Electrochemical reduction of NOx - DTU Orbit

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2.3 Materials compatibility 2 Materials One of the major concerns when impregnating the NOx-storage compounds into the electrodes is whether the storage compounds may react with the electrode materials. This concern is not significant with respect MnOx impregnation on LSM electrodes, as the LSM used for the LSM electrodes from the beginning was synthesized as being slightly overstochiometric with respect to MnOx, and thus already contained some MnOx. However with respect to both K2O and BaO impregnation on LSM15-CGO10 and LSF15-CGO10 electrodes the risk of alternative phase formation should be taken more seriously. In the work by Simonsen et al. 43 no reaction between BaO and LSM15 was observed, when a mixture of these two materials had been sintered for 12 h at 1250 °C, and neither was any reaction between Ba-containing materials and LSM observed in the work by Ai et al. 90 or Jin et al. 91 which altogether indicates no reaction between the BaO and the LSM should be expected. This is however contradicted by the study made by Yang et al. 92 , which showed diffusion of Ba from BSCF (Ba1-xSrxFe1-yCoyO3) into LSM. In conclusion, even though most studies indicate there will be no reaction between LSM and BaO, this cannot be entirely excluded. With respect to reactivity between BaO and CGO, several studies report on the use of Ba-containing electrodes together with CGO electrolytes, but none of the studies show reaction between Ba and CGO as long as the temperature stays below 900 °C 93-95 . To the best of the author’s knowledge no studies are reported on the compatibility between LSM and KNO3/K2O. The best study for comparison is for this reason likely the study by Kim et al. 96 , in which Mn3O4 impregnated with 0.5 wt% K was investigated as an hydrocarbon oxidation catalyst. The study showed no new phase formation after impregnation and calcination of the sample; however XPS indicated the K impregnated had increased the defect concentration in the Mn3O4 96 . Furthermore a number of potassium containing manganese compounds are known to exist, namely potassium permanganate (KMnO4) and potassium manganate (K2MnO4) 1 , which possibly could form during reaction between K and excess manganese in the electrode. With respect to the combination of LSF15 with BaO and K2O no information was found in literature, however due to the similarity in oxidation state and ionic radii of Mn and Fe ions in LSM15 and LSF15 respectively, similar behaviour with respect to BaO and K2O may be expected. 14
2.4 Material properties 2 Materials In addition to a possible reaction between electrode materials and impregnated NOx-storage compounds as described in the previous section, there is also the possibility the presence of the NOx-storage compounds may significantly alter the current passage through the electrodes and the cell stacks, especially if the impregnated compounds have a much higher conductivity than the electrodes and form an interconnected network through the cell stacks. Furthermore if the NOx- storage compounds are molten at the operating temperatures during electrochemical deNOx, an increased risk of degradation/coalescence must be expected. For these reasons it is important to know both the conductivities of electrode materials and the NOx-storage compounds, and the melting points of the NOx-storage compounds. The relevant conductivities and melting points found from literature are stated in Table 2. No conductivities could be found for K2O and BaO; likely since these two oxides can be considered insulators. When the conductivities in Table 2 are compared, the conductivities for LSF15 and LSM15 are consistently highest. By comparison of the conductivities for CGO10 and KNO3 it is evident, that in sections of an electrochemical cell with only CGO10, for instance through a porous CGO electrolyte, an easier current path may be established through molten KNO3 at 350-500 °C. In the temperature range 300-500 °C investigated for electrochemical deNOx it is also clear from Table 2, that only KNO3 will be present as a melt, while the other NOx storage compounds will remain in solid form. Table 2. Conductivities and melting points electrode materials and NOx-storage compounds. Compound Conductivity Ref. Melting point 97 [S/cm] [°C] LSM15 90-110 (300-600 °C) 52, 53 LSF15 110 (500 °C) 98 CGO10 0.005 (500 °C) 68, 99 K2O - KNO3 0.660-1.094 (350-500 °C) BaO - Ba(NO3)2 MnOx 1x10-6 (300 °C) 5x10 -7 - 3x10 -3 S/cm 2 at RT(MnO2) 15 740 100 334 1973 101 590 Mn(NO3)2 - * *Melting point cannot be determined, as Mn(NO 3) 2 decomposes to MnO x before melting 103 102 1842 (MnO), 1567 (Mn3O4), 1080 (Mn2O3)
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 and 12: 1 Introduction 3, 12 Table 1. Emiss
Page 13 and 14: O 1 Introduction 2 2 NOx storage: N
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: 2 Materials used promoter in conven
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
Page 57 and 58: 5 Improvement of LSM15-CGO10 electr
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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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Electrochemical reduction of NOx - DTU Orbit

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