Metal organic frameworks for photo-catalytic water splitting

Abstract

Metal organic frameworks (MOFs) have recently debuted as participants and solid supports in catalytic water splitting. Their porosity and structural versatility offer a tantalising consolidation of the components needed for solar light harvesting and water splitting. Herein, we describe a selection of relevant contemporary investigations that employ electrocatalysis, chemically introduced redox partners, and photo-catalysts to generate dioxygen and dihydrogen from water. The role of semiconducting MOFs in these systems is addressed, in tandem with band gap control by linker functionalisation and doping. Considered holistically, MOFs offer an impressive physical, spatial and chemical versatility with which to support and sustain water splitting reactions. Major challenges toward practical implementation do remain, but opportunities for development are evidently numerous. Broader contextGrowing experimental and computational evidence suggests that metal organic frameworks (MOFs) can make a meaningful contribution to catalytically promoted water splitting. They offer an impressive physical, spatial, chemical and electronic mutability with which to support and sustain water splitting halfreactions. Their classical features -thermal stability, large surface area, high porosity and modularity -define them as versatile solid supports. Building on a vast library of existing frameworks, post-synthetic modification techniques allow further tailoring of intrinsic MOF properties and functionality. Compelling parallels are now being drawn between semiconductors and a select group of MOFs that are capable of sustaining a photo-generated charge separated state and using the resultant charge carriers to perform chemical transformations. In this perspective we describe a selection of contemporary electrocatalytic, chemically promoted, and photo-catalytic investigations that apply MOFs to water oxidation and reduction half reactions. We highlight successful strategies employed by the scientific community for engineering of catalytic sites, managing redox half-reactions, introducing light sensitisation and band gap tuning -all within the confines of a framework. Major challenges toward practical application certainly remain, but the adaptive nature of MOFs stands to make a poignant contribution to the discourse on photo-catalysed water splitting.Scheme 1 Proton reduction (1) and water oxidation (2) half reactions (NHE, 25 1C, pH 0). ; Tel: +41 (0)56 310 5843 † During the period between submission and revision of our manuscript a couple of examples of water reduction carried out by, or in the presence of MOFs, have appeared in the literature. We refer readers to the papers by: Du et al. concerning a nickel mercaptopyrimidine MOF with a TOF of 10.6 h À1 in the presence of triethylamine and fluorescein under blue light, 124 and Chen et al. describing a photosensitised MOF encapsulating a platinum dihydrogen evolving catalyst (Ru-Pt@UiO-67). 125

Metal–Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production

“…[35] However,inc ontrast to zeolites that are transparent to most visible and UV radiations,M OFs can play an active role,a bsorbing photons by the organic linkers and converting the initial ligandlocalized exciton into ac harge-separate state by single electron transfer from the ligand to the metal nodes ( Figure 5). Theu se of MOFs as photocatalysts is an emerging field [39] owing to the versatility of these materials and there are recent Reviews describing the activity of MOFs for the degradation of dyes, [40] water splitting, [41] and H 2 production or photocatalytic CO 2 reduction. Other linkers can exhibit similar photochemical behavior and, therefore,t he framework nature with linkers coordinated to metal ions, allows photoinduced electron transfer from the linker as electron donors to the metal nodes as electron acceptor to be quite ag eneral process.…”

Metal–Organic‐Framework‐Based Catalysts for Photoreduction of CO2

“…[69][70][71][72][73][74] The high-density metal nodes provide abundant catalytic active sites. [70,71,74,78,80] Others mainly shed light on different CO 2 -conversion reactions, such as the cycloaddition of CO 2 , photocatalytic conversion of CO 2 , and electrocatalytic conversion of CO 2 . As a result, the photoinduced electrons migrate from the photoexcited organic linkers to the adjacent metal nodes rather than the external surface of the whole semiconducting MOFs, suggesting that the low bulk recombination of photoexcited electron-hole pairs occurs in the migration process.…”

Metal–Organic Framework Based Catalysts for Hydrogen Evolution

“…However, MOFs can also be used as precursors to synthesize a variety of metal compounds or metal/carbon composites with intentionally designed elemental compositions and structures. [35][36][37] For example, in 2015, Meyer et al published a perspective article that focuses on MOFs for photocatalytic water splitting, which offered a comprehensive review on the fundamentals and advances of MOFs for water oxidation and reduction reactions. In recent years, there is an explosive development of hydrogen-powered technologies in industries and a considerable number of published articles on MOF-based catalysts for HER.…”