A stable catalyst for the future hydrogen economy
This is how formic acid works as a hydrogen storage system: carbon dioxide (CO2) reacts with green hydrogen (H2) with the help of a catalyst to form formic acid (HCOOH). This allows hydrogen to be stored, kept and transported over the long term with less effort. Before reconversion, the hydrogen is released from the formic acid with the help of a catalyst. Regina Palkovits and her team have been working on optimising this catalyst. The captured carbon dioxide is then recycled to be recombined with hydrogen to form formic acid. Hydrogen is green if the water has previously been broken down into its constituent parts, hydrogen and oxygen, using electricity generated from renewable sources. Graphic: Forschungszentrum Jülich/Reisen
A tailor-made catalyst could help to boost the potential of hydrogen for tomorrow’s green energy supply. Researchers at RWTH Aachen, 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.
Hydrogen is highly volatile and has a very low volumetric density. The basic idea is to use larger molecules that contain hydrogen. These can be stored and transported with less effort. This will enable users to make better use of hydrogen’s strengths as a climate-friendly way to store large amounts of energy or as a raw material in industry.
There are already a number of well-researched molecules such as methanol that can be used to store hydrogen. The group from Aachen, Mülheim an der Rur and Jülich has set its sights on formic acid compounds, for which it has developed a catalyst. The catalyst has the important task of enhancing the dehydration, i.e. the release of hydrogen from the larger molecule.
Published in the Journal of Catalysis
The fact that methyl formate can be a worthwhile target was described in a publication by a team from the Leibniz Institute for Catalysis (LIKAT) in Rostock last year. Methyl formate can be produced in a 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 had achieved its results with the help of so-called homogeneous catalysis based on ruthenium. This means that the catalyst ruthenium and the methyl formate both participate in the reaction in the same phase. In this case, both are in a liquid state. For dehydration on an industrial scale, homogeneous catalysis poses challenges because the catalyst molecules are difficult to separate from the liquid, for example, when they lose activity. 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.
Catalyst remains active
‘In our study, we have demonstrated a solid catalyst that can have the potential to remain active during the release of hydrogen because it is not drawn into the liquid,’ says Prof. Regina Palkovits, director at the Institute for Sustainable Hydrogen Economy in Jülich and head of the Chair of Heterogeneous Catalysis and Technical Chemistry at RWTH Aachen University.
Regina Palkovits and her team have used such a heterogeneous catalysis for the release of hydrogen from formic acid, in which the formic acid remains 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 over the long term,’ explains Regina Palkovits.
Prof. Regina Palkovits’ research is in the field of heterogeneous catalysis. Foto: Forschungszentrum Jülich/Jansen
Adjust to methyl formate
That is why the research team added phosphorus to ruthenium as a stabiliser. This way, the ruthenium atoms retain their position instead of dissolving, clumping together and thus losing parts of their reaction surface. ‘We were able to achieve a constant flow of released hydrogen in the laboratory over the two-and-a-half-day test period,’ says chemist Sebastian Seidel, describing the results of the phosphorus-stabilised ruthenium catalyst.
In the laboratory, the team initially worked with formic acid, which is also a potential hydrogen carrier but binds two atoms of hydrogen less than methyl formate with four. Formic acid and methyl formate belong to the same group of molecules and have similar properties. ‘For our catalyst platform, formic acid is the simplest test molecule. However, the results we have obtained here suggest that our ruthenium-phosphorus catalyst can also be adapted for the dehydrogenation of methyl formate. We want to test this for other hydrogen carrier molecules in the future,’ explains Seidel.