How fine bubbles lead to more efficient catalysts

Newly discovered effect boosts productivity of green hydrogen transport processes

Jülich/Erlangen, 17 November 2022 Catalysts are of utmost relevance for the production of chemicals and energy storage with hydrogen. Researchers at the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN) and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have now discovered that the formation of bubbles in the pores of a catalyst can be essential for its activity. Their findings help to optimize catalyst materials for reactions in which gases are formed from liquids and which play a central role in a future green hydrogen economy.

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Video: Experiment: Activation of H₂ bubble formation in catalyst pellets by light pressing with a metal rod (Length1:19 min.)

Catalysts accelerate chemical reactions without being consumed by themselves. A large number of reactions in nature and industry only work thanks to their help. For example, catalysts are used in around 80 percent of all chemical production processes and also play an important role in technologies for the chemical storage of hydrogen.

Mit feinen Bläschen zu effizienteren Katalysatoren
Catalyst pellet for hydrogen release
HI ERN / Thomas Solymosi

In most cases, so-called heterogeneous catalysts are involved, which are in a different aggregate state than the actual reaction partners. Solid, porous catalysts are particularly important here, as they can be separated very well from liquid or gaseous reaction products.

Researchers at HI ERN and FAU have now discovered: The productivity of this type of catalysts for gas generation reactions can be markedly increased if gas bubbles form particularly easily in the catalyst pores.

Mit feinen Bläschen zu effizienteren Katalysatoren
Prof. Dr. Peter Wasserscheid
Forschungszentrum Jülich / Sascha Kreklau

"This additional factor, which significantly determines the reaction rate, was previously unknown. Until now, it was assumed that the reaction rate was determined only by the chemical surface reaction or by the transport of molecules to the active centers of the catalyst," explains Prof. Dr. Peter Wasserscheid, director of the Helmholtz Institute Erlangen-Nürnberg, a branch of the Jülich Research Center, and head of the Chair of Chemical Reaction Engineering at FAU.

Increasing productivity by a factor of 50

The discovery was made using a reaction that could play a key role in the transport of green hydrogen in the future. To this end, hydrogen is stored and transported bound to a liquid carrier medium - in this case LOHC ("liquid organic hydrogen carrier") - and later released from it.

Mit feinen Bläschen zu effizienteren Katalysatoren
Liquid carrier LOHC for easy and safe transport of hydrogen
Forschungszentrum Jülich

The technology is considered to be extremely safe and easy to handle. The faster hydrogen can be released from the carrier medium with the aid of a catalyst, the more compact and powerful that technology can be used.

The researchers at HI ERN and FAU were able to show that 50 times more hydrogen is released from the carrier medium per unit of time under the same conditions when the formation of gas bubbles in the pores of the catalyst is induced in the process.

The reason for the enormous difference: "Normally, the system produces only dissolved hydrogen during catalytic hydrogen release. Saturation then quickly sets in in the liquid phase around the active centers of the catalyst," says Peter Wasserscheid.

How fine bubbles lead to more efficient catalysts
Experimental setup
HI ERN / Thomas Solymosi

The bubbles in the catalyst pores, on the other hand, act like tiny pumps. They help to remove the released hydrogen. "Once a bubble has formed in a catalyst pore, the growing bubble collects the formed hydrogen. When the bubble then detaches into the surrounding liquid, the loaded hydrogen carrier flows back into the pore and the process starts all over again," Peter Wasserscheid explains the principle.

The formation of bubbles, known as nucleation, can also be induced artificially, for example by chemically modifying the catalyst surface or by a mechanical stimulus. The findings shed new light on performance-limiting factors in heterogeneous catalysis, which are of very high relevance especially for the green hydrogen economy of the future.

The results were obtained in collaboration between the teams of Professors Jens Harting, Matthias Thommes, Nicolas Vogel and Peter Wasserscheid in the DFG Collaborative Research Program 1452 "Catalysis at liquid interfaces" and have just been published in the prestigious journal Science Advances.

Schematic view of the "bubble pump“

Mit feinen Bläschen zu effizienteren Katalysatoren
A) Inhibited state without bubble nucleation, diffusive transport of dissolved H2. (B to E) Active state with bubble formation and different oscillating states of convective transport.
Thomas Solymosi et al., Science Advances, DOI: https://doi.org/10.1126/sciadv.ade3262 (CC BY 4.0)

Original publication

Thomas Solymosi, Michael Geißelbrecht, Sophie Mayer, Michael Auer, Peter Leicht, Markus Terlinden, Paolo Malgaretti, Andreas Bösmann, Patrick Preuster, Jens Harting, Matthias Thommes, Nicolas Vogel, Peter Wasserscheid
Nucleation as a rate-determining step in catalytic gas generation reactions from liquid phase systems
Science Advances (16 November 2022), DOI: 10.1126/sciadv.ade3262

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    Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN)

    The Helmholtz Institute Erlangen-Nürnberg for Renewable Energy (HI ERN) investigates and develops material- and process-based solutions for a climate-neutral, sustainable, and cost-effective utilization of renewable energies. The institute works on the structural and functional characterization, modeling, and processing of materials relevant to hydrogen and solar technology. The common goal is the understanding of structure-performance-process relationship on the mesoscale.

    The HI ERN was founded on 20 August 2013 as a branch office of Forschungszentrum Jülich in order to facilitate close cooperation between Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) and Helmholtz-Zentrum Berlin (HZB).

    The collaboration will enable research and development of material- and process-based solutions for climate-neutral, sustainable and cost-effective utilization of renewable energies. The collaboration relates to the areas of innovative materials and processes for photovoltaic energy systems and hydrogen as a storage and carrier medium for CO2-neutral energy. The collaboration relates to the areas of innovative materials and processes for photovoltaic energy systems and hydrogen as a storage and carrier medium for CO2-neutral energy.

    More information about HI ERN: https://www.hi-ern.de/en

    Last Modified: 21.11.2022