Water Oxidation by the All-Inorganic Homogeneous Catalyst [Co4(H2O)2(α-PW9O34)2]10-: System Optimization, Stability Considerations, and Kinetic Analysis Open Access
Vickers, James Wesley (2015)
Abstract
Since the initial report of homogeneous water oxidation activity from the polyoxometalate complex [Co4(H2O)2(α-PW9O34)2]10- (Co4PPOM) in 2010 it has been cited over 500 times, with studies examining its activity in a wide variety of systems under various conditions. Immediately following this publication, we show incorporation of the complex into a light-driven system where it maintains its activity and produces a turnover frequency several times faster than other homogeneous water oxidation catalysis (WOC) complexes to date under optimized conditions. Although this study reported several control experiments, supplemental to the seven in its initial report, work examining this catalyst in different systems suggested that the complex was not responsible for the WOC activity observed, but that this activity was instead derived from dissociation or decomposition products of the parent structure. Here we developed a series of experiments providing strong evidence that under the conditions initially reported for water oxidation using Co4PPOM it functions as a molecular catalyst, not a precursor for cobalt oxide (CoOx). Specifically, we quantify the amount of Co2+(aq) released from Co4PPOM by two methods, cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry, and show that this amount of cobalt - whatever speciation state it may exist in - cannot account for the observed water oxidation. We document that water oxidation catalyzed by Co4PPOM, Co2+(aq), and CoOx have different dependences on buffers, pH, and WOC concentration. Extraction of Co4PPOM, but not Co2+(aq) or CoOx into toluene from water, and other experiments further confirm that Co4PPOM is the dominant WOC. However, problems persist in studying some of the most basic aspects of WOCs including acquisition of satisfactory early-reaction-time kinetics and rapid quantification of O2 formation. To this end, two new methods for evaluating homogeneous WOCs by reaction with a stoichiometric oxidant are presented which eliminate problems of incomplete fast mixing and O2 measurement response time. These methods generate early-reaction-time kinetics that have previously been unavailable, and the data they produce is used to develop and evaluate mechanistic aspects of WOCs.
Table of Contents
Chapter 1 Introduction: Polyoxometalates as Water Oxidation Catalysts: A Step Towards Energy Sustainability. 1
1.1 Motivation - Earth in trouble. 2
1.2 Water Oxidation Considerations. 6
1.3 General POM background. 10
1.4 Background on POMs as WOCs. 13
1.5 Goals of this work and outline. 19
1.6 References. 20
Chapter 2 Efficient Light-Driven Carbon-Free Cobalt-Based Molecular Catalyst for Water Oxidation. 29
2.1 Introduction. 30
2.2 Experimental. 30
2.2.1 Instrumentation. 31
2.2.2 Materials and synthesis. 32
2.2.3 Light-driven system. 34
2.2.4 Quantum yield measurements. 34
2.2.5 Steady state luminescence quenching. 35
2.2.6 Stopped-flow. 36
2.3 System design. 36
2.3.1 System quantification. 36
2.3.2 Optimization. 38
2.3.3 Dependence on stirring rate. 45
2.4 Results and discussion. 45
2.5 References. 55
Chapter 3 Differentiating Homogeneous and Heterogeneous Water Oxidation Catalysis: Confirmation that [Co4(H2O)2(α-PW9O34)2]10- Is a Molecular Water Oxidation Catalyst. 59
3.1 Introduction. 60
3.2 Experimental. 61
3.2.1 General methods and materials. 61
3.2.2 Synthesis of Co4PPOM from Δ-PW9O34 and Co2+ in borate buffer. 63
3.2.3 Cathodic adsorptive stripping voltammetry. 64
3.2.4 Synthesis of tetraheptylammonium nitrate and extraction of Co4PPOM from post-reaction solution. 65
3.2.5 Measurement of Co2+(aq) from Co4PPOM. 67
3.2.6 Inductively coupled plasma mass spectrometry. 67
3.2.7 Dynamic light scattering. 68
3.2.8 Electronic absorption. 69
3.2.9 Measurement of Co2+(aq) from Co4PPOM. 69
3.2.10 Catalyst reusability test. 70
3.2.11 Electrochemical synthesis of CoOx. 70
3.2.12 Co4PPOM decomposition. 70
3.3 Results. 75
3.3.1 Quantification of active species leached from the initial molecular catalyst. 75
3.3.2 Behavioral distinction between a molecular catalyst and decomposition product catalysts. 78
3.4 Discussion. 86
3.4.1 Equilibrium aspects of POM systems. 86
3.4.2 Analysis of previous Co4PPOM studies. 87
3.5 Conclusions. 92
3.6 References. 93
Chapter 4 Collecting Meaningful Early-time Kinetic Data in Homogeneous Catalytic Water Oxidation with a Sacrificial Oxidant. 100
4.1 Introduction. 101
4.2 Experimental materials and methods. 102
4.2.1 Synthesis. 102
4.2.2 Instruments. 103
4.2.3 Fast mixing system. 103
4.2.4 Continuous-flow system. 104
4.3 Results and discussion. 105
4.3.1 Fast mixing of solutions. 105
4.3.2 Measurements of O2 concentration. 107
4.3.3 Continuous-flow system. 109
4.3.4 System validation. 110
4.3.5 Measurements of [Ru(bpy)3]3+ concentration. 112
4.3.6 The effect of precipitation on the reaction kinetics. 115
4.3.7 Simplified reaction mechanism. 117
4.3.8 Selectivity of the catalyst. 120
4.4 Conclusions. 125
4.5 References. 126
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