hydrogen economy

New catalysts could be the key to the hydrogen economy

Catalysts based on platinum group metals 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 has 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 catalytic converters. Instead, it draws energy from light sources such as sunlight or energy-efficient LEDs.

Chemical manufacturers have benefited from the fact that temperature in general accelerates chemical reactions by adding heat on an industrial scale for more than a century. 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 on them every year.

Study co-author Naomi Halas of Rice said: “Transition metals like 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 could be produced locally rather than in huge centralized plants.”

Platinum and other closely related noble metals such as palladium, rhodium and ruthenium are used to make the best thermal catalysts. Halas and Nordlander spent years creating plasmonic, or light-activated, metal nanoparticles. The best are often made from precious metals such as gold and silver.

Halas, Nordlander, their students, and collaborators have worked for years to find base metal alternatives for both the energy-harvesting and reaction-accelerating halves of antenna reactors. The new study is a culmination of this work. In it, Halas, Nordlander, Rice alumnus Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter, and others show that copper-iron antenna reactor particles are highly efficient at converting ammonia. The copper, an energy-harvesting piece of the particle, captures energy from visible light.

Robatjazi said “In the absence of light, the copper-iron catalyst showed about 300-fold lower reactivity than copper-ruthenium catalysts, which is not surprising since ruthenium is a better thermal catalyst for this reaction. 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-powered reactors. In laboratory tests at Rice, the copper-iron catalysts were illuminated with lasers. The Syzygy tests showed that the catalysts maintained their efficiency under LED lighting and on a scale 500 times larger than in the laboratory setup.

Halas said “This is the first report in the scientific literature showing that LED photocatalysis can generate gram-scale quantities of hydrogen gas from ammonia. This opens the door to completely replacing noble metals in plasmonic photocatalysis.”

Carter added, “Given their potential to significantly reduce carbon emissions from the chemical sector, plasmonic antenna reactor photocatalysts deserve further investigation. “These results are a great motivation. They suggest that other combinations of abundant metals could likely be used as inexpensive catalysts for a variety of chemical reactions.”

Magazine reference:

  1. Yogao Yuan et al. Earth’s abundant photocatalyst for H2 production from NH3 with LED lighting. Science. DOI: 10.1126/science.abn5

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