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
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 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 utilise 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 ofCatalysis.
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.
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. Photo: 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.
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