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Small, ambient, electricity-driven ammonia synthesis could produce fertilizer anywhere

By Tim Palucka August 13, 2018
Pd-C ammonia catalyst
Catalytic mechanism for N2 to NH3 reaction involving hydrogen atoms in the bulk of the Pd catalyst “hopping” to the surface to provide an activated surface site for N2 hydrogenation. Image courtesy of Xiaofeng Feng, University of Central Florida.

The demand for nitrogen fertilizer continues to increase as the need for plant-derived food rises with the world’s burgeoning population. According to the Food and Agriculture Organization of the United Nations, the world is projected to use 119.4 million metric tons of nitrogen in 2018, up from 117.9 million metric tons in 2017. To date, the Haber-Bosch process, which requires large industrial chemical facilities and high temperatures and pressures to produce ammonia (NH3)—the main form of nitrogen-based fertilizer—has been the primary method of synthesis.

Now researchers at the University of Central Florida and the Virginia Polytechnic Institute and State University have reported preliminary experimental and quantum-chemical simulation results in Nature Communications on a Pd-based catalytic system to produce ammonia electrocatalytically. Importantly, the synthesis is carried out at room temperature and ambient pressure. If this electrocatalytic hydrogenation of nitrogen can be successfully developed and commercialized, “this process will need only a small electrolytic device that can be powered by solar-generated electricity, which means that the electrocatalytic ammonia synthesis can be distributed, because there are nitrogen, water, and sunshine almost everywhere,” says Xiaofeng Feng, assistant professor of physics at the University of Central Florida and one of the lead researchers on this work. “This electricity-driven ammonia production can be sustainable and won’t release CO2. The ammonia can be produced near point-of-use so that its transportation will be minimized,” he says.

While other research groups have attempted similar electrocatalytic processes, this work succeeded because of two factors, Feng believes: the use of a Pd/C system as the catalyst and a neutral pH phosphate buffer solution (PBS) as the electrolyte.

First, Pd metal, in the form of nanosized islands supported on carbon black, can absorb hydrogen into its bulk thereby forming Pd hydride, unlike other catalytic metals. The hydrogen atoms stored in the bulk of the Pd catalyst can hop to the surface and interact with adsorbed N2 molecules to form the rate-determining *N2H intermediate. This reaction pathway requires a free energy approximately 0.3 eV lower than that without bulk hydrogen atoms. Hongliang Xin, assistant professor of chemical engineering at the Virginia Polytechnic Institute and State University and another primary researcher for this work, postulated that a mechanism inspired by Grotthuss-like proton hopping in a water network is at work here. The Grotthuss mechanism, proposed by Theodor Grotthuss in 1806, describes how protons tunnel from one water molecule to the next through hydrogen bonding.

Second, the neutral pH PBS electrolyte, as opposed to more commonly studied acidic or basic electrolytes, suppresses the competing hydrogen evolution reaction (HER) which typically consumes 99% of the electrons in the system. With so many electrons being consumed in producing hydrogen, few are available to activate the N2 molecule by splitting its triple bond. Feng and colleagues used linear sweep voltammograms with the catalyst in neutral PBS (pH = 7.2), acidic H2SO4 (pH = 1.2), and basic NaOH (pH = 12.9) electrolytes to show that the current density in the PBS solution was several times lower than that in the acidic and basic solutions, showing that the HER is greatly suppressed in PBS, leaving more electrons available for N2 hydrogenation. 

The results show that the Pd/C catalyst in the PBS electrolyte produces an NH3 yield rate of around 4.5 μg/mg h with a Faradaic efficiency of 8.2% at 0.1 V versus the reversible hydrogen electrode. This corresponds to an overpotential of 56 mV, which is approximately 300 mV lower than the best alternative catalytic solution to date.

“The neutral electrolyte creates a reaction environment that makes this Pd metal outstanding,” Feng says. “Each catalyst has its preferred reaction conditions. In this research, we have identified an optimized environment for this challenging reaction catalyzed by Pd metal.”

“Together with using renewable electricity, this approach represents an important step toward sustainable nitrogen transformations,” says Jingguang Chen, Thayer Lindsley Professor of Chemical Engineering at Columbia University, who was not involved in this research. “This work also provides understanding of reaction mechanisms enabled by hydride transfer and of the structure-activity relationships that might hold the promise for developing more efficient and cost-effective catalysts for this process.”

Recognizing the limitations imposed by the high cost of Pd, Feng, Xin, and colleagues are planning future research that might replace part of the Pd with another, less expensive catalytic metal.

Read the article in Nature Communications.