Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


Walpita, Janitha 2.pdf (7.08 MB)

ETD Abstract Container

Abstract Header

Iminium Based Electrocaralysts for Water Oxidation and Organic Photohydrides for Proton Reduction

Walpita, Janitha Kumara
http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1431967855

Abstract Details

2015, Doctor of Philosophy (Ph.D.), Bowling Green State University, Photochemical Sciences.
Earth-abundant catalysts for water splitting process are needed to facilitate the large-scale implementation of solar fuel cells. Most molecular model catalysts for oxygen and hydrogen evolution are made of transition metals. The approach used in our labs involves fully organic, bio-inspired catalysts for water splitting. While such model catalysts are likely easy to be prepared and inexpensive, the challenge is to control the reactivity of organic compounds and avoid catalyst degradation due to undesired chemical reactions. Our recent findings on simple flavin-based iminium ion (Et-Fl+) which facilitates the electrocatalytic water oxidation postulated that the catalysis occurs in cooperation with electrode surface oxides. Since a fully molecular system facilitates spectroscopic studies of the catalytic process, we are interested in developing such a system that consists of covalently-linked iminium ions. The mechanism of operation in this type of a catalyst is proposed in four major steps: (i) pseudobase formation via a reaction of flavinium ions with water; (ii) proton-coupled electron transfer (PCET) of pseudobases to generate alkoxyl radicals; (iii) coupling of alkoxyl radicals to generate the peroxide intermediate; (iv) oxidation of the peroxide to release molecular oxygen and regenerate the catalyst. Although the pseudobase formation step is common, the possibility of PCET of the formed pseudobase to generate alkoxy radical is not known. Therefore, we evaluate here the thermodynamic possibility of this process using two iminium-based pseudobases: 2,7-dimethyl-9-hydroxy-9-phenyl-10-tolyl-9,10-dihydroacridine (AcrOH) and 6-phenylphenanthridinol (PheOH). The comparative study reveals the importance of having the redox active –N center closer to –OH functionality in order to drive the PCET process. Pourbaix diagrams constructed using pKa values and standard reduction potentials show that only PheOH has a wide and mild range (pH= 2.8 – 13.3) of pH values in which both the alcohol and alkoxy radical can coexist in order to facilitate the coupled process. Furthermore, in an attempt to identify the functional groups responsible for catalytic oxidation from Et-Fl+ ion, a comparative research was conducted with a simple iminium ion derivative N-methyl-9-phenylacridinium perchlorate (Acr+). Although, some similarities were found in electrochemical behavior of Et-Fl+ and Acr+, the catalytic water oxidation was not facilitated by Acr+. In addition, an excited state hydride release from an organic hydride is proposed to reduce the protons. The proposed photohydride, 10-methyl-9-phenyl-9, 10-dihydroacridine (PhAcrH) was oxidized to PhAcr+, with 52% conversion, while only 2.5% of hydrogen is liberated. The reduction of the solvent (CH3CN/H2O mixture) was suggested as the reason for low yield of H2. The hydride transfer mechanism was identified as stepwise electron/hydrogen atom transfer process originating from the triplet excited state.
Ksenija Glusac, Dr. (Advisor)
Ray Larsen, Dr. (Other)
John Cable, Dr. (Committee Member)
R. Wilson, Dr. (Committee Member)
151 p.
Chemistry; Organic Chemistry; Physical Chemistry
water oxidation catalysis; proton-coupled electron transfer; organic photohydrides

Recommended Citations

Citations

  • Walpita, J. K. (2015). Iminium Based Electrocaralysts for Water Oxidation and Organic Photohydrides for Proton Reduction [Doctoral dissertation, Bowling Green State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1431967855

    APA Style (7th edition)

  • Walpita, Janitha. Iminium Based Electrocaralysts for Water Oxidation and Organic Photohydrides for Proton Reduction. 2015. Bowling Green State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1431967855.

    MLA Style (8th edition)

  • Walpita, Janitha. "Iminium Based Electrocaralysts for Water Oxidation and Organic Photohydrides for Proton Reduction." Doctoral dissertation, Bowling Green State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1431967855

    Chicago Manual of Style (17th edition)

bgsu1431967855
983
© 2015, all rights reserved.
This open access ETD is published by Bowling Green State University and OhioLINK.