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Catalytic combustion of methane over LaMnO3 perovskite supported on La2O3 stabilized alumina. A comparative study with Mn3O4, Mn3O4-Al2O3 spinel oxides
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2293–2299
CATALYTIC COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITE SUPPORTED ON La2O3 STABILIZED ALUMINA. A COMPARATIVE STUDY WITH Mn3O4, Mn3O4-Al2O3 SPINEL OXIDES
S. ARNONE,1 G. BUSCA,2 L. LISI,3 F. MILELLA,2, G. RUSSO3 and M. TURCO1 1Dipartimento
di Ingegneria Chimica, di Ricerche sulla Combustione, CNR Universita` di Napoli Federico II Naples, Italy 2Istituto di Chimica Facolta` di Ingegneria Universita` di Genova Genoa, Italy
3Istituto
Ten and 20 wt % LaMnO3 perovskites supported on La2O3-stabilized c-Al2O3 were studied for catalytic combustion of methane. A comparison with Mn3O4 and Mn3O4-Al2O3 spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O2 temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T 4 300–800 8C, space velocity 4 40000 h11, CH4 concentration 4 0.4% v/v, O2 concentration 4 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of c-alumina was retained after the treatment at 800 8C, the treatment at 1100 8C led to the transition to the h and b phases. TPR measurements suggested the presence of a fraction of Mn4` in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn2`, led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O2 desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH4 conversion within 650 8C with 100% selectivity to CO2. The activation energy value, evaluated from a methane firstorder rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO3. The preexponential factors of the catalysts treated at 800 8C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100 8C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO3 is available for the reaction. This was related to the stabilization of a fraction of Mn2`, probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600 8C with 100% selectivity to CO2. The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O2 was hypothesized.
Introduction Catalytic combustion is a useful technique for improving combustion efficiency and reducing NOx emissions in several applications, like as gas turbines, boilers, aircrafts, afterburners, domestic heaters, and volatile organic compound (VOC) removal [1,2]. The main goal of the research on catalytic combustion is to obtain materials with strong thermal resistance retaining high surface areas even at high temperature [3]. Manganese-based oxides showed the best performances among catalysts proposed for methane combustion. Manganese spinel-type oxides are able to catalyze the methane combustion even at
low temperature; however, they easily decompose at high temperature [4]. Lanthanum-manganese-based perovskites are active at higher temperature and heat resistant [5,6]. Nevertheless, their application is limited by the low specific surface areas. Some recent studies were devoted to obtain perovskites with high surface area by dispersion on suitable refractory support materials [3,7]. In this paper, LaMnO3 perovskites supported on La2O3 modified alumina were studied. The performances of these materials were compared with Mn3O4 spinel oxide to clarify the role of manganese ions in activating methane oxidation.
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Experimental Sample preparation Mn3O4 was prepared by precipitation from an aqueous solution of Mn(CH3COO)2. Al(NO3)3 with 1:2 Al/Mn ratio at pH 9.5 was added to obtained Mn3O4-Al2O3. In both cases, the solution was stirred overnight at room temperature, and the powder was dried 12 h at 110 8C and then calcined at 600 8C. Stabilized alumina used as support for perovskitebased catalysts was prepared by wet impregnation of high purity Al2O3 CK 300 supplied by AKZO Chemie with a La(NO3)3 aqueous solution. The dried material was calcined 3 h at 300 8C. Supported perovskites were prepared by adding the stabilized alumina to an aqueous solution of (CH3COO)2Mn and La(NO3)3 with 1:1 La/Mn ratio. The solution was stirred overnight at room temperature, and the dried powder (110 8C) was calcined 3 h at 800 8C or at 1100 8C. The samples contained 10 and 20 wt % LaMnO3, respectively, and will be referred to with the following codes according to the perovskite nominal content (weight percentage) and calcination temperature: LaMn 10-800, LaMn 20800, LaMn 10-1100 and LaMn 20-1100. Physicochemical Characterization X-ray diffraction (XRD) analysis was performed using a Philips PW 1710 diffractometer. Specific surface area of the catalysts was evaluated by N2 adsorption at 77 K according to the Brunauer–Emmett–Teller (BET) method using a Carlo Erba 1900 Sorptomatic apparatus. Energy-dispersive spectromety (EDS) microanalysis was performed by an EDAX instrument. TPR experiments were carried out in a Micromeritics 2900 temperature-programmed desorption (TPD)/TPR flow system equipped with a thermal conductivity detector (TCD). After a treatment in airflow at 600 8C, for spinel oxides, or at 800 8C for perovskites oxides, the sample was reduced with a 2% H2/Ar mixture (25 cm3 min11) heating 10 8C min11 up to 600 or 800 8C. TPD of O2 experiments were performed in the same apparatus described for TPR tests and the catalyst was pretreated under the same conditions. After pretreatment the sample was heated 10 8C min11 up to 600 or 800 8C in pure helium stream (25 cm3 min11). Catalytic Combustion of Methane Catalytic combustion of methane was studied in a fixed-bed quartz microreactor. The catalyst (particle size 4 300–400 lm), diluted 1:10 in quartz powder, was placed on a fritted disk. Quartz pellets upside
the catalytic bed and a narrowing of the reactor section both in the post and in the precatalytic zone reduced the homogeneous volume. The feed composition was 0.4% CH4 and 10% O2 in a balance of N2. A constant space velocity of 40000 h11 was ensured by Brooks 5850 TR Series mass flow controllers. The concentration of reactants and products was measured using a Hewlett-Packard 6890 gas chromatograph equipped with two capillary columns (poraplot Q and a molecular sieve 5A) and thermal conductivity and flame ionization detectors. Carbon balance was verified within 55%. Catalytic tests were carried out in the temperature range 300– 600 8C for spinel oxides and 300–800 8C for supported perovskite oxides after a 2 h treatment at 600 and 800 8C, respectively. Results and Discussion Physicochemical characterization XRD spectrum of the Mn3O4 sample shows the typical signals of the spinel phase. Mn3O4-Al2O3 shows the signals of both the Mn3O4 and c-Al2O3 phases. Specific surface area of Mn3O4-Al2O3 is higher than pure Mn3O4 (Table 1) due to the contribution of c-alumina. Samples of La2O3 stabilized alumina were treated at 800 and 1100 8C to compare the thermal behavior of the support with that of the supported perovskites treated at the same temperature. The addition of La2O3 inhibits the transition to a-Al2O3, leading to the formation of the b and h phases in the temperature range 1000–1200 8C [8–10]. XRD analysis of La2O3 stabilized alumina shows that the c-phase is still present after treatment at 800 8C and that traces of h phase appear after a treatment at 1100 8C. In agreement with these results, high specific surface area values are retained even after a treatment at 1100 8C (Table 1). These results indicate that under our experimental condition, the b phase is not yet formed. XRD analysis of supported perovskites shows the absence of a segregate perovskite phase, in agreement with the results of microanalysis showing a uniform composition of the samples. These results indicate a uniform dispersion of the active phase on the surface of the support, suggesting that the monolayer coverage is not complete. A rough evaluation of the perovskite content corresponding to the monolayer coverage can be obtained on the base of the unit cell dimensions of LaMnO3 perovskite (a 4 b 4 5.5 A˚, c 4 13.3 A˚, ASTM 32-484). If the ab face of the exhagonal structure of LaMnO3 [6] is exposed, the surface area would be 107 m2 g11; otherwise, if the ac face is exposed, the surface area would be 303 m2 g11. Thus, an average value of about 200 m2 g11 can be hypothesized as monolayer
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
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TABLE 1 Specific surface area of samples, hydrogen uptake in the TPR experiments, and oxygen desorption in TPD experiments Sample
Surface Area (m2 g11)
H2 Uptake (mol H2 mol11 Mn)
O2 Desorption 2 1011 (mol O2 cm12)
152 118 177 137 74 42 33 95
— — 0.56 0.46 0.52 0.22 0.53 0.47
— — — — — — 2.4 4.2
stab. Al2O3 (800 8C) stab. Al2O3 (1100 8C) LaMnO3 10–800 LaMnO3 20–800 LaMnO3 10–1100 LaMnO3 20–1100 Mn3O4 Mn3O4–Al2O3
Fig. 1. TPR profiles of Mn2O3 and Mn2-Al2O3 catalysts.
TABLE 2 Activation energy (Ea) and preexponential factor (A) values.
Catalyst LaMnO3 10–800 LaMnO3 20–800 LaMnO3 10–1100 LaMnO3 20–1100 Mn3O4 Mn3O4–Al2O3
Ea 2 1013 (Kcal mol11)
A 2 1018 (l h11 g11)
24 24 24 24 20 20
2.5 4.7 1.9 1.9 0.4 0.5
coverage on c-alumina corresponding to a LaMnO3 fraction of about 50 wt %. XRD analysis of catalysts treated at 800 8C shows the presence of the c phase, whereas treatment at 1100 8C gives rise to the transition to the h and b phases. High specific surface areas, comparable to that of the support, are shown
by samples treated at 800 8C. A decrease was observed after the treatment at 1100 8C. Therefore, the presence of LaMnO3 does not affect the thermal behavior of La2O3/Al2O3 up to 800 8C, whereas a promoting effect on the transition to the b and h phases can be supposed after the treatment at 1100 8C. The TPR profiles of the Mn3O4 and Mn3O4-Al2O3 samples treated at 600 8C are reported in Fig. 1. Two peaks, slightly superimposed, with the maximum at 385 and 518 8C, respectively, clearly appear in the TPR curve of Mn3O4. The XRD spectrum taken on this sample after the TPR experiment shows only the signal of the MnO phase, suggesting that manganese is completely reduced to the 2` oxidation state. Taking into account this result, an average initial oxidation state of manganese of 3` can be evaluated from H2 consumption (Table 2), then higher than that expected from the stoichiometry of the compound. This suggests that the oxidation to Mn2O3 can occur during the pretreatment, in agreement with the literature data reporting that Mn3O4 undergoes the transition to Mn2O3 in the oxidizing atmosphere at about 600 8C [11]. Therefore, the low temperature signal could be related to the reduction of Mn2O3 to Mn3O4, as also suggested by the H2 uptake of the first peak (about 0.17 mol mol11), corresponding to a variation of the oxidation state of about 0.3. A further confirmation was also given by the XRD analysis effected on the sample after the first TPR peak, showing the signals of Mn3O4 phase. However, the presence of a fraction of Mn4` in manganese oxides cannot be excluded on the base of the literature data [12]. The high temperature signal of the TPR profile of the Mn3O4 sample could be related to the reduction to MnO. Mn3O4 gives rise to a quite different TPR signal when alumina is present in the catalyst composition. Preliminary measurement carried out on pure cAl2O3 showed that it is not reduced under the same conditions of TPR experiments; therefore, the H2 uptake of the mixed sample can be supposed as a
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Fig. 2. TPR profiles of supported perovskites catalysts.
Fig. 3. O2 TPD profiles of Mn2O3 and Mn2O3-Al2O3 catalysts.
contribution of the Mn3O4 fraction only. The TPR curve of the Mn3O4-Al2O3 sample shows a complex signal starting at about 170 8C resulting from at least three contributions, the main signal, with the maximum at 570 8C, being absent in the pure Mn3O4 sample. XRD analysis effected on the sample after TPR measurement indicates the reduction to MnO.
The H2 consumption (Table 1) is close to 0.5, suggesting that the presence of alumina does not affect the average initial oxidation state of manganese in comparison with the Mn3O4 sample. TPR results can be explained by supposing that some Mn3O4 interacts with alumina. The low and medium temperature signals, occurring at the same temperature observed for Mn3O4 but with lower intensity, could be related to the reduction of the fraction of Mn3O4 that did not interact with alumina. The high temperature signal can be attributed to the reduction of manganese reacted with alumina. These results suggest that the interaction between manganese oxide and alumina can modify the redox properties of manganese, leading to the formation of Mn3` more stable towards reduction. In Fig. 2, TPR profiles of supported perovskites are reported. They are due only to the contribution of perovskite phase, because it was verified in preliminary measurements that stabilized alumina does not undergo reduction up to 800 8C. All samples show TPR profiles extending in a wide range of temperature due to the superimposition of two peaks. Reduction starts at about 150 8C for all catalysts, and the maximum of the peaks ranges from 420 to 455 8C. A shoulder at lower temperature, about 300 8C, is evident in all TPR curves. The results of TPR measurements can be interpreted supposing the presence of a fraction of Mn4` and the complete reduction of manganese to Mn2`, in agreement with the behavior of unsupported perovskites [5,6]. Moreover, the interaction of Mn with alumina leading to the stabilization of Mn2` can be supposed, this effect being enhanced by the temperature treatment [13,14]. The presence of two signals in TPR profiles, the first one appearing as a shoulder, could be due to the reduction Mn4` → Mn3`, and the subsequent reduction Mn3` → Mn2`, as reported for unsupported perovskites [15,16]. Therefore, the presence of a fraction of Mn4` can be hypothesized in all samples. The H2 uptake (Table 1), and consequently the manganese average initial oxidation state, decreases by increasing LaMnO3 content, mainly for catalysts treated at 1100 8C, suggesting the presence of some Mn2`, which reduces the average oxidation state to values lower than 3` in some cases. This effect is expected to increase with the LaMnO3 content and with the pretreatment temperature. After the TPR experiments, spinel oxides and supported perovskites were treated in airflow at 600 and 800 8C, respectively, and reduced again under the same conditions of the first experiments. The results, exactly comparable to the first experiments, suggested that catalysts undergo a reversible reduction. O2 desorption starts at 120 and 80 8C for the Mn3O4 and Mn3O4-Al2O3 samples, respectively (Fig. 3). Mn3O4 shows a peak with a maximum at
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
Fig. 4. CH4 conversion as a function of the temperature for Mn2O3 and Mn2O3-Al2O3 catalysts (dotted lines represent calculated curves).
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chemically bonded O2 can be evolved from both samples. In the Mn3O4-Al2O3 sample, the O2 desorption can occur from the pure Mn3O4 phase and from the solid solution formed by the interaction of manganese oxide with alumina, which can give rise to the signal at 182 8C absent in the spectrum of Mn3O4. Otherwise, the O2 desorption occurring at high temperature could be due to the reduction of some surface manganese that in the Mn3O4-Al2O3 sample could be present either in the solid solution and in the Mn3O4 phase. Therefore, the enhancement of the O2 evolution at high temperature in the Mn3O4-Al2O3 sample is probably related to the reduction of Mn3` formed by the interaction of manganese oxide with alumina. After the O2 TPD experiments, the samples were treated in airflow at 600 8C. The results, exactly comparable to those already described, suggest that the catalysts undergo reversible oxygen evolution as well. The O2 TPD experiments carried out on the supported perovskites show no detectable desorption up to 800 8C. The literature data report significant desorption from LaMnO3 unsupported perovskites [5,16]. Two kinds of oxygen species with different bonding strength were assumed on the surface of La–Mn perovskites: weakly bonded oxygen related to the presence of oxygen vacancies and oxygen ascribed to the reduction of tetravalent manganese to lower valences evolved at high temperature [5]. The absence of detectable O2 desorption signals in TPD curves of supported perovskites could be explained by the low content of perovskite phase in these samples. Catalytic Activity Measurements
Fig. 5. CH4 conversion as a function of the temperature for supported perovskites catalysts (dotted lines represent calculated curves).
244 8C superimposed to a wide signal at higher temperature. Mn3O4-Al2O3 shows a peak at 182 8C, absent in the spectrum of the Mn3O4 sample, and a more intense signal at higher temperature with a shoulder at about 360 8C. The amount of O2 (Table 1) suggests that surface-adsorbed O2 rather than
Preliminary tests, performed under the same conditions of the catalytic tests, but without catalyst, showed that homogeneous reactions are negligible under the experimental conditions investigated. The results of the catalytic tests on the Mn3O4 and Mn3O4-Al2O3 samples are reported in Fig. 4. The catalytic activity of c-Al2O3 pretreated at 600 8C was also evaluated under the same conditions and was found negligible up to 500 8C. The catalysts show comparable activity, and complete conversion is reached within 600 8C. One hundred percent selectivity to CO2 was observed for both samples. Mn3O4Al2O3 appears slightly more active than Mn3O4, giving 50% CH4 conversion at temperature of 470 8C, about 10 8C lower than that observed for Mn3O4. The results of the catalytic tests on supported perovskites are reported in Fig. 5. The activity of stabilized alumina treated at 800 and 1100 8C, tested in preliminary experiments, was found negligible up to 600 8C. All supported perovskites give complete conversion within 650 8C with 100% selectivity to
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CO2. Catalysts treated at 800 8C show activity increasing with LaMnO3 content. The temperature corresponding to the 50% conversion decreases from 520 to 500 8C, when the perovskite content increases from 10 to 20 wt %. The catalysts treated at 1100 8C are less active then those treated at 800 8C and the CH4 conversion appears not influenced by perovskite content. The temperature corresponding to the 50% conversion is about 550 8C, thus higher than that observed for catalysts treated at 800 8C. After a first cycle of tests, all catalysts were cooled down to room temperature, and a new cycle of experiments was performed. The results of the second cycle were the same of the first one, suggesting that catalysts do not undergo any modification or deactivation under the reaction conditions. From the preceding results, spinel oxides appear slightly more active than supported perovskites at low temperature. Moreover, supported perovskites treated at 1100 8C are less active than those treated at 800 8C. To compare the catalysts on the base of kinetic parameters, the catalytic activity data were elaborated assuming a methane first-order rate equation and supposing that the reactor behaves as a PFR. The conversions data, reported in Figs. 4 and 5, are fitted by the equation containing the estimated parameters. The values of the estimated activation energy and preexponential factor are reported in Table 2. The supported perovskites treated at 800 8C show the same activation energy value. This is within the range of literature data of unsupported LaMnO3 perovskites [17], suggesting that the same mechanism is promoted both by supported and unsupported perovskites. The preexponential factors are proportional to the perovskite content, suggesting that the nature of the supported active phase does not change by increasing the surface coverage, in agreement with a monolayer model. The supported perovskites treated at 1100 8C show the same activation energy value as that observed for samples treated at 800 8C. Both catalysts show the same values of the preexponential factors notwithstanding the different perovskite content. This result suggests that only a fraction of manganese in LaMn 20-1100 catalyst is available for the reaction, the remaining fraction being inactive. This effect could be related to the interaction of Mn with alumina, promoted by the temperature, with consequent stabilization of a fraction of Mn2`, as supposed from results of the TPR experiments. The fraction of Mn2` that is not involved in the reduction process could be not catalytically active toward the methane oxidation. Mn3O4 and Mn3O4-Al2O3 show the same activation energy value, suggesting that the catalysts promote the same reaction mechanism. Thus, the presence of alumina does not modify the nature of the
active phase. The preexponential factors have similar values, notwithstanding the lower percentage of Mn3O4 is in the alumina-containing sample, probably because of the higher dispersion of manganese oxide in the Mn3O4-Al2O3 sample. Manganese spinel oxides show an activation energy lower than that of supported perovskites, due to their higher activity at low temperature. This could be explained taking into account their ability to adsorb oxygen at low temperature. The reaction mechanisms proposed for metal oxide–based catalysts suppose that both adsorbed oxygen and lattice oxygen are active toward methane oxidation, the adsorbed oxygen being more active at lower temperature [3]. The catalysts can be also compared on the base of preexponential factors referred to the surface in order to obtain information on the surface activity. This comparison, however, cannot be extended to the Mn3O4-Al2O3 sample due to the uncertainty in evaluating the dispersion of manganese oxide on the alumina surface. An active surface of supported perovskites corresponding to about 20 and 40 m2 g11 for 10 and 20 wt % perovskite content, respectively, can be estimated assuming a monolayer coverage model. The preexponential factors of samples treated at 800 8C have, as expected, the same value (1.2 2 107 l h11 m12). LaMn 10-1100 show slightly lower value (1.0 2 107 l h11 m12), suggesting a surface activity comparable to that of the samples treated at 800 8C. However, a markedly lower preexponential factor was evaluated for LaMn 20-1100 (0.5 2 107 l h11 m12), probably because of the interaction of the LaMnO3 phase with the support leading to the inactivity of a fraction of LaMnO3, as discussed earlier. Finally, the preexponential factor of Mn3O4 is about one order of magnitude lower than those of supported perovskites (0.1 2 106 l h11 m12), suggesting a lower surface concentration of active sites.
Conclusions LaMnO3 perovskite supported on La2O3-stabilized alumina with high thermal resistance showed a uniform dispersion of the active phase that was retained even after treatment at 1100 8C. The presence of a fraction of Mn4` besides Mn3` was supposed. Redox properties similar to those of unsupported perovskites were observed, with evidence of a reversible reduction of manganese to a 2` oxidation state. These properties could be involved in methane oxidation. The catalytic activity of the supported perovskites was comparable to that of the unsupported LaMnO3 perovskites, notwithstanding the low perovskite content, due to the high dispersion of the active phase. However, the interaction of manganese with alumina, promoted by the
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
thermal treatment at 1100 8C, made a fraction of LaMnO3 inactive toward the reaction. The knowledge of the influence of severe thermal treatments on the interactions of perovskite phase with alumina deserves further investigation. Acknowledgments The authors acknowledge Dr. Giovanni Bagnasco for his precious contribution during the discussion of the results and Mrs. Clelia Zucchini and Mr. Sabatino Russo for the microanalyses.
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7. Twu, J. and Gallagher, P. K., in Properties and Application of Perovskite-type Oxioles (L. G. Tejuca and J. L. G. Fierro, eds.), Marcel Dekker, New York, 1993, pp. 1–23. 8. Arai, H. and Machida, M., Appl. Catal., A 138:161– 176 (1996). 9. Schaper, H., Doesburg, E. B. M., and van Raijen, L. L., Appl. Catal. 7:211–220 (1983). 10. Church, J. S., Cant, N. W., and Trimm, D. L., Appl. Catal., A 10:105–116 (1993). 11. Arnone, S., Bagnasco, G., Busca, G., Lisi, L., Russo, G., Turco, M., in Studies in Surface Science and Catalysis, vol. 119 (A. Parmaliana, D. Sanfilippo, F. Frusteri, A. Vaccari, and F. Arena, eds.), Elsevier Science B. V., Amsterdam, 1998, pp. 65–70. 12. Dorris, S. E. and Mason, T. O., J. Chem. Ceram. Soc. 71(5):379–385 (1988). 13. Prasad, R., Kennedy, L. A., and Ruckenstein, E., Combust. Sci. Technol. 22:271–280 (1980). 14. Machida, M., Eguchi, K., and Arai, H., J. Catal. 123:477–485 (1990). 15. Rojas, M. L., Fierro, J. L. G., Tejuca, L. G., and Bell, A. T., J. Catal. 124:41–51 (1990). 16. Bagnasco, G., Ciambelli, P., Lisi, L., G., Russo, G., Turco, M., De Rossi, S., Faticanti, M., Minelli, G., Pettiti, I., and Porta, P., in Proceeding of TCC-97 Tecnologie Chimiche Compatibili, XII Congresso Nazionale di Chimica Industriale e Simposio di Catalisi, Giardini Naxos, 22–25, June, 1997, pp. 203–206. 17. Arai, H., Yanmada, T., Eguchi, K., and Seiyama, T., Appl. Catal. 26:265–276 (1986).
CATALYTIC COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITE SUPPORTED ON La2O3 STABILIZED ALUMINA. A COMPARATIVE STUDY WITH Mn3O4, Mn3O4-Al2O3 SPINEL OXIDES
S. ARNONE,1 G. BUSCA,2 L. LISI,3 F. MILELLA,2, G. RUSSO3 and M. TURCO1 1Dipartimento
di Ingegneria Chimica, di Ricerche sulla Combustione, CNR Universita` di Napoli Federico II Naples, Italy 2Istituto di Chimica Facolta` di Ingegneria Universita` di Genova Genoa, Italy
3Istituto
Ten and 20 wt % LaMnO3 perovskites supported on La2O3-stabilized c-Al2O3 were studied for catalytic combustion of methane. A comparison with Mn3O4 and Mn3O4-Al2O3 spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O2 temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T 4 300–800 8C, space velocity 4 40000 h11, CH4 concentration 4 0.4% v/v, O2 concentration 4 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of c-alumina was retained after the treatment at 800 8C, the treatment at 1100 8C led to the transition to the h and b phases. TPR measurements suggested the presence of a fraction of Mn4` in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn2`, led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O2 desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH4 conversion within 650 8C with 100% selectivity to CO2. The activation energy value, evaluated from a methane firstorder rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO3. The preexponential factors of the catalysts treated at 800 8C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100 8C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO3 is available for the reaction. This was related to the stabilization of a fraction of Mn2`, probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600 8C with 100% selectivity to CO2. The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O2 was hypothesized.
Introduction Catalytic combustion is a useful technique for improving combustion efficiency and reducing NOx emissions in several applications, like as gas turbines, boilers, aircrafts, afterburners, domestic heaters, and volatile organic compound (VOC) removal [1,2]. The main goal of the research on catalytic combustion is to obtain materials with strong thermal resistance retaining high surface areas even at high temperature [3]. Manganese-based oxides showed the best performances among catalysts proposed for methane combustion. Manganese spinel-type oxides are able to catalyze the methane combustion even at
low temperature; however, they easily decompose at high temperature [4]. Lanthanum-manganese-based perovskites are active at higher temperature and heat resistant [5,6]. Nevertheless, their application is limited by the low specific surface areas. Some recent studies were devoted to obtain perovskites with high surface area by dispersion on suitable refractory support materials [3,7]. In this paper, LaMnO3 perovskites supported on La2O3 modified alumina were studied. The performances of these materials were compared with Mn3O4 spinel oxide to clarify the role of manganese ions in activating methane oxidation.
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Experimental Sample preparation Mn3O4 was prepared by precipitation from an aqueous solution of Mn(CH3COO)2. Al(NO3)3 with 1:2 Al/Mn ratio at pH 9.5 was added to obtained Mn3O4-Al2O3. In both cases, the solution was stirred overnight at room temperature, and the powder was dried 12 h at 110 8C and then calcined at 600 8C. Stabilized alumina used as support for perovskitebased catalysts was prepared by wet impregnation of high purity Al2O3 CK 300 supplied by AKZO Chemie with a La(NO3)3 aqueous solution. The dried material was calcined 3 h at 300 8C. Supported perovskites were prepared by adding the stabilized alumina to an aqueous solution of (CH3COO)2Mn and La(NO3)3 with 1:1 La/Mn ratio. The solution was stirred overnight at room temperature, and the dried powder (110 8C) was calcined 3 h at 800 8C or at 1100 8C. The samples contained 10 and 20 wt % LaMnO3, respectively, and will be referred to with the following codes according to the perovskite nominal content (weight percentage) and calcination temperature: LaMn 10-800, LaMn 20800, LaMn 10-1100 and LaMn 20-1100. Physicochemical Characterization X-ray diffraction (XRD) analysis was performed using a Philips PW 1710 diffractometer. Specific surface area of the catalysts was evaluated by N2 adsorption at 77 K according to the Brunauer–Emmett–Teller (BET) method using a Carlo Erba 1900 Sorptomatic apparatus. Energy-dispersive spectromety (EDS) microanalysis was performed by an EDAX instrument. TPR experiments were carried out in a Micromeritics 2900 temperature-programmed desorption (TPD)/TPR flow system equipped with a thermal conductivity detector (TCD). After a treatment in airflow at 600 8C, for spinel oxides, or at 800 8C for perovskites oxides, the sample was reduced with a 2% H2/Ar mixture (25 cm3 min11) heating 10 8C min11 up to 600 or 800 8C. TPD of O2 experiments were performed in the same apparatus described for TPR tests and the catalyst was pretreated under the same conditions. After pretreatment the sample was heated 10 8C min11 up to 600 or 800 8C in pure helium stream (25 cm3 min11). Catalytic Combustion of Methane Catalytic combustion of methane was studied in a fixed-bed quartz microreactor. The catalyst (particle size 4 300–400 lm), diluted 1:10 in quartz powder, was placed on a fritted disk. Quartz pellets upside
the catalytic bed and a narrowing of the reactor section both in the post and in the precatalytic zone reduced the homogeneous volume. The feed composition was 0.4% CH4 and 10% O2 in a balance of N2. A constant space velocity of 40000 h11 was ensured by Brooks 5850 TR Series mass flow controllers. The concentration of reactants and products was measured using a Hewlett-Packard 6890 gas chromatograph equipped with two capillary columns (poraplot Q and a molecular sieve 5A) and thermal conductivity and flame ionization detectors. Carbon balance was verified within 55%. Catalytic tests were carried out in the temperature range 300– 600 8C for spinel oxides and 300–800 8C for supported perovskite oxides after a 2 h treatment at 600 and 800 8C, respectively. Results and Discussion Physicochemical characterization XRD spectrum of the Mn3O4 sample shows the typical signals of the spinel phase. Mn3O4-Al2O3 shows the signals of both the Mn3O4 and c-Al2O3 phases. Specific surface area of Mn3O4-Al2O3 is higher than pure Mn3O4 (Table 1) due to the contribution of c-alumina. Samples of La2O3 stabilized alumina were treated at 800 and 1100 8C to compare the thermal behavior of the support with that of the supported perovskites treated at the same temperature. The addition of La2O3 inhibits the transition to a-Al2O3, leading to the formation of the b and h phases in the temperature range 1000–1200 8C [8–10]. XRD analysis of La2O3 stabilized alumina shows that the c-phase is still present after treatment at 800 8C and that traces of h phase appear after a treatment at 1100 8C. In agreement with these results, high specific surface area values are retained even after a treatment at 1100 8C (Table 1). These results indicate that under our experimental condition, the b phase is not yet formed. XRD analysis of supported perovskites shows the absence of a segregate perovskite phase, in agreement with the results of microanalysis showing a uniform composition of the samples. These results indicate a uniform dispersion of the active phase on the surface of the support, suggesting that the monolayer coverage is not complete. A rough evaluation of the perovskite content corresponding to the monolayer coverage can be obtained on the base of the unit cell dimensions of LaMnO3 perovskite (a 4 b 4 5.5 A˚, c 4 13.3 A˚, ASTM 32-484). If the ab face of the exhagonal structure of LaMnO3 [6] is exposed, the surface area would be 107 m2 g11; otherwise, if the ac face is exposed, the surface area would be 303 m2 g11. Thus, an average value of about 200 m2 g11 can be hypothesized as monolayer
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
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TABLE 1 Specific surface area of samples, hydrogen uptake in the TPR experiments, and oxygen desorption in TPD experiments Sample
Surface Area (m2 g11)
H2 Uptake (mol H2 mol11 Mn)
O2 Desorption 2 1011 (mol O2 cm12)
152 118 177 137 74 42 33 95
— — 0.56 0.46 0.52 0.22 0.53 0.47
— — — — — — 2.4 4.2
stab. Al2O3 (800 8C) stab. Al2O3 (1100 8C) LaMnO3 10–800 LaMnO3 20–800 LaMnO3 10–1100 LaMnO3 20–1100 Mn3O4 Mn3O4–Al2O3
Fig. 1. TPR profiles of Mn2O3 and Mn2-Al2O3 catalysts.
TABLE 2 Activation energy (Ea) and preexponential factor (A) values.
Catalyst LaMnO3 10–800 LaMnO3 20–800 LaMnO3 10–1100 LaMnO3 20–1100 Mn3O4 Mn3O4–Al2O3
Ea 2 1013 (Kcal mol11)
A 2 1018 (l h11 g11)
24 24 24 24 20 20
2.5 4.7 1.9 1.9 0.4 0.5
coverage on c-alumina corresponding to a LaMnO3 fraction of about 50 wt %. XRD analysis of catalysts treated at 800 8C shows the presence of the c phase, whereas treatment at 1100 8C gives rise to the transition to the h and b phases. High specific surface areas, comparable to that of the support, are shown
by samples treated at 800 8C. A decrease was observed after the treatment at 1100 8C. Therefore, the presence of LaMnO3 does not affect the thermal behavior of La2O3/Al2O3 up to 800 8C, whereas a promoting effect on the transition to the b and h phases can be supposed after the treatment at 1100 8C. The TPR profiles of the Mn3O4 and Mn3O4-Al2O3 samples treated at 600 8C are reported in Fig. 1. Two peaks, slightly superimposed, with the maximum at 385 and 518 8C, respectively, clearly appear in the TPR curve of Mn3O4. The XRD spectrum taken on this sample after the TPR experiment shows only the signal of the MnO phase, suggesting that manganese is completely reduced to the 2` oxidation state. Taking into account this result, an average initial oxidation state of manganese of 3` can be evaluated from H2 consumption (Table 2), then higher than that expected from the stoichiometry of the compound. This suggests that the oxidation to Mn2O3 can occur during the pretreatment, in agreement with the literature data reporting that Mn3O4 undergoes the transition to Mn2O3 in the oxidizing atmosphere at about 600 8C [11]. Therefore, the low temperature signal could be related to the reduction of Mn2O3 to Mn3O4, as also suggested by the H2 uptake of the first peak (about 0.17 mol mol11), corresponding to a variation of the oxidation state of about 0.3. A further confirmation was also given by the XRD analysis effected on the sample after the first TPR peak, showing the signals of Mn3O4 phase. However, the presence of a fraction of Mn4` in manganese oxides cannot be excluded on the base of the literature data [12]. The high temperature signal of the TPR profile of the Mn3O4 sample could be related to the reduction to MnO. Mn3O4 gives rise to a quite different TPR signal when alumina is present in the catalyst composition. Preliminary measurement carried out on pure cAl2O3 showed that it is not reduced under the same conditions of TPR experiments; therefore, the H2 uptake of the mixed sample can be supposed as a
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CATALYTIC COMBUSTION
Fig. 2. TPR profiles of supported perovskites catalysts.
Fig. 3. O2 TPD profiles of Mn2O3 and Mn2O3-Al2O3 catalysts.
contribution of the Mn3O4 fraction only. The TPR curve of the Mn3O4-Al2O3 sample shows a complex signal starting at about 170 8C resulting from at least three contributions, the main signal, with the maximum at 570 8C, being absent in the pure Mn3O4 sample. XRD analysis effected on the sample after TPR measurement indicates the reduction to MnO.
The H2 consumption (Table 1) is close to 0.5, suggesting that the presence of alumina does not affect the average initial oxidation state of manganese in comparison with the Mn3O4 sample. TPR results can be explained by supposing that some Mn3O4 interacts with alumina. The low and medium temperature signals, occurring at the same temperature observed for Mn3O4 but with lower intensity, could be related to the reduction of the fraction of Mn3O4 that did not interact with alumina. The high temperature signal can be attributed to the reduction of manganese reacted with alumina. These results suggest that the interaction between manganese oxide and alumina can modify the redox properties of manganese, leading to the formation of Mn3` more stable towards reduction. In Fig. 2, TPR profiles of supported perovskites are reported. They are due only to the contribution of perovskite phase, because it was verified in preliminary measurements that stabilized alumina does not undergo reduction up to 800 8C. All samples show TPR profiles extending in a wide range of temperature due to the superimposition of two peaks. Reduction starts at about 150 8C for all catalysts, and the maximum of the peaks ranges from 420 to 455 8C. A shoulder at lower temperature, about 300 8C, is evident in all TPR curves. The results of TPR measurements can be interpreted supposing the presence of a fraction of Mn4` and the complete reduction of manganese to Mn2`, in agreement with the behavior of unsupported perovskites [5,6]. Moreover, the interaction of Mn with alumina leading to the stabilization of Mn2` can be supposed, this effect being enhanced by the temperature treatment [13,14]. The presence of two signals in TPR profiles, the first one appearing as a shoulder, could be due to the reduction Mn4` → Mn3`, and the subsequent reduction Mn3` → Mn2`, as reported for unsupported perovskites [15,16]. Therefore, the presence of a fraction of Mn4` can be hypothesized in all samples. The H2 uptake (Table 1), and consequently the manganese average initial oxidation state, decreases by increasing LaMnO3 content, mainly for catalysts treated at 1100 8C, suggesting the presence of some Mn2`, which reduces the average oxidation state to values lower than 3` in some cases. This effect is expected to increase with the LaMnO3 content and with the pretreatment temperature. After the TPR experiments, spinel oxides and supported perovskites were treated in airflow at 600 and 800 8C, respectively, and reduced again under the same conditions of the first experiments. The results, exactly comparable to the first experiments, suggested that catalysts undergo a reversible reduction. O2 desorption starts at 120 and 80 8C for the Mn3O4 and Mn3O4-Al2O3 samples, respectively (Fig. 3). Mn3O4 shows a peak with a maximum at
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
Fig. 4. CH4 conversion as a function of the temperature for Mn2O3 and Mn2O3-Al2O3 catalysts (dotted lines represent calculated curves).
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chemically bonded O2 can be evolved from both samples. In the Mn3O4-Al2O3 sample, the O2 desorption can occur from the pure Mn3O4 phase and from the solid solution formed by the interaction of manganese oxide with alumina, which can give rise to the signal at 182 8C absent in the spectrum of Mn3O4. Otherwise, the O2 desorption occurring at high temperature could be due to the reduction of some surface manganese that in the Mn3O4-Al2O3 sample could be present either in the solid solution and in the Mn3O4 phase. Therefore, the enhancement of the O2 evolution at high temperature in the Mn3O4-Al2O3 sample is probably related to the reduction of Mn3` formed by the interaction of manganese oxide with alumina. After the O2 TPD experiments, the samples were treated in airflow at 600 8C. The results, exactly comparable to those already described, suggest that the catalysts undergo reversible oxygen evolution as well. The O2 TPD experiments carried out on the supported perovskites show no detectable desorption up to 800 8C. The literature data report significant desorption from LaMnO3 unsupported perovskites [5,16]. Two kinds of oxygen species with different bonding strength were assumed on the surface of La–Mn perovskites: weakly bonded oxygen related to the presence of oxygen vacancies and oxygen ascribed to the reduction of tetravalent manganese to lower valences evolved at high temperature [5]. The absence of detectable O2 desorption signals in TPD curves of supported perovskites could be explained by the low content of perovskite phase in these samples. Catalytic Activity Measurements
Fig. 5. CH4 conversion as a function of the temperature for supported perovskites catalysts (dotted lines represent calculated curves).
244 8C superimposed to a wide signal at higher temperature. Mn3O4-Al2O3 shows a peak at 182 8C, absent in the spectrum of the Mn3O4 sample, and a more intense signal at higher temperature with a shoulder at about 360 8C. The amount of O2 (Table 1) suggests that surface-adsorbed O2 rather than
Preliminary tests, performed under the same conditions of the catalytic tests, but without catalyst, showed that homogeneous reactions are negligible under the experimental conditions investigated. The results of the catalytic tests on the Mn3O4 and Mn3O4-Al2O3 samples are reported in Fig. 4. The catalytic activity of c-Al2O3 pretreated at 600 8C was also evaluated under the same conditions and was found negligible up to 500 8C. The catalysts show comparable activity, and complete conversion is reached within 600 8C. One hundred percent selectivity to CO2 was observed for both samples. Mn3O4Al2O3 appears slightly more active than Mn3O4, giving 50% CH4 conversion at temperature of 470 8C, about 10 8C lower than that observed for Mn3O4. The results of the catalytic tests on supported perovskites are reported in Fig. 5. The activity of stabilized alumina treated at 800 and 1100 8C, tested in preliminary experiments, was found negligible up to 600 8C. All supported perovskites give complete conversion within 650 8C with 100% selectivity to
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CATALYTIC COMBUSTION
CO2. Catalysts treated at 800 8C show activity increasing with LaMnO3 content. The temperature corresponding to the 50% conversion decreases from 520 to 500 8C, when the perovskite content increases from 10 to 20 wt %. The catalysts treated at 1100 8C are less active then those treated at 800 8C and the CH4 conversion appears not influenced by perovskite content. The temperature corresponding to the 50% conversion is about 550 8C, thus higher than that observed for catalysts treated at 800 8C. After a first cycle of tests, all catalysts were cooled down to room temperature, and a new cycle of experiments was performed. The results of the second cycle were the same of the first one, suggesting that catalysts do not undergo any modification or deactivation under the reaction conditions. From the preceding results, spinel oxides appear slightly more active than supported perovskites at low temperature. Moreover, supported perovskites treated at 1100 8C are less active than those treated at 800 8C. To compare the catalysts on the base of kinetic parameters, the catalytic activity data were elaborated assuming a methane first-order rate equation and supposing that the reactor behaves as a PFR. The conversions data, reported in Figs. 4 and 5, are fitted by the equation containing the estimated parameters. The values of the estimated activation energy and preexponential factor are reported in Table 2. The supported perovskites treated at 800 8C show the same activation energy value. This is within the range of literature data of unsupported LaMnO3 perovskites [17], suggesting that the same mechanism is promoted both by supported and unsupported perovskites. The preexponential factors are proportional to the perovskite content, suggesting that the nature of the supported active phase does not change by increasing the surface coverage, in agreement with a monolayer model. The supported perovskites treated at 1100 8C show the same activation energy value as that observed for samples treated at 800 8C. Both catalysts show the same values of the preexponential factors notwithstanding the different perovskite content. This result suggests that only a fraction of manganese in LaMn 20-1100 catalyst is available for the reaction, the remaining fraction being inactive. This effect could be related to the interaction of Mn with alumina, promoted by the temperature, with consequent stabilization of a fraction of Mn2`, as supposed from results of the TPR experiments. The fraction of Mn2` that is not involved in the reduction process could be not catalytically active toward the methane oxidation. Mn3O4 and Mn3O4-Al2O3 show the same activation energy value, suggesting that the catalysts promote the same reaction mechanism. Thus, the presence of alumina does not modify the nature of the
active phase. The preexponential factors have similar values, notwithstanding the lower percentage of Mn3O4 is in the alumina-containing sample, probably because of the higher dispersion of manganese oxide in the Mn3O4-Al2O3 sample. Manganese spinel oxides show an activation energy lower than that of supported perovskites, due to their higher activity at low temperature. This could be explained taking into account their ability to adsorb oxygen at low temperature. The reaction mechanisms proposed for metal oxide–based catalysts suppose that both adsorbed oxygen and lattice oxygen are active toward methane oxidation, the adsorbed oxygen being more active at lower temperature [3]. The catalysts can be also compared on the base of preexponential factors referred to the surface in order to obtain information on the surface activity. This comparison, however, cannot be extended to the Mn3O4-Al2O3 sample due to the uncertainty in evaluating the dispersion of manganese oxide on the alumina surface. An active surface of supported perovskites corresponding to about 20 and 40 m2 g11 for 10 and 20 wt % perovskite content, respectively, can be estimated assuming a monolayer coverage model. The preexponential factors of samples treated at 800 8C have, as expected, the same value (1.2 2 107 l h11 m12). LaMn 10-1100 show slightly lower value (1.0 2 107 l h11 m12), suggesting a surface activity comparable to that of the samples treated at 800 8C. However, a markedly lower preexponential factor was evaluated for LaMn 20-1100 (0.5 2 107 l h11 m12), probably because of the interaction of the LaMnO3 phase with the support leading to the inactivity of a fraction of LaMnO3, as discussed earlier. Finally, the preexponential factor of Mn3O4 is about one order of magnitude lower than those of supported perovskites (0.1 2 106 l h11 m12), suggesting a lower surface concentration of active sites.
Conclusions LaMnO3 perovskite supported on La2O3-stabilized alumina with high thermal resistance showed a uniform dispersion of the active phase that was retained even after treatment at 1100 8C. The presence of a fraction of Mn4` besides Mn3` was supposed. Redox properties similar to those of unsupported perovskites were observed, with evidence of a reversible reduction of manganese to a 2` oxidation state. These properties could be involved in methane oxidation. The catalytic activity of the supported perovskites was comparable to that of the unsupported LaMnO3 perovskites, notwithstanding the low perovskite content, due to the high dispersion of the active phase. However, the interaction of manganese with alumina, promoted by the
COMBUSTION OF METHANE OVER LaMnO3 PEROVSKITES
thermal treatment at 1100 8C, made a fraction of LaMnO3 inactive toward the reaction. The knowledge of the influence of severe thermal treatments on the interactions of perovskite phase with alumina deserves further investigation. Acknowledgments The authors acknowledge Dr. Giovanni Bagnasco for his precious contribution during the discussion of the results and Mrs. Clelia Zucchini and Mr. Sabatino Russo for the microanalyses.
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