By Paul Gabrielsen
Reprinted from UNews

Nearly a century ago, German chemist Fritz Haber won the Nobel Prize in Chemistry for a process to generate ammonia from hydrogen and nitrogen gases. The process, still in use today, ushered in a revolution in agriculture, but now consumes around one percent of the world’s energy to achieve the high pressures and temperatures that drive the chemical reactions to produce ammonia.

Today, University of Utah chemists publish a different method, using enzymes derived from nature, that generates ammonia at room temperature. As a bonus, the reaction generates a small electrical current. The method is published in Angewandte Chemie International Edition.

Although chemistry and materials science and engineering professor Shelley Minteer and postdoctoral scholar Ross Milton have only been able to produce small quantities of ammonia so far, their method could lead to a less energy-intensive source of the ammonia, used worldwide as a vital fertilizer.

“It’s a spontaneous process, so rather than having to put energy in, it’s actually generating its own electricity,” Minteer says.

How to make ammonia

Both the Haber-Bosch process (named also for Carl Bosch, who scaled up the process for industrial production) and the process developed by Minteer and Milton are based on fundamental chemical principles. To make ammonia, which consists of one nitrogen atom and three hydrogen atoms, chemists must break the strong bond that holds two nitrogen atoms together, and then reduce the nitrogen, or add electrons and protons to it in the form of hydrogen. In the Haber-Bosch process, hydrogen and nitrogen gas are pumped over beds of metal catalysts, which aid the reaction, at pressures up to 250 times atmospheric pressure and temperatures up to 500 degrees Celsius (932 F). The process currently produces nearly 500 million tons of ammonia every year.

PHOTO CREDIT: Ross Milton
A diagram of the chemistry used in a hydrogenase-nitrogenase ammonia producing fuel cell.

Simulating a cell

In biology, conversion of gaseous nitrogen to ammonia is called nitrogen “fixation” and is accomplished through several pathways, including through enzymes called nitrogenases. Used by some bacteria, nitrogenases are the only known enzymes to reduce nitrogen to ammonia. Nitrogenase is rarely studied in fuel cell applications, because the enzyme is not commercially available and must be handled in an oxygen-free environment.

“One of the things my group does well is designing the interface between the enzyme and the electrode, so the enzymes can communicate with electrode surfaces,” Minteer says.