Polyoxometalates on Functional Substrates: Concepts, Synergies, and Future Perspectives

Abstract

POMs were studied as heterogeneous acid catalysts and oxidation catalysts. However, this approach is often affected by low specific surface areas, leading to reduced reactivity. [10] Later studies, therefore, pioneered the immobilization of POMs on high surfacearea substrates including porous silica and other metal oxides, [10] as well as more advanced materials such as carbon nanostructures [11] and metal-organic frameworks (MOFs). [9] This strategy can enable maximized exposure of the individual POM molecules to the reaction environment, and in addition, allows for facile separation by filtration or centrifugation. [12] While early studies on POM immobilization used heterogeneous supports for mechanical stabilization and maximized surface-area, more recent studies have moved toward added properties introduced by the support. These include light-absorption and charge transport by semiconductors (TiO 2 , CdSe, etc.), electrical conductivity (by carbons, metals, conductive polymers) as well as specific POM binding sites, as introduced in MOFs and organofunctionalized supports. Many contemporary studies in this field share a common interest in exploring the synergism between POM and substrate to tune and optimize reactivity and stability of the composite. The concept has been theoretically and experimentally described several decades ago as the so-called "microenvironment effect" (electronic charge enhancement) where entrapment of POM species in a 3D matrix has been shown to induce huge improvement of their electrocatalytic activity. [13][14][15] However, to-date, the understanding of the fundamental processes which govern synergism of POM-substrate interactions is still in its infancy, as mole cular-level understanding of these complex processes by experiment or theory is far from trivial. This is due to the vast number of materials combinations, which are currently studied, and is further complicated by the often unknown chemical structures of the POM-substrate interface which make theoretical studies difficult. Also, in situ/operando studies of these materials are still challenging, and instrumentational approaches to address these issues are still being developed and not widely available. Most current experimental studies, therefore, focus on macroscopic reactivity, while providing less insight into the underlying causes of the processes observed.This Progress Report aims at raising awareness of the vast benefits of exploring this under-researched area to enable full use of POM-substrate interactions including charge and energy transfer, band-gap, and frontier orbital alignment as well as interface design for stability and reactivity. These fundamental concepts can affect both the thermodynamics as well as the kinetics of the chemical processes studied so that new reactivity can be tailored by understanding of the interactions between POM and substrate. Given the fast-paced progress in the synthesis, application, and characterization of this materials class, this Progress Report will serve as a focal point for ...

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