Inexpensive catalyst uses light energy to convert ammonia into hydrogen for fuel

Rice University researchers have developed an important light-activated nanomaterial for the hydrogen economy. Using inexpensive raw materials, a team from the Rice Laboratory for Nanophotonics, Syzygy Plasmonics Inc. and Princeton University's Andlinger Center for Energy and the Environment has developed a scalable catalyst that requires only the power of light to convert ammonia into clean hydrogen. The research results are online in the journal Science published.

The research follows government and industry investment to create infrastructure and markets for carbon-free liquid ammonia fuel that does not contribute to global warming. Liquid ammonia is easy to transport and contains a lot of energy with one nitrogen and three hydrogen atoms per molecule. The new catalyst splits these molecules into hydrogen gas, a clean-burning fuel, and nitrogen gas, the largest component of the Earth's atmosphere. And unlike traditional catalytic converters, it doesn't require heat. Instead, it derives energy from light, either from sunlight or from energy-saving LEDs.

The rate of chemical reactions tends to increase with temperature, and chemical manufacturers have been taking advantage of this by using heat on an industrial scale for more than a century. Burning fossil fuels to raise the temperature of large reaction vessels by hundreds or thousands of degrees has a huge carbon footprint. Chemical manufacturers also spend billions of dollars each year on thermal catalysts—materials that do not react but further accelerate reactions when heated to high temperatures.

"Transition metals like iron tend to be poor thermocatalysts," said Naomi Halas, co-author of the study at the Rice Institute. “This work shows that they can be efficient plasmonic photocatalysts. It also demonstrates that photocatalysis can be performed efficiently with inexpensive LED photon sources.”

"This discovery paves the way for sustainable, low-cost hydrogen that could be produced locally rather than in large central plants," said Peter Nordlander, also a co-author of Rice.

The best thermal catalysts are made from platinum and related noble metals such as palladium, rhodium, and ruthenium. Halas and Nordlander spent years developing light-activated, or plasmonic, metal nanoparticles. The best of them are also usually made from precious metals like silver and gold.

After their 2011 discovery of plasmonic particles that emit short-lived, high-energy electrons called "hot carriers," they discovered in 2016 that hot-carrier generators can be combined with catalytic particles to create hybrid "antenna-reactors" where one part captures energy from light and the other part uses the energy to control chemical reactions with surgical precision.

Halas, Nordlander, their students and collaborators have worked for years to find non-precious 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 graduate student Hossein Robatjazi, Princeton engineer and physical chemist Emily Carter, and others show that antenna reactor particles made of copper and iron are very efficient at converting ammonia. The copper, energy-absorbing part of the particles captures energy from visible light.

"In the absence of light, the copper-iron catalyst showed about 300 times lower reactivity than copper-ruthenium catalysts, which is not surprising since ruthenium is a better thermal catalyst for this reaction," said Robatjazi, a PhD student in Halas' research group , who is now a senior scientist at Syzygy Plasmonics in Houston. "Under illumination, the copper-iron showed similar efficiency and reactivity as the copper-ruthenium.

Syzygy licensed Rice's antenna reactor technology and the study included full-scale testing of the catalyst in the company's commercially available LED-powered reactors. During the laboratory tests in Rice, the copper-iron catalysts were illuminated with lasers. The Syzygy tests showed that the catalysts maintained their efficiency under LED lighting, on a scale 1 times larger than the laboratory setup.

"This is the first report in the scientific literature showing that photocatalysis with LEDs can produce gram-scale hydrogen gas from ammonia," Halas said. "This opens the door to completely replacing noble metals in plasmonic photocatalysis."

"Given their potential to significantly reduce carbon emissions from the chemical sector, plasmonic antenna reactor photocatalysts are worth further investigation," Carter added. “These results are a great incentive. They suggest that other combinations of common metals could also be used as inexpensive catalysts for a wide range of chemical reactions.”

Halas is Rice's Stanley C. Moore Professor of Electrical and Computer Engineering and Professor of Chemistry, Bioengineering, Physics and Astronomy, and Materials Science and Nanoengineering. Nordlander is the Wiess Chair and Professor of Physics and Astronomy at Rice University, and Professor of Electrical and Computer Engineering, Materials Science and Nanotechnology. Carter is Princeton's Gerhard R. Andlinger Professor of Energy and the Environment at the Andlinger Center for Energy and the Environment, Senior Strategic Advisor in Sustainability Science at Princeton Plasma Physics Laboratory, and Professor of Mechanical and Aerospace Engineering and Applied and Computational Mathematics. Robatjazi is also an Associate Professor of Chemistry at Rice University.

Halas and Nordlander are co-founders of Syzygy and hold an equity interest in the company.

Source: chemie.de and  Science; Nov 24, 2022; Vol 378, Issue 6622; pp. 889-893