Study reveals that cobalt ions, rather than the covalent organic skeleton itself, promote catalytic activity

Covalent organic frameworks (COFs) are less stable catalysts than previously thought, but they maintain high activity. COFs are promising designer catalysts, including in the sustainable production of chemicals and fuels. Their properties, both in terms of molecular structure and chemical composition, can be tailored very specifically to catalyze desired reactions based on precise tunability.

However, researchers from the Max Planck Institute for Solid State Studies (MPI-FKF) and the Institute for Sustainable Materials Research (MPI-SusMat) at the Ruhr-University Bochum in Germany have found that the catalytic activity is not generated by the COFs themselves. I showed that. Instead, the cobalt ions break off the scaffold and turn into oxide nanoparticles that actually promote catalysis. The research team describes these results in the journal Advanced Science.

“The knowledge gained from this study will allow us to design catalysts from organic frameworks and nanoparticles that are significantly more efficient than previously designed COFs,” said Ruhr-University Bochum and the RESOLV Cluster of Excellence. says Professor Kristina Tschulik. We developed the research idea in collaboration with Professor Bettina Lotsch from MPI-FKF.

“As an electrochemist, I’ve always been a little curious about how the catalytic activity of COFs actually occurs,” says Tschulik.

Is it stable under harsh reaction conditions?

Tschulik’s Bochum-based group has begun a collaboration with Stuttgart researchers led by Lotsch, an expert in COF synthesis. Both teams are part of the joint research center 1333, based at the University of Stuttgart.

Researchers Pouya Hosseini, Andrés Rodríguez-Camargo, and Liang Yao analyzed the catalytic activity of several cobalt-containing COFs in so-called oxygen evolution reactions. This reaction occurs in many industrially important reactions, such as the electrolysis of water to produce hydrogen.

“The reaction conditions for the oxygen evolution reaction are harsh,” explains Churik. “In reality, the only catalyst that remains stable is iridium oxide.” However, a growing body of research is reporting that COFs are long-term stable in this reaction as well.

In a first step, the research team electrochemically analyzed the COF during the oxygen evolution reaction. In fact, the conversion occurred with high activity over several cycles. However, Tschulik had already seen the recorded current-potential curves in a different context.

As part of Collaborative Research Center 247, scientists have been studying cobalt oxide nanoparticles as catalysts to produce precisely this curve shape for seven years. So the group, with help from a team of MPI-SusMat researchers led by electron microscopy expert Christina Scheu, began characterizing more complex materials.

Scaffold prevents nanoparticles from aggregating

These analyzes showed that cobalt oxide nanoparticles are formed from cobalt-containing scaffold compounds and take over the catalytic activity. This conversion occurs as soon as the electrode is immersed in a basic solution.

“However, the porous COF scaffold still serves an important purpose,” Tschulik said, explaining another result of the analysis. “They provide a suitable reaction environment and hold the nanoparticles in place. Normally, the particles tend to aggregate, which means less catalytic surface is accessible and the nanoparticles become inactive. That means.”

In their publication, the authors also propose a future method for the targeted production of catalyst-supported COFs to maintain stability and catalytic activity even under harsh reaction conditions. Masu.