Notre Dame team develops plasma-enabled catalysis for ammonia production at milder conditions at smaller scale
The Haber-Bosch process developed in the early 1900s relies on non-renewable fossil fuels and large, centralized chemical plants for the large-scale synthesis of ammonia at elevated temperatures (~700 K) and pressures (~100atm). Researchers at the University of Notre Dame have now developed a sustainable low-temperature and -pressure ammonia synthesis process using plasma-based catalysis.
The new process, described in a paper in Nature Catalysis, utilizes a plasma—an ionized gas—in combination with non-noble metal catalysts to generate ammonia at much milder conditions than is possible with Haber-Bosch.
The energy in the plasma excites nitrogen molecules, one of the two components that go into making ammonia, allowing them to react more readily on the catalysts. Because the energy for the reaction comes from the plasma rather than high heat and intense pressure, the process can be carried out at small scale. This makes the new process well-suited for use with intermittent renewable energy sources and for distributed ammonia production.
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. 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.—William Schneider, H. Clifford and Evelyn A. Brosey Professor of Engineering, affiliated member of ND Energy and co-author
The research team led by Schneider; David Go, Rooney Family Associate Professor of Engineering in aerospace and mechanical engineering; and Jason Hicks, associate professor of chemical and biomolecular engineering, discovered that because the nitrogen molecules are activated by the plasma, the requirements on the metal catalysts are less stringent, allowing less expensive materials to be used throughout the process.
This approach overcomes fundamental limits on the heat-driven Haber-Bosch process, allowing the reaction to be carried out at Haber-Bosch rates at much milder conditions.
Application of a non-thermal plasma to a catalytic system can selectively activate certain degrees of freedom of gas molecules, allowing manipulation of reaction energetics in a way that is not possible in conventional thermal catalysis. Using a microkinetic model that incorporates nitrogen vibrational excitations expected of a non-thermal N2:H2 plasma, we predict that it is possible to produce NH3 at low temperatures and atmospheric pressure in plasma-catalytic reaction environments at rates that can match those of the high-temperature and -pressure Haber–Bosch process. However, such rates cannot be achieved on catalyst motifs that are most active for thermal catalysis; instead, the optimal catalyst is shifted to sites that bind nitrogen more weakly.
The shift in optimal catalyst is confirmed by kinetic experiments in a plasma reactor, showing that the idealized model can provide the essential insights to help guide the design of plasma-catalytic systems. The work thus represents an important step forwards in connecting plasma catalysis to well- established concepts of conventional heterogeneous catalysis.
… 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.—Mehta et al.
Prateek Mehta, Patrick Barboun, Francisco A. Herrera, Jongsik Kim, Paul Rumbach, David B. Go, Jason C. Hicks & William F. Schneider (2018) “Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis” Nature Catalysis (2018) doi: 10.1038/s41929-018-0045-1