New catalyst releases H₂ from ammonia

To facilitate and accelerate the recovery of hydrogen from ammonia, researchers from the institute for inorganic chemistry of the university Christian-Albrechts-Universität zu Kiel (CAU) have developed, in its project AmmoRef (duration 04/2021-03/2025), a more active and cost-effective catalyst together with its cooperation partners. The results of this work are retained in the hydrogen lead project (Wasserstoff-Leitprojekt) TransHyDE of the Germany ministry for education and research (BMBF). AmmoRef is one of ten TransHyDE projects that are funded by the BMBF. In it, existing technologies for hydrogen transport are to be improved.

The ability to store energy from wind or solar power is playing a central role in the clean energy transition. “The storage of energy in the form of chemical compounds such as hydrogen has many advantages. The energy density is high, and the chemical industry also needs hydrogen for many processes,” says Malte Behrens, professor for inorganic chemistry at CAU Kiel and subproject manager in AmmoRef. In addition, green hydrogen can be produced through electrolysis using electricity from renewable energy sources, without producing CO₂.

Importing hydrogen from regions where wind and solar power is cheap is, however, not easy. One possibility is the chemical conversion of hydrogen into ammonia, which itself already contains a relatively large amount of hydrogen. For the transport of ammonia over long distances, a mature infrastructure already exists. “Ammonia can easily be liquefied for transport. It is already being manufactured on a megatonne scale and shipped and traded worldwide, and is therefore interesting for us,” says chemist Dr. Shilong Chen, scientist in the Kiel AmmoRef subproject of TransHyDE. Together Chen and Behrens are exploring how hydrogen can be released from ammonia after transport.

Image taken with a transmission electron microscope: nanoscale structure of the iron-cobalt catalyst. The many bimetallic particles, visible here as dark spots, are separated from each other at the nano level by the carrier material and thus contribute to a large active surface of the catalyst.
Source: Franz-Philipp Schmidt, Thomas Lunkenbein, adaptiert: Shilong, C.et al. Nature Communications (2024), https://creativecommons.org/licenses/by/4.0/

When hydrogen is transformed into ammonia, less gas is lost than with other processes. Ammonia allows itself, according to Behrens, to be liquefied already at a pressure of eight bar. Tankers could easily be filled with it. “A major advantage over other chemical processes, such as LOHC, is that hydrogen in liquid ammonia has a very high storage density,” says Behrens.

The problem for the scientists at the start of the project was to develop a catalyst that would allow ammonia to be quickly converted into hydrogen at the target location. “Large systems are required for this,” explains Behrens. However, there is currently no industrial application for reforming ammonia on this scale.

Cobalt for the activation of iron
The researchers’ goal was to find the cheapest possible materials for the catalysis. In addition, the expected application of the catalyst should be scalable. The material ruthenium is currently the benchmark in research. Iron is, according to Behrens, however, the most cost-effective metal used. “The problem, however, is that inexpensive iron catalysts suffer from low activity due to too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. However, this limitation can be overcome by adding cobalt,” he explains. By combining two base metals – iron and cobalt – which creates highly active, bimetallic surfaces with a lower metal-nitrogen binding energy and other properties that are otherwise only known from much more expensive precious metals, the catalyst, which has a metal content of more than 70 percent, is not only highly active but also affordable.

“Highly active” means that it has a very high conversion speed. “Our catalyst reaches over 90 percent that of ruthenium and is around 20 percent more powerful than our nickel benchmark,” says Behrens. The researchers have also developed a special manufacturing method that allows a very high metal load. Up to 74 percent of the material consists of active metal particles. These alternate with carrier particles, so that cavities in the nanoscale range are created between them – like a porous, metallic nano-sponge. The structure is stable enough to withstand the high temperatures of around 600 °C associated with the decomposition of ammonia.

Previous result
By alloying iron with cobalt, the nitration of iron, which led to weak binding energy and thus lower activity, was suppressed and the nitrogen binding energy additionally influenced in such a way that the binding energies move closer to the peak of the activity volcano, which leads to a highly active and catalytic performance. It was also shown that alloying iron with other metals with weak nitrogen adsorption energy provides a simple and general approach to producing a highly active and nitride-free catalyst for the ammonia decomposition reaction.

Fig. 3: Prof. Malte Behrens and Dr. Shilong Chen in their Kiel laboratory in front of a test stand for new catalysts
Source: Julia Siekmann, Uni Kiel

Ammonia synthesis and decomposition
The production of ammonia using the Haber-Bosch process changed the world, as it enabled the production of fertilizers on an industrial scale. In 2021, 235 million tonnes of ammonia were produced, making it the highest-volume chemical produced. This production could be further increased in the near future, because ammonia due to its high hydrogen content and energy density as well as favorable infrastructure for transport and storage, as a carrier and storage material for regeneratively produced hydrogen, could help mitigate the climate crisis. In this scenario, hydrogen could be released from ammonia through its decomposition.

In contrast to ammonia synthesis, its reverse reaction, ammonia decomposition, has not found a comparable large-scale industrial application, but has been used primarily academically for over half a century to study the reaction mechanism of ammonia synthesis at ambient pressure on catalysts designed for the ammonia synthesis reaction. The most active catalysts for this synthesis are ruthenium-based, but the commercial aspect makes the less active but much cheaper iron catalysts appear more attractive. The reason for their moderate activity is the nitration. The present AmmoRef subproject was able to show how nitration can be suppressed and nitrogen binding energy, similar to ruthenium, can be achieved by alloying iron with cobalt.

The current challenge is to reduce the cobalt content. This is necessary on the one hand for cost reasons, but also because of the current political conditions under which cobalt is extracted. The prerequisites for upscaling are already there, but further measures need to be identified. In addition, it must be determined what still needs to be done to further increase the stability and activity of the catalyst. The addition of promoters, substances that increase the activity of a catalyst, is being considered.

Synthesis efforts are currently being transferred from the 1-liter to the 100-liter scale. The catalyst will now be further investigated and transferred from basic research to application. The scientists’ goal is to achieve an industrial scale for the catalyst.