Simultaneous Ammonia and Nitrate Electrochemical Removal Using Carbon Supported

Electrodes

A thesis presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

Mohiedin Bagheri Hariri

August 2020

© 2020 Mohiedin Bagheri Hariri. All Rights Reserved. 2

This thesis titled

Simultaneous Ammonia and Nitrate Electrochemical Removal Using Carbon Supported

Electrodes

by

MOHIEDIN BAGHERI HARIRI

has been approved for

the Department of Chemical and Biomolecular Engineering

and the Russ College of Engineering and Technology by

Valerie Young

Associate Professor of Chemical and Biomolecular Engineering

Gerardine G. Botte

Professor of Chemical and Biomolecular Engineering

Mei Wei

Dean, Russ College of Engineering and Technology 3

Abstract

MOHIEDIN BAGHERI HARIRI, M.S., August 2020, Chemical Engineering

Simultaneous Ammonia and Nitrate Electrochemical Removal Using Carbon Supported

Electrodes (169 pp.)

Directors of Thesis: Valerie Young and Gerardine G. Botte

The goal of this study is to introduce a new undivided cell capable of simultaneous removal of nitrate and ammonia from wastewater streams using CuNi-PtIr supported on carbon (CuNi-PtIr/C) catalysts. The Environmental Protection Agency

- (EPA) considers ammonia (NH3) and nitrate (NO3 ) as a large-scale threat to environmental quality and human health, causing impaired air quality, surface water eutrophication, and other serious health problems. This project focuses on introducing new classes of CuNi-PtIr/C catalysts for efficiently simultaneous removal of ammonia and nitrate through a pulse electrolysis technique in an undivided flow cell system. A systematic study has been conducted to examine different compositions of PtIr/C and

CuNi/C catalysts using electrochemical techniques, and it was found that 40% Cu9Ni/C and 60% Pt9Ir/C are the most efficient catalysts for the nitrate reduction and ammonia oxidation, respectively. The introduced process enables the reduction of nitrate and the direct oxidation of ammonia to nitrogen with the co-generation of hydrogen in alkaline media. The co-generated hydrogen can be used for further recovery of the energy. Using the introduced technique, in some cases, after a few hours of electrolysis, no nitrate was left at the end. The ammonia concentration decreased from 2000 ppm to below 100 ppm, with an average current density of about 30-50 mA/cm2 (depending on electrolysis 4 condition and electrode properties). Using this undivided flow cell, we were successful in simultaneously removing nitrate and ammonia. The proposed undivided flow cell uses

65-90% less energy than the existing processes. Because of this advantage, this technique could be implemented for the nitrate/ammonia decontamination of multiple lakes, rivers, and ponds.

5

Preface

Either the entire or particular paragraphs of the Introduction, Literature Review,

Experimental Setup, Methodology, Results & Discussion, and Conclusion of this thesis will be submitted to the Journal of The Electrochemical Society or Catalysts journal under the title “CuNi and PtIr supported on carbon catalysts for synchronous nitrate removal and ammonia electro- oxidation using a pulse electrolysis technique in alkaline media”.

6

Dedication

Dedicated to my family

7

Acknowledgments

I would like to appreciate my advisor, Dr. Gerardine Botte, for all her guidance and support. I want to also thank my advisor, Dr. Valerie Young, for her support and help since Dr. Botte’s departure. I would like to thank my committee members, particularly

Dr. Kruse-Daniels, Dr. Che, and Dr. Crist for all their help and advice. I also appreciate

Dr. Harrington’s feedback on my proposal.

My special gratitude also goes out to Mr. John Goettge, our amazing lab coordinator, for all his help and assistance. I would like to thank the Center for Electrochemical

Engineering Research (CEER) staff. I also do appreciate our staff at the Chemical and

Biomolecular Engineering Department, particularly Mr. Thomas Riggs. Lastly, I would also like to thank my colleague Benjamin Sheets for all the valuable discussion and exchange of ideas during this project.

This project was funded through CEPro Tech, DOE, OWDA, and NSF. I do appreciate all of them for supporting this project financially. Most importantly, I would like to thank God for this opportunity. I am looking forward to more experiences and works in the future.

8

Table of Contents

Page

Abstract ...... 3 Preface...... 5 Dedication ...... 6 Acknowledgments...... 7 List of Tables ...... 11 List of Figures ...... 13 Chapter 1: Introduction ...... 19 1.1. Project Significance ...... 19 1.2. Project Overview ...... 20 1.3. Statement of Objectives ...... 22 Chapter 2: Literature Review ...... 24 2.1. Ammonia Electrolysis ...... 24 2.2. Nitrate Electro-Reduction ...... 27 2.2.1. Pulse Potential Electrolysis for Nitrate Electrochemical Removal...... 30 2.2.2. Copper-Based Cathodes for Nitrate Electro-Reduction in Alkaline Media by Constant Potential Electrolysis ...... 32 2.2.2.1.Voltammetric Study of Nitrate Reduction on Copper-Based Catalysts34 2.2.3. Effect of Cell Configuration on Nitrate Electro-Reduction Kinetics ...... 36 2.3.Simultaneous Removal of Nitrate and Ammonia ...... 39 2.3.1. A Paired Electrolysis in a Solid Polymer Electrolyte Reactor ...... 39 2.3.2. Airlift Inner-Loop Sequencing Batch Reactror (SBR) ...... 40 2.3.3. Combination of ANAMMOX and Hydrogenotrophic Denitrification ...... 41 2.3.4. Reduction of Nitrate and Oxidation of Ammonia in an Undivided Cell by Constant Current Electrolysis ...... 42 2.3.4.1. Influence of pH on Kinetics of Nitrate Reduction ...... 43 2.3.4.2. Influence of Temperature on Kinetics of Nitrate Reduction ...... 43 2.3.4.3. Effect of Current Density on Kinetics of Nitrate Reduction...... 44 2.3.4.4. Effect of Addition of NaCl on Kinetics of Nitrate Reduction and Oxidation of Ammonia ...... 45 2.3.4.5. Effect of the Ratio of Anode to Cathode Surface Area on Kinetics of Nitrate Reduction ...... 48 9

2.3.5. Mechanism of Nitrate Reduction and Ammonia Oxidation ...... 50 2.3.5.1. Mechanism of Nitrate Electro-Reduction ...... 51 2.3.5.2. Mechanism of Ammonia Electro-Oxidation ...... 55 2.3.5.3. Mechanism of Simultaneous Nitrate Reduction and Ammonia Oxidation in an Undivided Cell During Pulse Electrolysis ...... 58 Chapter 3: Experimental & Methodology...... 60 3.1. Materials & Apparatus ...... 60 3.2. PtIr-CuNi/C Catalyst Synthesis ...... 61 3.2.1. Synthesis of PtIr Supported on Carbon Catalyst ...... 61 3.2.2. Synthesis of CuNi Supported on Carbon Catalyst ...... 62 3.3. Electrodeposition of Bimetallic CuNi Catalysts ...... 63 3.4. Nitrate and Ammonia Analysis ...... 63 3.5. Electrochemical Measurements and Characterizations ...... 65 3.6. Characterization of the Catalysts...... 66 3.7. Electrolysis Process ...... 67 3.8. Quality Assurance and Reproducibility ...... 68 3.8.1. Uncertainties and Calibration ...... 68 3.8.2. Total Number of Measurements During Ammonia and Nitrate Analysis .. 69 Chapter 4: Results & Discussion ...... 70 4.1. Physical Characterization of Synthesized Catalysts ...... 70 4.1.1. XRD and TEM Characterization of Catalysts ...... 70 4.2. Ammonia Electro-Oxidation ...... 71 4.2.1. Characterization and Selection of a PtIr/C for Ammonia Oxidation...... 74 4.2.2. Electro-Catalytic Activity of PtIr/C Catalyst for Ammonia Oxidation ...... 76 4.2.3. Kinetics of Ammonia Oxidation ...... 80 4.2.4. Effect of pH and NaCl Addition ...... 81 4.2.5. Effect of Temperature ...... 86 4.2.6. Effect of Electrode Spacing on Electrolysis Current ...... 90 4.2.7. Effect of Flow Cell Size on Ammonia Electrolysis ...... 93 4.2.8. Electro-Sensitivity of PtIr/C Catalysts for Ammonia Detection...... 95 4.2.9. Effect of Substrate and Pulse Switching Time (Pulse Width) on PtIr/C Catalyst Durability ...... 96

4.2.10. Durability of 50% Pt3Ir/C Catalyst ...... 99 10

4.3. Nitrate Electro-Reduction ...... 101 4.3.1. Electrochemical Characterization of Three Different Candidates for Nitrate Electro-Reduction ...... 101 4.3.1.1. Obtaining Best CuNi Composition for Nitrate Reduction Reaction . 112 4.3.2. Comparison Between Electroplated CuNi and CuNi/C Catalyst for Nitrate Electro-Reduction ...... 115 4.3.3. Characterization & Selection of CuNi/C Cathode for Nitrate Reduction . 118 4.3.3.1. ElectroCatalytic Activity of CuNi/C Catalyst for Nitrate Reduction 120 4.3.4. Electro-Sensitivity of CuNi/C Catalysts for Nitrate/Nitrite Detection ..... 126 4.4. Simultaneous Ammonia and Nitrate Removal in an Undivided Cell ...... 127 4.4.1. Effect of Catalyst Loading ...... 127

4.4.2. Simultaneous Nitrate and Ammonia Removal Using a Mixed Cu9NiPt9Ir/C Catalyst (25 cm2 Flow Cell) ...... 135 4.4.3. Energy Consumption Calculations in New Developed Undivided Cell ... 138 Chapter 5: Conclusions and Future Works ...... 140 5.1. Conclusions ...... 140 5.2. Future Scope and Recommendations ...... 142 5.3. Cell Scale-up Considerations ...... 143 References ...... 144 Appendix 1: Ammonia Measurment & ISE Calibration ...... 157 Appendix 2: Nitrate Analysis...... 166 Appendix 3: References for the Appendices ...... 169

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List of Tables

Page

Table 2.1. Possible reaction pathways during nitrate/nitrite reduction ...... 29 Table 4.1. Variation of ammonia concentration, ammonia rate loss, and Faraday efficiency 2 versus time during ammonia electrolysis in a 300 cm flow cell with a pair of 60% Pt9Ir/C 2 electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) ...... 73

Table 4.2. Effect of catalyst loading on current density associated with ammonia oxidation at different temperatures in 225 cm2 flow cell ...... 89

Table 4.3. Effect of electrodes spacing on the normalized current associated with ammonia oxidation at different temperatures ...... 92

Table 4.4. Effect of cell size and NaOH concentration on average current density during ammonia electrolysis process in 8.3 g/l (NH4)2SO4 solution at 40 ◦C (2 LPM, pulse width

18 sec, pulse potential ± 0.925 V), all electrodes are similar 60% Pt9Ir/C, loading 0.2 mg/cm2 ...... 94

Table 4.5. Effect of electrodes spacing on the normalized current associated with ammonia oxidation at different temperatures ...... 95

Table 4.6. Average current density corresponding to ammonia oxidation on three consecutive days of pulse electrolysis in 8300 ppm (NH4)2SO4 + 6 g/l NaOH (potential 2 amplitude= ±0.925 V, T = 38 °C, loading: Pt3Ir/C-Ni = 0.301 mg/cm , Pt3Ir/C-Ti = 0.302 2 2 mg/cm , Pt3Ir/C cathode = 0.312 mg/cm ...... 97

Table 4.7. EDS analysis of four electroplated CuNi catalysts on Ni mesh ...... 112

Table 4.8. Six different synthesized grades of Cu9Ni supported on Vulcan ...... 119

Table 4.9.BET analysis of three synthesized Cu9Ni/C catalysts with the highest surface area ...... 120 12

Table 4.10. Ammonia and nitrate analysis during electrolysis process in 8.3 g/l 2 (NH4)2SO4+18g/l NaOH + 1.5 ppm nitrate a 225 cm cell with a pair of 60% Pt9Ir/C catalyst: loading 0.5 mg/cm2 (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) .. 134

Table A.2.1. Nitrate analysis calibration table ...... 167

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List of Figures

Page

Figure 2.1. Schematic representation of ammonia electrolysis process for hydrogen production...... 26 Figure 2.2. Cell configuration for ammonia electrolysis designed by Jiang et al. to monitor the rate of H2 production...... 27 Figure 2.3. A divided glass cell designed by De et al. for investigating the mechanism of nitrate reduction...... 29 Figure 2.4. Variation of nitrite, ammonia, and nitrogen gas concentration vs. electrolysis time under an asymmetrical square potential wave having the duration of positive and negative of 20% and 80%, respectively (positive and negative potential limits are 1.5 V and -1.5 V, respectively)...... 32 Figure 2.5. Linear sweep voltammogram of (a) copper electrode in 1M NaOH+10 mM NaNO3, and (b) graphene-modified copper electrode in 0.1 M NaOH + 40 mM NaNO3.34 Figure 2.6. Different reaction pathways during nitrate reduction on copper in alkaline media...... 36 Figure 2.7. Mechanism of nitrate reduction in SCC & DCC in the presence of Cl- ions...36 Figure 2.8. Mechanism of nitrate reduction in Pt|Nafion|Pt-Cu cell configuration and corresponding schematic representation of catalytic hydrogenation or electron transfer reactions...... 37 Figure 2.9. Schematic representation of micro-electrolysis reaction cell design based on Pd-Sn/AC catalyst particles...... 38 Figure 2.10. Zero gap solid polymer electrolyte reactor—1: catholyte; 2: end plate (stainless steel); 3: manifold plate (PTFE); 4: distributor (stainless steel mesh); 5: cathode (PdRh1.5/Ti mini-mesh); 6: Nafion® 117 membrane; 7: anode (Pt/Ti mini-mesh); 8: seal Oring (Tiron rubber); 9: anolyte. The cell dimension is 22 cm×14cm×3cm. (b) Flow diagram—10: gas adsorbent reservoir; 11: catholyte reservoir; 12: zero-gap solid polymer electrolyte reactor; 13: pump; 14: anolyte reservoir...... 40 Figure 2.11. Schematic representation of the airlift inter-loop sequencing batch reactor..41 Figure 2.12. Simultaneous removal of nitrate and ammonia using the ANAMMOX technique combined with hydrogenotrophic denitrification...... 42 Figure 2.13. Nitrate, nitrite, and ammonia concentrations vs. time during constant current electrolysis in (a) absence and in (b) presence of 0.3 g/l NaCl...... 46 Figure 2.14. Reaction pathways for nitrate reduction on BDD electrodes in the presence of chloride...... 47 14

Figure 2.15. Nitrate, nitrite, and ammonia concentration change versus time of electrolysis at different A/C ratios, Ielectrolysis = 20 mA/cm2 in absebce (A) and in presence (B) of 1 g/l NaCl ...... 49 Figure 2.16. Pathways for nitrate reduction mechanism which involves electron transfer...... 52 Figure 2.17. Hydrogenation reduction mechanism of nitrate on bimetallic catalysts...... 54 Figure 2.18. Different reaction pathways during nitrate reduction on Cu-cathode in 0.1 M NaNO3+0.01M NaOH+0.5M NaCl ...... 55 Figure 2.19. Electrochemical reactions occurring at the surface of the electrodes during positive and negative cycles of pulse electrolysis for simultaneous nitrate and ammonia removal in an undivided cell ...... 59 Figure 3.1. Ultra Turrax T18...... 60 Figure 3.2. Branson 2800 Ultrasonic ...... 61 Figure 3.3. 225 cm2, (b) 25 cm2 electrolysis flow cell, and schematic representation of (c) 225 cm2 and (d) 25 cm2 flow cell ...... 66 Figure 3.4. Ammonia/Nitrate electrolysis loop setup ...... 68

Figure 4.1. (a, b) XRD patterns and (c-f) TEM images of synthesized Cu9Ni, Pt9Ir and Pt3Ir supported on Carbon catalysts ...... 70 Figure 4.2. (a) Pulse potential electrolysis and (b) corresponding pulse current response 2 during ammonia electrolysis for 5 h in a 300 cm flow cell with a pair of 60% Pt9Ir/C 2 electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) ...... 72 Figure 4.3. Variation of ammonia concentration during ammonia electrolysis in a 300 cm2 2 flow cell with a pair of 60% Pt9Ir/C electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) ...... 73 Figure 4.4. (a) pulse potential electrolysis, (b) corresponding pulse current response, and (c) energy consumption plot during ammonia electrolysis in a 25cm2 flow cell with a pair 2 of Pt9Ir/C 60% electrodes (loading 0.5 mg/cm ) in 8.3 g/l (NH4)2SO4+18 g/l NaOH solution at 60 ◦C, flow rate 2 LPM (pulse width 18 sec, pulse potential ±0.925 V) ...... 75

Figure 4.5. (a) CVs of three different PtIr/C catalyst compositions in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution (b) correlation between current density during ammonia electrolysis process at 50 ◦C and NaOH concentration (loading for all catalysts tested is 0.5 mg/cm2 ...... 77

Figure 4.6. CV response of Pt9Ir/C catalyst in the absence and the presence of ammonia, scan rate = 25 mV/s ...... 78

Figure 4.7. Repetitive CV response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution, scan rate: 25 mV/s ...... 79 15

-3 3 Figure 4.8. (a) Voltammograms of Pt9Ir/C catalyst in 3.2×10 mol/cm NH3 solution at different scan rates, (b) extracted plot of peak current versus the square root of scan rate ...... 81

Figure 4.9. Voltammogram response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 at (a) pH range between 12-13 and (b) pH 9, 11, and 13. (pH was adjusted using NaOH) ...... 82

Figure 4.10. (a) Effect of NaCl addition on voltammogram response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18g/l NaOH (pH 12.85), (b) effect of NaCl addition of CV of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 6g/l NaOH (pH 11) ...... 84 Figure 4.11. Effect of NaCl addition on (a) average rate loss and (b) total mg loss of 2 ammonia during ammonia electrolysis process using 60% Pt9Ir/C (loading 0.25 mg/cm on 2 both electrodes) in a 225 cm flow cell (8.3 g/l (NH4)2SO4 + 18g/l NaOH solution) ...... 85 Figure 4.12. Effect of NaOH concentration on pH and average current density during 2 ammonia electrolysis in 8.3 g/l (NH4)2SO4 at 60 ◦C in a 25 cm flow cell with a pair of 2 60% Pt9Ir/C catalysts (loading 0.5 mg/cm , 2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) ...... 86 Figure 4.13. Ammonia pulse electrolysis in 300 cm2 flow cell at 38 °C and 60 °C in 8.3 g/l 2 (NH4)2SO4+6 g/l NaOH solution , both electrodes 60% Pt9Ir/C with loading 0.2 mg/cm (pulse width 18 sec, pulse potential ±0.925 V, flow rate = 2 LPM) ...... 87 Figure 4.14. Effect of temperature and NaOH concentration on average current density associated with ammonia removal during ammonia electrolysis in a 25 cm2 flow cell 2 consists a pair of 60% Pt9Ir/C electrodes (loading 0.5 mg/cm ), base solution: 8.3 g/l (NH4)2SO4...... 88 Figure 4.15. (a) schematic representation of the 25 cm2 flow cell (b) corresponding current response in a cell of Pt9Ir/C as cathode and anode (loading 0.5 mg/cm2) in 8300 ppm (NH4)2SO4 + 12 g/l NaOH (pulse width = 18s, potential amplitude= ±0.925 V, T = 60 °C), and (c) GC analysis of produced gas during ammonia oxidation ...... 89 2 Figure 4.16. (a) 25 cm flow cell comprising a pair of 60% Pt9Ir/C with an electrode- toelectrode spacing of 8 mm, (b) effect of electrode spacing on normalized average current density during ammonia pulse electrolysis in 8.3 g/l (NH4)2SO4 solution at 50 °C, flow rate 2 LPM (pulse width 18 sec, pulse potential ±0.925 V) ...... 90 Figure 4.17. Reproducibility of the data obtained for pulse electrolysis of ammonia in a 25 cm2 flow cell at 50 °C ...... 92 Figure 4.18. Large flow cell (300 cm2) and small flow cell (25 cm2) both includes a pair of 2 60% Pt9Ir/C catalysts loading 0.2 mg/cm ...... 93 Figure 4.19. Effect of cell size on average current density during ammonia electrolysis process in 8.3 g/l (NH4)2SO4 solution at 40 ◦C (2 LPM, pulse width 18 sec, pulse potential 2 ± 0.925 V), all electrodes are similar 60% Pt9Ir/C, loading 0.2 mg/cm ...... 94 2 Figure 4.20. CVs of 60% Pt9Ir/C catalyst (loading 0.25 mg/cm ) at different NH3 concentrations in 6 g/l NaOH solution, scan rate 25 mV/s ...... 96 16

Figure 4.21. Effect of pulse width and substrate on average current density associated with ammonia oxidation on three consecutive days in 8300 ppm (NH4)2SO4 + 6 g/l NaOH 2 (potential amplitude= ±0.925 V, T = 38 °C, (a) : Pt3Ir/C-Ni loading = 0.301 mg/cm , (b) : 2 2 Pt3Ir/C-Ti loading = 0.302 mg/cm , Pt3Ir/C cathode loading = 0.312 mg/cm )...... 98 Figure 4.22. Effect of pulse potential amplitude on durability and current decay percentage after three consecutive days of pulse electrolysis in 8300 ppm (NH4)2SO4 + 6 g/l NaOH 2 2 (T= 38 °C, Pt3Ir/C cathode loading = 0.222 mg/cm , Pt3Ir/C anode loading = 0.203 mg/cm ) ...... 99

Figure 4.23. XRD pattern of 50% Pt3Ir/C catalyst before electrolysis process and after 10 runs of 5 h (totally 50 h) electrolysis operation ...... 100 Figure 4.24 (a) Comparison of Raman spectra for GO and Cu-rRGO, (b) comparison of XRD patterns for Cu and CuNi modified GC electrodes ...... 102 Figure 4.25. LSV responses in the absence and the presence of nitrate for three different catalysts (a) Cu, (b) Cu-rGO, (c) CuNi, and (d) the LSV response of different catalysts - toward detection of nitrate in 0.1M NaOH+23ppm NO3 (Scan rate: 25 mV/s)...... 103 Figure 4.26. SEM and EDS analysis of (a) Cu, (b) Cu-rGO, and (c) CuNi modified GC electrodes ...... 105 Figure 4.27. Comparison of constant potential electrolysis for different catalysts at different - conditions in 0.1M NaOH + 32 ppm NO3 at -1.3 V vs. Hg/HgO, 38 °C for Cu and CuNi, and at -2.5 V, 23 °C for Cu-rGO ...... 107 - Figure 4.28. N-NH3 and N-NO3 concentration vs. time during constant potential electrolysis for (a) Cu at -1.3 V vs. Hg/HgO, and 38 °C, (b) Cu-rGO at -2.5 V vs. Hg/HgO, - and 23 °C and (c) CuNi at -1.3 V vs. Hg/HgO, and 38 °C all in 0.1M NaOH + initial NO3 concentration of 32 ppm ...... 108 Figure 4.29. Effect of nitrate concentration on LSV responses of (a) Cu, (b) Cu-rGO and (c) CuNi catalysts in alkaline media of 0.1M NaOH (scan rate: 25 mV/s) ...... 110 - Figure 4.30. LSV responses of Cu, Cu-rGO, and CuNi toward detection of nitrite, NO2 in - 1M NaOH+10mM NO2 (Scan rate: 25 mV/s) ...... 111 Figure 4.31. SEM images of four different compositions of electroplated CuNi catalysts on Ni mesh ...... 113 Figure 4.32. (a) CV and (b) chronoamperometric curves of four different compositions of - electroplated CuNi catalysts in 0.1M NaOH+23 ppm N-NO3 ...... 114

Figure 4.33. (a) CV responses for Cu9Ni supported on Carbon catalyst in the absence and - the presence of 23 ppm N-NO3 and (b) comparison of LSV responses for electroplated 2 2 Cu9Ni (loading 0.98 mg/cm ) and sprayed Cu9Ni/C (loading 0.98 mg/cm ) in the presence - of 23 ppm N-NO3 (blank solution 8300 ppm (NH4)2SO4 + 6 g/l NaOH): scan rate 25 mV/s ...... 116

Figure 4.34. Comparison of successive voltammograms for electroplated Cu9Ni and - sprayed Cu9Ni/C in 8300 ppm (NH4)2SO4 + 6 g/l NaOH + 23 ppm N-NO3 : scan rate 25 mV/s ...... 117 17

Figure 4.35. (a) XRD patterns of Six different synthesized grades of Cu9Ni supported on Vulcan, (b) TEM image of 40% Cu9Ni/C ...... 119 Figure 4.36. Quantity of absorbed nitrogen gas during BET analysis for 20%, 30%, and 40% Cu9Ni/C catalysts ...... 121 Figure 4.37. (a) LSV and (b) Chronoamperometric responses (applied potential -1.2 V vs. Hg/HgO) for six different grades of Cu9Ni/C catalysts in 8.3 g/l (NH4)2SO4+18 g/l NaOH - + 165 ppm N-NO3 , scan rate 25 mV/s ...... 122

Figure 4.38. LSV of 40 % Cu9Ni/C catalysts in the absence and the presence of 165 ppm - N-NO3 , Base solution: 8.3 g/l (NH4)2SO4 + 10 g/l NaOH, scan rate 25 mV/s ...... 123

Figure 4.39. (a) LSV of 40 % Cu9Ni/C catalysts in the absence and the presence of 205 - ppm N-NO2 (b) effect of ammonia addition on LSV response of 40 % Cu9Ni/C catalysts - toward nitrate reduction in 10 g/l NaOH +165 ppm N-NO3 solution: scan rate 25 mV/s ...... 124 2 Figure 4.40. Voltammograms of 40% Cu9Ni/C catalyst (loading 0.5 mg/cm ) in 8.3 g/l - (NH4)2SO4 + 6 g/l NaOH + 32 ppm N-NO3 solution at different scan rates ...... 125 - Figure 4.41. Effect of NO3 concentration on LSV response of 40% Cu9Ni/C catalyst (loading 0.5 mg/cm2) in 18 g/l NaOH solution, scan rate 25 mV/s ...... 126 Figure 4.42. Loading effect of (a) Pt9Ir/C catalyst on ammonia oxidation current density in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH at applied potential of +0.925 V vs. Hg/HgO and (b) Cu9Ni/C catalyst on nitrate reduction current density in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH + - 23 ppm N-NO3 at applied potential of -0.925 V vs. Hg/HgO, 38 °C ...... 128

Figure 4.43. Synthesized 40% Cu9Ni/C (as the cathode) and 60% Pt9Ir/C (as the anode) applicable for nitrate electro-reduction and ammonia electro-oxidation reactions, respectively ...... 129 Figure 4.44. Pulse current corresponding to the ammonia electrolysis in a 25 cm2 flow cell 2 with pairs of 60% Pt9Ir/C electrodes with different loadings of 0.25,0.35,0.5mg/cm . Solution: 8.3 g/l (NH4)2SO4+10 g/l NaOH at 60 °C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V) ...... 129

Figure 4.45. Ammonia electrolysis average current density versus 60% Pt9Ir/C catalyst loading (25 cm2 flow cell) ...... 130 Figure 4.46. (a) pulse electrolysis and (b) corresponding current response in a 25 cm2 cell 2 2 of Cu9Ni/C as cathode (loading 0.25 mg/cm ) and Pt9Ir/C as anode (loading 0.5 mg/cm ) - in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH + 23 ppm N-NO3 (pulse width = 18s, potential amplitude= ±0.925 V, T = 38 °C) ...... 131 Figure 4.47. (a) nitrate and ammonia concentrations during the pulse electrolysis in a 2 system of Cu9Ni/C as cathode (loading 0.25 mg/cm ) and Pt9Ir/C as anode (loading 0.5 2 mg/cm ). Effect of Cu9Ni/C catalyst loading on (b) average rate loss of nitrate/ammonia and (c) average ppm loss of nitrate/ammonia after 3 h of pulse electrolysis all in 8300 ppm - (NH4)2SO4 + 6 g/L NaOH + 23 ppm N-NO3 (pulse width = 18s, potential amplitude= 2 ±0.925 V, T = 38 °C, anode Pt3Ir/C loading was fixed at 0.5 mg/cm ) ...... 132 18

Figure 4.48. Concentration of ammonia and nitrate during pulse electrolysis process in 8.3 2 g/l (NH4)2SO4+18g/l NaOH + 1.5 ppm nitrate in a 225 cm cell with a pair of 60% Pt9Ir/C catalyst: loading 0.5 mg/cm2 (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V, electrode spacing of 8 mm) ...... 133 Figure 4.49. Schematic representation of the proposed mechanism for simultaneous nitrate/ammonia removal during pulse potential electrolysis ...... 135 Figure 4.50. Small 25 cm2 flow cell for nitrate/ammonia electrolysis. Both electrodes are 2 60% Pt9Ir - 40% Cu9Ni (v/v 50%), loading 0.25 mg/cm , electrode spacing: 0.8 mm .. 136 - Figure 4.51.Pulse electrolysis in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH + 32 ppm N-NO3 . 137 Figure A.1.1. Procedure Flowsheet for the use of Ammonia Ion-Selective Electrode ... 157

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Chapter 1: Introduction

1.1. Project Significance

Compared to the available technologies for ammonia oxidation and nitrate electro-reduction, the approach proposed in this study consumes 35-65% less energy per gram of nitrate reduction and about 90% less energy per gram of ammonia oxidation. The voltage in our system is about 1 V less than potential, which is usually applied commercially for the nitrate reduction reaction. Our proposed technique consumes 5×10-3

-1 -3 -1 - kWhg NH3 and 9×10 kWhg NO3 of energy, which is much lower than what is

-1 - reported elsewhere (e.g., 0.015-0.022 kWhg NO3 in a rotating cylinder electrode design

-1 for nitrate removal [1], 0.5 kWhg NH3 for ammonia electrolysis on a boron-doped-

-1 - −1 − diamond electrode [2], 0.04 kWhkg NO3 [3], and 2.66 kWhg NO3 using an electrocoagulation technique [4]). The main limitation of electrochemical application for nitrate removal is the generation of by-products, mainly ammonia and nitrite. The challenge is to introduce new operational conditions of electrolysis (cathode and anode materials, cell design, etc.) in an undivided-unbuffered cell to simultaneously conduct both nitrate reduction and ammonia oxidation [5]. At the same time, our newly proposed electrode architecture results in faster kinetics of nitrate removal and continuous monitoring of nitrate/ammonia concentration. It leaves no toxic and hazardous byproducts at the end, which is critical in environmental applications. No report is available on simultaneous nitrate and ammonia detoxification using such a cost-effective and economic electrochemical-based technique in a single undivided cell. The knowledge obtained by the proposed approach will be a great achievement that can be promoted to 20 the wastewater treatment industry and be extended for the recovery of energy from waste streams containing ammonia and nitrate.

1.2. Project Overview

Ammonia and nitrate contamination of groundwater, surface water, and the other water resources, which can cause lots of problems like algae bloom and fatal illnesses, especially for the infant under six months (baby blues), are considered severe environmental and human health issues [5-7]. The contamination of nitrate and ammonia is mainly generated from industrial waste and chemical fertilizers. According to the

World Health Organization, the maximum permitted limit of nitrate and ammonia contamination of drinking water is about 50 ppm and 0.5 ppm, respectively [8]. There are several techniques for detoxifying bodies of waters from ammonia and nitrates (e.g., the biological process of nitrification/denitrification, air-stripping, breakpoint chlorination, ammonia cracking, physiochemical techniques, ion exchange resins, electro-dialysis, and electrocoagulation, reverse osmosis [1,7,9-11]). These techniques usually use large amounts of energy and voltage, have slow kinetics of removal, in some cases lead to the formation of more toxic byproducts, and are time-consuming to carry out [9, 12]. On the other hand, electrochemical techniques for ammonia oxidation and nitrate reduction reactions are usually straightforward, cheap, energy-efficient, environmentally friendly, and easy to control. So, if a new single-cell design can perform both reactions simultaneously, a tremendous amount of time and energy can be saved, which immensely helps the wastewater treatment industry. Among a wide range of materials used for nitrate reduction and ammonia oxidation, Cu, Ni, and carbon-based electrodes [13-15], 21 and Pt and Ir metals have the highest catalytic activity for nitrate reduction and ammonia oxidation, respectively [13, 16]. Within this context, the overall objective of this project is to design a new carbon-supported electro-architecture flow cell that converts nitrate/ammonia to some less harmful products by consuming less energy in kWh per grams of nitrate (and ammonia). This undivided cell enables simultaneous ammonia and nitrate removal using electrochemical techniques.

The specific objectives are to 1. Synthesize a new Cu-based catalyst for nitrate electroreduction, which is cost-effective and commercializable, 2. Synthesize an upgrade class of the carbon-supported catalyst (CuNi-PtIr is an example) with high efficiency for both ammonia oxidation and nitrate reduction reactions, 3. Design a new undivided cell for performing simultaneous nitrate and ammonia removal with lower energy consumption.

In this project, first, a new catalyst for nitrate reduction is synthesized and characterized. Then a new developed carbon-supported catalyst with the highest efficiency for nitrate reduction and ammonia oxidation is synthesized, and the obtained catalysts are used in an undivided flow cell system to simultaneously treat nitrate and ammonia. The electrochemical characterization of the new catalysts is assessed using chronoamperometry, cyclic voltammetry, pulse potential technique, chrono- potentiometry, and several characterization techniques are applied to investigate the properties of the catalysts such as Scanning Electron Microscope (SEM), Transmission

Electron Microscope (TEM), X-ray Diffractometer (XRD), (Energy-Dispersive

Spectroscopy) EDS, Raman, Brunauer–Emmett–Teller (BET) Micromeritics Tristar II, 22

UV/vis spectrophotometry, and gas chromatography. Compared to other available techniques, the new introduced layout can remarkably decrease the costs and help the industry of wastewater treatment to efficiently detoxify nitrate and ammonia from wastes and protect the environment. This technique consumes much less kWh energy per grams of nitrate and ammonia removed.

1.3. Statement of Objectives

Over the past few decades, the ammonia electrolysis has attracted significant attention from researchers in the area of wastewater treatment and those who are concerned about its potential application in direct ammonia fuel cell (DAFC) [13,14,16].

Nitrate removal using an electro-reduction reaction has been widely investigated during past decades as a potential and cost-effective treatment approach for detoxifying nitrate from water. This approach does not require a reducing agent and is applicable in media with high concentrations of nitrate toxicity. The beauty of the electrochemical process of nitrate treatment is that it enables us to control the final products by controlling pH and applied potential [9]. There is no report available on simultaneous nitrate and ammonia simultaneous detoxifying using electrochemical techniques in an undivided cell. Within this context, the overall objective of this work is to fabricate and characterize different carbon-supported catalysts that perform effectively in terms of removing nitrate and ammonia with lower energy consumption in kWh per grams of ammonia (and nitrate).

Our new designed cell has the potential to use 35-65 % less energy per grams of nitrate removal and about 90 % less energy per grams of ammonia removal compared to the available techniques. The specific objectives of this project are to: 23

1. Synthesize some classes of Cu-based catalyst for nitrate electroreduction applications which has the potential of being commercialized,

2. Upgrade a well-established carbon-supported catalyst to achieve ammonia oxidation and nitrate reduction reactions,

3. Build an undivided flow cell system integrating an effective catalyst electrode for performing simultaneous nitrate and ammonia detoxification.

24

Chapter 2: Literature Review

2.1. Ammonia Electrolysis

The mechanism of ammonia oxidation to N2 is a complex multi-step reaction and is not well clarified. Researchers have examined several noble electrocatalysts to oxidize ammonia. Among them, Pt attracted enormous attention because of its promising catalytic activity. It reported that transition metals in 5d electron orbitals configuration

(e.g., Pt and Ir) have the highest catalytic activity [14]. The activity of Pt-Ir electrodes toward ammonia electro-oxidation has been investigated elsewhere [16]; most notably,

Botte et al. reported some carbon-supported catalysts based on Pt and Ir, which possesses the highest electrocatalytic activity toward ammonia oxidation [17]. During the ammonia electrolysis, the electro-oxidation of ammonia and the reduction of water occur concurrently through the reactions, which are presented below [18, 19]. Diaz et al. have successfully proposed a kinetic model for ammonia oxidation. They proposed a pathway that includes two parallel reactions; the dehydrogenation of ammonia to ammine and the dimerization of amine to hydrazine, which is finally oxidized to nitrogen gas [19].

The ammonia electro-oxidation on crystalline Pt is reported to be a surface- controlled-process where the decay in peak current during the successive cyclic voltammetry is because of the formation of deactivated adsorbed species like imine and 25 nitrogen ad-atoms [18, 19]. It is necessary to operate at the potential regions not so far away vs. SHE to prevent nitrogen evolution. At high anodic potentials, water oxidation may occur, which oxidizes Pt to PtOx [13]. In this condition, it is possible to have the

- simultaneous evolution of nitrogen and oxygen, which forms nitrogen oxides like NO2

- and NO3 which are not our desired [13]. So, it is critical to have a good understanding of the operational potential of ammonia electrolysis. Estejab et al. has studied the ammonia oxidation on Pt3-xIrx (x = 0–3) clusters using Density functional theory (DFT) [12]. They concluded that the N2H4-x bond formation decreases the stability of intermediates compared to the situation where NH3-x is formed. On Pt clusters, the ammonia oxidation

- occurs through a N2H4 formation-mechanism; while, at the surface of Ir, a N2-formation- mechanism is predominant [12]. When N2-mechanism is predominant, better performance for oxidation of ammonia oxidation was detected for Pt1Ir2 compared to that for Pt2Ir1 [12]. Xu et al. used electrodeposited NiCu on carbon paper as a stable candidate for ammonia oxidation [20]. They used an applied potential of about 1.0 V with a current efficiency of 92.8% in alkaline media with pH=12. pH has a remarkable effect on current efficiency. At higher applied potential and in more alkaline media, more nitrate ions are formed as a result of ammonia oxidation [20]; so, it is critical to have a good knowledge of what operational potential should be applied. Modisha et al. have investigated the effect of temperature and ammonia concentration on ammonia electro-oxidation reaction in a system shown in Figure 2.1, where a pair of Pt-Ir cathode and anode was applied

[21]. They predicted a pseudo-first-order reaction for ammonia electro-oxidation, where 26 the efficiency of the process increases with increasing the current density at elevated temperature [21].

Figure 2.1. Schematic representation of the ammonia electrolysis process for hydrogen production [21].

At high concentrations of ammonia, more adsorbed NH3 at the electrode surface leads to a high current density and likely blockage of catalyst sites as adsorption of NH and NH2 intermediates may happen [21, 22]. It is critical in some applications to measure the rate of hydrogen production during the ammonia electrolysis. Jiang and co-workers designed a new cell setup configuration (Figure 2.2) for monitoring the rate of hydrogen generation. They used electroplated Pt-It electrodes on Ni foam [11].

27

Figure 2.2. Cell configuration for ammonia electrolysis designed by Jiang et al. to monitor the rate of H2 production [11].

The voltage in the designed cell was 0.58 V, with a current density of 2.5 mA/cm2. The oxygen evolution reaction during ammonia electrolysis can decrease the local pH and affect the process [11]. During direct ammonia electro-oxidation, the local pH drop is always an issue. This issue can be eliminated by reducing the Nernstian diffusion layer thickness by stirring or circulating the solution while the process is in progress [22].

2.2. Nitrate Electro-Reduction

Many researchers have investigated the effect of support on catalytic activity for nitrate reduction. Vulcan carbon black provides a large surface area enabling us to obtain a high loading of catalytic metal [23]. Sakamoto et al. illustrated that PdCu supported on carbon provides a higher catalytic activity for nitrate reduction compared to other supports, such as TiO2, Al2O3, and ZrO2 [24]. The kinetics of nitrate electro-reduction is widely investigated on different electrodes, including, Ni, Ag, Rh, Pt, Cu, Pd, and some bimetallic alloys, e.g., CuZn, CuSn, CuPt, CuNi, and PdRh [15]. Among these 28 candidates, the copper exhibits the highest catalytic activity toward nitrate reduction reaction compared to other materials that have recently attracted lots of attention for the nitrate removal process [6]. Cu and Ni with the same crystal structure (Faced-Centered-

Cubic), together with their alloys with other noble metals, exhibits the outstanding catalytic properties for nitrate reduction; however, few attempts have been made for studying CuNi alloy as a cheap and cost-effective candidate for this purpose [25].

Oznuluer et al. investigated the nitrate reduction on graphene-modified copper electrodes in alkaline media. At high negative overpotentials, ammonia and nitrogen gases are most likely the product of nitrate reduction reaction, respectively [15]. Reyter et al. have widely investigated the nitrate reduction on copper in alkaline media [6]. Nitrate adsorption on Cu, which is the rate-controlling step during the nitrate reduction process on Cu, begins at a potential of about -0.6 V vs. Hg/HgO. At more negative potentials of about -0.9 V vs. Hg/HgO, nitrate is reduced to nitrite [6]. At even more negative potentials of about -1.1 V vs. Hg/HgO, nitrite is reduced to a short-life species of hydroxylamine, which immediately transforms into ammonia [6]. At the potential range of about -1.3 V vs. Hg/HgO, nitrate is more likely reduced to ammonia [6]. The higher reductive potentials beyond -1.5 V vs. Hg/HgO decreases the surface activity due to the surface poisoning and hydrogen ad-atom adsorption [6]. De et al. studied the mechanism of nitrate reduction using a divided glass cell design (Figure 2.3) for identifying the intermediate and final reaction products during nitrate reduction on a Pt-group electrode

[26]. 29

Figure 2.3. A divided glass cell designed by De et al. for investigating the mechanism of nitrate reduction [26].

De et al. reported the following pathways (Table 2.1) for nitrate and nitrite reductions. According to their proposed mechanism, the nitrogen oxide adsorption occurs through a single electron-transfer step which is considered to be the rate-controlling step.

This step is followed by two different pathways, each of which is likely to happen in a particular low and high potential region creating N2 and NH3, respectively [26].

Table 2.1. Possible reaction pathways during nitrate/nitrite reduction [26].

30

2.2.1. Pulse Potential Electrolysis for Nitrate Electrochemical Removal

Hourani et al. were the first who reported the electrochemical removal of nitrate from aqueous solutions using symmetrical square wave pulses on Pt by applying a potential window from -0.2 to 1.3 V [27]. Since then, the square wave potential pulse for nitrate removal has rarely been investigated, and there are just a few attempts available on studying pulse electrolysis for nitrate reduction. The oxidation reaction of ammonia to nitrogen is claimed to be challenging to occur at constant potential electrolysis [28]. It has been reported that at constant potential electrolysis, there is a high accumulation of

OH- and nitrite at the front of the cathode, which may result in passivation and deactivation of Pt-Cu cathode in long terms hindering the nitrate reduction [28]. Perez et al. designed a rotating cylinder electrode for nitrate removal with the mean value of cell voltage, energy consumption, the current efficiency for ammonia formation, and the nitrate conversion of about 10.9 V, 22.1 KWhkg-1, 92%, and 90%, respectively [1].

Polatides et al. reported a square asymmetrical pulse potential technique having a constant cathodic limit of -1.7 V vs. Ag/AgCl and an anodic limit of 0.6-3.0 V vs.

Ag/AgCl for nitrate removal on Cu60Zn40 cathode (Pt auxiliary electrode) [28]. At the frequency range of 30-50 Hz, the minimum selectivity for both nitrite and ammonia was obtained [28]. The nitrite/ammonia cannot follow the potential change at waves with higher frequencies than 200 Hz, which leads to a lower removal efficiency [28]. The main problem with constant potential nitrate removal technique is that it is likely to produce more toxic products, e.g., nitrate and ammonia; however, through pulse electrolysis, nitrate is first reduced to nitrite/ammonia during the cathodic cycle. And 31

- - afterward, the produced nitrite and ammonia will transform to OH , NO3 , and N2 gas during the anodic cycle [28]:

and the overall reaction of nitrate removal during a pulse technique would be [28]:

The overall concentration of nitrate decreases over time [28]. Compared to the sinusoidal and triangular potential waves, higher removal efficiency of nitrate/nitrite, and a lower selectivity of nitrite/ammonia were obtained for asymmetrical waves [28]. This might be because of the improved mass transfer rate under the pulsing potential condition, which generates pulsating concentration profiles of nitrate/nitrite or ammonia adjacent to the electrode surface [28]. Figure 2.4. shows the variation of nitrite, ammonia, and nitrogen gas concentration versus electrolysis time under an asymmetrical square potential wave having the duration of a positive and negative period of 20% and 80%, respectively (positive and negative potential limits are 1.5 V and -1.5 V, respectively)

[28]. 32

Figure 2.4. Variation of nitrite, ammonia, and nitrogen gas concentration vs. electrolysis time under an asymmetrical square potential wave having the duration of positive and negative of 20% and 80%, respectively (positive and negative potential limits are 1.5 V and -1.5 V, respectively) [28].

After 2 h pf electrolysis, the concentration profiles for nitrite and ammonia display a maximum while nitrogen gas concentration continuously increases. Hence, the asymmetrical pulse potential electrolysis having the duration of the positive and negative limit of 20% and 80%, respectively, is an efficient technique for nitrate reduction and conversion of by-products to nitrogen gas [28]. Nitrite as an intermediate can be reduced to N2 or NH3 at the cathode surface or be oxidized again to nitrate at the anode surface; thus, no considerable nitrite accumulation occurs during nitrate electrolysis [28].

2.2.2. Copper-Based Cathodes for Nitrate Electro-Reduction in Alkaline Media by

Constant Potential Electrolysis

Several studies have conducted the electrochemical reduction of nitrate on

Copper-based bimetallic alloys in alkaline media [6,7,15, 29-32]. Mattarozzi et al. investigated the constant potential electrolysis for nitrate electro-reduction using a broad range of compositions of CuNi bimetallic alloys in a solution of 1M NaOH containing 10 mM NaNO3/2 [29-31]. They proved that the CuNi bimetallic catalysts rich in Cu have the 33 highest activity for nitrate reduction with higher selectivity toward ammonia generation

- [29, 31]. At the same time, CuNi alloys rich in Cu have faster rates of NO3 elimination.

- Reduction on CuNi alloys rich in Cu (e.g., Cu80Ni20) produces less NO2 and no hydroxylamine was detected [29, 31]. The high activity of porous CuNi alloys rich in Cu

- is because of a synergistic mechanism in which Cu sites acts as the sites for NO3 adsorption and Ni sites are suitable for efficient H-atoms adsorption (Ni with high electroactivity for hydrogenation and copper with high activity for nitrate reduction) [7,

31]. The reduction mechanism involves the Hads to the O-moieties sites of oxyanion to eliminate hydroxyls [31]. Another advantage of copper as the cathode for nitrate reduction reaction is that it has a more negative potential value for hydrogen evolution (-

1.2V); therefore, less hydrogen is produced during the cathodic cycle, which leads to a higher rate of total nitrogen removal [33]. Reyter’s group investigated the constant potential electrolysis at -1.1. V for nitrate reduction and oxidation of produced ammonia on CuNi bimetallic alloys (cathode) in the presence of NaCl in an undivided flow cell

(with commercial Ti/IrO2 grids as anodes) [7]. About 105 ppm nitrate was removed after

24 h of electrolysis for monometallic Cu cathode while the removal of nitrate after just 3h of electrolysis using Cu70Ni30 and Cu90Ni10 were 570 ppm, and 532 ppm, respectively [7].

The energy consumption for a cell containing Cu70Ni30 electrode was 90% lower than

- that for monometallic Cu and Ni electrodes (20 kWh/kg NO3 for Cu70Ni30 cathodes

- vs. 220 kWh/kg NO3 for Cu or Ni cathodes) [7]. In addition to energy considerations, at monometallic Cu cathodes, nitrite is also produced (aside from ammonia), which will be oxidized to nitrate and decrease the efficiency of electrolysis [7]. To conduct the 34 simultaneous nitrate reduction and oxidation of byproducts (i.e., ammonia and nitrite), we must perform a paired electrolysis in an undivided cell configuration, because there are several issues in a divided cell configuration, e.g., membrane degradation and blockage by organic compounds/or other anions [7].

2.2.2.1. Voltammetric Study of Nitrate Reduction on Copper-Based

Catalysts

Figure 2.5 indicates the voltammetric responses of copper [6] and graphene-modified copper [15] electrodes in sodium hydroxide based alkaline media containing NaNO3.

a b

Figure 2.5. Linear sweep voltammograms of (a) copper electrode in 1M NaOH+10 mM NaNO3 [6], and (b) graphene modified copper [15] electrode in 0.1M NaOH+40 mM NaNO3.

It has been claimed that wave C1 corresponds to the reduction of nitrate to nitrite which initiates in a potential range of -0.65 to -0.9 V vs. Hg/HgO through the following reaction [6, 15]:

This step, which occurs at the potential of -0.9 V vs. Hg/HgO, is controlled by adsorption of nitrate ion and its reduction to nitrite, which is claimed to be the rate- 35 controlling step of overall nitrate reduction reaction [6, 15]. The linear dependency of peak current to the square root of scan rates confirm that the reduction of nitrate is a purely diffusion-controlled process. Peak C2 in potential range of about -1.1 V vs.

Hg/HgO relates to the reduction of nitrite to hydroxylamine via the following fast reaction [6]:

and peak C3 in a potential range of about -1.2 V to -1.4 V vs. Hg/HgO corresponds to the production of ammonia through the reaction [6]:

Beyond -1.4V, peak C4 shows the activity degradation of the catalyst due to the poisoning effect, i.e., the adsorption of hydrogen and surface blockage of the electrode, which hinders the kinetics of nitrate reduction [6].

At -0.9 V vs. Hg/HgO, the main product is nitrite, while at more negative potentials of about -1.4 V vs. Hg/HgO, nitrite is entirely reduced to ammonia [6]. It has been claimed that ammonia production is favored in the potential regions close to hydrogen evolution reaction where the possibility of reaction between adsorbed hydrogen and adsorbed nitrites to form ammonia is highest [6, 15]. In most cases, when we use a very negative reduction potential, the main product of nitrate reduction is ammonia. It is desirable to ultimately convert nitrate to N2 gas, increase the selectivity for N2 gas production, and minimize the produced amount of ammonia during nitrate reduction reaction [6, 15]. Figure 2.6 represents different reaction pathways during nitrate reduction on copper in alkaline media [6]. 36

Figure 2.6. Different reaction pathways during nitrate reduction on copper in alkaline media [6].

2.2.3. Effect of Cell Configuration on Nitrate Electro-Reduction Kinetics

Ding and co-workers investigated the effect of cell configuration (single-chamber cell (SCC) vs. dual-chamber cell (DCC)) on the rate of nitrate reduction reaction in the presence of Cl- anions (Figure 2.7) [34]. In DCC configuration, cathodic and anodic chambers are separated from each other by a cation exchange membrane (CEM), while in

SCC, both electrodes are in the same compartment in contact with the electrolyte.

Figure 2.7. Mechanism of nitrate reduction in SCC and DCC in the presence of Cl- ions [34].

Due to the ammonium concentration gradient between anodic and cathodic compartment in DCC, ammonium migrates from the cathodic compartment to anodic 37 chamber. In contrast, nitrite cannot migrate through CEM, thereby preventing nitrite oxidation to nitrate. Anodic compartment acts as a sink for ammonium ions, while in cathodic compartment no oxidation of nitrite occurs, and final product of nitrate reduction is ammonium which diffuses to the anodic chamber to get oxidized to N2 in the presence of Cl- ions via reaction [34]:

Ding et al. illustrated that nitrate removal efficiency in DCC was higher than that in SCC, because CEM prevents nitrite/nitrate diffusion to the anodic chamber where just ammonium oxidation to N2 occurs; hence the overall selectivity for N2 gas generation is higher than that in SCC [34].

Hasnat et al. introduced a sandwich-type membrane containing a cell with a configuration of Pt|Nafion|Pt-Cu for nitrate reduction reaction in the absence of any supporting electrolyte (Figure 2.8) using constant potential electrolysis [35].

Figure 2.8. Mechanism of nitrate reduction in Pt|Nafion|Pt-Cu cell configuration and corresponding schematic representation of catalytic hydrogenation or electron transfer reactions [35]. 38

+ + After H2O dissociation to O2 and H on the anode surface, H migrates through

H+-conducting Nafion-117 (180 μm thickness) to promote the nitrate/nitrite reduction at

Pt-Cu cathode surface [35]. The advantage of this configuration is its bi-functional mode of nitrate reduction, where both hydrogenation and electron transfer mechanisms occur concurrently to improve the rate of nitrate reduction [35]. It is proposed that nitrate

+ reduction occurs on Cu clusters on the cathode surface where Cu0/Cu2 redox couple

2+, - plays an active role in the formation of Cu which reduces NO3 to N2. At the same time,

+ Pt-H bond provokes electrons to convert Cu2 to Cu0, creating a catalytic cycle of reduction, as depicted in Figure 2.8 [35].

Lan and co-workers introduced a new reactor configuration in which Pd-

Sn/activated-carbon catalyst particles are suspended in the cathode cell compartment where a micro-electrolysis reaction occurs at particle surface under the applied electric field (Figure 2.9) [36].

Figure 2.9. Schematic representation of micro-electrolysis reaction cell design based on Pd-Sn/AC catalyst particles [36]. 39

After proton generation in the anode chamber, they diffuse toward the cathode compartment through the GEFC-107 proton exchange membrane. In this cell configuration, after 80 min of electrolysis under an applied current of 30 mA, 90% of the

- initial NO3 concentration (24.6 ppm) were converted to N2 [36].

2.3. Simultaneous Removal of Nitrate and Ammonia

2.3.1. A Paired Electrolysis in a Solid Polymer Electrolyte Reactor

Few attempts have been made so far to remove nitrate and ammonia from water resources simultaneously. Several strategies like single batch reactor [37], sequencing batch reactor [38], airlift inter-loop sequencing batch reactor [39], the combined

ANAMMOX and hydrogenotrophic denitrification [40], and a specific chemically based method like HDTMA-modified zeolite [41] are introduced to investigate the simultaneous nitrate and ammonia removal. These methods are usually time-consuming, use a considerable amount of energy, the kinetics of removal is slow, and produces toxic byproducts. Cheng et al. [3] developed a solid polymer electrolyte reactor enabling simultaneous nitrate and ammonia removal (Figure 2.10).

40

Figure 2.10. (a) Zero gap solid polymer electrolyte reactor—1: catholyte; 2: end plate (stainless steel); 3: manifold plate (PTFE); 4: distributor (stainless steel mesh); 5: cathode ® (PdRh1.5/Ti mini-mesh); 6: Nafion 117 membrane; 7: anode (Pt/Ti mini-mesh); 8: seal O-ring (Tiron rubber); 9: anolyte. The cell dimension is 22 cm×14 cm×3 cm. (b) Flow diagram—10: gas adsorbent reservoir; 11: catholyte reservoir; 12: zero gap solid polymer electrolyte reactor; 13: pump; 14: anolyte reservoir [3].

Their reactor successfully removed 16.1mM nitrate and 9.4 mM ammonia after 45

- -2 -1 hours of operation with the average rate of 0.057 mol NO3 cm h and 0.017 mol NH3 cm-2h-1 with a current efficiency of 24.5% in nitrate reduction and 1.4% in ammonia oxidation [3].

2.3.2. Airlift Inner-Loop Sequencing Batch Reactor (SBR)

In this method, an airlift inner-loop sequencing batch reactor using a biofilm carrier, which is poly(butylene succinate) operates under an alternant aerobic/anoxic approach for simultaneous nitrate and ammonia removal [39] (Figure 2.11). 41

Figure 2.11. Schematic representation of the airlift inter-loop sequencing batch reactor [39].

The biodegradable PBS granules are loaded in a 5.5 L cylindrical tube reactor, and air diffuses from the bottom of the inner tube to provide the driving force for wastewater circulation in the system. The reactor operates in sequencing mode for 4 h, including a feeding time of 6 min, followed by 1 min of aeration cycle, 8 min of intermittent setting time, and the final effluent charge time of 9 min [39]. The reactor operates in a dark artificial climate room at 25 °C. The average rate of ammonia and

-3 -1 nitrate removal in this technique are 47.35±15.62 gNH4-Nm d and 0.64±0.14 kgNO3-

Nm-3d-1, respectively [39]. In this technique, the complex microbial community in the system provides a high rate of denitrification [39].

2.3.3. Combination of ANAMMOX and Hydrogenotrophic Denitrification

Figure 2.12 indicates the schematic diagram of the experimental setup showing the combined ANAMMOX technique with hydrogenotrophic denitrification [40]. 42

Figure 2.12. Simultaneous removal of nitrate and ammonia using the ANAMMOX technique combined with hydrogenotrophic denitrification [40].

In this method, enriched ANAMMOX sludge is loaded in a 4.2 L rectangular reactor at 35 °C. Carbon dioxide and hydrogen gases are continuously supplied to the reactor from the bottom with the flow rate of 20 mL/min, which deactivates the

ANAMMOX bacteria and reacts with ammonia and nitrate pollutants. After 30 days of operation, the removal efficiencies of 95% and 90% for ammonium and nitrate are obtained, respectively [40].

2.3.4. Reduction of Nitrate and Oxidation of Ammonia in an Undivided Cell by

Constant Current Electrolysis

Ammonia and nitrite generation are the main drawback of the electrochemical process for nitrate reduction [7, 33]. Feng’s group and few other researchers have investigated the nitrate electro-reduction and oxidation of by-products (ammonia and nitrite) in an undivided cell in presence/absence of NaCl using a Ti/IrO2-Pt anode and a copper-based cathode during constant current electrolysis of 20 and 40 mA/cm [7,33, 42-

44].

43

2.3.4.1. Influence of pH on Kinetics of Nitrate Reduction

It has been illustrated that the nitrate reduction is not significantly impacted by pH and current density, and cathode/anode materials are the most crucial factors controlling the kinetics of nitrate reduction [44]. Low pH values are not favorable because it speeds up the rate of hydrogen evolution, which competes with nitrate reduction reaction at the cathode surface and decrease the kinetics of nitrate removal [42, 44]. pH range of 3-11 is introduced as an optimum range for constant current electrolysis in an undivided cell containing Ti/IrO2-Pt anodes [42]. The rate constant, k1, for nitrate conversion to nitrite

- - via reaction NO3 NO2 increases as the initial pH of the electrolyte become more

+ alkaline [42]. It has been claimed that low pH values accelerate the formation of NH4

- + because lower pH values favor the kinetics of the binding reaction between NO2 and H

[45]. Extreme high concentrations of OH- leads to the adsorption of OH- on active sites of catalyst, thereby inhibiting the available active sites for nitrate/nitrite reduction [46]. The highest activity for nitrate reduction was observed at pH values where bimetallic clusters

- became positively charged to effectively ads.orb NO3 anions [46].

2.3.4.2. Influence of Temperature on Kinetics of Nitrate Reduction

Increasing the temperature of electrolysis is favorable for nitrate reduction and

oxidation of by-product ammonia and nitrite [42]. Operating the ammonia/nitrate

electrolysis process at temperatures as close as possible to room temperature is favorable

for economic and energy-saving considerations, in this regard, optimizing the catalyst

material and the cell design are the main critical factors to be considered.

44

2.3.4.3. Effect of Current Density on Kinetics of Nitrate Reduction

The rate of nitrate reduction reaction increases as we operate in higher current densities [42]. It has been claimed that the ammonia oxidation rate is linearly related to the current density during electrolysis; however, due to the hydrogen evolution at cathode surface at high current densities, nitrate reduction kinetics does not linearly increase with increasing current density [42]. Li et al. declared that 20 mA/cm2 is the optimum current density for operating constant current electrolysis for simultaneous nitrate removal and ammonia oxidation [33, 42]. The nitrate reduction rate did not increase considerably with increasing current density in the range of 40-60 mA/cm2, because of the high rate of hydrogen evolution at these high current densities [43]. The potentials detected at current densities of 5, 10, 20, 40, and 60 mA/cm2 were in a range of 3.8-4.1, 5.6-6.0, 7.8-8.1, 9.8-10.1, and 11.9-12.2 V, respectively [43]. Zhang and co- workers reported that increasing the current density up to the optimized value of 25

2 - + - mV/cm favors both NO3/2 and NH4 removal [45]. During nitrate electrolysis, NO2 generation at low current densities is higher than that at high current densities because

- + NO2 is unstable and its conversion to NH4 and N2 is thermodynamically more feasible

- at high currents; however, at low current densities the kinetic of NO2 conversion is slow

- leading to the accumulation of NO2 at the cathode surface. At low current density values, the amount of produced hypochlorite anions is not enough to oxidize ammonia as well [45].

45

2.3.4.4. Effect of Addition of NaCl on Kinetics of Nitrate Reduction

and Oxidation of Ammonia

The effect of NaCl addition on simultaneous electrochemical reduction of nitrate and oxidation of produced ammonia is investigated by a few researchers [7,22,33, 42-44].

Compared to the condition without NaCl addition, total nitrogen sharply decreases in the

- presence of 0.5 g/l NaCl with energy consumption of 1.2-1.6 g NO3 -N/KWh [5,33, 43].

During electrolysis, chlorine is produced at the anode surface and immediately reacts with water creating hypochlorite acid (HClO), which would oxidize both nitrite and ammonia through the following reactions [5,33, 43]:

Reyter et al. claimed that during nitrate reduction, produced ammonia is immediately

- oxidized to N2 gas in the presence of hypochlorite anions (ClO ) via reaction [7]:

Figure 2.13 shows the variation of total nitrogen, nitrate, nitrite, and ammonia concentrations after 3 h of electrolysis at a constant current of 20 mA/cm2 in an undivided cell containing copper cathode and Ti/IrO2-Pt anode [33]. In the absence of

NaCl (Figure 2.13 a), the concentration of nitrite slightly increases first. It then decreases to zero after 3 h, confirming that nitrite is an intermediate anion during nitrate reduction reaction, and it is further reduced to nitrogen gas or ammonia or oxidized back to nitrate.

As it is shown in Figure 2.13 (a), ammonia as the main product of nitrate reaction is 46 continuously increased during electrolysis in the absence of NaCl [33]. In the presence of

NaCl (Figure 2.13 b), ammonia concentration firstly increases from 0 to 11.8 mg/l after 1 h and then decreases to 2.0 mg/l at the end of 3 h run of electrolysis. In the presence of chloride ion, hypochlorite ions will oxidize ammonia and nitrite to produce nitrogen gas and nitrate [33].

a b

Figure 2.13. Nitrate, nitrite, and ammonia concentrations vs. time during constant current electrolysis in (a) absence and in (b) presence of 0.3 g/l NaCl [33].

It must be considered that the extreme amount of hypochlorite or hydroxyl anions beyond the optimum concentration leads to the anion accumulation at the cathode surface that hinders the nitrate reduction reaction [44].

Zhang et al. have recently investigated the nitrate reduction and oxidation of by- produced ammonia in an undivided cell containing Cu/Ni composite cathode and Ir-

Ru/Ti anode during electrolysis at a constant current of 25 mA/cm2 in the presence of

NaCl [45]. They reported complete nitrate removal with a selectivity of about 100% to N2 gas in the presence of 1 g/l NaCl after 2.5 h electrolysis; however, a significant amount of ammonia and some N2 gas was detected after 2.5 h electrolysis in the absence of NaCl. 47

- This indicates the crucial role of NaCl in transforming NO3 to N2 gas [45]. Perez et al. have studied the simultaneous removal of nitrate and ammonia using boron-doped diamond (BDD) electrodes in solutions containing 14.1-28.2 mol/m3 NaCl at a constant current of 40 mA/cm2 [47]. They reported a significant effect of chloride presence on the oxidation rate of nitrite and ammonia in an undivided cell. After electrolysis, nitrite was detected only in solutions without chloride, which reveals that in the presence of chloride, nitrite/ammonia oxidation to nitrogen has a very fast kinetics [47, 48]. Perez and co- workers suggested the reaction pathways as depicted in Figure 2.14, for nitrate reduction on BDD electrodes in the presence of chloride [47]:

Figure 2.14. Reaction pathways for nitrate reduction on BDD electrodes in the presence of chloride [47].

Chloride has a positive influence on both nitrate reduction and ammonia oxidation and increases the total conversion of nitrate to nitrogen gas [47, 49]. Couto et al. investigated the nitrate reduction and oxidation of by-produced ammonia and nitrite in an undivided cell (BDD as anode and carbon fiber cathode) using constant current electrolysis at 25 and 50 mA/cm2 [50]. At the initial steps of electrolysis, the high 48

- quantity of generated OH anions favors the direct oxidation of ammonia to N2 gas,

+ thereby decreasing the selectivity to NH4 formation [50].

2.3.4.5. Effect of the Ratio of Anode to Cathode Surface Area on Kinetics

of Nitrate Reduction

The influence of the anode/cathode surface area ratio on nitrate electro-reduction in 0.01 M NaOH+ 0.1M NaNO3+ 0.5M NaCl solution, is investigated elsewhere using copper and Ti/IrO2 coupled electrodes [10]. The cathode/anode surface area ratio of 2.25 is reported to be the optimum for converting nitrate to N2, resulting in the highest

-1 - efficiency with the energy consumption of 14.7 kWhkg NO3 [10], much less than what

-1 - was reported by Cheng et al. (40.1 kWhkg NO3 ) [3]. Kuang et al. investigated the effect of the anode to cathode surface area (A/C ratio) on nitrate reduction and oxidation of by- product by applying constant current electrolysis of 20 mA/cm2 in an undivided cell using Cu-Zn cathode and Ti/IrO2-Pt anodes in the presence of 1 g/l NaCl [44]. An improvement in the rate of nitrate removal rate was observed by decreasing A/C ratio from 1.00 to 0.12 [44]. 49

Figure 2.15. Nitrate, nitrite, and ammonia concentration change versus time of 2 electrolysis at different A/C ratios, Ielectrolysis = 20 mA/cm in the absence (A) and the presence (B) of 1 g/l NaCl [44].

Figure 2.15 (A) shows that after 120 min of electrolysis in the absence of NaCl, N-NO3- concentration decreases from 50 ppm to 11.46, 9.35, 8.09, and 7.25 ppm at the A/C ratio of 1.00, 0.46, 0.23, and 0.12, respectively, thus nitrate reduction rate increases with decreasing A/C ratio [44]. In the absence of NaCl, the N-NH3 increases from zero to

19.4, 23.51, 25.95, and 27.04 ppm as A/C ratio decreases from 1.00 to 0.12. Similarly, N-

- NO2 increases from zero to 0.83, 0.92, 1.99, and 3.73 ppm as A/C ratio decreases from

1.00 to 0.12. In the absence of NaCl, the by-product of nitrate reduction is removed by oxidation at Ti/IrO2-Pt anode through dehydrogenation [44]. IrO2 creates highly oxidative hydroxyl radicals associated with Ir(III)/Ir(IV) and Ir(IV)/Ir(VI) redox couples of oxy- iridium groups which have the leading role in ammonia oxidation to N2 in the absence of 50

NaCl through the following reactions when using Ti/IrO2-Pt as anode for nitrate electrolysis [44]:

As A/C decreases, the conversion of M-OH to ●OH is promoted and thereby weakening the efficiency of direct ammonia oxidation to nitrate at Ti/IrO2-Pt anode surface in a solution containing 50 ppm (NH4)2SO4 [44]. Figure 2.15 (B) depicts the effect of adding 1 g/l NaCl on nitrate removal rate In different A/C ratios. After 2 h of electrolysis, the nitrate concentration decreases from 50 ppm to 14.83, 14.19, 13.14, and

11.88 ppm at A/C ratios of 1.00, 0.46, 0.23, and 0.12, respectively and no ammonia and nitrite by-products were detected at the end [44]. By decreasing A/C ratio from 1.00 to

0.12, the nitrate reduction efficiency increased from 70.3% to 76.2%, and at the same time, the average current efficiency of electrolysis increased [44].

2.3.5. Mechanism of Nitrate Reduction and Ammonia Oxidation

The main challenge during the nitrate electrolysis process is to obtain a 100 % selectivity to N2 in an undivided cell [50, 51]. Two pathways are proposed for oxidation of produced ammonia during nitrate reduction: the first is the direct oxidation of ammonia, and the second route is indirect oxidation of ammonia through its reaction with produced species at the anode surface [50]. In this sense, the anode electrode material plays an important role in the reaction mechanism involved in nitrate reduction and 51 simultaneous oxidation of undesired ammonia/nitrate by-product formed during the electrolysis [50]. During nitrate reduction reaction in alkaline media, the by-product

- + - + ammonium is mainly generated through the reaction NO3 + H2O + 6H + 3e →NH4 +

4OH- [50].

2.3.5.1. Mechanism of Nitrate Electro-Reduction

The reaction pathway during nitrate reduction strongly depends on the electrolysis parameters, including, the cathode/anode material, pH, the electrolysis potential, temperature, the cell configuration, presence of other anions, cathode/anode morphology, and the electrodes spacing/geometry [46]. A broad range of complexes, such as N2, N2O,

- NH3, NO, NO2 , NH2NH2, and NH2OH can be produced as a result of electrochemical reduction of nitrate [33]. Two fundamental explanations for nitrate reduction mechanism routes are, (1) pathways for nitrate reduction, which involves electron transfers via the following reactions (Figure 2.16) [46, 51]:

and (2) pathways that involve hydrogen atom [51]: 52

where the following hydrogen evolution reaction is the most competitive side reaction at the cathode surface:

Figure 2.16. Pathways for nitrate reduction mechanism, which involves electron transfer [51].

Due to a large amount of OH- production during nitrate electrolysis via reaction pathway mechanism (1), pH of the solution increases [51]. It is claimed that on cathode materials with empty d orbit which has an affinity to H atom adsorption (e.g., Pt, Rh, and

Ni), the mechanism (2) is more feasible, while on metals with a strong affinity to nitrate 53 adsorption (e.g., Cu), the mechanism (1) is more likely to occur [51]. Bimetallic alloys of

Pd and Pt with transition metals have a high activity to adsorb hydrogen as a reducing agent to initiate the nitrate reduction to nitrite through mechanism (2) [46]:

It is asserted that transition metal is oxidized after promoting the nitrate reduction.

By contrast, the role of noble metal is to facilitate the reduction of oxidized transition metal [46]. Zhang et al. proposed that nitrite reduction to NO(Ads.) species is the intermediate step during the reduction of nitrite to ammonia [52] (NO dissociation pathway mechanism [53]). In contrast, the second pathway could be NO hydrogenation mechanism, which generates HNO(Ads.), H2NO(Ads.), H2NOH(Ads.), NH2(Ads.), and NH3(Ads.) as intermediate species [53]. The most well-known mechanism for the hydrogenation mechanism of nitrate on bimetallic clusters is illustrated in Figure 2.17 [46]. 54

Figure 2.17. Hydrogenation reduction mechanism of nitrate on bimetallic catalysts [46].

It is assumed that encountering two of Pt/Pd-NO(Ads.) (nitrosyl) intermediates provides N-N bonds required for N2O formation and consequent N2 generation [32]. A single adsorbed Pt/Pd-nitrosyl intermediate ends up forming ammonia instead of N2 [32].

The more active sites for adsorption of Pt/Pd-nitrosyl, the more possibility for encountering two of NO, and thereby promoting the probability of N2 generation (instead of ammonia) because of the nitrate reduction reaction [32].

Kerkeni et al. suggested the following reaction for possible copper oxidation during nitrate/nitrite reduction on Cu active sites [54]:

Figure 2.18 schematically shows the different pathways for simultaneous nitrate reduction and ammonia oxidation in the presence of NaCl, which strongly depends on the cathode/anode surface area ratio and cathodic potential. At the potential of -0.9 V vs.

Hg/HgO, the only product at the copper surface is nitrite, which is oxidized to nitrate 55 again at Ti/IrO2 (nitrate is not desirable). At more negative potentials between -1 to -1.5

V vs. Hg/Hg, the production of ammonia and nitrogen is accelerated by means of ClO- anions. Beyond -1.5 V vs. Hg/HgO, hydrogen evolution reaction increases, thereby decreasing the rate of nitrate reduction, which is again not desired [10].

Figure 2.18. Different reaction pathways during nitrate reduction on Cu-cathode in 0.1 M NaNO3+0.01M NaOH+0.5M NaCl [10].

Part of nitrite intermediate could be reduced to N2 gas or ammonia or could be oxidized again to nitrate. Kuang et al. detected no nitrate accumulation at Cu-Zn cathode surface under constant current electrolysis of 20 mA/cm2 in an undivided cell in the presence of NaCl [44].

2.3.5.2. Mechanism of Ammonia Electro-Oxidation

In alkaline media, the equilibrium between ammonia and ammonium is

+ - established via reaction NH4 +OH ↔NH3+H2O [50]. The first mechanism for ammonia electro-oxidation in alkaline solutions on Pt was proposed by Oswin and Salomon, who 56 suggested a four-step mechanism for ammonia oxidation through the following reactions

[55]:

In alkaline media, ammonia is in the form of NH3(aq.) [19], which is firstly adsorbed on Pt active sites and then is decomposed to Pt-NH2(Ads.) and H2O. The next two steps are the dehydrogenation of Pt-NH2(Ads.) and Pt=NH(Ads.) species followed by the reaction between two Pt≡N(Ads.) to create N2 gas [55]. Gerischer and Mauerer [13] proposed another premier and well-established mechanism. According to this mechanism, NH3 is first dehydrogenated to the adsorbed NH2, followed by its dimerization to produce hydrazine (N2H4). Hydrazine is rapidly oxidized N2 gas [13]:

Muller and Spitzer proposed the successive mechanism of ammonia oxidation by

OH- anions in NaOH solution at a platinum anode during ammonia electrolysis [13]: 57

Ammonia oxidation to N2 gas preferably occurs on Pt(100) micro-domains because Pt(100) stabilizes NH2(Ads.) which is required for hydrazine formation and its rapid oxidation to N2, albeit Pt(111) and Pt(110) lattice surfaces stabilize NH(Ads.) species

- [13]. Azide ion (N3 ) formation through the following reactions, is the intermediate step for N2 generation on Pt anodes during ammonia oxidation reaction [56]:

The standard potential difference between the following two anodic and cathodic reactions is about 59 mV that is much lower than the potential of water electrolysis (1.23

V) [13]. This makes ammonia electrolysis as a promising reaction for H2 generation for energy purposes [13]:

Elemental nitrogen is formed at anodic potentials lower than 0.7 V, while the production of nitrite, nitrate, and other nitrogen oxides complexes are thermodynamically more feasible at positive potentials higher than 0.8 V (vs. SHE) [13]. It has been postulated that at low concentrations of ammonia, oxidation of ammonia occurs under the 58 control of mass transport of active species. In contrast, at high concentrations, mass transfer is not the predominant factor that favors the N2 gas production [13].

It has been claimed that the formation of active intermediate to generate N2 gas is only possible on Pt and Ir clusters [57]. Estejab et al. investigated the mechanism of ammonia oxidation to nitrogen on Pt3-xIrx(x=0-3) clusters using Density functional theory

(DFT) analysis [12]. Based on DFT model, two different mechanisms were found for ammonia oxidation on Pt and Ir clusters. On Pt clusters, hydrazine (N2H4) is firstly formed by the reaction between adsorbed N and adsorbed NH intermediates followed by dehydrogenation of hydrazine which generates N2 gas [12], however, on Ir clusters, successive dehydrogenation of ammonia generates atomic nitrogen, followed by N-N bond formation to produce N2 gas [12]. It has been claimed that Ir sites are active sites to adsorb toxic intermediates, leaving the Pt sites available for ammonia oxidation via hydrazine formation mechanism [12].

2.3.5.3. Mechanism of Simultaneous Nitrate Reduction and Ammonia

Oxidation in an Undivided Cell During Pulse Electrolysis

Up to now, the detailed mechanism of simultaneous nitrate and ammonia removal in an undivided electrochemical cell is not well understood.

Figure 2.19 schematically represents the possible electrochemical reactions occurring at the surface of the electrodes during positive and negative cycles of pulse electrolysis for simultaneous nitrate and ammonia removal in an undivided cell. 59

Figure 2.19. Electrochemical reactions occurring at the surface of the electrodes during positive and negative cycles of pulse electrolysis for simultaneous nitrate and ammonia removal in an undivided cell.

During the positive cycle, E1 behaves like an anode where the ammonia electrolysis and nitrite oxidation to nitrate are taking place, and E2 acts like a cathode where the nitrate reduction reactions to ammonia, nitrite, and nitrogen occur. In contrast, in the negative cycle, E1 becomes a cathode, and E2 acts as an anode, and during the whole electrolysis process, nitrate reduction and ammonia oxidation repeatedly occur at both electrodes. The sum of all reactions in each cycle leads to the following overall reaction [28]:

The main challenge is to find the most efficient electrode material enabling both cathodic reduction of nitrate and anodic oxidation of ammonia [25]. The rate of anodic oxidation of ammonia during constant potential electrolysis is much lower than that of pulse potential electrolysis [25].

60

Chapter 3: Experimental & Methodology

3.1. Materials & Apparatus

Carbon black Vulcan XC 72R was purchased from FuelCellStore. 10 gr of 10 wt.% of ionomer (FFA-3-film anion exchange ionomer in bromide form (part# M21951303)) was prepared by adding 1 gr of FFA-3-film anion exchange ionomer to 9 gr of Methanol

(Fisher Chemicals, A412-4 with Lot number: 174135). After synthesizing the catalyst, in a 125 mL Erlenmeyer flask 0.4 gr catalyst/C (which means 0.2gr C and 0.2 gr metal) was added to 65 gr isopropanol (2-Propamol, Fisher Chemical, A451SK-4 with Lot number:

157378) and was mixed with 1 gr of 10 wt.% of ionomer solution. Thus, the ratio of catalyst to ionomer was 2 to 1 mass fraction. The mixture was then sonicated using IKA

Ultra Turrax (T 18 digital) ultrasonic for 15 min at 9000 rpm (Figure 3.1).

Figure 3.1. Ultra Turrax T18.

Immediately after 15 min of sonicating in step 4, the mixture was sonicated in

Branson 2800 Ultrasonic Cleaner ¾ Gallon (Figure 3.2) for 1 hour. 61

Figure 3.2. Branson 2800 Ultrasonic.

Sodium nitrate (NaNO3), ACS 99.0% and sodium nitrite (NaNO2) ACS 97.0% were purchased from Alfa Aesar. Ammonium sulfate (NH4)2SO4 98% and sodium hydroxide NaOH 97% pellets ACS were purchased from Fisher chemicals. All other solvent chemicals were analytical grades, and all solutions were prepared using deionized

(DI) water (18 MΩ), which was prepared using an EVOQUA water instrument equipped with an ion exchange process with two ionized resin beds.

3.2. PtIr-CuNi/C Catalyst Synthesis

3.2.1. Synthesis of PtIr Supported on Carbon Catalyst

A mixture of H2PtCl6 hydrate (99.9 % metal basis, Alfa Aesar) and IrCl3 trihydrate (53-56% Ir, Acros Organics) precursors and Vulcan XC-72 (from

FuelCellStore) were used to synthesize the catalyst using a modified polyol technique.

The Vulcan was mixed with ethylene glycol (EG) using ultrasonic in a Branson

CPX2800 bath sonicator for 2 hours, then using IKA Ultra-turrax ultrasonic sonicator for

30 min. In order to obtain a catalyst that was 60% Pt9Ir metal and 40% Vulcan, 54 ml of

10 mg Pt/ml EG precursor and 15 mL of 4 mg Ir/ml EG was mixed with 10.8 ml of 1M 62

KOH (Fisher Chemical), and the mixture was heated on a heater stirrer at 180 °C for 16h.

After 16 hours, 100 ml of DI-water was added to the mixture, and the vacuum filter was used to filter the mixture. After filtration, the ink was collected from the funnel. DI-water was used to wash ink from the funnel and remove all the materials from the centrifuge tubes. The mixture of DI-water and ink was centrifuged for about 45 min at the speed of

5000 rpm. After 1 hour, the excess water was removed, and the collected catalyst was dried in a vacuum oven (MTI EQ-DZF-6050-UL) at 110 °C for 16 hours. 0.4 g of the catalyst was mixed with 65 g isopropanol and 0.1 g ionomer and again was sonicated using IKA Ultra-Turrax for 15 min, then with Branson CPX2800 bath sonicator for 1 hour. A Master-Air-Brush spray gun was used to spray a specific loading on Ni mesh

(gauze, Alfa Aesar 40 mesh woven), acting as anode for ammonia oxidation.

3.2.2. Synthesis of CuNi Supported on Carbon Catalyst

A mixture of Ni(NO3)2 hexahydrate (99.999%, Alfa Aesar) and Cu(NO3)2 pentahydrate (98%, Alfa Aesar) precursors and Vulcan XC-72 was used to synthesize the catalyst using a modified polyol method. The Vulcan was mixed with ethylene glycol

(EG) using ultrasonic in a Branson CPX2800 bath sonicator for 2 hours and afterward using IKA Ultra-turrax ultrasonic sonicator for 30 min. In order to prepare the CuNi/C catalyst, proper amounts of 10.93 mg Cu/ml EG and 10.48 mg Ni/ml EG was mixed with an appropriate amount of 1M NaOH (Certified ACS Pellets, Fisher Chemical) at 196

°C for 16 hours. After this step, the wet impregnation technique was carried out in accordance with reference [58] to load the colloidal CuNi particles (with a specific mass ratio of Cu to Ni) on carbon Vulcan, obtaining a particular wt.% of metallic particles to 63

Vulcan. The same steps for creating the Pt9Ir/C catalyst synthesis were followed, such that, after 16 hours, 100 ml of DI-water (18 MΩ) was added to the mixture, and the vacuum filter was used to filter the mixture. After filtration, the ink was collected from the funnel. DI-water is used to wash ink from the funnel and remove all materials from the centrifuge tubes. The mixture of DI-water and ink was centrifuged for about 45 min at the speed of 5000 rpm. After 1 hour, the excess water was removed, and the collected catalyst was dried in a vacuum oven (MTI EQ-DZF-6050-UL) at 110 °C for 16 hours.

0.4 g of the catalyst was mixed with 65 g isopropanol and 0.1 g ionomer. The mixture was sonicated using IKA ultra-turrax (for 15 min) and then using Branson CPX2800 bath sonicator (for 1 hour). A Master-Air-Brush spray gun was used to spray a specific loading on Ni mesh (gauze, Alfa Aesar 40 mesh woven), acting as cathode for nitrate/nitrite reduction.

3.3. Electrodeposition of Bimetallic CuNi Catalysts

The electroplating of Cu90Ni10 on Ni mesh electrodes was accomplished using galvanostatic (constant current density of 10 mA/cm2) for 300 seconds in a solution containing 0.05M NiSO4 + 0.7M CuSO4 + 0.26 M C6H5Na3O7.2H2O. A wide range of

CuNi alloy compositions (Cu91Ni9, Cu77Ni23, Cu60Ni40, and Cu25Ni75) were electroplated on Ni mesh, and their performance for nitrate reduction reaction were investigated.

3.4. Nitrate and Ammonia Analysis

All concentration measurements of nitrate and ammonia were determined according to the Environmental Protection Agency (EPA) standards. An Ammonia Ion-

Selective Electrode (ISE-THERMO) Orion 9512 was used to measure the concentration 64 of ammonia. It yielded a concentration ranging from 0.1-100 ppm of N-NH3 without dilution. For calibration, N-NH3 standard solutions with concentrations of 0.121, 1.21,

12.1, and 121 ppm N-NH3 were used. In a 50 mL beaker, 1 mL of pH-adjusting solution,

ISA was added to the first standard solution, and the reading of the ISE electrode was set at 0.121 ppm N-NH3. This step was repeated for each standard solution to obtain the calibration curve. The resulting value for the calibration curve should be between -54 to -

60 mV at room temperature (detailed information regarding the procedure for using ISE is provided in Appendix 1).

The analysis of nitrate was done in accordance with reference [59]. For this method, a premix solution consisting of premix 1 (vanadium (III) Chloride solution), premix 2 (sulfanilamide solution), and premix 3 (N-(1-naphthyl)-ethylenediamine dihydrochloride) (ratio of 2:1:1, respectively) was required. A standard solution of 40

- ppm NO3 was used to plot a calibration curve of absorbance versus concentration of nitrate. In this step, the spectrum for blanks and standards was measured in the UV/VIS spectrophotometer at 540 nm to prepare the calibration curve. To measure the absorbance spectra using UV/vis for samples, and the standard solution, a specific ratio of the premix solution with the samples was mixed. After plotting the calibration curve, the same procedure for samples was repeated to measure the UV/vis spectra for samples at 540 nm. Finally, based on the calibration plot of absorbance versus nitrate concentration, we obtained the nitrate concentration (detailed information regarding the procedure for nitrate analysis is provided in Appendix 2).

65

3.5. Electrochemical Measurements and Characterizations

All electrochemical characterization measurements were taken using an Arbin

BT2000 and a Solartron 1470E potentiostat. Hg/HgO reference electrodes (in basic media) and Ag/AgCl reference electrodes (in acidic media) and Pt plate counter electrode

(2×2 cm2) were used for our electrochemical measurements. The electrochemical techniques primarily applied in this study were cyclic voltammetry (CV), Linear Sweep

Voltammetry (LSV), pulse electrolysis, and chrono-amperometry.

This approach consisted of testing the catalysts in a 25 cm2 flow cell first, then moving them to a bench-scale 300 cm2 (or 225 cm2) flow cell to investigate the simultaneous nitrate and ammonia removal from the electrolyte in an undivided cell.

Figure 3.3 illustrates the assembled 25 and 225 cm2 electrolysis cells. The cell body was fabricated by a milling machined clear acrylic plexiglass. 66

Figure 3.3. (a) 225 cm2, (b) 25 cm2 electrolysis flow cell, and schematic representation of (c) 225 cm2 and (d) 25 cm2 flow cell.

3.6. Characterization of the Catalysts

Characterization of the synthesized catalysts was done using Ultima IV XRD

Rigaku, JEOL JSM-6390 SEM, 15kV, 14 mm working distance), EDS, and JEOL JEM

2100F TEM. UV/vis Hewlett Packard 8452A Diode-Array Spectrophotometer was used to collect the absorption spectra for nitrate analysis. The Raman spectra were collected using a Bruker Senterra Raman spectrometer and a connected microscope equipped with a 20X lens (power 20 mW and wavelength 532 nm). BET Micromeritics Tristar II connected to a FlowPrep 060 Sample Degas System was applied to measure the effective 67 electroactive surface area of synthesized catalysts. The analysis of produced gas was done using gas chromatography (GC) -SRI 8610C instrument equipped with a Mole

Sieve 5A column, and argon gas were used as the carrier gas. All resources required were available at the Center for Electrochemical Engineering Research, except for the proximate analysis.

3.7. Electrolysis Process

Figure 3.4 presents the experimental set up used for the ammonia and nitrate electrolysis process. The synthetic solution streams were fed to the electrolysis cell by a

Cole Parmer gear pump. A Blue-White Industries F-400 flowmeter was used to control the flow rate. The temperature was controlled by an Econo Temperature Controller model

12125-14. The solution was continuously circulated in this loop, and every 30 min, the water samples were taken from the feed tank output to monitor the ammonia and nitrate concentration. The cycles were performed until the concentration of ammonia was reduced from about 2000 ppm to below 100 ppm.

68

Figure 3.4. Ammonia/Nitrate electrolysis loop setup.

3.8. Quality Assurance and Reproducibility

3.8.1. Uncertainties and Calibration

It is critical to calibrate the ammonia ISE electrode every day before using it. Five different ranges of ammonia from 0.1214 ppm to 121.4 ppm were used for calibrating the

ISE electrode (detailed information regarding the procedure for using ISE is provided in

Appendix 1). The slope of ISE calibration was monitored to ensure an acceptable range of -57 to -60 mV/decade; otherwise, the calibration was repeated. In some cases, when the ISE reading dropped down to zero, it was probable that there was some leakage put of its membrane. Thus, the membrane and ISE filling solution needed to be changed accordingly based on the procedure mentioned in the instructions. pH is an essential factor in ammonia electrolysis and nitrate electro-oxidation reactions. An inaccurate pH- meter with an incorrect reading could considerably affect the results. Therefore, the pH- meter was calibrated for each procedure. For the nitrate analysis, a new calibration curve was plotted each time. The calibration for Arbin and Solartron Potentiostat- 69

Galvanostat instruments was conducted once per year. Abnormality of measurements may sometimes occur as a result of not defining the procedure properly. It is necessary to limit the range of current and potential in a specific range based on the operational condition of each particular experiment. Temperature, pH, and flow rate during the simultaneous removal of ammonia and nitrate are three crucial factors such that any unintentional error in their monitoring results in extreme uncertainty.

3.8.2. Total Number of Measurements During Ammonia and Nitrate Analysis

For each sample, the concentration of ammonia was measured twice, and the average was always reported. Any remarkable observation in terms of the performance of a catalyst for nitrate removal and ammonia oxidation was examined at least twice before reporting that.

70

Chapter 4: Results and Discussion

4.1. Physical Characterization of Synthesized Catalysts

4.1.1. XRD and TEM Characterization of Catalysts

The polyol technique based on ethylene glycol (EG) as a common technique for synthesizing a carbon-supported catalyst was used to synthesize Cu9Ni/C, Pt9Ir/C, and

Pt3Ir/C catalysts. The precursor solutions included CuSO4+NiSO4 with EG, and

H2PtCl6+IrCl3 with EG, respectively. 1 M KOH and 1M NaOH were used as reducing agents, respectively. Figure 4.1 indicates the XRD patterns and TEM images for Cu9Ni/C and Pt9Ir/C catalysts in powder form.

Figure 4.1. (a, b) XRD patterns and (c-f) TEM images of synthesized Cu9Ni, Pt9Ir, and Pt3Ir supported on Carbon catalysts. 71

The diffraction peaks of Cu (111), Cu(200), and Cu (220) together with Ni (111) indicates the presence of CuNi bimetallic alloy in CuNi/C catalyst. The presence of CuO peaks suggests that some part of copper is oxidized to copper(II)oxide during the polyol process. CuO formation can be prevented by adding more of NaOH than the stoichiometric ratios, ensuring that all CuSO4 and CuO is entirely reduced to copper. At the same time, by eliminating excess water or other oxidizing agents from the system by using a longer stirring time, we can minimize the CuO formation during the polyol process [58]. A well-dispersed Cu9Ni nanoparticles distribution with an average particle size of about 3.0 – 3.5 nm can be observed in TEM images. The XRD patterns of the powder Pt9Ir/C and Pt3Ir/C show the clear peaks of Pt(111), Pt(200), Pt(220), and Ir(311).

The addition of Ir element intensifies the reflection peaks, which is in good accordance with what was observed elsewhere [11]. Both Pt and Ir have a Face-Centered-Cubic

(FCC) crystal structure, and, as we increase the elemental Pt content from Pt3Ir to Pt9Ir is increased, Pt atoms tend to replace for Ir atoms in the FCC crystal structure [57, 60]. The average particle size of the synthesized PtIr catalyst is calculated to be about 2.2 – 2.9 nm

(as it is shown in Figure 4.1), which provides a sufficient surface area for the ammonia/nitrate electrolysis reactions.

4.2. Ammonia Electro-Oxidation

A systematic approach is used to examine the performance of different grades of

PtIr/C and CuNi/C catalysts for ammonia oxidation and nitrate reduction to select the best catalyst. Figure 4.2 illustrates the applied pulse potential and the corresponding pulse current response during the ammonia electrolysis process in 6.9 g/l (NH4)2SO4+10 g/l 72

NaOH solution. Figure 4.3 shows the variation of ammonia concentration versus time during 5 hours of electrolysis. The corresponding ppm and mg loss of ammonia, ammonia rate loss (in mg/in), and Faraday efficiency during the electrolysis process are listed in Table 4.1.

After 5 hours of electrolysis, ammonia concentration decreased from about 1725 ppm to 57 ppm, with an average current density of about 17.4 mA/cm2.

Figure 4.2. (a) Pulse potential electrolysis and (b) corresponding pulse current response 2 during ammonia electrolysis for 5 h in a 300 cm flow cell with a pair of 60% Pt9Ir/C 2 electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V). 73

2000

1500 3

1000 ppm NH ppm 500

0 0 100 200 300 400 time (min)

Figure 4.3. Variation of ammonia concentration during ammonia electrolysis in a 300 2 2 cm flow cell with a pair of 60% Pt9Ir/C electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V).

The calculated values of Faraday efficiency after 30 min of electrolysis is above 80% for the reaction 2NH → N + 3H , which indicates the effectiveness of this process in ammonia electro-oxidation throughout the three-electron transferred reaction process.

Table 4.1. Variation of ammonia concentration, ammonia rate loss, and Faraday efficiency versus time during ammonia electrolysis in a 300 cm2 flow cell with a pair of 2 60% Pt9Ir/C electrodes (loading 0.25 mg/cm ) in 6.9 g/l (NH4)2SO4+10 g/l NaOH solution at 38 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V).

74

4.2.1. Characterization and Selection of a PtIr/C Anode for Ammonia Oxidation

During this study, different PtIr/C catalysts were tested, and three grades with the most promising characteristics were selected for further examinations. A higher ratio of the metallic nanoparticle to carbon Vulcan results in higher electro-catalytic activity.

Based on the weight analysis during the synthesis process, it was found that the maximum amount of metallic PtIr nanoparticles that could be loaded on carbon Vulcan support is about 61.8% Pt9Ir catalyst. Thus, the loading on carbon support could not exceed 61.8% ratio of Pt9Ir metal to Vulcan support for the polyol technique in this study.

Under the constant potential/current electrolysis, the direct oxidation of ammonia to nitrogen is difficult due to the intense OH- accumulation at the electrode surface, while pulsing potential can facilitate the rate of mass transfer reactions such as ammonia/nitrate electro-oxidation/reduction [28]. Figure 4.4 illustrates an example of the square wave pulse potential electrolysis technique (±0.925 V, pulse width 18 s) and the corresponding pulse current response, as well as the energy consumption plot in a 25cm2 flow cell with

2 a pair of Pt9Ir/C 60% electrodes (loading 0.5 mg/cm ) in 8.3 g/l (NH4)2SO4+18 g/l NaOH solution at 60 ◦C (flow rate 2 LPM).

75

Figure 4.4. (a) pulse potential electrolysis, (b) corresponding pulse current response, and (c) energy consumption plot during ammonia electrolysis in a 25cm2 flow cell with a pair 2 of Pt9Ir/C 60% electrodes (loading 0.5 mg/cm ) in 8.3 g/l (NH4)2SO4+18 g/l NaOH solution at 60 ◦C, flow rate 2 LPM (pulse width 18 sec, pulse potential ±0.925 V).

At the pulse potential of ±0.925 V in this testing condition, the average current density of about 50 mA/cm2 was obtained, which is much higher than what was reported elsewhere [2, 11]. After 30 min of electrolysis, approximately 200 ppm ammonia (0.2 mg

N-NH3) was removed from the solution, which is equal to a Faraday efficiency of about

- - 100% for reaction NH3+3OH → 0.5N2+3H2O+3e . Figure 4.4 (c) shows an evident energy fluctuation, which corresponds to the pulsating potential (according to the formula 76 of E = V×I×t). The energy consumption in 25cm2 flow cell after about 30 min in our

-3 -1 undivided cell is approximately about 3×10 kWhg NH3, which is comparable with what was reposted elsewhere [2-4] for ammonia and nitrate removal. Further improvements in energy consumption were obtained as we enlarged the size of the flow cell (for example, for 225 cm2 cell), which will be further discussed in the following sections. At a pulse potential of 0.925 V, the corresponding average current was 1.25A, and based on the stoichiometric calculations for reaction 2NH3 → 3H2 + N2

(and considering 100% for ammonia oxidation), the electrical energy for 1 g H2

-1 production is about (0.0882 M H2 × 2 g/mole H2) ×0.003 kWh× (1000 W/1kW) = 17 Wh which is so in accordance with what was reported in reference [9] for the ammonia electrolysis process on Pt-Ir deposited on nickel foam (17.6 Wh per g of H2 production).

4.2.2. Electro-Catalytic Activity of PtIr/C Catalyst for Ammonia Oxidation

Three different grades of catalysts (50% Pt3Ir/C, 60% Pt9Ir/C, and 40% % PtIr/C) were synthesized using a modified polyol technique and their catalytic activities for ammonia oxidation were examined using CV and pulse electrolysis in a 25 cm2 flow cell system. Figure 4.5 (a) shows the voltammograms of 50% Pt3Ir/C, 60% Pt9Ir/C, and 40%

2 % PtIr/C (loading for all catalysts equal to 0.25 mg/cm ) in 8.3 g/l (NH4)2SO4 + 18 g/l

NaOH solution. 77

Figure 4.5. (a) CVs of three different PtIr/C catalyst compositions in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution (b) correlation between current density during ammonia electrolysis process at 50 ◦C and NaOH concentration (loading for all catalysts tested is 0.5 mg/cm2).

2 At the same loading of 0.25 mg/cm , 60% Pt9Ir/C, the highest peak current density of about 18 mA/cm2 was obtained. The peak potential observed between 0 to -0.2

V vs. Hg/HgO corresponds to the ammonia oxidation via reaction [60]:

NH() + 3OH → 0.5N + 3HO + 3

Figure 4.5 (b) illustrates the average current density for 50% Pt3Ir/C, 60% Pt9Ir/C, and 40% % PtIr/C catalysts at different NaOH concentrations during ammonia

2 electrolysis in 25 cm flow cell in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution. In all cases, there is an increasing trend in current density when more NaOH is added to the base

(NH4)2SO4 solution. More concentrated NaOH solutions shift NH() + 3OH →

- 0.5N + 3HO + 3 reaction to the right by providing more OH in access, thus increasing the current density associated with ammonia oxidation. As it is shown in

Figure 4.5 (b), at different NaOH concentrations, the kinetics of ammonia oxidation 78 during the electrolysis process at 50 °C for 60% Pt9Ir/C catalyst is higher than those for

40% Pt1Ir1/C and 50% Pt3Ir/C. A possible explanation for the higher activity of Pt9Ir/C

- may be that the N2H4 formation-mechanism is the predominant mechanism during ammonia oxidation. This mechanism is more feasible on Pt sites, while a N2-formation- mechanism is more achievable on Ir sites. Hence, for a dominant N2H4-mechanism process, better performance for oxidation of ammonia is observed for those grades of

PtIr/C catalysts rich in Pt [12].

The voltammogram responses of Pt9Ir/C catalyst in the absence and the presence of ammonia were measured in a basic solution of 18 g/l NaOH (Figure 4.6). The onset of ammonia oxidation during the positive potential sweeping is about -0.4 to -0.5 V vs.

Hg/HgO, which is in good agreement with what was observed by Diaz et al. [18, 60].

Figure 4.6. CV response of Pt9Ir/C catalyst in the absence and the presence of ammonia, scan rate = 25 mV/s. 79

No oxidation peak was observed in the absence of ammonia, while the addition of ammonia yielded a 70 mA rise in the current, which indicates the catalytic activity of

Pt9Ir/C catalyst toward the ammonia oxidation reaction [60]. The shape of CV in the ammonia-free solution is similar to what was observed by Diaz et al. [18, 19] (in terms of the underpotential deposition of hydrogen at the potential range between -0.7 to -0.4 V vs. Hg/HgO). Figure 4.7 represents the successive voltammograms of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution.

13 11 9 pH 13 1st cycle 7 ) 2 5 10th cycle 3 1 i (mA/cm -1 -3 -5 -7 -1 -0.5 0 0.5 E (V vs. Hg/HgO)

Figure 4.7. Repetitive CV response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution, scan rate: 25 mV/s.

Over ten cycles in a wide potential window ranging from -1 to +0.4 V vs.

Hg/HgO, the Pt9Ir/C catalyst possessed notable stability in such the alkaline environment

(pH ≈13). The slight decay in peak current during the repetitive scans is primarily caused 80 by the poisoning of Pt catalytic sites by adsorption of imine and nitrogen ad-atoms [18,

19].

4.2.3. Kinetics of Ammonia Oxidation

The CV response of Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH solution as a function of scan rate and the corresponding dependency of peak current on the square root of scan rate is presented in Figure 4.8. By increasing the sweep rate, the peak current magnitude increases, and the potential peak shifts to more positive potentials, which implies the rapid kinetics of ammonia oxidation on the Pt9Ir/C surface [61].

Considering the following Randles–Sevcik equation, the linear dependency of peak current versus the square root of the scan rate (ν0.5) implies that the oxidation of ammonia on Pt9Ir/C surfaces occurs through a diffusion-controlled process [19, 62].

= −(2.99 × 10 ) × ()

where n is the number of electrons involved in the reaction, α is the charge transfer coefficient, na is the number of electrons transferred in the rate-determining step, A is the

2 geometrical surface area of the electrode (in our case it was 4 cm ), DNH3 is the ammonia

-9 2 diffusion coefficient (≈ 1.217×10 cm /s, Ref. [60]), and CNH3 is the ammonia bulk concentration (3.2×10-3 mol/cm3). 81

40 40 5 mv/s 35 30 10 25 30 50 ) 20 100 2 25 20 10 Slope = 3.001 R2= 0.998

i (mA/cm 15

i (mA/cm2) 0 10 -10 5 0 -20 0 5 10 15 -1 -0.5 0 0.5 ν0.5(mV/s)0.5 E vs. Hg/HgO (V) -3 3 Figure 4.8. (a) Voltammograms of Pt9Ir/C catalyst in 3.2×10 mol/cm NH3 solution at different scan rates, (b) extracted plot of peak current versus the square root of scan rate.

The term αnc in the Randles–Sevcik equation was calculated to be about 0.62 at T

= 25°C.

.× − / = ()

Taking αna = 0.62, the n value is determined to be about 2.98 which is in accordance with the number of the electrons transferred during the ammonia oxidation reaction

(NH() + 3OH → 0.5N + 3HO + 3e ).

4.2.4. Effect of pH and NaCl Addition

Figure 4.9 illustrates the effect of pH on the kinetics of ammonia electro- oxidation on 60% Pt9Ir/C catalyst in pH range between 12-13 (Figure 4.9 (a)), and pHs of

9, 11, and 13 (Figure 4.9 (b)). The higher peak current densities were obtained at higher

pH values above 12, which is due to the shift of reaction NH() + 3OH → 0.5N +

3HO + 3 to the right side [63]. Observing no characteristic peaks at pHs 9 and 11 implies that the kinetics of ammonia oxidation via a diffusion-controlled step is so slow

(or not feasible) at these pH values. Sharp ammonia oxidation peaks were clearly 82 observed for pH values above 12 (especially at pH = 13, Figure 4.9 (b)). pH plays a vital role in the existent form of ammonia such that it can control the predominant form of

+ ammonia as NH4 or NH3 via the following reaction [64]:

NH + OH ↔ NH + HO

Figure 4.9. Voltammogram response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 at (a) pH range between 12-13 and (b) pH 9, 11, and 13. (pH was adjusted using NaOH).

At constant temperature, considering the Nernst equation of thermodynamic,

+ [NH4 ]/[NH3] ratio is the only factor that can change the ammonia oxidation potential

+ [64]. Hence, increasing pH can concurrently accelerate the rate of NH4 and NH3 electro- oxidation. The advantage of our proposed technique is that, by the circulation of the solution in a closed-loop, the thickness of the Nernstian diffusion layer can be minimized, thereby eliminating the possible local pH drop at the catalyst surface, which has been considered as the main challenge during the ammonia electrolysis process [22]. It is worth mentioning that an extremely high concentration of OH- causes the hydroxyl (or 83 nitrate) poisoning and blockage of the catalyst by eliminating the available active sites for the ammonia oxidation reaction [46].

The effect of NaCl addition on ammonia oxidation and nitrate reduction at constant current/potential has been investigated by a few researchers, particularly by

Feng’s group [33, 42-44]. A small amount of NaCl addition (in a range of 0.5 g/l) can have a significant effect on the rate of nitrate removal and ammonia oxidation. The

2 voltammograms of 60% Pt9Ir/C catalyst (loading 0.25 mg/cm ) in 8.3 g/l (NH4)2SO4 solution at different NaCl concentrations and two various pHs are depicted in Figure

4.10. There is a clear increasing trend of the kinetics of ammonia detoxification when

- adding NaCl. Cl in the solution can be oxidized to Cl2, which immediately reacts with water to form hypochlorite (ClO-). In the presence of hypochlorite, two additional reactions (the following reactions) provide more pathways for oxidation of NH3 and

+ NH4 species, thereby improving the current, corresponding to the ammonia oxidation reaction in LSV measurements (Figure 4.10 (a)) [65]:

2ClO + 2NH + 2OH ↔ N + 2Cl + 4HO

3ClO + 2NH ↔ N + 3Cl + 3HO + 2H

A beneficial effect of adding NaCl is that it enables us to operate in lower pH, which entails adding less NaOH to the wastewater to increase pH. No clear peak is detected at pH 11 in the absence of NaCl, while the oxidation peak of ammonia is distinguishable after adding NaCl (Figure 4.10 (b)).

84

Figure 4.10. (a) Effect of NaCl addition on voltammogram response of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 18g/l NaOH (pH 12.85), (b) effect of NaCl addition of

CV of 60% Pt9Ir/C catalyst in 8.3 g/l (NH4)2SO4 + 6g/l NaOH (pH 11).

We ran two sets of pulse electrolysis in the absence and in the presence of NaCl in

2 2 a 225 cm flow cell with a pair of 60% Pt9Ir/C catalyst (loading 0.25 mg/cm ) in 8.3 g/l

(NH4)2SO4 + 18 g/l NaOH solution (pulse width 18 sec, pulse potential ±0.925 V, flow rate = 2 LPM). The results of ammonia rate loss versus time are presented in Figure 4.11

(a). By integrating these data, the obtained total mg loss of ammonia is plotted in Figure

4.11 (b), showing that the rate loss and total mg loss of ammonia in the presence of 1 g/l

NaCl is higher than that in the absence of NaCl. 85

Figure 4.11. Effect of NaCl addition on (a) average rate loss and (b) total mg loss of 2 ammonia during ammonia electrolysis process using 60% Pt9Ir/C (loading 0.25 mg/cm 2 on both electrodes) in a 225 cm flow cell (8.3 g/l (NH4)2SO4 + 18g/l NaOH solution).

By adding 1 g/l NaCl to the system, the average current, the NH3 loss (after 2 h of electrolysis), and the Faraday efficiency improved from 4.45 A to 4.63 A, from 0.80 to

1.4 g, and from 86.4% to 96.4 %, respectively. This implies the beneficial effect of NaCl on the kinetics of ammonia removal. Kapalka et al. argued that the reaction between chlorine and ammonia occurs so rapidly that it can compete with the decomposition or oxidation reaction of chlorine to chlorate, leaving no harmful chlorate or perchlorate complexes in the system [66]. Another beneficial effect of NaCl addition is that it

- - accelerates the oxidation of NO2 /NO3 by-products due to the quick reaction between

- - NO2 /NO3 and hypochlorite, which produces ammonia and nitrogen gas [66]. Taken together these results, indicate that NaCl addition is beneficial to the removal of ammonia and NOx species from the alkaline wastewater.

The effect of NaOH concentration on pH and on average current density during

2 ammonia electrolysis in 8.3 g/l (NH4)2SO4 at 60 °C in a 25 cm flow cell with a pair of 86

2 60% Pt9Ir/C catalysts ( loading 0.5 mg/cm , 2 LPM, pulse width 18 sec, pulse potential ±

0.925 V) is illustrated in Figure 4.12.

Figure 4.12. Effect of NaOH concentration on pH and on average current density during 2 ammonia electrolysis in 8.3 g/l (NH4)2SO4 at 60 ◦C in a 25 cm flow cell with a pair of 2 60% Pt9Ir/C catalysts (loading 0.5 mg/cm , 2 LPM, pulse width 18 sec, pulse potential ± 0.925 V).

For as prepared 8.3 g/l (NH4)2SO4 solution the pH was about 6.7; adding 6 g/l

NaOH brought up the pH to about 11. As expected, adding more NaOH to the solution

shifts the reaction NH() + 3OH → 0.5N + 3HO + 3e to the right side, thereby accelerating the kinetics of ammonia oxidation and leading to a higher current value.

4.2.5. Effect of Temperature

The pulse current response of ammonia electrolysis in a 300 cm2 flow cell with a

2 pair of 60% Pt9Ir/C electrodes (loading 0.2 mg/cm ) in 8.3 g/l (NH4)2SO4+6 g/l NaOH solution at two different temperatures (38 °C and 60 °C) is represented in Figure 4.13. 87

Figure 4.13. Ammonia pulse electrolysis in 300 cm2 flow cell at 38 °C and 60 °C in 8.3 g/l (NH4)2SO4+6 g/l NaOH solution , both electrodes 60% Pt9Ir/C with loading 0.2 mg/cm2 (pulse width 18 sec, pulse potential ±0.925 V, flow rate = 2 LPM).

Based on the Arrhenius equation of reaction rate dependency on temperature, the average current associated with ammonia electrolysis increased from 1.18 A to 3.09 A, as the temperature elevated from 38 °C to 60 °C. Figure 4.14 also demonstrates the influence of temperature and NaOH concentration on average current density associated

2 with ammonia oxidation in a 25 cm flow cell with a pair of 60% Pt9Ir/C electrodes. The higher temperatures enhance the kinetics of electron transfer, thus improving the ammonia electrolysis reaction rate in alkaline media [11].

88

40 35 30 ) 2 25 20 15 40 C i (mA/cm 10 50 C 5 60 C 0 5 8 11 14 17 20 g/l NaOH

Figure 4.14. Effect of temperature and NaOH concentration on average current density associated with ammonia removal during ammonia electrolysis in a 25 cm2 flow cell 2 consists a pair of 60% Pt9Ir/C electrodes (loading 0.5 mg/cm ), base solution: 8.3 g/l (NH4)2SO4.

Higher temperatures increase the adsorption and the rate of diffusion-controlled processes. Wang et al. claimed that, at a higher temperature, NH3 is thermodynamically

+ more stable than NH4 and, consequently, oxidation of NH3 at higher temperatures favors the kinetics of ammonia oxidation. At higher temperatures, ammonia is first transformed to the hydroxylamine, and further oxidation of adsorbed hydroxylamine generates N2 gas

(or NOx species) according to [67]:

→ hydroxylamine →

Table 4.2 summarizes the effect of temperature on average current density during the ammonia electrolysis process in a 225 cm2 flow cell for two different catalyst loadings and at various NaOH concentrations.

89

Table 4.2. Effect of catalyst loading on current density associated with ammonia oxidation at different temperatures in 225 cm2 flow cell. j (mA/cm2) 40 ◦C 50 ◦C 60 ◦C NaOH (g/l) 0.5 mg/cm2 0.25 mg/cm2 0.5 mg/cm2 0.25 mg/cm2 0.5 mg/cm2 0.25 mg/cm2 6 13.19 7.75 15.32 8.83 16.66 9.73 10 17.31 12.19 20.98 14.04 22.93 15.51 14 21.47 15.25 26.69 17.56 27.61 19.54 18 27.66 20.25 31.48 23.52 34.18 25.34

As expected, higher loading, more elevated temperatures, and more concentrated

NaOH solutions lead to higher current densities. The effect of temperature on increasing current density is more significant at higher catalyst loadings. In 18 g/l NaOH concentration and for the catalyst loading of 0.5 mg/cm2, the current density increased from 27.6 mA/cm2 to 34.1 mA/cm2 when the temperature was increased from 40 °C to

60 °C. Figure 4.15 (a) indicates the schematic of the 25 cm2 flow cell fabricated from the clear acrylic plexiglass using a milling machine. Figure 4.15 (b) represents the corresponding current response where the Gas Chromatography (GC) analysis of produced gas is shown in Figure 4.15 (c).

Figure 4.15. (a) schematic representation of the 25 cm2 flow cell (b) corresponding 2 current response in a cell of Pt9Ir/C as cathode and anode (loading 0.5 mg/cm ) in 8300 ppm (NH4)2SO4 + 12 g/l NaOH (pulse width = 18s, potential amplitude= ±0.925 V, T = 60 °C), and (c) GC analysis of produced gas during ammonia oxidation. 90

Using this cell configuration, the average current of 1.3 A, which is equivalent to

52 mA/cm2 current density for ammonia electrolysis, was obtained. It was the highest current density that was observed, which is much larger than the reported current densities elsewhere [43]. The GC analysis in Figure 4.15 (c) shows the generation of N2 and H2 as gaseous products of the ammonia electrolysis process. The rate of gas production (N2 and H2) through the following reaction was measured to be about 50 cm3/min in our system.

2NH() → N + 3H

4.2.6. Effect of Electrode Spacing on Electrolysis Current

Figure 4.16 compares the normalized average current density during ammonia pulse electrolysis in a 25 cm2 flow cell at two different electrode-to-electrode spacing of

4 mm and 8 mm, in 8.3 g/l (NH4)2SO4 solution at 50 °C.

2 Figure 4.16. (a) 25 cm flow cell comprising a pair of 60% Pt9Ir/C with an electrode-to- electrode spacing of 8 mm, (b) effect of electrode spacing on normalized average current density during ammonia pulse electrolysis in 8.3 g/l (NH4)2SO4 solution at 50 °C, flow rate 2 LPM (pulse width 18 sec, pulse potential ±0.925 V). 91

Table 4.3 summarizes the variation of normalized current (normalized current density with loading) versus electrodes spacing. From these data, it can be seen that at 8 mm of electrode spacing, we were able to obtain a higher normalized current, which means faster kinetics of ammonia oxidation. This may be explained by the fact that a higher volume of solution between two electrodes can provide enough amount of ammonia to be oxidized, leading to a higher current. Figure 4.16 reveals a slight increase in the normalized current density by increasing the separation between two electrodes from 4 mm to 8 mm. As the NaOH concentration increases, the effect becomes more notable (e.g., at 18 g/l NaOH); the normalized current density rises from 65 A/g % PtIr/C to 92 A/g by increasing the electrode spacing from 4 mm to 8 mm. However, the % PtIr/C lower electrode spacing may decrease the resulting ohmic drop in constant potential/current electrolysis, but at the same time, it encourages the OH- accumulation at electrodes surface. The lowest OH- accumulation is obtained during pulse electrolysis where a more significant gap between two electrodes can prevent the diffusion fields overlapping, thereby promoting the kinetics of ammonia oxidation by hindering the OH- poisoning of the electrodes [28, 68]. Table 4.3 compares the normalized current of ammonia electrolysis at two different electrode-to-electrode spacing values at different temperatures and NaOH concentrations. The higher normalized current was obtained at

8mm electrode spacing, and the effect is more significant at higher concentrations of

NaOH and higher temperatures in a way that at 18 g/l NaOH, the normalized current increased from 68 A/g to 96 A/g at 60 °C. 92

Table 4.3. Effect of electrodes spacing on the normalized current associated with ammonia oxidation at different temperatures different size-loading-electrode spacings. normalized I (A/g) 40 ◦C 50 ◦C 60 ◦C NaOH (g/l) 225cm2-0.25-8mm 225cm2-0.35-4mm 225cm2-0.25-8mm 225cm2-0.35-4mm 225cm2-0.25-8mm 225cm2-0.35-4mm 6 32.08 32.09 36.68 35.83 40.16 38.51 10 47.68 42.49 55.16 47.17 60.08 51.14 14 58.08 49.97 66.21 54.54 73.2 57.69 18 74.56 59.46 88.08 64.31 96.36 68.43

Figure 4.17 illustrates the reproducibility of the data obtained for pulse electrolysis of ammonia in a 25 cm2 flow cell at 50 °C.

Figure 4.17. Reproducibility of the data obtained for pulse electrolysis of ammonia in a 25 cm2 flow cell at 50 °C.

The error bars denote the standard deviation for three independent runs of ammonia electrolysis. The electrolysis current data were well reproducible during repeated experiments. The maximum error was for 18 g/l NaOH with a current density change of about ±2.5 mA/cm2.

93

4.2.7. Effect of Flow Cell Size on Ammonia Electrolysis

We carried out the electrolysis process in two different flow cells with different sizes of 300 cm2 and 25 cm2, as depicted in Figure 4.18, to examine the effect of cell size on kinetics of ammonia electro-oxidation. The 60% Pt9Ir/C catalyst loading was precisely similar in two cells and was about 0.2 mg/cm2, and the electrolysis was carried out at 40

°C.

Figure 4.18. Large flow cell (300 cm2) and small flow cell (25 cm2) both includes a pair 2 of 60% Pt9Ir/C catalysts loading 0.2 mg/cm .

The comparison between current densities associated with pulse electrolysis for

these two flow cells in a solution of 8.3 g/l (NH4)2SO4 at different NaOH concentrations is shown in Figure 4.19. The flow rate (2 LPM), temperature (40 °C), pulsing width (18 sec.), and pulse potential (±0.925 V) for these two cells were similar. Table 4.4 also summarizes the average current density associated with the ammonia pulse electrolysis at different NaOH concentrations and at various temperatures of 40 °C, 50 °C, and 60 °C. In comparison with the small cell, a higher current density was obtained using the 94 large cell (up to 15 g/l NaOH). In all NaOH concentrations, the average current density for 300 cm2 cell was about 6 mA/cm2 higher than that for a 25 cm2 flow cell. At 18 g/l

NaOH, the average current density of ammonia electrolysis for 25 cm2 flow cell is higher than that for 300 cm2 cell, but the difference is not significant and is about 2 mA/cm2.

Figure 4.19. Effect of cell size on average current density during ammonia electrolysis process in 8.3 g/l (NH4)2SO4 solution at 40 ◦C (2 LPM, pulse width 18 sec, pulse 2 potential ± 0.925 V), all electrodes are similar 60% Pt9Ir/C, loading 0.2 mg/cm .

Table 4.4. Effect of cell size and NaOH concentration on average current density during ammonia electrolysis process in 8.3 g/l (NH4)2SO4 solution at 40 ◦C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V), all electrodes are similar 60% Pt9Ir/C, loading 0.2 mg/cm2.

95

At higher temperatures, the trend still stays the same, and a larger flow cell ended

up having a higher current density. By defining the cell size factor as , the ratio of the current density in the small cell to the current density in the large cell at different temperatures and NaOH concentrations were calculated and are listed in Table 4.5.

Table 4.5. Cell configuration factor at different temperatures and NaOH concentrations.

Regardless of NaOH concentration and the electrolysis temperature, the cell size factor in all situations does not significantly change. At the same time, almost in all cases, the 300 cm2 flow cell can lead to a higher average current density of ammonia electrolysis than that for 25 cm2 cell. This indicates that the cell size is a critical factor enabling us to obtain higher kinetics of ammonia removal at lower temperatures and pH values, which is so promising for further scaling-up steps for industrial-scale applications. This project is still on track to accomplish the necessary scaling-up steps for fabricating seven 3000 cm2 flow cells for electrolysis applications in wastewater treatment plants.

4.2.8. Electro-Sensitivity of PtIr/C Catalysts for Ammonia Detection

CVs of 60% Pt9Ir/C catalyst at different NH3 concentrations ranging from 128 ppm to 3840 ppm in 6 g/l NaOH solution is illustrated in Figure 4.20. The shape of CVs, in terms of the onset potential and the location of the characteristic peaks, are similar to 96 what was observed by Moran et al. [69] (for electrodeposition of Pt-Ir alloys in 0.1 M

KOH + NH3).

2 Figure 4.20. CVs of 60% Pt9Ir/C catalyst (loading 0.25 mg/cm ) at different NH3 concentrations in 6 g/l NaOH solution, scan rate 25 mV/s.

The peak current varies linearly with ammonia concentration, implying that the 60

% Pt9Ir/C catalyst effectively responses to the diffusion-controlled process of ammonia oxidation in a wide range of ammonia concentration from 128 to 3840 ppm.

4.2.9. Effect of Substrate and Pulse Switching Time (Pulse Width) on PtIr/C

Catalyst Durability

The durability of performance is a critical property of any synthesized catalysts and is necessary to be examined in laboratory scales before synthesizing the catalyst at a large industrial scale. To assess the effect of pulse width and the nature of the substrate on the average current density, Pt3Ir/C was sprayed on Ni and Ti mesh substrates (loading of about 0.30 mg/cm2 for both). They were tested in ten different pulse widths ranging 97 from 5 to 40 sec in a single cell on three consecutive days (cathode Pt3Ir/C loading kept

2 constant at 0.312 mg/cm ). The test solution was 8300 ppm (NH4)2SO4 + 6 g/l NaOH at

38 °C. Table 4.6 represents the average values of current densities, and Figure 4.21 (a) and (b) show the variation of current density versus pulse width in three consecutive days for Pt3Ir/C-Ni and Pt3Ir/C-Ti, respectively.

Table 4.6. Average current density corresponding to ammonia oxidation on three consecutive days of pulse electrolysis in 8300 ppm (NH4)2SO4 + 6 g/l NaOH (potential 2 amplitude= ±0.925 V, T = 38 °C, loading: Pt3Ir/C-Ni = 0.301 mg/cm , Pt3Ir/C-Ti = 0.302 2 2 mg/cm , Pt3Ir/C cathode = 0.312 mg/cm .

98

(b)

Figure 4.21. Effect of pulse width and substrate on average current density associated with ammonia oxidation on three consecutive days in 8300 ppm (NH4)2SO4 + 6 g/l

NaOH (potential amplitude= ±0.925 V, T = 38 °C, (a) : Pt3Ir/C-Ni loading = 0.301 2 2 mg/cm , (b) : Pt3Ir/C-Ti loading = 0.302 mg/cm , Pt3Ir/C cathode loading = 0.312 mg/cm2).

The durability of the Pt3Ir/C catalyst after three days of 30 runs of pulse electrolysis was suitable; interestingly, in some cases, the current density associated with ammonia electro-oxidation showed a slight increase. The trend of current density versus pulse width was similar regardless of what material was used as the substrate (i.e., Ni or

Ti substrate did not influence the trend). A pulse width of 18-30 sec was the optimum operational pulsing situations when there were both high current densities and low deteriorations of the catalyst due to the harsh pulsing conditions (lower than 10 sec). To investigate how the potential amplitude and changing the catalyst surface coverage may influence the current decay after three days of pulse electrolysis, a partially sprayed 300 cm2 Ni mesh electrode (inset of Figure 4.22) was tested at two different pulse amplitudes of ±0.500 and ±0.925 V (Figure 4.22). 99

Potential amplitude = ±0.500 V Potential amplitude = ±0.925 V

Figure 4.22. Effect of pulse potential amplitude on durability and current decay percentage after three consecutive days of pulse electrolysis in 8300 ppm (NH4)2SO4 + 6 2 g/l NaOH (T = 38 °C, Pt3Ir/C cathode loading = 0.222 mg/cm , Pt3Ir/C anode loading = 0.203 mg/cm2).

At the potential amplitude of ±0.925 V, the current decay is about 50% after three days of pulse electrolysis at ten different pulse widths. Decreasing the pulse width to ±

0.5 V increased the current decay to 70% in a situation when the Ni substrate was partially sprayed with Pt3Ir/C catalyst. It is possible that the areas with bare Ni mesh were hydroxidized in such a basic solution of 8300 ppm (NH4)2SO4 + 6 g/l NaOH, forming

Ni(OH)2 at the surface which can deteriorate the performance of the electrode in the long term. It is critical that the entire support structure (e.g., nickel mesh) is covered with the catalyst; otherwise, the system will suffer from passivation, and the performance of ammonia and/or nitrates removal will be affected.

4.2.10. Durability of 50% Pt3Ir/C Catalyst

Figure 4.23 displays the XRD spectrums of 50% Pt3Ir/C catalyst after ten runs of

5h ammonia electrolysis in 8.3 g/l (NH4)2SO4 + 10 g/l NaOH solution at 50 °C. The XRD pattern of 50% Pt3Ir/C catalyst after electrolysis shows characteristic peaks of iridium oxides (IrOx), which implies that the long-term electrolysis process may cause oxidation 100 of Ir, i.e., deterioration of the catalyst performance for ammonia electro-oxidation [18,

19].

Figure 4.23. XRD pattern of 50% Pt3Ir/C catalyst before the electrolysis process and after 10 runs of 5 h (totally 50 h) electrolysis operation.

In order to prevent the degradation of the PtIr/C catalyst due to the iridium oxidation in the long terms through continuous pulse electrolysis, a recommended strategy was to examine the performance of those grades of PtIr/C catalyst richer in Pt

(e.g., Pt9Ir/C). Thus, we started testing a Pt9Ir/C catalyst, and interestingly, Pt9Ir/C catalyst demonstrated superior performance in comparison with the Pt3Ir/C catalyst, as discussed in section 4.2.2.

101

4.3. Nitrate Electro-Reduction

4.3.1. Electrochemical Characterization of Three Different Candidates for Nitrate

Electro-Reduction

Copper is considered among the catalysts with the highest activity for the electroreduction reaction of nitrate to ammonia [29-32]. The same loading of copper

(Cu), copper-supported on reduced graphene oxide (Cu-rGO), and copper-nickel (Cu-Ni) bimetallic alloy were electroplated on glassy carbon (GC) using the potentiostat method at a constant potential of -0.55 V vs. Ag/AgCl, potentiostat method at a constant potential of -1.1 V vs. Ag/AgCl, and the galvanostat method at a constant current density of -10 mA/cm2 all for 300 seconds, respectively. Cu, Cu-rGO, and CuNi were electrodeposited on GC from 0.005M CuSO4+0.05M H2SO4, 0.005M CuSO4+0.1 g/l GO, and 0.7M

NiSO4+0.05M CuSO4+0.26M trisodium citrate dihydrate solutions, respectively. The

Raman spectra for D-band and G-band for GO and Cu-rGO can be observed in Figure

4.24 (a). The Raman spectrum for rGO displays no D mode, which validates the reduction of GO to rGO. The Raman band disappearing at around 2755 cm-1 is an indication of successful GO reduction to rGO during Cu-rGO catalyst synthesis.

102

Figure 4.24. (a) Comparison of Raman spectra for GO and Cu-rRGO, (b) comparison of XRD patterns for Cu and CuNi modified GC electrodes.

Figure 4.24 (b) compares the XRD patterns for Cu and CuNi modified GC catalysts. The diffraction peaks of (222), (311), (220), and (200), together with an intense peak of (111), clearly indicate the formation of CuNi bimetallic alloy. Figure 4.25 (a)-(c) shows the LSV responses of Cu, Cu-rGO, and CuNi catalysts, respectively, in the

- absence and the presence of NO3 in 0.1M NaOH. The abrupt increase of the cathodic current beyond -1.4 V is due to the hydrogen evolution reaction (HER) [70]; thus, the sweeping potential was stopped at -1.5 V. All three catalysts show an onset of cathodic current at around -0.8 to -1 V, which is followed by a continuous increase in the current, indicating the nitrate electroreduction. No remarkable peak was observed in blank 0.1 M

NaOH solutions. All three catalysts responded well to the presence of 23 ppm nitrate, and two main peaks can be seen at around -0.8 and -1 V vs. Hg/HgO, indicating a diffusion- controlled process of nitrate reduction. A catalyst with diffusion-controlled behavior is always our desired one because it encourages the nitrate/nitrate adsorbing and a consequent reduction of nitrate to the ammonia. It appears that two different potential 103 regions need to be investigated for the reduction of nitrate. The first region is near -0.8 V, and the second one is around -1 V to -1.4 V, which would be kinetically favorable for nitrate reduction such that the nitrate reduction mostly produces ammonia [30].

Figure 4.25. LSV responses in the absence and the presence of nitrate for three different catalysts (a) Cu, (b) Cu-rGO, (c) CuNi, and (d) the LSV response of different catalysts - toward detection of nitrate in 0.1M NaOH+23ppm NO3 (Scan rate: 25 mV/s).

The observation depicted in Figure 4.25 (d) suggests that the electrochemical activity for these catalysts toward nitrate reduction is different. The higher current density for nitrate reduction for CuNi compared to those for Cu and Cu-rGO could be attributed to a higher electro-active surface area on CuNi. Indeed, a rougher surface morphology with a smaller particle size of about 50 nm for CuNi particles, compared to those for Cu and Cu-rGO with a particle size of about 150-300 nm, was observed in SEM images is 104 displayed in Figure 4.26. The EDS analysis of Cu, Cu-rGO, and CuNi catalysts are also presented in Figure 4.26. According to SEM images, a more uniform of particle size for

Cu and CuNi catalysts was obtained than for Cu-rGO. Several coarse particles with dimensions of about 500 nm to 1 micrometer can be seen in SEM of Cu-rGO. A uniform surface morphology for Cu and CuNi might be due to an instantaneous nucleation mechanism during the electrodeposition of Cu and CuNi clusters on the GC surface. In contrast, progressive nucleation is more likely to occur for Cu-rGO [71]. For the same catalyst loading, a higher surface coverage was obtained for electrodeposited CuNi.

105

Figure 4.26. SEM and EDS analysis of (a) Cu, (b) Cu-rGO, and (c) CuNi modified GC electrodes.

As discussed by Reyter et al. [72], in alkaline media, the electroreduction of nitrate on Cu-based catalysts can be presented through the following steps: 106

NO + HO + 2 → NO + 2OH

NO + 5HO + 6 → NH + 7OH

2NO + 4HO + 6 → N + 8OH

2NO + 3HO + 4 → NO + 2OH

It has been reported that the wave at around -0.7 to -0.9 V corresponds to the

reduction of nitrate to nitrite through NO + HO + 2 → NO + 2OH which is the rate-controlling step during nitrate removal [72]. The ammonia oxidation as a nitrate- reduction product is favored at a potential region close to the hydrogen evolution reaction

(HER), where Hads and nitrite interactions are more probable [29, 30]. Therefore, prolonged electrolysis of nitrate for different catalysts was performed at the potential of about -1.3 V (close to HER potential region) to investigate this effect at different electrolysis conditions (Figure 4.27). The current transients during the electrolysis process at different conditions (various applied potential and temperatures) in 0.1 M

- NaOH + 32 ppm NO3 are illustrated in Figure 4.27 for Cu, Cu-rGO, and CuNi catalysts.

Meanwhile, the concentration of nitrate/nitrite and ammonia was continuously monitored.

107

Figure 4.27. Comparison of constant potential electrolysis for different catalysts at - different conditions in 0.1M NaOH + 32 ppm NO3 at -1.3 V vs. Hg/HgO, 38 °C for Cu and CuNi, and at -2.5 V, 23 °C for Cu-rGO.

Figure 4.28 represents the results of nitrate/nitrite and ammonia analysis during the prolonged constant potential electrolysis in Figure 4.27. Depending on the applied

- overpotential of electrolysis, different species (i.e., NO2 , NH3, Hads, or a mixture of them) can be produced. At around -1.3 V to -1.4 V, NH3 is the predominant product; however,

- at -0.9 V to -1 V, NO2 is more favorable. In between these two potential regions, a

- mixture of NO2 and NH3 is produced [29-32, 70]. At the same applied overpotential of -

1.3 V, the CuNi catalyst showed a more substantial integrated charge with a higher current density associated with nitrate reduction, which demonstrates that Ni has a synergistic effect on Cu, leading to a better performance of NH3 production. Simpson et

- al. reported that Cu sites act as spots for NO3 adsorption and that Ni sites accelerate the adsorption of H atoms, which efficiently increases the nitrate removal rate for a Cu-Ni combination [73]. In the alkaline solution with an initial concentration of 32 ppm nitrate, 108 using the CuNi catalyst led to higher ammonia and lower nitrate concentrations than those for Cu and Cu-rGO catalysts (after 4000 seconds of electrolysis). As discussed above, at the potentials beyond -1.4 V, the hydrogen evolution and surface poisoning predominantly affect the electrolysis, resulting in only small amounts of ammonia being detected at t -2.5 V.

- Figure 4.28. N-NH3 and N-NO3 concentration vs. time during constant potential electrolysis for (a) Cu at -1.3 V vs. Hg/HgO, and 38 °C , (b) Cu-rGO at -2.5 V vs. Hg/HgO, and 23 °C and (c) CuNi at -1.3 V vs. Hg/HgO, and 38 °C all in 0.1M NaOH + - initial NO3 concentration of 32 ppm. 109

Using CuNi catalysts, the concentration of nitrate after 4000 seconds of electrolysis is decreased from 32 ppm to a final concentration of about 5 ppm, and, interestingly, 17 ppm ammonia is successfully produced (Figure 4.28). During the same electrolysis process, the final concentration of nitrate was about 11 ppm when a Cu catalyst was used. A smaller amount of ammonia was produced at the end when Cu and

Cu-rGO catalysts were applied for electrolysis. The slight current decrement after 3500 seconds for CuNi and after 3800 seconds for Cu might be attributed to the nitrate concentration polarization, aging, poisoning, or unintentional temperature fluctuation at the electrode surface, which affects pH and the electrolysis process [72]. The final total

- balance of N as NH3 or as NO3 is lower than that of its initial value, indicating that part of the ammonia likely volatilized and was lost from our system. Overall, these results suggest that the capability of nitrate reduction and ammonia production is in the order of

Cu-rGO

Figure 4.29 (a)-(c) indicates the effect of nitrate concentration on peak current and compares the nitrate electro-detection of Cu, Cu-rGO, and CuNi catalysts, respectively. 110

Figure 4.29. Effect of nitrate concentration on LSV responses of (a) Cu, (b) Cu-rGO and (c) CuNi catalysts in alkaline media of 0.1M NaOH (scan rate: 25 mV/s).

By increasing the nitrate concentration, not only the cathodic peak potential for

Cu and CuNi shifts toward a more negative direction but also the cathodic peak current also raises. The linear dependency of current density on the nitrate concentration suggests that the reduction of nitrate on Cu and CuNi should be a first-order process with respect to the concentration of nitrate [15]. This linear dependency indicates that Cu and CuNi catalysts have the potential of nitrate electro-detection in the range of at least 0.01M to

- 0.1M of NO3 complexes. For Cu-rGO, by increasing the nitrate concentration, the cathodic current has been decreased, indicating probable catalyst poisoning due to adsorbed hydrogen. However, Cu and CuNi catalysts show proper nitrate detection performance. Still, the surface deactivation occurs in the case of Cu-rGO at high 111 concentrations of nitrate, due to the surface blockage with NOx intermediate species [6].

Figure 4.30 shows the sweep voltammograms of Cu, Cu-rGO, and CuNi catalysts in the

- presence of nitrite (NO2 ) in alkaline media.

Figure 4.30. LSV responses of Cu, Cu-rGO, and CuNi toward detection of nitrite, - - NO2 in 1M NaOH+10mM NO2 (Scan rate: 25 mV/s).

- Comparing Figure 4.30 with Figure 4.25, in the presence of NO2 , the waves of

- reduction at around -0.7 to -0.8 V is disappeared. This suggests that the reduction of NO2

- is an intermediate step during the reduction of NO3 in alkaline media on Cu and CuNi

[29, 30]. Hence, the reduction mechanism of nitrate has at least two steps. The

- mechanism of NO2 reduction occurs through a process that includes at least one step less

- than nitrate, resulting in a faster reduction rate for NO2 [31]. For Cu-rGO in the presence

- of NO2 , no peak is observed, which is in good accordance with our findings in Figure

4.25 (b) where just a single peak was detected, corresponding to the nitrate reduction.

- This demonstrates that Cu-rGO is likely not an appropriate candidate for NO2 electro-

- detection. Hence, the ability of NO2 electroreduction is in the order of Cu-rGO

CuNi. 112

4.3.1.1. Obtaining the Best CuNi Composition for Nitrate Reduction

Reaction

Taken together, these findings suggest that CuNi is the most promising catalyst for the nitrate electro-reduction reaction. To obtain the most effective catalyst, four different compositions of CuNi were electroplated on nickel mesh. Table 4.7 summarizes the elemental analysis of EDS and the corresponding chemical composition of each grade of CuNi bimetallic alloys. Figure 4.31 shows the SEM images of the obtained CuNi electroplated alloys.

Table 4.7. EDS analysis of four electroplated CuNi catalysts on Ni mesh. Electroplated CuNi #1 st #2 #3 #4 Catalysts Element Mass % Mass % Mass % Mass % Nickel 8.6 21.04 69.93 35.21 Copper 86.96 70.45 23.31 52.80 Composition = Ni9Cu91 Ni23Cu77 Ni75Cu25 Ni40Cu60

113

Ni9Cu91 Ni23Cu77

Ni Cu 40 60 Ni75Cu25

Figure 4.31. SEM images of four different compositions of electroplated CuNi catalysts on Ni mesh.

The CV and the obtained current transients at a constant applied potential of -1.2

- V vs. Hg/HgO in 0.1 M NaOH + 23 ppm N-NO3 solution are reported in Figure 4.32.

The reduction peaks at about -0.7 V and -1 V correspond to the nitrate reduction to nitrite, and nitrite reduction to ammonia or hydroxylamine, respectively [6]. The chronoamperometric transients show a sudden fall at the early stages, which corresponds to a charge transfer reaction. Once the nitrate reduction becomes diffusion-controlled, a diffusion plateau is obtained. 114

Figure 4.32. (a) CV and (b) chronoamperometric curves of four different compositions of - electroplated CuNi catalysts in 0.1M NaOH+23 ppm N-NO3 .

As Cu content in CuNi composition increases, the current associated with nitrate reduction (i.e., the current peak (Figure 4.32 (a)) and the current diffusion plateau (Figure

4.32 (b)) increases. Mattarozzi et al. demonstrated a synergistic mechanism in the reduction of nitrate to ammonia for Cu9Ni alloys, where Cu sites act as the active sites for nitrate adsorption and Ni sites have the highest efficiency for H atoms adsorption [31].

During the nitrate reduction to ammonia, the nitrate adsorption is the rate-determining step; hence, for those CuNi compositions rich in Cu, Cu provides a more negative potential for hydrogen evolution, leading to a higher rate of total nitrogen removal and a higher current at the end of electrolysis process [33]. The highest performance for nitrate reduction reaction is obtained for CuNi compositions rich in Cu (Cu91Ni9 or Cu9Ni), in agreement with what has been previously reported [29-31].

115

4.3.2. Comparison Between Electroplated CuNi and CuNi/C Catalyst for Nitrate

Electro-Reduction

The electroplating of Cu90Ni10 on Ni mesh electrodes was accomplished using a galvanostatic (constant current density of 10 mA/cm2) for 300 s in a solution containing

0.05M NiSO4 + 0.7M CuSO4 + 0.26 M C6H5Na3O7.2H2O. A wide range of CuNi alloy compositions (Cu91Ni9, Cu77Ni23, Cu60Ni40, and Cu25Ni75) was electroplated on Ni mesh, and their performance for nitrate reduction reaction was investigated. Figure 4.33 (a)

- shows CV responses of CuNi/C catalyst, in the absence and the presence of NO3 . The abrupt increase of the cathodic current beyond -1.4 V is due to the hydrogen evolution reaction. Therefore, the sweeping potential was stopped at about -1.4 V vs. Hg/HgO. No

- characteristic peak was observed in the absence of NO3 ; however, three diffusion- controlled peaks are observed in the presence of nitrate at the potential regions of about -

0.2, -0.8, and -1.2 V vs. Hg/HgO, which correspond to the electro-reduction of nitrate/nitrite and the other intermediate NOx species. This indicates that the CuNi/C catalyst responds well to the presence of 23 ppm nitrate in 8300 ppm (NH4)2SO4 + 6 g/l

NaOH solution. This was in accordance with the goal of obtaining a diffusion-controlled electrode, enabling the adsorbing and consequent reducing of nitrate/nitrate to ammonia or other intermediate species. Figure 4.33 (b) compares the LSV response of the electroplated CuNi (loading 0.98 mg/cm2) and the sprayed CuNi/C catalyst (loading 0.97

2 - mg/cm ) in 8300 ppm (NH4)2SO4 + 6 g/l NaOH solution + 23 ppm NO3 . At the same loading, the electrochemical activity of the CuNi/C catalyst for nitrate reduction is about

3 times higher than that for electroplated CuNi. It corresponds to a higher electro-active 116 surface area for CuNi/C compared to that for electroplated CuNi toward nitrate electro-

2 reduction. At the same amount of loading of about 0.98 mg/cm , the Cu9Ni/C sprayed catalyst shows a higher nitrate reduction peak current (about 15 mA) compared to that of electroplated Cu9Ni (5 mA).

Figure 4.33. (a) CV responses for Cu9Ni supported on Carbon catalyst in the absence and - the presence of 23 ppm N-NO3 and (b) comparison of LSV responses for electroplated 2 2 Cu9Ni (loading 0.98 mg/cm ) and sprayed Cu9Ni/C (loading 0.98 mg/cm ) in the - presence of 23 ppm N-NO3 (blank solution 8300 ppm (NH4)2SO4 + 6 g/l NaOH): scan rate 25 mV/s.

Figure 4.34 indicates a comparison of repetitive CVs (10 cycles) between electroplated Cu9Ni and sprayed Cu9Ni/C catalyst in 8300 ppm (NH4)2SO4 + 6 g/l NaOH

- + 23 ppm N-NO3 . 117

Figure 4.34. Comparison of successive voltammograms for electroplated Cu9Ni and - sprayed Cu9Ni/C in 8300 ppm (NH4)2SO4 + 6 g/l NaOH + 23 ppm N-NO3 : scan rate 25 mV/s.

- As discussed above, the reduction of NO2 is an intermediate step during the

- reduction of NO3 in alkaline media. It has been reported that two different potential regions need to be investigated for the electro-reduction of nitrate. Some of the most probable electrochemical reactions during nitrate reduction are provided in Figure 4.34.

The reduction mechanism of nitrate has at least two steps. The first step occurs near -0.8

V and the second one occurs around -1 V to -1.4 V. By sweeping the potential toward

- negative values up to -0.8 V, nitrate is first reduced to the intermediate species (e.g., NO2

- - , N2 , OH ), and, at more negative potentials of about -1.2 to -1.4 V, nitrite is reduced to ammonia which is the favorable reaction. In the potential region of between -1.2 V to -1.4

- V, the formation of NH3 is predominant; however, at -0.8 V to -1 V, NO2 is thermodynamically more favorable. In between these two potential regions, a mixture of

- NO2 and NH3 (or other intermediate species) are yielded. For electroplated CuNi, the 118 waves of reduction at around -1.2 V is almost disappeared; however, an intense diffusion- controlled peak at this potential region is observed for CuNi/C catalyst, suggesting that the Cu9Ni/C catalyst can be a more suitable option than electroplated CuNi. Additionally, for the CuNi/C catalyst, the current density associated with the reduction of nitrite to hydroxylamine (NH2OH) and ammonia is much greater than that for electroplated CuNi.

Hence, in terms of the electro-detection of nitrate/nitrate and regarding the decay of performance for the electroplated CuNi in basic media (8300 ppm (NH4)2SO4 + 0.1M

NaOH), the sprayed CuNi/C catalyst can be introduced as a potential replacement for nitrate electro-reduction reaction. Especially, in the end, an optimal combination of

2 Cu9Ni/C+Pt3IrC catalyst can be used to spray both electrodes in a 300 cm flow cell, enabling a pulse electrolysis technique for simultaneous nitrate/ammonia removal.

4.3.3. Characterization and Selection of a CuNi/C Cathode for Nitrate Reduction

At this point, it was found that metallic Cu9Ni had the highest ability to reduce nitrate to ammonia. In addition to this, it is vital to find the optimal ratio of metallic

Cu9Ni to the carbon Vulcan support during the polyol-impregnation synthesis procedure.

In order to achieve this goal, six different grades of synthesized CuNi/C catalysts with the various ratio of Cu9Ni to carbon Vulcan were synthesized (Table 4.8). A higher ratio of metallic Cu9Ni is associated with a higher nitrate reduction performance; however, loading more than a specific amount of metallic Cu9Ni nanoparticles on carbon support is impossible because the Vulcan has a particular capacity for Cu9Ni nanoparticle loaded.

As the weight measurement analysis in Table 4.8 shows, the minimum Cu9Ni catalyst loss (about 10%) during the polyol-impregnation process of synthesis was obtained for 40 119

% Cu9Ni/C. 46.2% and 55.5% of material loss for 60% and 50 % Cu9Ni/C catalysts, respectively, indicate that it is impossible to load beyond 50% or 60% of Cu9Ni nanoparticles on carbon Vulcan using this technique. Figure 4.35 shows the XRD patterns of synthesized Cu9Ni/C catalyst and the TEM image of 40% Cu9Ni/C.

Table 4.8. Six different synthesized grades of Cu9Ni supported on Vulcan.

Figure 4.35. (a) XRD patterns of Six different synthesized grades of Cu9Ni supported on Vulcan, (b) TEM image of 40% Cu9Ni/C.

The XRD spectrum of 40% Cu9Ni/C nanoparticles demonstrates the sharp peaks of Cu elements, which is consistent with a composition close to CuNi rich in Cu (Cu9Ni). 120

Using Scherrer’s equation, a uniform particle size distribution of about 4-5 nm for Cu9Ni nanoparticles was calculated for the Cu rich (111) peak located at around 44º, which is in agreement with TEM observations in Figure 4.35 (b). For other grades of catalysts, the

Cu (111) peak became very weak, implying that the polyol-impregnation synthesis technique was most successful for 40% Cu9Ni/C.

4.3.3.1. Electro-Catalytic Activity of CuNi/C Catalyst for Nitrate

Reduction

In order to calculate the total surface area of the catalysts, the N2 physisorption

BET technique was used. Based on BET surface area analysis, it was noted that 20%,

30%, and 40% Cu9Ni/C catalysts showed higher electro-active surface areas, among

2 which the 40% Cu9Ni/C catalyst with 92.02 m /g had the highest BET surface area

(Table 4.9). It is worth mentioning that these three catalysts were those with the lowest material loss during the polyol-impregnation synthesis procedure (Table 4.8).

Table 4.9. BET analysis of three synthesized Cu9Ni/C catalysts with the highest surface area.

Figure 4.36 indicates the quantity of adsorbed nitrogen gas versus relative pressure for three different grades of Cu9Ni/C catalysts. It is apparent that the quantity of the adsorbed N2 gas for the 40 % Cu9Ni/C catalyst is higher than those for 30% and 20% 121

Cu9Ni/C, indicating a higher active surface area available for interaction with species during nitrate reduction reaction.

Figure 4.36. Quantity of absorbed nitrogen gas during BET analysis for 20%, 30%, and 40% Cu9Ni/C catalysts.

A higher number of active sites can surpass the denitrification rate for the 40%

Cu9Ni/C catalyst [74]. Figure 4.37 shows the LSV and the amperometric responses of six different grades of Cu9Ni/C catalysts in 8.3 g/l (NH4)2SO4+18 g/l NaOH + 165 ppm N-

- NO3 .

122

Figure 4.37. (a) LSV and (b) Chronoamperometric responses (applied potential -1.2 V vs. Hg/HgO) for six different grades of Cu9Ni/C catalysts in 8.3 g/l (NH4)2SO4+18 g/l NaOH - + 165 ppm N-NO3 , scan rate 25 mV/s.

The voltammograms show an onset of cathodic current at about -0.2 V vs.

Hg/HgO. Two peaks at around -0.6 V and -0.9 V vs. Hg/HgO correspond to the reduction of nitrate to nitrite and the reduction of nitrite to ammonia, hydroxylamine, and N2 gas, respectively [75]. In both CV and chronoamperometric responses, the current increases in the order of 40% Cu9Ni/C > 20% Cu9Ni/C > 30% Cu9Ni/C catalysts, indicating that the reduction of nitrate is more favorable on 40% Cu9Ni/C than those for 20% and 30% 40%

Cu9Ni/C. These findings agree with BET data, wherein the highest geometric active surface area was obtained for 40% Cu9Ni/C. Aside from an improved electroactive surface area, a developed mass transfer feature could also increase the performance of the

40% Cu9Ni/C catalyst for nitrate reduction [15]. Figure 4.38 shows the LSV of 40 %

- Cu9Ni/C catalyst in the absence and in the presence of 165 ppm N-NO3 in a solution of

8.3 g/l (NH4)2SO4 + 10 g/l NaOH. 123

Figure 4.38. LSV of 40 % Cu9Ni/C catalysts in the absence and the presence of 165 ppm - N-NO3 , Base solution: 8.3 g/l (NH4)2SO4 + 10 g/l NaOH, scan rate 25 mV/s.

Interestingly, for 40% Cu9Ni/C, the electrooxidation peaks of ammonia can be seen at the potentials of about -0.10 V and -0.25 V vs. Hg/HgO in the absence and the presence of nitrate, respectively, which implies the electro-oxidation of ammonia is still favorable on the 40% Cu9Ni/C catalyst. Although the ammonia oxidation on Cu9Ni is feasible, in comparison with Pt9Ir/C catalyst, the peak potential for ammonia oxidation shifted to more negative values. This confirms that the ammonia oxidation on Pt9Ir/C requires much less energy in comparison with Cu9Ni/C. There are no characteristic peaks

- - in the absence of N-NO3 while adding 165 ppm N-NO3 led to a cathodic peak around -

0.8 V, corresponding to the nitrate reduction reaction. The LSV of the 40 % Cu9Ni/C

- catalyst in the presence and the absence of 205 ppm N-NO2 is illustrated in Figure 4.39

(a). No peak is obtained in the absence of nitrite, while in the presence of 205 ppm N-

- NO2 , a reduction peak is observed at -0.55 V vs. Hg/HgO, which correlates to the reduction of nitrite to hydroxylamine and other NxO1-x intermediate species [9]. When 124 further potential scanning to more negative potentials, the cathodic peak observed at about -0.9 V corresponds to the further reduction of produced intermediates to ammonia and N2 gas [31]. Vavilin et al. stated that nitrite cannot be directly reduced to ammonia in one single step and that nitrite is first transformed to NO and N2O through one or two charge transfer steps [76]. In the case of the synchronous removal of nitrate and ammonia, it is essential to examine how the electrode responds to the simultaneous presence of nitrate and ammonia. Figure 4.39 (b) shows the CVs obtained for the 40 %

Cu9Ni/C catalyst in 10 g/l NaOH containing 165 ppm nitrate in the absence and the presence of 2100 ppm ammonia in the potential range from -1.2 to +0.2 V vs. Hg/HgO.

Figure 4.39. (a) LSV of 40 % Cu9Ni/C catalysts in the absence and the presence of 205 - ppm N-NO2 (b) effect of ammonia addition on LSV response of 40 % Cu9Ni/C catalysts - toward nitrate reduction in 10 g/l NaOH +165 ppm N-NO3 solution: scan rate 25 mV/s.

In the presence of ammonia, the peak potential associated with nitrate reduction is shifted to more negative potential values with an apparent decrease in the nitrate reduction peak current. The presence of ammonia suppresses the rate of nitrate removal, 125 causing a rise in the required energy for nitrate reduction initiation. This entails a higher reduction potential [77]. The presence of ammonia does not considerably influence the performance of the40% Cu9Ni/C cathode in terms of nitrate reduction reaction; hence, the simultaneous removal of nitrate and ammonia in our developed undivided flow cell system integrating the 40% Cu9Ni as the cathode and the 60% Pt9Ir/C as the anode is attainable. The LSVs of 40% Cu9Ni/C in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH + 32 ppm N-

- NO3 solution at different scan rates is represented in Figure 4.40.

2 Figure 4.40. Voltammograms of 40% Cu9Ni/C catalyst (loading 0.5 mg/cm ) in 8.3 g/l - (NH4)2SO4 + 6 g/l NaOH + 32 ppm N-NO3 solution at different scan rates.

The relatively linear dependency of peak current on the square root of the scan rate confirms that based on the Randles–Sevcik equation [78], the nitrate reduction on the

40% Cu9Ni/C electrode is predominantly controlled by the mass transfer of the NOx species in the system. The results presented thus far in section 3.3.2 supports that, 40% 126

Cu9Ni/C is an efficient catalyst for nitrate reduction reaction and the subsequent oxidation of by-products.

4.3.4. Electro-Sensitivity of CuNi/C Catalysts for Nitrate/Nitrite Detection

LSVs of the 40% Cu9Ni/C electrode in 18 g/l NaOH solution and at different

- concentration of N-NO3 is shown in Figure 4.41.

10 5 0 -5 ) 2 -10 -15 -20 25 ppm N-NO3 i (mA/cm 50 ppm N-NO3 -25 100 ppm N-NO3 -30 150 ppm N-NO3 -35 300 ppm N-NO3 -40 -1.2 -1 -0.8 -0.6 -0.4 -0.2 E (V vs. Hg/HgO)

- 9 Figure 4.41. Effect of NO3 concentration on LSV response of 40% Cu Ni/C catalyst (loading 0.5 mg/cm2) in 18 g/l NaOH solution, scan rate 25 mV/s.

The reduction peaks corresponding to the two-step nitrate reduction reaction increases linearly as more nitrate is added to the system. This linear dependency suggests that, in a concentration ranging from 25 ppm to 300 ppm nitrate, the nitrate reduction on

40% Cu9Ni/C undergoes a two-step process, which is controlled by diffusion [79].

Hence, the 40% Cu9Ni/C electrode can detect and effectively respond to the presence of

25 ppm-300 ppm nitrate in wastewater.

127

4.4. Simultaneous Ammonia and Nitrate Removal in an Undivided Cell

Having identified the optimal catalysts for ammonia oxidation and nitrate reduction reactions, the last section of this paper integrates these two processes and examines the performance of the undivided flow cell for simultaneous nitrate and ammonia removal. The 225 cm2 and 25 cm2 flow cells were used to investigate the simultaneous nitrate and ammonia removal.

4.4.1. Effect of Catalyst Loading

The effect of Pt9Ir/C catalyst loading on ammonia electro-oxidation current density in 8.3 g/l (NH4)2SO4 + 0.1M NaOH solution is shown in Figure 4.42 (a).

Similarly, the effect of Cu9Ni/C catalyst loading on nitrate electro-reduction current

- density in 8.3 g/l (NH4)2SO4 + 0.1M NaOH + 23 ppm NO3 the solution is depicted in

Figure 4.42 (b). The electrolysis for both cases was accomplished using a constant potential electrolysis technique (E = ±0.925 V) at 38 °C.

128

Figure 4.42. Loading effect of (a) Pt9Ir/C catalyst on ammonia oxidation current density in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH at applied potential of +0.925 V vs. Hg/HgO and (b) Cu9Ni/C catalyst on nitrate reduction current density in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH - + 23 ppm N-NO3 at applied potential of -0.925 V vs. Hg/HgO, 38 °C.

2 Obviously, by increasing the Pt9Ir/C loading from 0.093 to 0.580 mg/cm , the steady-state current density associated with ammonia electro-oxidation in constant potential electrolysis increased from about 0.6 to 1.2 mA/cm2. Similarly, the current density of nitrate electro-reduction increased from about 0.5 to 1.5 mA/cm2 by increasing

2 the Cu9Ni/C loading from 0.25 to 1 mg/cm . To investigate how the loading of Cu9Ni/C and Pt9Ir/C catalyst influences the rate of nitrate/ammonia removal, two grades of 40%

Cu9Ni/C and 60% Pt3Ir/C catalysts were synthesized (Figure 4.43). 129

Figure 4.43. Synthesized 40% Cu9Ni/C (as the cathode) and 60% Pt9Ir/C (as the anode) applicable for nitrate electro-reduction and ammonia electro-oxidation reactions, respectively.

The pulse current corresponding to the ammonia pulse potential electrolysis

(pulse width 18 sec, pulse potential ± 0.925 V) in a 25 cm2 flow cell (flow rate = 2 LPM)

2 with pairs of 60% Pt9Ir/C electrodes with different loadings of 0.25, 0.35, 0.5 mg/cm in

8.3 g/l (NH4)2SO4+10 g/l NaOH solution at 60 °C is illustrated in Figure 4.44. Figure

4.45 shows the variation of the correlating average current density versus catalyst loading.

2.00 1.50 1.00 0.50 0.00 -0.50 30 80 130 180 Current(A) -1.00 -1.50 0.25 mg/cm2 Pt9Ir/C 60% -2.00 0.35 mg/cm2 Pt9Ir/C 60% 0.5 mg/cm2 Pt9Ir/C 60% -2.50 Time(s) Figure 4.44. Pulse current corresponding to the ammonia electrolysis in a 25 cm2 flow cell with pairs of 60% Pt9Ir/C electrodes with different loadings of 0.25, 0.35, 0.5 130

2 mg/cm . Solution: 8.3 g/l (NH4)2SO4+10 g/l NaOH at 60 °C (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V).

20 y = 23.342x + 7.7279 18 ) 2 16

14 i(mA/cm 12

10 0.2 0.3 0.4 0.5 0.6 2 Loading (mg/cm ) Figure 4.45. Ammonia electrolysis average current density versus 60% Pt9Ir/C catalyst loading (25 cm2 flow cell).

The average current density during ammonia pulse electrolysis increases roughly about 23.35 mA per mg of 60% Pt9Ir/C catalyst. Figures 4.42 and 4.44 show that during both constant and pulse potential electrolysis, the average current density shifts positively with catalyst loading. It has been claimed that the catalyst loading controls the current density associated with the conversion of nitrate and ammonia to N2 gas [80]. At higher catalyst loading, the pseudocapacitance of catalyst coating decreases, leading to a higher current density at a constant potential considering the following formula [81]:

C = ∫ IdV () ×(/)× where C is the pseudocapacitance of the catalyst, m is loading, Vstep is the potential step during the potentiostatic data gathering, tstep is the time duration step, ΔV is the potential range, and I is the current response. 131

Figures 4.46 (a) and (b) show the applied pulse square wave and the

2 corresponding current response, respectively, in a 25 cm cell configuration of Cu9Ni/C

2 2 as cathode (loading 0.25 mg/cm ) and Pt9Ir/C as anode (loading 0.5 mg/cm ) in 8.3 g/l

- (NH4)2SO4 + 6 g/l NaOH + 23 ppm N-NO3 . The electrode spacing was kept at 8 mm.

Meanwhile, the ppm of nitrate/nitrite and ammonia was measured according to the techniques described in section 2.2. Figure 4.47 (a) represents the corresponding ppm concentration of ammonia/nitrate in the abovementioned cell configuration for simultaneous nitrate and ammonia removal.

Figure 4.46. (a) pulse electrolysis and (b) corresponding current response in a 25 cm2 cell 2 2 of Cu9Ni/C as cathode (loading 0.25 mg/cm ) and Pt9Ir/C as anode (loading 0.5 mg/cm ) - in 8.3 g/l (NH4)2SO4 + 6 g/l NaOH + 23 ppm N-NO3 (pulse width = 18s, potential amplitude= ±0.925 V, T = 38 °C).

2 2 At Cu9Ni/C loading of 0.25 mg/cm (cathode) and Pt3Ir/C loading of 0.5 mg/cm

(anode), after 3 h of pulse electrolysis, the ppm of nitrate and ammonia is reduced from

23 to 21 ppm, and from 1870 to 1370 ppm, respectively. This 25 cm2 undivided flow cell set-up was successfully removed, both nitrate and ammonia. Figures 4.47 (b) and (c) manifest the CuNi/C loading effect on the average rate loss of ammonia/nitrate and total 132 ppm loss of ammonia/nitrate after 3 h of pulse electrolysis (pulse width = 18s, potential

2 amplitude = ±0.925 V, T = 38 °C, anode Pt9Ir/C loading was kept fixed at 0.5 mg/cm ).

2 As the loading of Cu9Ni/C catalyst increases from 0.25 to 1 mg/cm , the rate of nitrate

- and ammonia loss per min increases from about 0.005 to 0.03 ppm NO3 /min and from

0.42 to 0.52 ppm NH3/min.

Figure 4.47. (a) nitrate and ammonia concentrations during the pulse electrolysis in a 2 system of Cu9Ni/C as cathode (loading 0.25 mg/cm ) and Pt9Ir/C as anode (loading 0.5 2 mg/cm ). Effect of Cu9Ni/C catalyst loading on (b) average rate loss of nitrate/ammonia and (c) average ppm loss of nitrate/ammonia after 3 h of pulse electrolysis all in 8300 - ppm (NH4)2SO4 + 6 g/L NaOH + 23 ppm N-NO3 (pulse width = 18s, potential 2 amplitude= ±0.925 V, T = 38 °C, anode Pt3Ir/C loading was fixed at 0.5 mg/cm ).

During the pulse electrolysis, the average current increased as the loading of

2 Cu9Ni/C (cathode) increased (at a constant Pt9Ir/C loading of 0.5 mg/cm (anode)). So, an electrochemical system configuration integrating Cu9Ni/C as anode and Pt9Ir/C as 133 cathode enabled us to apply the pulse electrolysis for simultaneous nitrate and ammonia removal. We presume that an optimized combination of Cu9Ni/C+Pt9Ir/C catalyst performs much better in terms of nitrate/ammonia removal because Cu9Ni/C and Pt9Ir/C do not efficiently oxidize ammonia and reduce nitrate, respectively, during successive oxidation and reduction cycles. Figure 4.48 illustrates the results of nitrate and ammonia analysis during pulse electrolysis in 8.3 g/l (NH4)2SO4+18 g/l NaOH + 1.5 ppm nitrate in

2 a 225 cm cell with the electrode-to-electrode spacing of 8 mm with a pair of 60% Pt9Ir/C electrodes (loading 0.5 mg/cm2). Table 4.10 lists the corresponding data for simultaneous nitrate and ammonia removal. The applied pulse potential was similar to what is presented in Figure 4.46 (a).

Figure 4.48. Concentration of ammonia and nitrate during pulse electrolysis process in 2 8.3 g/l (NH4)2SO4+18g/l NaOH + 1.5 ppm nitrate in a 225 cm cell with a pair of 60% 2 Pt9Ir/C catalyst: loading 0.5 mg/cm (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V, electrode spacing of 8 mm).

134

Table 4.10. Ammonia and nitrate analysis during electrolysis process in 8.3 g/l 2 (NH4)2SO4+18g/l NaOH + 1.5 ppm nitrate a 225 cm cell with a pair of 60% Pt9Ir/C catalyst: loading .5 mg/cm2 (2 LPM, pulse width 18 sec, pulse potential ± 0.925 V).

After 3h of pulse electrolysis, the ammonia and nitrate concentrations decreased from 2355 ppm to 972 ppm, and from 1.17 ppm to almost zero, respectively. Even using the same catalyst of 60% Pt9Ir/C, which aimed for ammonia oxidation, we were able to detoxify our system from nitrate as well. Although there are reports on using Pt-Ir based catalysts for nitrate reduction [82, 83], their performance of nitrate removal is much weaker than Cu based catalysts. No previous study has investigated the detailed mechanism of simultaneous nitrate and ammonia removal in an undivided electrochemical cell during pulse electrolysis. Figure 4.49 represents a schematic representation of simultaneous nitrate reduction and ammonia oxidation during pulse potential electrolysis. 135

Figure 4.49. Schematic representation of the proposed mechanism for simultaneous nitrate/ammonia removal during pulse potential electrolysis.

During the positive cycle, the left-side electrode is anode where the ammonia electrolysis and nitrite oxidation to nitrate are taking place, and the right-side is the cathode, where we have nitrate reduction reactions to ammonia, nitrite, and nitrogen. In contrast, in the negative cycle, the left-side electrode becomes the cathode, and the right- side electrode acts as the anode. During the whole electrolysis process, nitrate reduction and ammonia oxidation repeatedly occur at both electrodes. The overall reaction for nitrate removal during a pulse is reported to be [28]:

2NO + 6HO + 10 → N + 12OH

4.4.2. Simultaneous Nitrate and Ammonia Removal Using a Mixed Cu9Ni-Pt9Ir/C

Catalyst (25 cm2 Flow Cell)

Replacing Pt9Ir/C with Cu9Ni/C or introducing a mixture of Pt9Ir/C+Cu9Ni/C catalyst is significantly cost-effective. Given that the prices for Pt, Ir, Cu, and Ni are 136

1 $53k, $102k, $6.17, and $13.79 USD/kg, respectively , the replacement of Pt9Ir/C with

Cu9NiC enables us to decrease the costs in the order of 8300 times; however, the effectiveness of Pt9Ir for ammonia electrolysis is remarkable, and even a combined mixture of Pt3Ir+Cu9Ni catalyst can lead to a considerable monetary saving. A catalyst batch of 60% Pt9Ir - 40% Cu9Ni (v/v 50%) was prepared and sprayed on Ni meshes to obtain the loading of 0.25 mg/cm2 for both electrodes. To compare the rate of nitrate/ammonia removal and the average current density associated with pulse potential electrolysis, the electrolysis experiment for 60% Pt9Ir-40% Cu9Ni (v/v 50%) was carried

2 out in a 25cm flow cell (Figure 4.50) in a solution of 8.3 g/l (NH4)2SO4 +18 g/l

- NaOH+32 ppm N-NO3 .

Figure 4.50. Small 25 cm2 flow cell for nitrate/ammonia electrolysis. Both electrodes are 2 60% Pt9Ir - 40% Cu9Ni (v/v 50%), loading 0.25 mg/cm , electrode spacing: 0.8 mm.

The corresponding pulse current response and the result of concentration analysis for ammonia and nitrate are represented in Figure 4.51 (25 cm2 flow cell with a pair of

1 - The prices are for January 2020 137

2 electrodes both sprayed with 60% Pt9Ir - 40% Cu9Ni (v/v 50%), loading 0.25 mg/cm , 2

LPM, pulse width 18 sec, pulse potential ± 0.925 V, electrode spacing of 8 mm). The average current density during 5 h of pulse electrolysis process was about 16 mA/cm2.

Although Cu9Ni catalyst has much less catalytic activity than Pt9Ir for ammonia oxidation reaction, the synergistic effect between Cu and Ni can significantly improve its capability even for ammonia oxidation reaction due to the formation of a mixed Cu1-xNixOOH intermediates with a remarkable ability to bind N atoms [78, 84].

- Figure 4.51. Pulse electrolysis in 8.3 g/l (NH4)2SO4 + 18 g/l NaOH + 32 ppm N-NO3 .

After 5 h electrolysis, the concentrations of ammonia and nitrate reduced from about 1950 to 1500 ppm, and from 32 ppm to 19 ppm, respectively. The final concentration of nitrate reached to 19 ppm. Hence, this 25 cm2 undivided flow cell with a pair of 60% Pt9Ir - 40% Cu9Ni (v/v 50%) is capable of simultaneously detoxifying the wastewater from nitrate and ammonia.

Up to 30 min, the nitrate concentration declined from 32 ppm to 19 ppm. After 30 min of nitrate electrolysis, an increment of nitrate was observed. The nitrate concentration was increased 8 ppm for the next 210 min and again started to continue the 138

+ decreasing trend after 4 h. It seems possible that NH4 species oxidize back to nitrate through the following reactions [85], which produce more nitrate/nitrite in solution, causing a slight rise in the concentration of detected nitrate/nitrite in solution.

NH + 1.5O → NO + 2H + HO

NH + 0.85O → 0.11NO + 1.08H + 1.43HO + 0.44N

Not having a continuous decreasing trend in nitrate/ammonia concentration could be attributed to the NH3 oxidation reaction, which produces excess nitrate or nitrite via the following reactions [46]:

NH + 7OH → NO + 6e + 5HO

NH + 9OH → NO + 8e + 6HO

Also, the hydroxylation of un-sprayed bare Ni areas of the electrode may deteriorate the performance of the electrode. Hence, it is critical to have a very uniform layer of the sprayed catalyst with a full surface coverage of the Ni mesh substrate. The applied pulse potential limit can be widened (higher range than ±0.925) to increase the rate of nitrate reduction notably. The volumetric ratio of Pt9Ir to Cu9Ni batch catalyst can also be optimized, and different composition for catalyst other than 60% Pt9Ir - 40%

Cu9Ni (v/v 50%) may have a better performance in terms of nitrate and ammonia detoxification.

4.4.3. Energy Consumption Calculations in Our New-Developed Undivided Cell

According to the electrical energy calculation in a 2 L flow cell system (average pulse current = 3.85 A, pulse potential = 0.92 V, electrolysis time =5hrs), the energy

-3 -1 consumption is determined to be 5.33×10 kWhg NH3 (based on the following 139 calculation). This value is about 90% lower electrical energy consumption compared to the other reported techniques for ammonia removal. We assumed an average removal of

1660 ppm NH3 after 5 hrs.

3.85 A × 0.92V × 5h × 10 Energy Consumption = = 5.33 × 10 kWhg NH g 1660 ppm NH × 10 mg × 2L

Similarly, in a 2 L flow cell system with an average current of 0.12 A under an applied potential of 0.96 V after 5hrs of electrolysis the energy consumption is

-3 -1 - determined to be 9.00×10 kWhg NO3 (based on the following calculation), which is about 65% lower energy consumption compared to the other reported techniques for

- ammonia removal. We assumed an average removal of 32 ppm NO3 after 5 hrs.

0.12 A × 0.92V × 5h × 10−3 −3 −1 Energy Consumption = g = 9.00 × 10 kWhg NO 32 ppm NO × 10−3 × 2L mg

The introduced technique discussed in this paper could be considered as a promising one to significantly reduce the energy consumption and the capital costs of wastewater treatment for simultaneous nitrate and ammonia detoxification.

140

Chapter 5: Conclusions and Future Works

5.1. Conclusions

The electrochemical removal of ammonia and nitrate in an undivided flow cell using

40% Cu9Ni/C (cathode) and 60% Pt9Ir/C (anode) catalysts through a pulse potential electrolysis technique was studied. The following conclusions are drawn.

 The kinetics of ammonia oxidation during the electrolysis process for 60% Pt9Ir/C

catalyst is higher than those for 40% Pt1Ir1/C and 50% Pt3Ir/C in alkaline media.

 For ammonia electro-oxidation reaction, 60% Pt9Ir/C catalyst composition

performs much better than 40% PtIr/C and 50% Pt3Ir/C. In some electrolysis

cases, enormous current densities in a range of 55 mA/cm2 were obtained using

pulse potential of ±0.925 V in 25 cm2 undivided cell.

 Adding NaCl in order of 1 g/l has a tremendously beneficial effect on the rate of

ammonia removal and further nitrate electro-reduction. At the same time, it

improves the Faraday efficiency during the electrolysis process.

 Higher pH in the range of 12.5-13.0, elevated temperature of 60 °C, catalyst

loading of 0.5 mg/cm2, NaCl addition in order of 1 g/l, the pulse width of 18-30

sec, and electrode-to-electrode spacing of 8 mm are introduced as the optimum

operational condition for electrolysis.

 Compared to Cu and Cu-rGO, bimetallic NiCu alloy showed higher

electrocatalytic activity with an enhanced capability of nitrate electro-detection.

The ability to perform nitrate removal and ammonia production is in order Cu-

rGO

 The highest performance for nitrate reduction reaction is obtained for CuNi

catalysts rich in Cu (Cu91Ni9 or Cu9Ni).

 40% Cu9Ni/C is an efficient catalyst for nitrate reduction reaction and the

subsequent oxidation of by-products.

 Ammonia oxidation on 60% Pt9Ir/C electrode and nitrate reduction on 40%

Cu9Ni/C electrodes are predominantly under the control of the diffusion.

 60% Pt9Ir/C and 40% Cu9Ni/C catalysts can electrochemically detect and respond

to the diffusion-controlled reactions of ammonia oxidation (in a range of <100

ppm to about 4000 ppm N-NH3) and nitrate reduction (in a range of <25 ppm to

- about 300 ppm N-NO3 ), respectively.

 An undivided flow cell integrating 40% Cu9Ni/C as the cathode and 60% Pt9Ir/C

as anode were successfully capable of simultaneous removal of nitrate and

ammonia during pulse electrolysis. After 3h of electrolysis, in some cases, no

nitrate was left in the solution, and the final ammonia concentration decreased

from about 2500 ppm to below 1000 ppm. Even using a mixed 60% Pt9Ir - 40%

Cu9Ni (v/v 50%) catalyst batch for both electrodes, we could remove ammonia

and nitrate simultaneously with much less energy consumption compared to the

existing techniques.

 The introduced undivided flow cell pulse electrolyzer process uses 65-90% less

energy than the existing technologies and can be implemented for the

nitrate/ammonia removal in the wastewater treatment plant.

142

5.2. Future Scope and Recommendations

1) Although the presence of Ir element can affect the durability of Pt9Ir catalyst

(regarding its oxidation), it plays a critical role in ammonia oxidation. NaCl

effectively increases the rate of ammonia oxidation. Testing the Pt1Ir1/C catalyst

with the loading of 0.5 mg/cm2 under the following condition is suggested:

- Solution: 8.3 g/l (NH4)2SO4+18g/l NaOH+ 1g/l NaCl

- pH: 13, T = 60 °C, Flow rate 2 LPM, spacing 8 mm, small size Ni mesh

2) Replacing NH4Cl instead of (NH4)2SO4 to eliminate the pH drop in our system

(sulfate complexes can make the media more acidic). Testing the Pt1Ir1/C catalyst

with the loading of 0.5 mg/cm2 under the following condition is suggested:

- Solution: 6.7 g/l NH4Cl+18g/l NaOH+ 1g/l NaCl

- pH: 13, T = 60 °C, Flow rate 2 LPM, spacing 8 mm, small size Ni mesh

3) Increase the pulse potential limit of cathodic reduction. We can test the following

potential limits (instead of ±0.925 V)

a) +0.925 to -1.1 V

b) ±1.1 V

c) ±1.2 V

4) Increase the switching time to 25 s, instead of 18 s

5) Test constant current electrolysis (25-35 mA/cm2), instead of pulse potential

electrolysis

6) Test pulse current electrolysis instead of pulse potential electrolysis (±30 mA/cm2

instead of ±0.925 V) 143

7) Regarding the high rate of Ni mesh degradation in concentrated NaCl solution,

SS316 from TWP Inc. can be an ideal replacement for Ni mesh.

5.3. Cell Scale-up Considerations

Operating the flow cell reactors at constant temperatures can lead to additional costs when cell size is scaled up. Keeping the solution temperature constant is particularly critical in this case. Indeed, at large scales, a sizable volume of hydrogen gas needs to be safely released and stored. Increasing the cell size can alter both the rate of heat generation and the rate of heat removal from the system. An important safety consideration is that, at increased cell sizes, the heat removal rates do not increase as fast as the heat generation rates [86, 87]; hence, it is necessary to use cooling systems to control the temperature. The electric resistance-heated technique is considered a safe approach to heat the flow cell at large scales. This type of heater enables adequate control over the temperature of the reactor at large scales [88]. In large-scale operations, the cooling water circulation is typically used to achieve temperature control. Additionally, Afaik Gelid Cryotheum is a suitable coolant for large-scale electrolysis [87, 88].

144

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157

Appendix 1: Ammonia Measurement & ISE Calibration

Procedures for Using the Ammonia Ion-Selective Electrode (Ref. 1 & 2)

The purpose of the Ammonia Ion-Selective Electrode (ISE) is to measure the concentration of ammonia in solution. Figure A1.1 depicts the overview of the procedure for using the Ammonia ISE. The following procedures list the methods for membrane changing (to occur once a month), calibration solution preparation and calibration of the electrode (to occur once a day), sample storage, sample dilution, and sample procedures, and electrode storage.

Membrane

Changing

- Solution

Preparation Calibration - (Once a Day) Calibration - Sample Storage Points ** Osmotic

- Sample Concentration Sampling Dilution ** Ammonia

Concentration

- Sample- Between Measurement Samples

Electrode - During non-use

Storage - Longer than 1 week

without use Figure A.1.1. Procedure Flowsheet for the use of Ammonia Ion-Selective Electrode. 158

These instructions need to be followed carefully for the measurements to be accurate and the lifetime of the electrode to be extended.

Fill out the “Ammonia Ion-Selective Electrode Usage Log” each time the instrument is used. It is essential to note the ammonia concentration limits for the measurement as well as the overall concentration limits. Be aware that solutions with high concentrations of ammonia and/or KOH will need to be diluted (see “High Concentration Solutions” section). For accurate measurements, it is also important to note the sample storage guidelines. Basic solutions containing ammonia lose the ammonia rapidly, so as time progresses, the accuracy of the ammonia concentrations to be tested will decrease. The section on “Sample Storage” explains how to reduce the amount of ammonia lost if the sample is not be tested immediately.

Membrane Changing

Membrane lifetime is based on both age and use. The expected lifetime of the membrane in this lab is one month. After this point, the age of the membrane may play a more significant factor. Refer to pages 4-6 of the “Orion Ammonia Electrode Instruction

Manual” (should be stored in the electrode box in cabinet #14) for instructions and diagrams on how to replace the membrane.

Calibration

Solution Preparation for Calibrating

1) Prepare 1000 mL of each solution below. This amount of solution should be

enough to last 1-3 weeks when stored in an airtight container.

2) Solution 1: 100 ppm as N (121.4±1.2 ppm as NH3). 159

a. From the 1000 ppm as N standard solution, measure out 100 mL in a 100

mL graduated cylinder.

b. Transfer to a 1000 mL volumetric flask. Rinse the cylinder with DI water

and add it to the flask. Fill the flask to volume with DI water. Transfer

solution to a clean, 1000 mL Nalgene bottle (labeled as 100 ppm as N,

121.4±1.2 ppm as NH3, preparer’s initials, and the date on which it is

prepared).

c. Rinse the volumetric flask with DI water.

3) Solution 2: 10 ppm as N (12.14±0.2 ppm as NH3).

a. From the 100 ppm as N solution (solution 1), measure out 100 mL in a

100 mL graduated cylinder.

b. Transfer to a 1000 mL volumetric flask. Rinse the cylinder with DI water

and add it to the flask. Fill the flask to volume with DI water. Transfer

solution to a clean, 1000 mL Nalgene bottle (labeled as 10 ppm as N,

12.1±0.2 ppm as NH3, preparer’s initials, and the date).

c. Rinse the volumetric flask with DI water.

4) Solution 3: 1 ppm as N (1.214±0.02 ppm as NH3).

a. From the 10 ppm as N solution (solution 2), measure out 100 mL in a 100

mL graduated cylinder.

b. Transfer to a 1000 mL volumetric flask. Rinse the cylinder with DI water

and add it to the flask. Fill the flask to volume with DI water. Transfer 160

solution to a clean, 1000 mL Nalgene bottle (labeled as 1 ppm as N,

1.21±0.02 ppm as NH3, preparer’s initials, and the date).

c. Rinse the volumetric flask with DI water.

5) Solution 4: 0.1 ppm as N (0.1214±0.002 ppm as NH3).

a. From the 1 ppm as N solution (solution 3), measure out 100 mL in a 100

mL graduated cylinder.

b. Transfer to a 1000 mL volumetric flask. Rinse the cylinder with DI water

and add it to the flask. Fill the flask to volume with DI water. Transfer

solution to a clean, 1000 mL Nalgene bottle (labeled as 0.1 ppm as N,

0.121±0.002 ppm as NH3, preparer’s initials, and the date).

c. Rinse the volumetric flask with DI water.

Note: In total, 4 solutions should be prepared (1 L each). These 4 L of solution should last for at least 10 calibration procedures.

Calibration of the Electrode

1) Rinse the electrode with DI water. Pat dry with a paper towel.

2) Measure out 100 mL of the 0.1 ppm as N solution (solution 4) with a 100 mL

graduated cylinder. Empty into a 150 mL beaker. The measurement does not

have to be exact.

3) Place a small stirring bar in the beaker. Place the beaker on the center of the

stirring plate.

4) Insert the electrode at a 20° angle in the 150 mL beaker. Have the electrode about

½” below the surface of the solution. 161

5) Turn on the stirring mechanism to about 300 rpm.

6) Using a 2 mL volumetric pipet, pipet 2 mL of the Ammonia pH Adjusting ISA

solution into the 150 mL beaker. The beaker solution should turn blue.

7) Turn on the meter by pressing the “power” button. Press the “Mode” key until the

arrow at the bottom is over the “conc.” This should normally be in this position

when the meter is turned on.

8) Press the “2nd” button on the meter. Press the “mode (in white letters)/cal (in

green letters)” button.

9) P1 will be displayed when the instrument is read to measure point 1 on the

calibration curve. The measurement will steady.

10) When the measurement has steadied, press the up and down arrows to position the

decimal point so that it would be in the correct position to enter the 0.1214 ppm

(e.g., if the numbers read 0.0679, position the decimal place before the 6).

When the decimal position has been reached, press the “yes” button.

11) Use the arrow keys to set the flashing digit so it will fit 0.121. Press “yes” when

that digit is correct. Adjust the digits in this manner until all are correct.

12) After “yes” has been pressed for the last digit, the display will hold for a couple of

seconds, then P2 will be displayed.

13) Remove the electrode from the calibration solution. Rinse the electrode with DI

water and pat dry. Dispose of the solution tested.

14) Repeat steps 2 through 6 with the 1 ppm as N solution (solution 3). 162

15) Once the electrode is in the solution, the reading will steady out. Repeat steps 10

and 11 for the ammonia concentration of 1.214 ppm as NH3.

16) Repeat 14 and 15 with the 10 ppm as N (12.14 ppm as NH3, solution 2) and then

the 100 ppm as N (121.4 ppm as NH3, solution 1).

17) When the P5 prompt appears on the screen, press the “measure” button. The

calibration slope will appear for a few seconds. Record the slope on the daily log.

18) The meter is now ready to test samples.

Sampling

Sample Storage

Basic samples should be tested immediately or be chemically preserved for later testing.

The electrode manual states that up to 50% of the ammonia in a basic solution can be lost every six hours!

1) To preserve samples, add 0.5 mL of 1M HCl to each liter of the sample (or until

the pH is 6).

2) Store in a tightly capped container.

High Concentration Solutions

IMPORTANT: Do not use 5M KOH and 1M NH3 directly in the ISE meter! This can result in bad measurements and could damage the instrument. The instrument is designed to operate with osmotic concentrations less than 1M (the sum of the K+ ion, the OH- ion, and the ammonia). For example, 5M KOH, 1M NH3 has an osmotic concentration of

+, - 11M (assuming NH3 is 1 ion, not NH4 and OH , then it has a strength of 12M). 163

1) To test samples of high osmotic strengths, dilute the sample until it is an

acceptable range for osmotic strength and ammonia concentration.

2) Dilute concentrated solution using a 2 mL volumetric class A pipet of the

concentrated solution. Transfer the 2 mL into a 250 mL volumetric flask.

3) Fill to volume with DI water.

4) If the concentrated solution had been 5M KOH, 1M NH3, the diluted solution

would have an osmotic concentration of 0.088M, and the ammonia concentration

would be 0.008M.

Sampling Procedure

1) Dilute sample so that the osmotic concentration is below 1M (see high

concentration section), and the ammonia concentration is below 0.01M (See

Example 1 of this in the appendix).

2) Measure out 100 mL of the diluted solution with a 100 mL graduated cylinder.

Empty cylinder into a 150 mL beaker.

3) Place a small stirring bar in the beaker. Place the beaker on the center of a stirring

plate.

4) Rinse the electrode with DI water. Pat dry with a paper towel.

5) Insert the electrode at a 20° angle in the 150 mL beaker. Have the electrode about

½” below the surface of the solution.

6) Turn on the stirring mechanism to about 300 rpm.

7) Using a 2 mL volumetric pipet, pipet 2 mL of the Ammonia pH Adjusting ISA

solution into the 150 mL beaker. The beaker solution should turn blue. 164

8) Turn on the meter by pressing the “power” button. Press the “Mode” key until the

arrow at the bottom is over the “conc.” This should normally be in this position

when the meter is turned on.

9) Once the electrode has stabilized, a small beep will be heard, and the reading on

the screen will have “ready” next to it. Record the reading. This reading should

be in ppm (See appendix Example 2 for conversion to molarity).

10) (Electrode Storage (between samples)) Remove the electrode from the solution.

Rinse well with DI water and pat dry. Place the electrode in a 50 mL beaker so

that it is just submerged in a solution of 10 ppm as N calibration solution with

added ISA solution (a few drops until it turns blue). The electrode cannot be

stored in air.

11) Repeat sampling procedure with more samples.

12) (Electrode Storage (when not using the electrode)) If sampling is complete,

remove the electrode from the 10 ppm as N solution (Solution 2), rinse, dry, and

store in a 50 mL beaker with 1000 ppm as N solution that does not have any ISA

solution. This is not savable for more than 1 day.

13) (Electrode Storage (for more than 1 week without use)) Electrodes may be stored

in the 1000 ppm as N solution without use for 1 week. After that point, the

electrode membrane and filling solution should be removed and disposed of. The

electrode should be stored in its packing box.

Example 1. Dilution to appropriate Ammonia Concentration

Original Solution: 0.1M NH3, 0.25M KOH 165

The original solution has an osmotic concentration of 0.6M, which is acceptable for the instrument usage, but the ammonia concentration is too high.

Dilute the solution by taking 10 mL of the original solution using a 10 mL class A volumetric pipet and transferring it into a 250 mL volumetric flask.

The new concentration is 0.004M NH3 using the equation

C1V1 = C2V2 where,

C1 = 0.1 M

V1 = 10 mL

V2 = 250 mL

C2 is solved for

This concentration is within the measurable range of 0.01M and 0.01mM NH3.

Example 2. Conversion of ppm NH3 to M NH3

Reading: 100 ppm

1 ppm = 1 mg/l

 1g  1mol  CM  C ppm    1000mg 17.03g 

mg  1g  1mol  CM  100     0.00587M L 1000mg 17.03g 

166

Appendix 2: Nitrate Analysis

The following is the procedure to measure the concentration of nitrate/nitrite (Ref. 3):

1. Prepare vanadium solution

a. In 100 ml volumetric flask, add 50 ml of 1M f HCL (4.15 ml of 36.5% HCl

bring to 50 ml)

b. Weigh out 0.80 g of vanadium (III) chloride (VCl3) (very hygroscopic)

c. Quickly add VCl3 to the volumetric flask containing the 1M f HCL

d. Flush with N2 and store solution in the refrigerator should be blue.

2. Prepare sulfanilamide solution

a. In 100 ml volumetric flask, add 2 g of sulfanilamide

b. Bring to volume with 5% (w/v) HCl (11.5 ml of 36.5% HCl bring to 100 ml)

c. Flush with N2 and store solution in the refrigerator. Discard if colored.

3. Prepare NEDD solution

a. In 100 ml volumetric flask, add 100 mg of N-(1-naphthyl)-ethylenediamine

dihydrochloride

b. Bring to volume with DI water

d. Flush with N2 and store the solution in the refrigerator. Discard if colored.

4. Prepare premix solution

a. Create a mixture for the same day measurement of the vanadium,

sulfanilamide, and NEDD solutions in a ratio of 2:1:1 (e.g., 20 ml VCl3: 10 ml SULF:

10 ml NEDD).

b. Store mixture in the refrigerator 167

5. Prepare 40 ppm nitrate standard solution

a. In 100 ml volumetric flask, add 4 ml of 1,000 ppm nitrate (NO3) standard

b. Bring to volume with “experimental solution without nitrate” (e.g., when preparing 1.5 L of 30ppm nitrate in 0.1 M NaOH solution first make 2 L of the 0.1 M

NaOH and remove 500 ml before adding the nitrates, this will be the “experimental solution without nitrate”)

6. Prepare calibration curve solutions

a. In 2 ml centrifuge tube, add 1 ml of premix solution and 1 ml of

“experimental solution without nitrate” for a blank, shake

b. Prepare the following standard dilutions:

Table A.2.1. Nitrate analysis calibration table.

c. In 2 ml centrifuge tubes, add 1ml of standard dilution to 1 ml of premix solution, shake

d. Place blank and standards in the oven for 15 minutes, the color should turn pink

e. Measure blanks and standards in UV/VIS spectrophotometer at 540 nm and prepare the calibration curve 168

7. Analyze experimental solutions for nitrate concentration

a. Remove 0.5 ml of sample from the electrolytic cell and dilute with 9.5 ml of

“experimental solution without nitrate,” shake.

b. In 2 ml centrifuge tube, add 1 ml of the diluted solution prepared in step 7a and 1 ml premix solution, shake, place in the oven for 15 minutes, measure in UV/VIS spectrophotometer at 540 nm.

169

Appendix 3: References for the Appendices

1. Elizabeth Cellar, “Procedure for Using Ammonia Ion-Selective Electrode,”

February 2005.

2. Thermo SCIENTIFIC, “User Guide for High Performance Ammonia Ion-

Selective Electrode”, © 2010 Thermo Fisher Scientific Inc.

3. Doane, T.A. & Horwath, W.R. “Spectrophotometric Determination of Nitrate

with a Single Reagent”, Analytical Letters 2003, 36, 2713–2722.

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