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Mathematical Modeling of Ammonia Electro-Oxidation on Polycrystalline Pt Deposited

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Mathematical Modeling of Ammonia Electro-Oxidation on Polycrystalline Pt Deposited Mathematical Modeling of Ammonia Electro-Oxidation on Polycrystalline Pt Deposited Electrodes A dissertation presented to the faculty of the Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Luis A. Diaz Aldana May 2014 © 2014 Luis A. Diaz Aldana. All Rights Reserved. 2 This dissertation titled Mathematical Modeling of Ammonia Electro-Oxidation on Polycrystalline Pt Deposited Electrodes by LUIS A. DIAZ ALDANA has been approved for the Department of Chemical and Biomolecular Engineering and the Russ College of Engineering and Technology by Gerardine G. Botte Professor of Chemical and Biomolecular Engineering Dennis Irwin Dean, Russ College of Engineering and Technology 3 ABSTRACT DIAZ ALDANA, LUIS A., Ph.D., May 2014, Chemical Engineering Mathematical Modeling of Ammonia Electro-Oxidation on Polycrystalline Pt Deposited Electrodes Director of Dissertation: Gerardine G. Botte The ammonia electrolysis process has been proposed as a feasible way for electrochemical generation of fuel grade hydrogen (H2). Ammonia is identified as one of the most suitable energy carriers due to its high hydrogen density, and its safe and efficient distribution chain. Moreover, the fact that this process can be applied even at low ammonia concentration feedstock opens its application to wastewater treatment along with H2 co-generation. In the ammonia electrolysis process, ammonia is electro-oxidized in the anode side to produce N2 while H2 is evolved from water reduction in the cathode. A thermodynamic energy requirement of just five percent of the energy used in hydrogen production from water electrolysis is expected from ammonia electrolysis. However, the absence of a complete understanding of the reaction mechanism and kinetics involved in the ammonia electro-oxidation has not yet allowed the full commercialization of this process. For that reason, a kinetic model that can be trusted in the design and scale up of the ammonia electrolyzer needs to be developed. This research focused on the elucidation of the reaction mechanism and kinetic parameters for the ammonia electro-oxidation. The definition of the most relevant elementary reactions steps was obtained through the parallel analysis of experimental 4 data and the development of a mathematical model of the ammonia electro-oxidation in a well defined hydrodynamic system, such as the rotating disk electrode (RDE). Ammonia electro-oxidation to N2 as final product was concluded to be a slow surface confined process where parallel reactions leading to the deactivation of the catalyst are present. Through the development of this work it was possible to define a reaction mechanism and values for the kinetic parameters for ammonia electro-oxidation that allow an accurate representation of the experimental observations on a RDE system. Additionally, the validity of the reaction mechanism and kinetic parameters were supplemented by means of process scale up, performance evaluation, and hydrodynamic analysis in a flow cell electrolyzer. An adequate simulation of the flow electrolyzer performance was accomplished using the obtained kinetic parameters. 5 DEDICATION To my support and pride (my beloved wife Sandra), my family, and the recovery of my brother Nelson. 6 ACKNOWLEDGMENTS I acknowledge the financial and technical support of the Center for Electrochemical Engineering Research and the Department of Chemical and Biomolecular Engineering at Ohio University. I would like to thank the support and teaching of my advisor Dr. Gerardine G. Botte. I will always be grateful for her support. I want to thank and the suggestions and recommendations of the committee members Dr. Savas Kaya, Dr. Gang Chen and specially Dr. Howard Dewald and Dr. Valerie Young from whom I got very important lessons about technical aspects related with the dissertation as well as for the future of my career. I wish to thank the contribution of Jim Caesar and John Goettge, who were fundamental for the successful completion of the experiments performed. Thank you all CEER members and staff, Shannon Bruce, Dr. Dan Wang, and Dr. Damilola Daramola, all the students, and special thank you to my friends and mentors Dr. Ana Valenzuela and Dr. Madhivanan Muthuvel. Thank you to Vedasri Vedharathinam, Santosh Vijapur, and Samy Palaniappan for being such good friends and make this work place completely enjoyable. I acknowledge your significant contributions to the successful completion of this work. Finally I would like to thank God and the support of my family, my loved wife Sandra and our little bundle of joy; you make my life so rich and happy making possible to get the best of every moment as Ph.D. student. 7 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Dedication ........................................................................................................................... 5 Acknowledgments............................................................................................................... 6 List of Tables .................................................................................................................... 11 List of Figures ................................................................................................................... 12 Chapter 1: Introduction ..................................................................................................... 19 1.1. Project Significance ............................................................................................... 19 1.2. Statement of Objectives ......................................................................................... 20 1.3. Project Overview ................................................................................................... 21 Chapter 2: Literature Review ............................................................................................ 24 2.1. Importance of Ammonia Electrolysis .................................................................... 24 2.1.1. Ammonia electro-oxidation for fuel cell applications .................................... 25 2.1.2. Ammonia electrolysis for hydrogen production ............................................. 25 2.1.3. Ammonia electrolysis for water remediation .................................................. 27 2.2. Ammonia Electro-Oxidation Kinetics ................................................................... 27 2.2.1. Oswin and Salomon’s mechanism ................................................................. 28 2.2.2. Gerisher and Maurer mechanism .................................................................... 31 8 2.2.3. Studies of reaction intermediates .................................................................... 32 2.2.4. Differential electrochemical mass spectrometry (DEMS) .............................. 34 2.2.5. Spectro-electrochemical studies ...................................................................... 35 2.2.6. Hydrodynamic electrochemistry studies ......................................................... 36 Chapter 3: Analysis of Ammonia Electro-Oxidation Kinetics Using a Rotating Disk Electrode ........................................................................................................................... 38 3.2. Introduction ............................................................................................................ 38 3.2. Experimental .......................................................................................................... 41 3.2.1. Electrode preparation ...................................................................................... 41 3.2.2. Morphological and surface characterization ................................................... 43 3.2.3. Electrochemical measurements ....................................................................... 44 3.3. Results and Discussions ......................................................................................... 46 3.3.1. Morphological characterization ...................................................................... 46 3.3.2. Electrochemical characterization .................................................................... 50 3.3.3. RDE experiments ............................................................................................ 54 3.3.4. Mathematical approach to the RDE experimental results ............................... 60 3.4. Conclusions ............................................................................................................ 63 Chapter 4: Mathematical Modeling of Ammonia Electro-Oxidation Kinetics in a Platinum Deposited Nickel Rotating Disk Electrode (RDE) System ............................... 66 9 3.2. Introduction ............................................................................................................ 66 4.2. Ammonia Electro-Oxidation. ................................................................................. 68 4.3. Mathematical Model of Ammonia Electro-Oxidation ........................................... 75 4.3.1. Combined surface-diffusion controlled process (SDCP) model in the RDE system ....................................................................................................................... 76 4.3.2. Surface confined process (SCP) model in the RDE system............................ 84 4.3.3. Solution
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