Pt_Rh密偶催化劑的制備及其催化控制汽車尾氣排放的性能_英

1、Article I D :025329837(20060820647203Communication :647649R eceived Date :2006203227.First Author :SHI Zhonghua ,male ,born in 1977,PhD student.Found ation Item :Supported by the K ey Project of the National Natural Science Foundation of China (20333030.Preparation of a Pt 2Rh Close Coupled C atalys

2、t and Its C atalyticPerform ance for Controlling AutomobileExhaust EmissionS HI Zhonghua ,GONG Maochu ,CHEN Y aoqiang(College of Chemist ry ,S ichuan U niversity ,Chengdu 610064,S ichuan ,China Abstract :A noble Pt 2Rh close coupled catalyst supported on a novel high surface area alumina was prepare

3、d to control the exhaust gas emission of automobiles.The results indicated that the prepared catalyst has good activity at low temperature and good stability at high temperature ,which can satisfy the requirements of a high performance close coupled catalyst.K ey w ords :platinum ;rhodium ;close cou

4、 pled catalyst ;cold start ;exhaust gas C LC number :O643Document code :AEurope standards require significantly higher emission reduction ,especially for hydrocarbons (HC .For a typical vehicle ,a large portion (up to 80%of the HC emissions occurs during the cold start phase.A variety of technologie

5、s are under devel 2opment to reduce cold start HC emissions ,including close coupled catalysts 1,electrically heated cata 2lysts 2,HC absorbers 3,and bypass catalysts 4.Among these ,the close coupled catalyst is gradually dominating the technology 5.The development of an inexpensive and stable close

6、 coupled catalyst is a key point to meet the requirement of the Europe stan 2dards.A high performance close coupled catalyst should have the ability to oxidize HC at low tempera 2ture and have good stability at high temperature for meeting the more stringent emission standards.Nowadays ,only several

7、 large catalyst companies ,such as Engelhard ,Degussa ,and Johnson Matthey ,can prepare catalysts that satisfy these conditions.The close coupled catalyst prepared by Hu et al 6(Engelhard had a HC light 2off temperature of 254after aging in 10%steam in air at 950for 12h.In this study ,a close couple

8、d catalyst containing a new high surface support was prepared ,and it was effective for removing HC in the exhaust gas.G enerally ,the catalyst must comprise a high surface support that can be used at a temperature as high as 1000.In the present communication ,we report the preparation of a La 2stab

9、ilized alumina that can satisfy these demands.The textural properties ofthe La 2stabilized alumina samples prepared by co 2pre 2cipitation are presented in Table 1.After aging at 1000for 10h ,the support still had a surface area and pore volume of 16219m 2/g and 0145cm 3/g ,re 2spectively.In contras

10、t ,a commercial 2Al 2O 3had on 2ly 8016m 2/g and 0126cm 3/g after aging only for 2h.The results showed that La 2Al 2O 3had a higher surface area and better thermal stability.T able 1T extural properties of the La 2stabilized and commercial2Al 2O 3supports aging under different conditions Sample Agin

11、gtemperature (Agingtime (h A BETm 2/g Vcm 3/g La 2Al 2O 3280.60.26The Pt 2Rh catalysts were prepared by the flow 2ing process.La 2Al 2O 3(2Al 2O 3powder was impreg 2nated with an aqueous chloroplatinic acid and rhodium nitrate solution sequentially.Then ,the alumina con 2taining platinum and rhodium

12、 was mixed with a zirco 2nium acetate solution and some amounts of lanthanum nitrate and strontium nitrate crystals.The resulting materials were ballmilled with deionized water to form slurry with a solid content about 40%by weight.A monolithic support made of cordierite con 2taining 400cells per sq

13、uare inch was dipped into the第27卷第8期催化學(xué)報Chi nese Journal of CatalysisAugust 2006slurry to form a washcoat.Excess slurry was blownoff the monolith using compressed air.The resultant monolithic Pt 2Rh close coupled catalyst contained 1g/L Pt and 011g/L Rh after calcination at 500.The Pt 2Rh catalyst p

14、repared using La 2Al 2O 3as the support is denoted C1,and the one with 2Al 2O 3as the support is denoted C2.The catalysts were aged in 3%H 2and 7%steam in air at 1000for 5h.The catalysts were evaluated using a laboratory reactor with a feed gas of the compositions 016%CO ,10%CO 2,10%H 2O ,0105%HC (C

15、 3H 8or half C 3H 8and half C 3H 6,0106%NO x ,and 0152%O 2with the balance being N 2,which is similar to the exhaust gas.The volume hourly space velocity (V HSV was 30000h -1.The conversion of HC ,CO ,and NO x was measured with an F G A 24100exhaust analysis in 2strument made in Foshan ,China.The re

16、actor tem 2perature was gradually raised from ambient tempera 2ture to 400.The activity of the catalysts was ex 2pressed interms of the light 2off temperature and the complete conversion temperature.The light 2off tem 2perature (T 50%for a specific feed component is de 2fined as the temperature at w

17、hich 50%of that com 2ponent has been converted.The complete conversion temperature (T 90%is defined as the temperature at which the conversion is 90%.The conversion of HC ,CO ,and NO over the Pt 2Rh close coupled catalyst versus the reaction tem 2perature is shown in Fig 1.The feed gas of S =1(S =(2

18、c (O 2+c (NO /(c (CO +10c (C 3H 8was used.It is clear that the fresh and aged catalysts have similar activity for removing HC ,CO ,and NO.All these compounds were converted quickly and com 2pletely within 20-30after the reaction was lit off.Fig 1Activity of the fresh (aand aged (bPt 2Rh close couple

19、d catalysts (sample C1(1HC ,(2CO ,(3NOT able 2Activity of the Pt 2Rh close coupled catalysts Catalyst T 50%/HC CO NO T 90%/250230240280240260The feed gas was C 3H 8+CO +NO.The catalytic activity of the fresh and aged cata 2lysts is listed in Table 2.When only C 3H 8was the HC component ,the fresh ca

20、talyst showed low con 2version temperatures with respect to HC ,CO ,and NO.The corresponding light 2off temperatures were 220,210,and 230,and the complete conversion temperatures were 240,220,and 240,respective 2ly.After the catalyst was aged ,it still showed good activity and conversion.The light 2

21、off temperatures for HC ,CO ,and NO were 250,230,and 240,and the complete conversion temperatures were 280,240,and 260,respectively.T able 3H C reactivity over the Pt 2Rh close coupled catalystsprep ared with different supportsCatalystT 50%/T 90%/Fresh C1220240Fresh C2255290Aged C1250280Aged C232040

22、0Table 3shows the HC reactivity over the Pt 2Rh close coupled catalysts prepared with different sup 2ports.C2had a higher conversion temperature than C1.The light 2off temperature of fresh C2was 35higher than that of C1.After aging ,the light 2off temperature of C2was 70higher than that of C1,while

23、the complete conversion temperature was 120higher.The catalysts were prepared with the same conditions except for the use of different supports.It846催化學(xué)報第27卷is obvious that the supports led to the difference in ac 2tivity.In a moving vehicle ,the exhaust gas tempera 2ture can reach 1050.At such a hi

24、gh temperature ,most support materials (such as activated alumina undergo thermal degradation caused by a phase transi 2tion with an accompanying volume shrinkage ,espe 2cially in the presence of steam ,and the catalytic metal will be aggregated and occluded in the shrunken sup 2port medium.This lea

25、ds to a loss of the exposed cata 2lyst surface area and a corresponding decrease in the catalytic activity.A very effective method to prevent such thermal degradation is through stabilizing the a 2lumina supports by the use of materials such as zirco 2nia ,alkaline earth metal oxides (such as baria

26、and strontia ,or rare earth metal oxides (such as ceria and lanthana .The La 2Al 2O 3had a larger surface area and better thermal stability than 2Al 2O 3.Thus ,C1catalyst had a higher activity and thermal stability than C2.Since there are many alkenes in the real exhaust gases ,the HC composed of ha

27、lf C 3H 8and half C 3H 6was used to evaluate the catalytic performance of the Pt 2Rh close coupled catalyst.The results listed in Table 4showed that the HC conversion temperature was much decreased ,while the conversion tempera 2ture of CO and NO was little changed.The light 2off temperatures for HC

28、 and CO were 180and 190,respectively.After the catalyst was aged ,the tem 2peratures were 210and 220,respectively.Theseresults clearly showed that when there is C 3H 6in the exhaust gas ,the catalyst has a higher CO conversion temperature than that for HC ,which showed that the Pt 2Rh close coupled catalyst can control the emission of CO.T able 4Activity of the Pt 2Rh close coupled catalysts Catalyst T