Lasers liberate hydrogen from ammonia water

Laser pulses that can shred ammonia molecules dissolved in water may offer a way to generate hydrogen gas without using heat, pressure, or catalysts (J. Am. Chem. Soc. 2024, DOI: 10.1021/jacs.3c13459). The proof-of-principle work by researchers at Sun Yat-sen University demonstrates an unusual way to exploit ammonia as a carrier for green hydrogen, an idea that is quickly gaining traction in many industries.

Most hydrogen is currently made by steam methane reforming, an energy-intensive process that releases huge amounts of carbon dioxide. But “green” hydrogen is produced by electrolyzing water with renewable electricity, and it can be used to decarbonize various industrial processes, to power fuel cells in vehicles, and even as a means to store excess renewable energy.

Delivering that hydrogen to where it is needed is tricky and expensive, not least because it is explosive and flammable. It can be transported as a highly compressed gas or as a liquid at –253 °C, both of which are relatively expensive options for a fuel.

Many companies are instead turning to ammonia as an easier way to ship hydrogen. Ammonia is much easier to liquefy—it can be stored at just –33 °C or at about 7 atmospheres of pressure—and it is richly packed with hydrogen atoms, which can be liberated by ammonia decomposition reactions.

Unfortunately, the most common ammonia decomposition method involves thermal crackers that operate at over 850 °C using nickel on aluminum oxide as the catalyst. Alternative methods, including electrochemistry or photochemistry, typically require expensive precious-metal catalysts. (Ind. Eng. Chem. Res. 2021, DOI: 10.1021/acs.iecr.1c00843).

The Sun Yat-sen researchers have now taken a completely different approach to recovering hydrogen from ammonia. They use laser pulses to cause brief yet intense heating within an ammonia-water solution held inside a purpose-built reactor. The laser delivers 10 pulses per second to a 1-mm-wide spot within the liquid. Each pulse lasts just 10 nanoseconds and delivers enough energy to instantaneously raise the temperature of the target spot tens of thousands of degrees, turning the molecules there into a bubble of plasma. This plasma consists of a soup of radicals, including ^•H, ^•OH, and ^•NH₂, which quickly react to form hydrogen, nitrogen, and nitrogen oxides.

The bubble cools and collapses almost immediately, releasing the newly formed gases. These are picked up by a stream of argon flowing through the reactor and passed through dilute acid and alkali solutions to remove any ammonia and nitrogen oxides. “Using a laser with a higher pulse energy and higher pulse frequency increases the hydrogen yield,” says Bo Yan, a member of the team.

“It’s quite an interesting finding,” says Jordi Llorca of the Technical University of Catalonia, who develops catalysts for clean energy applications and has studied ammonia decomposition reactions. “This is a way to activate chemical bonds, by putting in energy very locally in a very short time.”

However, Llorca points out that the energy that would be gained from burning the hydrogen is less than half the amount of energy the laser uses to produce it, making it impractical for industrial use. “You have to invest double the energy than you can get from this hydrogen,” he says. In contrast, hydrogen made by conventional thermal ammonia cracking carries about four times as much energy as is spent during the process (ACS Sustainable Chem. Eng. 2017, DOI: 10.1021/acssuschemeng.7b02219).

To overcome this drawback, the Sun Yat-sen researchers suggest that their new process could instead be powered by a solar-pumped laser. These devices, which are still in development, effectively turn sunlight into laser light, potentially offering a cheap source of energy for the reaction (Commun. Phys. 2020, DOI: 10.1038/s42005-020-0326-2). But for now, the researchers are optimizing their laser-based system and investigating whether it could be used to drive other chemical reactions.