Polyoxometalate-based Multi-Electron-Transfer Catalysts for Solar Energy Conversion Open Access

Lv, Hongjin (2015)

Permanent URL: https://etd.library.emory.edu/concern/etds/ws859g27v?locale=en
Published

Abstract

The chemistry of polyoxometalates (POMs) has been extensively and increasingly studied in recent years. The high tunability of POM redox, acid-base and other properties makes them attractive candidates for transition-metal-based multi-electron-transfer catalysis. The focus of this thesis is to design/synthesize functional transition-metal-substituted POMs, explore their structural and electronic features and investigate their applications in solar energy conversion field: mainly focusing on POM-based water oxidation catalysts (WOCs) and water reduction catalysts (WRCs).


This dissertation contains four major parts: The first part reports the synthesis, characterization, and catalysis of a novel banana-shaped hexanuclear cobalt-containing tungstovanadate complex, [(Co(OH2)Co2VW9O34)2(VW6O26)]17-. Considering the instability of such a complex in basic borate buffer solution that is used for water oxidation catalysis, its catalytic activity for the H2O2-based epoxidation of 1-hexene and cyclohexene has been studied instead in 1,2-dichloroethane solvent. The second major part focuses on the preparation, characterization and water oxidation activity of a carbon-free, tetra-Co-containing polyoxometalate, [Co4(H2O)2(VW9O34)2]10-. This complex shows remarkable stability and efficiency for homogeneously catalyzing water oxidation with turnover frequency (TOF) approaching 1000 s-1. Also, a family of N-alkylated derivatives of the complex [Ru(1)2]2+ (1 = 4'-(4-pyridyl)-2,2':6',2"-terpyridine) has been investigated in conjunction with [Co4(H2O)2(PW9O34)2]10- catalyst for catalytic water oxidation. The structure-activity correlation is established. The third important part presents a systematic, multifaceted approach to address one of the important problems in POM-based water oxidation catalysis, that is, distinguishing between homogeneous and heterogeneous catalysis. One specific new protocol (tetra-n-heptylammonium nitrate (THpA)-toluene extraction) involves quantitatively separating and quantifying different soluble species with water oxidation activity that are present simultaneously under turnover conditions. This conceptually-simple but powerful experiment has been widely used by other groups to evaluate the homogeneity of POM-based WOC systems. The forth significant part includes the use of transition-metal-substituted POM-catalysts for the multi-electron reduction of water to hydrogen under homogeneous, visible-light, and noble-metal-free conditions. In this part, several known and novel transition-metal-substituted POM-catalysts (e.g. [M4(H2O)2(XW9O34)2]10- (M = Mn, Ni, Cu; X = P, V), [{Ni4(OH)3AsO4}4(B-α-PW9O34)4]28-, and [{Ni4(OH)3PO4}4(A-α-PW9O34)4]28-) has been rationally prepared and systematically characterized using multiple spectroscopic and computational methods. Their activity towards catalyzing hydrogen evolution has also been thoroughly studied.

Table of Contents


Chapter 1 Introduction: Structures, Features, and Applications of Polyoxometalates in Renewable Energy Utilization 1
1.1 General Energy Concerns and Possible Solutions 2
1.2 Requirements and Rational Analysis of Viable WRCs and WOCs 5
1.3 Overview of Polyoxometalates 8
1.4 Applications of POMs as WRCs and WOCs 15
1.5 Goals of This Work and Outline 20
References 22
Chapter 2 Novel Banana-shaped Multi-Cobalt-Containing Tungstovanadate:[(Co(OH2) Co2VW9O34)2(VW6O26)]17- 38

2.1 Introduction 39
2.2 Experimental 40
2.2.1. General Methods and Materials 40
2.2.2. Synthesis 41
2.2.3. X-ray Crystallography 42
2.2.4. Catalysis 44
2.3 Results and Discussion 44
2.3.1. Synthesis and Structure 44
2.3.2. Physicochemical properties 47
2.3.3. Stability study and the conversion of 1 to 2 50
2.3.4. Catalysis 51
2.4 Conclusions 52
References 52
Chapter 3 An Exceptionally Fast Homogeneous Carbon-free Cobalt-based Water Oxidation Catalyst 58
3.1 Introduction 59
3.2 Experimental 59
3.2.1. Synthesis 59
3.2.2. X-ray Crystallography 62
3.2.3. UV-Vis spectroscopy 63
3.2.4. 51V NMR spectra and characterization of the post-catalysis solution 64
3.2.5. ESI Mass Spectrometry and Peak Assignments 65
3.2.6. Computational Procedures 66
3.2.7. Magnetochemical Characterization 67
3.2.8. Kinetics of Water Oxidation by [Ru(bpy)3]3+, O2 Yield Measurements and Turnover Frequency 71
3.2.9. Light-Driven Catalytic Water Oxidation 74
3.2.10. Quantum Efficiency Measurements 75
3.2.11. Dynamic Light Scattering Measurements 77
3.3 Results and Discussion 77
3.3.1. Synthesis, Crystal Structures and Characterization 77
3.3.2. Water Oxidation Activity using Chemical Oxidant in Dark 80
3.3.3. Visible-Light-Driven Water Oxidation 82
3.3.4. Stability Studies 84
3.4 Conclusions 89
References 90
Chapter 4 Bis(4'-(4-pyridyl)-2,2':6',2"-terpyridine)ruthenium(II) Complexes and Their N-alkylated Derivatives in Catalytic Light-Driven Water Oxidation 94
4.1 Introduction 95
4.2 Experimental 97
4.2.1. General Methods and Materials 97
4.2.2. Synthesis 98
4.2.3. Light-Driven Water Oxidation 98
4.3 Results and Discussion 99
4.3.1. Synthesis and characterization of [Ru(1)2][HSO4]2 99
4.3.2. Mechanistic Evaluation of Catalytic Activity 101
4.3.3. Rate Constants of [RuIII(L)2] Self-Decomposition at Elevated pH 107
4.3.4. Structure-Activity Correlation 110
4.4 Conclusions 113
References 113
Chapter 5 Differentiating Homogeneous and Heterogeneous Water Oxidation Catalysis: Confirmation that [Co4(H2O)2(α-PW9O34)2]10- Is a Molecular Water Oxidation Catalyst 116

5.1 Introduction 117
5.2 Experimental 119
5.2.1. General Methods and Materials 119
5.2.2. Synthesis of Co4PPOM from Δ-PW9O34 and Co2+ in Borate Buffer 120
5.2.3. Electrochemical Synthesis of CoOx 120
5.2.4. Catalytic Light-Driven Water Oxidation 121
5.2.5. Chemical Water Oxidation Kinetics Monitored by Stopped-Flow 121
5.2.6. Cathodic Adsorptive Stripping Voltammetry 122
5.2.7. Synthesis of Tetraheptylammonium Nitrate (THpANO3) and Extraction of Co4PPOM from Post-Reaction Solution 124
5.2.8. Inductively Coupled Plasma Mass Spectrometry 125
5.2.9. Dynamic Light Scattering 125
5.2.10. Co4PPOM Decomposition 126
5.3 Results and Discussion 128
5.3.1. Quantification of Active Species Leached from the Initial Molecular Catalyst 128
5.3.2. Behavioral Distinction Between A Molecular Catalyst and Decomposition Product Catalysts 133
5.3.3. Equilibrium Aspects of POM Systems 140
5.3.4. Analysis of Previous Co4PPOM Studies 141
5.4 Conclusions 146
References 148
Chapter 6 Visible-Light-Driven Hydrogen Evolution from Water Using a Noble-Metal-Free Polyoxometalate Catalyst 154
6.1 Introduction 155
6.2 Experimental 156
6.2.1. Materials and Instrumentation 156
6.2.2. Synthesis 158
6.2.3. X-ray crystallography 160
6.2.4. General Procedure for Light-Driven Catalytic Experiments 161
6.2.5. Isotope Labeling Experiment 162
6.2.6. Steady-State and Time-Resolved Fluorescence Decay Measurement of [Ru(bpy)3]2+ in the Presence of 1 or TEOA 162
6.2.7. Computational Procedure 163
6.2.8. Kinetics of Stoichiometric Reduction [Ru(bpy)3]3+ by TEOA 163
6.3 Results and Discussion 163
6.3.1. Preparation and Characterization of Complex 1 163
6.3.2. Photocatalytic H2 Formation 166
6.3.3. Determination of the Origin of H2 170
6.3.4. Catalyst Stability Tests 171
6.3.5. Mechanistic Studies 172
6.4 Conclusions 176
References 177
Chapter 7 A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution 182
7.1 Introduction 183
7.2 Experimental 184
7.2.1. Materials and Instrumentation 184
7.2.2. Synthesis of polyanion [Ni4(H2O)2(PW9O34)2]10- (Ni4P2) 186
7.2.3. X-ray Crystallography 187
7.2.4. ESI Mass Spectrometry and Peak Assignments 190
7.2.5. Visible-Light-Driven Catalytic Experiments and Characterization of the Post-Catalysis Solution 191
7.2.6. Steady-State and Time-Resolved Luminescence Decay and Nanosecond Transient Absorption Measurements of [Ir(ppy)2(dtbbpy)]+ in the Presence of Catalyst Ni4P2 or TEOA 192
7.2.7. Dynamic Light Scattering Measurements 192
7.2.8. TEM, SEM and EDS Measurements 193
7.3 Results and Discussion 193
7.3.1. Synthesis, Crystal Structures and Characterization 193
7.3.2. Computational Studies 195
7.3.3. Cyclic Voltammetry 202
7.3.4. Visible-Light-Driven Catalytic Activity of Ni4P2 203
7.3.5. Mechanistic Studies 207
7.3.6. Stability Studies 211
7.4 Conclusions 214
References 214
Chapter 8 [{Ni4(OH)3AsO4}4(B-α-PW9O34)4]28- a New Polyoxometalate Structural Family with Catalytic Hydrogen Evolution Activity 220

8.1 Introduction 221
8.2 Experimental 222
8.2.1. Materials and Instrumentation 222
8.2.2. Synthesis 223
8.2.3. X-ray Crystallography 227
8.2.4. Magnetochemical Characterization. 231
8.2.5. Visible-light-driven Catalytic Hydrogen Evolution Experiments 231
8.3 Results and Discussion 233
8.3.1. Synthesis, Crystal Structures and Characterization 233
8.3.2. Magnetic Properties of Ni16As4P4 241
8.3.3. Catalytic Hydrogen Evolution Activity and Evaluation of Stability 243
8.3.4. Quenching Mechanistic Studies 248
8.4 Conclusions 250
References 251
Chapter 9 A Cu-based Polyoxometalate Catalyst for Efficient Catalytic Hydrogen Evolution 257
9.1 Introduction 258
9.2 Experimental 259
9.2.1. Materials and Instrumentation 259
9.2.2. Synthesis of Na3K7[Cu4(H2O)2(PW9O34)2] (Na3K7-Cu4P2) 260
9.2.3. X-ray Crystallography 262
9.2.4. Computational Procedures 264
9.2.5. Photocatalysis Experiments 264
9.2.6. Dynamic Light Scattering Measurements 266
9.2.7. TEM, SEM and EDS Measurements 266
9.3 Results and Discussion 267
9.3.1. Synthesis, Crystal Structure and Characterization 267
9.3.2. Electronic and Geometrical Structure of Cu4P2, [Cu4P2]2- and [Cu4P2]2+ 270
9.3.3. Catalytic Activity for H2 Evolution 273
9.3.4. Stability Evaluation 278
9.3.5. Relevant Energetics and Photochemical Quenching Mechanisms 283
9.4 Conclusions 285
References 286

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