Low-tech ammonia production could help slash global CO2 emissions

Two ammonia wagons at the railway station of Oulu in Finland. (Reference image by Methem (Mikko J. Putkonen), Wikimedia Commons.)

A team at Stanford University has discovered a simple and environmentally sound way to make ammonia with tiny droplets of water and nitrogen from the air.

In a paper published in the journal Proceedings of the National Academy of Sciences, the researchers explain that ammonia is the starting point for producing chemical fertilizers for farm crops.

For over a century, the world has relied on the Haber-Bosch process to yield ammonia but this industrial procedure is energy intensive. To break nitrogen’s strong bonds, the Haber-Bosch process requires roughly 80-300 atmospheres of pressure and temperatures around 300-500 degrees Celsius. The steam-treating of natural gas involved in the process also releases large amounts of climate-changing carbon dioxide.

According to the research group, to satisfy the current annual worldwide demand for 150 million metric tons of ammonia, the Haber-Bosch process gobbles up more than 2% of global energy and accounts for about 1% of the carbon dioxide emitted into the atmosphere.

In contrast, their new method requires less specialized circumstances.

“We were shocked to see that we could generate ammonia in benign, everyday temperature-and-pressure environments with just air and water and using something as basic as a sprayer,” Richard Zare, senior author of the paper, said in a media statement. “If this process can be scaled up, it would represent an eco-friendly new way of making ammonia, which is one of the most important chemical processes that take place in the world.”

The novel method also uses little energy, thus pointing a way forward to potentially producing the valuable chemical in a sustainable manner.

New chemistry

The new chemistry discovered follows in the footsteps of pioneering work by Zare’s lab examining the long-overlooked and surprisingly high reactivity of water microdroplets.

In a 2019 study, Zare and colleagues demonstrated that caustic hydrogen peroxide spontaneously forms in microdroplets in contact with surfaces. Experiments since have borne out a mechanism of electric charge jumping between the liquid and solid materials and generating molecular fragments, known as reactive oxygen species.

Taking those findings further, the study’s lead author, Xiaowei Song, and Zare began a collaboration with study co-author Basheer Chanbasha, a professor of chemistry at King Fahd University of Petroleum and Minerals in Saudi Arabia. Chanbasha specializes in nanomaterials for energy, petrochemical and environment applications.

The research team zeroed in on a catalyst that they suspected could help blaze a chemical pathway toward ammonia. The catalyst consists of an iron oxide, called magnetite, and a synthetic membrane invented in the 1960s that is composed of repeating chains of two large molecules.

The researchers applied the catalyst to a graphite mesh that Song incorporated into a gas-powered sprayer. The sprayer blasted out microdroplets in which pumped water and compressed molecular nitrogen (N2) reacted together in the presence of the catalyst. Using a device called a mass spectrometer, Song analyzed the microdroplets’ characteristics and saw the signature of ammonia in the collected data.

Low tech

“Our method does not require the application of any electrical voltage or form of radiation,” Zare pointed out.

The method uses nitrogen as gas, water as liquid, and catalyst as solid.

“To our knowledge, the idea of using gas, liquid, and solid all at the same time to cause a chemical transformation is a first of its kind and has a huge potential for advancing other chemical transformations,” Zare said.

While promising, the ammonia production method revealed by Zare, Song, and Chanbasha is only at the demonstration stage.

The researchers plan to explore how to concentrate the produced ammonia as well as gauge how the process could potentially be scaled up to commercially viable levels.

While Haber-Bosch is only efficient when pursued at huge facilities, the new ammonia-making method could be portable and done on-site or even on-demand at farms. That, in turn, would slash the greenhouse gas emissions related to the transportation of ammonia from far-off factories.