Transition-metal-substituted polyoxometalates as water oxidation catalysts Open Access

Zhu, Guibo (2013)

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Several new polysilicotungstates substituted with earth-abundant elements (Co and Ni), K10.2Na0.8[{Co4(µ-OH)(H2O)3}(Si2W19O70)]·31H2O, K5Na4H4[{Na3(µ-OH2)2Co2(µ-OH)4}(Si2W18O66)]·37H2O, K6Na3[Na(H2O){Co(H2O)3}2{Co(H2O)2}(Si2W18O66)]·22H2O and K10H2[Ni5(OH)6(OH2)3(Si2W18O66)]·34H2O, have been synthesized and characterized by X-ray crystallography, infrared, elemental analysis, UV-vis spectroscopy and cyclic voltammetry. They can effectively catalyze water oxidation in a photo-driven system with [Ru(bpy)3]2+ as photosensitizer and Na2S2O8 as the sacrificial electron acceptor. Several lines of evidence show that none of these complexes decompose to insoluble metal oxide under the harsh catalytic conditions. Although [{Co4(µ-OH)(H2O)3}(Si2W19O70)]11- slowly hydrolyzes in aqueous solution, it is quite probable that the initial complex is the real catalyst, because the isolated hydrolysis products show lower WOC activity. Because of their low solubility in aqueous solution, [{Na3(µ-OH2)2Co2(µ-OH)4}(Si2W18O66)]13-, [Na(H2O){Co(H2O)3}2{Co(H2O)2}(Si2W18O66)]9-, and [Ni5(OH)6(OH2)3(Si2W18O66)]12- form solution ⇔ [Ru(bpy)3]n+-POM complex equilibria and remain molecular during the catalytic process.

A dodecazinc silicotungstate K20Na2[Zn6(OH)7(H2O)(Si2W18O66)]2·34H2O has been synthesized and characterized. DFT B3LYP calculations give HOMO-LUMO and (HOMO-1)-LUMO energy gaps of ~3.65 and 3.91 eV, which are larger than in ZnO (band gap = 3.35 eV).

The catalytic activity of tetra-n-butylammonium salt of [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10-, whose water soluble salt is a water oxidation catalyst, was explored in oxidation of 3,5,3',5'-tetrabutyl-biphenyl-4,4'-diol by dioxygen. Alcohols are stoichiometricaly oxidized under Ar by this compound to aldehydes/ketones in acetonitrile solution. The reaction kinetics and stoichiometry were studied by GC, NMR and UV-vis spectroscopies. The reaction kinetics are very complex, the reaction proceeds in two steps with the second step being significantly slower. Several reaction mechanisms were analyzed by fitting of the reaction kinetic curves obtained from changes of UV-Vis spectra versus time.

Table of Contents

Chapter 1 : Introduction to Transition-metal-Substituted Polyoxometalates (TMSPs) and TMSP-Catalyzed Water Oxidation 1

1.1 General Information about POMs 2
1.2 Catalytic applications of POMs 13
1.3 Goals of this thesis and outline 16

Chapter 2 : Water Oxidation Catalyzed by a Tetracobalt-Substituted Polysilicotungstate: [{Co4(µ-OH)(H2O)3}(Si2W19O70)]11- 17

2.1 Introduction 18
2.2 Experimental 19
2.3 Results and discussion 24
2.4 Conclusions 38

Chapter 3 : Synthesis and Characterization of Di- and Tri-Cobalt Polyoxometalates and Their Application in Photocatalytic Water Oxidation 39

3.1 Introduction 40
3.2 Experimental 41
3.3 Results and discussion 45
3.4 Conclusions 72

Chapter 4 : A nickel-containing polyoxo-metalate water oxidation catalyst 73

4.1 Introduction 74
4.2 Experimental 76
4.3 Results and discussion 82
4.4 Conclusions 100

Chapter 5 : A Dodecanuclear Zn Cluster Sandwiched by Polyoxometalate Ligands 101

5.1 Introduction 102
5.2 Experimental 103
5.3 Results and Discussion 107
5.4 Conclusions 121

Chapter 6 : Oxidation of Alcohols and Phenols in Acetonitrile by a Tetraruthenium Polyoxometalate Complex: Kinetics and Mechanism 122

6.1 Introduction 123
6.2 Experimental 126
6.3 Results and discussion 129
6.4 Conclusions 150
References 151

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