Mechanistic Study on Multi-Electron Processes in POM Catalysis Öffentlichkeit
Kim, Mooeung (Spring 2018)
Published
Abstract
Polyoxometalate (POM)-based catalytic oxidations have been attracting great attention and are even used in several commercialized POM-dependent processes owing to their distinctive natures, especially high stability toward oxidative and hydrolytic degradation as well as capability to transfer multiple electrons without structural alterations. POM-based aerobic oxidation generally comprise two steps: (1) oxidation of a substrate catalyzed by the form of POM and (2) the counterpart, reoxidation of reduced POM by O2. The latter step is usually slower than the former, and thus is rate-limiting, so that is generally considered the key reaction in POM-catalyzed O2-based oxidations. In addition, the development of efficient water oxidation catalysts (WOCs) has been challenging. The fact that water oxidation is a multi-electron process has led to utilizing POMs as WOCs. Cobalt-containing species including POMs, CoOx and others are known to be among the most active WOCs. However, understanding the behavior and mechanism of aqueous cobalt is necessary due to the lack of detailed investigations on aqueous cobalt. Given these facts, this dissertation is mainly on the focus of two parts: (1) O2-based POMred reoxidation to make it more efficient and (2) a thorough analysis on aqueous cobalt WOCs.
The reoxidation process is investigated in Chapters 2 through 4. The first part describes Cu catalysis of the reoxidation process. Chapter 2 addresses the fact that trace amounts of Cu efficiently catalyze oxidation of the one-electron-reduced Keggin heteropolytungstates, SiW12O405–. Chapter 3 involves further advances using both Cu(II) and Fe(II) as a more efficient POMred reoxidation catalyst, and Chapter 4 describes aerobic (O2-based) reoxidation of reduced POMs that have much higher potentials by nitrous acid.
The last chapter addresses the true catalysts in catalytic water oxidation using aqueous cobalt and [Co4(H2O)2(VW9O34)2]10–: key catalytic species and mechanisms are elucidated. Double-mixing stopped-flow kinetics and systematic computational calculations ascertain the possible active catalysts in water oxidation by aqueous solutions of cobalt ion, Co(II), that are present in many types of water oxidation catalysts.
Table of Contents
Table of Contents
Chapter 1: Introduction: Polyoxometalates in Redox Chemistry............................... 1
1.1 Fundamental Overview of Polyoxometalates................................................. 2
1.2 General Properties of Polyoxometalates......................................................... 5
1.3 Applications of POM Catalysis...................................................................... 7
1.4 Goal of This Work and Outline.................................................................... 15
1.5 References..................................................................................................... 17
Chapter 2: Cu(II)-Catalyzed Oxidation of Reduced Keggin Heteropolytungstates
with Dioxygen in Aqueous Medium.......................................................... 27
2.1 Introduction................................................................................................... 28
2.2 Experimental................................................................................................. 30
2.3 Results and Discussion................................................................................. 31
2.4 Conclusions................................................................................................... 47
2.5 References..................................................................................................... 49
Chapter 3: Synergistic Oxidation of Reduced Keggin Heteropolytungstates by
Dioxygen Catalyzed by a Combination of Copper and Iron................... 53
3.1 Introduction................................................................................................... 54
3.2 Experimental................................................................................................. 56
3.3 Results and Discussion................................................................................. 59
3.4 Conclusions................................................................................................... 82
3.5 References..................................................................................................... 83
Chapter 4: Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen
Catalyzed by Nitrous Acid......................................................................... 87
4.1 Introduction................................................................................................... 88
4.2 Experimental................................................................................................. 89
4.3 Results and Discussion................................................................................. 92
4.4 Conclusions................................................................................................. 108
4.5 References................................................................................................... 108
Chapter 5: Water Oxidation Catalyzed by [Co4(H2O)2(VW9O34)2]10– and Co2+aq:
Identification of the True Catalysts......................................................... 111
5.1 Introduction................................................................................................. 112
5.2 Experimental............................................................................................... 114
5.3 Results and Discussion............................................................................... 116
5.4 Conclusions................................................................................................. 131
5.5 References................................................................................................... 132
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