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Derivatisation of Cobalt Polypyridyl Based Water Reduction Catalysts: Influence on Mechanism and Performance in Photocatalysis


Müller, Peter. Derivatisation of Cobalt Polypyridyl Based Water Reduction Catalysts: Influence on Mechanism and Performance in Photocatalysis. 2020, University of Zurich, Faculty of Science.

Abstract

This work describes derivatizations of cobalt-polypyridyl based water reduction catalysts (WRCs) and their performance under photocatalytic conditions. Furthermore, mechanistic aspects were investigated by electrochemistry and other analytical methods.
Chapter 2.1 discusses the synthetic steps leading to the different ligand architectures. The intro-duction of a naphthyl substituent at the bridging position between the two bipyridyl units served as a potential anchoring group for cyclodextrin-derivatized surfaces for catalyst immobilization. Complexations of the ligands with Co(OTf)2 or CoBr2 led to the desired catalysts. Some of the derivatizations were not stable under ambient conditions and the oxidation of the bridging po-sition, mostly to a ketone was observed. The uncommon CoIII-precursor Na3[Co(CO3)3] · 3 H2O served as starting material for synthesizing directly a novel CoIII-complex. The diamagnetic properties of this complex extended the possible analytical techniques as compared to the com-
mon CoII-catalysts.
The performance of the respective WRCs under photocatalytic conditions is discussed in Chap-ter 2.3. Special attention has been paid to the investigation of ”proton-relay” functionalities at the bridging position which act in the second coordination sphere of the catalyst. These protic functionalities significantly increased the performance. A clear function of these derivatizations as ”proton-relay” could not be confirmed unambiguously, but it was shown that the hydroxy-functionalities in the bridging position are crucial to achieve high performances. Furthermore, some limitations of the photocatalytic system are shown by e.g. concentration dependent pho-tocatalytic experiments.
To gain a better insight into the mechanistic steps of the catalysis, it was investigated by cyclic voltammetry. Experiments in organic solvents showed that the catalysts generally undergo three reversible reductions. The first reduction at ca. -1.4 V vs. Fc/Fc+ is cobalt-based, followed by two ligand-based reductions. By CV in aqueous solutions three pH dependent mechanisms could be determined or postulated. In the range of pH < 3.6, the reaction follows an ECEC mechanism which changes to an EECC mechanism when the pH value is increased. Furthermore, the overpotential of the catalysts was determined to be about 500-800 mV.
In Chapter 2.6 the reduction of CO2 to CO by one of the catalysts is assessed and compared to proton reduction under similar conditions. Photocatalysis showed that the selective reduction of CO2 to CO is possible, whereby the pH has a decisive influence on the selectivity. In an optimum case, a selectivity of 71% was achieved.

Abstract

This work describes derivatizations of cobalt-polypyridyl based water reduction catalysts (WRCs) and their performance under photocatalytic conditions. Furthermore, mechanistic aspects were investigated by electrochemistry and other analytical methods.
Chapter 2.1 discusses the synthetic steps leading to the different ligand architectures. The intro-duction of a naphthyl substituent at the bridging position between the two bipyridyl units served as a potential anchoring group for cyclodextrin-derivatized surfaces for catalyst immobilization. Complexations of the ligands with Co(OTf)2 or CoBr2 led to the desired catalysts. Some of the derivatizations were not stable under ambient conditions and the oxidation of the bridging po-sition, mostly to a ketone was observed. The uncommon CoIII-precursor Na3[Co(CO3)3] · 3 H2O served as starting material for synthesizing directly a novel CoIII-complex. The diamagnetic properties of this complex extended the possible analytical techniques as compared to the com-
mon CoII-catalysts.
The performance of the respective WRCs under photocatalytic conditions is discussed in Chap-ter 2.3. Special attention has been paid to the investigation of ”proton-relay” functionalities at the bridging position which act in the second coordination sphere of the catalyst. These protic functionalities significantly increased the performance. A clear function of these derivatizations as ”proton-relay” could not be confirmed unambiguously, but it was shown that the hydroxy-functionalities in the bridging position are crucial to achieve high performances. Furthermore, some limitations of the photocatalytic system are shown by e.g. concentration dependent pho-tocatalytic experiments.
To gain a better insight into the mechanistic steps of the catalysis, it was investigated by cyclic voltammetry. Experiments in organic solvents showed that the catalysts generally undergo three reversible reductions. The first reduction at ca. -1.4 V vs. Fc/Fc+ is cobalt-based, followed by two ligand-based reductions. By CV in aqueous solutions three pH dependent mechanisms could be determined or postulated. In the range of pH < 3.6, the reaction follows an ECEC mechanism which changes to an EECC mechanism when the pH value is increased. Furthermore, the overpotential of the catalysts was determined to be about 500-800 mV.
In Chapter 2.6 the reduction of CO2 to CO by one of the catalysts is assessed and compared to proton reduction under similar conditions. Photocatalysis showed that the selective reduction of CO2 to CO is possible, whereby the pH has a decisive influence on the selectivity. In an optimum case, a selectivity of 71% was achieved.

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Additional indexing

Item Type:Dissertation (monographical)
Referees:Alberto Roger, Nevado Blazquez Cristina, Togni Antonio
Communities & Collections:07 Faculty of Science > Department of Chemistry
08 Research Priority Programs > Solar Light to Chemical Energy Conversion
UZH Dissertations
Dewey Decimal Classification:540 Chemistry
Language:English
Place of Publication:Zurich
Date:2020
Deposited On:09 Feb 2022 13:00
Last Modified:19 Dec 2023 16:18
OA Status:Closed