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Multifunctional catalyst for de-NOx processes: The selective reduction of NOx by methane
Catalysis Communications 8 (2007) 400–404 www.elsevier.com/locate/catcom
Multifunctional catalyst for de-NOx processes: The selective reduction of NOx by methane M.A. Go´mez-Garcı´a 1, Y. Zimmermann, V. Pitchon, A. Kiennemann
*
LMSPC, Laboratoire de Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, UMR 7515 du CNRS-ECPM-ULP, 25 Rue Becquerel, 67087 Strasbourg Cedex 2, France Received 8 February 2006; received in revised form 1 June 2006; accepted 27 June 2006 Available online 6 July 2006
Abstract A multifunctional catalyst has been applied for the reduction of NOx by methane in excess of oxygen. The catalyst includes a heteropolyacid (HPW) and a noble metal (Pt) deposited on a ceria–zirconia mixed oxide. The presence of hydrogen is necessary for reducing NOx into N2 with methane. During several alternate cycles of NOx storage (2 min) and reduction (1 min), the average NOx reduction percentage into nitrogen was of ca. 60% with a constant storage efficiency of ca.70%. A coherent picture of the catalytic process emerged from two separated experimental tests: water gas shift and steam methane reforming reactions. 2006 Elsevier B.V. All rights reserved. Keywords: NOx reduction; Multifunctional catalysts; Methane; HPW; Pt; Ceria–zirconia mixed oxides
1. Introduction Most of the studies about the catalytic reduction of NOx (NO and NO2) by hydrocarbons have been made since 1990. Methane has been used significantly from 1992 and since 1994 around 200 patents have been granted using either methane or natural gas, pure or mixed, for NOx reduction [1–4]. In fact, methane used as a reductant agent for fixed installations is an alternative solution for the selective catalytic reduction (SCR) with ammonia. The main problem is to promote the reduction of NOx in excess of oxygen, rather than the total oxidation of methane into CO2 and H2O. Recognized catalysts include some promoted zeolites such as Co-ZSM-5 [3], Ga-ZSM-5 [4] and protonated Pd/H-ZSM-5 [5–8]. The presence of stable cations (M+) seems to be the clue for the selective reduction of NOx performed with those catalysts [9–13]. The main role of NO in the NOx reduction into N2 had been confirmed for alumina [14], acid/basic zeolites [15] and more recently *
1
Corresponding author. Tel.: + 33 390 242766; fax: + 33 390 242768. E-mail address: [email protected] (A. Kiennemann). On leave from Universidad Nacional de Colombia, Sede Manizales.
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.029
for Co2+ cations [16,17]. For 12 tungstophosphoric acid (HPW) with noble metal based catalyst the interaction between HPW proton and noble metal, by forming a (H– Pt)d+ cationic site, seems to be the key for the de-NOx process [18]. Additionally, supports with marked redox properties, as ceria-based materials, can also play a main role in the reduction of NOx by cationic catalytic sites as Cex+ [19,20]. Methane activation can be achieved by dissociative adsorption and/or by forming CHy species and atomic hydrogen on metallic catalytic sites [21–23]. Then, NOx and CHy can react to produce CxHyNtOz species (e.g., isocyanates, nitriles, azo, azoxy, amides, etc.) [24,25] or CxHyOz species and NO [26–28]. Those oxygenated species will be completely oxidized by the oxygen remaining after NO decomposition, into N2, regenerating in this way catalytic active sites. It appears that, for different catalytic systems, there are specific roles played by the proton H+, by the metal (M0) and by metallic ions (M+) in one or several steps of NOx reduction into N2 by methane. Some previous works have shown the interest of using HPW for the NO and NO2 storage without forming nitrates [30–32]. The significance of adding a noble metal
M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
(e.g., Pt) and a support with suitable redox properties (CexZr4xO8 or TixZr1xO4) for the NOx reduction into N2 with CO/H2 has also been proved [33,34]. This work, presents the preliminary results for NOx reduction in excess of oxygen with methane and hydrogen and with a HPWnoble metal-support as multifunctional catalyst by using the NSR (NOx storage and reduction) concept [29]. The problem of NOx reduction is addressed by using a combination of two air/fuel ratios instead of a single fixed ratio. NOx storage stage occurs during lean excursion (in excess of oxygen). NOx elimination by reduction into N2 is performed during rich excursions. This work presents the preliminary results for NOx reduction with methane with a HPW-noble metal-support as catalyst. 2. Experimental 2.1. Preparation of the catalysts 2.1.1. Supports CexZr4xO8 oxide (with Zr/Ce molar ratio equals to 0.5) was prepared by dissolving in boiling propionic acid zirconium (IV) acetylacetonate (Avocado) and cerium (III) acetate hydrate (Strem). The resulting solution was mixed in the desired proportions and after 30 min of stirring the solvent was evaporated until a resin was formed. The resin obtained was calcined at 680 C for 4 h. Details about its characterization were already published [33]. 2.1.2. Supported catalysts Support was impregnated by an aqueous solution of HPW and metal precursor (H2PtCl6 – Strem) in order to obtain 65 wt.% HPW and 1 wt.% Pt [18,33,34]. Subsequently, the impregnated support was slowly dried under agitation at 80 C followed by drying at 100 C. 2.2. Test procedures The apparatus and test procedures are described elsewhere [33] and here a short description is presented. 2.2.1. Apparatus and gas composition A full computer driven reactor system was constructed for laboratory-scale test with powdered catalysts. Gas mixtures of NO, NO2, CO2, O2, H2, CH4, air and He are fixed using two independent sets of mass flow-meters, each one controlled individually. Tests were made using a lean gas mixture: NO = NO2 = 500 ppm, O2 = 10 vol.%, CO2 = 5 vol.%, H2O = 5 vol.% and He as a balance. This modified lean gas mixture is fed to an oxidation catalyst placed before the NOx storage system where hydrocarbons and CO are oxidised into CO2 and NO is partially transformed into NO2. Due to that oxidation catalyst and NO–NO2 thermodynamic equilibrium, the NO/NO2 ratio used was fixed. For reducing, a helium diluted mixtures of 0.4 vol.% methane, 5 vol.% water and 1 vol.% hydrogen were used (rich gas mixture). The gas
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mixtures were introduced into the quartz reactor of 1 cm diameter with 300 mg of catalysts (with space time 20,000 h1). NO and NO2 concentrations were monitored respectively by IR and UV analysers (Binos Rosemont Analytical) over a range of 0–3000 ppm (±10 ppm). Analytical instruments also included a gas phase micro-chromatograph (Agilent 3000, equipped with a molecular sieve column and a thermal conductivity detector) for N2 detection and a quadrupole mass spectrometer (HPR-20, Hiden Analytical). 2.2.2. Standard test procedure Test procedures were defined in order to assess the catalyst behaviour from two different points of view. NOx storage takes place while temperature rises to a pre-selected value and for the desorption cycle the temperature decreased (long cycle). This procedure is especially useful for understanding the interaction between NOx and the catalytic system. This procedure comprises a stabilization period, for the complete fuel–lean gas mixture, by using the bypass. The reactor is heated up to 80 C, where it is stabilized for 30 min. Afterwards, the furnace is heated from 80 C to the selected temperature, with a ramp rate of 4 C min1. The fuel–lean gas mixture is then rerouted from the bypass to the reactor. NOx storage manifests itself by a reduction in the signals of NO and NO2. At the end of the storage sequence, the original concentration of NOx is attained, which terminates the storage period. Desorption begins by changing the mixture of gases by wet air (5 vol.% water) and by decreasing the temperature to 80 C with a ramp rate of 4 C min1. NOx desorption begins around 120 C and reaches a maximum at 90 C. At 80 C, the temperature is kept constant until total desorption of NO and NO2 is accomplished. The isothermal procedure is a more realistic test for future applications and comprises alternate short cycles of storage and reduction at 250 C (this temperature was chosen from a previous work where the properties of NOx storage for HPW were optimised [33]). The furnace was heated from room temperature with a ramp of 4 C min1. At the same time, the complete mixtures of gases were stabilized by the by-pass. Afterwards, fuel lean mixture of gases was changed from by-pass to the reactor. After 2 min, gas mixture flowing through the reactor was changed to rich (methane or methane + H2) during 1 min completing the first cycle. Theses cycles were repeated successively. After 12 successive cycles, the final procedure consisted of cooling to room temperature in wet air (5 vol.% H2O). In this way, it could be verified if there was some NOx left in HPW structure. The amount of NOx stored and then released upon desorption was estimated by an integration of the curve below or above the baseline for storage or desorption, respectively, and expressed as mol NOx g1 HPW (with NOx calculated from the average molar weight between NO and NO2). The efficiency of the storage process was defined as the ratio between the amounts of NOx stored in 1 min to the total of NOx fed during the same period of time.
M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
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3. Results 3.1. Long cycles With HPW–Pt and 0.4 vol.% of methane, a very low NOx reduction was detected. A marked decrease in both the NOx storage and efficiency of HPW was observed after several long cycles. The average NOx storage efficiency was of ca. 50% at 250 C. To prevent that progressive lost of activity, 1 vol.% of hydrogen was added to 0.4 vol.% methane reductant mixture (as previously made for CO [32,34]). After switching to the rich mixture of gases, a fast, sharp and important signal of NO appears. This signal comes from both the NO2 reduction into NO (ca. 60%) and from the NO desorption from ½ðNOþ ÞHþ ðNO 2 Þ complex, which was formed in HPW during NOx storage [31]. The reduction of one NO2 molecule into NO implies, taking into account the stability of that complex, the desorption of two NO molecules. A NOx reduction into N2 of ca. 10% was detected (microGC and MS). Similar results were obtained with supported HPW–Pt. During long cycles, it seems that hydrogen assists the reduction of NO2 to NO and N2. and prevents loss of activity. The presence of noble metal is essential, although its role is limited. As concluded by Kato et al. [11], the role of hydrogen could be to reduce Pt allowing the formation/activation of CHx.
storage after 12 successive cycles is of ca. 0:54 mmol g1 HPW . At the end of cycles, an additional procedure of decreasing temperature under wet air allows to quantify the amount of NOx remaining stored in the catalyst. This supports the experimental evidence that the NOx storage prevails over its reduction. With 0.4 vol.% methane, the reduction of NOx into N2 was of ca. 6%. This reduction level shows the advantage of short cycles instead long ones. Subsequently, the reductant mixture of gases was changed in order to include hydrogen (0.4 vol.% methane + 1 vol.% hydrogen). Fig. 2 presents the NO and NO2 outlet concentrations obtained with short cycles. The presence of hydrogen highly modifies the performance of this catalyst as can be concluded after comparing Figs. 1 and 2. During the first cycle and after switching to rich mixture of gases, an important signal of NO was detected which corresponds with NO2 reduction (drop in the NO2 concentration in the gas mixture). The NO peak is sharp and high (up to ca. 1500 ppm) but it increases in the next two cycles (after three cycles it stabilise at ca. 3200 ppm). The higher NO concentration implies a reduction of the catalyst activity on NO reduction related with the first cycle: the reduction percentage of NOx into N2 passes from 6% (full long cycle) to 50% (first cycle), but reduction activity decreases with cycles and stabilizes at ca. 25% (from third cycles and subsequent). The NOx storage efficiency is stable at ca. 60%. These results support the hypothesis of hydrogen participation during NOx reduction process.
3.2. Short cycles 3.2.1. HPW–Pt The evolution of NOx during several short cycles, at 250 C, is presented in Fig. 1. It is possible to notice the ‘‘v’’ shape evolution of NO, NO2 and NOx curves resulting from the periodic switching between lean and rich mixtures of gases. Those peaks correspond to the decrease of NOx storage and efficiency capacities due to the progressive saturation of the HPW structure with NOx. The total NOx
3.2.2. Supported HPW–Pt The effect of the support on the NOx reduction is presented here. The NO and NO2 outlet concentrations obtained with (HPW–Pt)/(Zr/Ce = 0.5) are presented in Fig. 3. The induction period (first four cycles in Fig. 2) is not longer observed in the presence of supported catalyst. Additionally, the NO desorption peak is smaller than that observed in the absence of support (it rises up to ca. 1000 ppm). As compared with a non-supported catalyst, the NO2 concentration increases during storage phase 4000
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Fig. 1. NOx profiles on HPW–Pt after several cycles of lean (2 min) and rich (1 min: 0.4 vol.% CH4) at 250 C. Final NOx desorption with humid air during decreasing the temperature.
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Fig. 2. Absorption–reduction cycles (0.4 vol.% CH4 + 1 vol.% H2) with (HPW–Pt) at 250 C.
M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
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Fig. 3. Absorption–reduction cycles (0.4 vol.% CH4 + 1 vol.% H2) with (HPW–Pt)/(Zr/Ce = 0.5) at 250 C.
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NOx reduction,%
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Cycle Fig. 4. Reduction evolution during periodic cycles lean-rich at 250 C with HPW–Pt and (HPW–Pt)/(Zr/Ce = 0.5).
and recovers its base line (at ca. 500 ppm). For this catalyst, the average reduction percentage obtained was ca. 60% with a constant storage efficiency of ca. 70% after the third cycle. Fig. 4 compares the NOx reduction evolution for supported and unsupported HPW–Pt. The marked decrease in the reduction activity for HPW–Pt was not observed for supported catalysts evidencing the importance of the support. The stability of (HPW–Pt)/(Zr/Ce = 0.5) catalyst could be related to the specific support characteristics as oxygen mobility, oxygen storage capacity and also to its interaction with platinum. The switching between lean and rich conditions could generate the necessary conditions for regenerating catalytic sites. 4. Discussion The results presented above reveal the catalytic effect for NOx reduction of noble metal, support and hydrogen during lean reduction of NOx into N2. The first possible role of hydrogen is the reduction of the noble metal. H2 also allows both, the formation of cationic catalytic sites (M+) on the support (on CexZr4xO8 mixed oxides: Ce3+) and the cleaning of the excess of oxygen generated after using
a lean gas mixture. It is also helpful to refer to some other criteria concerning NOx reduction. In the literature, the beneficial role of metallic and/or support cations was discussed in relation to zeolites and some other systems [11– 13,16,17]. It was suggested that a proton will participate in the key step for NO2 and CH4 reaction (bifunctional catalyst: Pd + proton) [5,6], which seems to be also particularly important in the present case with HPW–metal (supported or not). Actually, one hypothesis developed for this catalytic system involves the interaction between metal particles and the proton of HPW structure by forming entities like (metal–H)d+ [34]. However, the possibility of interactions between noble metal and support can also be considered. Thus, the catalyst used in this work consists of several different catalytic sites: M0 (reduced metal particles) and cationic sites: M+ on the support and (metal–H)d+ in HPW. A remaining point to discuss is the evolution of methane in contact with the catalytic system. Two different reactions were tried separately: the steam methane reforming (SMR) and the water gas shift reaction (WGS). A series of experiments were performed using water mixtures with 1 vol.% methane or with 1 vol.% CO to simulate SMR and WGS mixtures, respectively (no additional treatment was performed on catalyst before test). Results are presented in Fig. 5. It is possible to observe that for SMR process, at 250 C, the methane conversion is ca. 2% (formation of ca. 200 ppm of H2 and CO2; CO was not detected). However, initially, the H2 peak is not in phase compared with that of CO2. Such a shift could be related to the hydrogen consumption to reduce the catalyst in-situ. This consumption explains the difference between SMR stoichiometric and experimental CO2/H2 values. For WGS process, after switching the gas mixture through the reactor, a fast peak of CO2 formation was observed in concomitance with the drop in the CO signal. The signal of H2 was also shifted forward. These facts could be related to a reduction process of both support and metal particles by CO and H2. Additionally, the 4000 Stabilization of 1% CH4and water mixture (by-pass)
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1% CH4 and water (reactor)
purge 1% CO with and water air by-pass
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Fig. 5. Activity for steam methane reforming (1 vol.% CH4 + 5 vol.% H2O) and water gas shift (1 vol.% CO + 5 vol.% H2O) with (HPW–Pt)/ (Zr/Ce = 0.5) at 250 C (CO concentration = CO · 101).
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M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
hydrogen formation was more important for WGS when comparing to SMR. The hydrogen formation detected during SMR and WGSR process is specific of (HPW–Pt)/Zr–Ce catalysts. No hydrogen production was detected in the case of (HPW–Pt). This difference in hydrogen production can explain the marked resistance to deactivation of the Zr– Ce based catalyst. In fact, as proposed in the literature, it could be expected that methane undergoes a dissociative adsorption on metal particles by generating adsorbed atomic hydrogen and CHx compounds [18,19]. The hydrogen formed in-situ and/or that from the gas mixture can reduce the noble metal resulting in the formation of active catalytic sites (as indicated above). It seems that cationic sites, like (H–Pt)d+ formed in HPW, prevent the total combustion of CH4 into CO2. 5. Conclusions From the results obtained using short lean-rich (CH4/ H2) cycles, several conclusions could be drawn. (a) The possibility of NOx reduction when using methane and hydrogen as reductant was proved for HPW– metal based catalytic systems. NOxreduction is very weak or absent without hydrogen. (b) The addition of noble metal has not influence in the NOx storage capacity but allows the formation of a new catalytic cationic site (metal–H)d+. Noble metal not related with HPW must be reduced by reductant agents (H2, hydrocarbons). It is also possible to form cationic sites M+ after reduction of support. (c) The dispersion of HPW–Pt on a support highly improves its catalytic performance towards NOx reduction. In fact, the outstanding behaviour of (HPW–Pt)/(Zr/Ce = 0.5) catalyst against deactivation could be related to support characteristics such as oxygen mobility, oxygen storage capacity and the possibility of in-situ hydrogen formation derived from steam methane reforming and/or water gas shift reaction processes. Acknowledgement Authors would like to thank ADEME (France) for the financial support to perform this research.
References [1] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, second ed., Wiley-AIChE, 2005. [2] R. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B: Environ. 39 (2002) 283. [3] Y. Li, J.N. Armor, Appl. Catal. B: Environ. 1 (1992) L31. [4] J.N. Armor, Catal. Today 31 (1996) 191. [5] Y. Nishizaka, M. Misono, Chem. Lett. (1994) 2237. [6] M. Misono, Y. Nishizaka, M. Kawamoto, H. Kato, Stud. Surf. Sci. Catal. 105 (1997) 1501. [7] M. Misono, Y. Hirao, C. Yokoyama, Catal. Today 38 (1997) 157. [8] H. Kato, C. Yakohama, M. Misono, Catal. Lett. 47 (1997) 189. [9] Y.A. Chin, A. Pisanu, L. Serventi, W.E. Alvarez, D.E. Resasco, Catal. Today 54 (1999) 419. [10] Y.A. Chin, D.E. Resasco, Catalysis, Royal Soc. Chem. London, vol. 14, 1999. [11] H. Kato, C. Yokohama, M. Misono, Catal. Today 45 (1998) 93. [12] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [13] V.I. Paˆrvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. [14] M.C. Kung, K.A. Bethke, J. Yan, J.-H. Lee, H.H. Kung, Appl. Surf. Sci. 121–122 (1997) 261. [15] C. Yokohama, M. Misono, J. Catal. 160 (1996) 95. [16] G. Dje´ga-Mariadassou, M. Boudart, J. Catal. 216 (2003) 89. [17] G. Dje´ga-Mariadassou, Catal. Today 90 (2004) 27. [18] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Appl. Catal. B: Environ., in press. Available online 30 June 2006. [19] O. Gorce, F. Baudin, C. Thomas, P. da Costa, G. Dje´ga-Mariadassou, Appl. Catal. B: Environ. 54 (2004) 69. [20] G. Dje´ga-Mariadassou, F. Fajardie´, J.F. Tempe`re´, J.M. Manoli, O. Touret, G. Blanchard, J. Mol. Catal. A 161 (2000) 179. [21] C.T. Au, C.F. Ng, M.S. Liao, J. Catal. 185 (1999) 12. [22] D.T.P. Watson, S. Titmuss, D.A. King, Surf. Sci. 505 (2002) 49. [23] A.D. Cowan, R. Du¨mpelmann, N.W. Cant, J. Catal. 151 (1995) 356. [24] G.R. Banwenda, A. Obuchi, A. Ogata, K. Misuno, Chem. Lett. 17 (1994) 2109. [25] D.C. Chambers, D.E. Angove, N.W. Cant, J. Catal. 204 (2001) 11. [26] M. Sazaki, H. Hamada, Y. Kintaichi, T. Ito, Catal. Lett. 15 (1992) 297. [27] T. Tanaka, T. Okuhara, M. Misono, Appl. Catal. B: Environ. 4 (1994) L1. [28] M. Iwamoto, H. Takeda, Catal. Today 27 (1996) 71. [29] N. Takahasi, H. Shinjoh, T. Ijima, T. Suzuki, T. Yamakazi, K. Yokota, H. Suzuki, S. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, Catal. Today 27 (1996) 63. [30] R.T. Yang, N. Chen, Ind. Eng. Chem. Res. 33 (1994) 825. [31] R. Be´langer, J.B. Moffat, J. Catal. 152 (1995) 179. [32] S. Hodjati, C. Petit, V. Pitchon, A. Kiennemann, J. Catal. 197 (2001) 324. [33] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Environ. Sci. Technol. 39 (2005) 638. [34] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Catal. Today 107– 108 (2005) 60.
Multifunctional catalyst for de-NOx processes: The selective reduction of NOx by methane M.A. Go´mez-Garcı´a 1, Y. Zimmermann, V. Pitchon, A. Kiennemann
*
LMSPC, Laboratoire de Mate´riaux, Surfaces et Proce´de´s pour la Catalyse, UMR 7515 du CNRS-ECPM-ULP, 25 Rue Becquerel, 67087 Strasbourg Cedex 2, France Received 8 February 2006; received in revised form 1 June 2006; accepted 27 June 2006 Available online 6 July 2006
Abstract A multifunctional catalyst has been applied for the reduction of NOx by methane in excess of oxygen. The catalyst includes a heteropolyacid (HPW) and a noble metal (Pt) deposited on a ceria–zirconia mixed oxide. The presence of hydrogen is necessary for reducing NOx into N2 with methane. During several alternate cycles of NOx storage (2 min) and reduction (1 min), the average NOx reduction percentage into nitrogen was of ca. 60% with a constant storage efficiency of ca.70%. A coherent picture of the catalytic process emerged from two separated experimental tests: water gas shift and steam methane reforming reactions. 2006 Elsevier B.V. All rights reserved. Keywords: NOx reduction; Multifunctional catalysts; Methane; HPW; Pt; Ceria–zirconia mixed oxides
1. Introduction Most of the studies about the catalytic reduction of NOx (NO and NO2) by hydrocarbons have been made since 1990. Methane has been used significantly from 1992 and since 1994 around 200 patents have been granted using either methane or natural gas, pure or mixed, for NOx reduction [1–4]. In fact, methane used as a reductant agent for fixed installations is an alternative solution for the selective catalytic reduction (SCR) with ammonia. The main problem is to promote the reduction of NOx in excess of oxygen, rather than the total oxidation of methane into CO2 and H2O. Recognized catalysts include some promoted zeolites such as Co-ZSM-5 [3], Ga-ZSM-5 [4] and protonated Pd/H-ZSM-5 [5–8]. The presence of stable cations (M+) seems to be the clue for the selective reduction of NOx performed with those catalysts [9–13]. The main role of NO in the NOx reduction into N2 had been confirmed for alumina [14], acid/basic zeolites [15] and more recently *
1
Corresponding author. Tel.: + 33 390 242766; fax: + 33 390 242768. E-mail address: [email protected] (A. Kiennemann). On leave from Universidad Nacional de Colombia, Sede Manizales.
1566-7367/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.029
for Co2+ cations [16,17]. For 12 tungstophosphoric acid (HPW) with noble metal based catalyst the interaction between HPW proton and noble metal, by forming a (H– Pt)d+ cationic site, seems to be the key for the de-NOx process [18]. Additionally, supports with marked redox properties, as ceria-based materials, can also play a main role in the reduction of NOx by cationic catalytic sites as Cex+ [19,20]. Methane activation can be achieved by dissociative adsorption and/or by forming CHy species and atomic hydrogen on metallic catalytic sites [21–23]. Then, NOx and CHy can react to produce CxHyNtOz species (e.g., isocyanates, nitriles, azo, azoxy, amides, etc.) [24,25] or CxHyOz species and NO [26–28]. Those oxygenated species will be completely oxidized by the oxygen remaining after NO decomposition, into N2, regenerating in this way catalytic active sites. It appears that, for different catalytic systems, there are specific roles played by the proton H+, by the metal (M0) and by metallic ions (M+) in one or several steps of NOx reduction into N2 by methane. Some previous works have shown the interest of using HPW for the NO and NO2 storage without forming nitrates [30–32]. The significance of adding a noble metal
M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
(e.g., Pt) and a support with suitable redox properties (CexZr4xO8 or TixZr1xO4) for the NOx reduction into N2 with CO/H2 has also been proved [33,34]. This work, presents the preliminary results for NOx reduction in excess of oxygen with methane and hydrogen and with a HPWnoble metal-support as multifunctional catalyst by using the NSR (NOx storage and reduction) concept [29]. The problem of NOx reduction is addressed by using a combination of two air/fuel ratios instead of a single fixed ratio. NOx storage stage occurs during lean excursion (in excess of oxygen). NOx elimination by reduction into N2 is performed during rich excursions. This work presents the preliminary results for NOx reduction with methane with a HPW-noble metal-support as catalyst. 2. Experimental 2.1. Preparation of the catalysts 2.1.1. Supports CexZr4xO8 oxide (with Zr/Ce molar ratio equals to 0.5) was prepared by dissolving in boiling propionic acid zirconium (IV) acetylacetonate (Avocado) and cerium (III) acetate hydrate (Strem). The resulting solution was mixed in the desired proportions and after 30 min of stirring the solvent was evaporated until a resin was formed. The resin obtained was calcined at 680 C for 4 h. Details about its characterization were already published [33]. 2.1.2. Supported catalysts Support was impregnated by an aqueous solution of HPW and metal precursor (H2PtCl6 – Strem) in order to obtain 65 wt.% HPW and 1 wt.% Pt [18,33,34]. Subsequently, the impregnated support was slowly dried under agitation at 80 C followed by drying at 100 C. 2.2. Test procedures The apparatus and test procedures are described elsewhere [33] and here a short description is presented. 2.2.1. Apparatus and gas composition A full computer driven reactor system was constructed for laboratory-scale test with powdered catalysts. Gas mixtures of NO, NO2, CO2, O2, H2, CH4, air and He are fixed using two independent sets of mass flow-meters, each one controlled individually. Tests were made using a lean gas mixture: NO = NO2 = 500 ppm, O2 = 10 vol.%, CO2 = 5 vol.%, H2O = 5 vol.% and He as a balance. This modified lean gas mixture is fed to an oxidation catalyst placed before the NOx storage system where hydrocarbons and CO are oxidised into CO2 and NO is partially transformed into NO2. Due to that oxidation catalyst and NO–NO2 thermodynamic equilibrium, the NO/NO2 ratio used was fixed. For reducing, a helium diluted mixtures of 0.4 vol.% methane, 5 vol.% water and 1 vol.% hydrogen were used (rich gas mixture). The gas
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mixtures were introduced into the quartz reactor of 1 cm diameter with 300 mg of catalysts (with space time 20,000 h1). NO and NO2 concentrations were monitored respectively by IR and UV analysers (Binos Rosemont Analytical) over a range of 0–3000 ppm (±10 ppm). Analytical instruments also included a gas phase micro-chromatograph (Agilent 3000, equipped with a molecular sieve column and a thermal conductivity detector) for N2 detection and a quadrupole mass spectrometer (HPR-20, Hiden Analytical). 2.2.2. Standard test procedure Test procedures were defined in order to assess the catalyst behaviour from two different points of view. NOx storage takes place while temperature rises to a pre-selected value and for the desorption cycle the temperature decreased (long cycle). This procedure is especially useful for understanding the interaction between NOx and the catalytic system. This procedure comprises a stabilization period, for the complete fuel–lean gas mixture, by using the bypass. The reactor is heated up to 80 C, where it is stabilized for 30 min. Afterwards, the furnace is heated from 80 C to the selected temperature, with a ramp rate of 4 C min1. The fuel–lean gas mixture is then rerouted from the bypass to the reactor. NOx storage manifests itself by a reduction in the signals of NO and NO2. At the end of the storage sequence, the original concentration of NOx is attained, which terminates the storage period. Desorption begins by changing the mixture of gases by wet air (5 vol.% water) and by decreasing the temperature to 80 C with a ramp rate of 4 C min1. NOx desorption begins around 120 C and reaches a maximum at 90 C. At 80 C, the temperature is kept constant until total desorption of NO and NO2 is accomplished. The isothermal procedure is a more realistic test for future applications and comprises alternate short cycles of storage and reduction at 250 C (this temperature was chosen from a previous work where the properties of NOx storage for HPW were optimised [33]). The furnace was heated from room temperature with a ramp of 4 C min1. At the same time, the complete mixtures of gases were stabilized by the by-pass. Afterwards, fuel lean mixture of gases was changed from by-pass to the reactor. After 2 min, gas mixture flowing through the reactor was changed to rich (methane or methane + H2) during 1 min completing the first cycle. Theses cycles were repeated successively. After 12 successive cycles, the final procedure consisted of cooling to room temperature in wet air (5 vol.% H2O). In this way, it could be verified if there was some NOx left in HPW structure. The amount of NOx stored and then released upon desorption was estimated by an integration of the curve below or above the baseline for storage or desorption, respectively, and expressed as mol NOx g1 HPW (with NOx calculated from the average molar weight between NO and NO2). The efficiency of the storage process was defined as the ratio between the amounts of NOx stored in 1 min to the total of NOx fed during the same period of time.
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3. Results 3.1. Long cycles With HPW–Pt and 0.4 vol.% of methane, a very low NOx reduction was detected. A marked decrease in both the NOx storage and efficiency of HPW was observed after several long cycles. The average NOx storage efficiency was of ca. 50% at 250 C. To prevent that progressive lost of activity, 1 vol.% of hydrogen was added to 0.4 vol.% methane reductant mixture (as previously made for CO [32,34]). After switching to the rich mixture of gases, a fast, sharp and important signal of NO appears. This signal comes from both the NO2 reduction into NO (ca. 60%) and from the NO desorption from ½ðNOþ ÞHþ ðNO 2 Þ complex, which was formed in HPW during NOx storage [31]. The reduction of one NO2 molecule into NO implies, taking into account the stability of that complex, the desorption of two NO molecules. A NOx reduction into N2 of ca. 10% was detected (microGC and MS). Similar results were obtained with supported HPW–Pt. During long cycles, it seems that hydrogen assists the reduction of NO2 to NO and N2. and prevents loss of activity. The presence of noble metal is essential, although its role is limited. As concluded by Kato et al. [11], the role of hydrogen could be to reduce Pt allowing the formation/activation of CHx.
storage after 12 successive cycles is of ca. 0:54 mmol g1 HPW . At the end of cycles, an additional procedure of decreasing temperature under wet air allows to quantify the amount of NOx remaining stored in the catalyst. This supports the experimental evidence that the NOx storage prevails over its reduction. With 0.4 vol.% methane, the reduction of NOx into N2 was of ca. 6%. This reduction level shows the advantage of short cycles instead long ones. Subsequently, the reductant mixture of gases was changed in order to include hydrogen (0.4 vol.% methane + 1 vol.% hydrogen). Fig. 2 presents the NO and NO2 outlet concentrations obtained with short cycles. The presence of hydrogen highly modifies the performance of this catalyst as can be concluded after comparing Figs. 1 and 2. During the first cycle and after switching to rich mixture of gases, an important signal of NO was detected which corresponds with NO2 reduction (drop in the NO2 concentration in the gas mixture). The NO peak is sharp and high (up to ca. 1500 ppm) but it increases in the next two cycles (after three cycles it stabilise at ca. 3200 ppm). The higher NO concentration implies a reduction of the catalyst activity on NO reduction related with the first cycle: the reduction percentage of NOx into N2 passes from 6% (full long cycle) to 50% (first cycle), but reduction activity decreases with cycles and stabilizes at ca. 25% (from third cycles and subsequent). The NOx storage efficiency is stable at ca. 60%. These results support the hypothesis of hydrogen participation during NOx reduction process.
3.2. Short cycles 3.2.1. HPW–Pt The evolution of NOx during several short cycles, at 250 C, is presented in Fig. 1. It is possible to notice the ‘‘v’’ shape evolution of NO, NO2 and NOx curves resulting from the periodic switching between lean and rich mixtures of gases. Those peaks correspond to the decrease of NOx storage and efficiency capacities due to the progressive saturation of the HPW structure with NOx. The total NOx
3.2.2. Supported HPW–Pt The effect of the support on the NOx reduction is presented here. The NO and NO2 outlet concentrations obtained with (HPW–Pt)/(Zr/Ce = 0.5) are presented in Fig. 3. The induction period (first four cycles in Fig. 2) is not longer observed in the presence of supported catalyst. Additionally, the NO desorption peak is smaller than that observed in the absence of support (it rises up to ca. 1000 ppm). As compared with a non-supported catalyst, the NO2 concentration increases during storage phase 4000
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Fig. 1. NOx profiles on HPW–Pt after several cycles of lean (2 min) and rich (1 min: 0.4 vol.% CH4) at 250 C. Final NOx desorption with humid air during decreasing the temperature.
0
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Fig. 2. Absorption–reduction cycles (0.4 vol.% CH4 + 1 vol.% H2) with (HPW–Pt) at 250 C.
M.A. Go´mez-Garcı´a et al. / Catalysis Communications 8 (2007) 400–404
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Temperature, ˚C
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Fig. 3. Absorption–reduction cycles (0.4 vol.% CH4 + 1 vol.% H2) with (HPW–Pt)/(Zr/Ce = 0.5) at 250 C.
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Cycle Fig. 4. Reduction evolution during periodic cycles lean-rich at 250 C with HPW–Pt and (HPW–Pt)/(Zr/Ce = 0.5).
and recovers its base line (at ca. 500 ppm). For this catalyst, the average reduction percentage obtained was ca. 60% with a constant storage efficiency of ca. 70% after the third cycle. Fig. 4 compares the NOx reduction evolution for supported and unsupported HPW–Pt. The marked decrease in the reduction activity for HPW–Pt was not observed for supported catalysts evidencing the importance of the support. The stability of (HPW–Pt)/(Zr/Ce = 0.5) catalyst could be related to the specific support characteristics as oxygen mobility, oxygen storage capacity and also to its interaction with platinum. The switching between lean and rich conditions could generate the necessary conditions for regenerating catalytic sites. 4. Discussion The results presented above reveal the catalytic effect for NOx reduction of noble metal, support and hydrogen during lean reduction of NOx into N2. The first possible role of hydrogen is the reduction of the noble metal. H2 also allows both, the formation of cationic catalytic sites (M+) on the support (on CexZr4xO8 mixed oxides: Ce3+) and the cleaning of the excess of oxygen generated after using
a lean gas mixture. It is also helpful to refer to some other criteria concerning NOx reduction. In the literature, the beneficial role of metallic and/or support cations was discussed in relation to zeolites and some other systems [11– 13,16,17]. It was suggested that a proton will participate in the key step for NO2 and CH4 reaction (bifunctional catalyst: Pd + proton) [5,6], which seems to be also particularly important in the present case with HPW–metal (supported or not). Actually, one hypothesis developed for this catalytic system involves the interaction between metal particles and the proton of HPW structure by forming entities like (metal–H)d+ [34]. However, the possibility of interactions between noble metal and support can also be considered. Thus, the catalyst used in this work consists of several different catalytic sites: M0 (reduced metal particles) and cationic sites: M+ on the support and (metal–H)d+ in HPW. A remaining point to discuss is the evolution of methane in contact with the catalytic system. Two different reactions were tried separately: the steam methane reforming (SMR) and the water gas shift reaction (WGS). A series of experiments were performed using water mixtures with 1 vol.% methane or with 1 vol.% CO to simulate SMR and WGS mixtures, respectively (no additional treatment was performed on catalyst before test). Results are presented in Fig. 5. It is possible to observe that for SMR process, at 250 C, the methane conversion is ca. 2% (formation of ca. 200 ppm of H2 and CO2; CO was not detected). However, initially, the H2 peak is not in phase compared with that of CO2. Such a shift could be related to the hydrogen consumption to reduce the catalyst in-situ. This consumption explains the difference between SMR stoichiometric and experimental CO2/H2 values. For WGS process, after switching the gas mixture through the reactor, a fast peak of CO2 formation was observed in concomitance with the drop in the CO signal. The signal of H2 was also shifted forward. These facts could be related to a reduction process of both support and metal particles by CO and H2. Additionally, the 4000 Stabilization of 1% CH4and water mixture (by-pass)
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Fig. 5. Activity for steam methane reforming (1 vol.% CH4 + 5 vol.% H2O) and water gas shift (1 vol.% CO + 5 vol.% H2O) with (HPW–Pt)/ (Zr/Ce = 0.5) at 250 C (CO concentration = CO · 101).
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hydrogen formation was more important for WGS when comparing to SMR. The hydrogen formation detected during SMR and WGSR process is specific of (HPW–Pt)/Zr–Ce catalysts. No hydrogen production was detected in the case of (HPW–Pt). This difference in hydrogen production can explain the marked resistance to deactivation of the Zr– Ce based catalyst. In fact, as proposed in the literature, it could be expected that methane undergoes a dissociative adsorption on metal particles by generating adsorbed atomic hydrogen and CHx compounds [18,19]. The hydrogen formed in-situ and/or that from the gas mixture can reduce the noble metal resulting in the formation of active catalytic sites (as indicated above). It seems that cationic sites, like (H–Pt)d+ formed in HPW, prevent the total combustion of CH4 into CO2. 5. Conclusions From the results obtained using short lean-rich (CH4/ H2) cycles, several conclusions could be drawn. (a) The possibility of NOx reduction when using methane and hydrogen as reductant was proved for HPW– metal based catalytic systems. NOxreduction is very weak or absent without hydrogen. (b) The addition of noble metal has not influence in the NOx storage capacity but allows the formation of a new catalytic cationic site (metal–H)d+. Noble metal not related with HPW must be reduced by reductant agents (H2, hydrocarbons). It is also possible to form cationic sites M+ after reduction of support. (c) The dispersion of HPW–Pt on a support highly improves its catalytic performance towards NOx reduction. In fact, the outstanding behaviour of (HPW–Pt)/(Zr/Ce = 0.5) catalyst against deactivation could be related to support characteristics such as oxygen mobility, oxygen storage capacity and the possibility of in-situ hydrogen formation derived from steam methane reforming and/or water gas shift reaction processes. Acknowledgement Authors would like to thank ADEME (France) for the financial support to perform this research.
References [1] C.H. Bartholomew, R.J. Farrauto, Fundamentals of Industrial Catalytic Processes, second ed., Wiley-AIChE, 2005. [2] R. Burch, J.P. Breen, F.C. Meunier, Appl. Catal. B: Environ. 39 (2002) 283. [3] Y. Li, J.N. Armor, Appl. Catal. B: Environ. 1 (1992) L31. [4] J.N. Armor, Catal. Today 31 (1996) 191. [5] Y. Nishizaka, M. Misono, Chem. Lett. (1994) 2237. [6] M. Misono, Y. Nishizaka, M. Kawamoto, H. Kato, Stud. Surf. Sci. Catal. 105 (1997) 1501. [7] M. Misono, Y. Hirao, C. Yokoyama, Catal. Today 38 (1997) 157. [8] H. Kato, C. Yakohama, M. Misono, Catal. Lett. 47 (1997) 189. [9] Y.A. Chin, A. Pisanu, L. Serventi, W.E. Alvarez, D.E. Resasco, Catal. Today 54 (1999) 419. [10] Y.A. Chin, D.E. Resasco, Catalysis, Royal Soc. Chem. London, vol. 14, 1999. [11] H. Kato, C. Yokohama, M. Misono, Catal. Today 45 (1998) 93. [12] Y. Li, T.L. Slager, J.N. Armor, J. Catal. 150 (1994) 388. [13] V.I. Paˆrvulescu, P. Grange, B. Delmon, Catal. Today 46 (1998) 233. [14] M.C. Kung, K.A. Bethke, J. Yan, J.-H. Lee, H.H. Kung, Appl. Surf. Sci. 121–122 (1997) 261. [15] C. Yokohama, M. Misono, J. Catal. 160 (1996) 95. [16] G. Dje´ga-Mariadassou, M. Boudart, J. Catal. 216 (2003) 89. [17] G. Dje´ga-Mariadassou, Catal. Today 90 (2004) 27. [18] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Appl. Catal. B: Environ., in press. Available online 30 June 2006. [19] O. Gorce, F. Baudin, C. Thomas, P. da Costa, G. Dje´ga-Mariadassou, Appl. Catal. B: Environ. 54 (2004) 69. [20] G. Dje´ga-Mariadassou, F. Fajardie´, J.F. Tempe`re´, J.M. Manoli, O. Touret, G. Blanchard, J. Mol. Catal. A 161 (2000) 179. [21] C.T. Au, C.F. Ng, M.S. Liao, J. Catal. 185 (1999) 12. [22] D.T.P. Watson, S. Titmuss, D.A. King, Surf. Sci. 505 (2002) 49. [23] A.D. Cowan, R. Du¨mpelmann, N.W. Cant, J. Catal. 151 (1995) 356. [24] G.R. Banwenda, A. Obuchi, A. Ogata, K. Misuno, Chem. Lett. 17 (1994) 2109. [25] D.C. Chambers, D.E. Angove, N.W. Cant, J. Catal. 204 (2001) 11. [26] M. Sazaki, H. Hamada, Y. Kintaichi, T. Ito, Catal. Lett. 15 (1992) 297. [27] T. Tanaka, T. Okuhara, M. Misono, Appl. Catal. B: Environ. 4 (1994) L1. [28] M. Iwamoto, H. Takeda, Catal. Today 27 (1996) 71. [29] N. Takahasi, H. Shinjoh, T. Ijima, T. Suzuki, T. Yamakazi, K. Yokota, H. Suzuki, S. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, Catal. Today 27 (1996) 63. [30] R.T. Yang, N. Chen, Ind. Eng. Chem. Res. 33 (1994) 825. [31] R. Be´langer, J.B. Moffat, J. Catal. 152 (1995) 179. [32] S. Hodjati, C. Petit, V. Pitchon, A. Kiennemann, J. Catal. 197 (2001) 324. [33] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Environ. Sci. Technol. 39 (2005) 638. [34] M.A. Go´mez-Garcı´a, V. Pitchon, A. Kiennemann, Catal. Today 107– 108 (2005) 60.