Catalysis: molecular “crown” improves CO₂ conversion

Transforming carbon dioxide (CO₂) into useful chemicals is one of the central challenges of sustainable energy technologies. While CO₂ is widely accessible and an attractive, inexpensive C1 feedstock, its high thermodynamic and kinetic stability makes it challenging to activate. A team led by LMU chemist Professor Ivana Ivanović-Burmazović, member of the e-conversion Cluster of Excellence, and Professor Ulf-Peter Apfel (Fraunhofer Institute for Environmental, Safety and Energy Technology UMSICHT) has demonstrated how this process can be significantly improved through the use of so-called crown ethers.

Rethinking catalysis

Many approaches to CO₂ conversion rely on solid-state catalysts made from metals such as gold, silver, or copper. Although these materials can be effective, they are often expensive and rely on scarce resources. In contrast, the LMU team pursues a molecular approach using so-called metalloporphyrins (which are structurally related to the heme group in hemoglobin) as catalysts.

In these systems, the cobalt center is responsible for converting CO₂ into carbon monoxide (CO), an important building block in the chemical industry. “However, the efficiency of catalysis depends not only on the active site, but also on its surrounding chemical environment,” explains Ivanović-Burmazović. “The local electrostatic environment plays a crucial role in stabilizing reaction intermediates and lowering the energy required for the reaction.”

Small changes, big effects

To influence this environment, the researchers introduced crown ethers – ring-shaped molecules of carbon and oxygen atoms that surround a metal ion like a crown – near the catalytic center. Crown ethers selectively bind positively charged ions (cations), thereby modifying the local electric field around the cobalt atom.

This second coordination sphere, as it is called, influences the electron distribution within the system and steers the reaction more efficiently toward the desired product: the conversion of CO₂ into CO.

Electrochemical experiments showed that the modified catalyst requires less energy while also exhibiting improved selectivity. “Through the targeted placement of the crown ether, we can control the reaction with significantly greater efficiency, such that it produces more of the desired product and fewer side products,” says Christian Wilhelm, doctoral researcher at LMU and one of the lead authors.

From concept to application

To test whether this concept also works under more realistic conditions, the researchers integrated the molecular catalyst into a so-called zero-gap electrolyzer. In this setup, electrodes and membranes are placed in direct contact, allowing gases like CO₂ to efficiently reach the catalytic surface.

Under these conditions, the system achieves high selectivity (96%) for CO production at moderate current densities, and a Faradaic efficiency of up to 43% at technologically relevant currents. According to the authors, this places the approach among the most efficient non-precious molecular catalysts in such devices.

Prospects for sustainable catalysis

Although noble-metal catalysts can still reach higher absolute efficiencies, the new approach offers an important advantage: sustainability. Cobalt is far more abundant and less expensive than metals like gold or silver.

Furthermore, the study demonstrates a general design principle: Even small modifications in the molecular surroundings of a catalyst, particularly changes in local charge, can have a large impact on performance. The authors are convinced that this concept could be extended to other catalytic systems and open up new avenues for efficient and sustainable energy technologies.

W. Wiesner et al.: Heavy is the Crown: Crown Ether Modulation of Cobalt Porphyrin CO2 Electroreduction in Zero-Gap Electrolyzers. Angewandte Chemie 2026