Sandwich polyoxometalates toward electrocatalytic water oxidation and organic transformation

Abstract

In today’s world, the scientific community is facing two major challenges namely energy crisis and global warming. Our energy sources are currently dependent on non-renewable energy mainly fossil fuels which are predicted to be depleted within the century and will significantly limit our quality of life. Additionally, these non-renewable energy resources emit greenhouse gases which result major global climate problems. In this scenario, the utilization of renewable and sustainable energy sources are the best alternatives for the economic, environmental and increased energy demand of industries and common households. Water splitting has long been perceived as Holy Grail in this regard because of the production of hydrogen (a clean, renewable fuel) and oxygen without any unwanted by-products. The water splitting occurs in two steps to produce H2 and O2, (1) water oxidation and (2) proton reduction. However, water oxidation is challenging and the bottleneck of the full process because of the high stability of water, multi-electron process, and sluggish oxygen evolution reaction (OER) kinetics at the electrode-electrolyte interface. Therefore, in last few decades, much more attention has been paid to develop the highly efficient water oxidation catalysts (WOCs). Recently, polyoxometalates (POMs) are extensively used in water oxidation because of their multi-electron redox properties and stability in harsh reaction conditions without much structural change. Furthermore, the oxidative resistant, all-inorganic ligands make these transition metal substituted POMs (TMSPs) more suitable towards water oxidation. The nonnoble metal-based tetra-cobalt POMs ([Co4(H2O)2(PW9O34)2]10- P-Co4 and [Co4(H2O)2(VW9O34)2]10- V-Co4) are reported as efficient catalyst towards water oxidation. However, these molecular WOCs are unstable in highly alkaline (pH>8) or acidic (pH<4) media. Moreover, the lower stability in highly acidic or alkaline media and poor conductivity restrict their water oxidation activity in neutral media with a very high overpotential. There are few supports like carbon nanotubes, graphene etc. are reported to stabilize POMs and accelerate electron transfer; however, the water oxidation activity has been reported only under neutral pH media. Therefore, the present work aimed to enhance the water oxidation activity by improving the stability and simultaneously the conductivity of such cobalt based sandwich POMs under highly alkaline conditions. Recently, we have shown the stabilization/heterogenization of sandwich zinc POM [WZn3(H2O)2(ZnW9O34)2]12- (Zn-WZn3) in harsh condition electrolysis, sensing, as well as in Li-S battery application. Here, we have shown the stabilization of ‘phosphorus’-centered tetra-cobalt POM (P-Co4) toward water oxidation in highly alkaline media (pH=14) by using the poly(ionic liquid) support. In addition, we have also studied the stabilization of ‘vanadium’-centered tetra-cobalt sandwich polyoxometalate [Co4(H2O)2(VW9O34)2]10- (V-Co4) by using the same poly(ionic liquid). Further, the stabilization and activation of penta-cobalt POM ([WCo3(H2O)2(CoW9O34)2] Co- WCo3 (analogous to P-Co4 and V-Co4) was studied towards water oxidation which was otherwise ‘chemically inert’. The activation of penta-cobalt POMs toward water oxidation in alkaline media was studied by encapsulating the POM inside the metal-organic framework (MOF) as well as by supporting the POM over the poly(ionic liquid) matrix. The true catalytic nature of penta-cobalt POM and poly(ionic liquid) composite catalyzed water oxidation was investigated by in-situ spectroelectrochemistry. Further, the effect of first row transition metal replacement on the physico-electrochemical properties and di-oxygen activation/binding of sandwich POM Zn-WZn3 was studied towards utilization of such POMs in organic transformations. Considering the di-oxygen activation/binding in non-coordinating solvent (toluene), the Zn-WZn3 catalyst was utilized towards highly selective imine synthesis and the possible mechanistic pathway was investigated. Chapter 1: Introduction to Polyoxometalates This chapter represents an overview of polyoxometalate chemistry; especially, the structural features, characterization techniques and properties of Keggin and Sandwich polyoxometalates. This chapter also represents a highlight of organic inorganic hybrid polyoxometalates. Further, this chapter emphasis on the application of polyoxometalates, mainly polyoxometalate catalyzed water oxidation and organic transformation. Chapter 2: Physico- and Electrochemical Properties of First-Row Transition Metal Substituted Sandwich Polyoxometalates This chapter focuses on the physicochemical and electrochemical properties of the first-row transition metal substituted zinc polyoxometalate [WZn3(H2O)2(ZnW9O34)2]12- (Zn-WZn3). The effect of transition metal substitution on these properties of sandwich zinc polyoxometalates has been narrated. The study of molecular oxygen activation/binding by the sandwich zinc polyoxometalates and the transition metal substituted zinc polyoxometalates is also highlighted. Additionally, this chapter represents the molecular oxygen activation/binding study of a penta-cobalt sandwich polyoxometalate [WCo3(H2O)2(CoW9O34)2]12- (Co-WCo3) and its transition metal substituted polyoxometalate complexes. Chapter 3: Stabilization and activation of cobalt based sandwich polyoxometalates toward electrocatalytic water oxidation in highly alkaline media This chapter describes the stabilization and activation of cobalt based sandwich polyoxometalates by poly(ionic liquid) and metal organic frameworks toward heterogeneous electrocatalytic water oxidation in highly alkaline media. This chapter is divided in two sections (A and B). Section A: Stabilization of tetra-cobalt sandwich polyoxometalates by poly(ionic liquid) toward electrocatalytic water oxidation in alkaline media This section emphasized on the stabilization of ‘phosphorus’ centered tetra-cobalt sandwich polyoxometalate ([Co4(H2O)2(PW9O34)2]10-, P-Co4) toward electrocatalytic water oxidation in highly alkaline media (pH 14). The poly(ionic liquid) poly(vinyl butyl imidazololium) bromide [PVIM]Br was utilized as a conductive support to prepare the polyoxometalatepoly( ionic liquid) composites for stabilization of the polyoxometalates. Additionally, this section also represents the post water oxidation stability study of the polyoxometalatepoly( ionic liquid) composite [PVIM][P-CO4]. Section B: Activation of penta-cobalt sandwich polyoxometalates toward electrocatalytic water oxidation This section describes the activation of a ‘chemically inert’ penta-cobalt polyoxometalate [WCo3(H2O)2(CoW9O34)2]10- (Co-WCo3) toward electrocatalytic water oxidation. Initially, the activation of the Co-WCo3 shows by utilizing the metal organic framework (ZIF-8) followed by the poly(ionic liquid) [PVIM]Br. More importantly, this section also represents the investigation of true catalytic nature for the polyoxometalate-poly(ionic liquid) [PVIM][Co- WCo3] composite catalyzed electrocatalytic water oxidation in highly alkaline media (pH 14) by performing spectroscopic as well as spectroelectrochemical study. Chapter 4: Sandwich polyoxometalate catalyzed organic transformation This chapter represents the sandwich zinc polyoxometalate [WZn3(ZnW9O34)2]12- (Zn-WZn3) catalyzed highly selective imine synthesis in presence of t-BuOK and di-oxygen; and describes the key findings of the investigation of mechanistic pathways of the reaction.Additionally, the zinc polyoxometalate catalyzed direct deoxygenation of primary alcohols are also shown. Chapter 5: Conclusion and future aspects In this chapter, the summary of the whole work has been narrated.