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Fundamental and Applied Science on Molecular Redox-Catalysts of Energy Relevance in Metal-Organic Frameworks

Periodic Reporting for period 4 - MOFcat (Fundamental and Applied Science on Molecular Redox-Catalysts of Energy Relevance in Metal-Organic Frameworks)

Reporting period: 2021-07-01 to 2021-12-31

The project deals with the development of a new class of catalytic materials for the production of hydrogen, reduction of carbon dioxide and oxygen, and the oxidation of water. All of these reactions are of vital importance for a sustainable society that is based on renewable fuels. Water oxidation is at the heart of natural photosynthesis, and provides electrons for the reduction of carbon dioxide in nature. Hydrogen is a carbon-free fuel, and its use for example in fuel cells produces water as the only by-product. All of these reactions are possible today, but require noble metal catalysts; a strategy that is hardly scalable. Thus, the project presented herein focuses mainly on cheap and abundant first row transition metals.

In general, there are two different kinds of catalysts: heterogeneous, solid-state materials and others that consist of discrete, atomically defined molecular entities. The interest in the latter is motivated by the high activity per metal center, as well as the diversity of catalyst structures that are possible in three dimensions, the level at which mechanisms can be determined, reactivity tuned, and selectivity achieved. However, homogenous catalysts almost always suffer from instability, and there are good reasons to suspect that the best catalysts in terms of practical considerations will be heterogeneous in nature. We therefore decided to incorporate the molecular catalysts into a heterogeneous material scaffold that is provided by so-called metal-organic frameworks (MOFs). MOFs are composed of metal fragments (secondary building units, SBUs) that are interconnected by organic linkers through coordination bonds to form three dimensional microporous crystalline materials. Due to their intrinsic topology and porosity, they have been studied for a range of applications in gas storage/separation, chemical sensing, and drug delivery. They exhibit unmatched high internal surface areas, and are thus ideally suited for the controlled incorporation of molecular catalysts of energy relevance.

The overall objective of the project is to expand our understanding of molecular catalysis in the confinements of porous crystalline scaffolds, and to use this knowledge for the production of new catalytic molecular/material hybrid catalysts (MOFcats) for the reactions discussed above. The catalysts should by structurally highly stable, and catalyze the reaction at fast rates and minimum energy input. Results from this project can contribute to our society’s transition to renewable fuel technologies.
We have explored different ways to immobilized molecular catalysts into different MOFs. This can be either be done directly during MOF synthesis by simply mixing a suitable catalyst linker with the metal salts for SBU assembly, or post-synthetically by first synthesizing a MOF which is then modified with the catalyst of choice.
Irrespective of the synthetic strategy and the catalyst of choice, we have discovered a few key design principles that are generally applicable. The first point is rooted in the fact that MOFs are crystalline materials with dimensions on the 0.1 to 100 micrometer scale. When molecular catalysts (typically about 1nm in size, i.e. at least 100 times smaller) are incorporated, it is often unclear whether these catalysts will reside primarily at the surface of the crystals, or evenly distributed throughout the material. Moreover, it is also often unclear whether all catalysts engage in turnover, or only a few catalysts at the surface, while the majority that is in the bulk of the crystals are merely spectators. We have investigated examples of both of these scenarios, and also observed secondary events such as pore clogging that slows down substrate access to catalysts in the interior of the MOFcat over time. Finally, as the catalytic reactions described above require the transport of charges, i.e. electrons for reductive, and electron holes for oxidative chemistry, charge transport through the MOFcats needs to be engineered. We have seen example where these processes are certainly limiting catalysis, while in other systems they work very well.
Over the course of the project, focus was directed more and more towards gaining understanding of transport phenomena in MOFs. Using theoretical models from related fields such as redox polymers, and translating these principles into MOFs, we have developed a profound understanding of electron hopping charge transport in MOFs. Importantly, we have contributed to the understanding that electron movement through the MOF is accompanied by translocation of counter ions. This is important to maintain overall charge neutrality.
The project draws a lot of its inspiration from naturally occurring enzymes. For example, in these enzymes, the active site is often protected from deleterious processes by being positioned deeply with in the peptide matrix, thus being physically isolated from the surrounding media. This means, in turn, that Nature also had to develop specific electron transport pathways to fuel the catalytic active sites. We have mimicked this scenario in a MOF for the first time, and have installed active sites as well as energy-matched “wires” in one MOF.
Finally, we have also shown that redox-active MOFs can be useful to interact with semiconductors. Semiconductors under illumination provide high-energy electrons, the energy of which is however often difficult to harness. We could show that redox-active MOFs can fulfill this function, and give rise to record-high photovoltages. We predict that this proof-of-concept work will be expanded to new semiconductor-MOF hybrid materials for the photoelectrocatalytic production of H2 or reduction of CO2 with minimum electrical energy input.
We have developed a profound understanding of conducting MOFs, in particular those where charges migrate by a hopping mechanism between discrete redox-active units that the MOF is composed of. The project is also at the international forefront in electrocatalysis in MOFcats.
In terms of method development that goes clearly beyond the state-of-the-art, we have pioneered the use of Rutherford backscattering spectrometry (RBS) as a means to obtain depth profiles of linkers that are post-synthetically introduced into pristine MOFs. The capacity to do so is unprecedented in the field, and we hope that this method report will ignite its use by other researchers.
Finally, and perhaps most importantly, we have learnt that the catalytic performance of MOFcats depends on an interplay between the intrinsic turnover frequency of the catalyst, substrate and product transport through the material, and charge transport (ions, electrons/holes). The latter is governed by experimental parameters such as the nature of the electrolyte and the solvent. We have also learnt that charge transport limitations can be mitigated by working with thin films and small MOF crystals.