F. Kinetics and reactionThe complete oxidation of methanol ispathways of methanol oxidation on platinum[J] Jclosely related to the adsorbed species of oxy-Phys. Chem. , 1986 ,90(7): 1428.[8] Wang Xingyi , Lu Guanzhong ，Wu Shanliang. Nitiegen on the catalyst surface. As in the case ofoxide reduction performance of automotive palladiumdoping lanthanum into the support , the elec-eatalysts[J] Rare Farths , 1995 ,132):99.tronic structure of Pd and the active component[9] Muraki H , Yokota K , Fujtani Y. Nitrie oxide reduc-change，and hence adsorbed species on thetion perormance of automotive plladium catalys[J ]catalyst surface is affected. Oxidation ofAppl. 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