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http://dx.doi.org/10.7464/ksct.2016.22.4.211

Catalytic Technology for NOx Abatement using Ammonia  

Park, Soon Hee (Super Ultra Low Energy & Emission Vehicle Center, Korea University)
Lee, Kwan-Young (Department of Chemical and Biological Engineering, Korea University)
Cho, Sung June (School of Chemical Engineering, Chonnam National University)
Publication Information
Clean Technology / v.22, no.4, 2016 , pp. 211-224 More about this Journal
Abstract
Three way catalyst has been used extensively for the exhaust gas treatment for the internal combustion gasoline engine. While, numerous research efforts have been directed to develop various technologies for the abatement of exhaust gas from diesel engine. Diesel engine operating under lean condition produces large amount of NOx and the corresponding catalytic technology employing vanadium supported titania using ammonia has been commercialized for heavy duty vehicle. Recently, the Cu catalyst supported on zeolite has been investigated for NOx abatement using ammonia because of its critical importance for ultra low emission vehicle. The current review shows the recent trend in research and development for zeolite based copper catalysts, which are mainly used as catalysts for selective catalytic reduction using ammonia, are one of the aftertreatment technologies for effectively removing nitrogen oxides from diesel exhaust.
Keywords
NOx; $NH_3$; Zeolite; Cu;
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Times Cited By KSCI : 3  (Citation Analysis)
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1 Saleh, R. Y., Wachs, I. E., Chan, S. S., and Chersich, C. C., "The Interaction of $V_2O_5\;with\;TiO_2$(Anatase): Catalyst Evolution with Calcination Temperature and O-Xylene Oxidation," J. Catal., 98, 102-114 (1986).   DOI
2 Oliveri, G., Ramls, G., Busca, G., and Escribano, V. S., "Thermal Stability of Vanadia-Titania Catalysts," J. Mater. Chem., 3(12), 1239-1249 (1993).   DOI
3 Madia, G., Elsener, M., Koebel, M., Raimondi, F., and Wokaun, A., "Thermal Stability of Vanadia-tungsta-titania Catalysts in the SCR Process," Appl. Catal. B: Environ., 39, 181-190 (2002).   DOI
4 Smirniotis, P. G., Sreekanth, P. M., Peña, D. A., and Jenkins, R. G., "Manganese Oxide Catalysts Supported on $TiO_2,\;Al_2O_3,\;and\;SiO_2$: A Comparison for Low-Temperature SCR of NO with $NH_3$," Ind. Eng. Chem. Res., 45, 6436-6443 (2006).   DOI
5 Li, Y., Cheng, H., Li, D., Qin, Y., Xei, Y., and Wang, S., "$WO_3/CeO_2-ZrO_2$, a Promising Catalyst for Selective Catalytic Reduction (SCR) of NOx with $NH_3$ in Diesel Exhaust," Chem. Commun., 1470-1472 (2008).
6 Qi, G., and Yang, R. T., "Performance and Kinetics Study for Low-temperature SCR of NO with $NH_3$ over MnOx-$CeO_2$ Catalyst," J. Catal., 217, 434-441 (2003).   DOI
7 Nie, J., Wu, X., Ma, Z., Xu, T., Si, Z., Chen, L., and Weng, D., "Tailored Temperature Window of MnOx-$CeO_2$ SCR Catalyst by Addition of Acidic Metal Oxides," Chin. J. Catal., 35, 1281-1288 (2014).   DOI
8 Peng, Y., Li, K., and Li, J., "Identification of the Active Sites on $CeO_2-WO_3$ Catalysts for SCR of NOx with NH3: An in situ IR and Raman Spectroscopy Study," Appl. Catal. B: Environ., 140-141, 483-492 (2013).   DOI
9 Lee, S. G., Lee, H. J., Song, I. H., Youn, S. H., Kim, D. H., and Cho, S. J., "Suppressed $N_2O$ Formation During $NH_3$ Selective Catalytic Reduction Using Vanadium on Zeolitic Microporous $TiO_2$," Sci. Rep. doi: 10.1038/srep12702.
10 Iwamoto, M., Yahiro, H., Tanda, K., Mizuno, N., Mine, Y., and Kagawa, S., "Removal of Nitrogen Monoxide through a Novel Catalytic Process. 1. Decomposition on Excessively Copper Ion Exchanged ZSM-5 Zeolites," J. Phys. Chem., 95, 3727-3730 (1991).   DOI
11 Centi, G., and Perathoner, S., "Nature of Active Species in Copper-Based Catalysts and Their Chemistry of Transformation of Nitrogen Oxides," Appl. Catal. A: Gen., 132, 179-259 (1995).   DOI
12 Tolonen, K. R., Maunula, T., Lomma, M., Huuhtanen, M., and Keiski, R. L., "The Effect of $N_2O$ on the Activity of Fresh and Aged zeolite Catalysts in the $NH_3$-SCR Reaction," Catal. Today, 100, 217-222 (2005).   DOI
13 Kwak, J. H., Tonkyn, R. G., Kim, D. H., Szanyi, J., and Peden, C. H. F., "Excellent Activity and Selectivity of Cu-SSZ-13 in the Selective Catalytic Reduction of NOx with $NH_3$," J. Catal., 275, 187-190 (2010).   DOI
14 Fickel, D. F., Addio, E. D., Lauterbach, J. A., and Lobo, R. F., "The Ammonia Selective Catalytic Reduction Activity of Copper-Exchanged Small-Pore Zeolites," Appl. Catal. B: Environ., 102, 441-448 (2011).   DOI
15 Colombo, M., Nova, I., Tronconi, E., SchmeiBer, V., Konrad, B. B., and Zimmermann, L., "$NO/NO_2/N_2O-NH_3$ SCR Reactions over a Commercial Fe-Zeolite Catalyst for Diesel Exhaust Aftertreatment: Intrinsic Kinetics and Monolith Converter Modelling," Appl. Catal. B: Environ., 111-112, 106-118 (2012).   DOI
16 Frey, A. M., Mert, S., Due-Hansen, J., Fehrmann, R., and Christensen, C. H., "Fe-BEA Zeolite Catalysts for $NH_3$-SCR of NOx," Catal. Lett., 130, 1-8 (2009).   DOI
17 Kwak, J. H., Tran, D., Burton, S. D., Szanyi, J., Lee, J. H., and Peden, C. H. F., "Effects of Hydrothermal Aging on $NH_3$-SCR Reaction over Cu/zeolites," J. Catal., 287, 203-209 (2012).   DOI
18 Ha, H.-J., Hong, J.-H., Choi, J.-H., and Han, J.-D., "Selective Catalytic Reduction of NOx with Ammonia over Cu and Fe Promoted Zeolite Catalysts," Clean Technol., 19(3), 287-294 (2013).   DOI
19 Long, R. Q., and Yang, R. T., "Selective Catalytic Reduction of NO with Ammonia over $Fe^{3+}$-Exchanged Mordenite (Fe-MOR): Catalytic Performance, Characterization, and Mechanistic Study," J. Catal., 207, 274-285 (2002).   DOI
20 Lee, J., Paratore, M., and Brown, D., "Evaluation of Cu-Based SCR/DPF Technology for Diesel Exhaust Emission Control," SAE Int. J. Fuels Lubr., 1(1), 96-101 (2009).
21 Cavataio, G., Jen, H., Warner, J., and Girard, J., "Enhanced Durability of a Cu/Zeolite Based SCR Catalyst," SAE Int. J. Fuels Lubr., 1(1), 477-487 (2009).
22 Bull, I., Boorse, R. S., Jaglowski, W. M., Koermer, G. S., Moini, A., Patchett, J. A., Xue, W. M., Burk, P., Dettling, J. C., and Caudle, M. T., "Copper CHA Zeolite Catalysts," U.S. Patent No. 0,226,545 (2008).
23 Shwan, S., Nedyalkova, R., Jansson, J., Korsgren, J., Olsson, L., Skoglundh, M., "Hydrothermal Stability of Fe-BEA as an $NH_3$-SCR Catalyst," Ind. Eng. Chem. Res., 51, 12762-12772 (2012).   DOI
24 Andersen, P. J., Collier, J. E., Casci, J. L., Chen, H.-Y., Fedeyko, J. M., Foo, R. K. S., and Rajaram, R. R., "SCR Method and System Using Cu/SAPO-34 Zeolite Catalyst," E.P. No. 2,150,328B1 (2008).
25 Zones, S. I., "Zeolite SSZ-13 and its Method of Preparation," U.S. Patent No. 4,544,538 (1985).
26 Moliner, M., Franch, C., Palomares, E., Grill, M., and Corma, A., "Cu-SSZ-39, an Active and Hydrothermally Stable Catalyst for the Selective Catalytic Reduction of NOx," Chem. Commun., 48, 8264-8266 (2012).   DOI
27 Wu, L., and Hensen, E. J. M., "Comparison of Mesoporous SSZ-13 and SAPO-34 Zeolite Catalysts for the Methanol-to-Olefins Reaction," Catal. Today, 235, 160-168 (2014).   DOI
28 Ma, L., Cheng, Y., Cavataio, G., McCabe, R. W., Fu, L., and Li, J., "Characterization of Commercial Cu-SSZ-13 and Cu-SAPO-34 Catalysts with Hydrothermal Treatment for $NH_3$-SCR of NOx in Diesel Exhaust," Chem. Eng. J., 225, 323-330 (2013).   DOI
29 Deimund, M. A., Harrison, L., Lunn, J. D., Liu, Y., Malek, A., Shayib, R., and Davis, M. E., "Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction," ACS Catal., 6, 542-550 (2016).   DOI
30 Baik, J. H., Yim, S. D., Nam, I.-S., Mok, Y. S., Lee, J.-H., Cho, B. K., and Oh, S. H., "Control of NOx Emissions from Diesel Engine by Selective Catalytic Reduction (SCR) with Urea," Top. Catal., 30/31, 1-4 (2004).   DOI
31 Jo, D., Ryu, T., Park, G. T., Kim, P. S., Kim, C. H., Nam, I.-S., and Hong, S. B., "Synthesis of High-Silica LTA and UFI Zeolites and $NH_3$-SCR Performance of Their Copper-Exchanged Form," ACS Catal., 6, 2443-2447 (2016).   DOI
32 Kwak, J. H., Tran, D., Szanyi, J., Peden, C. H. F., and Lee, J. H., "The Effect of Copper Loading on the Selective Catalytic Reduction of Nitric Oxide by Ammonia Over Cu-SSZ-13," Catal. Lett., 142, 295-301 (2012).   DOI
33 Fickel, D. W., and Lobo, R. F., "Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD," J. Phys. Chem. C, 114, 1633-1640 (2010).
34 Barrer, R. M., "Zeolites and Their Synthesis," Zeolites, 1, 130-140 (1981).   DOI
35 Deka, U., Juhin, A., Eilertsen, E. A., Emerich, H., Green, M. A., Korhonen, S. T., Weckhuysen, B. M., and Beale, A. M., "Confirmation of Isolated $Cu^{2+}$ Ions in SSZ-13 Zeolite as Active Sites in $NH_3$-Selective Catalytic Reduction," J. Phys. Chem. C, 116, 4809-4818 (2012).   DOI
36 Gao, F., Kwak, J. H., Szanyi, J., and Peden, C. H. F., "Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts," Top. Catal., 56, 1441-1459 (2013).   DOI
37 Moliner, M., Martínez, C., and Corma, A., "Synthesis Strategies for Preparing Useful Small Pore Zeolites and Zeotypes for Gas Separations and Catalysis," Chem. Mater., 26, 246-258 (2014).   DOI
38 Zones, S. I., and Nordstrand, R. A., "Further Studies on the Conversion of Cubic P Zeolite to High Silica Organozeolites," Zeolites, 8, 409-415 (1988).   DOI
39 Lobo, R. F., "Synthesis and Rietveld Refinement of the Small-Pore Zeolite SSZ-16," Chem. Mater., 8, 2409-2411 (1996).   DOI
40 Lewis, G. J., Miller, M. A, Moscoso, J. G., Wilson, B. A., Knight, L. M., and Wilson, S. T., "Experimental Charge Density Matching Approach to Zeolite Synthesis," Stud. Surf. Sci. Catal., 154, 364-372 (2004).   DOI
41 Blackwell, C. S., Broach, R. W., Gatter, M. G., Holmgren, J. S., Jan, D. Y., Lewis, G. J., Mezza, B. J., Mezza, T. M., Miller, M. A., Moscoso, J. G., Patton, R. L., Rohde, L. M., Schoonover, M. W., Sinkler, W., Wilson, B. A., and Wilson, S. T., "Open-Framework Materials Synthesized in the $TMA^{+]/TEA^{+}$ Mixed-Template System: The New Low Si/Al Ratio Zeolites UZM-4 and UZM-5," Angew. Chem. Int. Ed., 42, 1737-1740 (2003).   DOI
42 Itakura, M., Goto, I., Takahashi, A., Fujitani, T., Ide, Y., Sadakane, M., and Sano, T., "Synthesis of High-Silica CHA Type Zeolite by Interzeolite Conversion of FAU Type Zeolite in the Presence of Seed Crystals," Micropor. Mesopor. Mater., 144, 91-96 (2011).   DOI
43 Miller, M. A., Moscoso, J. G., Koster, S., Gatter, M. G., and Lewis, G. J., "Synthesis and Characterization of the 12-Ring Zeolites UZM-4 (BPH) and UZM-22 (MEI) via the Charge Density Mismatch Approach in the Choline-$Li_2O-SrO-Al_2O_3-SiO_2$ System," Stud. Surf. Sci. Catal., 170, 347-354 (2007).   DOI
44 Kerr, G. T., "Chemistry of Crystalline Aluminosilicates. 11. The Synthesis and Properties of Zeolite ZK-4," Inorg. Chem., 5, 1537-1539 (1966).   DOI
45 Chen, B. Xu, R., Zhang, R., and Liu, N., "Economical Way to Synthesize SSZ-13 with Abundant Ion-Exchanged $Cu^{+}$ for an Extraordinary Performance in Selective Catalytic Reduction (SCR) of NOx by Ammonia," Environ. Sci. Technol., 48, 13909-13916 (2014).   DOI
46 Zones, S. I., "Conversion of Faujasites to High-Silica Chabazite SSZ-13 in the Presence of N,N,N-Trimethyl-l-Adamantammonium Iodide," J. Chem. Soc. Faraday Trans., 87(22), 3709-3716 (1991).   DOI
47 Zones, S. I., and Nordstrand, R. A., "Novel Zeolite Transformations: The Template-Mediated Conversion of Cubic P Zeolite to SSZ-13," Zeolites, 8, (1988).
48 Ren, L., Zhu, L., Yang, C., Chen, Y., Sun, Q., Zhang, H., Li, C., Nawaz, F., Meng, X., and Xiao, F.-S., "Designed Copper-Amine Complex as an Efficient Template for One-Pot Synthesis of Cu-SSZ-13 Zeolite with Excellent Activity for Selective Catalytic Reduction of NOx by $NH_3$," Chem. Commun., 47, 9789-9791 (2011).   DOI
49 Papadakis, V. G., Pliangos, C. A., Yentekakis, I. V., Verykios, X. E., and Vayenas, C. G., "Development of High Performance, Pd-Based, Three Way Catalysts," Catal. Today, 29, 71-75 (1996).   DOI
50 http://www.dieselnet.com/standards (accessed Nov. 2016).
51 Storey, J. M. E., Sluder, C. S., Lance, M. J., Styles, D., and Simko, S., "Exhaust Gas Recirculation Cooler Fouling in Diesel Applications: Fundamental Studies, Deposit Properties and Microstructure," Proceedings of International Conference on Heat Exchanger Fouling and Cleaning, Crete Island, Greece (June 2011).
52 Takahashi, N., Shinjoh, H., Iijima, T., Suzuki, T., Yamazaki, K., Yokota, K., Suzuki, H., Miyoshi, N., Matsumoto, S., Tanizawa, T., Tanaka, T., Tateishi, S.-S., and Kasahara, K., "The New Concept 3-Way Catalyst for Automotive Lean- Burn Engine: $NO_x$ Storage and Reduction Catalyst," Catal. Today, 27, 63-69 (1996).   DOI
53 Elbouazzaoui, S., Corbos, E. C., Courtois, X., Marecot, P., and Duprez, D., "A Study of the Deactivation by Sulfur and Regeneration of a Model NSR $Pt/Ba/Al_2O_3$ Catalyst," Appl. Catal. B: Environ., 61, 236-243 (2005).   DOI
54 Xie, L., Liu, F., Ren, L., Shi, X., Xiao, F.-S., and He, H., "Excellent Performance of One-Pot Synthesized Cu-SSZ-13 Catalyst for the Selective Catalytic Reduction of NOx with $NH_3$," Environ. Sci. Technol., 48, 566-572 (2014).   DOI
55 Elbouazzaoui, S., Corbos, E. C., Courtois, X., Marecot, P., and Duprez, D., "A Study of the Deactivation by Sulfur and Regeneration of a Model NSR $Pt/Ba/Al_2O_3$ Catalyst," Appl. Catal. B: Environ., 61, 236-243 (2005).   DOI
56 Yang, M., Li, Y., Wang, J., and Shen, M., "NOx Removal Efficiency and Ammonia Selectivity During the NOx Storage-Reduction Process over Pt/BaO (Fe, Mn, Ce)/$Al_2O_3$ Model Catalysts. Part II: Influence of Ce and Mn-Ce Addition," Appl. Catal. B: Environ., 102, 362-371 (2011).   DOI
57 Takeuchi, M., and Matsumoto, S., "NOx Storage-Reduction Catalysts for Gasoline Engines," Top. Catal., 28, 1-4 (2004).   DOI
58 Chaugule, S. S., Yezerets, A., Currier, N. W., Ribeiro, F. H., and Delgass, W. N., "'Fast' NOx Storage on $Pt/BaO/{\gamma}-Al_2O_3$ Lean NOx Traps with $NO_2\;+\;O_2\;and\;NO\;+\;O_2$: Effects of Pt, Ba Loading," Catal. Today, 151, 291-303 (2010).   DOI
59 Luo, J.-Y., Kisinger, D., Abedi, A., and Epling, W. S., "Sulfur Release from a Model $Pt/Al_2O_3$ Diesel Oxidation Catalyst: Temperature-Programmed and Step-Response Techniques Characterization," Appl. Catal. A: General, 383, 182-191 (2010).   DOI
60 Park, S. M., "Selective Catalytic Reduction of Nitrogen Oxides Promoted by Storage Function," Ph.D. Dissertation, Chonnam National University, Gwangju (2010).
61 Burch, R., Breen, J. P., and Meunier, F. C., "A Review of the Selective Reduction of NOx with Hydrocarbon under Lean-Burn Conditions with Non-Zeolitic Oxide and Platinum Metal Catalysts," Appl. Catal. B: Environ, 39, 283-303 (2002).   DOI
62 Liu, Z., and Woo, S. I., Recent Advances in Catalytic $DeNO_x$ Science and Technology, Catal. Rev., 48(1), 43-89 (2006).   DOI
63 Wu, L., Degirmenci, V. D., Magusin, P. C. M. M,, Szyja B. M., and Hensen, E. J. M. "Dual Template Synthesis of a Highly Mesoporous SSZ-13 Zeolite with Improved Stability in the Methanol-to-Olefins Reaction," Chem. Commun., 48, 9492-9494 (2012).   DOI
64 Kaspar, J., Fornasiero, P., and Hickey, N., "Automotive Catalytic Converters: Current Status and Some Perspectives," Catal. Today, 77, 419-449 (2003).   DOI
65 Komvokis, V. G., Iliopoulou, E. F., Vasalos, I. A., Triantafyllidis, K. S., and Marshall, C. L., "Development of Optimized Cu- ZSM-5 DeNOx Catalytic Materials both for HC-SCR Applications and as FCC Catalytic Additives," Appl. Catal. A: Gen., 325, 345-352 (2007).   DOI
66 Shichi, A., Katagi, K., Satsuma, A., and Hattori, T., "Influence of Intracrystalline Diffusion on the Selective Catalytic Reduction of NO by Hydrocarbon over Cu-MFI Zeolite," Appl. Catal. B: Environ., 24, 97-105 (2000).   DOI
67 Shichi, A., Statsuma, A., and Hattori, T., "Influence of Hydrocarbon Molecular Size on the Selective Catalytic Reduction of NO by Hydrocarbon over Cu-MFI Zeolite," Appl. Catal. A: Gen., 207, 315-321 (2001).   DOI
68 Nova, I., Ciardelli, C., Tronconi, E., Chatterjee, D., and Weibel, M., "$NH_3-NO/NO_2$ SCR for Diesel Exhausts after Treatment: Mechanism and Modelling of a Catalytic Converter," Top. Catal., 42-43, 43-46 (2007).   DOI
69 Pârvulescu, V. I., Grange, P., and Delmon, B., "Catalytic Removal of NO," Catal. Today, 46, 233-316 (1998).   DOI
70 Grossale, A., Nova, I., and Tronconi, E., "Study of a Fezeolite-based System as $NH_3$-SCR Catalyst for Diesel Exhaust Aftertreatment," Catal. Today, 136, 18-27 (2008).   DOI
71 Grossale, A., Nova, I., and Tronconi, E., "Study of a Fe-zeolitebased System as $NH_3$-SCR Catalyst for Diesel Exhaust Aftertreatment," Catal. Today, 136, 18-27 (2008).   DOI
72 Yates, M., Martin, J. A., Martin-Luengo, M. A., Suarez, S., and Blanco, J., "$N_2O$ Formation in the Ammonia Oxidation and in the SCR Process with $V_2O_5-WO_3$ Catalysts," Catal. Today, 107-108, 120-125 (2005).   DOI
73 Schuler, A., Votsmeier, M., Kiwic, P., Gieshoff, J., Hautpmannb, W., Drochner, A., and Vogel, H., "$NH_3$-SCR on Fe Zeolite Catalysts-From Model Setup to $NH_3$ Dosing," Chem. Eng. J., 154, 333-340 (2009).   DOI
74 Chen, L., Li, J., Gea, M., and Zhu, R. "Enhanced Activity of Tungsten Modified $CeO_2/TiO_2$ for Selective Catalytic Reduction of NOx with Ammonia," Catal. Today, 153, 77-83 (2010).   DOI
75 Kim, M. H., "Performance Management of a DeNOx System for Stationary Sources and Regeneration Strategies of DeNOx Catalysts," Clean Technol., 22(3), 141-153 (2016).   DOI
76 Lietti, L., Nova, I., Ramis, G., Acqua, L. D., Busca, G., Giamello, E., Forzatti, P., and Bregani, F., "Characterization and Reactivity of $V_2O_5-MoO_3-TiO_2$ De-NOx SCR Catalysts," J. Catal., 187, 419-435 (1999).   DOI
77 Choo, S. T., Yim, S. D., Nam, I.-S., Ham, S.-W., and Lee, J.-B., "Effect of Promoters Including $WO_3$ and BaO on the Activity and Durability of $V_2O_5$/sulfated $TiO_2$ Catalyst for NO Reduction by $NH_3$," Appl. Catal. B: Environ., 44, 237-252 (2003).   DOI
78 Seo, C.-K., and Chio, B. C., "Physicochemical Characteristics According to Aging of Fe-Zeolite and $V_2O_5-WO_3-TiO_2$ SCR for Diesel Engines," J. Ind. Eng. Chem., 25, 239-249 (2015).   DOI
79 Shan, W., Liu, F., He, H., Shi, X., and Zhang, C., "A Superior Ce-W-Ti Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with $NH_3$," Appl. Catal. B: Environ., 115-116, 100-106 (2012).   DOI
80 Chen, L., Li, J., and Ge, M., "Promotional Effect of Ce-doped $V_2O_5-WO_3/TiO_2$ with Low Vanadium Loadings for Selective Catalytic Reduction of NOx by $NH_3$," J. Phys. Chem. C, 113, 21177-21184 (2009).   DOI
81 Wang, Z., Li, X., Song, W., Chen, J., Li, T., and Feng, A., "Synergetic Promotional Effects Between Cerium Oxides and Manganese Oxides for $NH_3$-Selective Catalyst Reduction Over Ce-Mn/$TiO_2$," Mater. Express, 1(2), 167-175 (2011).   DOI
82 Nova, I., Acqua, L., Lietti, L., Giamello, E., and Forzatti, P., "Study of Thermal Deactivation of a De-NOx Commercial Catalyst," Appl. Catal. B: Environ., 35, 31-42 (2001).   DOI