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Ammonia is used in fertilizer and many industrial processes. It is also seen as a promising way to store and transport energy, as it is safer and easier to handle ammonia than hydrogen gas. Using plasma, the fourth state of matter, scientists have created a material that boosts ammonia production.

"If one needs industrial hydrogen someplace else than where it is made, it will be easier and safer to transport hydrogen as ammonia and store it until it is needed. Ideally then one would decompose the ammonia where the hydrogen is needed, on demand," said Emily Carter, senior strategic advisor and associate laboratory director for Applied Materials and Sustainability Sciences (AMSS) directorate at the U.S. Department of Energy's (DOE) Princeton Plasma Â鶹ÒùÔºics Laboratory (PPPL).

"So, one needs methods to synthesize and decompose ammonia from and to hydrogen efficiently and cheaply, and we are working on both at PPPL in the electromanufacturing science division of AMSS."

The research was done by a multidisciplinary team from various institutions, including DOE's PPPL and Oak Ridge National Laboratory, Princeton University, Rutgers University and Rowan University. An about the work was recently published in ACS Energy Letters.

"The current method for making ammonia is expensive," said Zhiyuan Zhang, a doctoral candidate at Rutgers University-Newark and the lead author of the research. "You need a big factory to make the ammonia using processes that require extreme temperatures and pressures."

Storing and transporting hydrogen as ammonia

Ammonia can be used as a carrier for hydrogen, meaning it can store and transport the chemical before it is converted into hydrogen for energy. Hydrogen requires large manufacturing plants and storage facilities. This new method could create ammonia in far smaller facilities located closer to where it is needed—potentially even on-site. If the ammonia does have to be transported long distances, that, too, would be less expensive.

"Hydrogen has a very low energy density, and moving hydrogen around is extremely difficult. Ammonia has a higher energy density—twice compared to compressed hydrogen—and can be transported over long distances more easily than hydrogen," said Yiguang Ju, a principal investigator, managing principal research physicist and head of electromanufacturing science at PPPL, and a Princeton University professor. "This could open up a transformative change in energy storage and transportation."

Mark Martirez, the deputy advisor for sustainability science at AMSS and a research physicist, is now creating simulations of some of the experiments detailed in the new paper so the team can fully understand what's happening during the chemical reaction at an atomic level.

"Simulations are essential to fully understanding the mechanism that the chemical species undergoes to produce ammonia from water and nitrogen," Martirez said. "They could only guess the positions of the different atoms based on an image of the experiment." Martirez brings a rare understanding of the quantum chemistry involved in the process, which is broadly known as plasma catalysis and is a relatively new field.

Instead of using the high heat and pressure required for thermal catalysis—the old approach for making ammonia from hydrogen and nitrogen—the new method uses electricity, water, nitrogen and low-temperature plasma. In low-temperature plasma, the uncharged molecules are relatively cool or at room temperature. However, the electrons are very hot. The electrons have enough energy to change the surface of catalysts, knocking out certain atoms and implanting nitrogen or hydrogen atoms in their outermost layers.

A catalyst is an ingredient that speeds up chemical reactions without changing in the process. The catalyst used in the experiments has a unique structure which enables more energy-efficient chemical transformations. Scientists call this structure a heterogeneous interfacial complexion (HIC).

"The catalysts, tungsten oxide and tungsten oxynitride, are not new. What is new is the structure and the plasma-enabled method to fabricate it in a controllable and scalable way," said Huixin He, a Rutgers University professor who was one of the principal investigators of the research.

Structure of the catalyst is key to its efficiency

The special design of HIC helps create highly active hydrogen atoms right where they're needed to form tiny voids, known as nitrogen vacancies, that are a perfect fit for a nitrogen molecule. These features work together: The hydrogen atoms convert the nitrogen into ammonia, and the vacant spots attract more nitrogen from the air to keep the process going. This method significantly increases the amount of ammonia produced compared to older methods. It also minimizes unwanted side reactions, like the creation of hydrogen gas instead of ammonia.

"The process of producing this catalyst was reduced from approximately two days to 15 minutes," Zhang said. The process also outperformed other similar methods in terms of the amount of ammonia generated. The researchers will continue to study ways to improve ammonia production with the HIC catalyst.

Also involved in the research were: PPPL's Sophia Kurdziel; Christopher Kondratowicz, Yijie Xu, Elizabeth Desmet and Eddie Tang from Princeton University; Jacob Smith and Miaofang Chi from Oak Ridge National Laboratory; Pavel Kucheryavy, Junjie Ouyang and Michael Adeleke from Rutgers University; and Aditya Dilip Lele from Rowan University.