Light to Hydrogen: Photocatalytic Hydrogen Generation from Water with Molecularly-Defined Iron Complexes

Henrik Junge,Oliver Kühn,Angelika Brückner,Nils Rockstroh,Stefan Lochbrunner,Steffen Fischer,Matthias Beller,Ralf Ludwig

doi:10.3390/inorganics5010014

Henrik Junge, Oliver Kühn + Show 6 more

Open Access

https://doi.org/10.3390/inorganics5010014

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Abstract

Photocatalytic hydrogen generation is considered to be attractive due to its combination of solar energy conversion and storage. Currently-used systems are either based on homogeneous or on heterogeneous materials, which possess a light harvesting and a catalytic subunit. The subject of this review is a brief summary of homogeneous proton reduction systems using sacrificial agents with special emphasis on non-noble metal systems applying convenient iron(0) sources. Iridium photosensitizers, which were proven to have high quantum yields of up to 48% (415 nm), have been employed, as well as copper photosensitizers. In both cases, the addition or presence of a phosphine led to the transformation of the iron precursor with subsequently increased activities. Reaction pathways were investigated by photoluminescence, electron paramagnetic resonance (EPR), Raman, FTIR and mass spectroscopy, as well as time-dependent DFT-calculations. In the future, this knowledge will set the basis to design photo(electro)chemical devices with tailored electron transfer cascades and without the need for sacrificial agents.

Highlights

An increasing number of pilot plants combining wind mills or photovoltaic devices with classical water electrolysis is installed all over the world
Besides complex [Fe3 (CO) and CN ligands, the dinuclear iron complex contains an internal base, Iron is a very important and abundant base metal constituting, e.g., the active centers in which is of fundamental importance since it allows for the heterolytic splitting/formation of H2 by hydrogenases [47,48,49] and showing impressive activities for hydrogen generation and cleavage with metal ligand cooperative catalysis
CdTe quantum dots were applied resulting in the hitherto highest observed TONFe as high as 52,800 [111] for 11 (Scheme 2). All of these results might be compared, e.g., with the use of an isolated [NiFe] hydrogenase in visible light-driven hydrogen production on C3 N4 /TiO2 with an excellent productivity (TON) of Surprisingly, the promising application of copper (I) complexes with polypyridine ligands as photosensitizers has been neglected for a long time with only one early communication from the group of Sauvage [113]

Summary

Introduction

An increasing number of pilot plants combining wind mills or photovoltaic devices with classical water electrolysis is installed all over the world. Within the simplification of the complex system of overall water splitting and a more detailed understanding development of more efficient and stable catalyst systems for photocatalytic hydrogen generation of the basic processes as a prerequisite for improvements. Prominent examples of nickel have been outperformed by pentapyridyl cobalt complexes achieving a turnover number (TON) with catalysts constitute the DuBois catalyst [37], as well as tris(2-pyridylthiolate) [38], respect to Co of up to 11,000 with a Re-photosensitizer (PS) and ascorbic acid (SR) [36]. 1. The basic concept of hydrogen generation from water (hydrogen half reaction) via a reductive quenching pathway applying a photosensitizer (PS) and a proton reduction catalyst (WRC).

Overview of Molecularly-Defined Iron WRCs and the Respective Photosensitizers

Overview

Photosensitizers applied in iron-based reduction systems the WRCs

Light to Hydrogen

Findings

Summary and and Outlook