Porphyrin‐Sensitized Evolution of Hydrogen using Dawson and Keplerate Polyoxometalate Photocatalysts

Abstract

Hydrogen evolution using photocatalytic systems based on artificial photosynthesis is a major approach toward solar energy conversion and storage. In the polyoxometalate-based photocatalytic systems proposed in the past, middle/near UV light irradiation and noble-metal catalysts were mainly used. Although recently polyoxometalates were sensitized in visible light, photosensitizers or catalysts based on noble metals, and/or poor activity of polyoxometalates were generally obtained. Here we show the highly efficient [turnover number (TON)=215] hydrogen evolution induced by the zinc(II) mesotetrakis(N-methyl-pyridinium-4-yl)porphyrin (ZnTMPyP ) sensitization of a series of polyoxometalate catalysts (two Dawson type, P Mo O and P W O anions, and one Keplerate {Mo } cluster) in a visible-light-driven, noble-metal-free, and fully water-soluble system. We attributed the high efficiency for hydrogen evolution to the multi-electron reduction of polyoxometalates and found that: (a) both Dawson polyoxometalates exhibit higher hydrogen evolution efficiency upon ZnTMPyP sensitization in relation to the direct photoreduction of those compounds; (b) the P Mo O anion is more efficient (TON=65 vs. 38, respectively) for hydrogen evolution than the P W O anion; and (c) the high nuclearity Keplerate {Mo } cluster exhibits the highest efficiency (TON=215) for hydrogen evolution compared with the polyoxometalates studied.

A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal–Organic Framework for Water Oxidation

“…In that context, one of the most studied MOFs for encapsulation is the highly porous MIL-101, but HKUST-1, and more recently ZIF and Zr-based MOFs, have also been investigated as robust host structures. Besides, photocatalytic properties of POMs in the presence of molecular porphyrins were investigated in homogeneous conditions for various reactions such as reduction of silver cations, hydrogen, and oxygen evolution reactions. These studies evidenced that porphyrins may be efficiently used for visible light sensitization of POMs.…”

Polyoxometalate-Based Photoactive Hybrid: Uncover the First Crystal Structure of Covalently Linked Hexavanadate-Porphyrin Molecule

“…Polyoxometalates (POMs) are an exceptional family of early transition metal oxygen anion clusters, , which have aroused widespread attention owing to their unique reversible redox activity and structural versatility, as well as broad applications in catalysis, − energy, − materials, , and medicines. ,, Furthermore, light-active POMs, as photocatalysts, possess an exceptional ability toward facilitating multiple proton coupled redox reactions, such as the hydrogen evolution reaction (HER), − oxygen evolution reaction (OER), − carbon dioxide reduction reaction (CO2RR), − and oxygen reduction reaction (ORR) . However, POM clusters are mainly photoactive in the UV region (λ = 200–400 nm) because the ligand-to-metal charge transfer (LMCT) absorption band lies in the shorter wavelength region, limiting their utilizations in solar-driven catalysis. − Therefore, POMs functionalized with visible-light active organic ligands, such as phenanthrolines, organic dye cations, or porphyrins, − provide an alternative strategy to enhance the POM’s visible absorption.…”

Polyoxometalate‐Based Compounds for Photo‐ and Electrocatalytic Applications

“…However, traditional POM architypes often display light absorption in ultraviolet region (large band gap energy), which extremely limits the improvement of photocatalytic efficiency of HER. In this situation, additional photosensitizers (PS) such as noble metal Ru/Ir‐based molecular complexes (with visible light response) and noble metal‐free metalloporphyrins are chosen to add into reaction system for expanding the light absorption range of POM photocatalyst [5e,h] . Because POMs have remarkable and strong electron acceptability, which makes them effectively receive photo‐excited electrons transferred from the LUMO energy level of PS.…”