La-Hexaaluminate Catalyst Preparation and Its Performance for Methane Catalytic Combustion

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JOURNAL OF RARE EARTHS 24 (2006) 690 - 694

La-Hexaaluminate Catalyst Preparation and Its Performance for Methane Catalytic Combustion

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@ ) ' , Long Zhiqi (&,&*)', Cui Meisheng (&&&)'*, Wang Liangshi (3-&zk ) 2 , Zhao Na Li Dianqing Chen Aifan ( Fr$$l&)2 ( 1 . General Research Institute for Nonfrrous Metals , Grirem Advanced Materials Co . , Ltd . , Beijing 100088,

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China ; 2 . Beijing University of Chemical Technology, Chemical Engineering Resource State Key Laboratory, Beijing 100029, China ) Received 15 July 2006; revised 25 September 2006

Abstract : The La-hexaaluminate catalysts with high performance were synthesized by the modified controllable co-precipitation method with the buffer solution of NH4HC03 and NH40H mixture as the precipitation agent. The physicochemical properties of catalysts were characterized by the means of BET, XRD , and TPR techniques . With methane catalytic combustion as the probe reaction, the catalytic performances were also tested on a fixed bed, continual flow system. The results show that it is a good method to obtain chemical homogeneous hexaaluminate materials by the buffer solution as the precipitation agent. The La-hexaaluminate can be formed at low temperatures ranging from 1050 to 1200 'T . The cerium introduction plays a great role in the methane catalytic combustion on La-Mn hexaaluminate because of its high oxygen storage capacity property and the well synergic effect between Ce and Mn . However, the CeOz appears in hexaaluminate through the XRD pattern, which reveals that Ce can not enter the crystal lattice position. Mn introduction improves the methane catalytic activity to a large extent due to its high redox property. When Mn atomic substitution amount for A1 is 2 , the hexaaluminate shows the highest activity, and the catalyst possesses good H2 consumption and redox performance. Mn can easily occupy the hexaaluminate crystal position, which reveals that the Mn substitute La-hexaaluminate is a promising high temperature methane combustion catalyst with high activity and good stability, Key words : La-hexaaluminate ; methane combustion ; cerium oxides ; rare earths CLC number : 0643.2 Document code : A Article ID: 1002 - 0721 (2006)06 - 0690 - 05

High-temperature catalytic combustion has attracted much attention d u e to its high combustion efficiency as well as low emission of air pollutants such as NO,, CO , and unburned hydrocarbons. The key technology for methane combustion is to develop the catalytic materials with high temperature resistance and high catalytic activity"-3'. To d a t e , many extensive studies have been carried out for applications, such as

* Corresponding author (E-mail:

in gas turbines and jet engines. The related catalytic materials mainly focus on the perovskite and hexaaluminate catalysts. In particular, the hexaaluminate has special layered crystal structure whose A1,0,* - spinel is isolated by large ions ( La, B a , S r , C a ) formed mirror planes, which makes the catalyst maintain a high surface area and stable structure even under high temperature ( 1200 1400 "c ) treatments. The

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Foundation item: Project supported by the National Natural Science Foundation of China (20476012) and Beijing Natural Science Foundation (2062017) Biography: Cui Meisheng (1971 - ) , Male, Master Copyright 0 2 0 0 6 , by Editorial Committee of Journal of the Chinese Rare Earths Society. Published by Elsevier B . V . All rights resened .

Cui M S et a1 . La-Hexaaluminate Catalyst Preparation for Methane Catalytic Combustion

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hexaaluminate catalyst is regarded as the most promising high temperature methane combustion material for application in turbine and jet enginesi4’5 ’ , such as Lahexaaluminate and Ba-hexaaluminate . The hexaaluminate catalysts may be doped with active cations of transition metals (Cr , Mn, Fe , Co , Ni), leading to good combustion materials with high stability and activity. These active dopants improve the reaction between methane and crystal oxygen by the redox cycles, which plays a key role for high combustion activityi6-” . Zarur et al. synthesized highly Ce02 surface dispersive hexaaluminate catalysts whose methane combustion activity is near to P d , Pt noble metal catalysts’”. In the present study, the homogeneous hexaaluminate with good performance was prepared with the buffer solution of NH4HC03 and NH40H as the precipitation agent substitutive for the common ( NH4)*C03agent. The related physicochemical properties and methane combustion performance were all tested by means of BET, XRD , TPR, and micro-reacting fixed bed catalytic techniques.

fraction) H2 in nitrogen with a temperature ramp of 10 “c emin-’, starting from 50 “c up to 1000 “c . Hydrogen consumption was monitored by an online TCD detector. The catalyst sample was about 50 mg, which was pretreated under 10% (volume fraction) O2 in He at 400 “c for 0.5 h before measurements.

1 Experimental

2 Results and Discussion

1.1 Catalysts preparation

2.1

The La-hexaaluminate catalysts were synthesized by the parallel flow dripping method, starting from the mixed metal solution of La3+ , A13 , Mn2+ , or Ce3 at a designated atomic ratio, with the buffer solution of NH4HC03and NH3* H20 as the precipitants. The stoichiometric solution mixture of La ( N 0 3 ) 3 , A1 ( N 0 3 ) 3 , Ce(N03)3, or 50% Mn(N03)* was added slowly into a beaker with some distilled water, and the buffer solution of NH4HC03 and NH40H was dropped slowly into the beaker at the same time and the same speed. The pH of the final solution was controlled at 9 11 while aging for 1 h . The precipitate was then dried at 110 “c for 10 h and calcined at 1200 “c for 4 h in air. +

+

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1.3 Methane combustion activity measurement The catalytic activity in methane combustion was measured on a mixture of 0 . 3 0 g catalyst and 0.30 g quartz particles placed in a quartz microreactori’O’. The catalyst solids were sieved at 20 40 mesh, weighted, and mixed with the same weighted quartz particle. The mixture was fixed by quartz fibers at the both sides in a microreactor. The reactant gas mixture consisting of 1% CH4, 4% 02(volume fraction), and 95% ( volume fraction) N2 was admitted. The gas space velocity was about 50000 h - ’ . The temperature was increased by steps of 20 “c between 300 and 800 “c. The reactants were analyzed online by GC-900 chromatograph with FID as detector. The catalytic activity is noted by Tloa , TSo%, and T 9 0 R .

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Effects of calcinated temperature on La-hexaaluminates

In order to investigate the effect of calcinated temperature on the structure of catalysts, the XRD patterns at different temperatures are shown in Fig. 1 for the one Mn atomic-substituted catalyst. It shows that the calcinated temperature plays a great role in the formation of hexaaluminate catalysts . When the calcinated temperature is 800 “c, only the broad 7A1203 diffraction peaks appear. However, when the calcinated temperature is 1050 “c , the LaMnlAlil019 hexaaluminate crystal peaks appear [ JCPDS 3613171, and the crystal is completely formed at 1200 “c 7+

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1.2 Characterization of catalysts Specific surface areas were measured by nitrogen adsorption at 77 K using the BET method by the apparatus of ST-08 auto-physical adsorbing analyzer. The solids were treated at 300 “c in air for 2 h prior to nitrogen adsorption. X-ray diffraction patterns were recorded with a Rigaku D/max-2500 diffractometer using nickel-filtered Cu Ka line at 0. 15406 nm . The spectra were recorded between 5” and 80” ( 2 8 ) . Temperature programmed reduction (TPR) of the catalysts was performed on TPDROllOO under 5% ( volume

+

10

20

30

50

40 2

Fig. 1

B -Al20,

(3: 60

70

8

O / ( O )

XRD patterns of LaMnAll1019calcined at different temperatures (1) 1200 “c; ( 2 ) 1050 “c; (3) 800 “c

JOURNAL OF RARE EARTHS, Vol. 2 4 , N o . 6 , Dec . 2006

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with narrow, intensive hexaaluminate peaks, without any disturbing peaks. The XRD results reveal that the modified co-precipitation method with buffer solution precipitants may cause homogeneous and pure hexaaluminate at the relatively low temperature of 1050 "c . Xu Jinguang'"] reports that La-hexaaluminate is made from the reaction between the scattered, uncrystal La species and y-A1203. La ions are very evenly dispersed in y-A1203, which makes the migration of ions easier, and the hexaaluminate can be shaped under low temperature. Mn ions can enhance and accelerate the migration of La ions in y-A1203.

2.2

Effects of different substituted metals on La-hexaaluminates Fig. 2 shows that only the hexaaluminate , namely

P-Al203 crystals appear in catalyst when substituted metal M for A1 is Mn. The Mn ions can easily enter the hexaaluminates and occupy the proper crystal position, and promote crystal shaping. However, when M is Ce, although the hexaaluminate also appears, most of the peaks are intensive CeOz and CeA103, which indicates that Ce3+ cannot enter the hexaaluminates easily, but forms CeOz or CeA103. While M is both Ce and Mn, the hexaaluminate peaks in the patterns are stronger than that when M is Ce. It reveals that Mn can easily enter the structure of hexaaluminate , thereby improving La migration and accelerating the formation of hexaaluminate. The radii of Ce3+( 0 . 103 nm) and Ce4+(0.092 nm) are all bigger than Mn2' (0.08 nm) , and the stable Ce4+ is hard to substitute A13+ and also hard to occupy the position of hexaaluminate crystal structure, and only resists as the Ce02 with high oxygen storage capacity"*!

.

2.3 Methane catalytic combustion on Lahexaaluminate catalysts

ture on the surface area and methane combustion performance. The activity of sample calcinated at 800 "c is low, which may be due to the formation of hexaaluminate crystal being little, and the real reason needs to be further investigated. However, when the sample is calcinated at 1050 "c , the catalyst shows high catalytic performance because of the formation of hexaalumiante, even though it has a low surface area. Its light off temperature is as low as 470 "c , and 90% conversion temperature is only 700 "c . Due to its reduced surface area, the catalytic activity of the sample decreases when the temperature reaches 1200 "c . The methane catalytic combustion activity has been mainly affected by the redox capacity of active sites in catalysts. As active species, Mn can react with crystal oxygen through its Mn3+ /Mn2+ redox cycles. The cycles processed play a key role in the catalytic activity"3' . Table 2 shows the effects of different Mn amount substituted La-hexaaluminate catalysts on methane combustion activity. It reveals that substituted Mn amount has a positive effect on the catalytic combustion activity. There is the best performance on Lahexaaluminates when proper Mn atomic substituted amount for A1 is 2 , for which the light-off temperature is 480 "c. It was also found that there was no lineal relationship between the activity and the surface area or Mn-substituted amount, and both affect the activity * The effects of different substituted metals M on combustion activity for LaMAl11019( M = Mn, Ce, MnCe) were also studied as shown in Fig. 3 . The Mnsubstituted catalyst is much better than that of Ce-substituted catalyst; the reason for this could be due to the high redox characteristics of the Mn ions. Khen Table 1

Effects of calcinated temperatures of catalyst on methane catalytic combustion

Table 1 shows the effects of calcinated tempera-

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P -A1203 CeA10,

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116.12 29.15 18.63

800 "c

1050 "c 1200 "c

Table 2

10

20

30

40

2

50 (J

60

70

80

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XRD patterns of LaMAll1019and LHA catalysts calcinected at 1200 "c ( M = Mn, Ce, CeMn) (1) M = CeMn; ( 2 ) M = Ce; (3) M = Mn; (4) LHA

Fig.2

510 470 620

665 620 730

750 700 790

Effects of Mn-substituted amount on methane samples catalytic combustion ( LaMn,Al12- 019 were all calcined at 1200 "c for 4 h )

LaAhOls LaMnAl1101~ LaMn2AlloOl9 LaMn3AlgOl9 LaMn4 A18 019

32.14 18.36 12.47 11.71 27.64

625 620 480 505 590

765 730 635 650 735

790 790 760 740 820

Cui M S et a1 . La-Hexaaluminate Catalyst Preparation for Methane Catalytic Combustion

Mn and Ce are both introduced, the catalytic activity is greatly enhanced. It is known from above-mentioned factors that the active species is Mn, and Ce exists in the catalyst as CeOz with high oxygen storage capacity, which can afford rich active crystal oxygen, and the redox performance of Mn is improved. So the synergic effects of Mn and Ce are the main reasons for the enhanced catalytic activity.

2.4

Effects of substituted Mn amount on TPR performance

Temperature programmed reduction ( TPR ) can be used to test the redox performance of Mn ions in La-hexaaluminate catalyst. TPR curves in Fig. 4 show that the reduction performance is enhanced when the substituted Mn amount increases. However, the reduction temperature gets higher with Mn substituted increasing. Considering these two factors, the Mnsubstituted amount for the best catalytic performance should be about 2, which is in accordance with the methane combustion test (Table 2 ) . The redox capacity is related closely to the oxidation catalytic activity, and the active species are main-

693

ly the crystal oxygen under high temperature and the chemical adsorbed oxygen at low temperature in the methane catalytic combustion. Because Mn' + is hard to be reduced to Mno even at 1200 "c, the catalytic activity of Mn-substituted La hexaaluminate is affected by the redox of M I I ~ ' / M ~ and * ~ its reaction with crystal oxygen. Bellotto et a1 . showed that tetrahedron A13+ in hexaaluminate is substituted by Mn2+ when a small amount of Mn is introduced. The space in this position is large, and the distortion caused is small, and therefore the large and low valence Mn'' is easy to occupy. When more Mn substituted is introduced, octahedron A13' of hexaaluminate catalyst is replaced by Mn3+, which causes the decrease in surface area. The ratio of Mn3+/Mn2' is upgraded with increasing the Mn-substituted amount. Moreover, the catalytic activity is also enhanced with the increase of substituted Mn. The valence of Mn was calculated by the standard of reduc, According to Table 3 , the tion from Mn3' to results indicate that the mean valence of Mn increases when substituted Mn increases, and the H2 consumption amount also increases. Bible 3

Catalyst

Relationship between H2consumption of LaMn, Alll - *OI9and the mean Mn oxidation state H2 consumed per gram catalydmmol

LaMnAlilO19 0.102 LaMn2.41100~ 0.407 LaMn3AlsO~ 0.983

Mean Mn o-wdation state

2.16 2.33 2.56

3 Conclusion Temperature/"(: Fig. 3

Different substituted metals M on combustion activity for LaMAll1OI9

0

200

400

600

800

1000

Tcmperaturdc Fig.4

Effects of substituted Mn amount on TPR performance

The homogeneous and pure La-hexaaluminate catalysts can be synthesized by the modified precipitation method, with buffer solution of NH4HC03/NH,0H as the precipitant. The hexaaluminate is formed at a low temperature of 1050 "c, and the hexaaluminate crystal phase is completely shaped by the temperature of 1200 "c . Transition metal Mn occupies the position of hexaaluminate structure easily, which promotes crystal phase formation. The Ce introduction can improve the catalytic activity of hexaaluminate , which is hard to enter into the hexaalumiante structure, for existing as the CeO' with high oxygen storage capacity. The CeO' can offer influent active crystal oxygen, which also enhances the redox performance of Mn. The activity of LaMAlll OI9( M = MnCe) is improved largely because of the synergic effect between Mn and Ce . When Mn atomic substituted amount for A1 is 2, the activity of LaMn,A112_.019shows the best, which

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has high redox performance.

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