Rational design of photocatalysts

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Photo-redox catalysis has gained a lot of interest due to its potential of performing otherwise difficult chemical conversions and its ability to use visible or near UV light, that can be efficiently generated with LEDs. While it has been demonstrated in a range of applications, there is no clear concept for photocatalyst design and the exact catalyst used in an application appears to be by trial and error rather than guided by understanding and rational design criteria. This project aims at combining modelling and experimental work to design and optimize photocatalyst performance based on the following criteria (among others): - Light absorption: the energy available for lifting electrons of a substrate into an excited orbital will depend on the energy transferred from the photocatalyst to the substrate and therefore ultimately on the energy absorbed by the photocatalyst. - Sorption properties (heterogeneous catalysts): The adsorption of a substrate to a heterogeneous photocatalyst is a prerequisite to the energy transfer from the catalyst to the substrate. Assuming that the substrate is then ready to perform the reaction with a second reagent, the sorption properties of the latter to the catalyst surface will affect the reaction pathway and the required catalyst surface. And finally the desorption of the product (or the reagents) from the surface will affect the performance of the catalytic system, as it will free up the surface for the next set of reagent molecules to be converted into product- Coordination properties (heterogeneous catalysts): The coordination of substrates to photocatalytically active coordination metals will determine, whether a reaction is catalyzed and how the catalytic cycle looks like. - Electronic properties of the catalytic system: a typical redox photocatalytic system includes one catalyst for the reduction and another one for the oxidation reaction. This entails that electrons can move from the oxidizing catalyst to the site, where the corresponding reduction is to be performed (and to avoid electron-hole recombination). Identifying an appropriate pair of catalysts and combining them in an appropriate way, so the electrons can move as required will be crucial for the performance of the photo-redox-catalytic system. (Note that also mass transport of molecules/ions/moieties will be required to close the catalytic cycle.) This Ph.D. programme will be looking at the critical aspects of one to three exemplars for redox-photocatalytic systems - By modelling the above (and significant other) parameters - By experimentally testing, verifying, refining the models. This will include analyzing the product stream with respect to products and side products and develop concepts for the underlying reaction network.One goal will include the identification of the rate-/performance-limiting steps or features in a photocatalytic system and suggest measures to improve the effectiveness and efficiency (yield and selectivity, turn-over number, etc.) of the system and suggest appropriate catalyst combinations and formulations,The programme will aim at leveraging synergies with two other students within React by focusing on one photocatalytic system. One of the students is working on the synthesis and analysis of an oxidizing photocatalyst (Melanie Nutter), the other one (Janusz Siwek) on reactor design and scale up.