Rice University researchers designed a key light-activated nanomaterial for hydrogen economy. Using only inexpensive raw materials, a team from Rice’s Nanophotonics Laboratory and Syzygy Plasmonics Inc. Princeton University’s Andlinger Center for Energy and Environment has developed a scalable catalyst that requires only the power of light to convert ammonia into clean-burning hydrogen fuel.
The research was published online today in the journal Sciences.
The research follows government and industry investment to create the infrastructure and markets for carbon-free liquid ammonia fuel that will not contribute to global warming. Liquid ammonia is easy to transport and contains a lot of energy, with one nitrogen and three hydrogen atoms per molecule. The new catalyst splits those molecules into hydrogen gas, a clean-burning fuel, and nitrogen gas, the largest component of Earth’s atmosphere. And unlike conventional catalysts, it does not require heat. Instead, it harvests energy from light, whether from sunlight or energy-scarce LEDs.
The frequency of chemical reactions typically increases with temperature, and chemical producers have taken advantage of this for more than a century by applying heat on an industrial scale. Burning fossil fuels to raise the temperature of large reaction vessels by hundreds or thousands of degrees results in an enormous carbon footprint. Chemical producers also spend billions of dollars each year on thermal catalysts – substances that do not react but speed up reactions under intense heating.
“Transition metals such as iron are generally weak thermocatalysts,” said study co-author Naomi Halas, of Rice. “This work shows that they can be efficient plasmonic photocatalysts. It also demonstrates that photocatalysis can be performed efficiently using inexpensive LED photon sources.”
“This discovery paves the way for sustainable, low-cost hydrogen that can be produced locally rather than in huge centralized plants,” said Peter Nordlander, also a co-author at Rice.
The best thermal catalysts are made of platinum and related precious metals such as palladium, rhodium, and ruthenium. Halas and Nordlander have spent years developing plasmonic metal nanoparticles. The best ones are also made from precious metals such as silver and gold.
Following their discovery in 2011 of plasmonic particles that produce short-lived, high-energy electrons called “hot carriers,” they discovered in 2016 that hot-carrier generators can be coupled with catalyst particles to produce hybrid “air reactors,” where one part harvests energy from light while the other part uses energy. To trigger chemical reactions with surgical precision.
Halas and Nordlander and their students and collaborators have been working for years to find alternatives to non-precious metals for both energy harvesting and reaction half-acceleration for antenna reactors. The new study is the culmination of that work. In it, Halas, Nordlander, Rice alumnus Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter, and others show that copper-iron aerospace reactor particles are highly efficient at converting ammonia. The piece of copper that collects energy from particles captures energy from visible light.
“In the absence of light, the copper-iron catalyst showed about 300 times less reactivity than the copper-ruthenium catalysts, which is not surprising given that ruthenium is a better thermal catalyst for this reaction,” said Robatjazi, Ph.D. He is a graduate of the Halas Research Group and is now the Chief Scientist at Syzygy Plasmonics, based in Houston. Under illumination, iron and copper showed efficiencies and activities similar to and comparable to those of copper and ruthenium.
Syzygy licensed the antenna reactor technology from Rice, and the study involved extensive catalyst testing in the company’s commercially available LED reactors. In Rice’s lab tests, the copper-iron catalysts were illuminated with a laser. Syzygy tests showed that the catalysts retained their efficiency under LED lighting and on a scale 500 times greater than the laboratory setting.
“This is the first report in the scientific literature showing that photocatalysis with LEDs can produce large amounts of hydrogen gas from ammonia,” Halas said. “This opens the door entirely to substituting precious metals in plasmonic photocatalysis.”
“Because of their potential to significantly reduce carbon emissions in the chemical sector, plasmonic antenna reactor photocatalysts deserve further study,” added Carter. “These results are a great catalyst. They suggest that other groups of abundant metals can potentially be used as cost-effective catalysts for a wide range of chemical reactions.”
Yigao Yuan et al, Earth-abundant photocatalyst for H generation from NH3 with light-emitting diode illumination, Sciences (2022). DOI: 10.1126/science.abn5636. www.science.org/doi/10.1126/science.abn5636
Provided by Rice University
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