Application of Ceria and Lanthana in Catalyst for Cleansing Exhaust Gas of Car

JOURNAL OF RARE EARTHSoL22,spec.,Dec.2004,p.232Application of Ceria and Lanthana in Catalyst for Cleansing ExhaustGas of CarYang Chunsheng(杨春生), Chen Jianhua(陈建华), Dai Shaojun(代少俊)Department of Materials, Yancheng institute of Technology, Yancheng 224003, China)Abstract: The importan Ce of rare earths being applied in the catalyst for cleansing the exhaust gas out of car was intro-duced.The acting mechanism of ceria and lanthana in catalyst and its influencing factors were discussed, and its pros-pect was forecasted. Pt-Rh precious metals three-way catalyst is widely used for decontaminating the exhaust gas of carnow. Ceria and lanthana, which can decrease the content of Pt-Rh and increase the content of pd in the catalyst, areused as additive in the decontaminating catalyst in order to solve the problem of the supply and demand of Pt and RhIt is reported that increasing the activity of the coat on catalyst, regulating automatically the ratio of air and fuel, actingas catalyst-accelerator, and improving its properties such as thermal stability and strength may primarily amount for thecatalyzing. mechanism of ceria and lanthana. The factors, such as their interaction, additive methods, and effects ofcatalyst ZrO,, CuO, AgO, etc, will remarkably influence the catalyzing function of ceria and lanthanaKey words: inorganic nonmetallic materials; ceria; lanthana; catalyst; car; rare earthsCLC number: 0643. 36 Document code: A Article ID: 1002-0721(2004)-0232-04Nowadays, with the economic devCatalyst for cleansing exhaust gas of car arethe enhancing living level the sharp rising amount of mainly made up of alveolate pottery (or metal)sup-car gives big pressure to surroundings of cities in our porter and activating coat on surface. The activatingcountry. Cleansing exhaust gas of car becomes an coat are made up of big area y-Al2O3, some oxideexigent demand. Now HC and CO's are cleansed by acting as stabilizer and metals which have catalyticaloxidation of Pt and NO, are cleansed by deoxidation activity and disperse in coat For reducing the contentof Rh in the Pt-Rh precious metals three-way catalyst of precious Pt-Rh and increasing the content of cheapwhich are used widely. In order to meet the demand Pd, basing the precondition of not reducing the propof Pt and Rh, the developed countries are developing erties of catalyst for cleansing exhaust gas of caradvanced preparation methods of supporter, impro. some ceria and lanthana are added into Pt-Pd-Rh catving coating technique, reducing the amount of Pt alyst by co-impregnation method and rare earth-pre-and Rh of, catalyst, using cheap Pd-Rh catalyst and cious metals catalyst are prepared, the catalyst hasstrengthening the callback of Pt group metal. Those fine catalytical effect. Ceria and lanthana additionsteps have brought obvious effects, but they cannotan improve effect of the metal catalyst supported bymeet increasing demands of high effect catalyst for y-Al2O3. Their acting mechanism are as follows:cleansing exhaust gas of car. From 80s, both homeand eu1.1 Increase catalytical activity of coatire engaged in research of rare earth( mainly ceria and lan-Sintering can result in precious metals'reducingthana) as additive for reducing Pt-Rh content and enlattice site for catalyze and decreasing the catalyticallarging Pt-based catalyst using scope. Application activity, adding ceria can avoid those by keeping pre-of ceria and lanthana in catalyst for cleansing exhaust cious metal granula dispersed. Ceria can be monolay-gas of car was introduced in the articleer-dispersed( the largest content is 0.035 g CeO,/gActing Mechanism of Ceria and Y-Al,)and change superficial property of y-LanthanaALO3, then Pts degree of dispersion is increasedReceived date: 2004-09-30: revised date: 2004-1lTHGNMHGFoundation item: Project supported by JSSFC ( BK2002044)Biography: Yang Chunsheng( 1952-), Male, Master, ProfessoCorrespondingauthor(E-mail:dsj197315@yahoo.com.cn)Yang CS Application of Ceria and Lanthana in Catalyst for Cleansing Exhaust Gas of Car233olayer-dispersion capacity, Pt's degree of dispersioncrease transform of water gas and reforming of watercan reach extreme 2. Rh2O, generated by high tem- vapor, and reduce the content of H, and CO. La, O,perature oxidation can form solid solution with y- can increase reaction conversion of transform of waterAL2O, and lose the catalytical activity when Rh is in gas and reforming of water vapour, produced hydro-oxygen condition and temperature is above 600 Cgen gas does good to NO,'s reduction. AddingCeO, can reduce reaction between Rh and Al,,La, O, can increase Pd' reduction property of NO, inkeep activity of Rh. La2O, also can prevent the enlar- Pd catalyst or Pd-enriched catalyst, the Pd-La,Ogenet of Pt particle. Added into Pd/y-Al2O3, CeO 2alyst's catalytical effect is the same as that of Pt-an promote Pds dispersing in supporter and generateRh catalystco-deoxidize. High dispersion of Pd and the interThe effect of La,O, addition on catalytic proper-key role in high catalytical activin.( y-AL2O, play ty of Pd/ Ce02/y-AL,O, catalyst for methanol decom-tion between- Al,O, and CeO, in Pdpositior was investigated. The results showed thatCeo, is an effective catalyst for hydrocarbon's adding La, O, can reduce by-product( dimethyloxidation. When CO is oxidized in Pd/Y-Al2Oether) and increasing the catalytic activity. Methanoltice oxygen between Pt and CeO,s surface play imconversion reached a maximum( ca. 91. 4%)whenthe La, o, content increased to 10%. La,o could beportant role. CeO2's surface can generate low valence Ce in vacuum or reduce condition and oxygendispersed well on Y-AL,O, and promoted the disper-sion and the reduction of CeO, furthermore, the dis-bug which has high catalyze for redox and functionspersion of Pd and interaction betweenof adsorbing molecular exchange charge. CeO, canwas also increased, which results in high catalytic acgenerate low valence Ce and space oxygen because of tivity for methanol decomposition 6)hydrogens role. PV/y-A2O, can adsorb gas phaseydrogen and release it again. The oxygen absorbed1.4 Improving thermal stability of supby partial reduction CeO2 can generate dioxygen spe-portercies at room temperature. The absorbed oxygen canActivating Coat is mainly made up of Y-Al, O3retain and transform into lattice oxygen above 170Cl4. CeO, in AL, O, can promote the adsorptiontemperature above 800 C can transform y-Al,O,toa-A1,O, and induce increasing of density, reductionand reproduction of the surface absorbed oxygen and of surface and collperature is above 1200 C, activating coat can drop1.2 Regulate automatically ratio of air out from supporter, gas resistance is increased andand fuelcatalytic activity is decreased. La,O, addition canstabilize crystalline structure of Y-Al2O, and activatewhen the temperature of car engine is raised or coat at high temperature and prevent loss of catalyticdriving way and velocity are changed, velocity andactivity. After several hours at 1473 K, specific surcomponent of exhaust gas will change. Workingface area of CeO, is still 60 m'.g" in reducing orcondition's change of catalyst for cleansing exhaustneutral condition indicating that Ce mainly existsgas of car influences the catalytical activity. It is onlyCeAlO, and Ce prevent crystal,'s growth and transthe ratio of air and fuel are regulated between 14.5formation of Al, 0, 7)14. 6 that the catalyst can completely play role ofcleansing. Ceo's valence can change(Ce+/Ce + 1.5 Other functionsand have characteristic of n-semiconductor which haBesides those functions, CeO, can improve 15%0fine properties of oxygens absorption and release25 of pottery supporter's mechanical strengthWhen the ratio of air and fuel is changed, CeOz can increase three-way catalyst's absorption to sulfur,redynamically regulate the ratio of air and fuel: CeO2duce the content of carbon, reduce leads poison tolease oxygen to oxidize co and hydrocarbon whenprecious catalyst in those cars still using lead-contai-fuel is surplus or CeO2- deoxidize and remove NO,and CeO, is gotten when air is surplus中国煤化工1.3 Catalyst-acceleratorCNMHGCatalytic Func-uUu Ur CLiq quu anthanumWhen the ratio of air and fuel is stoichiometricamount, three-way can generate oxidation of H2, Co2. 1 Interaction of ceria and lanthanumand HC and reduction of No, Besides, ceO, can inActing as structure additive, La,O, plays theJOURNAL OF RARE EARTHS, VoL 22, Spec, Dec. 2004role of thermo-stabilization for superficial preciousZrO2-CeO2 catalyst is similar to Pt/ CeO, at 270 Cmetals and crystalline phase of Y-AL2O3. Atomic but they are different at high temperature peak tem-structure of Lanthanum is similar to cerium's, La, operature of PV/ ZrO2-Ce02 is 590 C, lower than thatcan come into CeOz's crystal lattice and generate so- of Pt/ CeO2 (650 C). ZrO, was added to reduce de-lidoid complex of La,O,,, favor CeO, distribusorption peak temperature to CO at high temperaturetion in y-AL2O3. Because La't valence is lower thanwith the rising of using temperature, sinteringoxygen vacancy is increased in CeO, crystalof oxide catalyst reduces sorption amount of oxygenlattice and the property of oxygen's absorption is in-Zro, addition can not only promote catalyst,s sorpcreased too(OSC). Added La,O, can reduce intertion of oxygen, but also slacken the reducing tendenaction between Ceo2 and y-Al2O3. Thereforecy of oxygens sorption with the rising temperatureLa,O, and CeO2's adding into y-Al, 0, /precious ZrO, not only prevent CeO2 crystal grains growthmetal catalyst can co-acceleration catalytical proper- but also can promote catalyst's sorttion of oxygen.ty. Interaction between CeO, and Pd cause the peak Ce can be deoxidized to Ce.in H2 or CO condiof the CeO2-Pdy-Al, 0, catalyst in TPR curve to tion, so it can easily absorb or release oxygen. ZrO2shift to lower temperature CeO2's existing promote is similar to Ce, ZrO2 adding into CeO2 can increasesurface oxygen adsorption of Pd/y-Althe oxidation activity of the catalyst,O,, improveredox and promote oxygens trasformation, sorptionamount of ZrO2-CeO2 is obvious bigger than CeO2O2CeO2 oSC at500℃2. 2 Additive methods600C, which shows ZrO, 's fine thermal stability isCeO2 and La,O3, added into Pd/y-A12O, cata- beyond CeOz. CO, PV ZrO2-CeO2's conversion onlyst by consecutive impregnation or co-impregnation'sHC and NO is obviously higher than that of Ptcan easily generate a solidoid complex of La2O3CeO,. The using temperature of catalyst for cleansingCeO,, favor CeO, distribution in y-Al, 0,, reduce exhaust gas is above 400 C: Those factors, such asinteraction between CeO2 and y-Al,O, and increassorption amount of oxygen and CO, temperature ofinteraction between Pd and CeO,. If La,O, is firstlysorption, affect catalytical activity in the temperatureadded, the degree of Pd dispersion is the biggest andinterval. Pt/ZrO2-CeO2 catalytical activity is high be-interaction between Pd and Ceo2( interface effectcause deoxidizing temperature of Pt/ Zro2-CeO2 issorption-desorption are better than Pt ceo, uthe biggest, there is biggest productivity of Co and low and properties of sorption ofH, in formaldehyde decompose reaction(10).Surfacearea of Pd-La/y-Al,O, prepared by Pd and La's co- When adding Ceo2 byhodimpregnation reduce in 1373K, but it still has high ac-Zro2 can promote property of sorption to oxygen oftivity. If La is firstly dipped and Pd later, La will ZrO2-Ceo2/Y-Al2O, catalyst 131firstly react with Al,O, and generate La_Al x O32)Cuo Pure CuO and CeO, have little oxidaWhen heating age, La,O, check pallation activity; but after CuO and CeO2 generated comgenerate by reaction between Pd and Al, O,, and plex, the oxidation activity on Co is promoted. ThisLa,O, plays a role of supporter stabilization. Solidoidmaybe due to Cus valence status( Cu*and Cucomplex of La, Al,-O, does not have enough ability on CeO2 surface, dispersion and reductibility. Somethat keep high activity and fine structure after heating Cuo whose radius is less than Ce"enter the Ceo,lattice when Cuo and CeO are mixed. At 5. 0% of2.3 Co-catalysis of other oxidesCuo loading, the Cuo crystalline has a minimal crys-alline size of 6. 1 nm and a maximal microstrain val(1)ZrO2 High speed and heavy load of car need ue of 2. 86x 10", high surface energy and bestcatalyst keep thermal stability for a long time at 10001000 tivity for Co oxidation(14. Adding Cu into Pd canC. Adding Zro, into CeO, can improve it's structure generate alloy and the property of removing CO andand property at 630-830 C and keep specific area NO, is reinforcedthe same, but pure CeO,'s specific area reduce at theYH中国煤化工 n be thermal decomsame temperaturectiveLoad of Pt reduces reduction temperature of catCNMHG and promote oxygenalyst and added ZrO2 promotes Pt and supporters re- diffuseness of Pd/ Ceoduction. Loaded Pt increases adsorption amount of(4)Other oxide Ceria's acting mechanism inCO. Desorption peak temperature and shape of Pt/ three-way catalyst is not clear so far, but it is clearYang Cs Application of Ceria and Lanthana in Catalyst for Cleansing Exhaust Gas of Cathat it is the oxygen vacancy located on interface beJournal of Hanzhou University, 1996, 23(2): 182tween metal and oxide plays key role and the inter-[6]Yan cheng, Ren Jie, Sun Yuhan. Study of CeO2 andface is generated by deoxidization state ceria andLa,O, modified Pdy-AlO, catalyst for methanolmetal. 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