Electrochemical reduction of NOx - DTU Orbit

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4 DRIFT study of NOx adsorption on CGO10 impregnated with K2O or BaO deNOx. The study will be made on CGO10 (Ce0.9Gd0.1O1.95), a material frequently used in electrodes for electrochemical deNOx. Moreover the effect on the NOx adsorption of impregnating the CGO10 with K2O or BaO is investigated as several studies show that addition of these compounds, known from the NSR catalyst, may increase the NOx conversion during electrochemical deNOx 43, 44, 46 . The findings in this work are in the results and discussion section related to literature on NOx adsorption on compounds similar to the electron conducting perovskites frequently used in deNOx-electrodes. Altogether this will offer a possibility of relating observations on activity and selectivity during electrochemical deNOx to the adsorption capabilities of the electrode materials. Even though the DRIFT measurements in this work are performed at relevant temperatures and gas compositions, they cannot be considered “true” in-situ measurements with respect to electrochemical deNOx since they are performed on powders and not electrodes under polarization. Martinez-Arias et al. pointed out that NO adsorption depends on the vacancy concentration at the surface of CeO2 114 . Since the vacancy concentration of CGO based electrodes likely is significantly altered during cathodic polarization, due to the pumping of O 2- away from the electrode surface, there may be some differences between the NO adsorption observed on CGO10 powders and on CGO10 in electrodes under polarization. However, since an important aim in this work is to study the influence of K2O and BaO impregnation on the NOx adsorption on CGO10, it is believed that altered properties of the powder sample, owing to impregnation with K2O or BaO, reflects similar effects on electrodes under polarization. 4.2.1 NOx adsorption on CeO2 To the best of the author’s knowledge there is no reporting on IR-studies of NOx adsorption on CGO10 in the temperature range 300-500 °C. However, several studies have been made of NOx- adsorption on CeO2 in the temperature range from RT to 400 °C 114-117 . Even though the assignment of the adsorption peaks observed is not unambiguous, the references in general agree on the presence of several different adsorbed NOx-species on CeO2, and usually assign the peaks to the presence of NO2 - (nitrite), N2O2 2- (hyponitrite) and NO3 - (nitrate) species. In the work by Philip et al. it was pointed out, how NO adsorption without oxygen mainly led to the formation of NO2 - , whereas in the presence of oxygen NO3 - was formed through the steps: NO adsorption/NO2 - formation → oxidation to NO3 - 116 . Philip et al. also showed, how the NOx, adsorption decreased, when the temperature was increased from 50 °C to 300 °C 116 . 26
4 DRIFT study of NOx adsorption on CGO10 impregnated with K2O or BaO 4.2.2 NOx adsorption on K2O and BaO Numerous studies have been made on the NOx adsorption on K- and Ba- containing catalysts, as both elements are of interest for the NSR-catalyst 69-71, 85, 118-122 . On Ba, NOx mainly adsorbs in the form of NO2 - or NO3 - 69, 70, 119 . Similar for K-containing catalysts, the NOx adsorption mainly leads to formation of NO2 - and NO3 - 72, 120 , but as pointed out by Lesage et al. 72 NO2 - formation is only detected at lower temperatures (200 °C). At higher temperatures (400 °C) only NO3 - formation is observed, implying that either NO3 - is formed directly at higher temperatures or that NO2 - conversion into NO3 - is so fast the NO2 - cannot be detected. 4.3 Experimental Methods 4.3.1 Sample preparation Three different samples were prepared for the measurements on CGO: 1) A non-impregnated CGO sample, 2) a K2O impregnated sample, and 3) a BaO impregnated sample. The CGO10 was a commercially available powder from Rhodia. For the preparation of the impregnated samples an aqueous solution of either KNO3 or Ba(NO3)2 was made. The nitrate solutions were mixed with CGO powder for several hours, and subsequently the mixtures were dried at 80 °C. After drying the samples were heated to 700 °C to decompose the nitrate into oxide. Amounts were chosen to give a weight percentage in the dry mixture of 3 wt% K2O and 1 wt% BaO, respectively. 4.3.2 IR measurements The DRIFT measurements were performed on a Biorad FTS 6000 FTIR spectrometer equipped with a high-temperature reaction cell (Harrick Scientific, Praying Mantis) with KBr windows. The gasses (NO, NO2, O2 and Ar) were supplied by mass flow controllers (Bronkhorst Hi-Tech) and the total flow to the cell during pre-treatment and measurements was 100 ml/min. The sample crucible was heated during the experiment and the temperature was controlled by a K-type thermocouple connected to a Eurotherm 2416 temperature controller. 27
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 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: 4 DRIFT study of NOx adsorption on
Page 37 and 38: 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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