Rice University researchers have developed an important light-activated nanomaterial for the hydrogen economy. Using only cheap raw materials, a team from Rice’s Laboratory for Nanophotonics, Syzygy Plasmonics Inc. and Princeton University’s Andlinger Center for Energy and the Environment 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 Science.
The research tracks government and industry investment to create infrastructure and markets for carbon-free liquid ammonia fuel that will not contribute to global warming. Liquid ammonia is easy to transport and high in energy, with one nitrogen and three hydrogen atoms per molecule. The new catalyst breaks those molecules down into hydrogen gas, a clean-burning fuel, and nitrogen gas, the largest component of Earth’s atmosphere. And unlike traditional catalytic converters, it does not require heat. Instead, it harvests energy from light, either sunlight or energy-hungry LEDs.
The rate of chemical reactions typically increases with temperature, and chemical manufacturers have capitalized on 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 a huge environmental footprint. Chemical manufacturers also spend billions of dollars each year on thermocatalysts — materials that don’t react, but accelerate reactions under intense heating.
“Transition metals like iron are typically poor thermocatalysts,” said Rice co-author Naomi Halas. “This work shows that they can be efficient plasmonic photocatalysts. It also shows that photocatalysis can be performed efficiently with low-cost LED photon sources.”
“This discovery paves the way for sustainable, low-cost hydrogen that can be produced locally rather than in massive centralized plants,” said Rice co-author Peter Nordlander.
The best thermocatalysts are made from platinum and related precious metals such as palladium, rhodium and ruthenium. Halas and Nordlander have spent years developing light-activated (plasmonic) metal nanoparticles. The best of these are also usually made with precious metals such as silver and gold.
Following their 2011 discovery of plasmonic particles that release short-lived, high-energy electrons called “hot carriers”, they discovered in 2016 that hot carrier generators can be combined with catalytic particles to produce hybrid “aerial reactors”, creating a part harvested energy from light and the other part used the energy to drive chemical reactions with surgical precision.
Halas, Nordlander, their students and collaborators have spent years finding non-precious metal alternatives to both the energy-harvesting and reaction-accelerated halves of antenna reactors. The new study is a culmination of that work. In it, Halas, Nordlander, Rice alumnus Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter and others show that aerial reactor particles made of copper and iron are highly efficient at converting ammonia. The copper, energy-harvesting part of the particles captures energy from visible light.
“In the absence of light, the copper-iron catalyst showed approximately 300 times lower reactivity than copper-ruthenium catalysts, which is not surprising given that ruthenium is a better thermocatalyst for this reaction,” said Robatjazi, a Ph.D. alumnus of Halas’s research group who is now chief scientist at Syzygy Plasmonics in Houston. “Under illumination, the copper-iron showed efficiencies and reactivities similar to and similar to that of copper-ruthenium.
Syzygy licensed Rice’s antenna reactor technology and the study included scaled-up testing of the catalyst in the company’s commercially available LED-powered reactors. In lab tests at Rice, the copper-iron catalysts had been illuminated with lasers. The Syzygy tests showed that the catalysts maintained their efficiency under LED lighting and on a scale 500 times larger than the lab setup.
“This is the first report in the scientific literature to show that photocatalysis with LEDs can produce gram-scale quantities of hydrogen gas from ammonia,” Halas said. “This opens the door to completely replace precious metals in plasmonic photocatalysis.”
“Given their potential to significantly reduce carbon emissions from the chemical sector, plasmonic aerial reactor photocatalysts are worthy of further study,” Carter added. “These results are a great motivator. They suggest that it is likely that other combinations of abundant metals could be used as cost-effective catalysts for a wide variety of chemical reactions.”
Yigao Yuan et al, Earth-abundant photocatalyst for H2 generation from NH3 with light-emitting diode illumination, Science (2022). DOI: 10.1126/science.abn5636. www.science.org/doi/10.1126/science.abn5636
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