Producing ammonia using the Haber-Bosch process requires fossil fuels as well as high heat and pressure. Now, a combination of plasma (ionized gas) and non-noble-metal catalysts can synthesize ammonia at much milder conditions than possible with Haber-Bosch, say researchers at the University of Notre Dame, Notre Dame, Ind.
The plasma’s energy allows the nitrogen molecules to react more readily on the catalysts, note the researchers. This means the plasma-generated reaction doesn’t require high heat and intense pressure and can take place at small scale, making the process well-suited for use with intermittent renewable energy sources and for distributed ammonia production, they add.
“Plasmas have been considered by many as a way to make ammonia that is not dependent on fossil fuels and had the potential to be applied in a less centralized way,” says William Schneider, a professor at the university who led the study. “The real challenge has been to find the right combination of plasma and catalyst. By combining molecular models with results in the laboratory, we were able to focus in on combinations that had never been considered before.”
Applying a non-thermal plasma to a catalytic system allows manipulating reaction energetics in a way that isn’t possible in conventional thermal catalysis, the researchers explain.
The team, consisting of Schneider along with professors David Go and Jason Hicks, discovered that the plasma-activated nitrogen molecules require less-severe conditions for the metal catalysts, which enable use of less-expensive materials throughout the process.
The team believes this method may enable producing ammonia at low temperatures and atmospheric pressure at rates that can match the Haber–Bosch process. However, achieving such rates requires catalyst motifs different from those of thermal catalysis — i.e., shifting to sites that more weakly bind with nitrogen,” they add.
An article in Nature Catalysis provides more detail.
“The goal of our work was to develop an alternative approach to making ammonia, but the insights that have come from this collaboration… can be applied to other difficult chemical processes, such as converting carbon dioxide into a less harmful and more useful product. As we continue studying plasma-ammonia synthesis, we will also consider how else plasma and catalysts could benefit other chemical transformations,” notes Hicks.
“Our analysis also highlights opportunities for further improvements in reaction rates through careful control of the plasma properties… With sufficient excitation of high-energy states, active sites for ammonia synthesis may transition from step sites to terrace sites, enabling more atom-efficient catalysis,” adds Schneider.