Something in the Air
Nitrogen makes up nearly 80 percent of Earth’s atmosphere, a concentration four times that of oxygen, its neighbor on the periodic table. Nitrogen is also a critical ingredient in the molecules of life, such as DNA and proteins. But before the abundant element can be incorporated into these molecules, it must be changed from nonreactive N2 into compounds like ammonia that can be used by living things. This transformation happens through a process known as nitrogen fixation. It’s a process that is being reimagined by Jonas C. Peters, Bren Professor of Chemistry at Caltech and director of the Resnick Sustainability Institute (RSI), Caltech’s ambitious multidisciplinary initiative to build a sustainable future.
Peters’s nitrogen fixation studies could someday lower the energy cost of growing crops and ease the environmental woes caused by the runoff of nitrogen-rich fertilizer, which can trigger algae super blooms that suck up oxygen and create “dead zones” in the oceans. These studies could also lead to the growing use of sustainably sourced, energy-rich ammonia as a liquid fuel to help decrease our dependence on fossil fuels. His research also seeks to capture carbon dioxide that otherwise would enter the atmosphere and convert that CO2 into useful feedstock chemicals such as ethylene, the precursor from which all plastic materials are ultimately made. Ethylene is currently produced on a massive scale globally for this purpose but via the high-temperature cracking of hydrocarbons that release lots of CO2 into the atmosphere. Generating ethylene from CO2 instead would help close the carbon-cycle loop.
“We get extremely excited by these fundamental problems that push the boundaries of chemistry,” Peters says. “Then, being opportunistic, we explore whether they have potential value in the context of sustainability.”
Better, Safer, Cleaner
There are two basic ways to turn nitrogen from the air into a biologically useable form, both of which link the element with hydrogen to create ammonia (NH3). The first is an industrial method invented a century ago, which requires high temperatures and pressures to mass-produce fertilizer; this energy-intensive method is called the Haber-Bosch process. The second method is nature’s way, accomplished by common bacteria that use an enzyme called nitrogenase to convert atmospheric nitrogen into ammonia.
“My fundamental interest has been to understand the molecular basis of how life does it,” Peters says. To unlock the puzzle, his lab considers the enzymes at play in natural reactions and builds synthetic molecules to test key hypotheses about the process. A deeper understanding of this chemistry could lead to less energy consumption in industrial plants making ammonia. Farms that use ammonia for fertilizer, for example, could create the chemical on-site by using the elements around them, including hydrogen from water, nitrogen from the air, and sunlight to drive the process.
What’s equally exciting to Peters is ammonia’s potential as a liquid fuel. “Ammonia is a high-energy-density molecule,” he says, “and when you burn it, you don’t put any CO2 into the atmosphere but rather nitrogen and water.” Because of ammonia’s widespread use as a fertilizer, an infrastructure already exists to create the chemical and move it around via pipelines. The problem, Peters says, is that industrial ammonia production requires too much energy; it doesn’t make sense to create ammonia this way to then burn it as a fuel. “Our work to make ammonia by a renewable energy process could someday allow one to store sunlight in the form of this high-density, easily transportable, easily combusted zero-carbon fuel.” Sustainable ammonia fuel could find uses such as powering cargo ships, which currently burn dirty diesel.
The N2 work is supported by the National Institutes of Health (NIH) and the Department of Energy (DOE). The CO2 work falls under the umbrella of the Joint Center for Artificial Photosynthesis (JCAP), a DOE Innovation Hub headquartered at Caltech. Last year, Peters and Caltech professor of chemistry Theodor Agapie collaborated with University of Toronto researchers on a project that showed how chemical plants might capture carbon dioxide from their exhaust streams and use it to create the ethylene needed for most plastics. While plastic gets a bad name from single-use products such as straws and cups, Peters notes, it is a remarkably useful material and a stable way to store carbon.
“Plastic isn’t going away, nor should it,” he says. “It’s a material that’s not easily replaced. But what you’d like to do is make plastic very sustainably and then contain or recycle the types of the plastics that find their way into and damage the environment.”
A Hub of Discovery
Nitrogen fixation combines valuable resources found in the environment to maximize their impact. That is, essentially, the charge of the Resnick Sustainability Institute. RSI brings together researchers from across Caltech who can maximize the Institute’s impact on sustainability research.
Founded in 2009, the RSI research mission was amplified by a landmark $750 million commitment from Caltech trustee Stewart Resnick and his wife, Lynda. The Institute and the RSI believe that the only way to study such globe-spanning challenges is by bringing together chemists, applied physicists, materials scientists, engineers, economists, and more. “That’s why this hub was created, to connect the network of scientists who are working at Caltech,” Peters says.
What unites researchers with different perspectives and innovative ideas from across Caltech is the same kind of curiosity that drives Peters and the chemists in his lab. “We are, by nature, opportunistic,” Peters says. “The more you understand a molecule or material or process, the more you realize it might have a broader context, and that can lead down a path to long-term impact.”
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