METAL-FREE ELECTROCATALYSTS FOR OXYGEN EVOLUTION REACTION AND PHOTOCATALYSTS FOR CARBON DIOXIDE REDUCTION REACTION Usha Pandey Kadel A Dissertation Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY May 2018 Committee: Ksenija D. Glusac, Advisor Liangfeng Sun Graduate Faculty Representative Alexander N. Tarnovsky R. Marshall Wilson © 2018 Usha Pandey Kadel All Rights Reserved iii ABSTRACT Ksenija Glusac, Advisor Oxygen evolving reaction (OER) and carbon dioxide reduction reaction (CO2RR) are the two important reactions related to energy conversion and storage. Both OER and CO2RR are multi electrons and protons transfer reactions with slower reaction kinetics. Thus catalysts are required to mediate the reactions in one step to desired products without forming the high energy .- intermediates, such as H2O2 in case of OER and CO2 in CO2RR. In case of OER, our previous study shows that the flavin-based catalysis depends on the type of working electrode. The oxides formed on the electrode surface assist the evolution of the oxygen. The disadvantage of this kind of catalysis is the difficulty in studying the catalytic mechanism by using conventional spectroscopic techniques. The proposed catalytic mechanism for a homogeneous system involves four different steps: (i) pseudobase formation via a reaction of xanthylium ion with water, (ii) proton coupled electron transfer (PCET) of pseudobase to form alkoxy radicals, (iii) coupling of alkoxy radicals to form peroxide intermediate and, (iv) oxidation of peroxide to release oxygen and regenerate the catalyst. The result from electrode-assisted mechanism suggests that a homogeneous catalyst can be developed, where two cation species are covalently linked with suitable linker. + The xanthylium dimer (Xan )2 containing two xanthene units covalently linked with + diphenyl ether was studied as the model electrocatalyst for water oxidation. The (Xan )2 readily reacts with water to form mono and di-hydroxylated species, which is the first step of proposed catalytic mechanism. For the formation of peroxide from the pseudobase, the two OH group should orient in favorable geometry (In-In conformer). The conformational flexibility of (Xan– iv OH)2 was studied by using NMR spectroscopy, X-ray crystallography and, DFT calculation methods. Although, DFT calculation and solid-state structure show the stability of In-Out conformer, the NMR study in solution shows that the conformers freely interconvert in NMR time scale, indicating the desired In-In conformer to be populated in millisecond timescale, which is required for catalysis. Moreover, the electrochemical study shows that the catalytic + + water oxidation occurs at lower potential in case of (Xan )2 compared to Xan . However, the + catalytic behavior of (Xan )2 depends on the type of working electrode In addition, the ground state hydride donating ability (hydricity) of organic hydrides, NADH analogues (BNAH, CN-BNAH, Me-MNAH and HEH), methylene tetrahydromethanopterin analogs (BIMH and CAFH), acridine derivatives (Ph-Acr, Me2N-AcrH, T-AcrH, 4OH, 2OH, 3NH), and a triarylmethane derivative (6OH) were studied by using theoretical (DFT) and experimental methods (potential –pKa and hydride transfer) in two different solvents (acetonitrile and dimethyl sulfoxide). The results show that the hydricity values of these organic hydrides are comparable to those of metal hydrides and most of the hydrides are capable to reduce proton in acetonitrile. v To my parents vi ACKNOWLEDGMENTS I would like to express my sincere appreciation to my advisor Dr. Ksenija D. Glusac for her patience, guidance, and continuous support during my PhD. I would never have been what I am today without her support and encouragement. I would like to take an opportunity to thank my committee members Dr. R. Marshall Wilson, Dr. Alexendar N. Tarnovsky, and Dr. Liangfeng Sun for their support, time and advice. I am also grateful to Dr. Thomas H Kinstle for his invaluable support, encouragement and advice. I also like to thank all my past and present lab members with whom I got an opportunity to work. Special thanks go to Dr. Janitha Walpita for his kind support during my initial phase in the lab, Marija Zoric and Stefan Ilic for their very helpful support during research. I have learnt so much from you people. In addition, special thanks go to Dr. Xin Yang, Dr. Suja Shyam, Yun Xie, Varun Singh, George Hargenrader, Aco Radujevic, Andrej Penavic, and Ravindra Weerasooriya. I take this opportunity to thank Dr. Peter Lu and his group, especially Dr. Bharat Dhital and Achyut P Silwal for allowing me to use their instrument during my studies. I would like to thank the faculty and staff of the Chemistry Department for their help, guidance, and advice. I would like to thank Bowling Green State University for the financial support. Finally, I would like to thank my parents, my brother Ubhar, my sister Urmila, my brother in law Dharma Raj, and my sister in law Kalpana for their unconditional love and support. Special thanks go to my brother in law Shreedhar for his help. I appreciate the love and support from my Kadel family during my studies. Most importantly, special thanks go to my husband Gokul for his patience, love, support and encouragement. Without Gokul, none of this would be possible. vii TABLE OF CONTENTS Page CHAPTER 1 INTRODUCTION ................................................................................................... 1 1.1 Oxygen Evolving Reaction (OER) .................................................................................... 2 1.2 Transition Metal Based Homogeneous Water Oxidation Catalysts .................................. 4 1.2.1 Ruthenium Based Catalysts .................................................................................................. 4 1.2.2 Iridium Based Catalysts ....................................................................................................... 9 1.2.3 Manganese Based Catalysts ............................................................................................ … 11 1.2.4 Cobalt Based Catalysts ......................................................................................................... 12 1.2.5 Iron and Copper Based Catalysts ........................................................................................ 14 1.3 Metal Free Water Oxidation Catalysts ..................................................................................... 16 1.4 Carbon dioxide Reaction Reactions (CO2RR) ........................................................................ 17 1.4.1 Metal Complexes .................................................................................................................. 19 1.4.2 Metal Free Catalysts ............................................................................................................. 21 1.5 Our Aim ........................................................................................................................................ 23 1.5.1 Electrocatalytic Water Oxidation Project .......................................................................... 23 1.5.2 Carbon Dioxide Reduction Project ..................................................................................... 25 1.6 References .................................................................................................................................... 26 CHAPTER 2 CONFORMATIONAL FLEXIBILITY OF XANTHENE BASED COVALENTLY LINKED DIMERS .................................................................................................................................. 36 2.1 Covalently Liked Dimers ...................................................................................................37 2.2 Experimental Section .................................................................................................................. 38 2.2.1 General Methods .................................................................................................................. 38 viii 2.2.2 Synthesis of Compounds ..................................................................................................... 38 2.2.3 Computational Methods ....................................................................................................... 40 2.2.4 Variable Temperature NMR Spectroscopy (VT-NMR) ................................................. 40 2.3 Result and Discussion ................................................................................................................. 42 + 2.3.1 Reaction of (Xan )2 with water ........................................................................................... 42 2.3.2 Conformational Flexibility of Molecules .......................................................................... 44 + 2.3.2.1 Conformational Study and Stability of (Xan )2 ...................................................... 45 2.3.2.2 Conformation Study of (Xan-OH)2 .......................................................................... 48 2.3.2.3 Conformation Study of Xan+-Xan-OH .................................................................... 54 2.4 Conclusions .................................................................................................................................. 59 2.5 References ...................................................................................................................................