Over 100 years after Fritz Haber won the Nobel Prize for the synthesis of ammonia from its elements nitrogen and hydrogen, the process he pioneered remains the main industrial method for making the foundational compound found in nitrogen-based fertilizers. More recently, a newer method—called the nitrogen reduction reaction (NRR)—is being touted as a more cost-effective and sustainable alternative.
In NRR, ammonia can be produced at standard conditions using electricity and two of the world’s most abundant compounds: nitrogen and water. But limiting factors are the sluggish adsorption of N2 and the high cleavage energy of the N-N triple bond. Another is the attendant hydrogen evolution reaction (HER), which occurs in competition with NRR.
In the past few years, the search for better catalysts has revealed that the element molybdenum—chemical symbol Mo—and Mo-based compounds like Mo2C have the ideal configuration of electrons needed to weaken the bond in N2. On their own, however, Mo and Mo2C catalysts have different selectivity and are suboptimal for NRR.
A team of researchers from Singapore and China, including co-corresponding author Xu Li, a Senior Scientist at A*STAR’s Institute of Materials Research and Engineering (IMRE), predicted that combining Mo and Mo2C into a composite catalyst could have a synergizing effect and ultimately boost ammonia production. Their new study provides evidence for this theory.
“Molybdenum-based materials are regarded as one of the most promising catalysts for NRR. In our catalyst system, the two active parts with different selectivity—Mo single atoms and Mo2C nanoparticles—are mutually compensated through a synergistic interaction,” explained Li.
Using theoretical calculations, the researchers first showed that Mo2C is more selective for NRR, while Mo single atoms are more selective for HER. They then went on to create and test three catalysts: Mo single atoms, Mo2C nanoparticles, and a composite catalyst called MoSAs-Mo2C/NCNT.
Performing NRR at room temperature and atmospheric pressure, the researchers found that MoSAs-Mo2C/NCNT boosted the reaction speed by up to 4.5 times and efficiency by up to seven times, compared to Mo single atoms. The composite catalyst was incredibly stable, maintaining the same level of activity for over ten hours.
Next, the researchers plan to test their catalyst in larger-scale reactions. However, they caution that more research is needed before these composite-type catalysts can be relevant in industrial applications.
“A substantial challenge for composite-type nanomaterials is to control the ratio and distribution of the two or more different phases,” Li said. “A deeper understanding of the mechanism in each case is urgently needed.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE).