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N2O decomposition in the presence of ammonia on faujasite-supported metal catalysts
Applied Catalysis B: Environmental 23 (1999) L79–L82
Letter
N2 O decomposition in the presence of ammonia on faujasite-supported metal catalysts Mathias Mauvezin a , Gérard Delahay a,∗ , Bernard Coq a , Stéphane Kieger b a
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS; ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex, France b Grande Paroisse S.A., Usine de Rouen, rue de l’Industrie – BP 204, 76121, Le Grand-Quevilly Cedex, France Received 7 March 1999; received in revised form 10 June 1999; accepted 10 June 1999
Abstract The reduction of N2 O to N2 was carried out by temperature programmed surface reaction (TPSR) from 473 to 873 K in He, He + 3% O2 , He + 0.02% NH3 and He + 3% O2 + 0.02% NH3 on H–FAU supported transition metal catalysts. In the absence of oxygen, the addition of NH3 considerably shifts the N2 O conversion profile towards lower temperatures. This is particularly true for the Ru– and Rh–FAU which exhibit the onset of N2 O conversion around 500 K. The addition of O2 inhibits this effect for most of the catalysts and NO formation due to NH3 oxidation is observed. However Fe–, Ni– and Co–FAU kept the best catalytic behaviour for N2 O reduction to N2 in the presence of O2 , when NH3 was added to the feed with respect to the decomposition reaction. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Nitrous oxide; Ammonia; Reduction; Zeolite FAU
1. Introduction Nitrous oxide (N2 O) is now considered as an environmental pollutant. The increase in atmospheric N2 O concentrations, mainly caused by anthropogenic activities [1], takes part in the destruction of stratospheric ozone and contributes to the greenhouse effect. Besides automotive exhaust, N2 O emissions come from some combustion processes and the nitric and adipic acid plants. Catalytic decomposition, and may be, selective catalytic reduction (SCR) are potential tech-
∗ Correspoding author. Tel.: +33-467-1443-93; fax: +33-467-1443-49 E-mail address: [email protected] (G. Delahay)
nologies for the abatement of these emissions. The former reaction has been studied intensively [1–14] and Rh–MFI seems to be most active although the Fe–FER catalyst has been recently claimed to be so [15]. N2 O is an efficient oxidant of hydrocarbons (e.g. benzene to phenol) and SCR by hydrocarbons is, therefore, a promising mode too [16]. On the other hand, Aika and Oshihara [17] have recently reported the promoting effect of NH3 on the conversion of N2 O to N2 in excess of oxygen over Co–MgO catalysts. This short report was thus aimed at showing the effect of ammonia on the reduction of N2 O to N2 on different FAU-supported noble and base metal catalysts. For this goal, the decomposition and reaction of N2 O with NH3 , either in the absence or in the presence of O2 , were studied on these materials.
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 7 2 - 7
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2. Experimental Metal–FAU catalysts were prepared by impregnating 2 g HFAU (PQ zeolite CBV 610, Si/Al = 2.7, SBET = 563 m2 g−1 ) with 15 ml of the acetylacetonate metal complex in chloroform solution. After drying at 353 K, the solid was first treated in He at 573 K and then calcined in air at 773 K. The metal loading (in wt.%) in the zeolite H–FAU were the following: Rh: 0.34 wt.%, Ru: 0.57 wt.%, Pd: 1.65 wt.%, Co: 4.02 wt.%; Cu: 4.35 wt.%, Ni: 4.53 wt.% and Fe: 4.42 wt.%. Before the catalytic experiments, 0.130 g of the catalyst was activated in situ at 773 K in air. After cooling the sample at 473 K under air, catalytic tests were carried out in temperature programmed surface reaction. Three catalytic experiments were carried out by decreasing the temperature by a step of 25 K, from 870 to 450 K (Co–FAU, Ni–FAU and Rh–FAU). The reaction mixture contained 0.2 vol% N2 O, 0 or 3.0 vol% O2 , 0 or 0.2% NH3 , balance with He. The total flow rate was 90 cm3 min−1 (space velocity: 30,000 h−1 ) and the temperature increased from 473 to 773 K (ramp: 5 K min−1 ). The composition of the effluents was monitored continuously by sampling on-line to a quadrupole mass spectrometer Balzers QMS421 equipped with an SEM detector (0–200 amu).
3. Results and discussion 3.1. N2 O decomposition The decomposition of N2 O in the absence and in the presence of O2 on metal–FAU catalysts is shown in Fig. 1 as a function of temperature. The data on Pd–FAU and Ni–FAU are not shown because these materials did not exhibit any catalytic activity. The examination of Fig. 1 allows us to put the catalysts in the sequence of decreasing activity (the values in brackets stand for the light-off temperature at 50% N2 O conversion): in the absence of O2 , Ru–FAU (638 K) > Rh–FAU (707 K) ≫ Fe–FAU (824 K) ≈ Co–FAU (828 K) > Cu–FAU (850 K) in the presence of 3% O2 , Ru–FAU (679 K) > Rh– FAU (716 K) ≫ Fe–FAU (828 K) > Co–FAU (852 K) > Cu– FAU (870 K)
Fig. 1. Decomposition of N2 O without (open symbol) or in the presence of O2 (full symbol) over M–FAU; M = Ru (—䊊—, · · · 䊉· · · ); Rh (—4—, · · · N· · · ), Fe (—5—, · · · H· · · ); Co (——, · · · 䉬· · · ); Cu (—䊐—, · · · 䊏· · · ). Conditions: 0.2 vol% N2 O, 0 or 3.0 vol% O2 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
In the presence of oxygen, the N2 O conversion profiles of Ru–FAU, Co–FAU and Cu–FAU are shifted towards higher temperatures by ca. 41, 24 and 20 K for Ru, Co and Cu, respectively. In contrast, the addition of oxygen does not significantly alter N2 O conversion on Rh–FAU and Fe–FAU. The higher reactivity exhibited by Ru–FAU and Rh–FAU catalysts is in agreement with the literature data [6,8,11,14]. N2 O decomposition has been studied in detail and the mechanism is generally considered as an oxidation-reduction cycle in which N2 O reacts on partially reduced sites, or oxygen vacancies, to yield N2 and adsorbed oxygen. In turn, the desorption of oxygen closes the catalytic cycle. This step, generally considered as rate determining, is easier on noble metal based catalysts than on the base metal ones. In this frame, it is obvious that the presence of O2 has an inhibiting effect on this step, more or less pronounced depending on the metal active phase. 3.2. N2 O reduction in the presence of NH3 and without O2 It is worthy to note that, in the course of these catalytic tests, neither NO nor NO2 were detected. The addition of NH3 to the N2 O/He mixture induces a
M. Mauvezin et al. / Applied Catalysis B: Environmental 23 (1999) L79–L82
Fig. 2. Reduction of N2 O by NH3 over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
very large shift of the N2 O conversion curves towards lower temperatures, and this is particularly true for catalysts which contain noble metals (Fig. 2). In this respect, Ru–FAU and Rh–FAU exhibit a similar profile of N2 O conversion with the light-off temperature at ca. 500 K, lower by 90 and 160 K, respectively, compared to N2 O decomposition. On the other hand, the Pd–, Ni–, Fe–, Cu– and Co–FAU catalysts show a peculiar shape of N2 O conversion profiles with a first wave which peaked between 600 and 670 K (Fig. 3). This behaviour was not a transient phenomenon due to the TPSR protocol and the conversion profiles obtained by decreasing back the temperature, from 870 to 450 K, superimpose nicely with the ascending profiles. Two explanations can be proposed tentatively to interpret the behaviour: 1. Two different mechanisms might operate: (i) the coupling between adsorbed NH3 and N2 O in the low temperature range, and (ii) an oxidation-reduction cycle at higher temperature where N2 O is decomposed to N2 and Oads . on a ‘reduced site’, which is in turn regenerated by NH3 . 2. Two different sites are active, depending on the temperature. It was indeed reported that the SCR of NO by NH3 in the presence of O2 on Cu–FAU samples also exhibits two waves of NO reduction to N2 as a function of temperature [18]. It was pro-
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Fig. 3. Reduction of N2 O by NH3 in the presence of oxygen over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 3.0 vol% O2 , 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
posed that Cu species located in the supercages and stabilised by NH3 ligands are very active at low temperature. These species migrate to the sodalite cavities upon desorption of NH3 ligands at high temperature, and become less active. A similar phenomenon may occur in the reaction of N2 O with NH3 on Pd–, Ni–, Fe–, Cu– and Co–FAU catalysts. Works are in progress to elucidate this point. 3.3. N2 O reduction in the presence of both NH3 and O2 In the presence of both NH3 and O2 , the profiles of N2 O conversion are clearly shifted back to high temperature (Fig. 3) and a positive effect of ammonia on N2 O conversion with respect to N2 O decomposition in oxygen is observed only for Fe–, Pd– and Ni–FAU catalysts. On the other hand, significant amounts of NO are formed in the course of the reaction (Fig. 4), very likely due to ammonia oxidation. The examination of Fig. 4 allows us to put the metals in the sequence of decreasing activity for NH3 oxidation: Ru, Pd, Rh ≈> Co, Ni, Cu > Fe A similar order of activity for ammonia oxidation has been reported by Ismagilov et al. [19]. Therefore,
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on which the conversion of N2 O is the lowest in the presence of NH3 and O2 (Fig. 5). 4. Conclusion
Fig. 4. NO formation during the reduction of N2 O by NH3 in the presence of oxygen over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 3.0 vol% O2 , 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
Fig. 5. Comparison of the different reactions on Rh–FAU: N2 O (—䊐—), N2 O/O2 (—䊏—), N2 O/NH3 (—䊉—), N2 O/NH3 /O2 (—H—), NH3 /O2 (—䊊—). Conditions: 0 or 0.2 vol% N2 O, 0 or 3.0 vol% O2 , 0 or 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
the low activity for N2 O removal of metal–FAU in the presence of NH3 and O2 can be accounted for in part by the consumption of ammonia in the side reaction of ammonia oxidation leading to NO, but it is also likely due to N2 O and N2 formation [20]. Moreover, this may explain the inhibition of N2 O decomposition by the presence of ammonia. This is illustrated for Rh–FAU
In the absence of oxygen, the addition of NH3 considerably enhances N2 O conversion to N2 , and does so more especially for the Ru– and Rh–FAU catalysts. The addition of O2 inhibits this effect and NO formation due to NH3 oxidation is observed for all the catalysts. Among the various catalysts studied, Fe–FAU is the material which presents the best behaviour in the reduction of N2 O to N2 in the presence of oxygen and NH3 with respect to the decomposition reaction.
References [1] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Appl. Catal. B-Environ. 9 (1996) 25. [2] S. Akbar, R.W. Joyner, J. Chem. Soc. Faraday Trans. 177 (1981) 803. [3] L.M. Aparicio, M.A. Ulla, W.S. Millman, J.A. Dumesic, J. Catal. 110 (1988) 330. [4] G. Centi, A. Galli, B. Montanari, S. Perathoner, A. Vaccari, Catal. Today 35 (1997) 113. [5] Y.F. Chang, J.G. Mccarty, E.D. Wachsman, V.L. Wong, Appl. Catal. B-Environ. 4 (1994) 283. [6] Y.F. Chang, J.G. Mccarty, Y.L. Zhang, Catal. Lett. 34 (1995) 163. [7] J. Leglise, J.O. Petunchi, W.K. Hall, J. Catal. 86 (1984) 392. [8] Y.J. Li, J.N. Armor, Appl. Catal. B-Environ. 1 (1992) L21. [9] Y.J. Li, J.N. Armor, Appl. Catal. B-Environ. 13 (1997) 131. [10] F. Kapteijn, G. Marban, J. Rodriguez-Mirasol, J.A. Moulijn, J. Catal. 167 (1997) 256. [11] K. Yuzaki, T. Yarimizu, S. Ito, K. Kunimori, Catal. Lett. 47 (1997) 173. [12] C.M. Decorrea, A.L. Villa, M. Zapata, Catal. Lett. 38 (1996) 27. [13] C.M. Fu, V.N. Korchak, W.K. Hall, J. Catal. 68 (1981) 166. [14] S. Imamura, T. Kitao, H. Hanai, S. Shono, K. Utani, H. Jindai, React. Kinet. Catal. Lett. 61 (1997) 201. [15] C. Hamon, K. Malefant, B. Neveu, French demand patent, Fr 97.16803, 1997. [16] C. Pophal, T. Yogo, K. Tanabe, K. Segawa, Catal. Lett. 44 (1997) 271. [17] K. Aika, K. Oshihara, Catal. Today 29 (1996) 123. [18] S. Kieger, G. Delahay, B. Neveu, B. Coq, J. Catal. 183 (1999) 267. [19] Z.R. Ismagilov, M.A. Kerzhentsev, T.L. Susharina, Russ. Chem. Rev. 59 (1990) 973–988. [20] L. Singoredjo, R. Korver, F. Kapteijn, J.A. Moulijn, Appl. Catal. B-Environ. 3 (1994) 173.
Letter
N2 O decomposition in the presence of ammonia on faujasite-supported metal catalysts Mathias Mauvezin a , Gérard Delahay a,∗ , Bernard Coq a , Stéphane Kieger b a
Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS; ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex, France b Grande Paroisse S.A., Usine de Rouen, rue de l’Industrie – BP 204, 76121, Le Grand-Quevilly Cedex, France Received 7 March 1999; received in revised form 10 June 1999; accepted 10 June 1999
Abstract The reduction of N2 O to N2 was carried out by temperature programmed surface reaction (TPSR) from 473 to 873 K in He, He + 3% O2 , He + 0.02% NH3 and He + 3% O2 + 0.02% NH3 on H–FAU supported transition metal catalysts. In the absence of oxygen, the addition of NH3 considerably shifts the N2 O conversion profile towards lower temperatures. This is particularly true for the Ru– and Rh–FAU which exhibit the onset of N2 O conversion around 500 K. The addition of O2 inhibits this effect for most of the catalysts and NO formation due to NH3 oxidation is observed. However Fe–, Ni– and Co–FAU kept the best catalytic behaviour for N2 O reduction to N2 in the presence of O2 , when NH3 was added to the feed with respect to the decomposition reaction. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Nitrous oxide; Ammonia; Reduction; Zeolite FAU
1. Introduction Nitrous oxide (N2 O) is now considered as an environmental pollutant. The increase in atmospheric N2 O concentrations, mainly caused by anthropogenic activities [1], takes part in the destruction of stratospheric ozone and contributes to the greenhouse effect. Besides automotive exhaust, N2 O emissions come from some combustion processes and the nitric and adipic acid plants. Catalytic decomposition, and may be, selective catalytic reduction (SCR) are potential tech-
∗ Correspoding author. Tel.: +33-467-1443-93; fax: +33-467-1443-49 E-mail address: [email protected] (G. Delahay)
nologies for the abatement of these emissions. The former reaction has been studied intensively [1–14] and Rh–MFI seems to be most active although the Fe–FER catalyst has been recently claimed to be so [15]. N2 O is an efficient oxidant of hydrocarbons (e.g. benzene to phenol) and SCR by hydrocarbons is, therefore, a promising mode too [16]. On the other hand, Aika and Oshihara [17] have recently reported the promoting effect of NH3 on the conversion of N2 O to N2 in excess of oxygen over Co–MgO catalysts. This short report was thus aimed at showing the effect of ammonia on the reduction of N2 O to N2 on different FAU-supported noble and base metal catalysts. For this goal, the decomposition and reaction of N2 O with NH3 , either in the absence or in the presence of O2 , were studied on these materials.
0926-3373/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 9 9 ) 0 0 0 7 2 - 7
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2. Experimental Metal–FAU catalysts were prepared by impregnating 2 g HFAU (PQ zeolite CBV 610, Si/Al = 2.7, SBET = 563 m2 g−1 ) with 15 ml of the acetylacetonate metal complex in chloroform solution. After drying at 353 K, the solid was first treated in He at 573 K and then calcined in air at 773 K. The metal loading (in wt.%) in the zeolite H–FAU were the following: Rh: 0.34 wt.%, Ru: 0.57 wt.%, Pd: 1.65 wt.%, Co: 4.02 wt.%; Cu: 4.35 wt.%, Ni: 4.53 wt.% and Fe: 4.42 wt.%. Before the catalytic experiments, 0.130 g of the catalyst was activated in situ at 773 K in air. After cooling the sample at 473 K under air, catalytic tests were carried out in temperature programmed surface reaction. Three catalytic experiments were carried out by decreasing the temperature by a step of 25 K, from 870 to 450 K (Co–FAU, Ni–FAU and Rh–FAU). The reaction mixture contained 0.2 vol% N2 O, 0 or 3.0 vol% O2 , 0 or 0.2% NH3 , balance with He. The total flow rate was 90 cm3 min−1 (space velocity: 30,000 h−1 ) and the temperature increased from 473 to 773 K (ramp: 5 K min−1 ). The composition of the effluents was monitored continuously by sampling on-line to a quadrupole mass spectrometer Balzers QMS421 equipped with an SEM detector (0–200 amu).
3. Results and discussion 3.1. N2 O decomposition The decomposition of N2 O in the absence and in the presence of O2 on metal–FAU catalysts is shown in Fig. 1 as a function of temperature. The data on Pd–FAU and Ni–FAU are not shown because these materials did not exhibit any catalytic activity. The examination of Fig. 1 allows us to put the catalysts in the sequence of decreasing activity (the values in brackets stand for the light-off temperature at 50% N2 O conversion): in the absence of O2 , Ru–FAU (638 K) > Rh–FAU (707 K) ≫ Fe–FAU (824 K) ≈ Co–FAU (828 K) > Cu–FAU (850 K) in the presence of 3% O2 , Ru–FAU (679 K) > Rh– FAU (716 K) ≫ Fe–FAU (828 K) > Co–FAU (852 K) > Cu– FAU (870 K)
Fig. 1. Decomposition of N2 O without (open symbol) or in the presence of O2 (full symbol) over M–FAU; M = Ru (—䊊—, · · · 䊉· · · ); Rh (—4—, · · · N· · · ), Fe (—5—, · · · H· · · ); Co (——, · · · 䉬· · · ); Cu (—䊐—, · · · 䊏· · · ). Conditions: 0.2 vol% N2 O, 0 or 3.0 vol% O2 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
In the presence of oxygen, the N2 O conversion profiles of Ru–FAU, Co–FAU and Cu–FAU are shifted towards higher temperatures by ca. 41, 24 and 20 K for Ru, Co and Cu, respectively. In contrast, the addition of oxygen does not significantly alter N2 O conversion on Rh–FAU and Fe–FAU. The higher reactivity exhibited by Ru–FAU and Rh–FAU catalysts is in agreement with the literature data [6,8,11,14]. N2 O decomposition has been studied in detail and the mechanism is generally considered as an oxidation-reduction cycle in which N2 O reacts on partially reduced sites, or oxygen vacancies, to yield N2 and adsorbed oxygen. In turn, the desorption of oxygen closes the catalytic cycle. This step, generally considered as rate determining, is easier on noble metal based catalysts than on the base metal ones. In this frame, it is obvious that the presence of O2 has an inhibiting effect on this step, more or less pronounced depending on the metal active phase. 3.2. N2 O reduction in the presence of NH3 and without O2 It is worthy to note that, in the course of these catalytic tests, neither NO nor NO2 were detected. The addition of NH3 to the N2 O/He mixture induces a
M. Mauvezin et al. / Applied Catalysis B: Environmental 23 (1999) L79–L82
Fig. 2. Reduction of N2 O by NH3 over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
very large shift of the N2 O conversion curves towards lower temperatures, and this is particularly true for catalysts which contain noble metals (Fig. 2). In this respect, Ru–FAU and Rh–FAU exhibit a similar profile of N2 O conversion with the light-off temperature at ca. 500 K, lower by 90 and 160 K, respectively, compared to N2 O decomposition. On the other hand, the Pd–, Ni–, Fe–, Cu– and Co–FAU catalysts show a peculiar shape of N2 O conversion profiles with a first wave which peaked between 600 and 670 K (Fig. 3). This behaviour was not a transient phenomenon due to the TPSR protocol and the conversion profiles obtained by decreasing back the temperature, from 870 to 450 K, superimpose nicely with the ascending profiles. Two explanations can be proposed tentatively to interpret the behaviour: 1. Two different mechanisms might operate: (i) the coupling between adsorbed NH3 and N2 O in the low temperature range, and (ii) an oxidation-reduction cycle at higher temperature where N2 O is decomposed to N2 and Oads . on a ‘reduced site’, which is in turn regenerated by NH3 . 2. Two different sites are active, depending on the temperature. It was indeed reported that the SCR of NO by NH3 in the presence of O2 on Cu–FAU samples also exhibits two waves of NO reduction to N2 as a function of temperature [18]. It was pro-
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Fig. 3. Reduction of N2 O by NH3 in the presence of oxygen over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 3.0 vol% O2 , 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
posed that Cu species located in the supercages and stabilised by NH3 ligands are very active at low temperature. These species migrate to the sodalite cavities upon desorption of NH3 ligands at high temperature, and become less active. A similar phenomenon may occur in the reaction of N2 O with NH3 on Pd–, Ni–, Fe–, Cu– and Co–FAU catalysts. Works are in progress to elucidate this point. 3.3. N2 O reduction in the presence of both NH3 and O2 In the presence of both NH3 and O2 , the profiles of N2 O conversion are clearly shifted back to high temperature (Fig. 3) and a positive effect of ammonia on N2 O conversion with respect to N2 O decomposition in oxygen is observed only for Fe–, Pd– and Ni–FAU catalysts. On the other hand, significant amounts of NO are formed in the course of the reaction (Fig. 4), very likely due to ammonia oxidation. The examination of Fig. 4 allows us to put the metals in the sequence of decreasing activity for NH3 oxidation: Ru, Pd, Rh ≈> Co, Ni, Cu > Fe A similar order of activity for ammonia oxidation has been reported by Ismagilov et al. [19]. Therefore,
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on which the conversion of N2 O is the lowest in the presence of NH3 and O2 (Fig. 5). 4. Conclusion
Fig. 4. NO formation during the reduction of N2 O by NH3 in the presence of oxygen over M–FAU; M = Ru (—䊉—); Rh (—N—), Pd (· · · ×· · · ); Ni (· · · · · · ); Fe (—H—); Co (—䉬—); Cu (—䊏—). Conditions: 0.2 vol% N2 O, 3.0 vol% O2 , 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
Fig. 5. Comparison of the different reactions on Rh–FAU: N2 O (—䊐—), N2 O/O2 (—䊏—), N2 O/NH3 (—䊉—), N2 O/NH3 /O2 (—H—), NH3 /O2 (—䊊—). Conditions: 0 or 0.2 vol% N2 O, 0 or 3.0 vol% O2 , 0 or 0.2% NH3 , balance with He; total flow rate = 90 cm3 min−1 (space velocity: 30,000 h−1 ) and temperature ramp (5 K min−1 ) from 473 to 773 K.
the low activity for N2 O removal of metal–FAU in the presence of NH3 and O2 can be accounted for in part by the consumption of ammonia in the side reaction of ammonia oxidation leading to NO, but it is also likely due to N2 O and N2 formation [20]. Moreover, this may explain the inhibition of N2 O decomposition by the presence of ammonia. This is illustrated for Rh–FAU
In the absence of oxygen, the addition of NH3 considerably enhances N2 O conversion to N2 , and does so more especially for the Ru– and Rh–FAU catalysts. The addition of O2 inhibits this effect and NO formation due to NH3 oxidation is observed for all the catalysts. Among the various catalysts studied, Fe–FAU is the material which presents the best behaviour in the reduction of N2 O to N2 in the presence of oxygen and NH3 with respect to the decomposition reaction.
References [1] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Appl. Catal. B-Environ. 9 (1996) 25. [2] S. Akbar, R.W. Joyner, J. Chem. Soc. Faraday Trans. 177 (1981) 803. [3] L.M. Aparicio, M.A. Ulla, W.S. Millman, J.A. Dumesic, J. Catal. 110 (1988) 330. [4] G. Centi, A. Galli, B. Montanari, S. Perathoner, A. Vaccari, Catal. Today 35 (1997) 113. [5] Y.F. Chang, J.G. Mccarty, E.D. Wachsman, V.L. Wong, Appl. Catal. B-Environ. 4 (1994) 283. [6] Y.F. Chang, J.G. Mccarty, Y.L. Zhang, Catal. Lett. 34 (1995) 163. [7] J. Leglise, J.O. Petunchi, W.K. Hall, J. Catal. 86 (1984) 392. [8] Y.J. Li, J.N. Armor, Appl. Catal. B-Environ. 1 (1992) L21. [9] Y.J. Li, J.N. Armor, Appl. Catal. B-Environ. 13 (1997) 131. [10] F. Kapteijn, G. Marban, J. Rodriguez-Mirasol, J.A. Moulijn, J. Catal. 167 (1997) 256. [11] K. Yuzaki, T. Yarimizu, S. Ito, K. Kunimori, Catal. Lett. 47 (1997) 173. [12] C.M. Decorrea, A.L. Villa, M. Zapata, Catal. Lett. 38 (1996) 27. [13] C.M. Fu, V.N. Korchak, W.K. Hall, J. Catal. 68 (1981) 166. [14] S. Imamura, T. Kitao, H. Hanai, S. Shono, K. Utani, H. Jindai, React. Kinet. Catal. Lett. 61 (1997) 201. [15] C. Hamon, K. Malefant, B. Neveu, French demand patent, Fr 97.16803, 1997. [16] C. Pophal, T. Yogo, K. Tanabe, K. Segawa, Catal. Lett. 44 (1997) 271. [17] K. Aika, K. Oshihara, Catal. Today 29 (1996) 123. [18] S. Kieger, G. Delahay, B. Neveu, B. Coq, J. Catal. 183 (1999) 267. [19] Z.R. Ismagilov, M.A. Kerzhentsev, T.L. Susharina, Russ. Chem. Rev. 59 (1990) 973–988. [20] L. Singoredjo, R. Korver, F. Kapteijn, J.A. Moulijn, Appl. Catal. B-Environ. 3 (1994) 173.