Preparation of metal supported hexaaluminate catalyst for methane combustion

Scientific Bases for the Preparation of Heterogeneous Catalysts E.M. Gaigneaux et al. (Editors) © 2006 Elsevier B.V. All rights reserved.

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Preparation of metal supported hexaaluminate catalyst for methane combustion Yanqing Zhai and Yongdan Li Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, China. Tel: +86-2227405613; Fax: 27405243; Email: ydli(a)tju.edu.en

Abstract: An active and stable FeCrAlloy supported hexaaluminate catalyst for the catalytic combustion of methane was designed. The pretreatment of support, the preparation of catalyst precursor and coating slurry all have strong effects on the activity and stability. It was found that a transition layer of a-Al2O3 formed in-situ during the final calcination is the best for holding the metal and the catalyst layers together, and that preformed hexaaluminate crystallites are better than those formed during the final calcination. A well crystallization of the hexaaluminate phase is important. Keywords: Methane combustion, metallic monolith, FeCrAlloy, hexaaluminate 1. Introduction The temperature of natural gas combustion is higher than 1300 °C, leading to an intensive NOX emission [1-5]. Catalytic combustion can be controlled at a low temperature, thus eliminates the emission problem, and is explored for application in several cases [6-11]. Monolithic structured catalyst is suitable due to its low-pressure drop [1,12-15]. Metallic monolith exhibits advantages over ceramic ones. However, it is not porous, and induces mismatch in the thermal expansion between the support and the coating layers, resulting in the peeling off of the active layer [16,17]. FeCrAlloy (FCA) monolith is promising due to its resistance to high temperature [7]. This behavior relates to the diffusion of Al to the surface and oxidation into a protecting layer of a-alumina [18-21]. Hexaaluminate (HA) has been used as combustion catalyst and possesses a superior thermal stability [22,23]. It was used either in a form of self-extruded or as an active coating layer on a ceramic monolith [24-26]. The temperature

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range for the formation of the A12O3 layer on FCA is similar to that of the formation of HA [7,27,28]. A good affinity between the layers is assumed. 2. Experimental 2.1. Preparation of samples HA precursor, with a formula LaMn2Al10Oi9, was prepared via co-precipitation, as described by Groppi et al. [28]. It was carried out in an open beaker, at pH 8, with 1 mol/L mixed solution of La, Mn and Al nitrates and with (NH4)2CO3 as the base, followed by aging 3 h at ambient temperature. The material was filtrated, washed and dried at 120 °C, and calcined at 900 or 1000 °C for 5 h.. 0.1 mm thick FCA, with Al 9.7 %, Cr 25.1 % and Fe in balance, was used as support. The sheets were polished using a 600-grit SiO2 sand-paper and cleaned ultrasonically in acetone and de-ionized water. The support was either oxidized in air at two temperatures, 900 °C and 1100 °C for 5 h, respectively, with ramping 10 °C/min or immersed in a solution of 100 g/1 H2SO4 and 100 g/1 NaCl at 60 °C for 5 min, and washed with de-ionized water afterwards. The coating slurry was prepared via two methods. The first, catalyst powder was crushed and sieved to below 300 meshes and dispersed with 10 wt% in 1:1 ethanol and acetylacetone. Then it was ball milled for 10 h. The second, after dispersion and ball milling under the same condition, boehmite sol and nitrates of La and Mn with a ratio to make up LaMn2Al10Oi9, and an amount to make 30 wt % in the final oxide were added, and stirred for 0.5 h. Pretreated support was dipped into the slurry for 3 min and withdrawn to ensure uniformity. The material was dried at 80 °C for 10 min and calcined at 900 °C for 1 h. The dip-coating and calcination was repeated three times. Finally it was calcined at 1000 °C another time for 5 h. The samples are described in Table 1. 2.2. Measurements XRD patterns were measured with a Rigaku D/max 2500 instrument with Cu Ka. A PHILIPS XL 30 SEM with an EDX detector was employed. The adhesion of the catalyst layer was measured by weight loss in ultrasonic bath test for 30 min, with immersion in petroleum ether in a sealed beaker. 10 cycles of thermal shock was applied by heating to 800 °C for 20 min with a rate 10 °C/min, dropping into water at 25 °C, and drying at 120 °C for each. The activity was measured with a tubular reactor under atmospheric pressure. The sheets coated with catalyst were cut into pieces of 0.2-0.3x2-3 cm. A bundle of pieces with ca 0.3 g catalyst were put in the tube vertically. A gas mixture of 1.5 vol% CH4 and 98.5 vol% air was used as feed at GHSV 60000 mlg^h"1, calculated according to the weight of the catalyst layer. An Angilent 4890 gas chromatograph with a thermal conductivity detector was used.

Preparation of metal supported hexaaluminate catalyst for for methane combustion

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Table 1 Conditions of pretreatment of metal support and preparation of slurry and the weight gains after dip-coating and calcination of the samples Sample

Pretreatment of

Coating

No

metal

Weight gains (%)

Calcination temperature of precursor (°C)

With sol or not

1

Oxidized at 1100 °C

1000°C

Non

3.0

2

Oxidized at 900 °C

1000 °C

Non

3.1

3

Treated with acid

1000 °C

Non

4.1

4

Oxidized at 1100 °C

1000 °C

With

4.5

5

Oxidized at 900 °C

1000 °C

With

4.8

6

Treated with acid

1000 °C

With

5.0

7

Oxidized at 1100 °C

900 °C

Non

3.0

8

Oxidized at 900 °C

900 °C

Non

3.1

9

Treated with acid

900 °C

Non

4.0

3. Results 3.1. Structure and characteristics of the catalyst precursor In Fig 1, the sample calcined at 900 °C exhibits broad and asymmetric peaks in the XRD pattern, assignable to microcrystalline y-Al2O3. La or Mn oxide was not detected, indicating well dispersion in alumina. For the sample calcined at 1000 °C, the pattern of HA with magnetoplumbite (MP) structure was observed. Calcination at 1200 °C resulted in a well crystallization of the MP phase. The surface areas of the three samples are 70.5, 55.4 and 21.8 m2/g, respectively. V Hexaaluminate _ -Al2O3 11200°C 200oC

UN

11000°C 000oC 9900°C 00oC

0 220 0 330 0 4 40 0 550 0 6 60 0 '7 070 8'0 80 '110

2 th e ta ( o° ) theta

Fig. 1. XRD patterns of the catalyst precursors calcinated at different temperatures

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3.2. Effect of the support pre-treatment Fig 2 (a), shows the acid eroded surface being no longer smooth. Fig 2 (b) and (c) are those oxidized at 900 °C and 1100 °C for 5 h, respectively. A coarse and dense oxide layer was anchored to the metal surface, increasing greatly the surface area. EDX analysis of the surface in Fig 2 (c) gives an element atomic ratio Al=38.0 %, Cr=5.21 %, Fe=7.04 %, 0=49.7 %, respectively. The acid eroded sample displays only the peaks of the alloy in the XRD pattern as shown in Fig 3. For the one oxidized at 900 °C, the peaks of a-Al2O3 appear. For the one oxidized at 1100 °C, the peaks of a-Al2O3 become obvious and the peaks of the metal become non observable in consistency with [29,30].

(a) treated with acid

(b) oxidized at 900 °C

(c) oxidized at 1100 °C

Fig. 2. SEM micrographs of metal support with different pre-treatment condition V a-Al2O3 T

Fe

•

treated with acid oxidized at 900 °C v

oxidized at 1100 °C V

10 20 30 40 50 60 70 80 2 theta ( ° ) Fig. 3. XRD patterns of metal support surface with different pre-treatment conditions

3.3. Property of the coated catalyst The XRD patterns of three coated catalysts, a metal support and a catalyst precursor are depicted in Fig 4. It shows that LaMn2Al10Oi9 with a MP structure

Preparation of metal supported hexaaluminate catalyst for for methane combustion

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formed in the catalyst layer after calcination at 1000 °C in despite of precursor history. It is likely that a-Al2O3 phase coexists. w

T a -A12O3 V Hexaaluminate

10 20 30 40 50 60 70 80 2 theta ( ° ) Fig. 4. The XRD patterns of samples, (a): Metal support oxidized at 1100 °C; (b): Sample 9; (c): Sample 3; (d): Sample 6; (e): Precursor powder calcined at 1000 °C

The data in table 1 show that with a same pretreatment condition, the loading is higher for the slurry containing boehmite sol, and is similar for the precursors calcined at different temperatures. The loading is higher for the support with acid pretreatment, and is almost the same for the supports oxidized at different temperatures, on the condition of same kind of coating slurry used. Three temperatures, light off temperature with 10 % conversion, with 50 % and 95 % conversions are listed in Table 2. The pre-treatment of the support and the coating slurry have effects on the activity. For the 9 samples, 1 -5 show better and similar activity, with 2 & 5 showing the best, while the other four show a sequence of activity 6 > 8 > 7 > 9. This sequence does not show any relevance to the loading of the active layer. The samples with the precursor calcined at 900 °C are the least active. For a specific coating slurry, the support oxidized at 900 °C gives the best activity among the three pre-treatment conditions. The weight losses of the samples after the two tests are summarized in table 2. For the ultrasonic test, it shows a sequence as: sample 3 < 2 < 1 < 6 < 5 < 4 . While, for the thermal test, it gives a sequence as: sample 3 < 2 < 6 < 5 < 1 < 4 . During the two different tests, sample 3 is found have the highest stability, while sample 4 is the least stable among the samples tested. A SEM analysis was done across the cross section of sample 3. It shows a typical multilayered structure with thin films stacked onto one another. A dense layer of CC-AI2O3 formed, during the calcination after the coating of the catalyst, in between the HA layer and the metal bulk. EDX analysis distinguishes the interfaces between the layers. The catalyst layer is thinner than 10 (am, while the medium layer is around 1-3 urn.

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Table 2 Temperatures for three critical conversion and weight loss during 30 min ultrasonic treatment and 10 times of thermal treatment Sample

Catalytic activity (°C)

Weight loss (%)

T(10%)

T(50%)

T(95%)

Ultrasonic test

Thermal shock test

1

457

594

734

27

13

2

443

580

708

17

5

3

457

580

720

15

4

4

460

592

737

80

14

5

434

586

720

50

8

6

478

620

32

6

7

540

721

-

-

8

520

686

-

-

9

613

761

-

-

Fig 5 gives the micrographs of the surface of sample 3, and that of after 10 times thermal treating and ultrasonic treating for 30 min. It can be seen from Fig 5 (a) that many obvious and randomized surface cracks exist in the catalyst layer. In Fig 5 (b), it shows a large patch of catalyst layer fallen off from the surface, whereas the exposed surface is still not the metal or the dense OC-A1203 layer and is likely an inner layer of catalyst exists. However, falling of catalyst layer during thermal shock test includes the inner layer as shown in Fig 5 (c).

(a) As prepared surface

(b) After ultrasonic adhesion test

(c) After thermal shock test

Fig. 5. SEM micrographs of sample 3 and after ultrasonic test and thermal shock test

4. Discussion Groppi et al. [28] proposed that MP phase LaMnxAl|2-xOi9 forms from coprecipitated precursor at a temperature higher than 1000 °C. The results in Fig 1, however, show calcination at 1000 °C leads already to a MP phase for the

Preparation of metal supported hexaaluminate catalyst for for methane combustion

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precursor. The data in table 2 indicate that the HA formed after coating is not as active as that formed before coating. It is noted that these samples were all calcined at 1000 °C after the coating, and that the results depicted in Fig 4 prove that MP phase were formed during the final calcination for all the samples. The support treatment with acid and salt solution does not induce an oxidation, but cleans and corrodes the surface. 900 °C oxidation is not enough for the formation of well crystallized A12O3 dense layer. However, at 1100 °C a well developed a-Al2O3 layer forms. EDX analysis of the surface oxidized at 1100 °C proves that the layer formed is a mixed oxide phase, with small amount of iron and chromium substituting the aluminum in the a-Al2O3 phase. The support treatment influences the adhesion of catalyst from coating slurry and has a strong effect on the resistance to ultrasonic and thermal shock tests. The support treated with acid is the best both for loading and stability. The adhesion is better on the support oxidized at 900 "C than that at 1100 "C. As for the activity, the one treated at 900 °C are better than that with acid. The least active one is with that oxidized at 1100 °C. Anyway, preparation of both active and stable catalyst is possible, e.g. samples 3 & 2 are both very active and stable. Addition part of the aluminum as boemite sol and Mn, La as salt into the coating slurry improves the adhesion of the oxides on the support during dipcoating, however, does not improve the adhesion in the final catalyst. Ultrasonic and thermal shock tests were employed to characterize the stability of catalytic coating layers [29-30]. Gradual peeling off of the catalytic layer was observed during the two tests. However, Peeling during ultrasonic test leaves a coarse surface and the inner layer of the oxide, while peeling in the thermal test leaves a smoother surface and goes into deep layer. It means that for ultrasonic test, the oxide peels as small pieces originating from local breaking due to localized stress, and that during thermal test, the oxide peels off along the interface due to thermal mismatch. The already existing random cracks, as shown in Fig 5 (a), enhance the stability, as they absorb some of the stress. However, they behave also as the origin of the peeling. The facts that the samples prepared with acid treated support have the best stability, and that the support oxidized at a lower temperature produces more stable catalyst than that oxidized at a higher temperature, indicate that a well formed a-Al2O3 layer before the coating is no good, then a high surface energy of support is necessary. EDX of locations along the section of sample 3 shows that a thin transition layer of alumina exists between the support and catalyst layers. The XRD patterns of sample 3, 6 and 9 all show very small peaks of aA12O3, which is formed during the final calcination. 5. Conclusions For FCA supported HA catalyst, the process factors during the pretreatment of the metal support, the preparation of the catalyst precursor and the coating slurry all have strong influence on the activity and stability. Support

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pretreatment is a key factor for a stable layered structure. Acid eroded surface produces the best stability and a lower temperature oxidation is better than a higher temperature. An a-Al2O3 transition layer formed in-situ during final calcination is the best for holding layers together. A well developed a-Al2O3 layer before coating is no good. The peeling of catalyst during ultrasonic treatment is as small pieces and leaves a coarse surface and the inner layer catalyst, while the peeling of the catalyst layer during thermal treatment originates from the interface and leads to the formation of large pieces. Acknowledgement NSF China is acknowledged for the contracts: 20576097 & 20425619. References 1. P. O. Thevenin, P. G. Menon and S. G. Jfirfis, CATTECH 7 (2003) 10 2. J. H. Lee, and D. L. Trimm, Fuel Processing Technology 42 (1995) 339 3. T. V. Choudhary, S. Banerjee and V. R. Choudhary, Appl. Catal. A 234 (2002) 1 4. P. Gelin and M. Primet, Appl. Catal. B 39 (2002) 1 5. G. Centi, J. of Mol. Catal. A 173 (2001) 287 6. E. Tronconi and G. Groppi, Chem. Eng. Sci. 55 (2000) 6021 7. I. Cerri, G. Saracco, F. Geobaldo et al., Ind. Eng. Chem. Res. 39 (2000) 24 8. I. Cerri, G. Saracco, V. Specchia et al., Chem. Eng. J. 82 (2001) 73 9. D. Ugues, S. Specchia and G. Saracco, Ind. Eng. Chem. Res. 43 (2004) 1990 10. I. Cerri, G. Saracco and V. Specchia, Catal. Today 60 (2000) 21 U . S . Cimino, R. Pirone and L. Lisi, Appl. Catal. B 32 (2002) 243 12. S. T. Kolaczkowski, W. J. Thomas, J. Titilgye et al. Combust. Sci. and Tech. 118(1996)79 13. S. Cimino, R. Pirone and G. Russo, Ind. Eng. Chem. Res. 40 (2001) 80 14. J. W. Geus and J. C. V. Giezen, Catal. Today. 47 (1999) 169 15. G. Centi and S. Perathoner, CATTECH. 7 (2003) 78 16. M. Ferrandon, M. Berg and E. Bjornbom, Catal. Today. 53 (1999) 647 17. M. V. Twigg and D. E. Webster, in: A. Cybulski and J. A. Mouliin (Eds.), structured catalysts and reactors, New York, 1998, p. 59 18. W. J. Quadakkers, A. Elschner, W. Specier et al., Appl. Surf. Sci. 52 (1991) 271 19. C. Badini and F. Laurella, Surf. Coat. Technol. 135 (2001) 291 20. K. Haas-Santo, M. Fichtner and K. Schubert, Appl. Catal. A 220 (2001) 79 21. A. Cybulski and J. A. Moulijn, Catal. Rev. -Sci. Eng. 36 (1994) 179 22. M.Machida, K. Eguchi and H. Aria, J. Catal. 103 (1987) 385 23. M.Machida, K. Eguchi and H. Aria, Chem. Lett. 185 (1987) 767 24. H. Inoue, K. Sekizawa, K. Eguchi et al., Catal. Today 47 (1999) 181 25. R. Kikuchi, K. Takeda, K. Sekizawa et al., Appl. Catal. A 218 (2001) 101 26. M. H. Han, Y. S. Ann, S. K. Kim et al., Mater. Sci. Eng. A. 302 (2001) 286 27. M. Machida, K. Eguchi and H. Arai, J. Catal. 123 (1990) 477 28. G. Groppi, C. Cristiani and P. Forzatti, Appl. Catal. B. 35 (2001) 137 29. M.Valentini, G. Groppi, C. Cristiani et al., Catal. Today 69 (2001) 307 30. Z. R. Ismagilov, V. V. Pushkarev, O. Yu. Podyacheva et al., Chem. Eng. J. 82 (2001) 355