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Wednesday, 13 December, 2017

Photocatalysis without noble metals

Renewable energy

H2 evolution with COF-based photocatalyst and cobaloxime. Picture: T. Banerjee

H2 evolution with COF-based photocatalyst and cobaloxime. Picture: T. Banerjee

This first noble metal-free covalent organic framework (COF)-based photocatalytic system uses cobaloximes instead of metallic platinum as the hydrogen evolution electrocatalyst. NIM scientist Prof Bettina Lotsch and her team have developed a new COF-based system for sustainable fuel generation from water by photocatalysis.

Cobaloxime hydrogen evolution electrocatalysts, instead of metallic platinum, with covalent organic framework (COF) photocatalysts, resulted in the first noble metal-free COF-based photocatalytic system. NIM scientist Professor Opens external link in new windowBettina Lotsch from Ludwig-Maximilians-Universität München, also heading the Nanochemistry Department at Max Planck Institute for Solid State Research in Stuttgart, and her team developed these promising systems for hydrogen evolution photocatalysis. They published their new system for photocatalytic hydrogen production from water in the Opens external link in new windowJournal of the American Chemical Society.

Solar energy for hydrogen generation from water
For generation of hydrogen (H2), water can be split photocatalytically in an artificial photosynthetic process. In such clean and renewable energy systems, solar energy is used as light source. Photocatalysis describes the process of acceleration of the rate of a photoreaction by addition of catalysts, which participate in the chemical reaction but do not get consumed. Conventional fossil fuel resources are limited: Hydrogen generated photocatalytically from water could be used as a non-fossil fuel in several applications. “Artificial photosynthesis, for driving thermodynamically uphill chemical reactions, is a fundamental approach to storing solar energy, and is perhaps most prominently represented by water splitting to make solar fuels like H2.” explains Dr Tanmay Banerjee.
Light absorbing photocatalysts and proton reduction co-catalysts are usually required for such hydrogen evolution photocatalysis. In the previously reported set-ups for photocatalytic H2 evolution with COF photocatalysts, noble metals such as platinum served the purpose of co-catalysts. The goal is to replace them by earth-abundant and cheap alternatives. Lotsch points the obstacles: “Devising such systems is challenging because molecular co-catalysts have limited photostability and are characterized by slow multi-electron diffusion-controlled proton reduction process which needs to be conjugated efficiently to the light-harvesting and charge percolation processes on the COF.”

COFs for photocatalytic hydrogen generation

Photocatalytic hydrogen evolution from water starts when light shines on the photoabsorber, in this case covalent organic frameworks. The COF is oxidized while electrons are transferred to the proton reduction co-catalyst and hydrogen is produced. The oxidized COF is then regenerated by a sacrificial electron donor such as triethanolamine (TEOA).
Covalent organic frameworks are two- or three-dimensional polymers with a porous and crystalline backbone of
covalently interlinked organic molecules. The robust structures are composed entirely of earth abundant elements hydrogen (H), carbon (C) and nitrogen (N). The atomically precise incorporation of organic units into the periodic structure makes them a variable and readily tunable platform, for applications such as photocatalysis.

Cobaloxime as new co-catalysts with COFs
The Lotsch group replaced metallic platinum as co-catalyst with the non-noble metal electrocatalyst cobaloxime (brown molecule in the schematic). Cobaloximes are coordination complexes of the earth abundant element cobalt with dimethylglyoxime ligands. In the photocatalytic H2 evolution2H process, they act as artificial hydrogenases for the reduction of protons.
“Our earth abundant COF
-molecular co-catalyst-based photocatalytic system highlights the prospects that lay in the possible precise control over the nature, the arrangement and the density of photocatalytically active sites in the robust and tunable COF backbone.” highlights Dr Tanmay Banerjee. Lotsch adds: “An earth-abundant molecular co-catalyst combined with the molecularly defined COF photoabsorber could provide a highly tunable, single-site heterogeneous photocatalytic scaffold fully accessible to the toolbox of organic synthetic chemistry. It would thus be a significant step toward sustainable and inexpensive photocatalytic systems.” This advancement is seminal as noble metals are very rare and expensive, and hence a potential limiting factor in the development of COF-based photocatalytic systems.

Single-Site Photocatalytic H2 Evolution from Covalent Organic Frameworks with Molecular Cobaloxime Co-Catalysts. Banerjee T, Haase F, Savasci G, Gottschling K, Ochsenfeld C, Lotsch BV. J Am Chem Soc. 2017 Nov 15;139(45):16228-16234. doi: Opens external link in new window10.1021/jacs.7b07489. Epub 2017 Nov 3.


Prof Dr Bettina Valeska Lotsch
Chemistry Department
Ludwig-Maximilians-Universität München
Butenandtstr. 5-13, Building D
81377 Munich

Tel: +49 (0)89 2180 - 77429

Email: Opens window for sending emailbettina.lotsch(at)cup.uni-muenchen.de

Web: Opens external link in new windowwww.cup.uni-muenchen.de/ac/lotsch/prof-lotsch.html


Prof Dr Bettina Lotsch
Nanochemistry Department
Max Planck Institute for Solid State Research
Heisenbergstr. 1
70569 Stuttgart

Tel: +49 (0)711 689 - 1610

Email: Opens window for sending emailb.lotsch(at)fkf.mpg.de

Web: Opens external link in new windowwww.fkf.mpg.de/171964/Prof_Dr_Bettina_V_Lotsch


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