The manufacturing of synthetic nitrogen fertilizer is an energy- and carbon-intensive process and creates nitrate-containing runoff. Researchers have long sought solutions to reduce emissions from the industry that accounts for three per cent of energy consumption each year.
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A collaboration between two labs at Northwestern University, partnering with the University of Toronto, has found that producing fertilizer urea using electrified synthesis could denitrify wastewater while enabling low-carbon-intensity urea production.
The process, which includes converting carbon dioxide and waste nitrogen by using a hybrid catalyst made of zinc and copper, could benefit water treatment facilities by reducing their carbon footprint and supplying a potential revenue stream.
“It’s estimated that synthetic nitrogen fertilizer supports half of the global population,” said Northwestern professor Ted Sargent. “A chief priority of decarbonization efforts is to increase quality of life on Earth, while simultaneously decreasing society’s net CO2 intensity. Figuring out how to use renewable electricity to power chemical processes is a big opportunity on this score.”
Sargent is a multidisciplinary researcher in materials chemistry and energy systems. In his field, many researchers have developed routes to make ammonia, a precursor to many fertilizers, but few have looked at urea, which is a shippable, ready-to-use fertilizer. It represents a $100 billion industry.
The team said the research stemmed from asking the question, “can we use waste nitrogen sources, captured CO2 and electricity to create urea?”
Yuting Luo, a post-doctoral fellow in the University of Toronto’s Sargent Group and a Banting postdoctoral researcher, said a deep dive into historical references helped identify what would become the “magic” hybrid catalyst. Typically, chemists use alloys or more complicated materials to trigger reactions, limiting them to favour a single reaction step at a time.
“It’s quite uncommon 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 pure metals like zinc and copper can be useful in processes involving carbon dioxide and nitrogen conversion.
These preliminary experiments, which the Sargent lab replicated, converted relatively little of the initial ingredients into the desired product. The team found about a 20-30 per cent conversion efficiency to urea.
Creating change within industries requires careful cost-benefit analyses that prove a new production route will pay off in both energy and cost savings. That’s where chemical engineering professor Jennifer Dunn’s research came in.
Chayse Lavallais, a fourth-year Ph.D. student in the Dunn lab, helped the team conduct a thorough life-cycle analysis, including each energy input and output in a variety of scenarios.
“Using an average U.S. grid, the energy emissions are about the same,” Lavallais said. “But when you go to renewable sources, several factors lower energy emissions, including CO2 sequestration and carbon credits stored in end-use polymers.
“In a water treatment facility, if it adds emissions or energy, they’re not encouraged to use the technology. We saw this doesn’t impact the daily operational costs significantly, and there’s potential to sell the product.”
They found the conversion efficiency would need to reach 70 per cent to be practical.
The researchers ultimately reached their target starting with a simple mistake. Their hypothesis was solid — a layer of zinc on copper would result in better performance. But initially, they didn’t find that because they were applying the layer of zinc too thick and using a one-to-one ratio of zinc to copper, so the material behaved as if it was only interacting with zinc.
At one point, someone added less binder than was typical to the mix, some zinc washed away, and the experiment worked well. The team then tuned the metals and determined a ratio of one part zinc to 20 parts copper resulted in optimal performance.
The Sargent group also applied a computational lens to uncover why copper and zinc worked so well together, and why it seemed synergy between the two reactions was needed.
Because it’s impossible to capture these reactions visually, they must be calculated to determine how electrons move across a reaction.
This process had two sections. First, the carbon must interact with zinc because a reaction with copper is weak. In the second stage, the opposite is true. Nitrogen and copper create an efficient reaction, while zinc does little.
Researchers said it will be awhile before the process can be commercialized. The reaction as it stands does not account for impurities found in a water treatment context. They also hope to increase the amount of time in which their process can operate.