A crucial light-activated nanomaterial for the hydrogen economy has been developed by researchers at Rice University. A group from Princeton University’s Andlinger Center for Energy and the Environment, Syzygy Plasmonics Inc., and Rice University’s Laboratory for Nanophotonics developed a scalable catalyst that requires only light to transform ammonia into clean-burning hydrogen fuel.
The study comes after government and business investments to build markets and infrastructure for carbon-free liquid ammonia fuel that won’t contribute to global warming. With one nitrogen and three hydrogen atoms per molecule, liquid ammonia is both transportable and highly energetic.
The new catalyst converts these molecules into hydrogen gas, a fuel that burns cleanly, and nitrogen gas, the most abundant gas in the Earth’s atmosphere. Moreover, unlike conventional catalysts, it does not require heat. Instead, it harvests energy from sunlight or energy-efficient LEDs.
Efficient Plasmonic Photocatalysts
Chemical producers have benefited for more than a century by applying heat on an industrial scale by recognizing that the speed of chemical reactions typically increases with temperature.
The use of fossil fuels to heat large reaction vessels to hundreds or thousands of degrees creates a massive carbon footprint. Chemical manufacturers also spend billions of dollars each year on thermocatalysts, which are materials that do not react but accelerate reactions when heated to high temperatures.
“Transition metals like iron are typically poor thermocatalysts. This work shows they can be efficient plasmonic photocatalysts. It also demonstrates that photocatalysis can be efficiently performed with inexpensive LED photon sources,”
said co-author Naomi Halas of Rice.
“This discovery paves the way for sustainable, low-cost hydrogen that could be produced locally rather than in massive centralized plants,”
said co-author Peter Nordlander, also of Rice.
The most effective thermocatalysts consist of platinum and related precious metals such as palladium, rhodium, and ruthenium. Halas and Nordlander spent years creating light-activated, or plasmonic, nanoparticles of metal. Typically, the finest of these are also crafted from precious metals such as silver and gold.
Following their discovery in 2011 of plasmonic particles that emit short-lived, high-energy electrons known as “hot carriers,” they discovered in 2016 that hot-carrier generators could be married with catalytic particles to produce hybrid “antenna-reactors,” in which one part harvested energy from light and the other part used the energy to drive chemical reactions with surgical precision.
For years, Halas and Nordlander, along with their students and collaborators, have been searching for non-precious metal alternatives for both the energy-harvesting and reaction-speeding halves of antenna reactors.
Copper-iron Catalyst Efficiency
Their previous work has culminated in the new study. In it, Halas, Nordlander, Rice alumnus Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter, and others demonstrate that copper and iron antenna-reactor particles are highly efficient at converting ammonia. The particles’ copper energy-harvesting piece captures energy from visible light.
“In the absence of light, the copper-iron catalyst exhibited about 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. from Halas’ research group who now serves as chief scientist at Houston-based Syzygy Plasmonics.
Under illumination, the copper-iron efficiencies and reactivities were comparable to and similar to those of copper-ruthenium.
Wide Potential Range Of Chemical Reactions
Rice’s antenna-reactor technology was licensed to Syzygy, and the study included scaled-up tests of the catalyst in the company’s commercially available, LED-powered reactors.
Lasers were used to illuminate the copper-iron catalysts in laboratory tests at Rice. The catalysts retained their efficiency under LED illumination and at a scale 500 times larger than the lab setup, according to Syzygy tests.
According to Halas, this is the first scientific paper to demonstrate that photocatalysis with LEDs can produce gram-scale quantities of hydrogen gas from ammonia. It paves the way for precious metals to be completely replaced in plasmonic photocatalysis.
“Given their potential for significantly reducing chemical sector carbon emissions, plasmonic antenna-reactor photocatalysts are worthy of further study. These results are a great motivator. They suggest it is likely that other combinations of abundant metals could be used as cost-effective catalysts for a wide range of chemical reactions,”
The work received support from the Welch Foundation, Syzygy Plasmonics, the U.S. Department of Defense, the U.S. Air Force Office of Scientific Research, and Princeton University.
Reference: Yigao Yuan, Linan Zhou, Hossein Robatjazi, Junwei Lucas Bao, Jingyi Zhou, Aaron Bayles, Lin Yuan, Minghe Lou, Minhan Lou, Suman Khatiwada, Emily A. Carter, Peter Nordlander, Naomi J. Halas. Earth-abundant photocatalyst for H2 generation from NH3 with light-emitting diode illumination. Science, Vol 378, Issue 6622, DOI: 10.1126/science.abn5636