Production of hydrogen and fertilizer at the same time

A research team from the Ruhr University Alliance, Germany, has found a catalyst that can be used to convert ammonia into the energy-carrying hydrogen and fertilizer precursor nitrite. Hydrogen production and fertilizer production have until now been separate chemical processes.

With the new approach, the team from the Ruhr University in Bochum and the University of Duisburg-Essen demonstrates that the two can be combined on a laboratory scale. The Bochum-based group led by Ieva Cechanaviciute and Professor Wolfgang Schuhmann reports on the results, with Bhawana Kumari and Professor Corina Andronescu from the University of Duisburg-Essen, in the journal Angewandte Chemie International Edition on June 23, 2024.

Hydrogen can be produced by splitting water (H₂O) to hydrogen (H₂) and oxygen (O₂) using electricity. To make this process sustainable, the energy must come from renewable sources.

“This can only be done in a place where there is a lot of space for wind power and a lot of sun for photovoltaics, for example in Namibia,” explains Schuhmann.

To build a hydrogen-based economy in Germany, it must be imported from distant countries. The crux of the matter is that it takes a lot of energy to liquefy hydrogen for transport, as it only becomes liquid at extremely low temperatures of minus 253 degrees Celsius or high pressures.

Ammonia is transported more easily than hydrogen

Therefore, alternative concepts envisage the conversion of hydrogen into ammonia at the production site, as it becomes liquid at minus 33 degrees Celsius. It also has a higher energy density.

“A tank full of liquid ammonia would carry about 2.5 times more energy than a tank full of liquid hydrogen,” explains Schuhmann.

Finally, the ammonia would have to be converted back to hydrogen at the point of use. This is usually done using the reverse Haber-Bosch reaction, in which ammonia (NH₃) turns into nitrogen (N₂) and hydrogen (H₂). Of the two products, however, only hydrogen can be used profitably.

Double the hydrogen yield

“Therefore we had the idea of ​​combining the reverse Haber-Bosch reaction with a second electrolysis of water to produce a product that can easily be used for the production of fertilizers, such as nitrite or nitrate, instead of nitrogen,” explains Ieva Cechanaviciute .

In this reaction, ammonia (NH₃) and water (H₂O) are consumed to produce nitrites (NO₂^–) and hydrogen (H₂). In contrast to the reverse Haber-Bosch reaction, hydrogen production is doubled and instead of unusable nitrogen, mainly nitrite is produced, which can be further processed into fertilizer.

For the reaction, the team used gas diffusion electrodes into which ammonia can be introduced as a gas. “This had never been done before,” explains Schuhmann. “Ammonia was always used in dissolved form.”

Crossing the thermodynamic canyon

A challenge for the researchers was to find a suitable catalyst with which to realize their idea. This is because the starting material NH₃ tends to convert to nitrogen because of the very strong nitrogen-nitrogen triple bond rather than to nitrite.

“At first we had to connect this huge thermodynamic canyon,” explains Cechanaviciute. In previous work, the team had already experimented with multi-metal catalysts, which proved suitable for this purpose. They were able to convert 87% of the transferred electrons into nitrites. The team also managed to avoid oxygen as an undesirable byproduct of water electrolysis.

“Our work shows that our Gedanke experiment can work in principle,” summarizes Schuhmann. “But we are still a long way from technical implementation on an industrial scale.”