A new process using light to reduce dinitrogen into ammonia, the main ingredient in chemical fertilizers could inspire development of new, more sustainable processes that eliminate the energy-intensive, lengthier processes now commonly in use. According to researchers at the Energy Department's National Renewable Energy Laboratory (NREL), photochemical (photon) energy can serve as a substitute for the adenosine 5'-triphosphate (ATP)-dependent electron transfer mechanism typically used in biology to drive nitrogenase to reduce dinitrogen (N2) to ammonia (NH3).
Scientists at NREL collaborated with researchers at Utah State University, University of Colorado Boulder, and Montana State University to develop the new ammonia-producing process, devising a way to harvest light to initiate the enzymatic reduction of N2.
"Not only were we able to prove the light-driven process successful for the first time, but the rates of ammonia production were shown to be a good approximation to those of the ATP-dependent reaction, at 60-70 percent," said NREL Research Scientist Paul King.
The study, "Light-driven dinitrogen reduction catalyzed by a CdS:nitrogenase MoFe protein biohybrid," appears in the journal Science, and was authored by Katherine A. Brown, Derek F. Harris, Molly B. Wilker, Andrew Rasmussen, Nimesh Khadka, Hayden Hamby, Stephen Keable, Gordana Dukovic, John W. Peters, Lance C. Seefeldt, and Paul W. King.
The researchers demonstrated that cadmium sulfide (CdS) nanocrystals can be used to harvest light, turning the energy from that light to energize electrons with sufficient potential to propel the reduction of N2 into ammonia, which takes place within the nitrogenase molybdenum iron (MoFe) protein. The new method essentially replaces the ATP hydrolysis-dependent enzymatic process with CdS nanorod light-harvesting and energy conversion.
Reducing N2 to ammonia is usually a very energy-intensive process. In the long-standing Haber-Bosch process used by industry, N2 reduction is accomplished using high temperatures and high pressure. In biology, the reduction is catalyzed by nitrogenase in a reaction that requires ATP to act as the energy source, which constrains how fast the reaction can take place. Both the biological and industrial processes require high energy input, and both result in emission of high levels of carbon dioxide. The new process requires far less energy and emits no carbon dioxide.
"Using light harvesting to drive difficult catalytic reactions has the potential to create new, more efficient chemical and fuel production technologies," said NREL Research Scientist Katherine Brown. "Light driven N2 reduction has been done before, just at very, very low rates and not by nitrogenase. We did it with nitrogenase and obtained much higher yields and rates."
In addition to its use in chemical fertilizers, ammonia is also a way to store energy (light energy in this case) that can then be used to power an ammonia fuel cell. Currently, there is high interest in storing solar energy (or solar electricity) in the form of biofuels or reduced chemicals like ammonia, and using these products as energy carriers to power vehicles and fuel cell devices. The new research is expected to inspire alternative concepts for meeting the demand for ammonia, but in a more energy-efficient and sustainable manner, with a lower impact on the environment than current commercial processes.
Because this study is the first to demonstrate the new process, the findings are important to help future studies evaluate the technological impact on a practical system, said the researchers. On a more fundamental level, there are many aspects of the process that make it useful as a tool to more fully understand enzyme functionality-for example, how enzymes catalyze N2 reduction and how efficiencies of using sunlight to drive difficult chemical reactions can be improved. In addition, the new process will allow researchers to explore the limit of how fast the nitrogenase enzyme can catalyze the ammonia-producing reaction.
Funding for this study was provided by NREL's Laboratory-Directed Research and Development program, which promotes innovations in science and technology, and by the DOE Office of Science.
"We followed an idea to create new science that we expect will make a significant impact on the industrial and scientific production of ammonia," King said. "In the case of this study, the type of funding allowed us to try something that we hadn't tested before, and it paid off."
NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.
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