Towards Artificial Photosynthesis - Solar Fuel Production in Metal-Organic Frameworks
Vanessa Ramm a, Philip M. Stanley a, Julien Warnan a, Roland A. Fischer a
a Chair of Inorganic and Metal-Organic Chemistry, Department of Chemistry, Technical University of Munich, Garching Germany.
Proceedings of Catalyst Design Strategies for Photo- and Electrochemical Fuel Synthesis (ECAT23)
Keele, United Kingdom, 2023 December 4th - 5th
Organizers: Charles Creissen, Qian Wang and Julien Warnan
Poster, Vanessa Ramm, 030
Publication date: 10th October 2023

Modern society faces the central challenge of fulfilling a steadily increasing energy demand by establishing efficient sustainable energy supplies and resolve the global energy and climate crisis, owing to the dominating dependency on fossil fuels. Herein, artificial photosynthesis towards solar fuel production constitutes a renewable and sustainable solution for long-term energy storage.[1,2] Homogeneous molecular photosystems present high performance (e.g., product selectivity, atom economy), which are, however, often defied by low stability, recyclability and controlled site positioning.[2,3] For this, metal-organic frameworks (MOFs) constitute an auspicious platform to build efficient photocatalysts, enabling photoinduced electron excitation, charge separation and migration to active sites and, ultimately, chemical conversion.[4] Photocatalytic reduction reactions towards high-value products have been widely researched and successful enhancement of catalytic performance has been shown in MOFs.[5] In this work, immobilization of molecular reduction catalysts (i.e., CO2-to-hydrocarbon, H2 evolution) in a light-harvesting MOF, NU-1000, has revealed synergy between enabled long-distance energy transfers, localized charge-separation and catalyst stabilization.[6] For this, MOF functionalization was pursued via solvent-assisted ligand-incorporation with the respective Re and Co coordination complexes and optimized linker-to-catalyst ratios achieved improved energy delivery during photocatalysis, resulting in state-of-the-art turnover numbers and incident photon conversion of up to 36%. However, sustainability of these processes has been so far defeated by the widespread requirement of high-value sacrificial electron donors as electron source. Thus, these reductions must be rendered scalable by coupling them to sustainable photocatalytic oxidations, resulting in atom economy and a closed redox cycle.[3,4,7]. The second part of my work has focused on establishing efficient photocatalytic oxidations with molecular catalysts-MOF-hybrid assemblies. For this, photocatalytic water and alcohol oxidation have been chosen as the model reactions, being supported by NU-1000 as a light-harvesting antenna. Here, establishing high catalytic performances requires devising MOF materials amenable for heterogenization of molecular catalysts, getting insights into underlying elementary mechanisms at integrated catalytic sites, as well as fundamental photophysical processes for in-depth understanding.

This work was supported by the German Research Foundation (DFG) Priority Program 1928 “Coordination Networks: Building Blocks for Functional Systems”, the research project MOFMOX (grant number: FI 502/43-1), and by the Excellence Cluster 2089 “e-conversion” (Fundamentals of Energy Conversion Processes).

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