Researchers from Princeton and Rice universities have combined iron, copper and a simple LED light to demonstrate an inexpensive technique that could hold the key to delivering hydrogen, a fuel that contains large amounts of energy without carbon pollution.
The researchers used experiments and advanced calculations to develop a technique using nanotechnology to separate hydrogen from liquid ammonia, a process that until now has been expensive and energy-intensive.
In an article published online in the journal Science, the researchers describe how they used light from a standard LED to break up ammonia without the need for high temperatures or expensive elements typically required by such chemistry. The technique overcomes a critical barrier to realizing hydrogen’s potential as a clean, low-emission fuel that could help meet energy demands without worsening climate change.
“We hear a lot that hydrogen is the ultimate clean fuel, if only it were cheaper and easier to store and retrieve for use,” said Naomi Halas, a Rice University professor and one of the main authors of the study. “This result demonstrates that we are rapidly moving towards that goal, with a new, simplified way to release hydrogen on demand from a convenient hydrogen storage medium using earth-abundant materials and the technological breakthrough of solid-state lighting.”
Hydrogen offers many advantages as a green fuel, including high energy density and zero carbon pollution. It is also used ubiquitously in industry, for example to make fertilizers, food, and metals. But pure hydrogen is expensive to compress for transport and difficult to store for long periods. In recent years, scientists have sought to use chemical intermediates to transport and store hydrogen. One of the most promising hydrogen carriers is ammonia (NH3), composed of three hydrogen atoms and one nitrogen atom. Unlike pure hydrogen gas (H2), liquid ammonia, although hazardous, has safe transport and storage systems.
“This discovery paves the way for sustainable, low-cost hydrogen that could be produced locally rather than in massive centralized plants,” said Peter Nordlander, a Rice professor and another lead author.
A lingering problem for proponents is that cracking ammonia into hydrogen and nitrogen often requires high temperatures to drive the reaction. Conversion systems may require temperatures in excess of 400 degrees Celsius (732 degrees Fahrenheit). It takes a lot of energy to convert the ammonia, as well as special equipment to run the operation.
Researchers led by Halas and Nordlander at Rice University, and Emily Carter, Gerhard R. Andlinger Professor of Energy and Environment and Professor of Mechanical and Aerospace Engineering and Applied and Computational Mathematics at Princeton, wanted to transform the fractionation process to make ammonia a more sustainable and economically viable carrier for hydrogen-based fuels. The use of ammonia as a hydrogen carrier has attracted considerable research interest due to its potential to stimulate a hydrogen economy, as shown in a recent study by the American Chemical Society.
Industrial operations often crack ammonia at high temperatures using a wide variety of materials as catalysts, which are materials that speed up a chemical reaction without being changed by the reaction. Previous research has demonstrated that it is possible to lower the reaction temperature by using a ruthenium catalyst. But ruthenium, a platinum group metal, is expensive. Researchers thought they could use nanotechnology to allow cheaper elements like copper and iron to be used as catalysts.
The researchers also wanted to tackle the energy cost of cracking ammonia. Current methods use a lot of heat to break the chemical bonds that hold ammonia molecules together. Researchers thought they could harness light to break chemical bonds like a scalpel rather than using heat to break them like a hammer. To do this, they turned to nanotechnology, as well as a much cheaper catalyst containing iron and copper.
Combining the tiny metallic structures of nanotechnology with light is a relatively new field called plasmonics. By shining light into structures smaller than a single wavelength of light, engineers can manipulate light waves in unusual and specific ways. In this case, the Rice team wanted to use this artificial light to excite the electrons in the metallic nanoparticles to split ammonia into its hydrogen and nitrogen components without the need for intense heat. Because plasmonics requires certain types of metals, such as copper, silver or gold, the researchers added iron to the copper before creating the tiny structures. When complete, the copper structures act as antennae to manipulate the light from the LED to excite electrons to higher energies, while the iron atoms embedded in the copper act as catalysts to speed up the reaction. performed by excited electrons.
The researchers created the structures and conducted the experiments in Rice’s labs. They were able to adjust many variables around the reaction such as pressure, intensity of light, and wavelength of light. But calibrating the exact settings was daunting. To investigate how these variables affected the reaction, the researchers worked with lead author Carter, who specializes in detailed investigations of reactions at the molecular level. Using Princeton’s high-performance computing system, the Terascale Infrastructure for Breakthrough Research in Engineering and Science (TIGRESS), Carter and his postdoctoral fellow, Junwei Lucas Bao, ran the reactions through his specialized quantum mechanical simulator, capable to study the catalysis of excited electrons. The molecular interactions of such reactions are incredibly complex, but Carter and his fellow researchers are able to use the simulator to figure out which variables need to be adjusted to favor the reaction.
“With quantum mechanical simulations, we can determine the rate-limiting reaction steps,” said Carter, who also holds positions at Princeton’s Andlinger Center for Energy and the Environment, in applied and computational mathematics, and Princeton Plasma Physics. Laboratory. “Those are the bottlenecks.”
By refining the process, while using the atomic-scale understanding provided by Carter and his team, the Rice team was able to consistently extract hydrogen from ammonia using only light from energy-efficient LEDs at room temperature. without additional heating. The researchers say the process is scalable. In further research, they plan to investigate other possible catalysts in an effort to increase process efficiency and reduce cost.
Carter, who also currently chairs the National Academies Committee on Carbon Utilization, said a crucial next step will be to reduce the costs and carbon pollution associated with creating the ammonia that starts the carbon cycle. transportation. Currently, most ammonia is created at high temperatures and pressures using fossil fuels. The process is both energy-intensive and polluting. Carter said many researchers are also working to develop green techniques for ammonia production.
“Hydrogen is used ubiquitously in industry and will increasingly be used as a fuel as the world seeks to decarbonize its energy sources,” she said. “However, today it is mostly unsustainably made from natural gas – creating carbon dioxide emissions – and is difficult to transport and store. Hydrogen must be made and transported sustainably there. If carbon-free ammonia could be produced, for example by electrolytic reduction of nitrogen using carbon-free electricity, it could be transported, stored and possibly serve as a source of hydrogen green on demand using the LED-lit iron-copper photocatalysts reported here.
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