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Substituted ferrite MxFe1 − xFe2O4 (M = Mn, Zn) catalysts for N2O catalytic decomposition processes
Catalysis Communications 26 (2012) 194–198
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Substituted ferrite MxFe1 − xFe2O4 (M = Mn, Zn) catalysts for N2O catalytic decomposition processes Rachid Amrousse ⁎, Toshiyuki Katsumi JAXA, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-Ku, Sagamihara, Kanagawa 252-5210, Japan
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 29 May 2012 Accepted 31 May 2012 Available online 7 June 2012 Keywords: Catalytic decomposition N2O Substituted ferrite catalysts
a b s t r a c t The N2O catalytic decomposition to N2 and O2 as gas products was carried out on the substituted ferrite MxFe1 − xFe2O4 (M= Mn and Zn with x = 0 − 0.8) catalysts. The obtained results showed that the partial substitution of Fe by Mn and Zn metal transitions in Fe3O4 spinel oxide led to a significant improvement in the catalytic activity for the N2O decomposition. Moreover, the catalytic activity depended on the degree of Fe replacement by Mn or Zn. The N2O conversion reached 100% over the Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts at 250 and 280 °C for pure N2O, respectively. The decomposition temperatures were relatively increased after O2 addition. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide (N2O) is a compound that during the last decade has been recognized as a potential contributor to the destruction of the ozone in the stratosphere and acknowledged as a relatively strong greenhouse gas [1,2]. The continuous increase of its concentration, both due to natural and anthropogenic sources (adipic acid production, nitric acid production, fossil fuels and biomass burning) and longer atmospheric residence time: 150 years, entails the need of developing efficient catalysts for its decomposition into nitrogen and oxygen. The catalytic decomposition of N2O has been intensively studied over several catalysts [3–6]. However, the catalytic activity towards N2O decomposition would be significantly affected by various gases that coexist in real exhaust or flue gases. For instance, the presence of excess oxygen is one of the causes for catalyst inhibition [5]. In recent years, spinel-type oxides based on 3d transition metals have been the subject of increasing fundamental and applied research because of their catalytic properties [7,8] Spinels are represented by the chemical formula AB2O4, in which A ions are generally divalent cations occupying tetrahedral sites and B ions are trivalent cations in octahedral sites; this is the structure of most chromites. For certain spinel structures the cations may shift between the A and B sites. This may result in the general formula B(AB) O4: A and half of B in the octahedral sites, half of B in the tetrahedral sites. This is actually the most structure of ferrites. To further add complexity, a mixed spinel structure is also possible, with wide variation in
composition. The most general formula of mixed spinels can therefore be (A1 − xBy)(AxB2 − y)O4. Ferrites are a group of technologically important materials that are used in the fabrication of magnetic, electronic and microwave devices. They exhibit relatively high resistivity at carrier frequencies, sufficiently low losses for microwave application and a wide range of other electrical properties [9]. The nanoparticles of ferrites, such as spinel ferrites, possess great potential for applications since they are relatively inert and their magnetic properties can be fine-tuned by chemical manipulations [10]. In recent years, substituted ferrites with the spinel structure have been widely investigated due to their considerable importance to the electronic materials industry [11]. By introducing relatively small amount of foreign ions (Zn, Mg, Co etc.) the structural and magnetic properties can be modified in ferrites [12–14]. Because the magnetic saturation is dependent on the site location and the d-electron structure of the transition metal cations, it is possible to systematically alter the net magnetic moment by chemical substitutions [15]. In present work, we reported Fe3O4 magnetite or spinel oxide with partial substitution of Fe by Mn and Zn which could completely decompose N2O into oxygen at 250 and 280 °C in the absence and presence of 15 vol. % oxygen. 2. Experimental 2.1. Catalyst preparation
⁎ Corresponding author at: Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan. Tel.: + 81 50 3362 5956; fax: + 81 42 759 8284. E-mail address: [email protected] (R. Amrousse). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.05.024
Two series of spinel MnxFe1 − xFe2O4 and ZnxFe1 − xFe2O4 catalysts were prepared via a highly exothermic and self-sustaining reaction named solution combustion synthesis method (SCS) [16]. Particularly, a concentrated aqueous solution of various precursors (metal
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
nitrates and urea) was located in an oven at 650 °C under air for a few minutes to decompose the very fast reaction. Under these conditions, nucleation of metal oxide crystals is induced, their growth is limited and nano-sized grains can be obtained.
195
to know the nature of each possible product (e.g. N2O, NO, NO2, N2 and O2). The N2O conversions were determined by analyzing N2O feed concentrations before (bypass) and after passing through the catalyst bed. The conversion was calculated using the following equation: h
2.2. Catalyst characterization All samples were characterized by XRD (X-ray diffraction); mass 200 to 400 mg of finely ground powder was placed in a holder standard sample. This holder rotates vertically during measurement. Therefore, crystallites are oriented in a random manner and no direction of diffraction is preferred. The XRD patterns were recorded with Cu detector, λCu Kα = 1.5406 Å, over a 2θ range of 15–80° with a step of 0.02° and an acquisition time of 2 s. The specific surface area was obtained through BET (Brunauer, Emett and Teller) measurements by using a Micromeritics Flowsorb II apparatus: 6 h pre-treatment at 250 °C under nitrogen flow; N2 in He with P(N2) = 0.3 bar. The composition of the catalysts was determined by AAS with a WFX-10 atomic absorption spectrometer. The TPD (temperature programmed desorption) experiment was used: 50 mL min− 1 flow of 15 vol. % O2 in He rate, and was carried out in “MicroVision 2 TPD” equipped with mass spectrometer on-line analysis.
2.3. Catalytic decomposition tests The N2O catalytic decomposition were performed on a quartz reactor by passing a gaseous mixture of pure N2O (1000 ppm) or N2O +15 vol. % O2 mixtures in a preheated catalytic bed as shown in Fig. 1. The gas flows of mixtures were passed into a He flow rate (100 mL min− 1) over 500 mg of catalyst to get a space velocity GHSV (Gas Hourly Space Velocity) of 20.000 h− 1. Before catalytic tests, the reactor and the catalytic bed were preheated at 400 °C for 2 h by He+ 15 vol. % O2 mixture to remove organic compounds, as impurities, adsorbed on the surface of catalysts, then the temperature was decreased. To obtain a reasonable N2O catalytic conversion, the reaction system was kept for 30 min at each temperature process, then started to analyze ejected gas by on-line gas chromatograph (GC-MS-2010 Plus, Shimadzu) equipped with molecular sieve 5 Å and Porapak Q columns, and TCD detector (SRI 310 CG). Fragment masses m/z of different ejected gas follow us
i N2 Obypass −½N2 Oreaction ppm h ppm i Conversion ð% Þ ¼ N2 Obypass ppm
3. Results and discussion 3.1. Catalyst characterization Characterization results of prepared samples with different amounts of Fe replaced by Mn and Zn metal transitions are shown in Table 1. From these results, the BET surface area, measured by Micromeritics Flowsorb II, increased strongly from 52 to 115 m2 g− 1 after replacement of x = 0.4 of Fe by Mn and from 52 to 104 m2 g− 1 after substitution of x = 0.4 by Zn. Consequently, the BET surface area decreased when more Fe substituted with Mn and Zn contents. Fig. 2 shows the XRD patterns of Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 prepared samples whose Fe substituted by Mn and Zn, respectively. The (220), (311), (400), (422), (511), and (440) diffraction peaks observed at curves can be indexed to the cubic spinel structure, and all peaks are in good agreement with the Fe3O4 phase (JCPDS file 19-0629). Moreover, XRD patterns show that no deviation in related diffraction peaks when Mn and Zn cations incorporate into the Fe3O4 lattice. 3.2. N2O catalytic decomposition Fig. 3 presents N2O catalytic conversion results over different substituted MxFe1 − xFe2O4 catalysts at 250 °C. It shows that the partial substitution of Fe by both metal transitions Mn and Zn significantly promoted the catalytic performance for N2O (1000 ppm) decomposition into N2 and O2 gas phases; the N2O catalytic decomposition was carried out at atmospheric pressure under He flow with a space velocity GHSV = 20.000 h − 1. For MnxFe1 − xFe2O4 catalysts, N2O conversion increased strongly from 7 to 74% when the Mn degree increased from x = 0 to x = 0.2, then slowly increased to 100% as maximum
N2O flow meter
O2 flow meter
He flow meter Oven Catalyst bed
Quartz reactor
GC N2 O
TCD He Fig. 1. Scheme of the N2O catalytic decomposition facility.
O2
196
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
100
Table 1 Surface area, porous volume and pore diameter of substituted ferrite MxFe1 − xFe2O4 catalysts. Surface area (m2 g− 1)
Porous volume (cm3 g− 1)
Pore diameter (Å)
Fe3O4 Mn0.2Fe0.8Fe2O4 Mn0.4Fe0.6Fe2O4 Mn0.6Fe0.4Fe2O4 Mn0.8Fe0.2Fe2O4 Zn0.2Fe0.8Fe2O4 Zn0.4Fe0.6Fe2O4 Zn0.6Fe0.4Fe2O4 Zn0.8Fe0.2Fe2O4
52 102 115 108 67 89 104 95 73
0.08 0.35 0.37 0.36 0.11 0.27 0.31 0.28 0.24
64 127 142 137 98 75 125 111 87
80
N2O conversion (%)
Catalyst
Mn Zn
60
40
20
0 x=0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x=1
Substituted degree Fig. 3. N2O conversion to N2 and O2 gaseous phase at 250 °C over Fe3O4 magnetite with different degree of substituted Mn and Zn metal transitions.
Pure N2O over Fe3O4 N2O + 15 vol. % O2 over Fe3O4 Pure N2O over Mn0.8Fe0.2Fe2O4 N2O + 15 vol. % O2 over Mn0.8Fe0.2Fe2O4 Data 1 12:07:59 06/05/2012
100
N2O catalytic conversion (%)
conversion value at x = 0.8. When x was increased (x = 1), the catalytic activity for N2O decomposition decreased. On the other hand, the substituted ferrite ZnxFe1 − xFe2O4 catalysts were approximately similar to MnxFe1 − xFe2O4 samples when Zn degree increased from x = 0 to x = 0.6. However, the ZnxFe1 − xFe2O4 synthesized catalysts showed the highest activity at x = 0.6, then the catalytic activity gradually decreased with a maximum of Zn degree (x ≥ 0.8). To understand the substitution effect, Sundararajan and Srinivasan [17] reported their studies about N2O decomposition over ZnCo2O4 spinel oxide in a batch reactor system. However, their results showed that the catalytic activity of ZnCo2O4 for N2O decomposition is worse than that of pure Co3O4 spinel catalyst. The difference in catalytic activity for N2O decomposition is due to the difference of preparation method and post synthesis treatment of the precursor compounds. On the weight basis of catalyst, the ZnxCo1 − xCo2O4 catalysts gave much higher activity than that of pure Co3O4. To conclude, the catalytic activity of MnxFe1 − xFe2O4 and ZnxFe1 − xFe2O4 catalysts depended on the substituted degree of Fe by Mn and Zn metal transitions. When the partial replacement of Fe element is different, the best degree was x = 0.8 of Mn and x = 0.6 of Zn. The catalytic activities of the samples, in which Fe was almost substituted completely by Mn and Zn, were less than that with partial replacement. Indeed, the existence of Mn or Zn in magnetite spinel structure might have a cooperative effect with Fe to form best active sites of N2O catalytic decomposition. Therefore, our research has identified that the substituted ferrite MxFe1 − xFe2O4 catalysts are much more active than pure magnetite. Therefore, it is fundamentally interesting to study the substitution effect on the morphology and physico-chemical properties of the magnetite.
80
60
40
20
(a)
0 0
100
200
300
400
500
Temperature (°C) Pure N2O over Fe3O4 N2O + 15 vol. % O2 over Fe3O4 Pure N2O over Zn0.6Fe0.4Fe2O4 N2O + 151vol. % O2 over Zn0.6Fe0.4Fe2O4 Data 12:15:34 06/05/2012
Zn0.6Fe0.4Fe2O4
Mn0.8Fe0.2Fe2O4
Fe3O4
N2O catalytic conversion (%)
(440)
(511)
(422)
(400)
(311)
(220)
Intensity (a. u.)
(111)
100
80
60 40
20
(b)
0 0
20
30
40
50
60
70
80
2 θ (°) Fig. 2. XRD patterns of Fe3O4, substituted ferrite Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts.
100
200
300
400
500
Temperature (°C) Fig. 4. N2O catalytic conversion in different feed compositions: pure N2O (1000 ppm) and N2O + 15 vol. % O2 over: (a) Mn0.8Fe0.2Fe2O4 and Fe3O4, and (b) Zn0.6Fe0.4Fe2O4 and Fe3O4.
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
197
100
80
60 GHSV = 20000 h-1 GHSV = 40000 h-1 GHSV = 60000 h-1
40
20
(a)
0 0
The present results have demonstrated that the introduction of Mn and Zn into the spinel structure of magnetite Fe3O4 significantly promoted the catalytic activity of N2O decomposition. The catalytic
300
400
500
GHSV = 20000 h-1 GHSV = 40000 h-1 GHSV = 60000 h-1
80
60
40
20
(b)
0 0
4. Conclusion
200
100
3.4. Effect of the space velocity on N2O catalytic decomposition Fig. 6 gives the influence of GHSV on the N2O conversion at different temperatures over Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 selected catalysts. Both curves showed that the N2O catalytic conversion shifted to higher temperature at higher space velocity. The N2O conversion reached 100% over Mn0.8Fe0.2Fe2O4 at 250, 320 and 360 °C at 20.000, 40.000 and 60.000 h− 1 GHSV, respectively, although the conversion affected 100% in the presence of Zn0.6Fe0.4Fe2O4 at 280, 330 and 375 °C at the same range of the space velocity.
100
Temperature (°C)
N2O catalytic conversion (%)
The addition of 15 vol. % O2 often inhibited the decomposition reaction of N2O decomposition [18,19,4–6,20]. Fig. 4 presents N2O conversion for reaction systems at different temperatures over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 synthesized catalysts. Although pure Fe3O4 magnetite had very low activity in the present study of N2O catalytic decomposition, indicating that high conversion was obtained at high temperature, the introduction of Mn and Zn into the spinel structure of Fe3O4 led to important improvement in catalytic performance for N2O decomposition, indicating that high conversion was obtained at low temperature, making the N2O conversion shift ~ 150 °C to lower temperatures. The addition of 15 vol. %O2 inhibited the N2O decomposition process, which suggested that oxygen flow decelerate the N2O adsorption in the catalytic surface (active sites) after gaseous competition. The inhibition process of O2 for N2O decomposition over Mn0.8Fe0.2Fe2O4 catalyst is lower than that over Zn0.6Fe0.4Fe2O4, indicating that the adsorption of O2 on Zn0.6Fe0.4Fe2O4 catalyst is stronger than that on Mn0.8Fe0.2Fe2O4 catalyst. The obtained TPD profiles (Fig. 5) of desorbed O2 over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 samples showed an O2-desorption peak at 350 °C over Fe3O4 spinel oxide. Furthermore, a second intense peak of O2-desorption was observed at 120 °C over Mn0.8Fe0.2Fe2O4 and a lower signal at low temperature (50–150 °C range) over Zn0.6Fe0.4Fe2O4, which allow as to understand the inhibition difference of added oxygen on the catalytic performance of selected catalysts. Moreover, when the temperature increased, oxygen was desorbed from the active sites of samples, and then the conversion of N2O decomposition significantly increased.
N2O catalytic conversion (%)
3.3. Addition effect of O2 on N2O catalytic performance
100
200
300
400
500
Temperature (°C) Fig. 6. Conversion of N2O catalytic decomposition at different GHSV over: (a) Mn0.8Fe0.2Fe2O4 and (b) Zn0.6Fe0.4Fe2O4.
activity of MxFe1 − xFe2O4 (M = Mn and Zn) spinel oxides depended on the degree of Fe replacement. In this paper, partial substituted samples present the best behavior in the N2O catalytic decomposition; the conversion of N2O decomposition to N2 and O2 reached 100% at lower temperature in comparison with N2O decomposition over pure Fe3O4 magnetite. Although the addition of molecular oxygen to the feed gases inhibited the N2O decomposition reaction, N2O can also be decomposed completely to N2 and O2 at 300 and 350 °C in the presence of 15 vol. % O2 over Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4, respectively. Acknowledgments Dr. Rachid Amrousse would like to thank Japan Aerospace Exploration Agency for financial support.
Zn Fe Fe O
Intensity (a.u.)
0.6
0.4
2
4
Mn Fe Fe O 0.8
0.2
2
4
Fe O 3
0
100
200
300
400
500
4
600
Temperature (°C) Fig. 5. TPD profiles of O2 over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts.
References [1] J.C. Kramlich, W.P. Linak, Progress in Energy and Combustion Science 20 (1994) 149. [2] M.A. Wojtowicz, J.R. Pels, J.A. Moulijn, Fuel Processing Technology 34 (1993) 1. [3] J. Perez-Ramirez, F. Kapteijn, G. Mul, J.A. Moulijn, Chemical Communications (2001) 693. [4] G. Centi, A. Galli, B. Montanari, S. Perathoner, A. Vaccari, Catalysis Today 35 (1997) 113. [5] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Applied Catalysis B: Environmental 9 (1996) 25. [6] K. Yuzaki, T. Yarimizu, S. Ito, K. Kunimori, Catalysis Letters 47 (1997) 173. [7] R. Sundararajan, V. Srinivasan, Applied Catalysis 73 (1991) 165. [8] Y. Liang, R. Tong, W. Xiaolai, J. Dong, S. Jishuan, Applied Catalysis B: Environmental 45 (2003) 85. [9] A. Goldman, Modern Ferrite Technology, Marcel Dekker, New York, 1993. [10] A.B. Van Groenou, P.F. Bongers, A.L. Stuyts, Materials Science and Engineering 3 (1968/1969) 317.
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[11] W.R. Busseu, Powder Metallurgy International 6 (1974) 120. [12] C.H. Lin, P.C. Kuo, J.L. Pan, D.D.R. Huang, Journal of Applied Physics 79 (1996) 6035. [13] K.P. Thummer, H.H. Joshi, R.G. Kulkarni, Journal of Materials Science Letters 18 (1999) 1529. [14] S. Li, V.T. John, C. O'Connor, V. Harris, E. Carpenter, Journal of Applied Physics 87 (2000) 6223. [15] Kirk-Othmer, Encyclopedia of Chemical Technology, in: Maito, Grayson (Eds.), 1980, p. 881, (New York). [16] N. Russo, D. Fino, G. Saracco, V. Specchia, Journal of Catalysis 229 (2005) 459.
[17] R. Sundararajan, V. Srinivasan, in: B. Viswanathan, C.N. Pillai (Eds.), Recent Developments in Catalysis, Technip, Paris, 1992, pp. 381–389. [18] J. Pérez-ramirez, F. Kapteijn, G. Mul, J.A. Moulijn, Chemical Communications (2001) 693. [19] F. Kapteijn, G. Marban, J. Rodriguez-Mirasol, J.A. Moulijn, Journal of Catalysis 167 (1997) 256. [20] Y. Chang, J.G. McCarty, E.D. Wachsman, Applied Catalysis B: Environmental 6 (1995) 21.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Substituted ferrite MxFe1 − xFe2O4 (M = Mn, Zn) catalysts for N2O catalytic decomposition processes Rachid Amrousse ⁎, Toshiyuki Katsumi JAXA, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-Ku, Sagamihara, Kanagawa 252-5210, Japan
a r t i c l e
i n f o
Article history: Received 7 May 2012 Received in revised form 29 May 2012 Accepted 31 May 2012 Available online 7 June 2012 Keywords: Catalytic decomposition N2O Substituted ferrite catalysts
a b s t r a c t The N2O catalytic decomposition to N2 and O2 as gas products was carried out on the substituted ferrite MxFe1 − xFe2O4 (M= Mn and Zn with x = 0 − 0.8) catalysts. The obtained results showed that the partial substitution of Fe by Mn and Zn metal transitions in Fe3O4 spinel oxide led to a significant improvement in the catalytic activity for the N2O decomposition. Moreover, the catalytic activity depended on the degree of Fe replacement by Mn or Zn. The N2O conversion reached 100% over the Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts at 250 and 280 °C for pure N2O, respectively. The decomposition temperatures were relatively increased after O2 addition. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrous oxide (N2O) is a compound that during the last decade has been recognized as a potential contributor to the destruction of the ozone in the stratosphere and acknowledged as a relatively strong greenhouse gas [1,2]. The continuous increase of its concentration, both due to natural and anthropogenic sources (adipic acid production, nitric acid production, fossil fuels and biomass burning) and longer atmospheric residence time: 150 years, entails the need of developing efficient catalysts for its decomposition into nitrogen and oxygen. The catalytic decomposition of N2O has been intensively studied over several catalysts [3–6]. However, the catalytic activity towards N2O decomposition would be significantly affected by various gases that coexist in real exhaust or flue gases. For instance, the presence of excess oxygen is one of the causes for catalyst inhibition [5]. In recent years, spinel-type oxides based on 3d transition metals have been the subject of increasing fundamental and applied research because of their catalytic properties [7,8] Spinels are represented by the chemical formula AB2O4, in which A ions are generally divalent cations occupying tetrahedral sites and B ions are trivalent cations in octahedral sites; this is the structure of most chromites. For certain spinel structures the cations may shift between the A and B sites. This may result in the general formula B(AB) O4: A and half of B in the octahedral sites, half of B in the tetrahedral sites. This is actually the most structure of ferrites. To further add complexity, a mixed spinel structure is also possible, with wide variation in
composition. The most general formula of mixed spinels can therefore be (A1 − xBy)(AxB2 − y)O4. Ferrites are a group of technologically important materials that are used in the fabrication of magnetic, electronic and microwave devices. They exhibit relatively high resistivity at carrier frequencies, sufficiently low losses for microwave application and a wide range of other electrical properties [9]. The nanoparticles of ferrites, such as spinel ferrites, possess great potential for applications since they are relatively inert and their magnetic properties can be fine-tuned by chemical manipulations [10]. In recent years, substituted ferrites with the spinel structure have been widely investigated due to their considerable importance to the electronic materials industry [11]. By introducing relatively small amount of foreign ions (Zn, Mg, Co etc.) the structural and magnetic properties can be modified in ferrites [12–14]. Because the magnetic saturation is dependent on the site location and the d-electron structure of the transition metal cations, it is possible to systematically alter the net magnetic moment by chemical substitutions [15]. In present work, we reported Fe3O4 magnetite or spinel oxide with partial substitution of Fe by Mn and Zn which could completely decompose N2O into oxygen at 250 and 280 °C in the absence and presence of 15 vol. % oxygen. 2. Experimental 2.1. Catalyst preparation
⁎ Corresponding author at: Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan. Tel.: + 81 50 3362 5956; fax: + 81 42 759 8284. E-mail address: [email protected] (R. Amrousse). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2012.05.024
Two series of spinel MnxFe1 − xFe2O4 and ZnxFe1 − xFe2O4 catalysts were prepared via a highly exothermic and self-sustaining reaction named solution combustion synthesis method (SCS) [16]. Particularly, a concentrated aqueous solution of various precursors (metal
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
nitrates and urea) was located in an oven at 650 °C under air for a few minutes to decompose the very fast reaction. Under these conditions, nucleation of metal oxide crystals is induced, their growth is limited and nano-sized grains can be obtained.
195
to know the nature of each possible product (e.g. N2O, NO, NO2, N2 and O2). The N2O conversions were determined by analyzing N2O feed concentrations before (bypass) and after passing through the catalyst bed. The conversion was calculated using the following equation: h
2.2. Catalyst characterization All samples were characterized by XRD (X-ray diffraction); mass 200 to 400 mg of finely ground powder was placed in a holder standard sample. This holder rotates vertically during measurement. Therefore, crystallites are oriented in a random manner and no direction of diffraction is preferred. The XRD patterns were recorded with Cu detector, λCu Kα = 1.5406 Å, over a 2θ range of 15–80° with a step of 0.02° and an acquisition time of 2 s. The specific surface area was obtained through BET (Brunauer, Emett and Teller) measurements by using a Micromeritics Flowsorb II apparatus: 6 h pre-treatment at 250 °C under nitrogen flow; N2 in He with P(N2) = 0.3 bar. The composition of the catalysts was determined by AAS with a WFX-10 atomic absorption spectrometer. The TPD (temperature programmed desorption) experiment was used: 50 mL min− 1 flow of 15 vol. % O2 in He rate, and was carried out in “MicroVision 2 TPD” equipped with mass spectrometer on-line analysis.
2.3. Catalytic decomposition tests The N2O catalytic decomposition were performed on a quartz reactor by passing a gaseous mixture of pure N2O (1000 ppm) or N2O +15 vol. % O2 mixtures in a preheated catalytic bed as shown in Fig. 1. The gas flows of mixtures were passed into a He flow rate (100 mL min− 1) over 500 mg of catalyst to get a space velocity GHSV (Gas Hourly Space Velocity) of 20.000 h− 1. Before catalytic tests, the reactor and the catalytic bed were preheated at 400 °C for 2 h by He+ 15 vol. % O2 mixture to remove organic compounds, as impurities, adsorbed on the surface of catalysts, then the temperature was decreased. To obtain a reasonable N2O catalytic conversion, the reaction system was kept for 30 min at each temperature process, then started to analyze ejected gas by on-line gas chromatograph (GC-MS-2010 Plus, Shimadzu) equipped with molecular sieve 5 Å and Porapak Q columns, and TCD detector (SRI 310 CG). Fragment masses m/z of different ejected gas follow us
i N2 Obypass −½N2 Oreaction ppm h ppm i Conversion ð% Þ ¼ N2 Obypass ppm
3. Results and discussion 3.1. Catalyst characterization Characterization results of prepared samples with different amounts of Fe replaced by Mn and Zn metal transitions are shown in Table 1. From these results, the BET surface area, measured by Micromeritics Flowsorb II, increased strongly from 52 to 115 m2 g− 1 after replacement of x = 0.4 of Fe by Mn and from 52 to 104 m2 g− 1 after substitution of x = 0.4 by Zn. Consequently, the BET surface area decreased when more Fe substituted with Mn and Zn contents. Fig. 2 shows the XRD patterns of Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 prepared samples whose Fe substituted by Mn and Zn, respectively. The (220), (311), (400), (422), (511), and (440) diffraction peaks observed at curves can be indexed to the cubic spinel structure, and all peaks are in good agreement with the Fe3O4 phase (JCPDS file 19-0629). Moreover, XRD patterns show that no deviation in related diffraction peaks when Mn and Zn cations incorporate into the Fe3O4 lattice. 3.2. N2O catalytic decomposition Fig. 3 presents N2O catalytic conversion results over different substituted MxFe1 − xFe2O4 catalysts at 250 °C. It shows that the partial substitution of Fe by both metal transitions Mn and Zn significantly promoted the catalytic performance for N2O (1000 ppm) decomposition into N2 and O2 gas phases; the N2O catalytic decomposition was carried out at atmospheric pressure under He flow with a space velocity GHSV = 20.000 h − 1. For MnxFe1 − xFe2O4 catalysts, N2O conversion increased strongly from 7 to 74% when the Mn degree increased from x = 0 to x = 0.2, then slowly increased to 100% as maximum
N2O flow meter
O2 flow meter
He flow meter Oven Catalyst bed
Quartz reactor
GC N2 O
TCD He Fig. 1. Scheme of the N2O catalytic decomposition facility.
O2
196
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
100
Table 1 Surface area, porous volume and pore diameter of substituted ferrite MxFe1 − xFe2O4 catalysts. Surface area (m2 g− 1)
Porous volume (cm3 g− 1)
Pore diameter (Å)
Fe3O4 Mn0.2Fe0.8Fe2O4 Mn0.4Fe0.6Fe2O4 Mn0.6Fe0.4Fe2O4 Mn0.8Fe0.2Fe2O4 Zn0.2Fe0.8Fe2O4 Zn0.4Fe0.6Fe2O4 Zn0.6Fe0.4Fe2O4 Zn0.8Fe0.2Fe2O4
52 102 115 108 67 89 104 95 73
0.08 0.35 0.37 0.36 0.11 0.27 0.31 0.28 0.24
64 127 142 137 98 75 125 111 87
80
N2O conversion (%)
Catalyst
Mn Zn
60
40
20
0 x=0
x = 0.2
x = 0.4
x = 0.6
x = 0.8
x=1
Substituted degree Fig. 3. N2O conversion to N2 and O2 gaseous phase at 250 °C over Fe3O4 magnetite with different degree of substituted Mn and Zn metal transitions.
Pure N2O over Fe3O4 N2O + 15 vol. % O2 over Fe3O4 Pure N2O over Mn0.8Fe0.2Fe2O4 N2O + 15 vol. % O2 over Mn0.8Fe0.2Fe2O4 Data 1 12:07:59 06/05/2012
100
N2O catalytic conversion (%)
conversion value at x = 0.8. When x was increased (x = 1), the catalytic activity for N2O decomposition decreased. On the other hand, the substituted ferrite ZnxFe1 − xFe2O4 catalysts were approximately similar to MnxFe1 − xFe2O4 samples when Zn degree increased from x = 0 to x = 0.6. However, the ZnxFe1 − xFe2O4 synthesized catalysts showed the highest activity at x = 0.6, then the catalytic activity gradually decreased with a maximum of Zn degree (x ≥ 0.8). To understand the substitution effect, Sundararajan and Srinivasan [17] reported their studies about N2O decomposition over ZnCo2O4 spinel oxide in a batch reactor system. However, their results showed that the catalytic activity of ZnCo2O4 for N2O decomposition is worse than that of pure Co3O4 spinel catalyst. The difference in catalytic activity for N2O decomposition is due to the difference of preparation method and post synthesis treatment of the precursor compounds. On the weight basis of catalyst, the ZnxCo1 − xCo2O4 catalysts gave much higher activity than that of pure Co3O4. To conclude, the catalytic activity of MnxFe1 − xFe2O4 and ZnxFe1 − xFe2O4 catalysts depended on the substituted degree of Fe by Mn and Zn metal transitions. When the partial replacement of Fe element is different, the best degree was x = 0.8 of Mn and x = 0.6 of Zn. The catalytic activities of the samples, in which Fe was almost substituted completely by Mn and Zn, were less than that with partial replacement. Indeed, the existence of Mn or Zn in magnetite spinel structure might have a cooperative effect with Fe to form best active sites of N2O catalytic decomposition. Therefore, our research has identified that the substituted ferrite MxFe1 − xFe2O4 catalysts are much more active than pure magnetite. Therefore, it is fundamentally interesting to study the substitution effect on the morphology and physico-chemical properties of the magnetite.
80
60
40
20
(a)
0 0
100
200
300
400
500
Temperature (°C) Pure N2O over Fe3O4 N2O + 15 vol. % O2 over Fe3O4 Pure N2O over Zn0.6Fe0.4Fe2O4 N2O + 151vol. % O2 over Zn0.6Fe0.4Fe2O4 Data 12:15:34 06/05/2012
Zn0.6Fe0.4Fe2O4
Mn0.8Fe0.2Fe2O4
Fe3O4
N2O catalytic conversion (%)
(440)
(511)
(422)
(400)
(311)
(220)
Intensity (a. u.)
(111)
100
80
60 40
20
(b)
0 0
20
30
40
50
60
70
80
2 θ (°) Fig. 2. XRD patterns of Fe3O4, substituted ferrite Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts.
100
200
300
400
500
Temperature (°C) Fig. 4. N2O catalytic conversion in different feed compositions: pure N2O (1000 ppm) and N2O + 15 vol. % O2 over: (a) Mn0.8Fe0.2Fe2O4 and Fe3O4, and (b) Zn0.6Fe0.4Fe2O4 and Fe3O4.
R. Amrousse, T. Katsumi / Catalysis Communications 26 (2012) 194–198
197
100
80
60 GHSV = 20000 h-1 GHSV = 40000 h-1 GHSV = 60000 h-1
40
20
(a)
0 0
The present results have demonstrated that the introduction of Mn and Zn into the spinel structure of magnetite Fe3O4 significantly promoted the catalytic activity of N2O decomposition. The catalytic
300
400
500
GHSV = 20000 h-1 GHSV = 40000 h-1 GHSV = 60000 h-1
80
60
40
20
(b)
0 0
4. Conclusion
200
100
3.4. Effect of the space velocity on N2O catalytic decomposition Fig. 6 gives the influence of GHSV on the N2O conversion at different temperatures over Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 selected catalysts. Both curves showed that the N2O catalytic conversion shifted to higher temperature at higher space velocity. The N2O conversion reached 100% over Mn0.8Fe0.2Fe2O4 at 250, 320 and 360 °C at 20.000, 40.000 and 60.000 h− 1 GHSV, respectively, although the conversion affected 100% in the presence of Zn0.6Fe0.4Fe2O4 at 280, 330 and 375 °C at the same range of the space velocity.
100
Temperature (°C)
N2O catalytic conversion (%)
The addition of 15 vol. % O2 often inhibited the decomposition reaction of N2O decomposition [18,19,4–6,20]. Fig. 4 presents N2O conversion for reaction systems at different temperatures over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 synthesized catalysts. Although pure Fe3O4 magnetite had very low activity in the present study of N2O catalytic decomposition, indicating that high conversion was obtained at high temperature, the introduction of Mn and Zn into the spinel structure of Fe3O4 led to important improvement in catalytic performance for N2O decomposition, indicating that high conversion was obtained at low temperature, making the N2O conversion shift ~ 150 °C to lower temperatures. The addition of 15 vol. %O2 inhibited the N2O decomposition process, which suggested that oxygen flow decelerate the N2O adsorption in the catalytic surface (active sites) after gaseous competition. The inhibition process of O2 for N2O decomposition over Mn0.8Fe0.2Fe2O4 catalyst is lower than that over Zn0.6Fe0.4Fe2O4, indicating that the adsorption of O2 on Zn0.6Fe0.4Fe2O4 catalyst is stronger than that on Mn0.8Fe0.2Fe2O4 catalyst. The obtained TPD profiles (Fig. 5) of desorbed O2 over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 samples showed an O2-desorption peak at 350 °C over Fe3O4 spinel oxide. Furthermore, a second intense peak of O2-desorption was observed at 120 °C over Mn0.8Fe0.2Fe2O4 and a lower signal at low temperature (50–150 °C range) over Zn0.6Fe0.4Fe2O4, which allow as to understand the inhibition difference of added oxygen on the catalytic performance of selected catalysts. Moreover, when the temperature increased, oxygen was desorbed from the active sites of samples, and then the conversion of N2O decomposition significantly increased.
N2O catalytic conversion (%)
3.3. Addition effect of O2 on N2O catalytic performance
100
200
300
400
500
Temperature (°C) Fig. 6. Conversion of N2O catalytic decomposition at different GHSV over: (a) Mn0.8Fe0.2Fe2O4 and (b) Zn0.6Fe0.4Fe2O4.
activity of MxFe1 − xFe2O4 (M = Mn and Zn) spinel oxides depended on the degree of Fe replacement. In this paper, partial substituted samples present the best behavior in the N2O catalytic decomposition; the conversion of N2O decomposition to N2 and O2 reached 100% at lower temperature in comparison with N2O decomposition over pure Fe3O4 magnetite. Although the addition of molecular oxygen to the feed gases inhibited the N2O decomposition reaction, N2O can also be decomposed completely to N2 and O2 at 300 and 350 °C in the presence of 15 vol. % O2 over Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4, respectively. Acknowledgments Dr. Rachid Amrousse would like to thank Japan Aerospace Exploration Agency for financial support.
Zn Fe Fe O
Intensity (a.u.)
0.6
0.4
2
4
Mn Fe Fe O 0.8
0.2
2
4
Fe O 3
0
100
200
300
400
500
4
600
Temperature (°C) Fig. 5. TPD profiles of O2 over Fe3O4, Mn0.8Fe0.2Fe2O4 and Zn0.6Fe0.4Fe2O4 catalysts.
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