Electrochemical reduction of NOx - DTU Orbit

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1 Introduction then evaporated to gasous NH3 before reaction in the SCR catalyst 15 . Even though NOx conversion higher than 95% can be achieved with NH3-SCR 16 , the technology has a significant drawback as it is necessary to have a separate supply system for the ammonia. Furthermore the ammonia supply must be carefully controlled to prevent ammonia slip in the exhaust. Despite of these drawbacks NH3-SCR is expected to be dominant technology for diesel exhaust cleaning in the future in combination with particle filters and oxidation catalysts units 3, 13 . 1.2.2 HC-SCR As explained in the previous section one of the disadvantages by NH3-SCR is the necessity of adding a separate ammonia supply system. In order to avoid a separate supply system for the addition of a NOx reductant, it has been suggested to use excess hydrocarbons from the fuel as the NOx reductant in the technology named HC-SCR. The overall reaction scheme for the reduction of NOx to N2 with hydrocarbons is illustrated with propene as an example: 2 NO + 3.5 O2 + C3H6 → N2 + 3 CO2 + 3 H2O (1.6) In the investigation of suitable catalysts for HC-SCR the main focus has been on zeolite-catalysts 17 , especially the ZSM-5 zeolite, and on noble metals, especially Ag, on alumina support 18 . Despite the extensive research in HC-SCR, the technology has not reached commercialization yet, mainly due to unsolved problems with activity and catalyst stability 17, 18 . 1.2.3 NSR catalysis The NSR catalyst for NOx removal from diesel exhaust was developed by Toyota in 1990’s 19 . The catalyst consist of a precious metal and a NOx storage compound (typically BaO) on an alumina support, and for the catalyst to work the engine must be cycled between 2 operating modes: 1) normal lean operation of the engine, during which the NOx in the exhaust is stored as nitrate on the NOx-storage compound, 2) operation with excess fuel-to-air ratio during which the excess fuel is used for reduction of the nitrate into N2. The storage and reduction of NOx is described by the following two equations 20 : 4
O 1 Introduction 2 2 NOx storage: NO→ NO 2 →Ba(NO 3 ) 2 (1.7) Pt HC, H2 , CO Reduction: ) ⎯⎯⎯⎯→ BaO + NO → N + CO + H O (1.8) Ba(NO3 2 2 2 2 2 Pt 5 O Ba HC, H2 , CO Even though the NSR catalyst has been introduced on the Japanese market 20 , the technology has some drawbacks/challenges to be solved with respect to fuel penalty, sensitivity to fuel sulfur levels and control under transient engine operation 21 . 1.2.4 Electrochemical deNOx Electrochemical deNOx is the NOx removal technology studied in this project. The principle in electrochemical deNOx is, the NOx is reduced to N2 in an electrochemical cell by means of the electrons supplied to the cell. Thereby addition of reductants becomes unnecessary. Typically electrochemical cells similar to solid oxide fuel cells are used, which means the electrolyte is an oxygen ion conductor and the desired reactions on the cathode and anode become: Cathode: NO + 2 e - → 0.5 N2 + O 2- Anode: O 2- → 0.5 O2 + 2 e - (1.9) (1.10) A competitive reaction on cathode in oxygen containing atmosphere is the reduction of O2 (equation 1.11), which will lead to a waste of energy during electrochemical deNOx. 0.5 O2 + 2 e - → O 2- (1.11) The idea electrochemical reduction could be used for NOx removal was initiated when Pancharatnam et al. in 1975 discovered NO could be reduced on polarised zirconia surfaces 22 . The active site for the NOx reduction was suggested to be F-centers i ,22 , which was later confirmed in a i An F-center is an oxygen ion vacancy with one electron trapped.
Page 1 and 2: Department of Energy Conversion and
Page 3 and 4: Preface Preface This thesis is subm
Page 5 and 6: Dansk resumé Dansk resumé NOx er
Page 7 and 8: Table of Contents 5.3 Experimental
Page 9 and 10: 1 Introduction 1 Introduction NOx i
Page 11: 1 Introduction 3, 12 Table 1. Emiss
Page 15 and 16: 1 Introduction the electrode materi
Page 17 and 18: 1 Introduction The main focus of th
Page 19 and 20: 2 Materials 2 Materials In this pro
Page 21 and 22: 2 Materials used promoter in conven
Page 23 and 24: 2.4 Material properties 2 Materials
Page 25 and 26: 3 Experimental Figure 1. Symmetric
Page 27 and 28: 3.2 Fabrication of electrodes 3 Exp
Page 29 and 30: 3 Experimental 3) Covering the cell
Page 31 and 32: 3 Experimental NO2* which decays to
Page 33 and 34: 4 DRIFT study of NOx adsorption on
Page 55 and 56: 4.5 Conclusions 4 DRIFT study of NO
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5 Improvement of LSM15-CGO10 electr
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6 NOx-conversion on Porous LSF15-CG
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7.1.3 SEM 7 NOx-conversion on LSF15
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7 NOx-conversion on LSF15-CGO10 cel
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8 NOx-conversion on LSM15-CGO10 cel
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8.5 Discussion 8 NOx-conversion on
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8.6 Conclusion 8 NOx-conversion on
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9.4 Conclusion 9 NOx-conversion on
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10 Measurements on LSM15-CGO10 elec
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11 Construction of an in situ XANES
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11.3 Experimental 11.3.1 Design of
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11.4 Results 11.4.1 Impedance measu
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11.4.2 XANES 11 Construction of an
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12 Overall Discussion, Conclusion a
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12.3 Outlook 12 Overall Discussion,
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References References 1 G. Rayner-C
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26 T. Hibino, Chem. Lett., 1994, 5,
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50 S. P. Jiang, J. Mater. Sci., 200
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References 73 T. J. Toops, D. B. Sm
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References 98 G. W. Coffey, J. Hard
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References 123 A. Laachir, V. Perri
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References 147 C. Lu, T. Z. Sholkla
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References 171 K. Adams, J. Cavatai
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Appendix A Appendix A The article
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temperature and in the presence of
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Table 1 Energy intervals and step s
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Figure 5 Impedance spectra, Nyquist
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Knibbe, R., Hauch, A., Hjelm, J., E
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