Methane combustion over B-site partially substituted perovskite-type LaFeO3 prepared by sol-gel method

~

AA PT PA LL E IY DSS C I A: GENERAL

Applied Catalysis A: General 156 (1997) 2941

ELSEVIER

Methane combustion over B-site partially substituted perovskite-type LaFeO3 prepared by sol-gel method Ziyi Zhong, Kaidong Chen, Yong Ji, Qijie Yan* Department of Chemistry, Nanjing University, Nanjing 210093, China Received 17 July 1996; received in revised form 2 December 1996; accepted 4 December 1996

Abstract

Ultrafine perovskite-type complex oxides LaFel yAyO3 A (A=Mn, A1, Co) were prepared by sol-gel method using citric acid as complex agent, and characterized by XRD, TPR, TG and XPS techniques. The catalytic activities for methane combustion were tested and compared with that of the large particle oxides having the same composition. It was shown that the catalytic activities of ultrafine LaFel_yAyO3 A complex oxides were much higher than that of LaFeO3 sample due to the increase of high valence B-site cations and lattice oxygen content by the substitution of B-site elements. Much higher catalytic activities were also observed for the ultrafine perovskite-type complex oxides compared to their large particle counterparts.

Keywords: Methane combustion; B-site partially substituted LaFeO3; Ultrafine particles

1. Introduction Perovskite-type complex oxides (ABO3) have a well-defined structure and high thermostability, and recently have been the subject of numerous studies on the combustion catalyst. Based on the study of LaMO3_;~ (M=Mn, Fe, Cu, Co, Ni, Cr) oxides in methane combustion, Arai et al. [ 1] pointed out that the substitution of the A-site with an element of different valence, e.g. substitution of La 3+ with S r 2+, led to the formation of oxygen vacancies and high-valence cations at B-site, which resulted in the significant change of catalytic activity. Nevertheless, it was found that the catalytic activity was generally determined by the B-site elements [2,3]. Mizuno et al. [4] reported that rather high activities and a pronounced synergetic * Corresponding author. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 0 0 3 - 3

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Z. Zhong et al./Applied Catalysis A: General 156 (1997) 29~tl

effect were found in the reaction of CO-O2 and NO-CO over LaMn~_xCuxO3_A oxides (x=0.3-0.5). Similarly, Salomonsson [5] found that Lal_xSrxByBtzO3 (B: Mn, Fe; Bt: Fe, Cu) oxides possessed significant activities in methane combustion and were able to exchange bulk oxygen in quantities corresponding to several hundred monolayers. Zhang et al. [6] mentioned that, for Lal_xSrxCOl_yFeyO3 oxides, partial substitution of Fe for Co enhanced adsorption and desorption of oxygen particularly in low-temperature region, and the substitution of Sr for La as well as of Fe for Co was found to promote catalytic activities of the oxides. However, the specific surface areas (SBET) of the perovskite-type oxides prepared from carbonates, acetates or metal oxides are very small (<10 mZ/g), limiting the improvement of their catalytic activity. Tasc6n et al. [7] and Zhang et al. [8] once reported that LaFeO3 oxides with a specific surface area of 10.7 and 14.4 m2/g, respectively, were prepared by citric acid process, and used in the catalytic oxidation reaction of CO, but their particle size and surface structure have not been further reported. So new preparation methods which give a catalyst with a sufficient large specific surface area are also eagerly awaited for practical purposes. In addition, it has been extensively reported that oxygen species on the surface of catalysts play an important role in the rate-limiting step of methane activation [9], but the role of different oxygen species is still an open question. Voorhoeve et al. [10] introduced the terms suprafacial and intrafacial catalysis in order to characterize the behavior processes of a series of perovskite-type oxide catalysts. Suprafacial processes are considered to be low-temperature processes proceeding through reactions with adsorbed oxygen (Oaas). Conversely, intrafacial or hightemperature processes involve the removal and incorporation of lattice oxygen (Olat). The results of kinetic study also suggest that the oxidation of methane are parallel reactions of adsorbed and lattice oxygen [ 1,11]. It is generally considered that adsorbed oxygen species results in the total oxidation of alkane, and the lattice oxygen for selective oxidation [12], e.g. for LaA103 [13], LaMnO3 and alkali doped LaMnO3 [14], it was reported that a gaseous or weakly adsorbed oxygen species is responsible for the complete oxidation of methane, etc. But recently, some studies show that lattice oxygen may be responsible for combustion of methane also. Omata et al. [15] reported that two kinds of lattice oxygen ('y- and/3oxygen) existed in the substituted SrCoO3 perovskite oxides, 7-oxygen is responsible for selective oxidation, and fl-oxygen leads to COx formation. By studying a series of perovskite oxides, Ding et al. [ 16] found a linear relationship between the ratios of Oads/Ola t and the selectivity of C 2 product, and pointed out that the adsorbed oxygen favors the formation of C2 product, and lattice oxygen for the complete oxidation of methane. Yu et al. [17] also found that lattice oxygen results in the complete oxidation of methane by an in situ IR spectra research for mixed oxides catalyst SrO-La203/CaO. The complexity may come from the fact that the active sites on a great variety of oxide systems might not be the same, and the determination of various oxygen species might be difficult at different reaction conditions.

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All above conclusions were obtained by studying the large particle oxides. Owing to the larger SBETand unique physico-chemical properties, ultrafine oxides are drawing research interests for catalysis in recent years. It is expected that the catalytic activity and reaction characteristics are different between ultrafine and large particle perovskite-type oxides. In this work, three series of substituted ultrafine perovskite-type oxides LaFel :, AvO3 A ( A : M n , A1 or Co) were first prepared by sol-gel method. The catalytic activities of methane combustion for these catalysts were investigated and compared to that of the large particle catalysts having the same composition. The role of lattice oxygen in the activation of methane was also discussed.

2. Experimental Ultrafine perovskite-type complex oxides were prepared by sol-gel method as described previously [18]. Citric acid (cit) was used as complex agent, the molar ratio of metal ion : cit was 1 : 2, and the preparation temperature of gel was in the range of 60-70°C. The gel was dried at 120°C and calcined at 650°C for 4 h (except mentioned especially). Large particle catalysts (LaFeo.9Ao.aO 3 A) were prepared in two methods. For the first one, the catalyst was prepared by sol-gel method first, and then followed by calcined at 900°C for 2 h. For the second one, nitrate solutions of component metals were mixed together in desired proportions, and then precipitated with NaOH and NazCO3 mixed solution. The as-prepared precipitate was then filtered and washed to sodium ions free and dried at 120°C. Finally it was calcined at 900°C for 2 h. Phase analysis of the samples was carried out with a Shimadzu XD-3A diffractometer using CuKc~ radiation and a Ni filter. TEM studies were performed with JEM-100s electronmicroscope for the particle size and shape determination. Micromeritics ASAP-2000 was used for the measurement of surface area by BET method (N 2 adsorption at 77 K). XPS experiment was carried out in an ESCALABMKII system. Binding energies was referenced to C1s at 285.0 eV. The samples were treated at 650°C for 1 h in the flow of oxygen before XPS test. Thermogravimetric analysis of the samples were determined in helium atmosphere at a heating rate of 20°C/min and the gravimetric measurement accuracy of 10 5 g. The weight loss of the sample (TG%) was taken as a probe for measuring amount of desorption oxygen. Temperature-programmed reduction (TPR) was performed at a heating rate of 16°C/min from room temperature up to 800°C in Hz-N 2 (5% volume H2) flow. Catalytic activity tests of methane combustion were carried out using a microreactor with 0.2 g catalyst pretreated in 02 stream for 1 h at 650°C. The reaction conditions were P=0.1 MPa, CH 4 • O 2 = 1 : 3 . 9 , and GHSV ca. 18 900 ml g-1 h-1.

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3. Results and discussion 3.1. Structure and BET surface areas

BET surface areas are listed in Table 1, which are generally larger than that of LaFeO3 prepared by citric acid process given [7,8]. XRD patterns of various catalysts are exhibited in Figs. 1-3. It is shown that, for LaFel_yMyO3_A (M--Co, A1) and LaFel_yMnyO3_;~ oxides prepared by sol-gel method with y in the range of 0.0-1.0 and 0.0-0.3, respectively, the pure orthorhombic perovskite-type structure is formed, and single component oxide such as La203, Fe203, etc., has not been detected, indicating that the perovskiteTable 1 Specific surface areas and catalytic activities of nanometer oxides LaFel yA,,O3 ~ for methane combustion A Catalyst

Mn 1

y 0.0 T5o% (°C) 555 Tloo~ (°C) 700 SBE-r (m2/g) 22.0 R 1.0 DI (nm) 22 D2 (nm) 23

A1

2

3

0.1 419 500 15.5 8.5 20 19

0.2 422 550 19.3 3.3 25 23

4

0.3 445 600 33.3 2.9 23 23

5

0.1 378 460 30.9 25.8 19 20

Co 6

0,2 419 480 22,9 4,1 20 22

7

8

0.3 424 480 26.7 3.5 24 23

0.1 411 500 16.5 10.0 28 31

9

0.2 432 500 14.6 8.7 36 38

10

0.3 500 600 19.4 1.9 42 46

R: specific activity, conversion of methane (mmol/s m2x4.72x 10 5) at 400°C. D1 and D2: average particle size obtained by TEM and XRD line width methods, respectively.

A A A_ cA b-)

J

aA t.

20

3"0 40

A A A 5"0 6"0 7~) 20P

80

Fig. 1. XRD patterns of perovskite-type oxides LaFel yMnyO 3 .x. a: ultrafine, y=0.0; b: ultrafine, y 0.1; c: ultrafine, y=0.4; d: y=0.1, prepared by sol-gel method and calcined at 900°C; e: y=0.1, prepared by precipitation method and calcined at 900°C.

Z. Zhong et al./Applied Catalysis A: General 156 (1997) 29-41

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e_Jl

A

d--/~

c_3

b-J~ a Jr 2b 3b

4'o 50 60 7"0 8b 2e/o

Fig. 2. XRD patterns of perovskite-type oxides LaFel_yAlyO3_a. a: ultrafine, y=0.1; b: ultrafine, 3,=0.2; c: ultrafine, y=0.3; d: y=0.1, prepared by sol-gel method and calcined at 900°C, e: y=0.1, prepared by precipitation method and calcined at 900°C.

JdA

e-~

tdA

dA

_

c_~

AL

b _A__~ L

20

A_A A

3b

4"0 5"0 6"0 7"0 8b 20/°

Fig. 3. XRD patterns of perovskite-type oxides LaFel yCOyO3 A' a: ultrafine, y=0.1; b: ultrafine, y=0.2; c: ultrafine, y=0.3; d: y=0.1, prepared by sol-gel method and calcined at 900°C; e: y=0.1, prepared by precipitation method and calcined at 900"C.

type structure is preserved with the substitution of B-site elements within certain amount. Figs. 4-6 show the TEM photographs of various samples. For LaFel yMnyO3 (2=0.0-0.5), LaFel_yAlyO3_;~ (2=0.0-0.3) and LaFel_yCoyO3_A (2=0.0-0.2) systems, the average particle sizes are about 20, 19 and 28 nm, respectively. But as y exceeds the above limits, some large particles ca. 100 nm are observed.

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b

C

i

i

200 n m Fig. 4. TEM images of LaFet_yMnyO3 A oxides, a: sol-gel method, calcined at 650-'C for 4 h; b: sol-gel method, calcined at 900°C for 2 h; c: precipitation method, calcined at 900°C for 2 h.

The average particle sizes are also calculated from line widths of XRD peak, and are listed in Table 1. These data are basically in agreement with that of the TEM results. XRD results show that perovskite structure is also formed for the large

Z. Zhong et al./Applied Catalysis A: General 156 (1997) 29-41

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a

b

C

!

!

200 nm Fig. 5. TEM images of LaFel_yAly03 A oxides, a: sol-gel method, calcined at 650°C for 4 h; b: sol-gel method, calcined at 900°C for 2 h; c: precipitation method, calcined at 900°C for 2 h.

particle oxides LaFeo.9Ao.lO3_A prepared by the above mentioned two methods. Nevertheless, because of the variety in preparation conditions, the average particle size of the samples are somewhat different but all over 100 nm.

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a

b

b

4

C

Fig. 6. TEM images of LaFel ~CoyO3_~ oxides, a: sol-gel method, calcined at 650~"C for 4 h; b: sol-gel method, calcined at 900°C for 2 h; c: precipitation method, calcined at 900~C for 2 h.

3.2. Catalytic properties Table 1 lists the catalytic properties of various samples. The catalytic activity is evaluated by the temperature (Ts0~ and Tloo~) at which 50% and 100% conversion of methane are obtained, respectively. The specific activity is defined as the mmol

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Table 2 Specific surface areas and catalytic activities for large particle LaFeo.9Ao.lO3_A oxides A

Mn

Catalyst

2a

Tso~'o (°C) R SBz'r (m2/g)

617 1.7 4.6

A1 2b

851 0.0 3.9

5a

619 0.0 4.8

Co 5b

520 1.9 8.3

8a

550 2.9 5.7

8b

538 0.0 4.2

a Sol-gel method, calcined at 900°C for 2 h. b Precipitation method, calcined at 900°C for 2 h.

converted methane per second per SBEV. From Table 1, it can be seen that the partial substitution of B-site ion by Mn, A1 and Co exhibits significant promotion effect on the catalytic activity of the samples. For the ultrafine LaFel_yAyO3_;~ oxides, the Tso~ and T1oo% are much lower than that of LaFeO3, while the specific activity at 400°C is much higher. On the other hand, different element substitution also gives different promotion effect on the catalytic activity in the following sequence AI>Co>Mn. The highest catalytic activity is obtained for the sample with y=0.1. Accordingly, the samples with y=0.1 are taken for further study. The SBET and catalytic activities of large particle oxides are listed in Table 2. It is shown that the BET surface area and the specific activity at 400°C of the large particle samples are much lower than that of the ultrafine particles, and the Tso~ is higher. For instance, the Tso~ of LaFeo.9Alo.lO3_~ ultrafine particles is 378°C, which is much lower than that of the other two large particle samples of 619 and 520°C, respectively. The large particle samples show little or no activity at 400°C. However, ultrafine oxides show much higher specific activity. This result suggests that the reaction temperature can be lowered greatly by using ultrafine particle oxides as catalysts. However, Mn substitution for Fe prepared by precipitation method gives rather high Ts0~. The reason for this phenomenon is still unknown.

3.3. Surface oxygen species It is well known that the surface oxygen species play an important role in catalytic oxidation reaction. In this work, XPS, TPR and TG tests are used to study the content and characteristic of surface oxygen species. XPS results of several samples are summarized in Table 3. Cls spectra indicate that carbonate species does not exist on the surface of the samples pretreated in oxygen flow. Ols spectra of ultrafine and large particle oxides LaFeo.9Ao.lO3_A were consistently analyzed by curve fitting technique [16], and the O~at/Oads ratios are shown in Table 3 too. It is evident that for all systems, the O l a t [ O a d s ratio of ultrafine particles is higher than that of the large particles, and the specific activity follows the tendency of the Olat/Oa~s ratio too, suggesting that at 400°C lattice oxygen has already played a major role in methane combustion.

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Table 3 XPS and TG results of various samples

Catalyst 1 2 2" 5 5a 8 8"

Olat/Mnet 1.34 1.78 1.28 1.77 1.41 1.65 1.03

Oads/mnel 0.64 0.11 0.33 0.21 0.22 0.17 0.47

OlatlOads 2.1 16.2 3.9 8.4 6.4 9.8 2.2

Binding energies (eV) Olat Oads 529.5 529.8 529.8 529.6 529.8 529.6 529.6

531.8 532.0 532.3 532.2 532.5 532.0 532.0

TG (%) 0.70 1.73 0.01 1.63 0.02 1.23 0.01

Mnet: total amount of metallic cations (La+Fe+A, A=Mn, Co or A1) on the surface. a Sot-gel method, calcined at 90ff'C for 2 h.

According to the surface composition and the value of Olat/Oaas, the atomic ratios of Olat~Mnet and Oads/Mnet (Mnet is the total amount of metal ions on the surface) are calculated and listed in Table 3 too. It is shown that ultrafine oxides LaFeo.9A0.103 A have higher Olat/Mnet and lower O~ds/Mnet than that of the LaFeo.9A0.103_A large particles with same composition and the ultrafine oxide LaFeO3. Since large Olat/rMnet requires more hypervalent metal cations to compensate the negative charge brought about by lattice oxygen ions, and the low value of Oads/Mnetreflects less oxygen ion vacancies, thus it is reasonable to suppose that B-site partially substituted ultrafine oxides contain more hypervalent metal ions M(4 5)+ (M=Fe, Mn, Co) and less oxygen ion vacancies on the surface of the sample than that of the ultrafine oxide LaFeO3. It is well established that the activation of methane is a rate limited step [9], hence the activation of methane may mainly occurred on the M(4-6)+-0 2- sites in low temperature region. In other words, it may further state that the lattice oxygen, not adsorbed oxygen, favors the complete oxidation of methane.TPR profiles of various samples are shown in Figs. 7-9, respectively. It can be observed that the first reduction peak of ultrafine oxide LaFeO3 is about 425°C. But for the ultrafine particles partially substituted by Mn, A1 and Co, the first reduction peak shifts to lower temperature. However, with the increase of the particle size, the first reduction peak temperature of various sample increases too. These results indicate that the M - O bond strength can be weakened by the substitution of the B-site elements with Mn, A1, Co, and decrease of the particle size as well. TG results are listed in Table 3 too. The observed weight loss is ascribed to the loss of oxygen from the bulk and surface. From Table 3, it is shown that the weight loss of substituted ultrafine oxides is much higher than that of their large counterparts, and that of pure ultrafine oxide LaFeO3. These results suggest that the oxygen desorption becomes easier due to the substitution of B-site element by Mn, A1, Co and the diminish of the particle size. This is in agreement with the results of XPS and TPR. As suggested by Zhang et al. [6], the rapid desorption of oxygen at low temperature is somehow related to the relative ease of redox process between

Z. Zhong et al./Applied Catalysis A: General 156 (1997) 29~ll

_J

39

dl j

C

b

240

420

60~0o

Temperoture(' C)

780

Fig. 7. TPR spectra of of LaFeO3 and LaFeo.9Mno.lO3_Aoxides, a: LaFeO3, sol-gel method, calcined at 650°C for 4 h; b: sol-gel method, calcined at 650°C for 4 h; c: sol-gel method, calcined at 900°C for 2 h; d: precipitation method, calcined at 900°C for 2 h.

C

I |

240

420

i

600_

Temperature(' C)

,

780

"°

Fig. 8. TPR spectra of LaFe0.9A10.103_~ oxides, a: sol-gel method, calcined at 650°C for 4 h ; b: sol-gel method, calcined at 900°C for 2 h; c: precipitation method, calcined at 900°C for 2 h.

40

Z. Zhong et al./Applied Catalysis A: General 156 (1997) 29~41

240

420

6%.

Temperoture(' C)

780

Fig. 9. TPR spectra of LaFeogCooAO3..~ oxides, a: sol-gel method, calcined at 650'C for 4 h; b: sol-gel method, calcined at 900°C for 2 h; c: precipitation method, calcined at 900cC for 2 h.

M(4-~)+--+M 3+. The chemical potential and reactivity of oxygen adjacent to M cations will increase with the increase of oxidation state and content of M cations. For the ultrafine oxides, partial substitution of B-site cations and their interaction lead to producing more hypervalent cations, higher ratio of Olat/Oads and weaker M-O bonding strength. Hence they tend to lost much more lattice oxygen, i.e. more hypervalent metal cations can be easily reduced. This may be the reason that they have higher catalytic activity for methane combustion, particularly in the low temperature region. For the partially substituted samples obtained by calcining at 900°C for 2 h, part of the particles grew up, and s o m e M ( 4 - ~ ) + cations were reduced to M 3+ due to the loss of surrounding oxygen and the formation of oxygen vacancies. This results in the lower Olat/Oads ratio, small amount of oxygen desorption and the decrease of catalytic activity.

4. Conclusion

Ultrafine perovskite-type complex oxides LaFel yAyO3 were prepared by solgel method using citric acid as complex agent. It is found that the catalytic activity for methane combustion decreases with the increase of the particle size. Further-

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more, the catalytic activity is promoted by the substitution of B-site ion, and reaches a maximum value at y=0.1. It seems reasonable to suppose that B-site partially substituted perovskite-type oxides contain more hypervalent ions, in which the adjacent lattice oxygen play an important role in the combustion of methane in low temperature region. Both substitution of B-site cations and decrease of the particle size lead to the formation of more surface lattice oxygen species and the weakening of M-O bond strength. We thus conclude that the large lattice oxygen content and easier reduction of B-site M n+ ions may be responsible for the higher catalytic activity.

Acknowledgements The support of the 85-06 NMS and NSFJ of China is gratefully acknowledged.

References [1] H. Arai, T. Yamada, K. Eguchi and T. Seiyama, Appl. Catal., 26 (1986) 265. [2] T. Seiyama, N. Yamazoe and K. Eguchi, Ind. Eng. Chem., Prod. Res. Dev., 24 (1985) 19. [3] T. Nitadori, T. Ichiki and M. Misono, Bull. Chem. Soc. Jpn., 61 (1988) 621. [4] N. Mizuno, Y. Fujiwara and M. Misono, J. Chem. Soc., Chem. Commun., (1989) 316. [5] E Salomonsson, T. Griffin and B. Kasemo, Appl. Catal. A, 104 (1993) 175. [6] H.M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura and N. Yamazoe, J. Catal., 121 (1990) 432. [7] J.M.D. Tasc6n, S. Mendioroz and L. Gonzalez Tejuca, Z. Phys. Chem., 124 (1981) 109. [8] H.M. Zhang, Y. Teraoka and N. Yamazoe, Chem. Lett., (1987) 665. [9] O.V. Krylov, Catal. Today., 18 (1993) 209. [10] R.J.H Voorhoeve, Advanced Materials in Catalysis, Academic Press, New York, 1977, p. 129. [11] J.G. McCary and H. Wise, Catal. Today, 8 (1990) 231. 1112] T. Shimizu, Catal. Rev. Sci. Eng., 34 (1992) 355. [13] T. Tagawa and H. Imai, J. Chem. Soc., Faraday Trans., 84 (1988) 923. [14] J.E. France, A. Shamsi and M.Q. Ahsan, Energy and Fuels, 2 (1988) 235. [15] K. Omata, O. Yamazaki, K. Tomita and K. Fujimoto, J. Chem. Soc., Chem. Commun., (1994) 1647. 1116] W.R Ding, Y. Chen and X.C. Fu, Catal. Lett., 23 (1994) 69. [17] L. Yu, Y.D. Xu and X. Guo, Acta Physico-Chimica Sinica, 11 (1995) 902. [18] Z.Y. Zhong, L.G. Chert, Q.J. Yan and X.C. Fu, Stud. Surf. Sci. Catal., 61 (1995) 647.