Researchers at Rice University have developed a key light-activated nanomaterial for the hydrogen economy. Using only inexpensive raw materials, a team from the Rice Nanophotonics Lab, Syzygy Plasmonics Inc., and Princeton University’s Andlinger Center for Energy and Environment has created a scalable catalyst that only needs than the power of light to convert ammonia into clean-burning hydrogen.
The research is published online today in the journal Science.
The research follows government and industry investments to create infrastructure and markets for carbon-free liquid ammonia that will not contribute to greenhouse warming. Liquid ammonia is easy to transport and contains a lot of energy, with one nitrogen atom and three hydrogen atoms per molecule. The new catalyst breaks these molecules down into hydrogen gas, a clean-burning fuel, and nitrogen gas, the largest component of Earth’s atmosphere. And unlike traditional catalysts, it does not require heat. Instead, it harvests energy from light, either sunlight or power-hungry LEDs.
The rate of chemical reactions generally increases with temperature, and chemical manufacturers 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 reactors by hundreds or thousands of degrees results in a huge carbon footprint. Chemical manufacturers also spend billions of dollars each year on thermocatalysts, materials that do not react but further accelerate reactions under intense heating.
“Transition metals like iron are generally poor 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 efficiently performed with inexpensive LED photon sources.”
“This discovery paves the way for sustainable, low-cost hydrogen that could be produced locally rather than in massive centralized plants,” said Peter Nordlander, also a Rice co-author.
The best thermocatalysts are made from platinum and related precious metals like palladium, rhodium and ruthenium. Halas and Nordlander have spent years developing light-activated (plasmonic) metal nanoparticles. The best ones are also usually made with precious metals like silver and gold.
Following their 2011 discovery of plasmonic particles that emit short-lived, high-energy electrons called “hot carriers”, they found in 2016 that hot carrier generators could be married to catalytic particles to produce “antennae- hybrid reactors, where one part harvested energy from light and the other part used the energy to cause chemical reactions with surgical precision.
Halas, Nordlander, their students and collaborators worked for years to find base metal alternatives for the energy harvesting and reaction accelerating halves of antenna reactors. The new study is the culmination of that work. In it, Halas, Nordlander, former Rice student Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter, and others show that copper and iron antenna-reactor particles are very effective at convert ammonia. The copper part collecting the energy of the particles collects the energy of the visible light.
“In the absence of light, the copper-iron catalyst exhibited approximately 300 times lower reactivity than copper-ruthenium catalysts, which is not surprising given that ruthenium is a better thermocatalyst for this reaction,” said said Robatjazi, a Ph.D. alumnus of Halas’ research group who is now chief scientist at Houston-based Syzygy Plasmonics. “Under illumination, copper-iron showed similar and comparable efficiencies and reactivities as copper-ruthenium.
Syzygy licensed Rice’s antenna-reactor technology, and the study included large-scale testing of the catalyst in the company’s commercially available LED reactors. In laboratory tests at Rice, the copper-iron catalysts had been illuminated by lasers. Syzygy tests showed that the catalysts retained their efficiency under LED illumination and at 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 amounts of hydrogen from ammonia,” Halas said. “This opens the door to the complete replacement of precious metals in plasmonic photocatalysis.”
“Given their potential to significantly reduce carbon emissions from the chemical sector, plasmonic antenna-reactor photocatalysts merit further study,” Carter added. “These results are a great source of motivation. They suggest that it is likely that other combinations of abundant metals could be used as cost-effective catalysts for a wide range of chemical reactions.”
Yigao Yuan et al, Earth-abundant photocatalyst for generation of H2 from NH3 with LED illumination, Science (2022). DOI: 10.1126/science.abn5636. www.science.org/doi/10.1126/science.abn5636
Provided by Rice University
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