How catalysts secretly lose their stability

How catalysts secretly lose their stability

Covalent organic framework compounds are more active than one would expect as catalysts. Researchers from the Ruhr University Bochum and the Max Planck Institutes for Solid State Research (MPI-FKF) and Sustainable Materials (MPI-NM) recently decoded what can be the reason.

Covalent organic framework compounds are significantly less stable than previously thought and yet active as catalysts. The so-called COFs are considered promising catalysts, for example for the sustainable production of chemicals and fuels. Their properties can be set very specifically due to their molecular structure. However, researchers from the Ruhr University Bochum and the Max Planck Institutes for Solid State Research (MPI-FKF) and Sustainable Materials (MPI-NM) showed that the catalytic activity does not arise from the COF itself. Instead, the cobalt ions detach from the scaffold and transform into nanoparticles that do the catalytic work. The results are described in the journal “Advanced Science”, published online on 26. November 2024.

“With the knowledge from this study, we will be able to design catalysts from organic framework compounds and nanoparticles that are significantly more efficient than COFs,” says Prof. Dr. Kristina Tschulik from the Ruhr University Bochum and the Cluster of Excellence RESOLV, who together with Prof. Dr. Bettina Lotsch from the MPI-FKF had the idea for the study. “As an electrochemist, I have always wondered a bit about where the catalytic activity of COFs actually comes from,” says Kristina Tschulik – she wanted to know more about it.

Stable under harsh reaction conditions?

The Bochum group led by Kristina Tschulik started a cooperation with Stuttgart researchers around Bettina Lotsch, who are experts in the synthesis of COFs. Both groups are part of the Collaborative Research Centre 1333, which is based at the University of Stuttgart. Together, the team led by Pouya Hosseini, Andrés Rodriguez Camargo and Liang Yao analysed the catalytic activity of several cobalt-containing COFs in the so-called oxygen development reaction. This partial reaction occurs in many industrially significant reactions, for example in the electrolysis of water for hydrogen production. “The reaction conditions in the oxygen development reaction are harsh,” explains Kristina Tschul. “Actually, there is only one catalyst – iridium oxide – which remains stable.” However, studies increased, which reported that cobalt-containing COFs are also long-term stable in the reaction.

In the first step, the research team analysed the COFs electrochemically in the oxygen development reaction. In fact, the fabric conversion with high activity ran over several cycles. Kristina Tschulik had ever seen the recorded current potential curves in a different context. As part of the Collaborative Research Centre 247, the scientist has been working with cobalt oxide nanoparticles as catalysts for seven years, which produce exactly this curve form. Therefore, the group started a more elaborate material characterization, together with electron microscopy experts from the MPI-NM led by Christina Scheu.

Scaffolding preventing lumps of nanoparticles

These analyses showed that oxidic cobalt nanoparticles are formed from the cobalt-containing frameworks, which are taking over the catalytic work. The conversion takes place immediately when the electrode is immersed in the alkaline solution. “The COF scaffolds still serve a purpose,” explains Tschulik, which explains another result of the analyses. “They hold the nanoparticles. Normally, the particles tend to aggregate, making it less accessible from their catalytic surface.”

In their publication, the authors also give suggestions on how COFs could be produced in a targeted manner in the future, so that the scaffold compounds remain stable and catalytically active even under real reaction conditions.