How to develop bio-inspired catalysts

Learning from 3 billion years of evolution

Catalysis is a process in which certain molecules (catalysts), are used to accelerate reactions and thus transform substances. “In contrast to nature, chemists often use rare metals as catalysts, an unsustainable source for processes at a global scale," Mougel outlines the problem. Most building blocks for chemical production are currently derived from fossil sources, presenting environmental challenges, including the problematic accumulation of carbon dioxide and nitrates. Electrochemistry is an appealing option to sustainably convert back these troublesome molecules.

“A key element of this approach is designing new electrocatalysts that enable this transformation with high activity and selectivity, and, for sustainability, only use earth-abundant elements." Here, Nature’s ingenuity shows the way: "For over three billion years, nature has been developing enzymatic catalysts to efficiently utilize abundant molecules like N₂ and CO₂, essential compounds for constructing complex molecules and materials”, Mougel states enthusiastically, "we can exploit this and develop bio-inspired catalysts that can help to solve our most pressing problems."

Creating artificial leaves and bio-inspired CO₂ reduction

For his purpose, Mougel and his group follow two approaches: first, they try to replicate the structure of the active sites of enzymes; second, they imitate functions found in enzymes, striving to reproduce these functions without limiting themselves to structures found in nature.

Furthermore, the group developed efficient catalysts for the reduction of CO₂ to formic acid, an important industrial compound. To do this, the group mimicked the active site of the enzyme carbon monoxide dehydrogenase (CODH), which contains two metals (Mouchfiq et al. 2020).

Controlled production of metal hydrides

Recently, the group focused on a key feature of enzymatic systems: electron transfers. In nature, electron transfers are typically mediated by iron-sulfur clusters. These clusters are essential for most living organisms, being involved in processes such as photosynthesis, mitochondrial energy production, and DNA replication. "They act as natural electrical wires and transfer electrons across the protein structures, while also being involved in proton transfers and in the activation of small molecules".

Synthetic iron-sulfur clusters could be exploited to design better electrocatalytic systems, Mougel explains: "We could prove, for example, that if we combine known catalysts for the reduction of carbon dioxide with iron-sulfur clusters, we could not only strongly enhance their catalytic activity, but also completely change their selectivity”.

The group demonstrated that the clusters are promoting a so-called concerted proton-electron transfer (CPET), where a proton and an electron are stored and transferred simultaneously from the cluster to a substrate. Mougel and his group succeeded for the first time in producing a metal hydride in a controlled manner in this way and use this hydride for the conversion of CO₂ to formic acid (Dey et al. 2022). This constituted the first experimental demonstration of that important concept that is expected to have broad implications for electrocatalysis, since metal hydrides are central intermediates in many catalytic transformations.

Mimicking the electrical wires of nature

Such examples show: Understanding natural systems is key. This is why in Mougel’s laboratory, application and basic research always go hand in hand. The group has also investigated the fundamental redox properties of iron-sulfur clusters in detail. "What’s exciting is the following: if you want to convert CO₂ into industrially useful compounds such as long-chain hydrocarbons, ethylene, or ethane, up to 14 electron reductions are necessary.

However, all biological CO₂ reductase enzymes are restricted to two electron processes," Mougel explains, "but the nitrogenase enzyme complex has an iron protein with an iron-sulfur cluster that can, in principle, bypass this limitation, although this does not occur in biological systems. However, an artificial model to study this in more detail had been missing until now."

Mougel and his group have succeeded for the first time in isolating and stabilizing such extremely reduced iron-sulfur, and finally synthesizing and characterizing a complete series of so-called iron-sulfur cubane redox clusters in all oxidation states (Grundwald et al. 2022). This allowed a thorough analysis of the varying structural and electronic properties.

In the next step, the group was able to show that even small changes in the environment of these clusters can have a major impact on their dynamics and redox potentials (Grundwald et al. 2023). This allows the generation of extreme reducing potentials – so facilitated oxidation and reduction – in situ and on-demand (gating concept).

For his research, in particular for his contributions in the field of iron-sulfur clusters, Victor Mougel is being awarded the 2023 Ruzicka Prize – an honor that means a lot to him, but which, as he emphasizes, he is accepting only as a representative of many minds. "I would like to thank my team, because ultimately, none of these research successes would have been possible without their commitment. The people in my group are the driving force behind this research and an important motivation for me to continue setting up projects together.