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Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters

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Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters 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 Oludamilola A. Daramola November 2011 © 2011 Oludamilola A. Daramola. All Rights Reserved. 2 This dissertation titled Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters by OLUDAMILOLA A. DARAMOLA 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 DARAMOLA, OLUDAMILOLA A., Ph.D., November 2011, Chemical Engineering Theoretical Characterization of Ammonia Oxidation Species on Platinum Clusters (166 pp.) Director of Dissertation: Gerardine G. Botte Ammonia oxidation is being considered as a viable technology for hydrogen production for use in fuel cells. This study was undertaken to gain insight into current issues related to catalytic inactivity with time. Density Functional Theory was used in modeling the chemical species present during ammonia oxidation: NHx (x = 0 – 3), OHy (y = 1 & 2) and N2Hz (z = 0 - 4) and the adsorption of these molecules on the surface of platinum clusters. Using comparison with experimental measurements where possible, it was found that the strength of adsorption for these molecules followed this trend: N2 < H2O < NH3 < N2H2 < N2H4 < N2H < N2H3 < OH < NH2 < NH < N. This suggests that the species present towards the right of this spectrum were especially relevant to surface blockage and could play a role in catalytic inactivity. In addition, the formation and oxidation of the N2Hz molecules could possibly be tracked by spectrochemical analysis of the position of the N – N bond, which went from single (N2H4) to double (N2H2) to triple (N2) as the oxidation of ammonia progressed. The presence or absence of this peak is an indicator of the orientation of the molecule formed and an indicator of the progress of the reaction. Finally, an exploratory investigation of a mechanism of ammonia oxidation, where ammonia is deprotonated in successive steps, predicted that the conversion of the 4 imide radical to nitrogen, although thermodynamically favorable, exhibits slow kinetics in comparison to deprotonation of ammonia or amidogen. Approved: _____________________________________________________________ Gerardine G. Botte Professor of Chemical and Biomolecular Engineering 5 DEDICATION To Olalekan and Funmilayo Daramola For all that I am and will become, I have you to thank. 6 ACKNOWLEDGMENTS First and foremost, thanks go to my parents Lekan and Funmi Daramola, who had a vision of sending their son to Ohio University so he could create his future, just like they had created theirs. This dissertation is a testament of that investment and I am forever indebted and forever grateful. Dad, thank you for the countless times you reminded me: “Don’t forget who you are and where you are coming from.” Mum, thank you for always keeping me in your prayers and ensuring that I kept a balance between the intellectual with the spiritual. Secondly, to my wife Brandi, our life together is just beginning, but our relationship has always involved me staying up late nights to run tests and perform calculations. Your relocation to Ohio helped in keeping me balanced and sane. Thank you for the countless times you wielded a big stick, just so I would get the ample sleep and the right diet. Your patience through this process has now been rewarded. The research contained within would never have been possible without the guidance, support and enthusiasm of an amazing advisor. Dr. Botte, you have provided me a template of what a research professor is supposed to do: allow students to take ownership of their work by creating an avenue for them to test their ideas, inspire students by continually receiving accolades based on your own research progress and create an environment that fosters creativity and interaction by ensuring students have mentors. Thank you for giving me the opportunity to work with you and allowing me to develop professionally as an author, a researcher and a teacher. I am fortunate enough to have taken a class with most of the professors present on my committee and I am thankful that I am able to share my work with you. Dr. Young, your statistics course was the most useful class I ever took in my undergraduate career; I am still 7 applying the analyses you spent so much time harping on 8 years ago. Your husband started me on the path to research by letting me work with him in my sophomore year, so one can say I have now come full circle. Dr. Sandler, I want to especially thank you for accepting to be on my dissertation committee on such short notice and providing the relevant feedback needed. Dr. Gulino, as my undergraduate advisor, you ensured that I was taking the appropriate courses and was on track to finish in four years. Your jovial approach to life as always helped me feel at ease in your presence. Dr. Dewald, your approach to teaching by tying textbook material with fundamental journal articles ensured the ideas taught in class were crystallized by further study. I would be remiss if I didn’t mention administrative staff at the Department of Chemical and Biomolecular Engineering and the Center for Electrochemical Engineering Research: from making copies (Carrie Linscott) to installing software (Jim Caesar) to stocking up on coffee and processing orders (Leisa Ostermann and Shannon Bruce). These are the non-academic things that ensure a student’s academic life goes smoothly. I would also like to thank my uncle, Dr. Simbo Odunaiya who was my guardian when all this began in 2000 as a freshman at Ohio University. Your support provided the valuable foundation for my academic career. Finally, to my colleagues at the Center for Electrochemical Engineering, I am thankful for all the discussions we have during our monthly research meetings. It reminds me that “Iron sharpens iron.” 8 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Dedication ........................................................................................................................... 5 Acknowledgments............................................................................................................... 6 List of Tables .................................................................................................................... 11 List of Figures ................................................................................................................... 14 Chapter 1: Introduction ............................................................................................... 17 1.1 Project Overview .............................................................................................. 17 1.2 Statement of Objectives .................................................................................... 18 1.3 Significance of Research................................................................................... 20 1.4 References ......................................................................................................... 21 Chapter 2: Literature Review...................................................................................... 22 2.1 Background ....................................................................................................... 22 2.2 Experimental Observations ............................................................................... 22 2.3 Molecular Modeling of Ammonia Oxidation ................................................... 25 2.4 Summary ........................................................................................................... 28 2.5 References ......................................................................................................... 29 Chapter 3: Characterization of NHx (x = 0 - 3) and OHy (y = 1 & 2) molecules on platinum clusters ............................................................................................................... 30 The contents of this chapter have been submitted to the Chemistry of Materials for publication consideration. ............................................................................................. 30 3.1 Abstract ............................................................................................................. 30 3.2 Introduction ....................................................................................................... 30 3.3 Computational Details ...................................................................................... 35 3.4 Results and Discussion ..................................................................................... 38 3.4.1 Bare Clusters ................................................................................................. 38 3.4.2 Gas Phase Adsorbate Molecules ................................................................... 42 3.4.3 Adsorption Energies and Geometry .............................................................. 43 3.4.4 Atomic Spin Density ..................................................................................... 54 3.4.5 Frequency Calculations ................................................................................
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