A tractor spraying fertilizer. Photo: Etienne Girardet.
A tractor spraying fertilizer. Photo: Etienne Girardet.

Agriculture relies on synthetic nitrogen fertilizer, which is manufactured using energy- and carbon-intensive processes and can produce nitrate-containing runoff when applied to crops. Researchers have long sought ways to reduce emissions from the agricultural industry, which accounts for 3% of annual energy consumption.

A collaboration between two labs at Northwestern University, partnering with the University of Toronto in Canada, has now found that using electrified synthesis to produce the fertilizer urea could both reduce carbon dioxide (CO2) emissions and denitrify wastewater. The process, which involves using a hybrid catalyst made of zinc and copper to transform carbon dioxide and waste nitrogen into urea, could benefit water treatment facilities by reducing their carbon footprint and supplying a potential revenue stream. The researchers report their work in a paper in Nature Catalysis.

“It’s estimated that synthetic nitrogen fertilizer supports half of the global population,” said Northwestern professor Ted Sargent, a corresponding author of the paper. “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 co-executive director of the Institute for Sustainability and Energy and a multidisciplinary researcher in materials chemistry and energy systems. He has appointments at the department of chemistry in the Weinberg College of Arts and Sciences and the department of electrical and computer engineering in the McCormick School of Engineering.

In Sargent’s field, many researchers have developed alternate routes for making ammonia, a precursor to many fertilizers, but few have looked at urea, which is a shippable, ready-to-use fertilizer that comprises a $100 billion industry. This 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Yuting Luo, a post-doctoral fellow in the Sargent Group and the paper’s first author, said a deep dive into historical references helped identify what would become their ‘magic’ hybrid catalyst. “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 suggested pure metals — like zinc and copper — can be useful for catalysing reactions involving carbon dioxide and nitrogen. However, these preliminary experiments, which the Sargent lab went on to replicate, converted relatively little of the initial ingredients into the desired product (the team found a 20–30% conversion efficiency to urea).

Creating change within industries requires careful cost-benefit analyses that definitively prove a new production route will ultimately pay off in terms of both energy and cost savings. That’s where chemical engineering professor Jennifer Dunn’s research came in. Chayse Lavallais, a fourth-year PhD student in the Dunn lab, helped the team conduct a thorough life-cycle analysis, carefully including each energy input and output in a variety of scenarios.

“Using an average US 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.” This analysis also determined that the conversion efficiency would need to reach 70% for the catalytic process to be practical.

The researchers ultimately reached this target by taking advantage of a simple mistake. Their research suggested that adding a layer of zinc to copper should result in a better catalytic performance, but initially they couldn’t make this approach work.

Then, someone accidentally added less binder than was typical to the mix, causing some of the zinc to wash away, and the experiment worked very well. It turned out they were applying the layer of zinc too thick and using a one-to-one ratio of zinc to copper, which caused the catalyst to act as if it only contained zinc. The team then tuned the metals accordingly and determined that a ratio of one part zinc to 20 parts copper resulted in optimal performance.

Using computational calculations, the Sargent group was also able to determine why copper and zinc worked so well together. While carbon atoms interact mainly with the zinc, nitrogen atoms interact most efficiently with the copper.

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.