Photochemical Sciences Ph.D. Dissertations

Metal-free electrocatalysts for oxygen evolution reaction and photocatalysts for carbon dioxide reduction reaction

Date of Award

2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Ksenija Glusac (Advisor)

Second Advisor

Liangfeng Sun (Other)

Third Advisor

Marshall Wilson (Committee Member)

Fourth Advisor

Alexander N Tarnovsky (Committee Member)

Abstract

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–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.

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