Synthetic nitrogen fertilizers such as urea help feed about half of the world’s population. But making urea is a multistep endeavor that consumes copious amounts of energy and emits large amounts of greenhouse gases. A new one-step electrochemical method promises to make urea production more sustainable and affordable (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0481-9).
Commercial urea production relies on a complex reaction between ammonia and carbon dioxide at temperatures of 200 °C. Around 80% of the ammonia produced globally goes to urea production. And the Haber-Bosch process that makes ammonia requires temperatures of about 500 °C and high pressures. Together these reactions consume more than 2% of the world’s energy and emit more CO2 than any other industrial chemical reaction.
Shuangyin Wang, a chemical engineer at Hunan University, and his colleagues have bypassed these steps and come up with an electrochemical reaction that converts nitrogen gas and CO2 directly into urea in water under ambient temperature and pressure.
The reaction relies on a special catalyst consisting of palladium-copper nanoparticles on titanium dioxide nanosheets. The reaction takes place in a flow reactor cell containing a cathode made of carbon paper loaded with the catalyst and a nickel-based anode. The electrodes, separated by a membrane, sit in a chamber filled with an aqueous potassium bicarbonate electrolyte. The researchers pump N2 and CO2 through the cell so that both gases are adsorbed on the catalyst and react to produce urea.
A theoretical analysis of the reaction mechanism suggests that on the catalyst surface, N2 promotes the reduction of CO2 to produce carbon monoxide, Wang explains. The CO then reacts with N2 to generate a few intermediate species. Further interactions between CO and these intermediates hydrogenate N2 and create C–N bonds, giving urea.
The titanium dioxide support also plays a key role in the urea synthesis by stabilizing the intermediates. The system’s efficiency, which is a measure of the share of electricity that goes into producing urea, is around 9%. Increasing the electrolyte concentration and flow rate improves the efficiency, the researchers found.
The reaction’s efficiency and production rate are still low, and there’s a long way to go to make it practical, says Feng Jiao, a chemist at the University of Delaware. But he’s still excited by the reaction’s prospects. The feasibility of making urea directly in an electrochemical reactor opens up the possibility of producing fertilizer on a small scale rather than relying on large centralized ammonia and urea plants that are capital and energy intensive. “If you could make urea at a smaller scale, investment can be much smaller and more countries could afford to make it,” Jiao says.
Wang says they plan to design structured electrodes with other more efficient electrocatalysts to increase urea yield. They have also received commercial interest from companies and hope to scale up to a larger prototype.