Integrating polyoxometalate-single atom catalyst based (photo-) electrodes in flow reactors for reductive and oxidative nitrogen activation

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In this project, we propose the development of photo-electrochemically driven reactors capable of coupling the nitrogen-to-ammonium reduction reaction (NRR) and the nitrogen-to nitrate oxidation reaction (NOR), leading to autonomous flow photoreactors for ammonium nitrate production from nitrogen, water, sunlight, and sustainable electricity. The project combines concepts from novel catalyst synthesis and stable electrode deposition to reactor design and materials-in-reactor integration. The project combines expertise in metal oxide catalysts and (photo-)electrode functionalization (PI Streb) with expertise in reaction engineering, mass transport optimization and photoreactor design (PI Ziegenbalg). In the first funding period (36 months), the project will develop noble-metal-free single atom catalysts (SACs, 1 metal reaction center) and single-site catalysts (SSCs 2 metal reaction centers) anchored to molecular metal oxides (polyoxometalates, POMs). Variation of the SAC/SSC metal sites will allow control of NRR/NOR performance, while POM-anchoring will provide a stable, well-defined all-oxo coordination environment. Deposition of these molecular precursors on high-porosity (photo-)electrodes is achieved by wet-chemical, microwave or hydrothermal POM-conversion to solid-state oxides to facilitate stable mechanical and electrical linkage between catalyst and electrode. Electrode integration into 3D-printed flow photoreactors will enable rapid prototyping and fast matching of chemical and reaction-engineering requirements. The systems will be studied by in situ/operando (photo-)electrochemical studies and – in collaboration – by theoretical modelling. This will provide insights from the atomic to the reactor-level on the catalytic performance, its limitations, and enable us to identify key optimization parameters. The first funding period is focused on studying NRR and NOR in half-cell setups for mechanistic investigations and materials design/optimization, while the second funding period will target the integrated full-cell photoreactors. In sum, this project will develop flow photoreactors for decentralized ammonium nitrate production starting from molecular nitrogen, and uses modern materials design, advanced electrode fabrication and reaction engineering to address challenges from the molecular to the reactor level.