A tailor-made catalyst is to help increase the potential of hydrogen for the green energy supply of tomorrow. Researchers at RWTH Aachen University, the Max Planck Institute for Coal Research in Mülheim an der Ruhr and Forschungszentrum Jülich are working on a new solution to make hydrogen, which is difficult to handle in its natural state, more usable.
This is how formic acid works as a storage system for hydrogen: carbon dioxide (CO2) reacts with green hydrogen (H2) using a catalyst to form formic acid (HCOOH). This allows hydrogen to be stored, stored and transported in the long term and with less effort. Before being converted back into electricity, the hydrogen is released from the formic acid using a catalyst. The separated carbon dioxide is then recycled to be combined with hydrogen again and formic acid is formed
Hydrogen is highly volatile and has a very low volumetric density. The basic idea is to use larger molecules that contain hydrogen. They can be stored, stored and transported with less effort. This gives users the opportunity to make better use of hydrogen's strengths as a climate-friendly storage medium for large quantities of energy or as a feedstock in industry.
There are already a number of well-researched molecules such as methanol that can be used as carriers for hydrogen. The group from Aachen, Mülheim an der Ruhr and Jülich is focusing on formic acid compounds as carrier molecules, for which they have developed a catalyst. The catalyst has the important task of enhancing dehydrogenation, i.e. the release of hydrogen from the larger molecule.
The newly developed catalyst is based on a compound of ruthenium and phosphorus. In the laboratory, it has shown that it can also permanently release hydrogen from formic acid without losing its effectiveness. In the next step, the team wants to demonstrate this property for methyl formate, which is similar to formic acid, and thus harness the beneficial properties of the molecule. In particular, methyl formate releases the stored hydrogen faster than other carrier molecules such as methanol. The team's study has been published in the Journal of Catalysis. A team from the Leibniz Institute for Catalysis (LIKAT) in Rostock described in a publication last year that methyl formate can be a worthwhile target. Methyl formate can be produced ina CO2-neutral way, it is non-toxic in contrast to ammonia and methanol and, with the right catalyst, it releases hydrogen 20 times faster than methanol. It is produced from methanol and formic acid.
The LIKAT team achieved its results using so-called homogeneous ruthenium-based catalysis. This means that the ruthenium catalyst and the methyl formate both take part in the reaction in the same phase. In this case, both are in a liquid state. Homogeneous catalysis poses challenges for dehydrogenation on an industrial scale because the catalyst molecules are difficult to separate from the liquid if they lose activity, for example. Heterogeneous catalysis, on the other hand, uses solids that can be easily separated from liquids and gases. This is a major advantage for technical processes.
"In our study, we have demonstrated a solid catalyst that may have the potential to remain active during the release of hydrogen because it is not entrained in the liquid," says Prof. Regina Palkovits, Director at the Institute for Sustainable Hydrogen Economy in Jülich and Chair of Heterogeneous Catalysis and Technical Chemistry at RWTH Aachen University.
Regina Palkovits and her team have used this type of heterogeneous catalysis for the release of hydrogen from formic acid, in which the formic acid is still present in liquid form, but the ruthenium takes part in the reaction in a solid environment. Here, too, the challenge is to prevent the catalyst from being deactivated. "We want to ensure that our catalyst does not get carried away and does its job in the long term," explains Regina Palkovits.
The research team has therefore added phosphorus to the ruthenium as a stabilizer. In this way, the ruthenium atoms retain their position instead of dissolving, clumping together and thus losing parts of their reaction surface. "We were thus able to achieve a constant gas flow of released hydrogen in the laboratory over the test period of two and a half days," says chemist Sebastian Seidel, describing the results of the ruthenium catalyst stabilized with phosphorus.
In the laboratory, the team initially worked with formic acid, which can also be used as a hydrogen carrier, but with two atoms it binds less hydrogen than methyl formate with four. Formic acid and methyl formate belong to the same group of molecules and have similar properties. "Formic acid is the simplest test molecule for our catalyst platform. However, the results we have achieved here suggest that our catalyst made of ruthenium and phosphorus can also be adapted for the dehydrogenation of methyl formate. We want to use this for other hydrogen carrier molecules in the future," explains Seidel.
Original publication:
Porous BINAP-based polyaromatic polymers for the continuous base-free Ru-catalyzed decomposition of aqueous formic acid, by Sebastian Seidel, Isabella Kappel, Claudia Weidenthaler, Peter J. C. Hausoul, Regina Palkovits
Journal of Catalysis, Volume 438,October 2024, 115712
