The agricultural industry heavily relies on synthetic nitrogen fertilizer, a product manufactured through energy-intensive and carbon-heavy processes, often leading to nitrate-laden runoff when applied to crops. This sector’s contribution to global energy consumption hovers around 3%, prompting a prolonged quest by researchers to find sustainable solutions for reducing emissions.
A groundbreaking collaboration between two research labs at Northwestern University, in partnership with the University of Toronto in Canada, has unveiled an innovative method utilizing electrified synthesis to produce urea-based fertilizer. This approach not only offers a potential avenue for reducing carbon dioxide (CO2) emissions but also addresses wastewater denitrification. The process employs a hybrid catalyst composed of zinc and copper, transforming carbon dioxide and waste nitrogen into urea, a development that could have profound implications for water treatment facilities by lowering their carbon footprint and potentially generating revenue streams.
The findings of this research have been published in a paper featured in Nature Catalysis. As Northwestern professor Ted Sargent, a corresponding author of the paper, emphasized, “It’s estimated that synthetic nitrogen fertilizer supports half of the global population. A chief priority of decarbonization efforts is to increase the 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, who also serves as the co-executive director of the Institute for Sustainability and Energy, has a multidisciplinary background in materials chemistry and energy systems, with affiliations in both the department of chemistry within the Weinberg College of Arts and Sciences and the department of electrical and computer engineering in the McCormick School of Engineering.
While many researchers in Sargent’s field have explored alternative pathways for ammonia production, a precursor to various fertilizers, few have delved into urea, a readily transportable, market-dominating fertilizer with a valuation of approximately $100 billion. The inspiration for this research derived from a compelling question: “Can we use waste nitrogen sources, captured CO2, and electricity to create urea?”
The critical breakthrough came in the form of a ‘magic’ hybrid catalyst, a discovery unearthed through an extensive exploration of historical references. Yuting Luo, the paper’s first author and a post-doctoral fellow in the Sargent Group, highlighted the significance of this catalyst: “It’s quite uncommon to put two catalysts together that cooperate in a relay mode. The catalyst is the real magic here.”
Historical references dating back to the 1970s suggested the efficacy of pure metals, such as zinc and copper, in catalyzing reactions involving carbon dioxide and nitrogen. Nonetheless, early experiments demonstrated a modest conversion efficiency, ranging from 20% to 30%, in the transformation of initial ingredients into urea.
Implementing change within established industries necessitates meticulous cost-benefit analyses to unequivocally demonstrate that a new production route will eventually yield energy and cost savings. Chemical engineering professor Jennifer Dunn’s research played a pivotal role in this aspect. A fourth-year PhD student in the Dunn lab, Chayse Lavallais, contributed to a comprehensive life-cycle analysis, taking into account every energy input and output under various scenarios.
Lavallais explained, “Using an average US grid, the energy emissions are about the same. 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.” The analysis also pinpointed a threshold of 70% conversion efficiency as the practical requirement for the catalytic process.
The researchers eventually achieved this milestone by capitalizing on an unexpected turn of events. Their research suggested that applying a zinc layer on top of copper should enhance catalytic performance, but initial attempts proved unsuccessful. Serendipitously, a reduction in binder volume, causing some of the zinc to wash away, led to the experiment’s success. It turned out that the layer of zinc was initially applied too thick, causing the catalyst to behave as if it only contained zinc. The team refined the metal ratios, ultimately concluding that a 1:20 ratio of zinc to copper yielded optimal performance.
Furthermore, computational calculations conducted by the Sargent group shed light on the synergistic interaction between copper and zinc. Carbon atoms predominantly interact with zinc, while nitrogen atoms exhibit the most efficient interaction with copper.
