A new method uses LED light, iron and copper to produce hydrogen from ammonia. In contrast to conventional ammonia cracking, expensive materials and high heat are not necessary.
Houston (USA). Hydrogen can be produced in a climate-neutral manner using electricity from renewable energy sources through the electrolysis of water. In order to be able to store and transport the hydrogen, however, it has to be cooled down considerably and liquefied or chemically bound. Liquid ammonia (NH3), which only has to be cooled to minus 40 degrees Celsius in order to bind hydrogen, is considered to be particularly suitable for this.
However, in order to split the liquid ammonia back into hydrogen and nitrogen, a temperature of 400 to 600 degrees Celsius is required. Alternatively, the reaction temperature can also be lowered with catalysts made of platinum, ruthenium, ruthenium or palladium. However, these raw materials are rare and expensive.
Iron and copper plasmonic photocatalysts
Scientists from Rice University have now developed an alternative that can be used to split ammonia using plasmonic photocatalysts. These reaction aids made of metallic nanoparticles bundle the energy of the incident light and thereby strongly stimulate electrons in the material. A second metal forms a short-term partner for nitrogen in this process, thereby enabling the bonded hydrogen to be split off.
According to their publication in the journal Science , the researchers working with Yigao Yuan have created a photocatalyst using iron as the chemical cleavage aid and copper as the light antenna. Together with ammonia, the particles were bombarded with laser pulses in a light reactor. As Hossein Robatjazi explains, this triggers ammonia cracking.
“Under the application of light, the copper-iron catalyst showed a reactivity and efficiency comparable to that of the common copper-ruthenium catalyst.”
Light reactor without laser?
The researchers then investigated whether the splitting also works on an industrial scale without a laser. They carried out their experiment in a light reactor that was 500 times larger, using LEDs with a wavelength of 470 nanometers. During operation, ammonia was continuously fed past the photocatalyst through two reaction tubes. In this experiment, too, ammonia was only split at room temperature due to the energy of the LEDs.
“The copper-iron catalyst also shows very high photocatalytic reactivity under LED lighting, with a 72 percent ammonia conversion and a yield of 14 grams of hydrogen per day.”
According to Naomi Halas, this is the first study in which hydrogen was produced from ammonia in this way.
“This is the first report in the literature showing that gram quantities of hydrogen gas can also be obtained from ammonia by photocatalysis with LEDs.”
Optimization and scaling needed
The process currently requires even more electricity from conventional ammonia crackers. However, the researchers consider it likely that optimizing and scaling the process will reduce the energy consumption. According to them, only a few kilowatt hours per kilogram of hydrogen are possible.
This would make the process suitable for a cheap and decentralized cracking of ammonia. According to Peter Nordlander, many simple reactors could produce hydrogen from ammonia instead of central cracker plants.
“Our discovery paves the way to sustainable, low-cost hydrogen produced locally rather than in centralized plants.”
Halas sees the technology as a further step in making hydrogen a cheap and clean fuel with high availability.
“Hydrogen can only become a clean fuel if it is cheap, easy to store and recover. With our approach, we are moving closer to this goal by being able to recover hydrogen from a storage medium on demand and using abundantly available materials and light emitting diodes.”
Science, doi: 10.1126/science.abn5636