Platinum group metal catalysts have been a major focus of the chemical industry for decades. 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 convert ammonia into clean-burning hydrogen fuel.

The new catalyst splits these molecules into nitrogen gas, which makes up most of the Earth’s atmosphere, and hydrogen gas, a clean-burning fuel. In addition, it does not require heat like conventional catalysts. Instead, it draws its power from light sources, such as sunlight or energy-efficient LEDs.

For more than a century, chemical manufacturers have benefited from the fact that temperature generally speeds up chemical reactions by applying heat on an industrial scale. A significant carbon footprint is left when fossil fuels are burned to raise the temperature of massive reaction vessels by hundreds or thousands of degrees. Thermocatalysts are materials that do not react but accelerate processes when heated to high temperatures, and chemical manufacturers spend billions of dollars annually on them.

Study co-author Naomi Halas of Rice said: “Transition metals such as iron are typically poor thermocatalysts. This work shows that they can be efficient plasmonic photocatalysts. It shows that photocatalysis can work 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.”

Platinum and other closely related precious metals such as palladium, rhodium and ruthenium are used to make the best thermocatalysts. Halas and Nordlander have been making plasmonic or light-activated metal nanoparticles for years. The best are often made using precious metals such as gold and silver.

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, an energy-harvesting piece of the particles, captures energy from visible light.

Robatjazi said, “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. Under illumination, the copper-iron efficiencies and reactivities were comparable with and similar to 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.

Halas said, “This is the first report in the scientific literature to show that LED photocatalysis can produce gram-scale quantities of hydrogen gas from ammonia. This opens the door to completely replace precious metals in plasmonic photocatalysis.”

Carter added, “Given their potential to significantly reduce carbon emissions from the chemical sector, plasmonic antenna-reactor photocatalysts are worthy of further study. “These results are a great motivator. They suggest that it is likely that other combinations of abundant metals can be used as cost-effective catalysts for a wide variety of chemical reactions.”

Magazine reference:

  1. Yogao Yuan et al. Earth-abundant photocatalyst for H2 generation from NH3 with light-emitting diode illumination. Science. DOI: 10.1126/science.abn5