Use carbon dioxide as a raw material

Chemists from TU Berlin led by Prof. Dr. Peter Strasser in the field of "Electrochemical Catalysis and Materials" were inspired by biocatalysis to develop a new catalyst for carbon dioxide reduction.

Carbon dioxide (CO2) is widely known as a climate-damaging exhaust gas. The recurring question is whether and how this gas could also be used as a raw material. This is a topic of interest, among other industries, that require large quantities of carbon monoxide (CO) for the production of polycarbonate or polyurethane. These are thermoplastic plastics used, for example, in the manufacture of CDs, glasses, and protective lenses (polycarbonate) or foams, dashboards, and foam materials (polyurethane).

"The carbon monoxide needed in the production process is so far obtained from methane, a fossil fuel," explains Prof. Dr. Peter Strasser from TU Berlin. "A process that consumes not only fossil fuels but also produces additional carbon dioxide. The question was whether it is absolutely necessary to obtain carbon monoxide from methane or if there is also a way to efficiently produce it from CO2? The answer is: yes, electrochemically it is possible, and this process is currently being intensively studied, among others by industry partners like Covestro. The problem: the best known catalyst for this electrolysis so far is still relatively non-specific and additionally requires gold or silver in its reactive center – making it relatively expensive."

In a joint project funded by Horizon2020 involving the Technical University of Dresden, Ruhr University Bochum, University of Copenhagen, and TU Berlin as the consortium leader, the working group around Peter Strasser focused on a bio-inspired catalyst whose active center is modeled after the active center of hemoglobin, called porphyrin. It contains four nitrogen atoms in its active center and a metal atom in the middle. This active center was manufactured as a solid. There were already theoretical predictions that these so-called porphyrin motifs could electrochemically not only reduce oxygen but also very selectively convert carbon dioxide into carbon monoxide. One of the few by-products is, for example, hydrogen. The central metal plays a crucial role. It binds the CO2 molecule and converts it through various intermediate steps into carbon monoxide. The efficiency of this CO production largely depends on the metal in the catalyst's active center.

"We investigated, among others, nickel and iron as the central atom in the catalyst. Nickel, for example, is a particularly interesting derivative because it binds carbon monoxide only weakly and releases it relatively easily as a gas. If the active center contains iron instead, the initial production of carbon monoxide is higher than with nickel, but the CO is bound much more strongly. As a result, the catalyst becomes blocked much faster. Comparing the different catalyst derivatives, we were able to demonstrate that at least in the laboratory scale, a 99 percent carbon-based catalyst with nickel in its active center produces carbon monoxide from carbon dioxide more effectively and selectively than the known gold and silver catalysts," describes Peter Strasser the experimental results.

While these catalysts have so far been tested in the laboratory scale, they are now being produced in gram quantities and tested at the industry partner Covestro in a mini-test plant.

Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2
Wen Ju, Alexander Bagger, Guang-Ping Hao, Ana Sofia Varela, Ilya Sinev, Volodymyr Bon, Beatriz Roldan Cuenya, Stefan Kaskel, Jan Rossmeisl & Peter Strasser
Nature Communications 8, Article number: 944(2017), DOI:10.1038/s41467-017-01035-z