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Urea fertilizer production using electrified synthesis can remove nitrogen from wastewater while enabling low carbon density urea production. Credit: Etienne Girardet

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Urea fertilizer production using electrified synthesis can remove nitrogen from wastewater while enabling low carbon density urea production. Credit: Etienne Girardet

Agriculture relies on synthetic nitrogen fertilizers, which are made using energy- and carbon-intensive processes and produce runoff containing nitrates. Researchers have long sought solutions to reduce emissions from industry, which account for 3% of energy consumption each year.

A collaboration between two laboratories at Northwestern University, in partnership with the University of Toronto, has found that producing urea fertilizer using electrified synthesis can remove nitrogen from wastewater while enabling low-carbon intensity urea production. The process, which involves converting carbon dioxide and nitrogen waste using a hybrid catalyst made of zinc and copper, could benefit water treatment facilities by reducing carbon emissions and providing a potential revenue stream.

The results are published today in the journal Stimulating nature.

“It is estimated that synthetic nitrogen fertilizers support half the world’s population,” said Ted Sargent, a Northwestern University professor and co-author of the study. “The main priority of decarbonization efforts is to improve the quality of life on Earth, while at the same time reducing net carbon dioxide in society.2 intensity. “Finding out how to use renewable electricity to power chemical processes represents a huge opportunity in this regard.”

Sargent is co-executive director of the Paula M. Trienens Institute for Sustainability and Energy (formerly ISEN) and an interdisciplinary researcher in materials chemistry and energy systems, with appointments in the Department of Chemistry in the Weinberg College of Arts and Sciences and Weinberg University. Department of Electrical and Computer Engineering in the McCormick School of Engineering.

In Sargent’s field, many researchers have developed alternative methods for making ammonia, a precursor to many fertilizers, but few have looked to urea, a chargeable, ready-to-use fertilizer. It represents a $100 billion industry. The team said the research stemmed from asking the question: “Can we use waste sources of nitrogen and captured carbon dioxide?”2And electricity to produce urea?

Look back to move forward

A deep dive into historical references helped determine what would become the “magical” hybrid catalyst, said Yuting Lu, the paper’s first author, a postdoctoral fellow in Sargent’s group and a Banting postdoctoral researcher. Typically, chemists use more complex alloys or materials to catalyze reactions, which limits them to favoring one reaction step at a time. “It is unusual to put two catalysts together that cooperate in a relay mode,” Luo said. “The catalyst is the real magic here.”

The team saw references dating back to the 1970s that implied that pure metals – such as zinc and copper – could be useful in processes involving the conversion of carbon dioxide and nitrogen.

These initial experiments, which Sargent’s lab continued to repeat, converted relatively few of the starting components into the desired product (the team found about 20-30% conversion efficiency to urea).

Renewable energy sources tip the scales

Bringing about change within industries requires careful cost-benefit analyzes that conclusively prove that the new production route will ultimately pay off in terms of energy and cost savings. That’s where chemical engineering professor Jennifer Dunn’s research came in. Chase Lavallee, fourth-year Ph.D. A student in Dan’s lab helped the team conduct a comprehensive life cycle analysis, carefully including all energy inputs and outputs in a variety of scenarios.

“Using an average U.S. grid, the energy emissions are roughly equal,” Lavallee said. “But when you go to renewables, there are several factors that reduce energy emissions, including carbon dioxide2 Sequestration and credits of stored carbon in end-use polymers. In a water treatment facility, if it adds emissions or energy, they are discouraged from using the technology. We have seen that this does not impact day-to-day operating costs significantly, and there is potential to sell the product.”

They found that conversion efficiency must reach 70% to be practical.

Improved “Magic Catalyst” ratio.

Starting with a simple mistake, the researchers eventually reached their goal. Their hypothesis was strong: applying a layer of zinc to copper would lead to better performance. But initially, they didn’t find that at all because they were applying a very thick layer of zinc and using a one-to-one ratio of zinc to copper, which resulted in the material behaving as if it was only reacting with zinc. At some point, someone added less binder than usual to the mix and some of the zinc was removed, and the experiment was very successful. The team then adjusted the metallurgy accordingly and determined the ratio of 1 part zinc to 20 parts copper resulted in optimal performance.

Sargent’s group also applied a computational lens to uncover why copper and zinc work so well together, and why synergy between the two reactions appears to be needed. Since it is impossible to capture these interactions optically—they occur on the nanosecond scale—one must calculate them and determine how the electrons move through the reaction.

This process had two distinct parts. First, carbon must react with zinc, because reaction with copper produces a weak reaction. In the second stage, the opposite is true: nitrogen and copper create an effective reaction, while zinc does very little.

The researchers said there is a way to go before this process can be commercialized. In the first place, the reaction as it stands does not take into account the impurities present in the course of water treatment. They also hope to increase the amount of time their process can operate.

more information:
Yuting Luo et al., Selective electrochemical synthesis of urea from nitrate and carbon dioxide via sequential catalysis on hybrid catalysts, Stimulating nature (2023). doi: 10.1038/s41929-023-01020-4

Magazine information:
Stimulating nature

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