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

Periodic Reporting for period 2 - MOFcat (Fundamental and Applied Science on Molecular Redox-Catalysts of Energy Relevance in Metal-Organic Frameworks)

Reporting period: 2018-07-01 to 2019-12-31

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 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 catalyst 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. To the best of our knowledge, we have reported the MOFcat with the highest so-called apparent diffusion coefficient, i.e. the fastest electron transport mechanism, reported to-date.
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.

The project is on a good projection, and continued knowledge gain is expected for the second half of the funding period. We have recently learnt to control one parameter that is most likely of vital importance for the performance of the MOFcat, namely the size of the MOFcat crystallites. If the MOFcats are grown on surface, it is possible to control MOFcat thicknesses by a so-called layer-by-layer approach, where the substrate is alternately exposed to solutions of SBUs and linkers. Using this method, the film thickness is controlled by the number of exposure cycles. We have built an automated system for this method, and are now in a position to control film thickness of MOFcats in the future.

In general, the catalytic performance of MOFcats depends on an interplay between the intrinsic turnover frequency of the catalyst, substrate and product transport through the material, charge transport (ions, electrons/holes) and finally the thickness of the films. In the first half of the funding period, we have gathered substantial knowledge on all of these factors as well as we developed crucial infrastructure like the RBS depth profiling method. We are now in a very good position to bring together all of these factors, and to assemble highly active catalysts that are based on abundant and cheap first row transition metals.