Electrochemical reduction of NOx - DTU Orbit

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4 DRIFT study of NOx adsorption on CGO10 impregnated with K2O or BaO 4 Diffuse Reflectance Infrared Fourier Transform Study of NOx Adsorption on CGO10 Impregnated with K2O or BaO This chapter is the manuscript “Diffuse Reflectance Infrared Fourier Transform Study of NOx Adsorption on CGO10 Impregnated with K2O or BaO” accepted for publication in the Journal of Physical Chemistry A. The experiments described in this chapter were conducted at Chalmers University of Technology in Sweden. 4.1 Abstract In the present work Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy is applied to study the adsorption of NOx at 300-500 °C in different atmospheres on gadolinium doped ceria (CGO), an important material in electrodes investigated for electrochemical NOx removal. Furthermore, the effect on the NOx adsorption when adding K2O or BaO to the CGO is investigated. The DRIFT study shows mainly the presence of nitrate species at 500 °C, while at lower temperature a diversity of adsorbed NOx species exists on the CGO. Presence of O2 is shown to have a strong effect on the adsorption of NO, but no effect on the adsorption of NO2. Addition of K2O and BaO dramatically affects the NOx adsorption and the results also show that the adsorbed NOx species are mobile and capable of changing adsorption state in the investigated temperature range. 4.2 Introduction Nitrogen oxides (NOx) affect the respiratory system negatively, increases the formation of ozone at ground level, cause formation of acid rain, contribute to smog formation and also act as green house gas 11 . Even though the NOx emission from road transport in Europe has been significantly reduced during the last 10 years, road transport is still the largest single contributor to the NOx pollution 2 . The reduction of NOx emission from road transport has mainly followed from the introduction of the three-way-catalyst for cleaning of the exhaust from gasoline engines 2 . Unfortunately, the three-way catalyst cannot remove NOx from diesel engine exhaust due to the higher oxygen content in diesel exhaust. Much research is currently focused on improving the technologies for removing NOx from diesel exhaust, with main focus on the three technologies: 1) selective catalytic reduction with urea (urea-SCR), 2) selective catalytic reduction with 24
4 DRIFT study of NOx adsorption on CGO10 impregnated with K2O or BaO hydrocarbons (HC-SCR) and 3) the NOx-storage and reduction (NSR) catalyst, for a review on the different techniques see Kaspar et al. 7 or Skalska et al. 108 . An alternative method under development for NOx-removal is the electrochemical reduction of NOx 109-111 , where NOx reduction to N2 and O2 occurs in an electrochemical cell. The NOx is reduced by the electrons supplied to the cell during operation, which makes it unnecessary to add other reducing agents like urea or extra hydrocarbons, as necessary in urea-SCR, HC-SCR and NSR catalysis. In the electrochemical cell the NOx is reduced on the cathode and afterwards the O 2- migrates through an oxygen ion conducting electrolyte and is subsequently oxidized to O2 on the anode, a sketch of the principle in electrochemical deNOx is shown in Figure 7. Figure 7. Principle of electrochemical deNOx (reprinted with permission from 37 ) In the investigation of suitable electrode materials for electrochemical deNOx a major challenge is to find electrode materials which have a sufficiently high activity and selectivity, i.e. which are capable of reducing NOx but not O2, as reduction of O2 is a competing reaction on the cathode 111 . Several electrode materials have been investigated for electrochemical deNOx, for example noble metals 22, 27, 30 , spinels 112 and perovskites 35, 39, 45 . Frequently a composite electrode is applied, i.e. an electrode with both an electron conducting phase (for instance a perovskite) and an O 2- conducting phase (for instance yttria stabilized zirconia (YSZ) or CGO) 39, 45, 113 . An obstacle in the investigation and understanding of electrochemical deNOx is a limited knowledge of the adsorption of NOx on the electrode materials, and how this adsorption changes in the temperature ranges investigated for electrochemical deNOx (usually 300-600 °C) and depending on the presence of NO or NO2. The aim in this work is to apply DRIFT (Diffuse Reflectance Infrared Fourier Transform) spectroscopy to study the NOx adsorption at conditions relevant for electrochemical 25
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: 3 Experimental NO2* which decays to
Page 35 and 36: 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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